Publications

The ability to control the icing temperature of supercooled water (SCW) is of supreme importance in subfields of pure and applied sciences. The ice freezing of SCW can be influenced heterogeneously by electric effects, a process known as electrofreezing. This effect was first discovered during the 19th century; however, its mechanism is still under debate. In this Account we demonstrate, by capitalizing on the properties of polar crystals, that heterogeneous electrofreezing of SCW is a chemical process influenced by an electric field and specific ions. Polar crystals possess a net dipole moment. In addition, they are pyroelectric, displaying short-lived surface charges at their hemihedral faces at the two poles of the crystals as a result of temperature changes. Accordingly, during cooling or heating, an electric field is created, which is negated by the attraction of compensating charges from the environment. This process had an impact in the following experiments. The icing temperatures of SCW within crevices of polar crystals are higher in comparison to icing temperatures within crevices of nonpolar analogs. The role played by the electric effect was extricated from other effects by the performance of icing experiments on the surfaces of pyroelectric quasi-amorphous SrTiO₃. During those studies it was found that on positively charged surfaces the icing temperature of SCW is elevated, whereas on negatively charged surfaces it is reduced. Following investigations discovered that the icing temperature of SCW is impacted by an ionic current created within a hydrated layer on top of hydrophilic faces residing parallel to the polar axes of the crystals. In the absence of such current on analogous hydrophobic surfaces, the pyroelectric effect does not influence the icing temperature of SCW. Those results implied that electrofreezing of SCW is a process influenced by specific compensating ions attracted by the pyroelectric field from the aqueous solution. When freezing experiments are performed in an open atmosphere, bicarbonate and hydronium ions, created by the dissolution of atmospheric CO₂ in water, influence the icing temperature. The bicarbonate ions, when attracted by positively charged pyroelectric surfaces, elevate the icing temperature, whereas their counterparts, hydronium ions, when attracted by the negatively charged surfaces reduce the icing temperature. Molecular dynamic simulations suggested that bicarbonate ions, concentrated within the near positively charged interfacial layer, self-assemble with water molecules to create stabilized slightly distorted \u201cice-like\u201d hexagonal assemblies which mimic the hexagons of the crystals of ice. This occurs by replacing, within those ice-like hexagons, two hydrogen bonds of water by C−O bonds of the HCO₃⁻ ion. On the basis of these simulations, it was predicted and experimentally confirmed that other trigonal planar ions such as NO₃⁻, guanidinium⁺, and the quasi-hexagonal biguanidinium⁺ ion elevate the icing temperature. These ions were coined as \u201cice makers\u201d. Other ions including hydronium, Cl⁻, and SO₄⁻² interfere with the formation of ice-like assemblies and operate as \u201cice breakers\u201d. The higher icing temperatures induced within the crevices of the hydrophobic polar crystals in comparison to the nonpolar analogs can be attributed to the proton ordering of the water molecules. In contrast, the icing temperatures on related hydrophilic surfaces are influenced both by compensating charges and by proton ordering(Figure Presented).

By performing icing experiments on hydrophilic and hydrophobic surfaces of pyroelectric amino acids and on the x-cut faces of LiTaO3, we discovered that the effect of electrofreezing of super cooled water is triggered by ions of carbonic acid. During the cooling of the hydrophilic pyroelectric crystals, a continuous water layer is created between the charged hemihedral faces, as confirmed by impedance measurements. As a result, a current of carbonic acid ions, produced by dissolved environmental CO2, flows through the wetted layer towards the hemihedral faces and elevates the icing temperature. This proposed mechanism is based on the following: (i) on hydrophilic surfaces, water with dissolved CO2(pH 4) freezes at higher temperatures than pure water of pH 7. (ii) In the absence of the ionic current, achieved by linking the two hemihedral faces of hydrophilic crystals by a conductive paint, water of the two pH levels freeze at the same temperature. (iii) On hydrophobic crystals with similar pyroelectric coefficients, where there is no continuous wetted layer, no electrofreezing effect is observed.

Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO3 is not pyroelectric, the (100) surface of TiO2-terminated SrTiO3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO3. The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO2 capping layer. The observation of pyroelectricity emerging at the SrTiO3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo- and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.

Crystals are physical arrays delineated by polar surfaces and often contain imperfections of a polar nature. Understanding the structure of such defects on the molecular level is of topical importance since they strongly affect the macroscopic properties of materials. Moreover, polar imperfections in crystals can be created intentionally and specifically designed by doping nonpolar crystals with \u201ctailor-made\u201d additives as dopants, since their incorporation generally takes place in a polar mode. Insertion of dopants also induces a polar deformation of neighboring host molecules, resulting in the creation of polar domains within the crystals. The contribution of the distorted host molecules to the polarity of such domains should be substantial, particularly in crystals composed of molecules with large dipole moments, such as the zwitterionic amino acids, which possess dipole moments as high as ∼14 D. Polar materials are pyroelectric, i.e., they generate surface charge as a result of temperature change. With the application of recent very sensitive instruments for measuring electric currents, coupled with theoretical computations, it has become possible to determine the structure of polar imperfections, including surfaces, at a molecular level. The detection of pyroelectricity requires attachment of electrodes, which might induce various artifacts and modify the surface of the crystal. Therefore, a new method for contactless pyroelectric measurement using X-ray photoelectron spectroscopy was developed and compared to the traditional periodic temperature change technique. Here we describe the molecular-level determination of the structure of imperfections of different natures in molecular crystals and how they affect the macroscopic properties of the crystals, with the following specific examples: (i) Experimental support for the nonclassical crystal growth mechanism as provided by the detection of pyroelectricity from near-surface solvated polar layers present at different faces of nonpolar amino acid crystals. (ii) Enantiomeric disorder in dl-alanine crystals disclosed by detection of anomalously strong pyroelectricity along their nonpolar directions. The presence of such disorder, which is not revealed by accurate diffraction techniques, explains the riddle of their needlelike morphology. (iii) The design of mixed polar crystals of l-asparagine·H2O/l-aspartic acid with controlled degrees of polarity, as determined by pyroelectricity and X-ray diffraction, and their use in mechanistic studies of electrofreezing of supercooled water. (iv) Pyroelectricity coupled with dispersion-corrected density functional theory calculations and molecular dynamics simulations as an analytical method for the molecular-level determination of the structure of polar domains created by doping of α-glycine crystals with different l-amino acids at concentrations below 0.5%. (v) Selective insertion of minute amounts of alcohols within the bulk of α-glycine crystals, elucidating their role as inducers of the metastable β-glycine polymorph. In conclusion, the various examples demonstrate that although these imperfections are present in minute amounts, they can be detected by the sensitive pyroelectric measurement, and by combining them with theoretical computations one can elucidate their diverse emerging functionalities.

The riddle of anomalous polar behavior of the centrosymmetric crystal of α-glycine is resolved by the discovery of a polar, several hundred nanometer thick hydrated layer, created at the {010} faces during crystal growth. This layer was detected by two independent pyroelectric analytical methods: (i) periodic temperature change technique (Chynoweth) at ambient conditions and (ii) contactless X-ray photoelectron spectroscopy under ultrahigh vacuum. The total polarization of the surface layer is extremely large, yielding ≈1 μC·cm^-2, and is preserved in ultrahigh vacuum, but disappears upon heating to 100 °C. Molecular dynamics simulations corroborate the formation of polar hydrated layers at the sub-microsecond time scale, however with a thickness of only several nanometers, not several hundred. This inconsistency might be reconciled by invoking a three-step nonclassical crystal growth mechanism comprising (i) docking of clusters from the supersaturated solution onto the evolving crystal, (ii) surface recognition and polar induction, and (iii) annealing and dehydration, followed by site-selective recrystallization.

This cooperative endeavour first describes early studies in chemical crystallography, encompassing molecular packing modes, characterization of weak hydrogen bonds, the engineering of functional crystals and monitoring of reaction pathways in molecular crystals by x-ray and neutron diffraction. With the design of 'tailor-made' auxiliary molecules, it became possible to correlate molecular enantiomerism and crystal enantiomorphism, to control the early stages of crystal nucleation, to resolve enantiomers by crystallization, induce the precipitation of metastable polymorphs, and shed light on the role played by solvent on crystal growth. With such auxiliaries, the structure of mixed crystals was revised and the ability to perform 'absolute' asymmetric synthesis in host centrosymmetric crystals demonstrated. With the introduction of grazing incidence synchrotron x-ray diffraction from liquid surfaces it also became possible to design and characterize crystalline thin film architectures at the air-water interface providing a general insight on the mechanism of crystal nucleation at the molecular level, in particular that of ice and cholesterol. Finally the collective knowhow from these studies were crucial for obtaining homochiral peptides prepared from the polymerization of racemates of amphiphilic amino acids dissolved in aqueous solution, and for experiments towards elucidating the pathological crystallization of cholesterol and the malaria pigment in Plasmodium-infected red blood cells.

Crystal specimens of the γ-polymorph of achiral glycine which crystallize in space groups P3₁ and P3₂ as determined by the anomalous X-ray scattering method are shown to be laevorotatory and dextrorotatory, respectively, as determined by optical rotation of the crystals.

Rub-a-dub-dub: The hypothesis of contact electrification through the transfer of cryptoelectrons was tested by scrutinizing the evidence for the reduction of Pd²⁺ and Cu²⁺ by static charges on rubbed Teflon. X-ray photoelectron spectroscopy studies indicated that neither of these ions is reduced (see picture; black: Pd²⁺ adsorbed, red: Pd ⁰ adsorbed and then reduced by formadehyde, green: mixture of Pd ²⁺ and Pd⁰ arising from partial reduction after 2 h under the XPS probe) by the static charge. The presented alternative interpretation challenges the role of cryptoelectrons.

Glycine nanocrystals, grown in aligned nanometer-scale cylindrical pores of nanoporous polystyrene-poly(dimethyl acrylamide) monoliths by evaporation of imbibed aqueous solutions, adopt preferred orientations with their fast-growth axes aligned parallel with the pore direction. X-ray diffraction analysis revealed the exclusive formation of the metastable beta-polymorph, with crystal size comparable with the 22 nm pore diameter, in contrast to the formation of alpha-glycine in the absence of nanoscale confinement. When grown from aqueous solutions alone, the nanocrystals were oriented with their [010] and [0 (1) over bar0] axes, the native fast growth directions of the (+) and (-) enantiomorphs of beta-glycine, respectively, aligned parallel with the pore direction. In contrast, crystallization in the presence of racemic mixtures of chiral auxiliaries known to inhibit growth along the [010] and [0 (1) over bar0] directions of the enantiomorphs produced beta-glycine nanocrystals with their [001] axes nearly parallel to the pore direction. Enantiopure auxiliaries that inhibit crystallization along the native fast growth direction of only one of the enantiomorphs allow the other enantiomorph to grow with the [010] axis parallel to the cylinder. Collectively, these results demonstrate that crystal growth occurs such that the fast-growing direction, which can be altered by adding chiral auxiliaries, is parallel to the pore direction. This behavior can be attributed to a competition between differently aligned crystals due to critical size effects, the minimization of the surface energy of specific crystal planes, and a more effective reduction of the excess free energy associated with supersaturated conditions when the crystal grows with its fast-growth axis unimpeded by pore walls. These observations suggest that the beta-glycine nanocrystals form by homogeneous nucleation, with minimal influence of the pore walls on orientation.

As part of our studies on the biochirogenesis of peptides of homochiral sequence during early evolution, the formation of oligopeptides composed of 14-24 residues of the same handedness in the polymerization of DL-leucine (Leu), DL-phenylalanine (Phe), and DL-valine (Val) in aqueous solutions, by activation with N,N-carbonyldiimidazole and then initiation with a primary amine, in a one-pot reaction, was demonstrated by MALDI-TOF MS using deuterium enantio-labeled α-amino acids. The formation of long isotactic peptides is rationalized by the following steps occurring in tandem: (i) creation of a library of short diasteroisomeric oligopeptides containing isotactic peptides in excess in comparison to a binomial kinetics, as a result of an asymmetric induction exerted by the N-terminal residue of a given handedness; (ii) precipitation of the less soluble racemic isotactic penta- and hexapeptides in the form of β-sheets that are delineated by homochiral rims; (iii) regio-enantiospecific chain elongation occurring heterogeneously at the β-sheets/solution interface. Polymerization of L-Leu with L-isoleucine (Ile) or L-Phe with L-¹N-Me-histidine yielded mixtures of copeptides containing both residues. In contrast, in the polymerization of the corresponding mixtures of L- + D-α-amino acids, the long oligopeptides were composed mainly from oligo-L-Leu and oligo-D-Ile in the first system and oligo-D-Phe in the second. Furthermore, in the polymerization of mixtures of hydrophobic racemic α-amino acids DL-Leu, DL-Val, and DL-Phe and with added racemic DL-alanine and DL-tyrosine, copeptides of homochiral sequences are most dominantly represented. Possible routes for a spontaneous "mirror-symmetry breaking" process of the racemic mixtures of homochiral peptides are presented.

Symmetry violation: Recent results by Tremel and co-workers on the phase selection of calcium carbonate through the chirality of adsorbed amino acids appear to provide a deterministic route to "mirror-symmetry breaking", which may have ramifications for the emergence of homochirality on Earth. In our view, however, the crystallization experiments violate basic rules of symmetry and we suspect the presence of chemical, biological, or other homochiral or achiral contaminants in the system.

This chapter contains sections titled:IntroductionGeneration and Amplification of Chirality in the Solid StateSelfAggregation of Amphiphilic Molecules at AirSolution InterfacesMolecular Recognition During Crystal NucleationChiral Amplification in The System GlycineSoluble CYAmino AcidsConclusion and Outlook

In this lecture we present a stereochemical approach to crystal growth and dissolution and processes related to topochemical polymerizations in the solid phase. Understanding of such processes should take into consideration the packing arrangements of the crystals involved, the texture and kinetics of growth of the faces that delineate these crystals as well as the nature of the solvent where the crystals are grown or reacted and the structures of possible supramolecular architecures formed in the solution by the molecules undergoing crystallization. This stereochemical methodology is represented by the following three examples: (i) The role played by the addition of methanol or ethanol in the precipitation of the metastable β-form of glycine, (ii) The resolution of several α-amino acids salts that crystallize in enantiomorphous space groups, however, are composed form alternating lamellae of opposite handedness, and (iii) Polymerization reaction in crystals where the short chains formed in the course of the reaction self-assemble and induce the formation of isotactic polymers.

In a recent paper, Srinivasan and Sherwood reported that α-resorcinol crystals grow unidirectionally at the {011} faces in the vapor phase and suggested that this phenomenon of unidirectional growth is intrinsic to polar crystals. Here, we present a molecular modeling study suggesting a plausible "self-poisoning" mechanism that may explain the unidirectional growth of α-resorcinol in the vapor phase, a process that does not exclude the inhibition of crystal growth by solvent.

The control of crystal polymorphism of the trimorphic crystals of glycine (Gly) grown in aqueous solutions in the presence of α-amino acids operating as stereospecific nucleation inhibitors is reported. The presence of enantiopure α-amino acids phenylalanine (Phe), methionine (Met), and tryptophan (Trp) in the crystallizing aqueous solutions induces changes in the morphology of α-Gly leading to the formation of pyramidal instead of bipyramidal crystals. Increased concentrations of racemic Phe and Met inhibit both the α- and β-polymorphs of glycine and induce precipitation of the thermodynamically most stable γ-polymorph. α-Amino acids that bear bulky side groups such as racemic tryptophan (Trp), N-CH₃-Trp, and α-naphthylalanine induce precipitation of the least stable β-Gly polymorph. Quasi-racemic mixtures of R-Trp and S-Phe (or S-Met), for example, lead to the precipitation of one of the enantiomorphs of β-Gly. The roles played by the different α-amino acids in affecting morphology and polymorphism are discussed in terms of their interactions with and stereoselective occlusion in the various sectors of the {010} faces of the β-Gly crystals.

(Graph Presented) Crystallization of the thermodynamically metastable β glycine is explained on the basis of its growth at the opposite poles of the crystal (see graph) and by considering the structure of the relevant surfaces of the three polymorphs and their interactions with solvent molecules.

Cholesteryl-L-glutamate (CLG) thin films were investigated when deposited on pure water and on saturated solutions of water soluble amino acids, to test the capacity of the CLG monolayer to recognize and incorporate amino acids from the subphase. Analysis of grazing incidence X-ray diffraction measurements indicates that the hydrophobic amino acids, leucine, isoleucine, and valine, are adsorbed between the glutamate moieties, within the CLG crystalline monolayer, but also around their domains, forming, for L-Leu and D-Val, crystalline bilayers. The presence of Cu²⁺ ions in the aqueous subphase enhances the crystallinity of the CLG monolayer without overly affecting the lateral packing, by bridging glutamate moieties on neighbor molecules. When amino acids are injected in a Cu²⁺ containing solution, the domain size of the mixed monolayers formed with the CLG molecules and hydrophobic amino acids is drastically increased.

Reactions within crystals: Homochiral oligo(phenylalanine)s (see MALDI-TOF mass spectrum) were obtained from a racemic monomer by utilizing a "zipper-like" mechanism in the solid state (see packing diagram). By generating such species within crystalline architectures, the formation of oligopeptides of up to 17 units in length is plausible.

In this review, the delicate interplay between stereochemical control, monitoring at the subnanometer level, and an understanding of crystal nucleation is probed. Control of crystal nucleation may be achieved employing tailor-made auxiliaries, which are either nucleation inhibitors or promoters. The process may be monitored at an interface via grazing incidence X-ray diffraction (GIXD). By these means, we can glean experimental knowledge of crystal nucleation in various molecular systems. A hypothesis was invoked that supersaturated solutions containing molecular clusters adopt various arrangements and shapes, some of which resemble the crystals into which they develop. This hypothesis was taken advantage of for the design of tailored inhibitors in achieving kinetic resolution of enantiomers and induced precipitation of particular crystal polymorphs. The control and behavior of polymorphic crystallization may be understood at the molecular level through the interplay between inhibitor, solvent, solute, crystal and layer lattice energies, as well as surface layer structures. With respect to promotion of crystal nucleation, it may be achieved by Langmuir monolayers at the air-aqueous solution interface, acting as a templating agent. Determination of the monolayer crystal structure by GIXD yields the extent and nature of the complementary fit between nucleator and nucleant. Finally, GIXD was applied to monitor by a snapshot technique the layer-by-layer crystalline assembly of cholesterol molecules at the air-water interface, which involved changes in molecular packing as the film grew in thickness.

Racemic S-ethyl thioesters of N^ε-stearoyllysine (= S-ethyl (R,S)-2-amino-6-(stearoylamino)hexanethioate) and S-ethyl thioesters of γ-stearyl glutamic acid (=stearyl (R,S)-4-amino-5-(ethylsulfanyl)-5-oxopentanoate) self-assemble as separated two-dimensional crystalline monolayers within an achiral phospholipid environment of racemic 1,2-dipalmitoylglycerol (DPG) and 1,2-dipalmitoylglycero-3-phosphoethanolamine (DPPE), as demonstrated by grazing-incidence X-ray-diffraction (GIXD) measurements performed on the surface of H₂O. Lattice-controlled polycondensation within these crystallites with deuterium-enantiolabeled monomers was initiated by injecting aqueous solutions of Ag⁺ or I₂/KI beneath the monolayers, which yielded mixtures of diastereoisomeric oligopeptides containing up to six to eight repeating units, as analyzed by MALDI-TOF mass spectrometry. Analysis of the diastereoisomeric distribution showed an enhanced relative abundance of the oligopeptides with homochiral sequences containing three or more repeating units. Within the DPPE monolayers, the nucleophilic amino group of the phospholipid operates as an initiator of polymerization at the periphery of the monomer two-dimensional crystallites. Enhanced relative abundance of enantiomerically enriched homochiral oligopeptides was obtained by the polycondensation of nonracemic monomers. This enhancement indicated a phase separation into racemic and enantiomorphous monomer crystallites within the phospholipid environment, although this separation could not be observed directly by GIXD. A possible role that might have been played by crystalline assemblies for the abiotic generation and amplification of oligopeptides with homochiral sequences is discussed.

We investigated the electronic properties of hybrid inorganic/organic thin films where the inorganic components are monodisperse CdS quantum size particles (diameters 2.5 and 5 nm). For this purpose, low-energy photoelectron spectroscopy was applied in which the energy of photoelectrons, ejected from the metal substrate or from the films themselves, is measured. The electronic properties of hybrid Langmuir-Blodgett films were studied, when the films were deposited on gold or silicon surfaces and contained particles of different sizes arranged in layers at different distances from the substrate. For comparison films containing only the organic matrix were examined, so the effect of the quantum particles (QPs) on the electronic properties of the whole film could be resolved. The present study proves the importance of the organization of hybrid structure on its electronic properties. It is clearly demonstrated that the QPs cannot be considered as insolated species and that their electronic properties are affected by the structure of the film as a whole and by the substrate. It is shown that the QP size affects both the photoelectron emission yield and the scattering efficiency of electrons from the QPs.

The advent of intense X-rays from synchrotron sources made possible to probe, at the molecular level, the structural aspects of self-assemblies generated at interfaces. Here we present the two-dimensional (2-D) packing arrangements of two-, three- and multi-component organo-metallic self-assemblies formed via interfacial reaction at the air-aqueous solution interface, as determined by grazing incidence X-ray diffraction (GIXD) and X-ray specular reflectivity techniques. GIXD yields structural information on the crystalline part of the Langmuir film, including the ions and counterions lateral order. Specular reflectivity yields the electron density profile of the film in the direction normal to the air-solution interface and, by modeling, gives independent information on the structure of both crystalline and amorphous parts of the film. In particular, we focus on the following systems: ordering of metal ions bound to the polar head groups of amphipilic molecules; use of bolaamphiphiles to generate oriented thin films with metal ions arranged in periodic layers; delineation of differences in the lateral organization of metal ions at interfaces as induced by racemates and enantiomerically pure amphiphiles; organization of ionophores in the presence of metal ions in membrane-like environment; self-assembly of 2×2 and 3×3 silver(I) grid-type complexes generated at the air-solution interface.

Composite materials of quantum particles (Q-particles) arranged in layers within crystalline powders of π-conjugated, rodlike dicarboxylic acids are reported. The synthesis of the composites, either as three-dimensional crystals or as thin films at the air-water interface, comprises a two-step process: 1) The preparation of the Cd salts 6(Cd), 8(Cd) or Pb salts 6(Pb), 8(Pb) of the oligo(p-phenyleneethynylene)dicarboxylic acids 6(H), 8(H), in which the metal ions are arranged in ribbons and are separated by the long axis of the organic molecules, as demonstrated by X-ray powder diffraction analysis of the solids and grazing incidence X-ray diffraction analysis of the films on water. 2) Topotactic solid/gas reaction of these salts with H₂S to convert the metal ions into Q-particles of CdS or PbS embedded in the organic matrix that consists of the acids 6 (H) and 8 (H). These hybrid materials have been characterized by X-ray photoelectron spectroscopy and transmission electron microscopy.

The spectroscopic properties of new molecular-wire/nanocrystalline (NCs) superstructures have been examined. The molecular-wires consisted of (phenylene)_n-(acetylene)_n-1-dicarboxylate conjugated chains, with a length varying between 1.9 nm and 3.3 nm. The NCs component consisted of either PbS or CdS, with an average diameter ranging between 2.0 and 5.0 nm. The photoluminescence (PL) spectra at 1.4 K, the PL-excitation, and the time-resolved PL measurements showed a typical quantum size effect of the individual NCs, and additional collective effects of the superstructure. Simulation of the luminescence decay processes suggested an energy transfer between the molecular wires and the NCs, by an exciton diffusion mechanism. Furthermore, direct excitation into the NCs band edge showed an unexpected red-shift, a consequence of a molecular wires-NCs electronic interaction.

Hybrid organic/inorganic Langmuir-Blodgett (LB) films were prepared by the synthetic solid/gas reaction. Addition to the thiocarboxylic acid to the LB film stabilized the ordered phase.

Keywords: MOLECULAR-DYNAMICS SIMULATIONS; OCTYL-D-GLUCONAMIDE; ALPHA-RESORCINOL; SUPERSATURATED SOLUTIONS; SOLUTION INTERFACE; ORGANIC-CRYSTALS; SURFACE; WATER; RECOGNITION; WETTABILITY

Tailor-made molecules have been designed and used as additives, in crystallization processes from solution, to control the nucleation and growth of molecular crystals. These additives can be either inhibitors or promoters of crystal growth. Tailor-made inhibitors can be used for a variety of purposes that include crystal morphology engineering, kinetic resolution of racemates, reduction of crystal symmetry, assignment of absolute configuration of chiral molecules and polar crystals and control of crystal polymorphism. As for crystal growth promoters tailor-made additives, that self-assemble at the air-aqueous solution interface due to their intrinsic hydrophobic nature, have been used to induce oriented crystallization by means of a structural match.

Structural studies on two-dimensional films of chiral amphiphiles at the air-solution interface, as investigated in situ primarily by grazing incidence x-ray diffraction (GIXD) using synchrotron radiation, yielded the crystalline packing arrangement at almost the molecular level. Results regarding three different topics are described. (1) It has proven possible to establish whether racemic mixtures of amphiphiles spread on water self-organize into 2-D crystals in which the two enantiomers either form heterochiral domains or spontaneously separate into enantiomorphous islands composed of homochiral molecules. Diastereoisomeric acid-base interactions between two different chiral amphiphiles were also used to achieve spontaneous chiral separation in two dimensions. (2) Ordered binding of solute molecules to the chiral amphiphiles could be applied in order to study their enantioselective interactions with chiral solutes present in the aqueous subphase. (3) Crystalline multilayer films of supramolecular architecture composed of water-soluble and water-insoluble chiral components may be formed at the air-solution interface and their packing arrangement determined by GIXD.

Insertion of the 1,3-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 Å, induces chain folding at the air-water interface. These folded molecules self-assemble into crystalline monolayers at this interface, with the plane of the folding unit almost perpendicular to the water surface, as determined by synchrotron grazing-incidence X-ray diffraction. Three distinct molecular shapes, of the types U, inverted U, and M, were obtained in the two-dimensional crystal-line state, depending upon the number of spacer units, and the number and position of the hydrophilic groups in the molecule. The molecules form ribbons with a higher crystal coherence in the direction of stacking between the molecular ribbons, and a lower coherence along the ribbon direction. A similar molecule, but with a spacer unit that imposes a 5 Å separation between alkyl chains, yields the conventional herring-bone arrangement.

Oriented crystalline monolayers, ~ 14 Å thick, of a 2 x 2 Ag⁺ grid complex, self-assembled at the air-solution interface starting from an waterinsoluble ligand 3,6-bis[2-(6-phenylpyridine)]pyridazine spread on silver-ion-containing solutions, were examined by grazing-incidence X-ray diffraction and specular X-ray reflectivity using synchrotron radiation. The monolayer structure was refined, including a determination of the positions of the counterions, with the SHELX-97 computer program. The monolayers were transferred from the interface onto various solid supports and visualized by scanning force microscopy, and characterized by X-ray photoelectron spectroscopy in terms of molecular structure. On surface compression, the initial selfassembled monolayer undergoes a transition to a crystalline bilayer in which the two layers, almost retaining the original arrangement, are in registry. Such a phase transition is of relevance to the understanding of crystal nucleation.

In 1848 Louis Pasteur manually separated enantiomorphous three- dimensional crystals. A 2D analogue of this experiment involving clusters of 1-nitronaphthalene bound to a gold surface has been recently performed by the research groups of Berndt and Schneider. The general principles governing separation of enantiomers from 3D crystals to 2D clusters at interfaces, and their characterization, are discussed.

On studies of large scale optical resolution by crystallization, it was found that α-amino acid salts crystalline in the form of conglomerates. The conglomerates were composed of lamellar twins, hence are not amenable to large scale resolution. By conducting further studies, it was found that the occurrence of the lamellar twins can be prevented with the assistance of 'tailor-made' enantioselective crystal nucleation inhibitors that allow the resolution of hydrophobic racemic α-amino acids by kinetic crystallization.

The assembly, orientation, and structural features of nanoscale tubes composed of cyclic peptides, formed at the air-water interface, were detected by grazing incidence X-ray diffraction (GIXD). The peptide cyclo-[(L-Phe-D- N-MeAla-)₄] (1) exhibits two-dimensional crystallinity in which the plane of the peptide ring is parallel to the water interface. The peptide cyclo-[(L- Trp-D-Leu)₃-L-Ser-D-Leu] (2) forms predominantly planar aggregates composed of several tubes, lying with their long axes parallel to the air-water interface. In contrast, the peptide cyclo-[(L-Trp-D-Leu)₄] (3) exhibits a very low tendency to form ordered two-dimensional arrays of nanotubes. Films of peptides 2 and 3 as well as their mixtures with the phospholipid DPPA were transferred onto a solid support and visualized by scanning force microscopy (SFM).

Model molecules, designed to simulate a chain fold, self-assemble into crystalline monolayers at the air - water interface, the structure of which has been determined, in situ, by synchrotron grazing-incidence X-ray diffraction. The two-dimensional packing arrangement in such monolayers is consistent with a structure composed of ribbons containing translationally related molecules adopting an inverted U-shape. The distance between these molecules along the ribbon is optimum for creating an additional chain fold, leading to the possibility that a polymeric molecule will undulate on the water surface.

Oriented crystalline films, ~11-20 Å thick, of metal ion complexes of the grid type [Co₄L₄]⁸⁺·8PF₆^- and [Ag₉L₆]⁹⁺ 9·CF₃SO₃^-, based on various ligands L, were prepared in-situ at the air-aqueous solution interface by the interaction of the free ligand molecules spread onto aqueous solutions containing Co²⁺ or Ag⁺ ions. The structure of the complex architectures composed of a 2 x 2 Co²⁺ grid coordinated to four ligand molecules and a 3 x 3 Ag⁺ grid coordinated to six ligand molecules as well as their molecular organization in thin films were characterized by grazing incidence synchrotron X-ray diffraction (GIXD) and specular X-ray reflectivity (XR) measurements performed at the air-aqueous solution interface and by UV, X-ray photoelectron spectroscopy (xPs), and scanning force microscopy (SFM) after film transfer onto various solid supports. The results open perspectives toward an implementation of the air-water interface for the self-assembly and subsequent deposition of organized arrays of complex inorganic architectures onto solid support.

A direct relationship between the structures of two-dimensional (2-D) crystallites of 4-octadecyloxy-E-cinnamic acid and 4-octadecyloxy-E- cinnamamide amphiphiles at the air-water interface and their photochemical behavior, is presented. The detailed packing arrangements of the monolayers were determined, close to the atomic level, from the diffraction pattern measured, in situ on the water surface, by the grazing incidence X-ray diffraction (GID) technique using synchrotron radiation. The products of the photochemical reaction, performed by irradiating the monolayer films directly on the water surface, were collected and analyzed. While the acid crystallites yield a β-truxinic acid product together with the cis-isomer, the amide undergoes only trans-cis isomerisation, in full agreement with the 2-D crystalline packing arrangements of the two amphiphiles on the water surface.

We present a study of two-dimensional (2D) crystallites of cholesterol formed at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) measurements performed along the surface pressure-area isotherm revealed a transition from a monolayer to a highly crystalline rectangular phase, about two layers thick. This variation in the film thickness was confirmed by ellipsometry measurements. Films transferred onto solid support by the Langmuir-Blodgett technique were seen by atomic force microscopy (AFM) to display elongated and faceted crystallites about 10 layers thick. The effect of the phospholipid dipalmitoylphosphatidylcholine (DPPC) on the 2D crystallization of cholesterol was studied by GIXD and AFM with three cholesterol:DPPC mixtures in molar ratios 1:1, 2:1, and 5:1. The phospholipid additive reduced the crystallinity of the cholesterol in the 5:1 and 2:1 mixtures, totally suppressing it in the 1:1 mixture.

Grazing incidence X-ray diffraction (GID) has been applied to the study of structural characteristics of two-dimensional crystallites of the ionophores valinomycin (VM) and nonactin (NA) when complexed with various cations at the air-solution interface. The VM complexes assume a bracelet shape that packs in a two-dimensional hexagonal unit cell. Other crystalline phases were formed on potassium iodide and barium perchlorate solutions. The presence of particular lipids induced ordered stacking of VM-potassium chloride complexes into three to four layers. NA packs in a pseudotetragonal unit cell on solutions of NH₄SCN and KSCN. Upon compression of the NA-NH₄SCN film, crystallites seven to eight layers thick were detected and the GID data enabled the determination of the structure. The tendency of these complexed ionophores to form multilayer crystallites at interfaces may have bearing on ion transport through membranes, via stacking.

The formation of Langmuir monolayers at the air water interface has long been believed to be limited to amphiphilic molecules containing a hydrophobic chain and a hydrophilic headgroup. Here we report the formation of crystalline mono- and multilayer self-assemblies of oligothiophenes, a class of aromatic nonamphiphilic molecules, self-aggregated at the air water interface. As model systems we have examined the de position of quaterthiophene (S₄), quinquethiophene (S₅), and sexithiophene (S₆) from chloroform solutions on tile water surface. The structures of the films were determined by surface pressure area isotherms, by scanning force microscopy (SFM after transfer of the films onto atomically smooth mica, by cryo-transmission electron microscopy (Cryo-TEM) on vitreous ice, and by grazing incidence synchrotron X-ray diffraction (GII) directly on the water surface. S₄ forms two polymorphic crystalline multilayers. In polymorph χ, of structure very similar to that of the three-dimensional solid, the molecules are aligned with their long molecular axis tilted by about 23 from the normal to the water surface. In polymorph β the long molecular axis is perpendicular to the water surface. S⁵ self-aggregates at tile water surface to form mixtures of monolayers and bilayers of the β polymorph: S₆, forms primarily crystalline monolayers of both χ and β forms. The crystalline assemblies preserve their integrity during transfer from the water surface onto solid supports. The relevance of the present results for the understanding of the early stages of crystal nucleation is presented.

Biography: Meir Lahav studied at the Hebrew University of Jerusalem, and received the M.Sc. in 1963 in the field of organic polymers, and the Ph.D. in 1967 for studies of organic solid state reactions. After postdoctoral work with P.D. Bartlett at Harvard he joined the Weizmann Institute of Science in 1971, where he is currently Head of the Department of Materials and Interfaces, and Director of The Minerva G.M.J. Schmidt Interdisciplinary Center of Supramolecular Architectures. He is incumbent of the M. Thatcher chair of chemistry. He has been the recipient of the Kolthoff Prize of Chemistry from the Technion in Haifa and the Prelog Medal for Stereochemistry (with L. Leiserowitz) from the E.T.H. in Zurich, and was Centenary Lecturer of the Royal Society of Chemistry. He has research interests in the origin of chirality, stereochemistry, mechanistic study of solid state reactions, crystal engineering, thin organic films, and related fields.

Structure, morphology, and mechanical properties of mono- and several-layer structures of amphiphiles or pure n-alkane crystallites generated by spontaneous self-assembly on aqueous subphase have been analyzed by scanning force microscopy (SFM). Pure-component and heterogeneous mixtures of molecules were allowed to spread and self-assemble without compression on an aqueous subphase. The self-assembled films were transferred to an atomically smooth mica substrate by drainage for measurement using SFM. Results were compared with a variety of techniques including cryo-transmission electron microscopy, grazing incidence X-ray diffraction. X-ray reflectivity, and reflectance/absorption infrared spectroscopy. Whereas the collaborative techniques provide spatially-averaged information, we find that the SFM accesses both individual crystallites and amorphous material, thus providing unique information on the morphology, number of layers, and complementary structural features.

A systematic analysis of grazing incidence synchrotron Xray diffraction data of uncompressed amphiphilic alcohols C_nH_2n+1 OH (n = 31, 30, 23, 20, 19, 18, 16, 14, 13) on a water subphase at 5°C is presented. Pronounced structural changes were observed on reduction of chain length from n = 31 to 13. The relative amount of two dimensional (2D) crystalline material formed fell drastically; shorter crystalline coherence lengths were also observed. For n3118 the molecules are arranged in a rectangular cell (a ≈ 5 Å, b increases from ca. 7.4 to ca. 8.2 Å) with plane symmetry p1 g1. For n 2 and in the atomic displacement parameter parallel to the water surface; it increases from 0.1 Ų for n = 31 to 0.30 Ų for n = 19. We explain the preference for chain tilt along the b axis, rather than a, in terms of hydrogen bonding to the water subphase. The various structural properties of the C_nH_2n+2OH (n = 3113) monolayer series, such as degree of crystallinity and coherence length, lattice dimensions, chain orientation, and molecular motion, may be correlated with the icenucleating efficiency of these alcohol monolayers as a function of n.

Double layers are formed by α,ω-docosanediol (C22 diol) when it is spread on a water surface. The space group (a slightly relaxed form of P21/a) and the lattice parameters of the unit cell were determined by X-ray diffraction. Furthermore, multilayer formation was inhibited by addition of 10% of C24H49OH or 5% of HO(CH2)22OCH2Ph to the spreading solution. The coverage with the double layer was reduced to less than 15%; the rest was occupied by a monolayer.

We enjoyed reading the article by Ron Dagani (C&EN, Aug. 29, 1994, page 28) on the current field of catenanes. We were dismayed by his reference to "a chemist" who synthesized the first catenane in 1960. That pioneering study was performed by Ed Wasserman, who is considered as the originator of this field of science. Wasserman not only synthesized the first catenane, but also elaborated on the topological aspects of this class of materials. Wasserman, who is well known for his original contributions in pure and applied chemistry, is currently at DuPont Experimental Station, Wilmington, Del. [1st paragraph]

Crystal polymorphism is important because of the different physical and mechanical properties of the polymorphs. Based on a stereochemical approach to controlling crystal polymorphism using stereospecific nucleation inhibitors, auxiliaries have been designed for the precipitation of the yform of glycine (see Figure) from aqueous solutions. The ideas behind this work are presented, together with results. (Figure Presented.)

Crystallization of R,S-alanine in the presence of small amounts of other resolved α-amino acids results in the precipitation of the racemic compound as twinned crystals displaying a propeller-like morphology of 222 symmetry, in contrast to needlelike crystals of mm2 symmetry when grown from pure aqueous solution. This unusual twinning is explained by assuming that the supersaturated solutions contain ordered clusters of structures akin to the mature crystals of (R,S)-, (R)-, and (S)-alanine. The resolved α-amino acid additive, e.g., R, stereospecifically inhibits growth of the R,S and R nuclei but not of the S nuclei. The latter serve as a template onto which the (R,S) crystals nucleate and grow. A model which takes into consideration the structural similarity between the racemic and chiral resolved forms of alanine to explain the twinning is presented. An X-ray diffraction analysis of a twinned crystal specimen is in agreement with a gradual change in structure, from a chiral crystal of space group P2₁2₁2₁ to the racemic compound of space group Pna2₁. To substantiate the model, crystals of alanine were grown from optically enriched solutions containing excess of one of the alanine enantiomers, which also induce the twinning phenomenon and an overall reduction in crystal symmetry. The relevance of these observations to the early stages of crystal nucleation is discussed.

The two-dimensional (2D) crystal structures of self-aggregated clusters of amphipbilic alcohols (C_nH_2n+1OH, n = 23, 30, 31, and C₁₉H₃₉CO₂(CH₂)_nOH, n = 9, 10) on pure water at a temperature of 5 °C have been determined to near-atomic resolution in order to understand the relative abilities of these monolayers to induce ice nucleation. The structures were solved making use of grazing incidence synchrotron X-ray diffraction (GID) data, complemented by lattice energy calculations. The GID data of the different monolayers within each of the two series (C_nH_2n+1OH, C₁₉H₃₉CO₂C_nH_2nOH) are very similar. The molecules pack in a rectangular unit cell of average dimensions a = 5.0 Å, b = 7.5 Å for the normal alcohols C_nH_2n+1OH and a = 5.7 Å, b = 7.5 Å for C₁₉H₃₉CO₂C_nH_2nOH. The plane group symmetry is essentially p1g1 for the normal alcohols C_nH_2n+1OH and essentially p1g1 for the other group. The molecular chains are tilted from the vertical by an average angle of 9°, in the direction of the b axis, for C_nH_2n+1OH and by 29°, in the direction of the a axis, for the other molecular type. The molecular chains related by glide (g) symmetry are arranged in a herringbone pattern. The fit to the Bragg rod intensity data of C_nH_2n+1OH permitted a reliable estimate of 0.07 Ų for the molecular mean-squared motion parallel to the water surface. The a bsolute orientations of the molecules C₁₉H₃₉CO₂C_nH_2nOH were determined by lattice energy calculations. The anisotropic coherence lengths of the crystallites of C₁₉H₃₉CO₂C_nH_2nOH derived from the widths of the two observed Bragg peaks have been correlated with the binding energies of molecules in different directions.

The spontaneous formation and coexistence of crystalline polymorphic trilayer domains in amphiphilic films at air-liquid interfaces is demonstrated by grazing incidence synchrotron x-ray diffraction. These polymorphic crystallites may serve as models for the early stages of crystal nucleation and growth, helping to elucidate the manner in which additives influence the progress of crystal nucleation, growth, and polymorphism and suggesting ways of selectively generating and controlling multilayers on liquid surfaces. Auxiliary molecules have been designed to selectively inhibit development of the polymorphs, leading primarily to a single phase monolayer.

The structure of an uncompressed monolayer of C₃₁H₆₃OH, which induces freezing of supercooled water drops at ∼-2°C, was studied by grazing incidence X-ray diffraction in the temperature range +6°C to freezing of the water subphase. As the water was cooled, the hydrocarbon chains underwent a change in tilt angle. The monolayer preserved its structure on the epitaxially-grown surface of hexagonal ice (after freezing) and, after the ice was melted, subsequent to the temperature being raised above 0°C. The coherence length of the observed ice crystallites parallel to the interface was about 25 Å, which is in agreement with the extent of match between the lattice of the monolayer and the ab lattice of ice. This yields some information on the critical size of the ice nucleus bound to the monolayer, just below 0°C.

Principles are outlined for symmetry lowering of a mixed crystal composed of host and tailor-made additive molecules, based on selective occlusion of the latter through a subset of surface sites of the growing crystal, the symmetry of the surface generally being lower than that of the bulk. A survey is given of the various methods and approaches used to detect the reduction in symmetry. These include changes in crystal morphology, detection of enantiomeric segregation of chiral additives in 'centrosymmetric' crystals, generation of second-harmonic optical signals, optical birefringence, asymmetric photoreactions in the crystalline state and X-ray and neutron diffraction. The last two methods are applied to mixed crystals of cinnamamide (host) and thienylacrylamide (additive). The diffraction analysis demonstrated that the mixed crystals are composed of six sectors of reduced symmetry, from monoclinic centrosymmetric P2₁/c to triclinic P1 in four sectors and possibly Pc in the remaining two. The X-ray diffraction data were not sufficiently accurate to permit assignment of the absolute structures of the P1 sectors with the use of anomalous X-ray scattering. Thus, by this method one could not ascertain the absolute orientation of the guest molecules on the surface sites through which they were selectively occluded. This ambiguity was resolved by assignment of the absolute configuration of the chiral heterophotodimers, between host and guest, in enantiomeric excess in the P1 sectors, after irradiation with UV light. These results led to the definite conclusion that the selective occlusion of thienylacrylamide arises from a replacement of attractive C-H⋯π (electron) interactions between host molecules by repulsive sulfur (lone-pair electron)⋯π (electron) interactions between guest and host at the crystal surfaces.

Mixed monolayers containing two different amphiphiles C₁₉H₃₉CONHCH₂CH₂CO₂H (A) and C₁₉H₃₉CONH₂ (B) were designed in order to form a two-dimensional (2-D) crystalline solid solution in which the A-type molecules form domains within the sea of B-type molecules. A "continuous" 2-D arrangement of the aliphatic chains was expected, driven by the amide hydrogen bonding requirement; a tendency for the formation of the embeded A-type domains should be provided by the interactions between the -CH₂CH₂CO₂H head group moieties. The mixed monolayers served to promote the oriented nucleation of silver propionate 3-D crystals attached at the monolayer-solution interface. Only the A-type domains induced silver propionate crystallization whereas the B-type domains were essentially inert. The mixed A + B monolayers were found to be efficient nucleators down to a 1:10 molar ratio, providing proof for the existence of A-type domains. Additional information such as the structure and ion-binding properties of the mixed monolayers was furnished by specular X-ray reflectivity and grazing incidence X-ray diffraction using synchrotron radiation.

A new approach is described for probing domains of ordered self-assemblies of amphiphilic monolayers at the aqueous solution interface. The method has potential importance for the study of membrane structure, Langmuir-Blodgett films, and nucleation processes of two-and three-dimensional crystals. Electron diffraction (ED) patterns indicative of two-dimensional crystalline self-assembly were obtained from samples, which were examined by cryo-electron microscopy, of monolayers of water-insoluble amphiphiles on vitrified aqueous substrates. The apparent hexagonal symmetry of an ED pattern from a C ₁₆H₃₃OH monolayer was interpreted in terms of multiple twinning. Monolayers of the C₃₁H₆₃OH and cadmium salt of C₁₉H₃₉CO₂H that were studied by dark-field techniques displayed faceted two-dimensional crystallites with a maximal size of 1 to 2 micrometers. Epitaxial nucleation of hexagonal ice by the C ₃₁H₆₃OH monolayer has also been demonstrated by ED.

The advent of wellcollimated, highintensity synchrotron Xray sources and the consequent development of surfacespecific Xray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the airliquid interface. These methods allowed for the first time the determination of the inplane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence Xray diffraction, specular reflectivity, Bragg rods, standing waves, and surface fluorescence techniques, and review recent results obtained from them for Langmuir films. The methods have been successfully applied in the elucidation of the structure of crystalline aggregates of amphiphilic molecules such as alcohols, carboxylic acids and their salts, αamino acids, and phospholipids at the water surface. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline selfaggregates as well as structural changes occurring by phase transitions. Furthermore, the surface Xray methods shed new light on the structure of the underlying ionic layer of attached solvent or solute species. Examples are given where singly or doubly charged ions bound to the twodimensional (2D) crystal form either an ordered or diffuse counterionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals, which had been successfully accomplished by epitaxiallike crystallization both in organic and inorganic crystals.

The hydrophobic faces of single crystals of a series of pairs of racemic and chiral-resolved hydrophobic α-amino acids were used as a substrate, onto which water vapor has been cooled to freezing. The morphologies and molecular packing arrangements within each crystal pair are similar but only one of each pair exhibits a polar axis, parallel to the hydrophobic face exposed to water. Those crystals that have a polar axis induce a freezing point higher by 4° to 5°C than the corresponding crystals that do not have a polar axis. The results are interpreted in terms of an electric field mechanism that helps align the water molecules into ice-like clusters en route to crystallization.

Monolayers of aliphatic long chain alcohols (C_nH_2n+1OH) were found to induce nucleation of ice at temperatures approaching O°C in contrast to watersoluble alcohols which are efficient antifreeze agents. The corresponding fatty acids and alcohols with bulky hydrophobic groups, induce ice nucleation at temperatures lower by as much as 12°C. The freezing point induced by the amphiphilic alcohols was found to be sensitive not only to area per molecule but also to chain length and parity, reaching higher temperatures for monolayers with n odd. Grazing incidence Xray diffraction studies performed on some of these alcohols at the air/water interface (at 5°C and at zero pressure), demonstrated the formation of crystalline twodimensional clusters with a distorted hexagonal cell whose dimensions resemble those of hexagonal ice. The catalysis of ice nucleation by these alcohol monolayers may be rationalized in terms of the structural match between the monolayer domains and the ab layer of hexagonal ice.

A crystalline counterionic layer at the interface between an electrolyte solution and a charged layer of insoluble amphiphilic molecules was observed with grazing incidence synchrotron x-ray diffraction. Uncompressed arachidic films spread over 10^-3 molar cadmium chloride solution (pH 8.8) spontaneously form crystalline clusters with coherence lengths of ∼1000 angstroms at 9°C. Ten distinct diffraction peaks were observed, seven of which were attributed to scattering only from a crystalline Cd²⁺ layer and the other three to scattering primarily from the arachidate layer. The reflections from the Cd²⁺ layer were indexed according to a 2 x 3 supercell of the arachidate lattice with three Cd²⁺ ions per cadmium unit cell.

Monolayers of aliphatic long-chain alcohols induced nucleation of ice at temperatures approaching 0°C, in contrast with water-soluble alcohols, which are effective antifreeze agents. The corresponding fatty acids, or alcohols with bulky hydrophobic groups, induce freezing at temperatures as much as 12°C lower. The freezing point induced by the amphiphilic alcohols was sensitive not only to surface area per molecule but, for the aliphatic series (C_nH_{2n + 1}OH), to chain length and parity. The freezing point for chains with n odd reached an asymptotic temperature of 0°C for an upper value of n = 31; for n even the freezing point reached a plateau of-8°C for n in the upper range of 22 to 30. The higher freezing point induced by the aliphatic alcohols is due to formation of ordered clusters in the uncompressed state as detected by grazing incidence synchrotron x-ray diffraction measurements. The diffraction data indicate a close lattice match with the ab layer of hexagonal ice.

In order to provide, on the molecular level, information on crystal nucleation of monolayers at air-water interfaces, the self-aggregation of lH,lH,2H,2H-perfiuorododecyl aspartate CF3(CF2)9(CH2)20COCH2CH(NH3+)C02~ (PFA) over water subphases at various pH values was studied using synchrotron X-ray grazing incidence diffraction (GID), including Bragg rods (BR) and reflectivity (XR) measurements. Two-dimensional crystalline domains with coherence lengths exceeding 1 500 A were detected for low surfactant surface densities and zero surface pressure. GID measurements reveal structural changes with subphase pH and composition. Structural models are proposed at high, neutral, and low pH. For water subphases containing KOH at pH > 11.2, the diffraction is consistent with molecules arranged in a hexagonal net and vertically aligned. Over pure water and acidic subphases containing HC1 at pH = 1.5, the molecules pack in a distorted hexagonal net with the fluorocarbon chains tilted from the vertical. The growth in time of the uncompressed crystallites over aqueous glycine solutions was directly monitored by GID. Compression and subsequent decompression of the monolayers over pure water and HC1 (pH = 1.5) subphases, for which the fluorocarbon chains are originally tilted, were found to reduce the crystallinity of the system considerably. By contrast, over KOH at pH > 11.2, the hexagonal net with vertically aligned molecules is preserved at all surface pressures and the crystalline order of the system is reduced upon compression but increases again upon release of pressure. Estimates of the degree of crystallinity of the monolayer were made over water for various states of compression and over KOH at pH >11.2 in the uncompressed state. The packing characteristics and the dynamics involved in the formation and partial destruction of the crystallites can be understood in terms of interaction between the hydrophilic ionic head groups of the monolayer and, if present, the attached molecules or ions (water, K+ or Cl"). Additional support for the packing arrangements proposed at high, neutral, and low pH was obtained from studies of the oriented growth of sodium chloride under PFA monolayers.

In recent studies we have reported that the insertion of one, or preferably, two amide groups in the hydrocarbon chain of amphiphilic molecules yields porous monolayers which deposit in a genuine Z-type mode. In this paper, we extend our studies to amphiphiles with an amide group and a polymerizable diacetylene function in the chain. Monolayers of monomers and polymers of 10,12-non-acosadiynoyl-S-lysine (1) and 10,12-nonacosadiynoyl-S-ornithine (2) were deposited in a Z-type mode. The differences in the photopolymerization of these films seem to depend on the odd or even location of the amide group along the chain with respect to the headgroup. These Langmuir-Blodgett (LB) films were characterized by UV-visible and Fourier transform (FT) IR (reflection-absorption mode) spectroscopies and by low-angle X-ray diffraction measurements.

A network of hydrogen bonds determines the structure of the complex A of nylon66 and perfluoroglutaric acid, HO₂C(CF₂)₃CO₂H. This explains why shorter or longer dicarboxylic acids do not form complexes with nylon66. The nylon partial structure differs from the structure of pure nylon not in the CC bond lengths but rather in the CCC angles, which result in a longer b axis. Complexes of type A are unusual, because they consist of a molecular mixture of otherwise immiscible hydrocarbons and fluorinated hydrocarbons. (Figure Presented.)

Controlled reduction in crystal symmetry has been accomplished and exploited to perform an absolute asymmetric photoreaction. The principle is based on selective introduction of a guest molecule into a centrosymmetric host structure, thus reducing the symmetry of the mixed crystal. Crystallization of (E)-cinnamamide (space group P2₁/c) in the presence of (E)-cinnamic acid results in a mixed crystal composed of two enantiomorphous halves each containing 0.5-1.0% of the acid. The replacement of N-HO hydrogen bonds between host molecules by OO lone-pair repulsion between host and guest at the chiral surfaces of the growing crystal is responsible for the stereoselective occlusion of the guest and the reduction in symmetry. Irradiation of each half separately yielded the optically active cyclobutane dimer la or lb in excess, with an enantiomeric yield in the range of 40-60%, varying from one single crystal to another. The absolute configuration of the chiral dimer la formed in excess at the +b pole (1R,2R,3R,4R) was independently assigned by the Bijvoet method, NMR, and crystal-etching experiments and is in agreement with our proposed mechanism of molecular recognition.

We have demonstrated, for the first time by diffraction methods, that a solid solution composed of host and guest molecules can exhibit a crystal symmetry lower than that of the host. The study proves that the symmetry of a solid solution is dependent not only upon the host crystal structure and the guest molecular structure but also upon the surface structure and symmetry of the host crystal. The crystal structures of (S)-asparagine monohydrate [D₂NCOCH₂CH(ND₃)CO₂.D₂O] and of the solid solution (0.848:0.152) (S)-asparagine/(S)-aspartic acid [DO₂CCD₂CD(ND₃)CO2] monohydrate were refined by using neutron diffraction data obtained at 15 K. The space group of the pure host crystal is P2₁2₁2₁(Z = 4), whereas that of the host/guest crystal is monoclinic P12₁1 with two molecular sites per asymmetric unit. The ratios of guest/host occupancies of the two independent sites are 0.173:0.827 and 0.132:0.868. The reduction in symmetry is in accordance with the preferred adsorption of guest aspartic acid on the (010) crystal face at half of the orthorhombic, symmetry-related surface sites. Aspartic acid mimics, at the preferred (010) surface sites, molecular asparagine, participating in all hydrogen bonds. At the less-favored (010) surface sites a \u201cnormal\u201d N-H.O(host) hydrogen bond is replaced by O(hydroxyl).O(host) repulsion between lone-pair electrons.

We have determined the packing arrangement of a floating monolayer of palmitoyl-(R)-lysine at the air-water interface by grazing incidence X-ray diffraction and reflection measurements. These techniques utilize the unique properties of synchrotron radiation: high intensity within a small natural collimation. In the grazing angle diffraction experiment two peaks were detected in the "twodimensional powder" pattern from a monolayer of palmitoyl-(R)-lysine. Their widths indicated coherence lengths of 500 J~. The positions and intensities of these peaks allowed us to choose between various models and to determine the monolayer structure. The packing arrangement of the Qt-amino acid headgroups in the model proved to be very similar to that found in the crystal structures of the 0t form of glycine and several hydrophobic at-amino acids, thus providing a simple explanation for the oriented crystallization of 0t-glycine at monolayer-solution interfaces. The tilt of the molecule calculated from the model is consistent with the results from reflectivity measurements and X-ray powder diffraction data of the crystalline powder material. Reflectivity measurements indicate that at surface pressures as high as 30 mN m- 1 the monolayer covers only about 90% of the surface. Reflectivity measurements of the monolayer over water and over solutions of 29/o (S)-glutamine showed significant differences, indicating binding of the solute molecules to the monolayer.

A measurement and interpretation on a molecular level of a phase transition in an ordered Langmuir monolayer is reported. The diagram of surface pressure (π) versus molecular area of a monolayer of chiral (S)-[CF3-(CF2)9-(CH2)2-OCO-CH2-CH (NH3+)CO2-] over water shows a change in slope at about πs= 25 millinewtons per meter. Grazing-incidence x-ray diffraction and specular reflectivity measurements indicate a solid-solid phase transition at πs. The diffraction pattren at low pressures reveals two diffraction peaks of equal intensities, with lattice spacings d of 5.11 and 5.00 angstroms; these coalesce for π ≥πs. Structural models that fit the diffraction data show that at π> πs the molecules pack in a two-dimensional crystal with the molecules aligned vertically. At π

The structures of the partially reacted crystals of the two complexes (5:2) deoxycholic acid (DCA)-acetophenone and (8:3) DCA-p-fluoroacetophenone were determined by low-temperature (-170 °C) X-ray diffraction. the structure analyses indicated that both independent guest acetophenone molecules G and G in DCA-acetophenone react and that in DCA-p-fluoroacetophenone the guest molecule G reacts whereas the other guest molecule G remains unreacted because of packing of G and G molecules along the channel. the observed guest photoconversion in DCA-acetophenone is 40%; the maximum value according to packing considerations is 50%. In DCA-p-fluoroacetophenone the observed guest photoconversion is 35%, very close to the maximum theoretical yield of 33% based on packing considerations. the analysis demonstrated that during the course of photoconversion there is minimal motion of the (guest) phenyl ring, pronounced rotation of the acetyl group about the exocyclic C-C bond, and displacement of unreacted and reacted molecules. Atom-atom potential energy calculations showed that in DCA-acetophenone a guest ketyl radical with a pyramidal geometry may rotate by a full 180° without inducing prohibitively short intermolecular contacts.

The two inclusion complexes of guest acetophenone and m, chloro-acetophenone with host deoxycholic acid (DCA) each yielded on UV irradiation a single diastereomeric photoaddition product at steroid atom C5, of configuration S at the generated stereogenic centre (DCAOHCH₃C₆H₄). Comparison of the host-guest arrangement at the sites of reaction with the S configuration at the new chiral atom C of the photoproducts indicates a net rotation of 180° by the guest acetyl group prior to C-C bond formation since the guest molecules expose their Re faces to the C5-H5 steroid bonds¹. To elucidate this rotation, photochemical and crystallographic studies were performed on four DCA complexes with engineered guest ketone arrangements. The guests were p-fluoroacetophenone and (p-substituted) propiophenones (XC₆H₄CO₂C₂H₅, X=H, F, Cl). The crystal structures of two isolated photoproducts are also reported. The channels of DCA-fluoroacetophenone contain two independent guest molecules G' and G, respectively exposing their Re and Si faces to the steroid C5-H5 bond vectors at the potential sites of reaction. Only one diastereomeric photoproduct was obtained with addition at atom C5, with absolute configuration S at the generated stereogenic centre, akin to that of the DCA-acetophenone complexes. The reaction is interpreted in terms of a 180° rotation of the acetyl group of the G' molecule; the absence of the R diastereomer is explained by a preclusion of photoaddition of guest G to DCA because of guest packing along the channel. The guest arrangements in the two complexes DCA-propiophenone and DCA-p-fluoropropiophenone are almost isomorphous. There are two independent guest molecules G and Ḡ related by a pseudo centre of inversion. Each of the two guest molecules exposes its Re face to a steroid C5-H5 bond at a site of reaction. Thus photoaddition takes place with and without 180° rotation of the propionyl group since the two complexes each yield the two diastereomeric photoproducts at C5. DCA-p-chloropropiophenone yields only one diastereomeric photoproduct at C5, with absolute configuration R about the new stereogenic centre. The crystal structure analysis suggests three independent guest molecules G, G' ana Ḡ; G and G' are related by pseudo two fold screw symmetry along the channel axis; Ḡ is related to G and G' by pseudo centre of inversion; once again the guests G, G' and Ḡ appear to expose their Re faces to the C5-H5 bonds at the sites of reaction.

Photochemical reactions in crystals proceed under control of the structure, including absolute configuration, of the bulk, unless preceeded by energy transfer, in which case defects may be important. The reaction may be followed by phase separation. Two-phase systems allow wider scope for reactant selection. The presence during growth of additives, matched stereochemically to the crystal, modifies habit and growth-rate, with far-reaching consequences. Some partially ordered systems are considered.

A new stereochemical approach for the controlled nucleation. growth and dissolution of organic crystals with well designed auxiliaries is described. The method is comprised of the use of appropriate stereospecific inhibitors of growth and dissolution of one or more preselected faces of the crystal. This approach has been successfully exploited for the engineering of organic crystals with desired morphologies, for the kinetic resolution of conglomerates by the process of crystallization, and for inducing etch-pits at preselected faces of crystals. A correlation has been established between crystal morphology and crystal purity. Oriented growth of organic and inorganic crystals at interfaces has been accomplished with the assistance of designed Langmuir and Langmuir Blodgett films. The approach has recently been extended to the study of solvent effects on the morphology of crystals in general, and of polar crystals in particular.

When crystals of organic compounds are grown in the presence of growth inhibitors, there is a change in crystal morphology. A stereochemical correlation exists between the crystal structure, its modified morphology, and the molecular structure of the inhibitor. This correlation has been successfully exploited for the efficient resolution of conglomerates, the engineering of organic crystals with desired morphologies, the direct and relative assignment of the absolute configurations of chiral molecules and crystals, and for the design of a new model for the spontaneous generation of optical activity. In an analogous way dissolution of organic crystals in the presence of these growth inhibitors induces etch pits at preselected faces. The effect of solvent on crystal growth has been analyzed in some model systems. The experimental results are complemented by atomatom potential energy calculations.

The centrosymmetric monoclinic crystals of glycylglycine were grown in the presence of three dipeptides (racemic and resolved GlyAla, AlaGly and GlyLeu); the presence of each yielded a different morphology. It was found by a combined theoretical and experimental study that the conformation of the guest molecule is determined by the highly stereospecific intermolecular interactions between adsorbed additive and the molecules of particular surface layers of the growing crystals. Theoretical calculations predicted the preferred conformation of the leucine side chain of GlyLeu consistent with the change in morphology. Occluded racemic GlyLeu segregates along the b axis of the single crystals of GlyGly.

A new general and efficient method for kinetic resolution of racemic conglomerates by crystallization in the presence of "tailor-made" additives is described. The process is explained in terms of stereoselective adsorption of the resolved additive at the surface of the growing crystals of the enantiomer of the same absolute configuration, resulting in a drastic decrease in their rate of growth and thus allowing preferential crystallization of the opposite enantiomer ("rule of reversal"). Some empirical r olutions reported in the literature are rationalized through this mechanism, and appropriate additives for the resolution of new systems are designed and successfully applied. The crystallization of the conglomerates (R,S)-glutarr1ic acid hydrochloride (GluHCl), (R,S)-threonine (Thr), (R,S)-(p-hydroxyphenyl)glycine p-toluenesulfonate (pHpgpTs)J and (R ,S )-aspa ragine hydrate (AsnH2O) in the presence of other amino acids, used as additives, has been studied in particular. It is demonstrated that the additives are occluded in the bulk of the homochiral crystal in typical amounts of 0.5-1.5%, while they are not found in the bulk of the crystals of the antipode. The possible role of the additives in nucleation and dissolution of the affected crystals is considered. A new method for the assignment of absolute configuration of chiral molecules is proposed.

An \u201cabsolute asymmetric synthesis\u201d with quantitative enantiomeric yield, via the process of crystallization of a nonchiral compound in a chiral crystal followed by a topochemical photoreaction, has been successfully executed. The needed crystalline chiral phases, composed of unsymmetrically disubstituted dienes and with the two different double bonds correctly juxtaposed for asymmetric (2_π + 2_π) photodimerization and photopolymerization along a translational axis, were designed. The starting point for this matrix engineering was the crystal structure of the chiral monomer 1. After inspection of the shortest contacts made by the chiral sec-butyl group inside the crystal, we examined some hypothetical transfers of methyls to and from this group, with the nearest neighboring molecules; such transfers might generate phases isomorphous to 1 but composed of achiral monomers. Four systems were considered promising candidates, namely the 1:1 mixture of monomers 3 and 4 and monomers 8, 9, and 11. Three of these behave in the predicted way. Large single crystals of achiral monomer 9 were grown and irradiated, yielding, in a number of independent experiments, dimers and oligomers of either chirality, with a quantitative enantiomeric yield within the limits of experimental error.

The differences in luminescence properties between the chiral resolved and the racemic compounds of some 9-anthroates have been exploited for the determination of the enantiomeric excess in these esters.

A method for resolution of conglomerates, based on morphological differences between the enantiomorphic crystals induced by tailor-made resolved impurities is described. The relationship between the crystal structure of the affected enantiomorph and the stereochemistry of the added impurity is analysed in the systems glutamic acid. HCl, asparagine. H₂O and threonine in the presence of L- or D-amino acids, and it is found that the additive binds stereoselectively at the surface of the enantiomer of the same absolute configuration, occupying the same site of the amino acid groups of the host molecule. The modified side chains perturb in a second stage the regular growth of the crystal in the directions along which the side chains of the host are hydrogen bonded.

Monomeric dienes 17, which pack in chiral crystal structures and yield upon irradiation chiral cyclobutane dimers, trimers, and oligomers, have been crystallized in the presence of small amounts of their photoproducts (\u201cimpurities\u201d). An efficient asymmetric induction was observed in these crystallizations; the absolute configuration of the phase precipitating in excess is always found to be opposite to that of the crystalline phase from which the impurity was generated. A mechanism is proposed in which small amounts (∼1%) of the chiral additives are stereospecifically adsorbed onto the growing sites of the stereochemically similar crystal, thereby delaying its growth. Experiments yielding evidence in support of this mechanism are presented. Possible applications of the results of this study to the optical resolution of conglomerates in general in the presence of \u201ctailor-made\u201d impurities are discussed.

An \u201cabsolute\u201d asymmetric synthesis of chiral dimers and polymers of variable optical yield and an asymmetric synthesis with quantitative enantiomeric yield starting from a monomer of low optical purity are described. Irradiation of polycrystalline ethyl 2-cyano-3-(p-sec-butyl-3-'(E)-propenoate)phenyl(E)-propenoate (1) of 60% optical purity or above leads to the formation of chiral dimers and oligomers with quantitative enantiomeric yield. Carefully crystallized samples of 1 of 22-60% optical purity lead to enantiomeric excesses of 90% or above. Racemic 1 crystallizes from the melt either in the form of a stable eutectic mixture or in the form of an isomorphous, chiral, metastable phase of different microcomposition. A method for the characterization of the two phases, based on photodimerization and NMR analysis of the diastereomeric dimers, is described. Irradiation of samples of the chiral metastable phase with one enantiomorphous crystal form in excess leads to an \u201cabsolute\u201d asymmetric synthesis with variable optical yield, of both signs. Possible routes to the achievement of an \u201cabsolute\u201d asymmetric synthesis with quantitative enantiomeric yield, in systems related to 1, are discussed.

In recent years there has been a growing interest in asymmetric synthesis via topochemically-controlled reactions in chiral crystak2 In such reactions the sole chiral influence derives from the asymmetric environment of the molecules in the crystal.

Solid-state thermolysis at 90°C or photolysis at ambient temperature of the molecular complex of deoxycholic acid with di-t-butyl diperoxycarbonate leads to a one-step regiospecific and stereospecific hydroxylation at position 5 of the acid.

The stereocourse of the reaction of solid dihalogenoadipodinitriles with gaseous ammonia has been investigated in terms of the conformation of the reacting molecules in the crystal; under the heterogeneous conditions of the reaction, in certain systems, an equilibrium between different rotamers occurs before an anti-type elimination takes place.