DFT- and post-HF-study on structure and electronic excitation of acyclic and cyclic sulfur diimides

Abstract

A series of representative organic acyclic and cyclic sulfur diimides were studied by Hartree–Fock-plus-correlation ab initio quantum chemistry and by density functional theory using Becke's three-parameter functional along with the LYP functional. The widely used formula representation –NSN– for these compounds suggests octet expansion of sulfur. This is not confirmed by theory. Although d-polarization functions significantly improve the numerical results, sulfur d-orbitals are hardly occupied. The calculated electronic charge distribution derived by population analysis and by the atoms-in-molecules topological theory favors charge separation resulting in a more or less ylidic structure with –NS+–N−– and –N−=S+–N– resonance contributors. This structure does not exclude relatively short SN bond lengths. The characteristics of the bonds in the parent structures is preserved in some non-Kekulé-type NSN heterocyles. Strong SN bond charge separations of organyl sulfur diimides are accompanied by short SN bond distances and narrow S0/T1 and S0/S1 energy gaps. The experimentally well known naphtho[1,8-cd][1,2,6]thiadiazine and the unknown 3,4-dimethylene-1,2,5-thiadiazole belong to this series. The calculations confirm that 1,2,3-thiadiazole is the diaza analog of thiophene rather than the vinylene-bridged sulfur diimide, while more complex heterocycles such as benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole take an intermediate position between classical and non-classical structures. Two closely related minimum structures are defined on the DFT Born–Oppenheimer energy surface of naphtho[1,8-cd;4,5-c′d′]bis[1,2,6]thiadiazine, but so far only one compound is experimentally known. The lowest energy structure is the quinoid form corresponding to the experimentally known compound. © 1997 John Wiley & Sons, Ltd.