GUEST LECTURES
Dr. Haiyan Fan
Associate Professor
Department of Chemistry
School of Science and Humanities
Nazarbayev University
The impact on the ring related vibrational frequencies of
pyridine by the hydrogen bonds with haloforms
Abstract
Hydrogen bond between pyridine (Py) and haloforms (CHX 3 , X = F, Cl, Br, I) and its impact on the ring
related vibrational frequencies of pyridine was studied using a combination of solution phase FTIR and
quantum mechanical DFT and ab initio calculations. With various possibilities of the dimer that could
potentially formed between pyridine and haloform, the calculation identified a intermolecular ring structure
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 haloform ([CHX 2 –]X) as the most energetically stable form.
The formation of the ring between 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 ν 12 , and ν 10 only experiences a moderate blue shift upon hydrogen bonding. The magnitude
of the shift in ν 10 is in an order of CHI 3 > CHBr 3 > CHCl 3 > CHF 3 according to calculation. The FTIR
experiments with pyridine and CHCl 3 /CHBr 3 /CHI 3 in solution of cyclohexane showed a consistent
sequence. A strong correlation was observed between the values of ν 10 and various interatomic distances
among [Py–]N, [Py–]H, [CHX 2 –]X and [CX 3 –]H, as well as other topological parametres involving the two
bond critical points (BCP1 and BCP2) and the ring critical point (RCP). The percentages of the contribution
from the internal coordinates were also estimated and closely related to the magnitude of ν 10 . Moreover, the
occupied frontier molecular orbitals of hydrogen bonding complex (from HOMO–4 to HOMO) were
analyzed to explain their roles in the pyridine ring vibrations and their sensitivity to hydrogen bonding.