Theoretical study on effects of hydrogen bonding on the ring stretching modes of pyridine

Pyridine generally acts as the proton acceptors in the hydrogen bonding interaction by using its lone pair n(N) or π -electrons. Some previous research indicated that for the N-type H-bond, the ring breathing mode v 1 , the N- para -C stretching mode v 6 a and the meta -CC stretching mode v 8 a of pyridine showed a frequency blueshift but the triangle mode v 12 had no change in frequency. Both electrostatic interaction and charge transfer caused by intermolecular hyperconjugation n ( N ) → σ ∗ ( HX ) have contributions to the frequency blue shifts, while charge transfer is predominant at equilibrium intermolecular distance. An intramolecular hyperconjugation between the lone pair n(N) and the two σ ∗ ( m e t a -CC ) orbitals in the pyridine ring provides a reasonable interpretation for the effect of charge transfer on the ring stretching modes upon formation of the N-type H-bonding.