THE ROLE OF HYDROGEN-BONDING IN THE $\eta\rightarrow n^{*}$ BLUE-SHIFT $PHENOMENON^{\dagger}$
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Date
1954
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Ohio State University
Abstract
The change in the position of the $\eta\rightarrow n^{*}$ 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 $\eta\rightarrow n^{*}$ 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.
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Author Institution: Department of Chemistry, Florida State University