AB-INITIO CALCULATIONS ON 1,2-DITHIINS AND VALENCE ISOMERS

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 la 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 la 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 la amount to about 54 degrees and 28 degrees, respectively. Whereas the parent compound 1a is more stable by at least 15 kcal 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-1,2-dithiin (2a). In view of these results the synthesis of 3,6-diamino-1,2-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.