The unimolecular dissociation of 2-butenenitrile: measurements of the CN elimination channel using FM Doppler spectroscopy

Abstract

The Doppler profiles of the nascent CN (X 2Σ +) fragments from the photodissociation of 2-butenenitrile, CH 3CHCHCN, at 193 nm have been measured using transient frequency-modulation absorption spectroscopy. State-dependent translational energy distributions were determined for selected CN states and can be modeled assuming a statistical partitioning of the available energy. The translational energy distributions suggest that the allyl radical rather than the 1-propenyl isomer is formed. No significant fragment v · j correlation was observed for any of the CN states measured. The results are consistent with dissociation on the ground electronic state without an exit barrier. Ab initio calculations employing MP2 gradient geometry optimizations and QCISD(T) single point energy calculations were performed for the relevant product species and transition states associated with isomerization and CN elimination. The calculations indicate that H-atom migration occurs prior to dissociation. The elimination of CN is a minor channel with a quantum yield of 0.01±0.01 at 193 nm.