The effects of solvents and structure on the electronic absorption spectra of the isomeric pyridine carboxylic acid N-oxides

Abstract

The ultraviolet absorption spectra of the carboxyl group of three isomeric pyridine carboxylic acids N-oxides (picolinic acid N-oxide, nicotinic acid N-oxide and isonicotinic acid N-oxide) were determined in fourteen solvents in the wavelength range from 200 to 400 nm. The position of the absorption maxima (λmax) of the examined acids showed that the ultraviolet absorption maximum wavelengths of picolinic acid N-oxide are the shortest, and those of isonicotinic acid N-oxide acid are the longest. In order to analyze the solvent effect on the obtained absorption spectra, the ultraviolet absorption frequen­cies of the electronic transitions in the carboxylic group of the examined acids were correlated using a total solvatochromic equation of the form Vmax = V0 + + Sπ*+ aα+ bβ, where Vmax is the absorption frequency (1/λmax), π*is a measure of the solvent polarity, β represents the scale of solvent hydrogen bond acceptor basicities and α represents the scale of solvent hydrogen bond donor acidities. The correlation of the spectroscopic data was carried out by means of multiple linear regression analysis. The solvent effects on the ultra­violet absorption maximums of the examined acids were discussed.

UV apsorpcioni spektri pikolinske kiseline N-oksida, nikotinske kiseline N-oksida i izonikotinske kiseline N-oksida određeni su u 14 protičnih i aprotičnih rastvarača u opsegu od 200-400 nm. Položaji maksimuma apsorpcije bili su najniži za pikolinsku kiseline N-oksid, a najviši za izonikotinsku kiseline N-oksid. Da bi se analizirao uticaj ratvarača, apsorpcione frekvence su korelisane Kamlet-Taftovom jednačinom, kojom se uticaj polarnosti/polarizabilnosti, proton-donorskog i proton-akceptorskog dejstva rastvarača može kvantitativno izraziti.