FTIR spectra and conformational structure of deutero-β-alanine isolated in argon matrices

Low temperature FTIR spectra of β-alanine-d₃ isolated in argon matrices are used to determine the conformational composition of this compound. UV irradiation of the matrix samples is found to change the relative populations of the β-alanine-d₃ conformers. The populations of conformers I and II with an N-D⋯O intramolecular H-bond decrease after the UV irradiation while the populations of conformer V with an N⋯D-O H-bond and conformer IV which has no intramolecular H-bonds increase. This behavior of the β-alanine-d₃ conformers are used to separate the bands of the different conformers. The analysis of the experimental FTIR spectra is based on the calculated harmonic B3LYP/6-311++G(df,pd) frequencies and on the MP2/aug-cc-pVDZ frequencies calculated with a method that includes anharmonic effects. Polynomial scaling of the calculated frequencies is used to achieve better agreement with the experimental data. The observation of the wide band of the OD stretching vibration at 2201 cm^-1 is a direct evidence of the presence of the β-alanine-d₃ conformer V in the Ar matrix. In total ten bands of conformer V are detected. The influence of the matrix environment on the structures and the IR spectra of the β-alanine and β-alanine-d₃ conformers is investigated. This involves performing calculations of the β-alanine conformers embedded in argon clusters containing from 163 to 166 argon atoms using the M06-2X and B3LYP(GD3BJ) density-functional methods. Good agreement between the calculated and the experimental matrix splitting is demonstrated.