Pseudo Jahn–Teller distortion for a tricyclic carbon sulfide (C6S8) and its suppression in S-oxygenated dithiine (C4H4(SO2)2)

Abstract

The tricyclic carbon-sulfide, C6S8 molecule containing two S-atoms in the 1,4-position of the central six-membered ring and one disulfide (SS) and one thione (CS) bond on the five membered rings on its either side (1) possesses a “butterfly flapping” type distorted ground state in the gas-phase and also in β-phase of the crystal. For the isolated molecule, better consideration of the S…S non-bonding interactions in the dithiine ring in the bent form at the M06-2X/6-31+G(d,p) level leads to a significant barrier for inversion of 2.4kcal/mol which is 2–3 times more than that previously obtained by Weber and Dolg at the B3LYP/cc-pVTZ level due to underestimation of dispersion interactions at the B3LYP level. The origin of the distortion leading to lowering of symmetry for 1 (C2h→C2) is traced to vibronic mixing between the ground state (Ag) and the low lying excited states of Au symmetry through the au normal mode, a (1Ag+1Au+2Au+3Au)×au pseudo Jahn–Teller effect (PJTE) problem. Based on fitting of the ground state APES to the lowest root of the 4×4 secular determinant, we calculate the linear vibronic coupling constants (F0i) between the relevant states. Similar in class to 1, the S-oxygenated derivative of dithiine, C4H4(SO2)2 (2) unlike most other dithiines, remains planar. The absence of the butterfly-type puckered structure in 2 is traced to the enhanced gap (Δ0) and very small vibronic coupling (F01) between the ground and PJT active state along the au distortion which suppress the PJT instability. Considering the separation of the occupied molecular orbital (OMO) and unoccupied molecular orbitals (UMO) energy levels provide a qualitative understanding for the contrasting behavior of 1 and 2. In effect, the PJT effect is shown to be a fruitful and general tool to describe the presence or lack of molecular distortion.