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

Abstract

An EPR study was carried out to investigate the structure and motional dynamics of the [(CH₃)₃N–CH₂]^+• radical formed in a γ-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 a_1^′=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 7.0×10⁶ s⁻¹ (140 K) to 4.1×10⁸ 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.