The quenching of isopropyl group rotation in van der Waals molecular solids

Abstract

X-ray diffraction experiments are employed to determine the molecular and crystal structure of 3-isopropylchrysene. Based on this structure, electronic structure calculations are employed to calculate methyl group and isopropyl group rotational barriers in a central molecule of a ten-molecule cluster. The two slightly inequivalent methyl group barriers are found to be 12 and 15 kJ mol −1 and the isopropyl group barrier is found to be about 240 kJ mol −1 , meaning that isopropyl group rotation is completely quenched in the solid state. For comparison, electronic structure calculations are also performed in the isolated molecule, determining both the structure and the rotational barriers, which are determined to be 15 kJ mol −1 for both the isopropyl group and the two equivalent methyl groups. These calculations are compared with, and are consistent with, previously published NMR 1 H spin-lattice relaxation experiments where it was found that the barrier for methyl group rotation was 11Ϯ 1 kJ mol −1 and that the barrier for isopropyl group rotation was infinite on the solid state NMR time scale.

A proton spin-lattice relaxation rate study of methyl and t-butyl group reorientation in the solid state

“…1) is E DCc = 12.9 ± 1.3 kJ mol -1 , which is in the range expected for methyl groups in a t-butyl group where the intra-tbutyl group interactions are dominant [3,[5][6][7]17]. Indeed, this activation energy is approximately the same as that for the methyl groups in an isopropyl group (which are out of the plane of an aromatic ring) [18,19] or the methyl group in an ethyl group (which is also out of the plane of an aromatic ring) [15,17,19].…”

Nonexponential 1H spin–lattice relaxation and methyl group rotation in molecular solids

“…Although we have no X-ray data for 4, we assume that Z' = 1 which is consistent with the relaxation rate data (this work and [45]). Ab initio electronic structure calculations in clusters of molecules based on the X-ray diffraction structure show, in agreement with the NMR relaxation experiments, that methoxy group rotation over a barrier in 2 [71] and in 3 [80], and isopropyl group rotation over a barrier in 5 [81] is quenched by intermolecular interactions in the solid state as a consequence of the rotational asymmetry of these groups. Librations (rotational vibrations) of these groups (which are very fast on the NMR time scale) over a small angle [71,80,81] play no role in the nuclear spin-lattice relaxation process other than adding a time dependence to the already present spatial dependence of methyl group rotation axes.…”

“…The two methyl groups in 5, though not identical (as a consequence of intermolecular interactions), have environments so similar that an NMR relaxation experiment would never detect the difference [81]. Z' = 1 in 6 so here all t-butyl groups are equivalent.…”

“…[71], 3 [80], 5 [81], and 6 [74]). In 1 and 2, there is one methyl group per molecule and Z' = 1, meaning that all molecules and therefore all methyl groups have the same environment (that is, they are equivalent and only one correlation time τ is needed).…”

1H and 19F spin-lattice relaxation and CH3 or CF3 reorientation in molecular solids containing both H and F atoms

“…For rotationally asymmetric groups like methoxy, ethyl, and isopropyl groups whose reorientational barriers are very small in many isolated molecules, 2,5,8,10 these reorientational barriers, due entirely to intermolecular interactions in the solid state, are so high that reorientation is completely quenched. 2,5,8,10 We are very careful to call the parameter determined in the NMR relaxation experiments the NMR activation energy E NMR and not the barrier V. The latter for the case of CH 3 and CF 3 groups has been computed for several systems similar to 1-3 shown in Fig. 1, both in the isolated molecules and for molecules in the solid state.…”