Is the S₂N₂ ring a singlet diradical? Critical analysis of alternative valence bond descriptions



Penotti, Fabio E, Cooper, David L ORCID: 0000-0003-0639-0794 and Karadakov, Peter B
(2019) Is the S₂N₂ ring a singlet diradical? Critical analysis of alternative valence bond descriptions. International Journal of Quantum Chemistry, 119 (7).

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Abstract

Rival valence bond (VB) descriptions are investigated for the π-electron system of the S₂N₂ ring and of another sulfur-nitrogen ring, S₄N₄²⁺, near equilibrium geometry. The lowest-energy compact spin-coupled generalized VB (SCGVB) description is provided in each case by the variational optimization of two configurations that are found to be symmetry related to one another. Optimization instead of symmetry-pure single-configuration SCGVB wavefunctions leads to the involvement of three-center SNS or NSN orbitals, which seems to be an unnecessary complication. In neither case is very much achieved from the mixing of competing solutions. Breathing orbital VB (BOVB) calculations for S₂N₂ confirm a structure with NN singlet diradical character to be more important than one with SS singlet diradical character, but the largest contribution (ca. 60%) turns out to be due to the symmetry-determined linear combination of four symmetry-equivalent structures that lack any obvious diradical character. Much the same pattern was consistently found when we used a simple but robust projection of our various SCGVB wavefunctions for S₂N₂ onto the basis of BOVB structures (plus an orthogonal complement).

Item Type: Article
Depositing User: Symplectic Admin
Date Deposited: 03 Mar 2020 16:04
Last Modified: 18 Jan 2023 23:59
DOI: 10.1002/qua.25845
URI: https://livrepository.liverpool.ac.uk/id/eprint/3077096