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: D3h 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry