Dipole Moment Surfaces of the CH4+•X → CH3•+ HX (X = F, Cl) Reactions from Atomic Dipole Moment Surfaces, and the Origins of the Sharp Extrema of the Dipole Moments near the Transition States

Abstract

The partitioning of the dipole moment of an isolated molecule or that of a reacting system is reviewed and applied to a dynamic reacting system whereby the system's dipole moment surface is constructed in parallel to its potential energy surface. The dipole moment surface is then decomposed into two origin-independent surfaces: (1) an atomic polarization (AP) surface and a charge transfer (CT) surface. The dipole moment surface as well as its two composing AP and CT surfaces are all further broken down into atomic and/or group contributions with the aid of the quantum theory of atoms in molecules (QTAIM). This approach is applied to the title's laser-induced chemical reactions [CH4 + (•)X → CH3(•) + HX (X = F, Cl)] previously studied by Bandrauk et al. [ J. Chem. Phys. 2004 , 121 , 7764 - 7775 ], and which were found to exhibit marked peaks in the dipole moment and in the polarizability tensor component at (or near) the transition state. These peaks afford a means to control the kinetics of these reactions with the proper adjustment of an external laser field intensity and phase. The entrance channel potentials of these reactions have recently been probed by photodetachment spectroscopy by Bowman and collaborators [ J. Chem. Phys. 2011 , 134 , 191102_1 - 4 ]. The understanding of the origin of the peaks in the dipole moment can provide, eventually, an additional layer of control in the design of reactions tunable by external fields through the proper selection of the reactants to maximize the field-molecule interaction.