PNNL

Abstract

Transient absorption spectroscopy (TAS) was used to measure the rate of formation and the rate of self-termination of the main group metal hydride, tri-n-butyltin hydride (Bu3SnH). Irradiation of di-t-butyl peroxide in the presence of Bu3SnH generates the tri-n-butylstannyl radical (Bu3Sn•) by a rapid hydrogen atom abstraction pathway. Analysis of the growth of the Bu3Sn• at 400 nm, corrected for radical termination, yields (kH-abs = 3.5 ± 0.3 x 108 M-1s-1). The extinction coefficient of the Bu3Sn• (e = 1620 ± 40 cm-1 M-1 at 400 nm), measured for the first time in this work, is 3.5 times greater than the literature estimate. The experimental value for e? (Bu3Sn•) ? permits the quantitative measurement of the rate of Bu3Sn• self-termination, (2kt = 3.6 ± 0.3 x 109 M-1 s-1) in benzene at 296 K. This experimental value is greater than previous literature estimates but less than predicted by the Smoluchowski equation (2kts = 4.4 ± 0.2 x 109 M-1 s-1) using our measured diffusion coefficient for tin hydride (D = 1.34 ± 0.08 x 10-5 cm2 s-1 at 296 K in benzene). This work shows that the rates of hydrogen abstraction from Bu3SnH by t-butoxyl radical and self-termination of Bu3Sn• are both more rapid than previously reported. A mechanistic kinetic model was developed with this new kinetic information to fit the time dependent TAS signal of Bu3Sn•. This approach provides the optimum reaction conditions, both low hydride and halide concentrations, to measure the rate of bromine atom abstraction from 2-phenethyl bromide (kBr-abs = 3.7 ± 0.3 x 107 M-1s-1).