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

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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.