Low-temperature FTIR spectra and hydrogen bonds in polycrystalline adenosine and uridine

FTIR spectra of polycrystalline samples of adenosine and uridine, pure and containing small (<10%) quantity of N(O)H or N(O)D groups, 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 narrow isotopically decoupled proton stretching vibration ν ₁ mode of NH and OH groups were found and assigned to ordered hydrogen bonds according to crystal structural data for both nucleosides. The FTIR adenosine spectra in the out-of-plane bending proton ν ₄ mode range (lower than 1000 cm ^-1) of N(O)H groups revealed at low temperature at least twice more bands, than in the ν ₁ range, which are influenced by isotopic exchange and (or) cooling. Almost all of them have their counterparts in the N(O)D substance spectrum with an isotopic frequency ratio of 1.30-1.40. These bands were assigned to the differently H-bound disordered NH and OH protons, which could not be seen with crystal structural methods. The energy and length of different H-bonds were estimated from peak positions of both mode bands (as the red shift of ν ₁ or blue shift of ν ₄ relatively free molecules) with well-established empirical correlations between spectral, thermodynamic and structural parameters of hydrogen bonds. The results were compared with independent experimental data.