Is the S2N2 ring a singlet diradical? Critical analysis of alternative valence bond descriptions. Issue 7 (7th December 2018) - Penotti, Fabio E.
Abstract: Rival valence bond (VB) descriptions are investigated for the π ‐electron systems of the sulfur‐nitrogen rings S2 N2 and S4 N4 2+ near equilibrium geometry. The lowest‐energy compact spin‐coupled generalized VB (SCGVB) descriptions are provided by variational optimization of two configurations that are found to be symmetry related to one another. Optimization instead of symmetry‐pure single‐configuration SCGVB wave functions introduces three‐center SNS or NSN orbitals, which seem to be an unnecessary complication. For neither system is much achieved by mixing competing solutions. Breathing orbital VB (BOVB) calculations for S2 N2 confirm NN singlet diradical character to be more important than SS singlet diradical character, but the largest contribution ( ca . 60%) comes from the symmetry‐determined linear combination of four symmetry‐equivalent structures that lack any obvious diradical character. Much the same pattern was consistently found using a simple but robust projection of various SCGVB descriptions for S2 N2 onto the basis of BOVB structures (plus an orthogonal complement). Abstract : The electronic structure of the deceptively simple inorganic ring disulfur dinitride, S2 N2, has been a puzzle to theoretical chemists for many years. A novel approach that utilizes a robust projection of the spin‐coupled generalized valence bond (SCGVB) wave function onto the basis of breathing orbital VB structures helps to reconcile one of the long‐standing arguments in the applications of modern VB theory and to establish a consistent description of the electronic structure of S2 N2 .