Vacuum ultraviolet absorption spectra of methyl-substituted cyclopentanones and cyclohexanones

Abstract

The vacuum ultraviolet absorption spectra of methyl-substituted cyclohexanones and cyclopentanones are reported and analysed in terms of Rydberg series. Apart from an anomaly in the n= 4 region, which is attributed to interference by a molecular valence state, long series are assigned in all cyclohexanone spectra allowing accurate determination of the ionization energies. The cyclopentanone spectra are similare but less extensive. The Rydberg structure differs with substituent position in the length and the type of series that dominates the spectra, and in the width of the individual bands. Two distinct substituent effects are identified; an ‘internal pressure’ effect causing increased bandwidth and increased Rydberg–valence state interaction; an orbital mixing effect causing transfer of transition intensity from a d-series in the parent molecules to a p-series of the substituted compounds as the substituent approaches the chromophore. The absolute intensities of the spectra are reported and discussed, and molecular orbital calculations on the ground states of the molecules and their ions used to interpret aspects of the spectra. The vibronic structure of the lowest energy, n= 3s, transitions of selected compounds both from photoabsorption and resonance-enhanced multiphoton ionization (REMPI) spectra are compared with an REMPI study of cooled molecules.