CONFORMERS OF GASEOUS ALPHA-ALANINE

Accurate geometries, relative energies, rotational and quartic centrif ugal distortion constants, dipole moments, harmonic vibrational freque ncies, and infrared intensities have been determined from ab initio ca lculations for 13 conformers, corresponding to minima on the potential energy surface, of the neutral form of the amino acid a-alanine. The theoretical relative energy predictions, even after zero-point vibrati onal energy corrections, differ significantly from the experimental lo wer limits deduced from millimeter wave spectra. Energetic and structu ral results indicate necessary model improvements for a gas-phase elec tron diffraction study of a-alanine. The structural results obtained s upport the molecular constants measured for two conformers of low rela tive energy. The accurate calculated rotational and quartic centrifuga l distortion constants and vibrational frequency data for the other co nformers should aid in identification of the conformers by rotational and vibrational spectroscopies, respectively.