Thionitroso or thiazyl? Density functional studies of relative stabilities between the two structural isomers

Density functional calculations (B3LYP/6-31G*) have been performed in order to determine the energies of a series of thionitroso molecules (R-N=S), and their structural isomers, the thiazyl molecules (R-S dropN). Our computational results show that, in general, the thionitroso form is thermodynamically more stable than its thiazyl counterpart, except for compounds where the bonding atom in R is highly electron-withdrawing. However, in all cases, both forms are stationary points on the potential energy surface, regardless of the higher energies of the less stable isomers. A simple correlation between the electronegativity of the bonding atom in R, and the size of the relative stability between the two isomers is established, and presented. Frontier molecular orbital theory is used to successfully account for this correlation as well as predict the preferred connectivity. (C) 2001 Elsevier Science B.V. All rights reserved.