The study of proton-donor proton-acceptor interactions by infrared spectroscopy and computer modelling
Infrared spectroscopy has been used to determine thermodynamic parameters for the reaction between ethanol and acetone in CC14 and in the vapour phase in which a hydrogen-bonded complex is formed. In carbon tetrachloride, AHf^ = -15.3 kJ mol"1 and ASf"0" = -50.8 J mol"1 K" 1 . These values are consistent with data reported in the literature for related alcohols. Equilibrium constants for the vapour phase reaction were not reproducible, possibly due to a combination of adsorption and pressure effects.The IR spectra of propionic and acetic acids was studied indilute CCl^ and for acetic acid (at low acid pressure) in gaseous argon mixtures. Bands were assigned to the monomer and dimer. Spectra confirm that carboxylic acids are much more strongly hydrogen-bonded than the corresponding alcohols. The IR spectra of acetic, propionic, butyric, benzoic, omethoxyland o-ethoxy benzoic, p-methoxyl and p-ethoxybenzoic and o-acetyl salicylic acids was studied in CCl^.The effect of inter and intramolecular hydrogen-bonding is discussed. Models of monomer and dimer carboxylic acids were constructed using the Chem-X computer modelling package(Chemical Design Ltd.). Molecular energies were generated for the most stable acid conformations. For o-methoxybenzoic acid, the variation of molecular energy with the degree of internal rotation for the four bonds for which internal rotation was feasible was plotted using Chem-X.The J-CAMP.DX file protocol was used to import IR spectra into the Chem-X package. The future potential of combining modelling and spectroscopic packages is discussed.
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