Violation of the weak noncrossing rule between totally symmetric closed-shell states in the valence-isoelectronic series O3, S3, SO2, and S2O

The ground and next excited 1A1 potential energy surfaces of the valence-isoelectronic molecules O3, S3, SO2, and S2O have been studied using full-valence-space-multiconfiguration self-consistent-field calculations. An open and a ring minimum were found for all species. In each case, interconversion between the C2v open and closed isomers proceeds via a C2v transition state which lies on a very sharp ridge separating the basins surrounding the minima. In the immediate vicinity of these transition states, the upper state for each species is found to have a minimum, also in C2v. Furthermore, not far away on the C2v ridges, the 1 1A1 and 2 1A1 surfaces intersect conically for all molecules.