π-Deficient N-heteroaromatics as proton acceptors in hydrogen-bonding: Infrared spectral shifts vs. p Ka's as measures of “basicity”

Abstract

IR spectral shifts (Δν) due to hydrogen-bonding between a common proton donor, methanol, and a wide variety of π-deficient N-heteroaromatics, including pyridine, alkylpyridines, halo- and cyanopyridines, benzologues of pyridine, and related molecules containing two N atoms were determined. With 3- and 4-cyanopyridines as proton acceptors, two methanol bonded OH peaks were observed, corresponding to hydrogen-bonding to the nitrile nitrogen and to the pyridine nitrogen; all other compounds gave symmetrical bonded OH peaks. The Δν's obtained were compared with the p Ka's of the bases and the equilibrium constants (log K's) for the associations: phenol + base ⇄ hydrogen-bonded complex. p Ka and log K as well as log K and Δν for all bases correlated reasonably well. A good linear relation between Δν and p Ka for the limited class of alkyl pyridines also was found, but when all bases were considered, this correlation was not as successful. However, the compounds which deviated most significantly were probably exceptional cases whose behavior could be understood in terms of special structural features present. The abnormally high p Ka of 1,10-phenanthroline was attributed to a strong intramolecular hydrogen-bond in the protonated base, while the high p Ka's found with azines with adjacent N atoms were similarly ascribed to intermolecular hydrogen-bonding. These exceptions illustrate the essential difference between p Ka and Δν as a measure of the “basicity” of molecules. 2,6-Di-t-butylpyridine did not associate with methanol, even at high concentration. Solvation effects in this molecule must also be very much inhibited.