Investigating the H‑Bond Network of Pyridine-Water Aqueous Solutions by Raman Spectroscopy and DFT

A pyridine (C₅H₅N)-water (H₂O) binary solution under different mole fractions was investigated by using Raman spectroscopy (spontaneous Raman spectroscopy and stimulated Raman scattering) combined with density functional theory (DFT). The results showed that the Raman spectra of O: H nonbond and O-H stretching vibrations of H₂O have irregular blue shifts as the molar fraction of C₅H₅N increases in the C₅H₅N-H₂O binary solution. Meanwhile, the wavenumbers of the symmetric ring breathing mode, triangle mode, and C-H stretching vibration mode of C₅H₅N move toward a higher wavenumber upon dilution. This phenomenon mainly resulted from the variation of intermolecular hydrogen bonds (HBs) and was affected by the vibrational coupling between different chemical bonds. Furthermore, we discussed the intramolecular Fermi resonance (FR) of C₅H₅N and the effect of HBs on the FR (ν₁ and ν₁₂). In addition, the DFT simulation was used to optimize the geometrical structure corresponding to four modes with stoichiometric ratios ranging from PyW₃, PyW₂, PyW, to Py₂W (Py: pyridine, w: water). The calculated results are in good agreement with the experimental findings, providing a valuable reference for studying the HB networks in N-heterocyclic binary solutions.