Theoretical Study of the Photodecomposition of Methyl Hg Complexes

Abstract

Methyl mercury, an important pollutant, decomposes photolytically when exposed to sunlight. Methyl mercury chloride has been shown to yield CH3 and HgCl radicals upon photodecomposition under UV irradiation. We have calculated spectral transition energies for a number of methylmercury species using quantum mechanical methods, specifically the Hartree−Fock method, the Moller−Plessett second order perturbation theory method (MP2), and the configuration interaction singles method using polarized double-ζ relativistic effective core potential basis sets. We find that singlet to triplet absorptions occur at lower energy than that of singlet to singlet absorptions, by about 2−3 eV. The calculated singlet to triplet (S−T) energy is much lower for 1-coordinate CH3Hg+ than for the 2-coordinate species CH3HgL, where L=CH3-, OH2, OH-, Cl-, or SH-. Of the 2-coordinate species studied, CH3HgOH2+ and CH3HgSH show the lowest energy S−T transitions, with calculated maxima just below 5.0 eV (at the MP2 level). They should...