ISSN:
0894-3230
Keywords:
Chemistry
;
Theoretical, Physical and Computational Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
---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.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
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