Non-statistical chemical reactions: The isomerization over low barriers in methyl and ethyl cyclohexanones

At room temperature, methyl cyclohexanone exists in two dominant conformations in which the methyl group is either axial or equatorial to the six membered ring in the chair conformation. In addition to the axial and equatorial conformations, the ethyl cyclohexanones have several ethyl rotor positions. Both theoretical and experimental information indicate that the barriers for axial-equatorial and for the ethyl rotor interconversion are on the order of 4-5 kcal/mol. According to the transition state theory (TST), such low barriers lead to interconversion rates of 10(9) sec(-1). Yet, it has been demonstrated that the room-temperature concentrations are frozen out during the cooling in a pulsed supersonic expansion. This means that vibrational relaxation is much more rapid than interconversion of the various conformations. The analysis of the results indicates that the axial-equatorial or ethyl rotor interconversion rates must be at least 3 orders of magnitude less than predicted by the TST. It is proposed that at the low energies associated with these reactions, most of the vibrational oscillators are in their ground states in which the anharmonic coupling to other modes is minimal. As a result, intramolecular vibrational redistribution (IVR) is insufficient to permit this reaction to proceed at its statistically expected rate. It is also noted that the same reaction in solution phase appears to proceed at the statistical rate which suggests that the participation of the solvent modes enhances IVR.