The dynamics of the H-atom abstraction reactions between chlorine atoms and the methyl halides

The rotational state distributions of nascent HCl(nu' = 0) products of the reactions between ground spin-orbit state Cl(P-2(3/2)) atoms and C(H)4 and CH3X (X = F, Cl, Br, I) molecules are reported. Reactions were initiated by photolysis of molecular chlorine at 355 nm and the nascent HCl(nu' = 0) products were probed using 2 + 1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. In accord with previous measurements, the HCl(nu' = 0) products for the Cl + CH4 reaction are rotationally very cold. A much greater degree of rotational excitation is observed in the HCl(nu' - 0) products of the reactions involving the methyl halides, which increases such that CH3I < CH3Br < CH3CI < CH3F, correlating with the magnitudes of the dipole moments of the radical fragment co-products. Ab initio calculations were performed at the G2//MP2/6-311G(d,p) level to characterize molecular complexes and transition states on the reaction pathways for all the reactions studied except Cl + CH3I. All proceed over barriers and have weakly bound complexes in the pre- and post-transition state regions of the potential energy surfaces. Comparisons are drawn between the dynamics of these reactions and those of Cl atoms with other organic molecules containing heteroatoms. (C) 2003 Elsevier B.V. All rights reserved.