AB INITIO MO CALCULATIONS ON 1,2-DITHIETES AND VALENCE ISOMERS

摘要

The valence isomerization of the 1,2-dithiete parent compound to the open-chain dithial was studied by CASSCF multiconfiguration methods including the CASPT2 perturbational treatment. The isomerization energy remains small at the highest level of theory. In agreement with Jonas and Frenking, the cyclic structure is only then preferred over the acyclic ones if f-functions on the sulfur atoms are considered. Lf they are included, the 1,2-dithiete is more stable by 3.8 kcal/mol and the barrier amounts to 24.9 kcal/mol at the CASPT2(8, 8)/6-31G(2df)//MF2/6-31G* level of theory. According to MP2/6-31G* geometry optimizations, substitution of H by NH2 and CH3 reduces the stability of the 1,2-dithiete ring structures relative to the open-chain dithiocarbonyl structures, whereas the inverse holds for acceptor substitution by CN and CF3. A higher stability of benzodithiete relative to ortho-dithiobenzoquinone is predicted at all employed levels of theory. This is in good agreement with conclusions drawn from experimental results. Whereas the experimental microwave geometry of 1,2-dithiete is well reproduced theoretically, the experimental electron diffraction geometry of 3,4-bis-(trifluoromethyl)1,2-dithiete differs from the calculated one. (C) 1996 John Wiley & Sons, Inc. [References: 44]