Low temperature FTIR spectra and hydrogen bonds in polycrystalline cytidine

Abstract

FTIR spectra of polycrystalline samples of cytidine, pure and containing a small quantity of N(O)H or N(O)D groups (<20%), were measured in KBr pellets from 4000 to 400 cm −1 at temperatures from 300 to 20 K. For the first time the bands of the narrow isotopically decoupled proton stretching vibration mode ( ν 1) of OH and NH groups were found; their number corresponds to the number of H-bonds in crystal according to structural data. The FTIR spectra at low temperature in the out-of-plane bending ν 4 proton mode range (lower than 1000 cm −1) of N(O)H groups revealed narrow bands, which correspond to ν 1 bands together with several “extra” bands, which are influenced by the isotopic exchange and (or) cooling. All of them have their counterparts in the N(O)D-substance spectrum with an isotopic frequency ratio of 1.30–1.40. The “extra” bands are assigned to the H-bound OH and NH protons, which are disordered and cannot be seen with X-ray crystal structure analysis. The peak positions of both mode bands (expressed as the red shift of ν 1 or blue shift of ν 4 modes relatively free molecules) were used for the estimation of the energy of different H-bonds using previously established empirical correlations between spectral and thermodynamic parameters of hydrogen bonds. The correlation of the red shift and H-bond length is also confirmed for all five H-bonds of cytidine.