Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52074 Aachen, Germany.
Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), CNRS UMR 7583, Université Paris-Est Créteil, Université de Paris, Institut Pierre Simon Laplace, 61 avenue du Général de Gaulle, F-94010 Créteil, France.
J Chem Phys. 2019 Sep 14;151(10):104310. doi: 10.1063/1.5116304.
Low barriers to internal rotations are especially challenging for both the experimental and theoretical determinations because they result in large tunneling splittings which are hard to assign and in potential functions that can be difficult to model. In the present work, the internal rotations of two methyl groups of 2,4-dimethylanisole were analyzed and modeled using a newly developed computer code, called ntop, adapted for fitting the high-resolution torsion-rotation spectra of molecules with two or more methyl rotors. The spectrum was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range of 2.0-26.5 GHz, revealing internal rotation tunneling quintets with splittings of up to several gigahertz. The V potential barriers are 441.139(23) cm and 47.649(30) cm for the o- and p-methyl groups, respectively. Quantum chemical calculations predicted only one conformer with the methoxy group in the anti position related to the neighboring o-methyl group. While the results from geometry optimizations were reliable, ab initio calculations at the MP2 level did not reproduce the low torsional barriers, calling for further experiments on related systems and additional theoretical models.
内部旋转的低势垒对实验和理论确定都特别具有挑战性，因为它们导致了难以分配的大隧道分裂和难以建模的势能。在本工作中，使用一种新开发的计算机代码 ntop 分析和模拟了 2,4-二甲基苯甲醚的两个甲基基团的内部旋转，该代码适用于拟合具有两个或更多甲基转子的分子的高分辨率扭转-旋转光谱。该光谱是使用工作在 2.0-26.5 GHz 频率范围内的脉冲分子喷射傅里叶变换微波光谱仪测量的，揭示了内部旋转隧道五重态分裂高达数十千兆赫。o-和 p-甲基的 V 势垒分别为 441.139(23)cm 和 47.649(30)cm。量子化学计算仅预测了一种构象，其中甲氧基与相邻的 o-甲基处于反式位置。虽然几何优化的结果是可靠的，但 MP2 水平的从头算计算没有再现低扭转势垒，需要对相关系统进行进一步的实验和额外的理论模型。
