Beckmann Peter A, McGhie Andrew R, Rheingold Arnold L, Sloan Gilbert J, Szewczyk Steven T
Department of Physics, Bryn Mawr College , 101 North Merion Avenue, Bryn Mawr, Pennsylvania 19010-2899, United States.
Department of Chemistry and Biochemistry, University of California, San Diago , 5128 Urey Hall, 9500 Gilman Drive, La Jolla, California 92093-0358, United States.
J Phys Chem A. 2017 Aug 24;121(33):6220-6230. doi: 10.1021/acs.jpca.7b06265. Epub 2017 Aug 9.
Using solid-state H nuclear magnetic resonance (NMR) spin-lattice relaxation experiments, we have investigated the effects of several solid-solid phase transitions on tert-butyl and methyl group rotation in solid 1,3,5-tri-tert-butylbenzene. The goal is to relate the dynamics of the tert-butyl groups and their constituent methyl groups to properties of the solid determined using single-crystal X-ray diffraction and differential scanning calorimetry (DSC). On cooling, the DSC experiments see a first-order, solid-solid phase transition at either 268 or 155 K (but not both) depending on thermal history. The 155 K transition (on cooling) is identified by single-crystal X-ray diffraction to be one from a monoclinic phase (above 155 K), where the tert-butyl groups are disordered (that is, with a rotational 6-fold intermolecular potential dominating), to a triclinic phase (below 155 K), where the tert-butyl groups are ordered (that is, with a rotational 3-fold intermolecular potential dominating). This transition shows very different DSC scans when both a 4.7 mg polycrystalline sample and a 19 mg powder sample are used. The H spin-lattice relaxation experiments with a much larger 0.7 g sample are very complicated and, depending on thermal history, can show hysteresis effects over many hours and over very large temperature ranges. In the high-temperature monoclinic phase, the tert-butyl groups rotate with NMR activation energies (closely related to rotational barriers) in the 17-23 kJ mol range, and the constituent methyl groups rotate with NMR activation energies in the 7-12 kJ mol range. In the low-temperature triclinic phase, the rotations of the tert-butyl groups and their methyl groups in the aromatic plane are quenched (on the NMR time scale). The two out-of-plane methyl groups in the tert-butyl groups are rotating with activation energies in the 5-11 kJ mol range.
通过固态氢核磁共振（NMR）自旋晶格弛豫实验，我们研究了几种固-固相变对固态1,3,5-三叔丁基苯中叔丁基和甲基旋转的影响。目的是将叔丁基及其组成甲基的动力学与使用单晶X射线衍射和差示扫描量热法（DSC）测定的固体性质联系起来。冷却时，DSC实验根据热历史在268 K或155 K（但不是两者都有）处观察到一级固-固相变。通过单晶X射线衍射确定，155 K转变（冷却时）是从单斜相（高于155 K）转变而来，其中叔丁基是无序的（即，以6重旋转分子间势为主导），到三斜相（低于155 K），其中叔丁基是有序的（即，以3重旋转分子间势为主导）。当使用4.7 mg多晶样品和19 mg粉末样品时，这种转变显示出非常不同的DSC扫描结果。使用大得多的0.7 g样品进行的氢自旋晶格弛豫实验非常复杂，并且根据热历史，在许多小时和非常大的温度范围内可能会出现滞后效应。在高温单斜相中，叔丁基以17-23 kJ/mol范围内的NMR活化能（与旋转势垒密切相关）旋转，其组成甲基以7-12 kJ/mol范围内的NMR活化能旋转。在低温三斜相中，叔丁基及其甲基在芳香平面内的旋转被淬灭（在NMR时间尺度上）。叔丁基中的两个面外甲基以5-11 kJ/mol范围内的活化能旋转。
