Investigation of the Quenching of Excited Mercury Atoms by the Alkanes

Abstract

The quenching of Hg(63P1), Hg(63P0), and Hg(61P1) by the alkanes was investigated by a theoretical model based on the formation of a relatively long-lived (Hg*HR) complex on a potential energy surface, most likely a polar one, which leads to the quenching of Hg* primarily by C–HHg bond rupture. The decomposition of the (Hg*HR) complex via C–HHg bond scission was treated as a unimolecular decomposition. The potential-energy surfaces associated with the reaction coordinate were defined by thermochemical and spectroscopic data, semiempirical calculations and, whenever not critical, by reasonable estimates. The unimolecular decomposition rate constants were calculated making use of the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. This approach correlates the nature and magnitude of the C–H vs C–D isotope effects and the large variation in quenching efficiencies of the various alkanes satisfactorily with the C–HHg bond strength, zero point energy, and structural effects (vibrational degrees of freedom). Consequently, both the relative increase of the quenching cross section and the relative diminution of the effect of C–H bond strength with increasing excitation energy of the mercury atom is accounted for in terms of the relative increase in the active energies of the (Hg*HR) molecule and its activated complex.