Theoretical and experimental study of the vibrational spectrum of N-acetyl-L-alanine

The energies, vibrational frequencies and IR intensities of cis- and trans- N-acetyl-L-alanine (NAAL) are computed using the density functional theory (B3LYP) combined with the 6-311G(d, p) basis set. The trans conformer is ch aracterized by an intramolecular NH ... O hydrogen bond leading to the form ation of a five-membered ring and is by 23 kJ mol (-1) more stable than the cis conformer. The difference between the vibrational frequencies and IR i ntensities computed for the two conformers is discussed. The IR spectra at different temperatures and the Raman spectra of solid NAAL and its deuterat ed counterpart are investigated and discussed. The frequencies of the nu(OH ) vibration and the isotopic ratio suggest the formation of short OH ... O hydrogen bonds in the solid state. The NH group seems also to be involved i n a weak hydrogen bond. (C) 2000 Elsevier Science B.V. All rights reserved.