Temperature phase changes in solid bicyclo[3.3.1]nonane-2,6-dione and bicyclo[3.3.1]nonane-3,7-dione from powder X-ray diffraction data

Abstract

The crystal structures of bicyclo[3.3.1]nonane-2,6-dione and bicyclo[3.3.1]nonane-3,7-dione have been solved by direct methods and by direct-space simulated annealing, respectively, from powder synchrotron X-ray diffraction data. Both compounds have a transition to a face-centred-cubic orientationally disordered phase (phase I) near 363 K, as shown by differential scanning calorimetry and powder diffraction measurements. Phase II of bicyclo[3.3.1]nonane-2,6-dione, which occurs below 363 K, is monoclinic, space groupC2/c, witha= 7.38042 (4),b= 10.38220 (5),c= 9.75092 (5) Å and β = 95.359 (1)° at 80 K. Phase II of bicyclo[3.3.1]nonane-3,7-dione, which occurs below 365 K, is tetragonal, space groupP41212, witha= 6.8558 (1) andc= 16.9375 (1) Å at 100 K. This phase coexists in a biphasic mixture with a minor monoclinic phase II′ [a= 11.450 (6),b = 20.583 (1),c= 6.3779 (3) Å, β = 94.7555 (5)°, at 100 K] detected in the sample, which impeded indexing with standard programs. The crystal structures of phases II were solved by direct methods and by direct-space simulated annealing, employing powder synchrotron X-ray diffraction data of increased instrumental intensity and resolution from the ID31 beamline at the ESRF, and novel indexing algorithms.Ab initiomolecular orbital calculations on the two systems are reported. In the solid state, the molecules pack in chair–chair conformation; molecular structures and packing are discussed.