Theoretical Study On Effects Of Hydrogen Bonding On The Ring Stretching Modes Of Free And Halogen-Substituted Pyridine

In this paper, ab initio quantum mechanics density functional theory (DFT) was employed to investigate the effects of hydrogen bond N…X-X(X=O, N, C, F, Cl) between free or substituted pyridine and H2O, HCONH2, CH3COOH and HX on normal vibrational modes of pyridine. Meanwhile, the ChelpG method were performed to simulate the electrostatic field of proton donor to insight into the contribution of the frequency blue shift. This systematically investigated the origin of frequency blue shift of the pyridine ring stretching modes. The main contents are the following:1. 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 v6a and the meta-CC stretching mode v8a 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.2. Density functional method B3LYP plus the AUG-cc-pVDZ and AUG-cc-pVTZ basis sets is employed to investigate ring normal modes of halogen-substituted pyridines involved in the N…H-X H-bonds with HX(X=F, Cl). The results demonstrated that the formation of hydrogen bond leads to an increase in the frequencies of the ring breathing mode v1, the N-para-C stretching mode v6a and the meta-CC stretching mode v8a, whereas there is no change in the triangle mode v12 for free pyridine and a smaller blue shift for substituted pyridines. There is a strong coupling between the C-Y stretching vibration and the triangle mode (ortho and para substituted) or the breathing mode (meta substituted) in substituted pyridines, which leads to the frequency decrease in the triangle or breathing modes. The natural bond orbital analysis suggests that electrostatic interaction as well as charge transfer caused by the inter-and intramolecular hyperconjugations are the origin of the frequency blue shift in the ring stretching modes. Moreover, we find that the order of the frequency blue shift in the substituted H-bond complexes are as following:para>meta>ortho.