IDENTIFICATION OF CONFORMATIONAL ISOMERS OF METHYL-SUBSTITUTED CYCLOHEXANONE AND TETRAHYDROPYRAN FROZEN IN A MOLECULAR-BEAM

Examination of the n → 3s Rydberg transition in several methyl-substituted cyclohexanones and tetrahydropyrans (THP's) cooled in a free-jet expansion reveals the presence of two distinct molecular species. These have been assigned as pairs of conformational isomers that freeze out in the molecular beam at their room temperature populations as a result of the substantial inversion barrier of the saturated six-membered ring system. Included in this study are 3-methylcyclohexanone, 4-methylcyclohexanone, 3-methyltetrahydropyran, 4-methyltetrahydropyran, and cis-3,4-dimethyltetrahydropyran. The approximate equilibrium concentration for the conformer pairs has been determined from the ratio of peak intensities of the electronic transition origins. The derived ΔG°'s for vapor-phase ring inversion are compared to similar ΔG°'s for the liquid phase. © 1990 American Chemical Society.