Vibrational analysis of alpha-D-glucose trapped in Ar matrix.

The FT-IR spectra of alpha-D-glucose have been investigated by means of matrix-isolation FT-IR spectroscopy in the 3700-200 cm-1 range and by B3LYP/6-311++G** density functional calculations. The joint analysis of the experimental and computed IR spectra supported the expected predominance of the most stable gg and gt conformers in the vapor phase and the minor contribution of the less stable tg conformer. Weak absorption bands at the edge of the OH stretching region are in agreement with small amounts of analogous conformers with clockwise arrangement of the hydrogen bonds (c-gg, c-gt, c-tg). The assignment of the IR spectra was carried out on the basis of the computed harmonic force field using the scaled quantum mechanical method of Pulay. Scale factors developed previously for the B3LYP/6-311++G** level proved to be well transferable except that for the OH torsion. The latter scale factor was adjusted on the basis of the present experimental data. On the basis of the calculated results, 99 bands have been assigned to the gg, gt, and tg conformers of alpha-D-glucose with average deviations of 5.0, 5.9, and 6.1 cm-1, respectively.