A periodic DFT study of IR spectra of amino acids: An approach toward a better understanding of the N-H and O-H stretching regions

• DOI: 10.1016/j.vibspec.2021.103280
• Publication Date: 2021-07-12T15:54:10Z
• Authors: Abdelali Boukaoud; Younes Chiba; Djamel Sebbar
• Keywords: 01 natural sciences; 0104 chemical sciences

Abstract Despite the effort deployed, the decryption of the vibrational spectra of amino acids in the high wavenumber region (2600–3600 cm−1) remains a challenge for both spectroscopists and computational scientists. In this challenging context, the structures and vibrational spectra of five amino acid crystals ( l -tyrosine, l -histidine, l -serine, l -serine monohydrate and DL-serine) have been investigated. Based on the results of DFT calculations performed in the solid state, the calculated vibrational spectra have been analyzed in order to clarify some controversies concerning previously published band assignments for the O H and N H stretching modes. Except for l -serine, which reveals three calculated IR bands at 3456, 3506 and 3507 cm−1 ascribed to different OH stretching vibrations in the unit-cell, all the calculated fundamental bands for the other four compounds, including those due to the l -serine molecule as it exists in l -serine monohydrate crystal, are predicted to be below 3201 cm−1. Because the structural and vibrational properties of the investigated molecules are strongly correlated to their own H-bonding networks, the previous theoretical studies based on non-periodic approaches have failed to give a correct description of the vibrational behaviors in the N H and O H stretching regions. In addition, a careful comparison of some previously published IR and Raman spectra of the studied anhydrous amino acids confirms the presence of certain additional bands. The possible origin of these bands is revealed in this study.