Hydrogen bonding in different pyrimidine-methanol clusters probed by polarized Raman spectroscopy and DFT calculations

We report on the hydrogen bonding between pyrimidine (Pd) and methanol (M) as H-donor in this study. Hydrogen bonds between pyrimidine andmethanolmolecules as well as those between differentmethanolmolecules significantly influence the spectral features at high dilution. The ring-breathing mode ν1 of the reference system Pd was chosen as a marker band to probe the degree of hydrogen bonding. Polarized Raman spectra in the region 970–1020 cm−1 for binary mixtures of (pyrimidine + methanol) at 28 different mole fractions were recorded. A Raman line shape analysis of the isotropic Raman line profiles at all concentrations revealed three distinct spectral components at mole fractions of Pd below 0.75. The three components are attributed to three distinct groups of species: ‘free Pd’ (pd), ‘Pd with low methanol content’ (pd1) and ‘Pd with high-methanol content’ (pd2). The two latter species differ considerably in the pattern and the strengths of the hydrogen bonds. The results of density functional theory calculations on structures and vibrational spectra of neat Pd and eight Pd/Mcomplexes with varying methanol content support our interpretations of the experimental results.A nice spectra–structure correlation for the different cluster subgroups was obtained, similar to earlier results obtained for Pd and water. Apart from N· · ·H and O· · ·H hydrogen bonds between pyrimidine andmethanol, O· · ·H hydrogen bonds formed among the methanol molecules in the cluster at high methanol content also play a crucial role in the interpretation of the experimental results.