Heat capacity and electron spin echo evidence for low frequency vibrational modes and lattice disorder in L-alanine at cryogenic temperatures

With the view of understanding the low frequency (40–50 cm⁻¹) motional processes in L-alanine around 4 K, we have carried out heat capacity $(CP)$ and electron spin echo (ESE) measurements on L-alanine and L-alanine-$d7.$ The obtained $CP$ data show the so-called boson peak (seen as a maximum in $CP/T3$ versus T plots) in the low temperature region (1.8–20 K). The phase memory time, $TM,$ and spin lattice relaxation time, $T1,$ of the spin probe, the so-called first stable alanine radical (SAR1), $⋅CHCH3COOH,$ have been measured between 4 and 105 K. The obtained relaxation rate $1/T1$ shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4–20 K). Together, the ESE and the $CP$ data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the $CP$ data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures.