Translational diffusion of a carbene and radicals derived from carbenes

Abstract

Translational diffusion constants (D) of a persistent carbene (hexachlorodiphenylcarbene) and, simultaneously, that of the precursor compound (2,2′,4,4′,6,6′-hexachlorodiphenyldiazomethane) are determined, after the laser photolysis of the diazo-compound in benzene, by using the transient grating (TG) method. D of the carbene is found to be anomalously (ca. five times) smaller than that of the parent molecule. At the same time, the TG signals of some short-lived carbenes (diphenylcarbene, fluorenylidene, α-(and β-)naphthylphenylcarbene) are investigated in various solvents (cyclohexane, methanol and acetonitrile). In methanol, monotonic decays of the population grating signals are observed and, from the rate constants, Ds of the precursors, diazo-compounds, are determined. In cyclohexane, rise and decay signals are observed as the TG signal and these components are interpreted in terms of the diffusion of the diazo compounds and radicals created from the carbenes by hydrogen abstraction from the solvents. Ds of the radicals are generally three–four times smaller than those of the parent molecules. The smaller Ds of the transient radicals indicate a larger friction of the radicals during the diffusion process and the origin of the friction with the matrix is attributed to an attractive intermolecular interaction between the radicals and the surrounding molecules. From these results, it is concluded that the intermolecular interaction between the carbene and the matrix is stronger than that between the radicals and the matrix. This could be further evidence for the important role of the unpaired electron in diffusion processes in solution.