Determination of reactivities of primary, secondary, and tertiary C-H bonds in saturated hydrocarbons from S2-xanthione fluorescence quenching

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 (tau(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 kH values show significant differences and are kH = 1.4 x 10(10) s(-1), 5 x 10(10) s(-1) and 20 x 10(10) s(-1), for the primary, secondary, and tertiary bonds. The values of kH 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.