Ab initio calculations on 1,2-dithiins and valence isomers

Abstract

Ab initio molecular orbital calculations were performed at the RHF and MP2 level with split valence basis sets. As recently shown for 1,2-dithietes, inclusion of f-functions has a noticeable effect on the isomerization energies of 1,2-dithiins to dithiones. 1,2-Dithiins constitute a series of nonplanar heterocyclic compounds with pronounced bond fixation. The parent structure 1a has C 2-symmetry and the energy barrier of inversion via the fully conjugated planar structure is rather low (5.7 and 8.3 kcal mol −1 at the RHF/6-31G ∗ and at the MP2/6-31G ∗ level, respectively). The MP2/6-31G ∗ geometry of 1a is in line with the structural features revealed by X-ray analysis with some derivatives. According to the theoretical and experimental data, the CSSC and CCCC dihedral angles of 1a amount to about 54 ° and 28 °, respectively. Whereas the parent compound 1a is more stable by at least 15kcal mol −1 with respect to the open chain dithiones 1b-1d at the MP2 level (with basis sets including d- and f-functions), some substituted dithiins are energetically not favored compared to the respective isomeric dithiones. Thus dithiomaleinic and dithiofumaric amids ( 2d-2f) are predicted to be more stable than 3,6-diamino-l,2-dithiin ( 2a). In view of these results the synthesis of 3,6-diamino-l,2-dithiins reported in the literature should be considered with cation. Substitution of the 3,6-positions by acceptor groups such as nitrile, however, favors the ring structure ( 5a) over open chain structures. According to isodesmic reactions (“methyl stabilization energies”) cyano and amino substitution of the dithiin ring results in a stabilization of the ring by 9 and 32.5 kcal mol −1, respectively. Because of the strong stabilization effect by the amino groups in the acyclic dithione, the bisthioamides are energetically favored.