ESR studies of barriers to ring inversion in cyclic monocarboxylic acid radicals

Abstract

Activation energies for ring inversion have been estimated for a number of cyclic monocarboxylic acid radicals, of structure (CH2)n≳ĊCOOH (I) with n=3,4, and 5, using electron spin resonance (ESR) methods. These radicals were produced from the corresponding cyclic monocarboxylic acids, by γ irradiation (at 195 K) and subsequent annealing. The ESR spectra of these radicals vary with temperature. The variations are attributed to ring inversion of the cyclic group in these radicals. Computer simulations of the temperature dependent ESR spectra were employed using the solutions of modified Bloch equations for a two-jump process. From these simulations the rate of ring inversion (τ) as well as the ESR parameters were estimated as a function of temperature (T). Arrhenius parameters found for the ring inversion in cyclohexane carboxylic acid radical (i.e., I with n=5) are Ea=4.0±0.3 kcal/mol and ln(1/τ0) =29.0±0.7 (τ0 in sec). The radical species with five-membered ring (n=4) was studied in a number of matrices: (i) sodium cyclopentane carboxylate, (ii) 12M LiCl(D2O), (iii) 8M NaOD, (iv) cycloleucine and (v) cyclopentane carboxylic acid. In the last two matrices, the five-membered ring was found to have twisted configuration yielding four nonequivalent β-proton splittings. In the other matrices a symmetrical configuration was found revealing two pairs of equivalent splittings. The barrier (Ea) to ring inversion for this radical (n=4) varies only slightly (from 1 to 1.5 Kcal/mole) in different matrices.