Integrated Calendar

London dispersion (LD) interactions, the attractive part of the van-der-Waals interaction1,2 hold somewhat of a unique position in the chemical world. Although their role in influencing macroscopic phenomena (such as the higher boiling points of larger alkanes) is well recognized, they are usually overlooked when discussing molecular phenomena. Substituents in reactions are generally considered as a source of “steric hindrance” and not as “steric attractors”, better termed dispersion energy donors (DEDs). As such, their influence on reaction outcomes was quantified and presented by classic steric factors such as the A-value. We have shown, using computational quantum mechanical tools, that these well recognized steric factors have also an attractive LD component that balance part of the steric repulsion. By focusing on the LD component we can explain various non-intuitive trends between substituents, such as the inconsistency between the size of the halogens and their A-values.3 In addition, a systematic analysis of both the steric and dispersion interactions of the same molecules allows us to quantify the relative weights of the two effects and form a new DED scale.4 Such corrected steric and LD factors could later be applied to explore the role of LD interactions also in other reactions. Our computations show that LD interactions have a significant influence on the overall relative stabilities and energetics in cyclohexane chair conformers, and also in related concerted reactions, and must not be ignored in reaction design.
  
Bibliography
(1) Eisenschitz, R.; London, F. Z. Phys. 1930, 60, 491–527.
(2) London, F. Trans. Faraday Soc. 1937, 33, 8–26.
(3) Solel, E.; Ruth, M.; Schreiner, P. R. London Dispersion Helps Refine Steric A-Values: The Halogens. J. Org. Chem. 2021, 86 (11), 7701–7713.
(4) Solel, E.; Ruth, M.; Schreiner, P. R. London Dispersion Helps Refine Steric A‑Values: Dispersion Energy Donor Scales. J. Am. Chem. Soc. 2021, Accepted.

Zoom: https://weizmann.zoom.us/j/97229318195?pwd=d0lyRVJUTkFWcGpoVTdpd2lzVDAvQT09
Passcode: 598488


Organic radicals and paramagnetic transition metal complexes can serve as molecular electron spin qubits for potential applications in quantum information science and technology. A long electron spin decoherence time is an important prerequisite for these applications, as well as for many electron paramagnetic resonance (EPR) experiments. We present an experimental and computational investigation into the decoherence mechanism of nitroxides and other organic radicals in protonated and deuterated matrices. The experiments reveal that the decoherence time depends critically on the bulk proton concentration and on the degree of clustering of the protons. Quantum dynamics simulations of the electron spin and several hundred surrounding hydrogen nuclei (protons and deuterons) quantitatively reproduce the measured coherence decays, showing that electron spin decoherence is driven by many small clusters of magnetic nuclei that interact among themselves and with the electron spin. These insights provide design rules to develop systems with longer decoherence times.