Publications

In this study, we perform quantum calculations of the spin dynamics of a small spin system that includes nine coupled electrons and one nucleus placed in an external magnetic field and exposed to microwave irradiation. This is an extension of a previous work in which we have demonstrated on a system of 11 coupled electron spins the dynamics of the electron polarizations composing the electron paramagnetic resonance (EPR) line during static dynamic nuclear polarization (DNP) experiments. There we have shown that the electron polarizations are determined by a spectral diffusion process, induced by the dipolar interaction and cross-relaxation. Additionally, we showed that a distinction had to be made between strong and weak dipolar-coupled systems relative to the inhomogeneity of the EPR line with only the first behaving according to the thermal mixing DNP (with two electron spin temperatures) description. The EPR spectra in the weak and strong dipolar interaction cases show different types of spectral features. In the extended spin system, we again make a distinction between weak and strong electron electron interactions and show that the DNP spectra for the two cases are different in nature but that the DNP spectra can be derived in all cases from the EPR line shapes using the indirect cross effect.

A deeper understanding of parameters affecting Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP), an emerging nuclear magnetic resonance hyperpolarization method, is crucial for the development of new polarizing agents and the successful implementation of the technique at higher magnetic fields (>10 T). Such progress is currently impeded by computational limitation which prevents the simulation of large spin ensembles (electron as well as nuclear spins) and to accurately describe the interplay between all the multiple key parameters at play. In this work, we present an alternative approach to existing cross-effect and solid-effect MAS-DNP codes that yields fast and accurate simulations. More specifically we describe the model, the associated Liouville-based formalism (Bloch-type derivation and/or Landau-Zener approximations) and the linear time algorithm that allows computing MAS-DNP mechanisms with unprecedented time savings. As a result, one can easily scan through multiple parameters and disentangle their mutual influences. In addition, the simulation code is able to handle multiple electrons and protons, which allows probing the effect of (hyper) polarizing agents concentration, as well as fully revealing the interplay between the polarizing agent structure and the hyperfine couplings, nuclear dipolar couplings, nuclear relaxation times, both in terms of depolarization effect, but also of polarization gain and buildup times.

This work presents a deuterium magic angle spinning (MAS) NMR study of the adsorption-desorption dynamics of glycine-(2,2)-d(2)-alanine dipeptide adsorbed at the inner surfaces of mesoporous SBA-15 silica under different hydration levels and temperatures. The experimental and theoretical challenges posed by the strong quadrupolar interaction of the rigid CD2 group, 3-fold bigger than that of the rotating methyl CD3, were addressed. Deuterium MAS NMR spectra modulated by exchange were analyzed using theoretically calculated exchange spectra based on the two-site Bloch-McConnel exchange equation represented in Floquet space. To solve this equation, which is composed of a high dimensional Floquet exchange matrix, our former computational approach was modified to reduce the overall computation time by orders of magnitude so as to yield more accurate exchange parameters from the spectral analysis. The adsorption-desorption kinetics of minutely hydrated silica surfaces is understood to originate from the diffusion of water molecules into and out of adsorbate binding sites, thereby gating the dynamic behavior of the adsorbate via increase or reduction of the size of the surrounding water cluster. Molecular dynamic (MD) simulations were employed to model the dynamic behavior of the adsorbate at the two states. Deviations between the MD and experimental observations are attributed to the simplified surface modeling, thereby highlighting the importance of experimental MAS NMR data to improve future modeling of realistic functional surfaces.

Gd(iii) complexes have emerged as spin labels for distance determination in biomolecules through double-electron-electron resonance (DEER) measurements at high fields. For data analysis, the standard approach developed for a pair of weakly coupled spins with S = 1/2 was applied, ignoring the actual properties of Gd(iii) ions, i.e. S = 7/2 and ZFS (zero field splitting) ≠ 0. The present study reports on a careful investigation on the consequences of this approach, together with the range of distances accessible by DEER with Gd(iii) complexes as spin labels. The experiments were performed on a series of specifically designed and synthesized Gd-rulers (Gd-PyMTA-spacer-Gd-PyMTA) covering Gd-Gd distances of 2-8 nm. These were dissolved in D₂O-glycerol-d₈ (0.03-0.10 mM solutions) which is the solvent used for the corresponding experiments on biomolecules. Q- and W-band DEER measurements, followed by data analysis using the standard data analysis approach, used for S = 1/2 pairs gave the distance-distribution curves, of which the absolute maxima agreed very well with the expected distances. However, in the case of the short distances of 2.1 and 2.9 nm, the distance distributions revealed additional peaks. These are a consequence of neglecting the pseudo-secular term in the dipolar Hamiltonian during the data analysis, as is outlined in a theoretical treatment. At distances of 3.4 nm and above, disregarding the pseudo-secular term leads to a broadening of a maximum of 0.4 nm of the distance-distribution curves at half height. Overall, the distances of up to 8.3 nm were determined, and the long evolution time of 16 μs at 10 K indicates that a distance of up to 9.4 nm can be accessed. A large distribution of the ZFS parameter, D, as is found for most Gd(iii) complexes in a frozen solution, is crucial for the application of Gd(iii) complexes as spin labels for distance determination via Gd(iii)-Gd(iii) DEER, especially for short distances. The larger ZFS of Gd-PyMTA, in comparison to that of Gd-DOTA, makes Gd-PyMTA a better label for short distances.

Dynamic Nuclear Polarization (DNP) experiments on solid dielectrics can be described in terms of the Solid Effect (SE) and Cross Effect (CE) mechanisms. These mechanisms are best understood by following the spin dynamics in electron-nuclear and electron-electron-nuclear model systems, respectively. Recently it was shown that the frequency swept DNP enhancement profiles can be reconstructed by combining basic SE and CE DNP spectra. However, this analysis did not take into account the role of the electron spectral diffusion (eSD), which can result in a dramatic loss of electron polarization along the EPR line. In this paper we extend the analysis of DNP spectra by including the influence of the eSD process on the enhancement profiles. We show for an electron-electron-nuclear model system that the change in nuclear polarization can be caused by direct MW irradiation on the CE electron transitions, resulting in a direct CE (dCE) enhancement, or by the influence of the eSD process on the spin system, resulting in nuclear enhancements via a process we term the indirect CE (iCE). We next derive the dependence of the basic SE, dCE, and iCE DNP spectra on the electron polarization distribution along the EPR line and on the MW irradiation frequency. The electron polarization can be obtained from ELDOR experiments, using a recent model which describes its temporal evolution in real samples. Finally, DNP and ELDOR spectra, recorded for a 40 mM TEMPOL sample at 10-40 K, are analyzed. It is shown that the iCE is the major mechanism responsible for the bulk nuclear enhancement at all temperatures.

Over the last two decades solid state Nuclear Magnetic Resonance has witnessed a breakthrough in increasing the nuclear polarization, and thus experimental sensitivity, with the advent of Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP). To enhance the nuclear polarization of protons, exogenous nitroxide biradicals such as TOTAPOL or AMUPOL are routinely used. Their efficiency is usually assessed as the ratio between the NMR signal intensity in the presence and the absence of microwave irradiation epsilon(on/off). While TOTAPOL delivers an enhancement epsilon(on/off) of about 60 on a model sample, the more recent AMUPOL is more efficient: >200 at 100 K. Such a comparison is valid as long as the signal measured in the absence of microwaves is merely the Boltzmann polarization and is not affected by the spinning of the sample. However, recent MAS-DNP studies at 25 K by Thurber and Tycko (2014) have demonstrated that the presence of nitroxide biradicals combined with sample spinning can lead to a depolarized nuclear state, below the Boltzmann polarization. In this work we demonstrate that TOTAPOL and AMUPOL both lead to observable depolarization at approximate to 110 K, and that the magnitude of this depolarization is radical dependent. Compared to the static sample, TOTAPOL and AMUPOL lead, respectively, to nuclear polarization losses of up to 20% and 60% at a 10 kHz MAS frequency, while Trityl OX63 does not depolarize at all. This experimental work is analyzed using a theoretical model that explains how the depolarization process works under MAS and gives new insights into the DNP mechanism and into the spin parameters, which are relevant for the efficiency of a biradical. In light of these results, the outstanding performance of AMUPOL must be revised and we propose a new method to assess the polarization gain for future radicals.

We report magic angle spinning, dynamic nuclear polarization (DNP) experiments at magnetic fields of 9.4 T, 14.1 T, and 18.8 T using the narrow line polarizing agents 1,3-bisdiphenylene-2-phenylallyl (BDPA) dispersed in polystyrene, and sulfonated-BDPA (SA-BDPA) and trityl OX063 in glassy glycerol/water matrices. The H-1 DNP enhancement field profiles of the BDPA radicals exhibit a significant DNP Overhauser effect (OE) as well as a solid effect (SE) despite the fact that these samples are insulating solids. In contrast, trityl exhibits only a SE enhancement. Data suggest that the appearance of the OE is due to rather strong electron-nuclear hyperfine couplings present in BDPA and SA-BDPA, which are absent in trityl and perdeuterated BDPA (d(21)-BDPA). In addition, and in contrast to other DNP mechanisms such as the solid effect or cross effect, the experimental data suggest that the OE in non-conducting solids scales favorably with magnetic field, increasing in magnitude in going from 5 T, to 9.4 T, to 14.1 T, and to 18.8 T. Simulations using a model two spin system consisting of an electron hyperfine coupled to a H-1 reproduce the essential features of the field profiles and indicate that the OE in these samples originates from the zero and double quantum cross relaxation induced by fluctuating hyperfine interactions between the intramolecular delocalized unpaired electrons and their neighboring nuclei, and that the size of these hyperfine couplings is crucial to the magnitude of the enhancements. Microwave power dependent studies show that the OE saturates at considerably lower power levels than the solid effect in the same samples. Our results provide new insights into the mechanism of the Overhauser effect, and also provide a new approach to perform DNP experiments in chemical, biophysical, and physical systems at high magnetic fields. (C) 2014 AIP Publishing LLC.

To study the solid state ¹H-DNP mechanism of the biradical TOTAPOL under static conditions the frequency swept DNP enhancement spectra of samples containing 20 mM and 5 mM TOTAPOL were measured as a function of MW irradiation time and temperature. We observed that under static DNP conditions the biradical TOTAPOL behaves similar to the monoradical TEMPOL, in contrast to MAS DNP where TOTAPOL is considerably more effective. As previously done for TEMPOL, the TOTAPOL DNP spectra were analyzed taking a superposition of a basic SE-DNP lineshape and a basic CE-DNP lineshape with different amplitudes. The analysis of the steady state DNP spectra showed that the SE was dominant in the 6-10 K range and the CE was dominant above 10 K. DNP spectra obtained as a function of MW irradiation time allowed resolving the individual SE and CE buildup times. At low temperatures the SE buildup time was faster than the CE buildup time and at all temperatures the CE buildup time was close to the nuclear spin-lattice relaxation time, T_1n. Polarization calculations involving nuclear spin-diffusion for a model system of one electron and many nuclei suggested that the shortening of the T_1n for increasing temperatures is the reason why the SE contribution to the overall enhancement was reduced.

A Floquet description of a phase alternated homonuclear recoupling scheme for perdeuterated systems is presented. As a result, we demonstrate improvements in the recoupling efficiency of the DOuble Nucleus Enhanced Recoupling [DONER; J. Am. Chem. Soc. 131 (2009) 170541 technique by utilizing Phase Alternated Recoupling Irradiation Schemes [PARIS; Chem. Phys. Lett. 469 (2009) 342]. The effect of proton and deuterium radio frequency irradiation during recoupling has been systematically studied and theoretical observations have been verified experimentally using a deuterated model compound, L-Alanine, at 10 and 20 kHz magic angle spinning frequency. Experimental results are well in agreement with theoretical observations, thereby significantly increasing the recoupling efficiency of conventional DONER in perdeuterated systems. (C) 2013 Elsevier Inc. All rights reserved.

The ¹³C solid state Dynamic Nuclear Polarization (DNP) mechanism using trityl radicals (OX63) as polarizers was investigated in the temperature range of 10-60 K. The solutions used were 6 M ¹³C urea in DMSO/H ₂O (50% v/v) with 15 mM and 30 mM OX63. The measurements were carried out at ∼3.5 T, which corresponds to Larmor frequencies of 95 GHz and 36 MHz for the OX63 and the ¹³C nuclei, respectively. Measurements of the ¹³C signal intensity as a function of the microwave (MW) irradiation frequency yielded ¹³C DNP spectra with temperature dependent lineshapes for both samples. The maximum enhancement for the 30 mM sample was reached at 40 K, while that of the 15 mM sample at 20-30 K. Furthermore, the lineshapes observed showed that both the cross effect (CE) and the solid effect (SE) DNP mechanisms are active in this temperature range and that their relative contribution is temperature dependent. Simulations of the spectra with the relative contributions of the CE and SE mechanisms as a fit parameter revealed that for both samples the CE contribution decreases with decreasing temperature while the SE contribution increases. In addition, for the 15 mM sample the contributions of the two mechanisms are comparable from 20 K to 60 K while for the 30 mM the CE dominates in this range, as expected from the higher concentration. The steep decrease of the CE contribution towards low temperatures is however unexpected. The temperature dependence of the OX63 longitudinal relaxation, DNP buildup times and ¹³C spin lattice relaxation times did not reveal any obvious correlation with the DNP temperature dependence. A similar behavior of the CE and SE mechanism was observed for ¹H DNP with the nitroxide radical TEMPOL as a polarizer. This suggests that this effect is a general phenomenon involving a temperature dependent competition between the CE and SE mechanisms, the source of which is, however, still unknown.

Deuterium magic angle spinning (MAS) NMR is used to study the dynamics of an organic molecule, N-[triethoxysilylpropyl]acetamide-d₃, grafted at the inner surface of the mesoporous silica material, MCM-41. The grafted molecule has a deuterated methyl group at its free terminus to monitor its local mobility through changes in its dynamic ²H-MAS NMR spectrum. Different spectra were recorded as a function of temperature from two different water containing samples. Observation shows that a major part of the grafted molecule remains static, irrespective of the temperature and hydration state of the sample, whereas the rest shows spectral changes indicative of a two-site jump motion of the methyl groups. Experimental observations were substantiated using molecular dynamic (MD) simulations of the grafted molecule. Subsequently, the MD results corroborate a model for the grafted molecules experiencing an exchange between two conformations consistent with the analysis of the ²H-MAS NMR spectra.

Proton Dynamic Nuclear Polarization (DNP) experiments were conducted on a 3.4 T homebuilt hybrid pulsed-EPR-NMR spectrometer, on static samples containing 10 mM or 40 mM TEMPOL in frozen glassy solutions of DMSO/water. During DNP experiments proton-NMR signals are enhanced with the help of microwave (MW) irradiation on or close to the Electron Paramagnetic Resonance (EPR) spectrum of the free radicals in the sample, transferring polarization from the free electrons to the nuclei. In the solid state a distinction is made between three DNP enhancement mechanisms: the Solid Effect (SE), the Cross Effect (CE) and Thermal Mixing (TM). In an effort to determine the dominant DNP mechanisms responsible for the enhancement of the nuclear signals, electron and nuclear spin-lattice relaxation rates, enhancement buildup times and microwave (MW) swept DNP spectra were measured as a function of temperature and MW irradiation strength. We observed lineshape variations of the DNP spectra that indicated changes in the relative contributions of SE-DNP and CE-DNP with temperature and MW power. Using a theoretical model describing the SE-DNP and CE-DNP the DNP spectra could be analyzed without involving the TM-DNP mechanism and the relative SE-DNP and CE-DNP contributions to the nuclear enhancement could be determined. From this analysis it follows that lowering the temperature beyond 20 K increases the SE-DNP and decreases the CE-DNP contributions. Possible explanations for this behavior are suggested.

We present a Floquet theory approach for the analysis of homonuclear recoupling assisted by radio frequency (RF) irradiation of surrounding heteronuclear spins. This description covers a broad range of systems from fully protonated to deuterated proteins, focusing in detail on recoupling via protons and deuterons separately as well as simultaneously by the double nucleus enhanced recoupling (DONER) scheme. The theoretical description, supported by numerical simulations and compared to experimental results from a partially deuterated model compound, indicates that in perdeuterated systems setting the RF amplitude equal to the magic angle spinning (MAS) frequency is not necessarily optimal for recoupling via ¹H and/or ²H nuclei and modified recoupling conditions are identified.

The application of Floquet theory in solid-state nuclear magnetic resonance is discussed. The Floquet approach has been used in the study of various effects related to quadrupolar nuclei varying from the basic magic-angle spinning (MAS) Hamiltonian, rotational resonance, and rotary-resonance conditions to the effect of quadrupole nuclei on spin 1/2 coupled to them. An extension of Floquet theory, multimode-multipole Floquet theory (MMFT), in which a multipole operator basis has been used is applied to the analysis of heteronuclear decoupling, the calculation of the R² condition width, and depolarization effects in double-quantum recoupling experiments. Cross polarization (CP) is one of the most important techniques for the detection of rare spins in solid-state nuclear magnetic resonance (NMR). MAS leads to improvement in the spectral resolution making the various sites distinguishable and improving the signal to noise ratio but it complicates the analysis of the spectra resulting from dynamic processes.

Magnetic susceptibility measurements of WO3 crystals hydrogen doped on the surface suggest 2D local non-percolated superconductivity with an onset temperature of 120 K. Observed zero field cooled vs. field cooled magnetization response is characteristic of type II superconductivity. The diamagnetic response at the critical temperature is field dependent, and is suppressed by a magnetic field of similar to 1700 Oe.

High resolution pulse EPR techniques applied to half integer high spin systems, such as Mn²⁺ (S = 5/2), usually focus only on the central |-1/2〉→ |1/2〉 transition. The reason is that at high fields, where the zero field splitting is considerably smaller than the Zeeman interaction, the spectrum of this transition is intense and narrow. However, because the experiments are carried out at low temperatures, the low lying levels are heavily populated and the signal of the central transition is nevertheless diminished. This, in turn affects the sensitivity of the pulse EPR technique applied. A transfer of populations from the lower lying levels, which for Mn²⁺ are the |-3/2〉 and |-5/2〉 levels, to the |-1/2〉 level will therefore increase the sensitivity. Here we describe such an experiment, where a rapid magnetic field sweep over the |-3/2〉→ |-1/2〉 sub-spectrum is carried out, concomitantly with a low power microwave (mw) irradiation, which results in population inversion. After this sweep any pulsed EPR sequence can be applied to the central transition that now has a population difference that deviates from the equilibrium value. The feasibility of the experiment is demonstrated at W-band (95 GHz) on Mn ²⁺ doped in MgO for echo-detected EPR measurements and the dependence of the signal enhancement on the rate and range of the magnetic field sweep and on the mw power is described. The results are then accounted for theoretically by considering a simple fictitious spin 1/2 system. In addition, preliminary enhanced ⁵⁵Mn pulse ENDOR electron nuclear double resonance (ENDOR) spectra are presented.

The elimination by HF of the silica matrix from the composites obtained by the two-step reaction deposition of CsxH3-xPW12O40 (CSHPW) salt nanocrystals with a CS/W-12 ratio equal 2.5 on SBA-15 yields materials with substantially lower Cs/W-12 ratios of 1.7-2.0. The value of the CS/W-12 ratio in the nanocasts is determined by the Cs-precursor (Cs n-propoxide or Cs-acetate) used at the first stage of materials preparation. The surface area of the CsHPW nanocasts is 41-45 times higher than their co-precipitated analogs at the same Cs/W-12, ratios. We report here that implementation of the nanocasting preparation technique yields for the first time a bulk CsHPW material that combines a high concentration of acid sites (CS/W-12= 1.7-2.0) with a high surface area of 41-93 m(2) g(-1). Co-precipitated analogues at the same CS/W-12 ratios are nonporous and exhibit a surface area smaller than 5 m(2) g(-1). Our nanocasted CsHPW materials are stable against leaching and colloidization in polar solvents, and their catalytic performance exceeded that of bulk Cs2.5H0.5PW12O40, known as the most active among the acidic HPW salts. The catalytic activity of CsHPW nanocasts in MTBE synthesis and in the isopropanol dehydration reactions is shown to be higher by a factor of 2-3 than that of the standard Cs2.5H0.5PW12O40 material. (c) 2007 Elsevier B.V. All rights reserved.

A theoretical treatment of heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance is presented here based on bimodal Floquet theory. The conditions necessary for good heteronuclear decoupling are derived. An analysis of a few of the decoupling schemes implemented until date is presented with regard to satisfying such decoupling conditions and efficiency of decoupling. Resonance conditions for efficient heteronuclear dipolar decoupling are derived with and without the homonuclear H1 - H1 dipolar couplings and their influence on heteronuclear dipolar decoupling is pointed out. The analysis points to the superior efficiency of the newly introduced swept two-pulse phase-modulation (SWf -TPPM) sequence. It is shown that the experimental robustness of SWf -TPPM as compared to the original TPPM sequence results from an adiabatic sweeping of the modulation frequencies. Based on this finding alternative strategies are compared here. The theoretical findings are corroborated by both numerical simulations and representative experiments.

The effects of grafting the alumina species at the surface of silica support on the H3PW12O40 (HPW) immobilization capacity and performance of loaded HPW in acid-catalyzed reactions were studied with regular silica-gel and ordered mesostructured silica SBA-15. The states of alumina and HPW, in terms of the acidity/basicity, material texture, and structure of adsorbed HPW species, were characterized by XRD, XPS, NH3-TPD, UV-vis-, and FTIR-spectroscopy, as well as H-1 and P-31 MAS NMR, NH3- and CO2-TPD, and N-2 adsorption. It was demonstrated that grafting of silica with small alumina clusters at partial to full surface coverage produces isolated basic sites of the same strength as at the surface of pure alumina, but with a surface concentration that is an order of magnitude lower. These sites anchor the molecular species of HPW, retaining their polyanion structure and acidity-catalytic activity patterns in several acid-catalyzed reactions. The reason for the low activity of HPW moieties immobilized at the surface of pure alumina is a polydentate adsorption of polyanions yielding a high extent of HPW acidity neutralization. The use of mesostructured SBA-15 as a support yielded catalysts with 30-70% higher activity compared with that based on regular silica gel. This is a result of higher surface area and surface concentration of silanols in SBA-15, which makes it possible to immobilize more HPW as molecular species. © 2005 Elsevier Inc. All rights reserved.

A high resolution two-dimensional solid state NMR experiment is presented that correlates half-integer quadrupolar spins with protons. In this experiment the quadrupolar nuclei evolve during t(1) under a split-t1, FAM-enhanced MQMAS pulse scheme. After each t(1) period ending at the MQMAS echo position, single quantum magnetization is transferred, via a cross polarization process in the mixing time, from the quadrupolar nuclei to the protons. High-resolution proton signals are then detected in the t(2) time domain during wPMLG5* homonuclear decoupling. The experiment has been demonstrated on a powder sample of sodium citrate and Na-23-H-1 2D correlation spectra have been obtained. From the HETCOR spectra and the regular MQMAS spectrum, the three crystallographically inequivalent Na+ sites in the asymmetric unit were assigned. This MQMAS-wPMLG HETCOR pulse sequence can be used for spectral editing of half-integer quadrupolar nuclei coupled to protons. (C) 2004 Elsevier Inc. All rights reserved.

The proton-carbon polarization exchange in Lee-Goldburg cross-polarization magic angle spinning (LG-CP MAS) nuclear magnetic resonance experiments on uniformly C-13-labeled compounds at high spinning frequency is studied. It is shown that the multiple carbon labels in the samples greatly influence the spin dynamics during the LG-CP mixing times. The zeroth order effective LG-CP MAS spin Hamiltonian is a sum of zero quantum dipolar interaction terms. These pairwise dipolar terms generally do not commute with each other, making it impossible to factorize the evolution operator. Consequently, the frequencies of the dipolar oscillations as well as the polarization transfer amplitudes become strongly dependent on the configuration of the spins involved in the multiple heteronuclear couplings. The strong carbon-proton couplings usually attenuate polarization transfers between weakly coupled spins. In practice, this implies that except for strongly coupled or isolated heteronuclear C-13-H-1 spin pairs, it is difficult to unambiguously extract structural constraints from experimental data. To better understand the complexity of the LG-CP processes, experiments on simple three- and four-spin systems are simulated and analyzed. More specifically, it is shown that in (CH)-C-13-(CH)-C-13 and (CH2)-C-13-C-13 spin systems, a significant amount of the proton polarization can be transferred to both carbons, despite the fact that the individual proton-carbon heteronuclear couplings between each proton and the carbon spins are very different. The dependence of the polarization transfer on the position of the proton carrier frequency is analyzed and it is shown that by an appropriate choice of this frequency, specific polarization transfer pathways can be selected. Experimental results from [U-C-13] tyrosine.HCl and [U-C-13, N-15] histidine.HCl.H2O samples are in satisfactory agreement with simulations. (C) 2003 American Institute of Physics.

We demonstrate here a new windowed multiple pulse sequence for the detection of protons in solid state NMR. Acquisition windows are inserted in the phase modulated Lee-Goldburg scheme (PMLG). This enables one-dimensional acquisition as against two-dimensional in windowless schemes. The pulse scheme, theoretical justifications and experimental spectra of a few samples are presented. In addition two-dimensional proton-proton and carbon-proton correlation experiments are performed with windowed PMLG proton detection. (C) 2002 Published by Elsevier Science B.V.

Reintroducing dipolar coupling between spin-1/2, nuclei (e.g., C-13, N-15) and spin-1 H-2, using phase-modulated deuterium dephasing pulses, provides a simple and efficient basis for obtaining peptide backbone torsion angles (phi, psi) in specific stable-isotope enriched samples, Multiple homonuclear spin-1/2 interactions due to isotopic enrichment can arise between neighboring molecules or within a multiply labeled protein after folding. The consequences of C-13 homonuclear interactions present during C-13-observed, H-2-dephased REDOR measurements are explored and the theoretical basis of the experimentally observed effects is investigated. Two tripeptides are taken to represent both the general case of H-2(alpha)-alanine (in the tripeptide LAF) and the special case of H-2(2)alpha-glycine (in the tripeptide LGF). The lyophilized tripeptides exhibit narrowed spectral linewidths over time due to reduced conformational dispersion. This is due to a hydration process whereby a small fraction of peptides is reorienting and the bulk peptide fraction undergoes a conformational change. The new molecular packing arrangement lacks homonuclear C-13 spin interactions, allowing determination of (phi, psi) backbone torsion angles. (C) 2001 Academic Press.

Dipolar recoupling techniques of homonuclear spin pairs are commonly used for distance or orientation measurements in solids. Accurate measurements are interfered with by broadening mechanisms. In this publication narrowband RF-driven dipolar recoupling magnetization exchange experiments are performed as a function of the spinning frequency to reduce the effect of zero-quantum T-2 relaxation. To enhance the exchange of magnetization between the coupled spins, a fixed number of rotor-synchronous pi-pulses are applied at spinning frequencies approaching the rotational resonance (R-2) conditions. The analysis of the powder averaged dipolar decay curves of the spin magnetizations as a function of the spinning frequency provides a quantitative measure of the dipolar coupling. An effective Hamiltonian for this experiment is derived, taking into account all chemical shift parameters of the spins. The length of the nbRFDR mixing time and the number of rotor cycles per pi-pulse are optimized by numerical simulations for sensitive probing of the dipolar coupling strength. The zero-quantum T-2 relaxation time can easily be taken into account in the data analysis, because the overall exchange time is almost constant in these experiments. Spinning-frequency-dependent nbRFDR experiments near the m = 1 and m = 2 R-2 condition are shown for doubly C-13-labeled hydroxybutyric acid. (C) 2000 Academic Press.

Phase modulated pulses for deuterium recoupling in H-2-C-13 REDOR NMR spectroscopy have been introduced to improve dephasing of the detected C-13 nuclei. The deuterium inversion properties of phase modulated recoupling pulses have been studied experimentally on L-alanine-2-d(1) and theoretically using average Hamiltonian theory and exact simulations of the equation of motion of the density matrix. The best C-13 dephasing was observed when XYXYX (PM5) deuterium recoupling pulses were applied. A comparison to the 900-180 degrees-90 degrees (CPL) composite pulse scheme revealed an improvement of recoupling on the order of 2.5. Simple CW recoupling pulses of the same length of PM5 and CPL pulses showed the weakest C-13 dephasing. Simulations have shown that the H-2 recoupling efficiency of PM5 REDOR experiments approach the very efficient REAPDOR results. However, in our case a REAPDOR study of L-alanine-2-d(1) resulted in a significant decrease of the C-13 signal intensity due to pulse imperfections of C-13 pi-pulses. The new PM5-REDOR technique has been employed to study the torsion angle between C1/2 and C5 in ethylmalonic acid-4-d(2). (C) 2000 Academic Press.

This paper presents a theoretical description of continuous wave (CW) high frequency Lee-Goldburg cross polarization magic angle spinning (LG-CPMAS) nuclear magnetic resonance experiments. The full time-dependent LG-CPMAS Hamiltonian is replaced by its zero order time-independent Hamiltonian in the interaction representation. Carbon signal enhancements of LG-CPMAS experiments are calculated for spin systems consisting of six H-1 nuclei coupled to one C-13 nucleus. These simulations are based on Floquet theory calculations, explicitly taking into account the time dependence because of magic angle spinning, and calculations based on the zero-order Hamiltonian. The good agreement between these calculations justifies the use of the zero-order Hamiltonian. The time-dependent intensities of the cross peaks in heteronuclear C-13-H-1 correlation spectra, extracted from 3D LG-CPMAS experiments on a natural abundant DL-alanine sample with increasing CP mixing times, are in good agreement with the theoretical intensities simulated by using the zero-order Hamiltonian. The approximated LG-CPMAS Hamiltonian can be used to obtain structural information about a proton coupled to a single carbon. The simulated intensities of the carbon signals of an isolated C-13-H-1 group and a C-13-H-1 group that is coupled to additional protons, measured by LG-CPMAS experiments with increasing CP mixing times, are compared. This study suggests that the buildup curve of each LG-CPMAS carbon signal and its Fourier transformed CP spectrum can be interpreted in terms of a single distance between the observed C-13 and its nearest proton, if the additional protons are removed from this carbon by at least 1.2 times this distance. (C) 2000 American Institute of Physics. [S0021-9606(00)00116-1].

CdS nanoparticles precipitated from aqueous solution were studied by ¹H NMR. The nanoparticles were deliberately not capped by any surface termination agent. The samples had porous structure. Proton high spinning speed magic angle spinning (MAS) NMR spectra revealed that there are three abundant proton species in nanoparticle samples prepared with an excess of Cd, having different chemical shifts: a relatively narrow peak due to hydroxyl groups and two broader lines resulting from adsorbed water molecules with different chemical environments. The relative intensities of the lines were temperature-dependent, reflecting a complicated chemical equilibration processes. A small fraction of water molecules (∼5%) experiences anisotropic dynamic motion as deduced from the analysis of the dipolar sideband patterns. The main part of the spectrum was due to rapidly exchanging protons. The exchange between protons of the same type occurs on a time scale not much longer than 1 μs. The exchange between different lines was studied by 2D exchange MAS spectroscopy. The analysis of the proton-proton correlation spectra as a function of the mixing time led us to the conclusion that the nanocrystalline surface is covered by clusters of water. The clusters are well separated from each other and are formed by protons occupying sites with the same chemical environment. The size of the pores was also estimated to be of the order of a few nanometers.

Multiple-quantum magic-angle-spinning (MQMAS NMR) spectroscopy has become a routine method to obtain high-resolution spectra of quadrupolar nuclei. One of the main problems in the performance of this experiment has been the poor efficiency of the radio-frequency pulses used in converting multiple-quantum coherences to the observable single-quantum signals. As the MQMAS experiment is basically an echo experiment this problem can be related to the efficiency with which continuous wave pulses can normally achieve the multiple- to single-quantum conversion for different crystallites in a spinning powdered sample. In this paper we investigate various aspects involved in this multiple-to-single quantum conversion, in the hope to facilitate the devise of new experimental schemes that can lead to significant MQMAS signal enhancements. We examine in particular a recently suggested experiment for MQMAS spectroscopy which employs amplitude-modulated radio-frequency pulses, and which can yield substantial signal and even resolution enhancements over the commonly used pulse schemes in MQMAS experiments. The mechanisms of operation of continuous-wave and of amplitude-modulated pulses as applied to the selective manipulation of spin-3/2 coherence elements are examined in detail, with the aid of the fictitious spin-1/2 formalism in combination with quadrupolar adiabaticity arguments. New insight into the nature of the MQMAS experiment is thus revealed, and the superior performance of suitable amplitude modulations toward the formation of MQMAS powder echoes is justified. Experimental results highlighting the utility of this scheme in samples possessing multiple quadrupolar sites with varying quadrupolar anisotropies and chemical shift offsets are demonstrated, as is the relative insensitivity of the new signal-enhancement technique to the actual level of rf irradiation. Further implications and uses of this new irradiation scheme are also briefly discussed.

Diluted magnetic semiconductor samples Cd0.99Fe0.01S and Cd0.994Co0.006S were investigated by Cd-113 magic angle spinning NMR spectroscopy in the temperature range of 180-400 K. These alloys were prepared by mixing Cd0.97M0.03S (M = Co, Fe) and the binary compound CdS in the appropriate molar ratios and maintaining the mixtures at about 1000 degrees C for seven-days. The macroscopic homogeneity of the samples was determined by energy dispersive spectroscopy. The microscopic homogeneity of the paramagnetic ion distribution in these samples can be studied by analyzing their Cd-113 NMR spectra. These spectra contain a set of Cd* bands (the * sign indicates the observed cadmium atoms) that are shifted by the transferred hyperfine (THF) interaction between the cadmium nuclei and their neighboring paramagnetic ions. Using the temperature dependence of the positions, the relaxation times, and anisotropies of these bands, we assigned each band to a well-defined next-nearest neighbor M(2)-S-Cd(1)-S-Cd* conformation, and correlated the THF interaction constants of all conformations to the M(2)-Cd* distances. All cadmium bands in the spectra comprise a set of lines that correspond to Cd* atoms in conformations of the type M(3)-S-Cd(2)-S-Cd(1)-Cd*-S-Cd(1)-S-M(2). The relative intensities of the lines can be calculated for given x values, when we assume a random magnetic ion impurity distribution. For the Cd0.994C0.006S sample, we found that the calculated line intensities were not consistent with the experimental intensities. To explain this deviation and to simulate a spectrum that fits the experimental data, we assumed that this sample consists of clusters with different concentrations of the impurity. Good agreement was obtained when about half of the cadmium atoms do not interact with the paramagnetic ions, and the other half interacts with ions that are randomly distributed in an alloy composed of Cd0.987Co0.013S. [S0163-1829(99)14335-2].

The application of short composite pulse schemes (90(x)degrees-90(x)degrees-90(x)degrees and 90(x)degrees-180(x)degrees-90(x)degrees) to the rotational echo double-resonance (REDOR) spectroscopy of X-H-2 (X: spin 1/2 observed) systems with large deuterium quadrupolar interactions has been studied experimentally and theoretically and compared with simple 180 degrees pulse schemes. The basic properties of the composite pulses on the deuterium nuclei have been elucidated, using average Hamiltonian theory, and exact simulations of the experiments have been achieved by stepwise integration of the equation of motion of the density matrix, REDOR experiments were performed on N-15-H-2 in doubly labeled acetanilide and on C-13-H-2 in singly H-2-labeled acetanilide. The most efficient REDOR dephasing was observed when 90(x)degrees-180(x)degrees-90(x)degrees composite pulses were used. It is found that the dephasing due to simple 180 degrees deuterium pulses is about a factor of 2 less efficient than the dephasing due to the composite pulse sequences and thus the range of couplings observable by X-H-2 REDOR is enlarged toward weaker couplings, i.e., larger distances. From these experiments the H-2-N-15 dipolar coupling between the amino deuteron and the amino nitrogen and the H-2-C-13 dipolar couplings between the amino deuteron and the alpha and beta carbons have been elucidated and the corresponding distances have been determined, The distance data from REDOR are in good agreement with data from X-ray and neutron diffraction, showing the power of the method. (C) 1999 Academic Press.

The spin dynamics of an S(1/2)I-N system during the CP mixing time of continuous wave and variable amplitude cross-polarization magic angle spinning (CWCPMAS and VACPMAS) experiments is discussed. The signal enhancement of a low abundant S spin, coupled to a set of N = 6 coupled spins with I = 1/2, is evaluated as a function of the length of the mixing time. For CWCPMAS this signal is first evaluated in the frequency domain and then transformed to the time domain. These calculations provide some additional insight into the CP spin dynamics and enable a practical approach toward the evaluation of CP signals of large spin systems. In addition the adiabatic character of the ramped VACPMAS experiments is discussed and S-spin signals of a spin system with N = 6 are simulated, Estimates of the upper bounds of the CP signals as a function of the number of I spins in an S(1/2)I-N system are given and compared with the calculated values. (C) 1998 Academic Press.

CdS and CdS:Mn nanoparticles were studied by ¹¹³Cd and proton NMR. Nanoparticle samples were synthesized by precipitation of CdS from an aqueous solution of Na₂S and CdSO₄. The nanoparticles were not deliberately capped by any surface-termination agent. The NMR spectra of CdS nanoparticles prepared with an excess of Cd consist of three frequency bands: a bulk line corresponding to the inner Cd atoms with full sulfur coordination, a broad band due to surface Cd atoms, and a sharp line of CdSO₄ in solution. The assignment of these lines is accomplished by experiments on S-rich samples, by the study of the temperature dependence of the spectra, and by comparing the chemical shift values of the lines with tabulated chemical shift values of different cadmium compounds. A significant amount of water was found in the samples. This water is trapped in microscopic pores resulting in a lowering of the water-ice phase transition and a significant shortening of the proton T₁ and T₂ relaxation times. The NMR spectra of CdS:Mn do not show any additional spectral structure. Mn²⁺ ions remain in the included water and/or are located at the nanocrystallite surface and do not appear to be incorporated inside the nanoparticles.

We have investigated the reorientational mobility of n-hexane and n-pentanol guest molecules in Dianin's compound and in zeolite 5A by molecular mechanics/dynamics calculations (MM/MD) and temperature-dependent solid-state H-2 NMR. To choose the proper motional models for the different host-guest systems, MM calculations were first performed and minimum-energy structures for the compounds were established. The calculations were performed on a crystallographic unit cell, with periodic boundary conditions to minimize edge effects. MD methods were then applied to the minimized structures and provided an insight into the different dynamic modes experienced by the guest molecules in their host cages. Several structural parameters of the host-guest systems were monitored for a total simulation period of up to 100 ps both at low temperature (77 K) and high temperature (300 K). H-2 NMR spectra of perdeuterated and specifically deuterated guest molecules in their inclusion compounds were acquired as a function of temperature. After detection the observed Line shapes were analyzed in terms of motional models, taking into account experimental parameters and the results of the MD calculations. The solid-state H-2 NMR showed a very high degree of agreement with the results of the molecular modeling calculations.

Solid extractants for metal ions have been prepared by chemical bonding of jojoba wax to a polystyrene backbone, followed by phosphonation or sulfur-chlorination of the jojoba moiety. In this study, the intermediates and final solid products of the reactions were characterized by solid-state C-13 and P-31 nuclear magnetic resonance spectroscopy. The spectra showed the expected chemical shifts of the atoms involved in the chemical reactions, as well as other parts of the reacting molecules. Thus, the carbonyl carbon of the jojoba chain appears at 175 ppm, the methyl carbons at 15 ppm, tie polystyrene backbone at 40-42 ppm (aliphatic carbons) and 128, 137, 143-147 (aromatic carbons). Carbons adjacent to N, S, and P appear at 45-55, 60, and 48 ppm, respectively.

The p-xylene adduct of Dianin's compound was studied by solid state NMR and molecular modelling. Molecular potential calculations were performed considering a p-xylene guest molecule in a single host cage. These calculations confirmed the existence of six different possible orientations of the guest molecules in the cage. The orientations correspond to two types of guests, which are not related by any symmetry, each consisting of three C-3 symmetry related sites. The two types induce significant distortions to the local structure of the host crystal cage, and these are most pronounced at the bottle-neck of the cage. In the calculations no diffusion paths were found that transferred guest molecules of one type to the other. X-Ray studies confirmed that the dimensions of the p-xylene/Dianin's cages are essentially equal to those reported previously. In our case, the R (3) over bar space groups failed to describe the X-ray diffraction data accurately. Single crystal H-2-NMR of the p-xylene/Dianin's compound, specifically deuterated in the hydroxyl groups of the cage, provided detailed information on the distortions of the hexagons of hydroxyl groups interconnecting two cages. The hexagon distortions were correlated to the orientation of the guest molecules. C-13 CP-MAS NMR confirmed that the methyl groups of the host, in the middle of the cage, are influenced strongly by the inclusion of the p-xylene molecules. Based on the experimental observations, a model describing the distortions in the crystal is proposed.

The results of various N-15 solid-state NMR experiments performed on solid samples of doubly N-15-labeled 3,5-dimethylpyrazole, 5-methyl-3-phenylpyrazole, (PMP), and 3,5-diphenylpyrazole are reported, In the solid state, these compounds form various hydrogen-bonded complexes, The principal values of the N-15 chemical-shift tensors (CST) of amine and imine nitrogen atoms are derived by lineshape analysis of the N-15 NMR spectra of the static powders obtained under the conditions of H-1-N-15 cross polarization and H-1 decoupling, The orientations of the N-15 CST in the molecular principal axis system are obtained by taking into account the N-15-N-15 dipolar interactions and the N-15-D dipolar interaction after deuteration of the mobile proton sites, The relative orientations of the amine and imine CST in PMP are independently checked by one-dimensional off-magic axis sample spinning magnetization transfer experiments, The isotropic chemical shifts, the principal elements, and the orientations of the CST of both nitrogen atoms and the ND distances depend only slightly on the chemical structure and the associated hydrogen-bonded structure, The intramolecular structure of pyrazoles, therefore, does not vary substantially when different types of hydrogen-bonded complexes are formed. (C) 1996 Academic Press, Inc.

The sensitivity of one-dimensional dynamic magic-angle spinning (MAS) and off-MAS H-2 nuclear magnetic resonance spectra to changes in the parameters of jump-type molecular motions is studied. The Floquet theory approach is used to simulate spectra of spins with I = 1, which are involved in exchange processes in rotating solids. The solution of the Bloch-McConnell equations for rotating samples are derived and some simulated frequency spectra are shown. The dependence of the lineshapes of the center and sidebands of the MAS and off-MAS spectra on the exchange parameters are discussed. Experimental results of H-2 spectra of perdeuterated dimethyl sulfone, obtained in the temperature range 20-55 degrees C, are demonstrated. The methyl groups in this molecule undergo pi flips at rates that can be detected by MAS and off-MAS NMR. The shapes of the experimental sidebands are compared with simulated results.

Multinuclear solid-state nuclear-magnetic-resonance (NMR) experiments have been performed on a single-crystal sample of bulk composition Hg0.78Cd0.22Te. The NMR spectra of the three distinct lattice species (Hg, Cd, and Te) have been observed and assigned. Some double-resonance Hg-Te and Hg-Cd experiments clarify the assignment of Te resonance to chemical environment. These experimental results confirm our previous assignments of Te shifts to distinct chemical environments. They further suggest that in melt-grown, homogeneously distributed samples, cation distributions differ little from predictions of a statistical model.

The Floquet theory for the description of magic angle sample spinning (MASS) nuclear magnetic resonance (NMR) experiments is introduced. MASS NMR signals of single spin systems and homonuclear two-spin system in the presence of direct and indirect dipolar couplings are evaluated. The single spin system is utilized to develop our Floquet formalism for the MASS NMR experiments and the coupled spin system is discussed to demonstrate its methodology and its applicability. This theoretical approach enables the calculation of the positions, the amplitudes, and line shapes of MASS center and sideband. The diagonalization of the MASS Floquet matrices of the different spin systems provides eigenvalues and eigenvectors which in turn determine the frequencies and intensities of the various bands in the MASS spectra. Their frequency shifts can be explained in terms of anticrossing of Floquet states and their amplitudes can be calculated via the matrix elements of transition-amplitude Floquet operator. New Floquet operators are introduced and the commutation relations between them utilized to enable the evaluation of diagonalization matrices. The Floquet theory is applied to determine the rotational resonance conditions for homonuclear dipolar-coupled spin pairs. It is shown that significant band shifts are expected when degeneracies between Floquet states occur. These shifts endow the centerband and sidebands with power line shapes from polycrystalline samples. Simulations of experimental line shapes can provide direct and indirect dipolar coupling constants, including their relative signs. This approach is valid for all possible dipolar coupling strengths as well as for all sample spinning speeds and isotropic and anisotropic chemical shift parameters. The decay of zero-quantum coherences in the coupled spin-systems is also considered in terms of the Floquet approach. The sensitivity of the MASS line shapes close to the rotational resonance conditions with respect to the re

Solid-state NMR techniques were employed in order to study the structure and the dynamics of porphine. The changes observed in the line width of the ¹H NMR signal between 173 and 443 K suggest that the porphine macrocycles rotate in the crystals. This was confirmed by recording the ¹³C CPMAS NMR spectra at different temperatures which showed, in addition to the expected coalescence of signals due to the central hydrogens tautomerism, a broadening of the resonances due to the overall molecular rotation. These studies, coupled to measurements at different temperatures and fields of the relaxation times of the ¹H magnetization in the rotating frame, allowed us to obtain activation parameters for the motion which are, within experimental error, equal to those made available by CPMAS NMR for the tautomerism of the central hydrogens. These results suggest an explanation for what seems to be a contradiction between the structure of porphine observed by X-ray, according to which the central hydrogens are localized in opposite pairs of nitrogens, and the structure observed by CPMAS in which the hydrogens migrate between the four central nitrogens. If it is assumed that the migration of the central hydrogens is coupled to a 90° rotation of the molecules, the translational symmetry of the crystal will not be changed by the tautomerism, and an X-ray analysis would always detect a single tautomer.

Deuterium and carbon-13 NMR of specifically labeled benzenehexoyl hexa-n-hexanoate in the various solid-state phases are reported. The spectra exhibit dynamic line shapes which change discontinuously at the phase transitions. The results are interpreted in terms of sequential \u201cmelting\u201d of the side chains on going from the low-temperature solid phases IV, III, etc., toward the liquid. In phase IV the molecules are very nearly static, except for fast rotation of the methyl groups about their C₃axes. The results in phase III were quantitatively interpreted in terms of a two-site isomerization process involving simultaneous rotation by 95° about C!-C₂and transition from gtg to g'g't (or equivalently g^ftg^fto ggt) for the rest of the chain. The specific rate of this reaction at 0 °C is ~ 10⁵s^{-12 3 4 5 6 7 8 910}. In phase II additional chain isomerization processes set-in which were, however, not analyzed quantitatively. Further motional modes, involving reorientation of whole chains about their C^ar-0 bonds, appear on going to phase I. In all solid phases the benzene ring remains static.