Infrared spectra of monomeric L-alanine and L-alanine-N-d3 zwitterions isolated in a KBr matrix

The mid-IR spectrum of monomeric L-alanine-N-d(3) zwitterions in a matrix of KBr has been obtained by means of a newly developed infrared sampling technique. The spectrum of zwitterionic L-alanine has been remeasured under similar conditions but with improved sensitivity. The molecular structure and vibrational spectra of the zwitterions have been determined by ab initio molecular orbital calculations of the non-aqueous self-consistent reaction field (SCRF) type, using the Onsager dipole-sphere model at both the density functional theory and Hartree-Fock levels. Using one frequency scaling factor, overall good agreement in terms of both frequencies and intensities was established between the predicted and measured spectra. Only one conformer of zwitterionic alanine was located by SCRF calculations in contrast to more than 10 conformers of neutral unionized alanine found by SCF ab initio studies. (C) 2000 Elsevier Science B.V. All rights reserved.