Bhattacharyya Kalishankar, Surendran Ammu, Chowdhury Chandra, Datta Ayan
Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur - 700032, Kolkata, West Bengal, India.
Department of Chemistry, Cochin University of Science and Technology (CUSAT), Cochin - 682 022, Kerala, India.
Phys Chem Chem Phys. 2016 Nov 16;18(45):31160-31167. doi: 10.1039/c6cp05237a.
The structures of molecules form the cornerstone of our chemical knowledge. Lowering of symmetry in closed-shell molecules is often attributed to the Pseudo Jahn-Teller (PJT) distortions wherein non-adiabatic coupling (NAC) between the ground state and excited states creates vibrational instability along specific normal modes. Nevertheless, other factors like steric interactions are also well known in the literature to induce structural distortions. In this article, we consider two specific cases of molecular distortions - the first one being spontaneous for contorted polyaromatic hydrocarbons (c-PAH) where non-bonded repulsions between the two pairs of syn H-atoms in tribenzopyrene, TBP (1), can enforce either a C → C or C → C distortion. PJT-effects account for the correct preference of the C structure over C (by 4.6 kcal mol). The second case (non-spontaneous symmetry breaking) is that of benzene (2) and coronene (3) which upon application of sufficiently strong static external electric field develop vibrational instability along q(a) to cause D → C and D → C distortions for 2 and 3 respectively. An external electric field (F) was applied parallel to the aromatic ring of 2-3 for investigation of non-spontaneous symmetry breaking. Such electric field induced structural distortion is understood on the basis of excess charge accumulation of the planar rings which is circumvented by symmetry lowering. PJT effects seem to have significant consequences for identification of global minima amongst several local minimal molecular structures.
分子结构构成了我们化学知识的基石。闭壳层分子对称性的降低通常归因于赝 Jahn - Teller（PJT）畸变，其中基态和激发态之间的非绝热耦合（NAC）会沿着特定的简正模式产生振动不稳定性。然而，文献中也熟知其他因素，如空间相互作用，也会引发结构畸变。在本文中，我们考虑分子畸变的两种特定情况——第一种情况是扭曲的多环芳烃（c - PAH）的自发畸变，在三苯并芘（TBP，1）中两对顺式 H 原子之间的非键排斥作用可以导致 C→C 或 C→C 畸变。PJT 效应解释了 C 结构相对于 C 结构的正确偏好（相差 4.6 kcal/mol）。第二种情况（非自发对称性破缺）是苯（2）和 coronene（3）的情况，在施加足够强的静态外部电场时，它们会沿着 q(a)方向产生振动不稳定性，分别导致 2 和 3 发生 D→C 和 D→C 畸变。为了研究非自发对称性破缺，将外部电场（F）平行于 2 - 3 的芳香环施加。这种电场诱导的结构畸变是基于平面环的过量电荷积累来理解的，而这种积累通过对称性降低得以避免。PJT 效应似乎对在几个局部最小分子结构中识别全局最小值具有重要影响。
