VIBRATIONAL ANALYSIS OF L-ALANINE AND DEUTERATED ANALOGS

Article abstract of DOI:10.1016/0022-2860(80)85305-1

Raman spectra of the polycrystalline L-alanine analogs CH3CH(NH3⁺)COO^-, CH3CH(ND3⁺)COO^-, CD3(NH3⁺)COO^-, and CD3(ND3⁺)COO^- have been obtained.A normal copordinate analysis is carried out based on the experimental frequencies of the four isotopic analogs and a 34 parameter valence-type force field defined in terms of local symmetry coordinates.The final refinement, in which five stretching force constants are constrained to fixed values obtained from bond length data, results in an average error of 7 cm^-1 (0.9percent) for the observed frequencies of the four isotopically substituted molecules.Band assignments are given in terms of the potential energy distribution for local symmetry coordinates.For non-deuterated L-alanine, the vibrations above 1420 cm^-1 and below 950 cm^-1 may be described as localized group vibrations.By contrast, the eight modes in the middle frequency range, viz. the three skeletal stretching, the COO^- symmteric stertching, one NH3⁺ rocking, the symmetric CH3 deformation, and the two methyne CH deformation vibrations, are very strongly coupled to one another.Some decoupling appears to take place in the perdeutero molecule, and all but five modes can be described as localized group vibrations.