THERMOCHEMISTRY OF BRIDGED-RING SUBSTANCES - ENTHALPIES OF FORMATION OF SOME DIAMONDOID HYDROCARBONS AND OF PERHYDROQUINACENE - COMPARISONS WITH DATA FROM EMPIRICAL FORCE-FIELD CALCULATIONS

The enthalpies of combustion and of sublimation of nine bridged-ring hydrocarbons of the diamondoid class and of pcrhydroquinacene have been determined. The derived solid and gas-phase enthalpies of formation in kcal/mol at 298.15 K follow : adamantane, -46.02 ± 0.07, -31.76 ± 0.32; 1-methyladamantane. -56.72 ± 0.30, -40.57 ± 0.34; 2-methyladamantane, -51.80 ± 0.29, -35.66 ± 0.62 (recommended value -36.89); 1, 3, 5, 7-tetramethyladamantane, -86.54 ± 0.47, -67.15 ± 0.50; protoadamantane, -36.04 ± 0.41,-20.54 ± 0.60; diamantane, -57.80 ± 0.61, -34.87 ± 0.64 (recommended value -34.54); 4-methyldiamantane, -62.51 ± 0.17, -43.53 ± 0.30; 3-methyldiamantane, -62.24 ± 0.48, -37.60 ± 0.58 (recommended value -39.67); 1-methyldiamantane, -59.12 ± 0.78. -39.85 i 0.85; perhydroquinacene, -37.99 ± 0.76, -24.46 ± 0.86. Trends and methyl group increments within the diamondoid series are discussed. Enthalpy differences within the methyladamantanes and methyldiamantanes are compared with existing data from equilibration studies. The enthalpy of isomerization of protoadamantane into adamantane is discussed in the light of indirect estimates from existing experimental data on derivatives in solution. Comparisons are made between the experimental gas-phase enthalpies of formation and those obtained by molecular mechanics calculations based on the MM1 (1971), EAS (1973), WB (1977), and MM2 (1977) force field models. With the exception of perhydroquinacene, the MM2 force field gives predictions in excellent agreement with the experimental data. The enthalpy of formation of perhydroquinacene is discussed in the light of experimental data for cyclopentanc and m-bicyclo[3.3.0]octane and force field predictions for peristylane and the unknown hydrocarbon dodecahedrane. © 1979, American Chemical Society. All rights reserved.