α-Hydroxyalkyl radicals by photoreduction in aprotic solvents: ESR spectra, AB initio potential function, inversion—internal rotation dynamics and temperature dependence of coupling constants of 1-hydroxyethyl

Abstract

A study of the formation of α-hydroxyalkyl radicals by photoreduction of formaldehyde, acetaldehyde, acetone and cyclobutanone in aprotic solvents (methylcyclohexane, isopentane,n-octane) by means of ESR spectra is reported. ESR spectra of 1-hydroxyethyl (CH3CHOH) and 1-hydroxy-1-methylethyl (CH3COHCH3) resolved to second order are observed, whereas from the other two carbonyl compounds only unstructured spectra are observed. Extended ab initio CI calculations were carried out for 1-hydroxyethyl within the framework of a molecular model featuring finite degrees of freedom for inversion at the α-carbon center and for internal rotation of the OH group, providing a two dimensional electronic potential function and values for the isotropic coupling constants for the hydroxylic, the α and the methyl protons. The ESR spectra of CH3CHOH show a characteristic temperature dependence of the coupling constants of each of the three types of protons between 150 and 360 K. For interpretation the inversion—internal rotation problem has been treated in two dimensions. The nuclear motion states were then used for calculation of the T dependence of spin densities and coupling constants within the framework of Boltzmann averaging. Theoretical predictions suggest that inversion is essential for interpretation of the observed T dependence of the coupling constants. For the hydroxylic proton the temperature dependence is predicted considerably too low, whereas for the α and CH3 protons it is predicted approximately correct. Possible effects of hydrogen bonding on ESR spectra are briefly discussed.