Electron Spin Resonance Studies of γ-Irradiated Ferroelectric Methylammonium Alum Crystals

Abstract

⁶⁰Co γ irradiation of methylammonium alum crystals at room temperature produces the ·CH₂NH₃⁺ radical ion which was observed by ESR for the first time. Measurements were made at temperatures between 100° and 300°K. The radical, which undergoes hindered rotation at room temperature, displays isotropic hyperfine splittings of 70 Mc/sec for the two α protons, 53.8 Mc/sec for the three β protons and 11.6 Mc/sec for the β nitrogen. These values are approximately 20% higher than those measured previously in the glycine radical ·CH(CO₂⁻)NH₃⁺ in which the CO₂⁻ group extracts π‐electron spin density from the remainder of the molecule. The isoelectronic ethyl radical ·CH₂CH₃ shows a much larger β‐proton splitting and a slightly smaller α‐proton splitting, indicating that hyperconjugation is approximately 60% more effective for the CH₃ group. ESR measurements below the ferroelectric Curie temperature (177°K) indicate that below ≈ 140°K the molecule is rigidly held, but that it begins gradual motion between 140°K and the Curie point. Just at the Curie temperature a transition occurs which liberates additional degrees of freedom and allows virtually free rotation at higher temperatures.