Steric and electric field driven distortions in aromatic molecules: spontaneous and non-spontaneous symmetry breaking.
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 C2v→ C2or C2v→ Csdistortion. PJT-effects account for the correct preference of the Csstructure over C2(by 4.6 kcal mol-1). 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(a2u) to cause D6h→ C6vand D6h→ C2distortions for 2 and 3 respectively. An external electric field (FZ) 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.
Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur - 700032, Kolkata, West Bengal, India. spad@iacs.res.in.
Kalishankar
K
Bhattacharyya
Department of Chemistry, Cochin University of Science and Technology (CUSAT), Cochin - 682 022, Kerala, India.
Ammu
A
Surendran
Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur - 700032, Kolkata, West Bengal, India. spad@iacs.res.in.
Chandra
C
Chowdhury
Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur - 700032, Kolkata, West Bengal, India. spad@iacs.res.in.
Ayan
A
Datta