Experimental and theoretical investigation into the structural and thermodynamic properties of the mixed methyl halide association radical cations

An experimental and theoretical study of the ion/molecule association reactions of methyl halide radical cations with mixed methyl halides (X, Y = I, Br, Cl, F) has been carried out. MS/MS unimolecular and collision-induced dissociation experiments were performed on the association products and provide strong evidence for formation of a two-center three-electron (2c-3e) bonded structure, [CH3X therefore YCH3](.+), for all association products excluding [C2H6FCl](.+). The [C2H6FCl](.+) results suggest the following atomic connectivity for this association product: [CH2Cl-H-FCH3](.+). Two metastable fragmentation pathways were observed for [CH3I therefore BrCH3](.+) and [CH3Br therefore ClCH3](.+): direct cleavage of the 2c-3e bond and elimination of a methyl group. Only direct cleavage was observed in [CH3I therefore ClCH3](.+), [CH3I therefore FCH3](.+), and [CH3Br therefore FCH3](.+). Kinetic energy release distributions were measured and unimolecular kinetic modeling studies were performed on the metastable reaction pathways. Unimolecular kinetic modeling was carried out using phase space calculations and molecular parameters from density functional theory (DFT) calculations. DFT was also used to investigate the potential energy surfaces for each system. (C) 1999 Elsevier Science B.V.