Conformers of gaseous alpha-alanine

Accurate geometries, relative energies, rotational and quartic centrifugal distortion constants, dipole moments, harmonic vibrational frequencies, and infrared intensities have been determined from ab initio calculations 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 vibrational energy corrections, differ significantly from the experimental lower limits deduced from millimeter wave spectra. Energetic and structural results indicate necessary model improvements for a gas-phase electron diffraction study of a-alanine. The structural results obtained support the molecular constants measured for two conformers of low relative energy. The accurate calculated rotational and quartic centrifugal distortion constants and vibrational frequency data for the other conformers should aid in identification of the conformers by rotational and vibrational spectroscopies, respectively.