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

The tricyclic carbon-sulfide, C₆S₈ molecule containing two S-atoms in the 1,4-position of the central six-membered ring and one disulfide (S-S) and one thione (C=S) 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 2.4 kcal/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 (C_2h → C₂) is traced to vibronic mixing between the ground state (Ag) and the low lying excited states of A_u symmetry through the a_u normal mode, a (1A_g + 1A_u + 2A_u + 3A_u) × a_u 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 (F_0i) 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 a_u 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.