Effect of Hydrogen Bonding on the Near Ultraviolet Absorption of Naphthol

Abstract

The effect of hydrogen bonding on near ultraviolet absorption spectra of α‐ and β‐naphthol, which were used as the proton donor, was studied. Triethylamine, ethyl ether, and nitromethane were employed as the proton acceptor. Absorption measurements were made with ternary solutions which contain small amounts of proton donor and acceptor in n‐heptane. The results show that both L_a and L_b bands decrease gradually with the increasing concentration of proton acceptor and new bands, which are caused by the hydrogen bonded proton donor molecule, appear on the longer wavelength sides of original bands. Using this change of absorption spectrum, the equilibrium constant between proton donor and acceptor and wavelength shifts due to hydrogen bonding were evaluated. From the evaluated values of these two quantities, it was concluded that the proton accepting power decreases in the order of triethylamine [N(C₂H₅)₃]>ethyl ether [O(C₂H₅)₂]>nitromethane (H₃CNO₂). On the basis of this proton accepting ability, it was discussed that hydrogen bonding is not purely electrostatic in its origin but the charge transfer or delocalization mechanism contributes it to an important extent.