A QUANTUM ELECTRONIC THEORY OF CHEMICAL PROCESSES - THE INVERTED ENERGY PROFILE CASE - CH3-2 REACTION(+H)

A quantum approach to chemical processes is developed. The chemical in terconversion is described as an electronic process. The reaction corr esponds to histories involving quantum states belonging to different s tationary molecular Hamiltonians. The system may be embedded in a weak (thermal) and/or external electromagnetic field. The electromagnetic transverse fields lead to transition moments yielding finite probabili ty amplitudes for the system to change from one quantum state to anoth er. Bottleneck subspaces (transition states) are defined; they mediate the interconversions in generic unimolecular and bimolecular processe s. Active precursor and successor complexes are introduced to help bri dge reactant and product electronic states. The stationary states are modeled with Born-Oppenheimer Hamiltonians. At a qualitative level, th e theory is general. The rate, measured as a time derivative of produc t concentration, is expressed in terms of concentrations of active pre cursor and successor complexes. The kinetic coefficients are given in terms of quantum processes involving electronic bottleneck states. Sta tionary structures and vibrational zero-point energies characterizing the reactive CH3++H-2 system are determined at a Hartree-Fock level of theory with 6-31++G* basis set. The vibrational levels are corrected with anharmonicity effects. The saddle point of index one for hydroge n scrambling reactions has been obtained and shown to be related to th e CH5+ molecular complex together with the precursor and successor com plexes geometries. The unusual properties of the system with respect t o standard transition-state theory are fairly well described within th is approach, in particular, isotope scrambling as well as photon emiss ion during formation of the carbocation. The theory suggests that thes e types of reactions, which are found in outer space, may contribute t o the scattering of the cosmic microwave background. (C) 1997 John Wil ey & Sons, Inc.