Electronic Spectra and Hydrogen Bonding. I. Phenol and Naphthols

Abstract

The effect of hydrogen bonding on the electronic absorption spectra of phenol, α naphthol, and β naphthol has been investigated with particular attention to the relation between the nature of electronic transitions and their behavior in hydrogen bond formation. The spectra were obtained down to 2000 A in isooctane solution in the presence of varying concentrations of dioxane. From the analysis of the observed spectra, the equilibrium constants for the hydrogen bonds and the spectra of the hydrogen-bonded species were determined. The hydrogen bond energies are given for the ground and excited states of the solute molecules. The experimental results clearly indicate that effects of hydrogen bonding on electronic spectra differ markedly with transitions. Both the frequency shifts and the intensity changes differ in magnitude and even in sign according to the properties of the transitions concerned. It is shown that the transition at 47 000 cm−1 of α naphthol is displaced to higher frequencies upon formation of the hydrogen bond. No appreciable changes are produced by hydrogen bonding in the spectral patterns of the individual transitions, aside from broadening or smoothing of the vibrational structure. The behavior of the transitions in hydrogen bond formation is interpreted on the basis of the electronic structure of the solute molecules. Two factors are shown to be important for accounting for the mechanism of the hydrogen bonding effect: (a) a change in the electron density at the oxygen atom of the O–H group accompanying an electronic transition; (b) a decrease in the electronegativity of the same oxygen atom resulting from hydrogen bond formation.