Structures and energies of C4S4q (q = +1, 0, −1) isomers. A theoretical study

Quantum chemical ab intio calculations at the MP2/6-311++G(2d2f)//MP2/6-311G(df) level have been carried out for the neutral and charged cyclobutane-1,2,3,4-tetrathione C4S4q species 3aq (q = −1, 0, +1) and for the tricyclic bis-1,2-dithiete isomers 3bq. The equilibrium geometries of 3aq possess a square-planar D4h symmetry where the CC bonds of neutral 3a become longer in 3a+ and shorter in 3a− while the CS distances become shorter in the cation 3a+ and longer in the anion 3a−. The bis-1,2-dithiete isomers 3bq are significantly higher in energy than the tetrathione forms 3aq. Neutral 3b which is 67.9 kcal/mol less stable than 3a possesses a trans-ladder arrangement of the four-membered rings with two short (1.390 Å) and two longer (1.515 Å) CC bonds. The cation 3b+ which is 57.4 kcal/mol less stable than 3a+ has a distorted boat-shaped arrangement of the four-membered rings where the CS bond lengths of the 1,2-dithete rings are clearly different. The anion 3b− which is 108.3 kcal/mol higher in energy than 3a− has a trans-ladder type structure like the neutral parent system 3b.