Transition state vibrational level thresholds for the dissociation of triplet ketene

Abstract

Rate constants for the unimolecular dissociation of ketene (CH2CO) and deuterated ketene (CD2CO) have been measured at the threshold for the production of CH2 (X̃ 3B1) or CD2 (X̃ 3B1) and CO (X̃ 1Σ+) by photofragmentation in a cold jet. The rate constant increases in a stepwise manner as energy increases. This is in accord with the long-standing premise that the rate of a unimolecular reaction is controlled by flux through quantized transition-state thresholds at each energy level for vibrational motion orthogonal to the reaction coordinate. The first step in rate constant and/or photofragment excitation (PHOFEX) spectrum gives accurate values for the barrier to dissociation above the zero-point energy of the products, 1281±15 cm−1 for CH2CO and 1071±40 cm−1 for CD2CO. The measured rate constants are fit by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The vibrational frequencies at the transition state obtained from the fits are compared with ab initio results. Vibrational motions at the transition state orthogonal to the reaction coordinate are also revealed in CO product rotational distributions. Calculations using an impulsive model which includes vibrational motions at the transition state reproduce the experimental dependence of the PHOFEX spectra on the CO J state quite well. The small dependence of rate constant on jet temperature (4–30 K) indicates that the Ka quantum number for rotation about its symmetry axis is conserved in the energized ketene molecule.