Thermochemical and theoretical studies on cyclohexanediones

The following standard molar enthalpies of formation in the gaseous state at 298.15 K were determined from the enthalpies of combustion of the crystalline solids and the enthalpies of sublimation: 1,3-cyclohexanedione, -335.6 ± 1.6 kJ mol^-1; 1,4-cyclohexanedione, -332.6 ± 1.2 kJ mol^-1; 4,4-dimethyl-1,3-cyclohexanedione, -400.4 ± 2.7 kJ mol^-1; and 5,5-dimethyl-1,3-cyclohexanedione, -383.6 ± 1.9 kJ mol^-1. Ab initio calculations were made of the enthalpies of isomerization of the three cyclohexanediones and of the isomerization of 1,3-cyclohexanedione to its enol forms: the keto form of 1,3-cyclohexanedione in the gaseous state was predicted to be energetically more stable than the enol form by 18.0 kJ mol^-1. NMR spectra, however, showed the compound in the crystalline state to exist in the enol form with the molecules held together by intermolecular hydrogen bonds. The enthalpy of formation of 1,2-cyclohexanedione in the gaseous state, not amenable to experimental measurement, was predicted to be -300 ± 5 kJ mol^-1, and the conventional strain energies in the three cyclohexanedione isomers were estimated to be of similar magnitude, ca. 13 kJ mol^-1.