A quantitative scale of the biradical charater (0 = � = 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 < � < 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 binding in these molecules can be described by a mixture of 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. The analysis of the MRCI wavefunctions provides a simple measure of the relative mixture of the two bonding scenarios, yielding a biradical character of 3.5% for OSO, 4.4% for SSO, 11% for S3, 18% for O3, 26% for SOO, and 35% for SOS. 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.

Analysis of bonding patterns in molecular systems exhibiting partial biradical character

247th National Meeting (2014)

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