An ab initio molecular dynamics study of S ketene fragmentation

The dynamical origins of product state distributions in the unimolecular dissociation of S0 ketene, CH2CO (X̃ 1A1)→CH2(ã 1A1)+CO, are studied with ab initio molecular dynamics. We focus on rotational distributions associated with ground vibrational state fragments. Trajectories are integrated between an inner, variational transition state (TS) and separated fragments in both the dissociative and associative directions. The average rotational energy in both CO and CH2 fragments decreases during the motion from the TS to separated fragments. However, the CO distribution remains slightly hotter than phase space theory (PST) predictions, whereas that for CH2 ends up significantly colder than PST, in good agreement with experiment. Our calculations do not, however, reproduce the experimentally observed correlations between CH2 and CO rotational states, in which the simultaneous formation of low rotational levels of each fragment is suppressed relative to PST. A limited search for nonstatistical behavior in the strong interaction region also fails to explain this discrepancy.