Infrared spectra and hydrogen bonding of pentitols and pyranosides at 20 to 300 K

Infrared spectra in the range 400-4000 cm^-1 of three pentitols - ribitol, xylitol, D-arabinitol, and of three pyranosides - methyl α-D-manno- , methyl α-D-gluco- and methyl β-D-galactopyranoside, as polycrystalline solids of both the pure OH and >90% isotopically substituted OD compounds, were recorded at 20-300 K. In the low temperature spectra of the OH substances, at least three isolated narrow bands in the stretching mode and about ten narrow bands in the out-of-plane-bending mode range (<1000 cm^-1) are affected by cooling. Almost all have counterparts in the respective OD spectra with frequency ratios of 1.30-1.40. On this basis, they are assigned to OH groups bonded in H-bonds of different strengths (from 10 to 50 kJ mol^{- 1}). The average number of the OH···O hydrogen bonds is found to be two to three times larger than indicated by the stretching mode only or by structural data. The newly measured peak frequencies of the very narrow decoupled stretching mode bands show a correlation between the red shift (Δv) and the H-bond length. As previously found for tetritols, the presence of weak H-bonds (bond energy <14 kJ mol^-1) is related to the different water sorption capabilities of the pentitols. (C) 2000 Elsevier Science Ltd.