The 2 + 1 REMPI spectra of cyclic ketones in a cold molecular beam. 2. The n .fwdarw. 3s Rydberg transition of methyl-substituted cyclohexanones and cyclopentanones

Abstract

The two-photon excitation of the n ..-->.. 3s Rydberg transition in monomethyl and dimethyl isomers of cyclohexanone and cyclopentanone has been studied by resonance enhanced multiphoton ionization (2 + 1 REMPI). This transition has been found to be highly sensitive to both the orientation and position of methyl substituents on the cyclic ketone ring. Cis and trans configurations of dimethyl isomers show characteristic 3s transition energy shifts which are additive in nature and can be used to predict the transition energy of other structures. The spectroscopic energies of configurational isomers appear to reflect relative ground-state stabilities. This correlation has been supported by thermodynamic data and molecular mechanics calculations of ground-state energies. Laser polarization effects are found to be useful in determining the degree of chromophore asymmetry in chiral molecules.