Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. III. Evidence for a kinetic isotope effect in the competitive process of atomic magnesium insertion into a C–H bond

Abstract

The formation of the C–H bond insertion product methylmagnesium hydride (CH3MgH) and the simultaneous emission of atomic triplet magnesium are observed following photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in solid methane matrices at 12 K. Isotopic variation of the solid methane hosts produces observable differences in the relative branching ratios into the photophysical (atomic triplet emission) and photochemical (insertion product formation) channels. In solid perdeuteromethane (CD4), the intensity of the atomic emission is approximately five times that in solid methane (CH4) while the rate of formation of the insertion product shows the opposite behavior in the two solids. No singlet atomic magnesium emission is observed in the solid Mg/methane systems and the rise time of the atomic triplet emission is deduced to occur on a time scale of less than 10 ns. A simple model derived from spin and orbital correlations between reactants and products is presented which considers the effect of the low symmetry of an insertive reaction coordinate in the approach geometry of atomic magnesium to methane yielding the linear product CH3MgH. Using this model, the absence of the singlet atomic emission is explained in terms of the attractive nature of the singlet surface with respect to the formation of a bent, inserted intermediate. The observation of an enhanced rate of atomic magnesium intersystem crossing is thought to occur as a result of the symmetry-induced participation of the repulsive triplet surface in the process leading to the linear insertion product. The model also suggests an origin for the observed kinetic isotope effects. Differences in the observed behavior of the 1P state of atomic magnesium in gas-phase and solid-phase quenching experiments (explicitly the formation of fragmented products only with no unreacted atomic triplet in the former case and the formation of the insertion product with intense atomic triplet emission in the latter) are discussed in relation to the presence of efficient relaxation pathways in the solid phase and the absence of such pathways in the single-collision conditions of the gas-phase experiments.