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Theory of damped quantum rotation in nuclear magnetic resonance spectra. II. Numerical simulations for the benzene rotor.

In Part I of this series of papers, the damped quantum rotation (DQR) theory, formulated originally for hindered threefold molecular rotors in solids, was generalized to the N-fold case. The stochastic dynamics of such objects, evidenced in NMR line shapes, was shown to be more complicated than in the standard model of classical jumps between the wells of the N-fold torsional potential. Actually, it comprises certain quantum rate (i.e., coherence-damping) processes subject to the requirements of the Pauli principle. The jump picture is recovered only when the quantum rates fit specific patterns. In this work, one of the ways of approaching such a classical limit is identified for the benzene rotor. This is inferred from a quantum mechanical model whose validity was earlier confirmed for a methyl group. Based on that model, theoretical calculations for the benzene ring dynamics in a clathrate crystal, 1-(9-anthryloxy)anthraquinone/benzene-d6, confronted with the pertinent literature data, point to possible deviations from the classical limit. However, the predicted DQR effects are too small to be observed in solid echo 2H NMR spectra of the C6D6 isotopomer. The chances of detecting the effects are improved when Carr-Purcell echo 1H spectra of a single crystal of the isotopomer including C6H6 as a guest are considered. The substantial differences in the sensitivity to the DQR effects of the spectra of protonated and deuterated benzene are concerned with different magnitudes of the intramolecular dipolar spin couplings. The dynamic isotope effect (C6D6 vs C6H6), which is small in this case, is only of secondary importance. Legitimacy of the use of the jump model in 2H NMR line shape studies of benzene-d6 is fully confirmed by the present considerations. However, the physical significance of the dynamic parameters extracted from such studies is shown from a new perspective.