engleski

A pulse EPR study of longitudinal relaxation of the stable radical in gamma-irradiated L-alanine

The longitudinal relaxation rate of the first stable alanine radical, SAR1, was studied by employing pulse EPR technique over a wide temperature interval (5-290 K). The complex nonexponential recovery of the longitudinal magnetization in this temperature interval has been described with two characteristic relaxation times, 1 / T-1a*. as the faster component and 1 / T-1b*, as the slower component, respectively. It was shown that 1 / T-1a*, is strongly affected by the CH3 group dynamics of the SAR1 center. The complete temperature dependence of 1 / T-1a*, was described by invoking several relaxation mechanisms that involve hindered motion of the CH3 group from classical rotational motion to coherent rotational tunneling. It was shown that all relevant relaxation mechanisms are determined by a single correlation time with the potential barrier (DeltaE / k = 1570 K). On the other hand the temperature dependence of 1 / T-1b* is related to the motional dynamics of the neighborly NH3 and CH3 groups. We found a larger average potential barrier for this motion (DeltaE / k = 2150 K) corresponding to smaller tunneling frequencies of the neighbor groups. (C) 2001 Academic Press. [References: 24]

Pulse epr. Spin-lattice relaxation. L-alanine. Paramagnetic center. Proton tunneling

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