Conformational behavior of α-alanine. Matrix-isolation infrared and theoretical DFT and ab initio study

Two conformers of the nonionized α-alanine and its isotopomer N,N,O-d₃-alanine have been observed in low-temperature Ar matrixes. Their infrared spectra have been analyzed and assigned using DFT/B3LYP/ aug-cc-pVDZ and MP2/aug-cc-pVDZ geometry and frequency theoretical calculations. Two different intramolecular H-bonds, bifurcated NH₂⋯O=C and N⋯H-O, were found in the observed α-alanine conformers, I and IIa. We found that the DFT/B3LYP/aug-cc-pVDZ method yields vibrational frequencies of the α-alanine conformers in excellent agreement with the experimental data.