EFFECT OF ISOTOPIC-SUBSTITUTION ON THE ELECTRON-SPIN DYNAMICS OF THE CH3(C)OVER-DOT(COOH)(2) RADICAL IN X-IRRADIATED METHYL MALONIC-ACID POWDER - INTRINSIC POTENTIALS AND ACTIVATION-ENERGIES

The temperature-dependent EPR line shapes from the methyl rotor of the X irradiation-induced CH3C(COOH)(2) radical in powder MMA (methyl mal onic acid) and the X irradiation-induced radical CD3C(COOH)(2) in meth yl specifically deuterated powder MMA are studied experimentally for t he temperature range 4.8 K (5 K) to 77 K (65 K). The hydrogenated syst em is simulated using a quantum inertial dynamical model with a hinder ing potential and three-site exchange rotation. The deuterated system is simulated using a classical three-site exchange model. The results show that due to the increase in moment of inertia, the tunneling freq uency is negligible for the deuterated rotor, resulting in a stopped r otor low-temperature spectrum, while being sufficiently large for the hydrogenated system for this to exhibit tunneling, From the low-temper ature deuterated analogue spectrum, the potential twist angle is estim ated to delta = +/-50 degrees +/- 2 degrees (+n.60 degrees, n is an el ement of Z). The site-exchange activation energy of the deuterium roto r is observed to be 387 K, substantially lower than the hindering pote ntial depth of 618 K, The hydrogenated system exchange rotation rate a ssumes a linear behavior with 754 K activation energy in the classical region of temperatures above 50 K.