Thermochemical Studies on Double- and Triple-layered [2.2]Paracyclophanes. Estimation of Molecular Strain Energies.

Abstract

Abstract The measurements of enthalpies of combustion by use of precision oxygen-bomb combustion calorimeter together with the measurement of carbon dioxide recovery were made for double- and triple-layered [2.2]paracyclophanes. The enthalpies of sublimation were also determined by the measurement of temperature dependence of the vapor pressures for the two compounds. The following values are reported for the standard enthalpy of combustion ΔHco(298.15 K)/kJ mol−1 and standard enthalpy of sublimation ΔHso(298.15 K)/kJ mol−1, respectively: double-layered [2.2]paracyclophane, 8729.48±1.80, 99.6±1.8; triple-layered [2.2]paracyclophane, 14230.87±3.59, 125.9±2.5. From analyses of the derived standard enthalpy of formation in gaseous state the following two values for total strain energy Us⁄kJ mol−1 of a molecule of triple-layered [2.2]paracyclophane were obtained: 251.9±5.6 with group method and 245.6±9.8 with chemical similarity method, respectively. These values are approximately twice the magnitudes of the corresponding quantities for double-layered [2.2]paracyclophane. From the analysis of the total strain energy by use of the result of Boyd’s molecular mechanical calculation, the contribution from the twisting of the inner benzene ring to the total strain energy of triple-layered [2.2]paracyclophane was estimated to be almost twice that from one outer bent benzene ring.