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 CH 3 X (X = F, Cl, Br) have been examined by quantum mechanical and molecular dynamics methods. When CH 3 X (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 + , H 2 + , and H 3 + 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 H 3 + formation ratio is unexpectedly smaller despite its low energy barrier. This is attributed to the dynamic effects of the H 2 roaming that always occur prior to the reaction in question. Molecular dynamics simulations showed that the H 2 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 H 3 + , 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 H 3 + can be explained by the dynamical probability of the transition state structure formation.