Photodissociation of the vinoxy radical through conical, and avoided, intersections

The mechanism for predissociation of the vinoxy radical after the excitation $X̃ 2A″→B̃ 2A″$ is studied theoretically. Critical points on the $Ã 2A′$ and $B̃ 2A″$ potential energy surfaces are determined at the multireference configuration interaction level using expansions of five million configuration state functions. A mechanism consisting of a sequence of internal conversions to the ground state is proposed. $B̃ 2A″–Ã 2A′$ conical intersections are shown to be too high in energy to participate in this process. The $B̃ 2A″–Ã 2A′$ radiationless transition is enabled by an avoided crossing which is accessible after a barrier of ca. 2000 cm⁻¹, in agreement with experimental observations. Subsequently the $Ã 2A′→X̃ 2A″$ radiationless transition can occur very efficiently through easily accessible conical intersections. A one-dimensional coupled adiabatic state model of the process is developed. The radiationless decay rates of the vibrational levels obtained from this model serve to validate the proposed mechanism.