An ab initio molecular dynamics study of S ketene fragmentation

Abstract

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.

Theoretical and Experimental Investigation of the Dynamics of the Production of CO from the CH3 + O and CD3 + O Reactions

“…The implementation of a direct dynamics procedure requires the choice of (i) an underlying ab initio methodology, (ii) a procedure for generating initial conditions, (iii) an algorithm for propagating the phase space variables in time, and (iv) a set of conditions for ending the propagations. The particular algorithm employed here is based on that employed in a recent study of the dissociation of CH 2 CO …”

CASPT2 Study of Photodissociation Pathways of Ketene

“…In the near-ultraviolet absorption region (300–360 nm), the mechanism has been understood well both experimentally and theoretically. − ,,− ,− H 2 CCO is excited to the first excited singlet state (S 1 ), then undergoes a nonadiabatic transition to the lower electronic state, i.e., the ground singlet state (S 0 ) or the ground triplet state (T 1 ), and finally dissociates to CH 2 and CO with the CC bond fission on either S 0 or T 1 , which correspond to pathway 2 and pathway 1, respectively.…”

“…The photodissociation dynamics of ketene (H 2 CCO) has been studied extensively both experimentally − and theoretically − in the past several decades. Experimentally, the following five channels have been studied so far. left rightauto fittrue100% H 2 CCO + h v → CH 2 false( X̃ B 1 3 false) + CO (1) CH 2 false( ã A 1 1 false) + CO (2) CH 2 false( b̃ B 1 1 false) + CO (3) HCCO false( X̃ normalA ″ 2 false) <...…”

Insight into Selected Reactions in Low-Temperature Dimethyl Ether Combustion from Born−Oppenheimer Molecular Dynamics

“…HPMF Decomposition. According to our conventional transition state theory results, the HPMF decomposition path with the lowest barrier to reaction (HPMF to CH 2 OO‚‚‚HC(d O)OH) has a lower bound to the HPMF decomposition time at 600 K of 0.2 ms. 30 75 Figure 9 illustrates similarities in the concerted reactions involving HPMF via the b-TS2, b-TS5, and b-TS5′ transition states. From the damped dynamics calculations done in previous work, 5 we see a lengthening of the C-O bond followed by H-transfer through b-TS2, b-TS5, and along the path leading to CH 2 OO‚‚‚HC(dO)OH via b-TS5′.…”