ELECTRON-SPIN RESONANCE STUDIES OF BARRIERS TO HINDERED ROTATION IN ACETIC-ACID, ACETAMIDE, AND PEPTIDE RADICALS

Activation energies for methyl group rotation in the radicals of type H3C-Ċ<, produced from acetic acid, acetamide, and peptides, have been found by using ESR techniques. Reactions of electrons at 77 K with acetic acid and acetamide in 12 M LiCl (D2O) as well as in 8 M NaOD glasses have produced the acetic acid anion, the acetate dianion, and the acetamide anion. ESR spectra of these have revealed a doublet (ca. 32 G) at 90 K reversibly interconverting to a 1:3:3:1 quartet, of ca. 15-G hyperfine splitting, at higher temperatures (170 K). This interconversion has been attributed to the hindered internal rotation of the methyl group about the H3C-Ċ< bond. The ESR spectra are analyzed using modified Bloch equations for the three-jump process. The mean lifetime (τ) at each temperature (T) has been estimated by a comparison of experimental and simulated ESR spectra. The activation energy (Ea) for the sixfold barrier to the rotation is found to be 5.0 ± 0.5 kcal/mol in the acetate dianion and is about 3.0 kcal/mol for the acetate and acetamide as well as N-acetylamino acids. ESR spectra characteristic of the tunneling methyl group were observed at low temperature (20 K < T < 100 K) for the radicals produced in γ-irradiated polycrystalline samples of glycyl-L-alanine and L-alanyl-L-alanine. © 1979 American Chemical Society.