Determination of Reactivities of Primary, Secondary, and Tertiary C-H Bonds in Saturated Hydrocarbons from S_2-Xanthione Fluorescence Quenching

摘要

A procedure for determination of reactivity of primary, secondary, and tertiary C-H bonds in molecules of saturated hydrocarbons has been proposed. The procedure involves measurements of picosecond lifetimes of xanthione in the S_2 state (τ_(s2)) from the fluorescence decay (by the TCSPC method) in selected hydrocarbons as solvents. On the basis of the lifetimes measured, the rate constants of hydrogen abstraction, (k_H), are determined for hydrocarbons having only primary C-H bonds, primary and secondary C-H bonds, and primary and tertiary C-H bonds. The obtained highly accurate k_H values show significant differences and are k_H = 1.4 * 10~(10) s~(-1), 5 * 10~(10) s~(-1) and 20 * 10~(10) s~(-1), for the primary, secondary, and tertiary bonds. The values of k_H determined by the procedure proposed can be used to assess the relative energy of C-H bonds and to infer about the presence of steric hindrance for the reactive tertiary C-H bonds. They can also be of interest in the studies of primary photochemical processes in hydrocarbon solvents and in the proper choice of solvents for photophysical and photochemical studies.