2-METHYL EFFECTS IN THE RYDBERG SPECTRA OF METHYL-SUBSTITUTED CYCLOHEXANONES

Rydberg spectra corresponding to n --> 3s transitions for a homologous series of cis- and trans-2,x-dimethylcyclohexanones (x = 2-6) have been collected using a resonance-enhanced multiphoton ionization (REMPI) spectroscopic technique. Two photons (390-410 nm) were employed to reach the resonant Rydberg state while absorption of a third photon ionized the molecules via a 2 + 1 REMPI process. The gaseous samples were diluted in an argon carrier gas and cooled to less than 40 K in a supersonic expansion from a pulsed valve. The resulting spectra produced sharp transition origins (fwhm of 5 cm-1). All the onset energies could be accounted for by previously derived additivity rules except for those molecules containing an axial 2-methyl group. The analysis of these molecules showed that a simple second tier of rules allows their origins to be predicted as well. All of the data on the methyl- and dimethylcyclohexanones, which includes 24 structures having transition origins over an 1800-cm-1 range, have been analyzed using a linear regression analysis. The correlation coefficient for predicted and measured transition energies is 0.9954. The extension of this analysis to more complex heterocyclic ketones and ethers is discussed.