Origin of the blue‐shifted hydrogen bond in the vibrational Raman spectra of pyridine–water complexes: A density functional theory study

Abstract Despite extensive scientific efforts to understand the exact nature of hydrogen bonding in Pyridine (Py) aqueous solutions, the exact mechanism causing the blue‐shift in Py ring breathing mode has remained unclear so far. The present study is an attempt to investigate the possible causes of the experimentally observed blue‐shifts using density functional theory (DFT) by computing the complexation‐induced electronic charge redistribution due to hydrogen bonding in distinct Py–water complexes. Electron density difference map (EDDM) calculations, natural population analysis (NPA), and natural bond orbital (NBO) analysis were used to explore the blue‐shift in Py ring breathing mode upon complexation with water. The combined NPA and NBO analysis yielded that interaction between the nitrogen lone pair in Py and hydrogen atom of W causes a net charge transfer from Py to W, which—through hyperconjugation—depopulates the σ* (C–C) orbitals. This finally leads to the strengthening of the local C–C force constants. The relative contributions of normal coordinates were obtained by potential energy distribution (PED) calculations. Our computation reveals that increasing contributions of C–C stretching motions to the Py ring breathing mode (ν 1 ) are de facto causing a blue‐shift in the ν 1 vibration upon water addition. Overall, this analysis suggests that complementary information from both (a) electronic charge redistribution and (b) potential energy distribution of normal coordinates is required to explain the blue‐shift in ν 1 (Py) mode in aqueous solutions.