HYDROGEN-BONDING BETWEEN A N-PYRIDINIUM PHENOLATE BETAINE AND O-H DONORS IN ACETONITRILE

Hydrogen bonding between 4-(2,4,6-triphenylpyridinio)-2,6-diphenylphenolate (1, Dimroth's dye) and water, alcohols, and phenols has been measured spectrophotometrically in acetonitrile at 25-degrees-C. Association constants for 1:1 hydrogen-bonded complexes range from K(AB) = 2 M-1 for water to K(AB) = 6.06 x 10(3) M-1 for m-nitrophenol with a Bronsted slope alpha = 0.38 +/- 0.04 for substituted phenols with pK(a) > 23. The acidity constant of 4-(2,4,6-triphenylpyridinio)-2,6-diphenylphenol was determined by potentiometric titration: pK(a) = 22.1 +/- 0.2. Hydrogen bonding of phenols is analyzed within the framework of the Hine equation that describes changes in the strength of the hydrogen bond with changes in the pK(a) of the hydrogen-bond donor and acceptor. A value of the Hine interaction coefficient tau = 0.016 +/- 0.002 is consistent with a double-minimum potential for the ArOH...1 hydrogen bond.