Gas-phase measurement of ΔH0 between axial and equatorial conformations of 3-methylcyclopentanone Original Research Article

The energy difference between axial-methyl and equatorial-methyl isomers of 3-methylcyclopentanone (3MCPO) was measured for the first time using resonance-enhanced multiphoton ionization spectroscopy. Variable temperature molecule beam spectroscopy was used to determine a ΔH0 of 1.19±0.14 kcal/mol which is considerably lower than the corresponding value for 3-methylcyclohexanone. This is because the 1,3-diaxial interaction between the methyl group and hydrogen atom is smaller in 3MCPO. The transition state for interconversion between the isomers of 3MCPO is calculated to lie 1.4 kcal/mol above the axial conformer energy. In spite of this small activation energy, the van’t Hoff plot shows that vibrational cooling of the conformers is much more efficient than isomerization during the supersonic expansion up to the highest temperature investigated. This means that in the molecular beam expansion, the two conformers are frozen out instantaneously.