Conformational enthalpies, Δ H°[axial/equatorial], of 3- and 4-methylcyclohexanone

Abstract

Methylcyclohexanone exists primarily in its most stable chair form at ambient temperatures. Two chair conformers exist in which the methyl group may be either axially or equatorially oriented. The conformational energy difference between these is dominated by non-bonded interactions. Several theoretical methods were undertaken to model the conformational enthalpy, Δ H°, including molecular mechanics (MM), ab initio molecular orbital, and density functional theories (DFT). Ab initio methods include HF/6-31G*, HF/6-311G**, HF/6-311+G(3 df,2 p), MP2-fu/6-31G*, MP2-fu/6-31+G*, and G2(MP2) levels of theory. A hybrid theoretical model, G2(MP2,SVP), was found to give nearly identical results to G2(MP2) theory. The DFT methods included B3-LYP/6-31G*, B3-LYP/6-311+G(3 df,2 p), B3-PW91/6-31G*, and B3-PW91/6-311+G(3 df,2 p). The theoretical results are compared with recent gas-phase experimental measurements from our laboratory. The conformational energy between the more stable equatorial conformer and the less stable axial conformer has been measured by resonance-enhanced multiphoton ionization spectroscopy to be 1.55±0.12 and 2.1±0.2 kcal mol −1 for 3- and 4-methylcyclohexanone, respectively. There is excellent agreement between these experimental values and the MM and DFT methods. A less satisfactory agreement is made for the MP2 and G2(MP2) methods.