Structures and energies of C₄S₄^q (q = +1, 0, -1) isomers. A theoretical study This paper is dedicated to the memory of Detlef Schröder.

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 C₄S₄^q species 3a^q (q = -1, 0, +1) and for the tricyclic bis-1,2-dithiete isomers 3b^q. The equilibrium geometries of 3a^q possess a square-planar D_4h 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 3b^q are significantly higher in energy than the tetrathione forms 3a^q. 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.