Abstract
The electronic structure of isothiocyanic acid, HNCS, was determined by an ab initio LCAO-MO-SCF calculation. Isocyanic acid, HNCO, was also calculated and the two molecules are compared. The basis set consisted of uncontracted, atom-optimized Gaussian functions, with nine s plus five p exponents to describe the sulfur and five s plus two p apiece for the carbon, nitrogen, and oxygen. The hydrogen was represented by three s-type functions. An additional calculation of HNCS was carried out by adding a d set to the description of the sulfur. The molecular orbitals of HNCS are analyzed using simple perturbation theory applied to the localized valence-bond functions and the SCF orbitals are displayed using planar electron-density plots. The electronic structure of HNCS is compared with that of HNCO and it is concluded that (1) the π system in HNCS involves a nitrogen lone pair stabilized by a higher-lying C–S π bond while the π system of HNCO consists of a C–O π bond stabilized by the higher-energy nitrogen lone pair and (2) the d orbitals of sulfur are accepting density in a σ rather than a π fashion.
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