A THEORETICAL-STUDY OF MERCURY PHOTOSENSITIZED REACTIONS

Some reactions of fundamental importance in mercury photosensitization have been studied by high level quantum chemical calculations. The re actions of Hg (P-3(1)) With H-2, CH4, C3H8, and SiH4 go by initial fo rmation of exciplexes, followed by insertion of Hg into the reactive X-H bond ultimately giving X radicals and H atoms. Intermediates and t ransition states have been located, notably the bent triplet (3)[H-Hg- H] species. The hydrogen and silane reactions are found to proceed wit hout barriers, whereas the methane and propane reactions require activ ation energies. In the case of methane the barrier is so high, 12.8 kc al/mol, that the reaction is very inefficient while for propane the ba rrier is small, 4.9 kcal/mol, leading to the breaking of one of the ce ntral C-H bonds. Triplet sensitization of alkenes and the bonding of H g with lone-pair ligands are also studied. Very good agreement with t he general picture obtained from experiments is obtained. The consider ation of both spin-orbit and zero-point vibrational effects are very i mportant for a qualitative understanding of the reactions. Similaritie s and differences to the corresponding transition metal reactions are discussed.