Density functional studies of nitrosyl halides, thiazyl halides and related molecules

Density functional calculations at the B3LYP/6-31G* level have been perform ed on a series of nitrosyl (Z-N=O) and thiazyl (Z-S=N) compounds where Z is F, Cl, Br and H, in order to understand the electronic properties and natu re of bonding in these molecules. The energies at the optimized geometries, together with their less stable isomers were calculated and reported. All of the structural isomers studied were found to be minima on the potential energy surface. The energy differences between the isomers are 26.3-43.6 kc al/mol in the nitrosyl series and 13.7-26.2 kcal/mol in the thiazyl series. Based on the frontier molecular orbital (MO) theory and the second order p erturbation theory, MO diagrams were constructed to account for the major o rbital interactions which chiefly contribute to the stabilization of the mo lecules. The qualitative approach has shown to be successful in providing i nsights into the connectivities of the atoms, the bond orders and the relat ive stabilities of the isomers. Simple correlations between the electronega tivity of Z and the magnitude of the energy difference between the two isom ers have been established. (C) 2000 Elsevier Science B.V. All rights reserv ed.