Absolute entropies, conformation, and debye temperatures of bicyclo[2.2.2]octane- and of bicyclo[3.2.2]nonane-type molecules

The spectroscopic entropies of bicyclo[2.2.2]octane (BCO), 1-azabicyclo[2.2.2]octane (ABCO), 1,4-diazabicyclo[2.2.2Joctane (DABCO), 3-oxabicyclo [3.2.2 Jnonane (OXBN), and 3-azabicyclo [3.2.2 Jnonane (AZBN) were calculated. For AZBN the ir spectra between 200 and 4000 cm^-1 are reported, together with a calculation of frequencies of the skeletal normal modes and the unit cell dimensions and space group of the high- and low-temperature crystalline phases (at 20°, orthorhombic, Aba2 with a = 21.3, b = 11.3, and c = 6.15 Å, Z = 8; at 50°, face-centered cubic, a = 9.47 Å, Z = 4) obtained by X-ray diffraction. The conclusions reached from the comparison between calculated and experimental third-law entropies are that there is residual entropy in the low-temperature phase of ABCO and OXBN but not in BCO and AZBN, and that the calculated frequency for the torsional mode of these compounds, ∼60 cm^-1, should be at least 15% higher for the bicyclo-octanesi and at least 15% smaller for the bicyclononanes. Furthermore, an analysis of the previous information in conjunction with the published values and the interpretation of the solid-solid phase transition entropy increments of these compounds leads us to propose the following symmetries for the molecular conformations: D_3h for BCO, C₃ for ABCO, and C₃ for OXBN (with the two twisted forms coexisting at all temperatures), and C₁ for AZBN. Finally, the experimental entropy vs. temperature curves are analyzed in terms of temperature-dependent Debye temperatures. The resulting curves, having a pronounced downward slope before the transition temperature, are regarded as characteristic of plastic crystals and as evidence of the onset of molecular disorder before the transition.