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

The Doppler profiles of the nascent CN (X(2)Sigma(+)) fragments from the ph otodissociation of 2-butenenitrile, CH3CH=CHCN, at 193 nm have been measure d using transient frequency-modulation absorption spectroscopy. State-depen dent translational energy distributions were determined for selected CN sta tes and can be modeled assuming a statistical partitioning of the available energy. The translational energy distributions suggest that the allyl radi cal rather than the 1-propenyl isomer is formed. No significant fragment nu .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 optimi zations and QCISD(T) single point energy calculations were performed for th e relevant product species and transition states associated with isomerizat ion and CN elimination. The calculations indicate that H-atom migration occ urs prior to dissociation. The elimination of CN is a minor channel with a quantum yield of 0.01 +/- 0.01 at 193 nm. (C) 2000 Published by Elsevier Sc ience B.V. All rights reserved.