The vacuum-ultraviolet absorption spectra of substituted p-benzoquinones
Abstract
The vacuum-ultraviolet spectra of methyl-substituted p-benzoquinones are reported and, where possible, their Rydberg structure is analysed. The spectra of methyl-p-benzoquinone and 1,6-dimethyl-p-benzoquinone are found to be closer to that of a single ketonic chromophore compound than to that of p-benzoquinone. An ‘internal-pressure’ effect is proposed to account for this observation. It is consistent with the spectra of 1,5-dimethyl-tetramethyl- and tetrafluoro-p-benzoquinone, which all show no Rydberg structure, and with the spectrum of 1,4-naphthaquinone, which has Rydberg structure originating from the benzenoid ring rather than the ketone groups. The proposed effect is possibly a very general phenomenon accounting for the loss of Rydberg structure with substitution found in many molecular spectra. The π*â†�π absorptions are assigned and their intensity variations with orbital character and symmetry are discussed.