Knowledge Bank

The change in the position of the $\eta \to n\ast$ absorption bands on changing from a hydrocarbon to a hydroxylic solvent has been investigated for a number of molecules. The large shift to shorter wavelengths (blue-shift) is shown to be mainly due to hydrogen bonding of the `$\eta$’ electrons by the hydroxylic solvent which causes a greater stabilization of the ground state compared to the excited state of the molecule. Pyridazine and benzophenone have been examined in detail in a series of different mixtures of hexane and ethanol. The families of spectra obtained indicate that essentially two species are involved, a hydrogen-bonded and a non-hydrogen-bonded form and it is the formation of the hydrogen-bonded species that causes the main shift of the $\eta \to n\ast$ transition to the blue. From the ultraviolet data, an association constant of hydrogen bonding can be obtained and this agrees well with the association constant found by a study of the association of ethanol with the molecule in the infrared. The infrared work makes use of the shift in the $O-H$ stretching frequency on formation of a hydrogen bond.