Dynamic Effects on the Ionization-Induced Reactions of Methyl Halides: Quantum Mechanical and Molecular Dynamics Studies.

Abstract

The mechanisms of the ionization-induced fragmentation and H migration of methyl halides CH3X (X = F, Cl, Br) have been examined by quantum mechanical and molecular dynamics methods. When CH3X (X = F, Cl, Br) is vertically ionized into a divalent cation, it can obtain enough excess energy to overcome the energy barrier of subsequent reaction channels for the formation of H+, H2+, and H3+ species and intramolecular H migration. The product distributions of these species greatly depend on the halogen atoms. The H+ formation decreases in the order of F > Cl > Br, which is inversely proportional to the increase in the magnitude of the energy barrier in the order of Br > Cl > F. This is attributable to the change in the charge distribution of the entire molecule by the halogen atoms. Meanwhile, the small H migration ratio for Cl and Br, despite their low energy barriers, was explained by the small sum of states at the transition state based on the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The H3+ formation ratio is unexpectedly smaller despite its low energy barrier. This is attributed to the dynamic effects of the H2 roaming that always occur prior to the reaction in question. Molecular dynamics simulations showed that the H2 roaming was restricted in a certain area due to an initially produced driving force on the hydrogen atoms in a certain direction by vertical ionization; this phenomenon suppresses the formation of H3+, which requires the hydrogen atoms to be in motion over a relatively wide range to enter the transition state region. Thus, the low proportion of the observed H3+ can be explained by the dynamical probability of the transition state structure formation.