Structure and dynamics of [(CH3)3N-CH2]+· radical generated in ?- irradiated Al-offretite

• An EPR study was carried out to investigate the structure and motional dynamics of the [(CH3)3N-CH2]+· radical formed in a ?-irradiated Al- offretite. Strongly temperature-dependent EPR spectra were observed in the temperature range between 4 and 300 K. The observed 1H hyperfine (hf) splitting was a1 = 0.46 mT for three magnetically equivalent hydrogens, one of each methyl group at 110 K and a1 = 0.15 mT for three of each methyl group at 300 K, the hf splittings due to two equivalent CH2 hydrogens and the central 14N were a2 = 2.24 and a3 = 0.35 mT, respectively, independent of the temperature. This result suggests that rotation of the methyl group around the N-CH3 bond is responsible for the temperature dependent line shapes. Assuming a three-site jump model in which three methyl protons interchange their positions with each other, the EPR line shapes were successfully analyzed by a simulation method using the jump rate as a variable parameter in the temperature range from 140 to 270 K. The rate was found to increase from 7.0 x 106 s-1 (140 K) to 4.1 x 108 s-1 (270 K) with temperature. From an Arrhenius plot of the rate constants, an activation energy of 8.1 kJ mol-1 was evaluated for the methyl group rotation of [(CH3)3N-CH2]+· in Al-offretite.

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