PNNL

Abstract

We investigate the bonding mechanism in ozone (O3) and its sulfur-substituted analogues, SO2, OS2, and S3. By analyzing their ground state Multi Reference Configuration Interaction (MRCI) wavefunctions, we demonstrate that the bonding in these systems can be represented as 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 (biradical). The biradical character (ß) further emerges as a simple measure of the relative contribution of those two bonding scenarios emanating from the interpretation of the respective wavefunctions. Our analysis yields 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. The size of the end atoms relative to the central one is the prevalent factor for determining the magnitude of ß: larger end atoms better accommodate the localization of electrons therefore increasing the weight of the second picture in the bonding scenario (larger ß). The adopted mixture of bonding scenarios allows for the definition of the bond order of the covalent bonds being (3-ß)/2 and this accounts for the different O–O, S–S or S–O bond lengths in the triatomic series. The biradical character was furthermore found to be an effective means of explaining several structural and energetics trends in the series: larger values of ß mark a smaller singlet-triplet splitting, closer bond lengths in the ground 1A? and the first excited 3A? states and larger atomization and bond dissociation energies in the ground state.