jp981312f_si_001.pdf (240.38 kB)
Theoretical Studies on the CH3CO + Cl Reaction: Hydrogen Abstraction versus CO Displacement
journal contribution
posted on 1998-09-25, 00:00 authored by Raman Sumathi, Minh Tho NguyenThe geometries, energies, and vibrational frequencies of the reactants, transition structures, intermediates,
and products of the reaction of the acetyl radical with atomic chlorine have been determined by ab initio
molecular orbital theory at the second-order Møller Plesset perturbation (MP2) level. Energies have been
recalculated at the quadratic configuration interaction QCISD(T) level by using geometries obtained at MP2
level. The energy of the initial acetyl chloride adduct CH3COCl (1), formed by barrier-free combination,
lies 78 kcal/mol below the reactants. Two major reaction routes are open to the chemically activated adduct
1: molecular dissociation to H2CCO + HCl (3), and the secondary formation of ketene via 1-chlorovinyl
alcohol (2). Both these processes are energetically feasible to the thermal reactants and should hence lead to
a spontaneous emission of a vibrationally hot HCl molecule as observed by Maricq et al. (Int. J. Chem.
Kinet. 1997, 29, 421). The thermodynamically most stable products, CH3Cl + CO, should preferably be
formed via direct displacement of CO from CH3CO by Cl; this reaction proceeds via a loose complex between
Clδ- and CH3COδ+, which explains the delayed emission of CO in the diode laser study of the Cl + CH3CO
reaction. The energy barrier for decarbonylation of the adduct 1 is quite high and thereby is not accessible
to the thermal reactants. The present potential energy surface reveals this reaction to be a capture-limited
association−elimination reaction with a very high and pressure-independent rate coefficient.