Evidence for disorder in l-alanine lattice detected by Pulsed-EPR spectroscopy at cryogenic temperatures

The unusual behavior of lattice dynamics of l-alanine has been assigned to intermolecular dynamics and localization of vibrational energy. Recent heat capacity and Pulsed-EPR measurements support presence of thermally activated dynamic orientational disorder in the l-alanine lattice below 20 K. In the present study, the additional evidence for possible thermally activated disordered behavior of l-alanine lattice have been obtained by investigating dependences of longitudinal relaxation time of first stable l-alanine radical, SAR1, on sample cooling rates for the same low temperature interval. The obtained relaxation time by Pulsed-EPR shows clear dependence on cooling rates and this behavior can be explained within two types of suggested spin-lattice relaxation mechanisms for the paramagnetic centers in the hydrogen-bonded organic crystal.