The impact on the ring related vibrational frequencies of pyridine of hydrogen bonds with haloforms-a topology perspective

Hydrogen bonds between pyridine (Py) and haloforms (CHX₃, X = F, Cl, Br, I) and their impact on the ring related vibrational frequencies of pyridine were studied using a combination of solution phase FTIR and quantum mechanical DFT and ab initio calculations. With various possibilities for dimers that could potentially be formed between pyridine and haloforms, the calculations identified an intermolecular ring structure, which was established based on both the [Py-]N-involved hydrogen bond and the hydrogen bond between the alpha H on pyridine ([Py-]H) and the halogen atom on the haloform ([CHX₂-]X), as the most energetically stable form. The formation of a ring between the two molecules makes the entire ring structure more rigid on one hand, and weakens the [Py-]N-involved hydrogen bond on the other hand. As a result, no significant shift was observed for ν₁₂, and ν₁₀ only experiences a moderate blue shift upon hydrogen bonding. The magnitude of the shift in ν₁₀ is in the order: CHI₃ > CHBr₃ > CHCl₃ > CHF₃, according to calculations. FTIR experiments with pyridine and CHCl₃/CHBr₃/CHI₃ in cyclohexane solution showed a consistent sequence. Strong correlation was observed between the values of ν₁₀ and the various interatomic distances among [Py-]N, [Py-]H, [CHX₂-]X and [CX₃-]H, as well as other topological parameters involving the two bond critical points (BCP1 and BCP2) and the ring critical point (RCP). The percentage contributions from the internal coordinates were also estimated and were closely related to the magnitude of ν₁₀. Moreover, the occupied frontier molecular orbitals of the hydrogen bonding complexes (from HOMO-4 to HOMO) were analyzed to explain their roles in the pyridine ring vibrations and their sensitivity to hydrogen bonding.