A study of the temperature effect on the IR spectra of crystalline amino acids, dipeptids, and polyamino acids. VI. L-alanine and DL-alanine

The results of IR and single crystal X-ray diffraction studies on the dynamics of molecular groups and structural changes in L -alanine and DL -alanine (NH 3 + -CH(CH 3 )-COO − ) with temperature variation are given. An analysis of changes in the 4000–600 cm −1 frequency range of the IR spectra with temperature variation reveals the occurrence of the anomaly for the ∼974 cm −1 band in DL -alanine, which is similar to the anomaly for the 955 cm-1 band, previously described for L -alanine. The X-ray diffraction data for L and DL -alanine show that no dramatic changes in the unit cell parameters, conformations of amino acid molecules themselves, and hydrogen bond lengths occur with temperature variation, which would indicate the structural phase transition. Changes in the IR spectra of L -alanine and DL -alanine with temperature variation are compared to the changes in the vibrational spectra of other amino acids on cooling.