Molecular Complexes of Amino Acid (α-Alanine) and Their Manifestation in the Raman Spectra, Ab initio Calculations

Abstract

The purpose of this work is to determine the role of hydrogen bonds in the crystallization of alanine, which includes carbooxyl COOH and amino NH2 groups, and the manifestation of these intermolecular interactions in the Raman spectra. FT-Raman spectra were analyzed and ab initio calculations by the RHF and B3LYP methods were performed in order to consider in more detail the possibility of the formation of aggregated intermolecular complexes in the crystalline state of α-alanine. The intra-molecular and inter-molecular interactions, the dynamics of molecular groups and structural changes of α-alanine have been studied by the method of Raman light scattering and ab initio calculations. Ab initio calculations have shown that the change in the Raman spectra is explained by the formation of several types of hydrogen bonds. These hydrogen bonds play a special role in the formation of the crystal structure of α-alanine. In Raman spectra, the presence of a hydrogen bond between molecules is manifested in the form of asymmetry and splitting of vibrational bands. An analysis of the Raman spectra of alanine with water shows that with an increase in the strength of the hydrogen bond leads to increase in the bond energy in the OH group. The hydrogen atom in the N-H group of the alanine molecule is actively involved in the formation of a hydrogen bond. It was shown by ab initio calculations that dimeric, trimeric and other chain molecular complexes exist in alanine. The complexes formed due to the hydrogen NH2 bond are weaker than those formed due to the OH bond. Upon the formation of a complex of alanine with water, the O-H vibration band shifts to the low-frequency side by 320 cm-1 and 415 cm-1, respectively. It was shown that an intramolecular hydrogen bond is formed between the NH and CO groups. It was found that in the Raman spectra of O-H and N-H, the alanine bands in the region of 2900-3050 cm-1 are complex and consist of several bands. The structure and connectivity of the bands is explained by the fact that the spectrum of the O-H bond consists of a symmetric valence band of vibrations and overtones of bending vibrations.