Abstract

A quantitative scale of the biradical charater (0 ≤ β ≤ 1) of a molecular system based on a Multi-Reference Configuration Interaction (MRCI) wavefunction is introduced and used to analyze its underlying electronic structure and bonding pattern. Triatomic ions in the FX2 + series, where X = O, S, Se, Te and Po are the terminal atoms, were found to exhibit unusually high biradical characters (0.76 < β < 0.92), the largest among the homologous, 18 valence electron molecules CX22 -, NX2 −, X3 and OX2 (X = O, S, Se, Te and Po). The concept of biradical character was further used to investigate the bonding mechanism in ozone (O3) and its sulfur-substituted analogues, SO2 , OS2, and S3. We demonstrate that the bonding in these molecules can be described by a mixture between a closed shell structure with one and a half bond between the central and terminal atoms and an open-shell structure with a single bond and two lone electrons on each terminal atom. Our analysis further offers an explanation for the different O-O, S-O and S-S bond lengths and singlet-triplet splittings of these species, the stabilization of OSO and SSO over the SOO and SOS isomers as well as the (X-YZ) relative binding energies (X=S, O), all based on their different biradical character. These concepts are finally used to analyze the biradical character of the Criegee Intermediate (CI) and address the apparent contradiction between its small biradical character and its rich reactivity with regards to atmospheric particle formation.

E. Miliordos, K. Ruedenberg and S. S. Xantheas, Ang. Chem. Int. Ed. 52, 5736 (2013) E. Miliordos and S. S. Xantheas, J. Amer. Chem. Soc. 136, 2808 (2014) E. Miliordos and S. S. Xantheas, Ang. Chem. Int. Ed. 55, 1015 (2016)