An AB initio study of amino-sulfur compounds

Abstract

The geometries of amino hydrogen sulfide (thiohydroxylamine), amino hydrogen sulfoxide (sulfinamide ), and hydrogen sulfonamide were optimized ab initio at the spin-restricted Hartree-Fock (RHF) and second-order Møller-Plesset perturbation theory (RMP2) levels with a variety of basis sets (i.e., STO-3G, STO-3G ∗, 3-21G, 3-21G ( ∗)5D, 3-21G ( ∗)6D, 4-31G, 4-31G ∗, 6-31G, 6-31G ∗, 6-31G ∗∗). The microwave geometries and relative stabilities of cis and trans NH 2-SH are reproduced well at the highest levels of theory utilized herein. The performance of each theoretical model was assessed in terms of the root-mean-square (rms) difference in the geometric parameters relative to those derived at the highest level of theory in this study. There are substantial changes in the geometry when d-type polarization functions are added to the basis set or when electron correlation is calculated. Satisfactory geometries may be obtained at the RHF/ STO-3G (for NH 2-SH) or the RHF/STO-3G ∗ (for NH 2-SHO and NH 2-SHO 2) levels. The 3-21G ( ∗) basis set (5D or 6D ) generally performs too poorly to serve as an alternative to the 6-31G ∗ basis set. At either the RHF or RMP2 level, the 4-31G ∗ basis set provided structures which agree very well with those derived from the use of the more extensive 6-31G ∗ and 6-31G ∗*basis sets. The rms differences in only the RMP2 bond angles and dihedral angles monotonically decrease as a function of the number of basis functions if the two 3-21G ( ∗) basis sets are excluded. Consideration of correlation effects with any of the basis sets or the addition of d-type polarization to the 4-31G and 6-31G basis sets (on all heavy atoms) increases the non-planarity of the nitrogen atom, whereas the addition of d-type functions to the sulfur atom in the STO-3G or 3-21G basis set resulted in an increase in planarity of the nitrogen atom.