The kinetics of the bimolecular sel‐reaction of t‐butyl radicals in solution. I. Termination rates

Abstract

SummaryTermination rate constants 2Kt for the self-reaction of t-butyl radicals in twelve different solvents are determined as a function of temperature by kinetic electron spin resonance. In the entire range measured, 7 . 107<2k,<2 . 10'O dm3 mol-' s-I, they are well described by the von Smoluchowski equation for diffusion controlled reactions, using a spin statistical factor, a temperature and solvent independent, isotropic reaction diameter, and diffusion coefficients for t-butyl estimated following empirical or semiempirical prescriptions. Methods for the prediction of diffusion coefficients are critically discussed, and rules for the estimation of termination rate constants of steric unhindered alkyl radicals are suggested.

Cyclization of the 4-cyanobutyl radical

“…The measured rate constant for decay, 2k,4 = 5.9 x lo9 M -I s-l, indicates that this reaction is diffusion controlled, just as we would expect for this sterically unhindered ketiminyl (12). The variation of 2k,4 with temperature was computed from the temperature coefficient of viscosity of the solvent (21). Table 1.…”

A homopropargyl radical rearrangement. Kinetics of the rearrangement of the 2,2,5,5,-tetramethyl-3-hexyn-1-yl radical

“…gave 2k,(t-Bu') = 3.0 x loy M-' s-I, (lit. (17), 2k,(t-Bu') = 2.9 x loy M-I s-I), for the same conditions. This excellent agreement serves to confirm that the light was uniformly distributed across the sample in our epr cavity.…”

Time-Resolved CIDNP Investigation of the Cross-Relaxation Mechanism of 1H Nuclear Polarization

“…12 In both solvents the size of the polarization is that expected from the geminate polarization and the product distribution, with some losses from pure nuclear relaxation. The kinetics could also be simulated with rate constants that agreed with those measured by kinetic EPR experiments 42 and without the inclusion of cross-relaxation. A similar value of T 1 N ∼ 100 µs was required for both solvents.…”