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

摘要

An EPR study was carried out to investigate the structure and motional dynamics of the [(CH3)(3)N-CH2](+.) radical formed in a gamma-irradiated Al-offretite. Strongly temperature-dependent EPR spectra were observed in the temperature range between 4 and 300 K. The observed H-1 hyperfine (hf) splitting was a(1)=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 N-14 were a(2)=2.24 and a(3)=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 7.0x10(6) s(-1) (140 K) to 4.1x10(8) 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)(3)N-CH2](+.) in Al-offretite. [References: 32]