Thermodynamic properties of pentacyclo[5.4.0.0 2,6.0 3,10.0 5,9]undecane, C 11H 14

Abstract

Thermodynamic properties of pentacyclo[5.4.0.0 2,6.0 3,10.0 5,9]undecane have been determined. The heat capacity was measured by vacuum adiabatic calorimetry ( T= 5.0 K to T= 320.6 K) and by the triple heat-bridge method ( T= 300 K to T= 480 K). One solid-to-solid transition was discovered at T= 164.4 K with molar enthalpy of transition: Δ trs H m° = (4.861 ± 0.038)kJ·mol −1. The fusion temperature is T= 475.8 K, and the molar enthalpy of fusion is Δ fus H m° = (6.38 ± 0.12)kJ·mol −1. Standard molar thermodynamic functions of the crystal obtained from the experimental heat capacity at T= 298.15 K are C sat,m=(184.4 ± 0.7)J·mol −1; Δ T 0 S m° = (212.1 ± 0.9)J·K −1·mol −1; and Φ m° = Δ T 0 S m°− Δ T 0 H m°/ T= (102.7 ± 0.4)J·K −1·mol −1. Comparison of the thermodynamic characteristics of fusion and solid-to-solid transition confirms the existence of a plastic crystalline state in the range T= 164.4 to T= 475.8 K. The entropy of the solid-to-solid transition at T= 164.4 is about 25 per cent of the free-rotation entropy of isolated molecules. The enthalpy of sublimation was measured with a heat-conduction differential microcalorimeter: Δ sub H m°(336.86 K) = (54.71 ± 0.94)kJ·mol −1. The vapor pressure, measured by means of the integral effusion Knudsen method in the range T= 273.21 K to T= 323.40 K, may be expressed by the equation: ln( p/Pa) = (25.74 ± 0.44) − (6598 ± 131)·(K/ T). The weight-averaged value of the molar sublimation enthalpy: Δ sub H m°(298.15 K) = (55.85 ± 1.00)kJ·mol −1, was obtained using Δ g cr C p, m = 49 J·K −1·mol −1. The third-law standard molar entropy at the pressure p= 101 325 Pa is (333.6 ± 3.4)J·K −1·mol −1, based upon the weight-averaged value of the molar sublimation enthalpy. The massic energy of combustion in oxygen: Δ c u° = −(43332.2 ± 31.3)J·g −1, and the molar enthalpy of combustion: Δ c H m° = −(6345.2 ± 4.8)kJ·mol −1, were determined from the results of five experiments. The standard molar enthalpy of formation in the crystalline state: Δ f H m°(cr,298.15K) = (15.8 ± 4.9)kJ·mol −1. The standard molar enthalpy of formation in the gas state (obtained by using the weight-averaged sublimation enthalpy) is Δ f H m°(g,298.15 K) = (71.7 ± 5.0) kJ·mol −1. It was shown that the total strain energy simply reflects the sum of the strain energies in the independent rings which comprise the compound. I.r. and Raman spectra were recorded, and a vibrational analysis was performed on the spectral quantities. The standard thermodynamic properties in the ideal-gas state were calculated in the temperature range 100 K to 1000 K. The calculated standard molar entropy at T= 298.15 K: Δ T 0 S m°( T, g) = 330.77 J·K -1·mol -1, is in satisfactory agreement with the experimental value.