Theoretical study on the ring stretching modes of pyridine in the hydrogen bonding with H2O, HCONH2 and CH3COOH

Density functional theory was applied to study the effects of H-bonds N⋯H–X between pyridine and H₂O, HCONH₂ and CH₃COOH on normal vibrational modes of pyridine at the B3LYP/AUG-cc-pVDZ and B3LYP/AUG-cc-pVTZ levels. The results show that the formation of H-bonds leads to an increase in frequencies of the ring breathing mode v₁, the N-para-C stretching mode v_6a and the meta-CC stretching mode v_8a of pyridine but there was no change in triangle mode v₁₂. The natural bond orbital analysis shows that the frequency blue shift in the ring stretching modes of pyridine is a corporate result of the intermolecular charge transfer caused by the intermolecular hyperconjugation n(N) → σ∗(HX) and the intramolecular charge redistribution caused by intramolecular hyperconjugation n(N) → σ∗(meta-CC) in the pyridine ring. We also found that the magnitude of the frequency blue shift increases with the strength of the hydrogen bonding.