Conformational and Energetic Analysis of Saturated Organic Ring Compounds by 2 + 1 Resonance-Enhanced Multiphoton Ionization Spectroscopy

The UV transition to the 3s Rydberg state in a number of saturated cyclic compounds has yielded a wealth of structural and energetic information. The HOMO → 3s spectra of pulsed molecular beam cooled samples can be conveniently investigated by 2 + 1 resonance-enhanced multiphoton ionization (REMPI) using pulsed laser light in the 360−420 nm region. The transition origins for a variety of methyl- and ethyl-substituted cyclohexanes, cyclohexanones, cyclopentanones, and tetrahyrdopyrans, as well as bicyclic compounds such as decalone and various camphors, are highly sensitive to the location of alkyl substitution as well as the orientation of the alkyl group (axial vs equatorial). In addition, the orientation of the ethyl rotor can be determined from the spectra. This technique is especially useful for investigating molecules in which several equilibrating conformations make the analysis of room-temperature spectra difficult. By rapid cooling in the molecular beam, higher energy conformers can be frozen out and investigated individually. By varying the temperature of the nozzle, it is possible to alter the conformational equilibria and to determine the Δ H ° for the various structures. These studies demonstrate that, in many cases, the concentrations of the various conformations are cooled so rapidly that no equilibration of their populations is possible, even up to 150 °C. This shows that vibrational relaxation in the molecular beam is much more rapid than the conformer equilibration reaction.