Density Functional Study of the Proton Transfer Effect on Vibrations of Strong (Short) Intermolecular O-H•••N/O-•••H-N+ Hydrogen Bonds in Aprotic Solvents

The structure and spectroscopic properties of the 1:1 complexes of substituted pyridines with benzoic acid and phenol derivatives in aprotic solvents are studied using B3LYP functional combined with the polarizable continuum model approximation. Two extreme structures are investigated: the state without (HB) and with proton transfer (PT). In the presence of an external electric field the O center dot center dot center dot N distance is contracted and the PT state does appear. The PT state of both the pyridine-benzoic and the pyridine-phenol complexes displays the only IR-active band in the 2800-1800 frequency region, which is located around 2000 cm(-1). However, the nature of the band is different for these two complexes. In the pyridine-benzoic acid complex it is practically a pure stretching vibration of the HN+ group, while in the pyridine-phenol complex it is the mixed vibration of the bridging proton. A specific feature of the PT state in the pyridine-phenol complex is an IR-intensive band near 600 cm(-1), associated with the asymmetric stretching vibrations of the O-center dot center dot center dot HN+ fragment. Its intensity is reciprocally proportional to the O center dot center dot center dot N distance. The appearance of this band provides an efficient criterion to differentiate between the HB and PT states of the 1:1 complexes of phenols with pyridines in aprotic solvents.