Photochemical Ring Opening of Oxirane Modeled by Constrained Density Functional Theory

A constrained density functional theory/classical trajectory surface hopping study of the photochemical dissociation of oxirane (CH₂)₂O is presented. The calculations confirm the Gomer-Noyes mechanism for the initial reaction and agree largely with experimental photolysis data including reaction yields. The calculated yields, however, depend both on temperature and its modeling. The timescales of the various reaction steps are well below 100 fs, similar to previous time-dependent density functional calculations. At variance with those, however, the present calculations obey Kasha's rule, i.e., the photoreaction is initiated in the energetically lowest excited state.