Generalized Valence Bond Description of Chalcogen–Nitrogen Compounds. I. NS, F(NS), and H(NS)

2015-03-05T00:00:00Z (GMT) by Tyler Y. Takeshita Thom H. Dunning
The electronic structures of the ground states (X<sup>2</sup>Π) of NS and those (X<sup>1</sup>A′) of F­(NS) and H­(NS), where X­(NS) collectively refers to the XNS and NSX isomers, were analyzed within the framework of generalized valence bond theory. The ground state of NS has a recoupled pair π bond, which has a profound effect on its reactivity. For example, the lowest-energy isomer of F­(NS) is NSF, which has a recoupled pair bond dyad with N–SF and NS–F bonds lengths and strengths similar to their covalent counterparts in NS and SF. The ground state of NSH, on the other hand, is only weakly bound with a NS–H bond energy 40.20 kcal/mol smaller than that in SH and a N–SH bond energy 40.20 kcal/mol less than that in NS. At its equilibrium geometry, the NSH molecule is best viewed as derived from the N­(<sup>2</sup>D) + SH­(X<sup>2</sup>Π) separated fragments, with the weak NS–H bond resulting from unfavorable interactions between the SH bond pair and the nitrogen lone pair. Addition of F/H atoms to the nitrogen atom in NS disrupts the NS recoupled pair bond, which weakens both the FN–S/HN–S and F–NS/H–NS bonds. In contrast to the formation of recoupled pair σ bonds, formation of the recoupled pair π bond in NS is expressed as a change in the spin-coupling coefficients, rather than an interchange of the orbitals.