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 characterized by an intramolecular NH ... O hydrogen bond leading to the formation 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 intensities computed for the two conformers is discussed. The IR spectra at different temperatures and the Raman spectra of solid NAAL and its deuterated counterpart are investigated and discussed. The frequencies of the v(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 in a weak hydrogen bond.