Methyl and t-butyl group reorientation in planar aromatic solids: Low-frequency nuclear magnetic resonance relaxometry and x-ray diffraction

Abstract

We have synthesized 3-t-butylchrysene and measured the Larmor frequency /2 ͑ϭ 8.50, 22.5, and 53.0 MHz͒ and temperature T ͑110-310 K͒ dependence of the proton spin-lattice relaxation rate R in the polycrystalline solid ͓low-frequency solid state nuclear magnetic resonance ͑NMR͒ relaxometry͔. We have also determined the molecular and crystal structure in a single crystal of 3-t-butylchrysene using x-ray diffraction, which indicates the presence of a unique t-butyl group environment. The spin-1/2 protons relax as a result of the spin-spin dipolar interactions being modulated by the superimposed reorientation of the t-butyl groups and their constituent methyl groups. The reorientation is successfully modeled by the simplest motion; that of random hopping describable by Poisson statistics. The x-ray data indicate near mirror-plane symmetry that places one methyl group nearly in the aromatic plane and the other two almost equally above and below the plane. The NMR relaxometry data indicate that the nearly in-plane methyl group and the entire t-butyl group reorient with a barrier of 24.2 Ϯ0.9 kJ mol Ϫ1 , and the two out-of-plane methyl groups reorient with a barrier of 14.2Ϯ0.6 kJ mol Ϫ1 . Following a brief review of methyl group rotation in simple ethyl-, and isopropyl-substituted one-and two-ring aromatic van der Waals molecular solids, the barriers for the out-of-plane methyl groups and the t-butyl group in 3-t-butylchrysene are compared with those barriers in three related molecular solids whose crystal structure is known: 4-methyl-2,6-di-t-butylphenol, 1,4-di-t-butylbenzene, and polymorph A of 2,6-di-t-butylnaphthalene. A trend is observed in the reorientational barriers for the t-butyl and the out-of-plane methyl groups across this series of four compounds: as the t-butyl barriers decrease, the out-of-plane methyl barriers increase.

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].…”

“…The previous studies either had no additional "lone" methyl groups [3,5,6] or the relaxation rate from the rotation of the lone methyl groups was observed in the temperature range studied [4].…”

“…The relaxation rate R is given by [4] a sum of several terms of the form R = E i tend to be determined with reasonable accuracy by the experiment (± 10%). They should fall within well-defined norms for the kinds of motions being studied here [3,5,7,8]. Once the E i are determined, a somewhat overly simplistic, but nevertheless helpful model [3,9,10] For motions like those studied here, the spectral density should characterize random uncorrelated hopping ("reorientation") since there is no experimental evidence that correlated motions play any role (the relaxation is strictly exponential) [11] nor is there any structural evidence that there would be any dynamical bottlenecks [12].…”

“…The molecular structure in this class of compounds (as determined by X-ray diffraction) shows the t-butyl groups oriented such that one methyl group is in the aromatic plane, or nearly so, and two methyl groups are out of the plane [3]. See Fig.…”

“…Models developed for the reorientation of "isolated" methyl groups [2] have been expanded to include the reorientation of t-butyl groups and their constituent methyl groups on planar aromatic molecules [3]. The molecular structure in this class of compounds (as determined by X-ray diffraction) shows the t-butyl groups oriented such that one methyl group is in the aromatic plane, or nearly so, and two methyl groups are out of the plane [3].…”