Effects of diffusion on the self‐termination kinetics of isopropylol radicals in solution

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Abstract
Rate constants for the bimolecular self‐reaction of isopropylol radicals [(CH3)2ĊOH] in various solvents are determined as functions of temperature by kinetic electron spin resonance. For hydrocarbon solvents they are well described by theoretical equations for reactions controlled by translational diffusion if diffusion coefficients of 2‐propanol, a constant reaction distance, and a spin statistical factor of 1/4 are applied. Deviations from 2kt ∼ D at high diffusion constants agree with trends expected from recent theoretical models. For hydrogen‐bonding solvents large negative deviations are observed. They are attributed to steric constraints and slower rotational diffusion of radical–solvent aggregates. The disproportionation‐to‐combination ratio of isopropylol increases with solvent viscosity. As previously for tert‐butyl, this is explained by anisotropic reorientation during encounters. Further, rate data are given for the decarbonylation of the 2‐hydroxy‐2‐methylpropanoyl radical and for several hydrogen abstraction reactions of isopropylol.