The effects of substituent substitution on the locus of a seam of conical intersection and the importance of conical intersections in the associated low symmetry subspaces are considered. For molecules with more than three atoms and with some symmetry the seam of conical intersection may well include an accidental symmetry-allowed portion involving two states of different symmetry. However, in regions of reduced point group symmetry, conical intersections involving two states of the same symmetry may exist. This later class of conical intersections is rarely considered although it could significantly alter the predicted outcome of a nonadiabatic process. The efficient quenching of OH(A 2Σ+)by H2, a consequence of OH–H2 conical intersections, is particularly compelling in this regard. Previous analyses have considered only the C2v2A12B2 accidental symmetry-allowed portion of the seam of conical intersection. It is demonstrated that when intersections of states of the same symmetry are considered conical intersections with Cs symmetry are found that are likely to play an important role in the quenching process.

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