The Origin of the Color of 1,2-Dithiins - Interpretation by Kohn-Sham Orbitals

Abstract

According to calculations by time-dependent density-functional theory (TD-DFT) the wine-red color of the half-chair (C2) 1,2-dithiin (1) is mainly due to a one-electron transition between the frontier orbitals. These orbitals are characterized by local two-center CC π bonds. The symmetric HOMO of the twisted structure 1 correlates with the fully delocalized HOMO of the planar reference structure with C2v symmetry. The anti-symmetrical LUMO correlates both with the essentially localized σ* (SS) of the LUMO and the π* of the LUMO+1 of the planar compound. Thus the dominating one-electron transition of 1 has some π→σ* character in the terminology of the planar system with a strong admixture of the anti-bonding combination of the CC bond orbitals to the σ*-type LUMO. The color of 1,2-dithiins are unique because of the simultaneous presence of a non-coplanar C=C bond system and the SS bond. For the sake of comparison calculations were also performed at ab initio levels of theory (TD-RHF, SCI, EOM-CCSD and CASPT2). Although the absorption wavelengths are strongly underestimated by SCI and TD-RHF the order and the nature of the lowest-excited states are the same as those found by TD-DFT. The electron excitation of 1,2-dithiin is compared with that of 1,2-dithiane (3). In addition, the structure and the energetics of 1,2-dithiin are compared with 1,4-dithiin (4) and thiepin (5).