Zachariou Andrea, Hawkins Alexander P, Collier Paul, Howe Russell F, Lennon David, Parker Stewart F
School of Chemistry, University of Glasgow, Joseph Black Building, Glasgow G12 8QQ, U.K.
UK Catalysis Hub, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, Chilton, Oxon OX11 0FA, U.K.
ACS Omega. 2020 Feb 7;5(6):2755-2765. doi: 10.1021/acsomega.9b03351. eCollection 2020 Feb 18.
How the methyl torsion transition energy in unsaturated systems is affected by its environment is investigated. It is strongly influenced by both its immediate neighborhood, (the number of methyl groups present in the molecule) and the intermolecular interactions. It is clear that the intermolecular interactions have a major influence on the torsion transition energy, as demonstrated unambiguously previously for mesitylene and also seen here for other systems. In part, this may be caused by the fact that the methyl torsion is rarely a pure mode (unless enforced by symmetry). Where the crystal structure is available, the assignments have been supported by CASTEP calculations of the unit cell. The agreement between the observed and calculated spectra is generally good, although not perfect, toluene being a case in point, and highlights just how demanding it is to obtain accurate transition energies for low energy modes. The disagreement between observed and calculated inelastic neutron scattering spectra for -xylene and 9,10 dimethylanthracene is so severe that it would suggest that there are additional phases to those presently known. Comparison between the full periodic calculations and those for the isolated molecule shows that intermolecular interactions raise the methyl torsion transition energy by at least 8% and in some cases by more than 50%. The presence of more than one methyl group in the molecule generally raises the average torsion energy from the <100 cm seen for single methyl groups to 150-200 cm.
研究了不饱和体系中甲基扭转跃迁能如何受其环境影响。它受到紧邻环境（分子中甲基的数量）和分子间相互作用的强烈影响。很明显，分子间相互作用对扭转跃迁能有重大影响，正如之前对均三甲苯明确证明的那样，此处对其他体系也有体现。部分原因可能是甲基扭转很少是一种纯模式（除非由对称性强制）。在有晶体结构的情况下，归属得到了晶胞的CASTEP计算的支持。观察到的光谱与计算光谱之间的一致性总体良好，尽管并不完美，甲苯就是一个例子，这突出了获得低能模式精确跃迁能的难度。对二甲苯和9,10 - 二甲基蒽的观察到的和计算的非弹性中子散射光谱之间的差异非常严重，这表明存在目前已知之外的其他相。完整周期计算与孤立分子计算的比较表明，分子间相互作用使甲基扭转跃迁能至少提高了8%，在某些情况下提高了50%以上。分子中存在多个甲基通常会使平均扭转能从单个甲基的<100 cm升高到150 - 200 cm。
