An ab initio molecular dynamics study of S-0 ketene fragmentation

The dynamical origins of product state distributions in the unimolecular di ssociation of S-0 ketene, CH2CO ((X) over tilde (1)A(1))--> CH2((a) over ti lde (1)A(1))+CO, are studied with ab initio molecular dynamics. We focus on rotational distributions associated with ground vibrational state fragment s. Trajectories are integrated between an inner, variational transition sta te (TS) and separated fragments in both the dissociative and associative di rections. The average rotational energy in both CO and CH2 fragments decrea ses during the motion from the TS to separated fragments. However, the CO d istribution remains slightly hotter than phase space theory (PST) predictio ns, whereas that for CH2 ends up significantly colder than PST, in good agr eement with experiment. Our calculations do not, however, reproduce the exp erimentally 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. ( C) 2001 American Institute of Physics.