Solvent and Structure Effects on Electronic Absorption Spectra of the Isomeric Pyridinecarboxylic Acids

Ultraviolet absorption spectra of three isomeric pyridinecarboxylic acids (picolinic, nicotinic, and isonicotinic acids) were measured in 18 various solvents, in the wavelength range from 200 to 400 nm. In order to analyze the solvent effect on the obtained absorption maxima, the ultraviolet absorption frequencies of the electronic transitions in the carbonyl group of the examined acids were correlated using a total solvatochromic equation in the forms nu(max) = nu(0) + s pi(*) + a alpha + b beta and nu(max) = nu(0) + s pi(*) + b beta, where nu(max) is the absorption frequency (1/nu(max)), pi(*) is a measure of the solvent polarity, alpha represents a scale of the solvent hydrogen bond donor acidity, and beta represents a scale of the solvent hydrogen bond acceptor basicity. Correlation of the spectroscopic data was carried out by means of multiple linear regression analysis. The effects of two different solvent types on the ultraviolet absorption maxima of the examined acids were compared and discussed.