UV Absorption Spectroscopy of the Conformer-Dependent Reactivity of the Four Carbon Criegee Intermediate of Methyl Vinyl Ketone Oxide: An Ab initio Quantum Dynamics Study.

An extended theoretical analysis of the photodissociation dynamics of the four-carbon Criegee intermediate (CH 2 ═CH(CH 3 )COO) or methyl vinyl ketone oxide, which has four conformers, following excitation to the B state, is presented. Our analysis relies on multireference electronic wave functions combined with a wavepacket propagation treatment for the two coupled B 1 A' and C 1 A' electronic states and two nuclear degrees of freedom. For each conformer, the 2D model depends on potential energy surfaces (PESs) along the O-O and C-O-O bending modes for the two lowest excited states, B 1 A' and C 1 A', and is sufficiently accurate to reproduce the experimental B 1 A' ← X 1 A' absorption spectrum with unprecedented accuracy. It is found that the roles of each conformer are essential in producing a cumulative spectrum, which is close to the recent experimental spectrum. The anti-trans and anti-cis conformers make contributions at the longer and shorter wavelengths of the cumulative spectrum, respectively, while the syn-cis and syn-trans conformers have contributions in the middle wavelength range of the cumulative spectrum of MVK-oxide. The existence of a deep well for each conformer on the PESs of the (diabatic) B state causes a considerable amount of the wavepacket to be reflected by the B state wells, which can explain the oscillatory structures appearing in the long wavelength range of 360-480 nm of the spectrum. The weakly avoided crossings between the B-state and C-state PESs of each conformer appearing within the range of 2.80-3.08 eV excitation energy cause considerable disturbance in the vibronic fine structure of the bands. The results give novel insight into the complex interactions governing this intriguing process.