Medium and temperature effects on the infrared spectra and structure of carboxylic acid pyridine complexes: acetic acid

Transformations of the infrared spectrum of strongly hydrogen bonded (H-bon ded) complex pyridine-acetic acid, caused by transition from the gas phase to solution, by increase of solvent polarity and by temperature decrease in solutions have been investigated. For comparison, spectrum of acetic acid dimer has been studied under similar conditions. It has been found that in contrast to the dimer, transition of the complex from the gas to condensed phase, as well as an increase of polarity of the complex solution, leads to significant low frequency shift, broadening and intensity increase of vOH band, high frequency shift of v(sigma) band and low frequency shift of vC=O band. All these changes can be treated as spectral manifestation of essent ial strengthening of the H-bond in the complex, accompanied by shifting of the bridging proton toward the nitrogen atom. Temperature behaviour of vOH band in polar solvents, such as butyl chloride or methylene chloride, is de termined mainly by two factors: (a) changing of population of the low frequ ency levels of hydrogen bond vibrations; (b) ordering and polarization of t he solvent in complex vicinity, which provides I-I-bond strengthening. In t he inert unpolar solvents, like hexane, temperature decrease in some cases leads to an increase of contribution of the complexes with more complicated composition than 1:1. The data obtained are analysed using the earlier dev eloped theory of the solvent effects on vibrational spectra of H-bonded com plexes. (C) 1999 Elsevier Science B.V. All rights reserved.