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

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.