Density functional study of the infrared spectrum of glucose and glucose monohydrates in the OH stretch region

Density functional theory (DFT) has been used to calculate the structures and infrared spectra of glucose and glucose monohydrates. Both α- and β-anomers were studied, including all hydroxymethyl rotamers (gg, gt, and tg) and both hydroxyl orientations (clockwise c and counter-clockwise r). A total of 69 glucose monohydrates were studied. The lowest-energy monohydrates correspond to complexes that require little distortion of the glucose structure in order to accommodate the water molecule. As was found in vacuum glucose calculations, the lowest-energy α-anomer is more stable than the lowest-energy β-anomer for the monohydrates. The vibrational modes of the infrared spectrum studied here are in the OH stretch region (3300−3800 cm−1). Peaks in the spectra produced by the hydroxymethyl rotamer when in the tg conformation, are generally red-shifted by ∼30 cm−1 relative to the peak location when in the gt and gg rotamer states. A second signature red-shift (also ∼30 cm−1) is found to characterize the glucose α-anomers relative to the β-anomer. The extent to which the hydroxyl peaks are conformation dependent depends strongly on the location of the water molecule. DFT calculations on specific phenyl-glucose derivatives allow comparison to recent experimental studies on the OH stretch region of these molecules and their monohydrates.