A VIBRATIONAL MOLECULAR-FORCE FIELD OF MODEL COMPOUNDS WITH BIOLOGICAL INTEREST .3. HARMONIC DYNAMICS OF ALPHA-D-GALACTOSE AND BETA-D-GALACTOSE IN THE CRYSTALLINE STATE

The infrared spectra of alpha- and beta-D-galactose were recorded, both in the mid-IR range (4000-500 cm-1) and in the far-IR (500-50 cm-1). The Raman spectra were also obtained. These spectra constitute the basis of a crystalline-state force field established for these two molecules through a normal coordinate analysis. A modified Urey-Bradley-Shimanouchi force field was combined with an intermolecular potential energy function which includes van der Waals interactions, electrostatic terms and an explicit hydrogen bond function. The force constants were varied, so as to obtain an agreement between the observed vibrational frequencies and the calculated ones of alpha-D-galactose. The force field obtained was then applied to alpha-D-galactose O-d5 and beta-D-galactose, in order to test its transferability. The computed potential energy distribution was found to be compatible with previous assignments for D-glucose, particularly for the modes involving C6 and C-OH groups. For beta-D-galactose the same force field was used with changing the force constants due to the C1 and C6 groups.