A structural investigation of the four phases of 7-oxabicyclo[2.2.1]heptane

Powder synchrotron X-ray diffraction studies confirm that 7-oxabicyclo[2.2.1]heptane, (oxanorbornane), forms three solid phases on cooling and four on warming, as previously indicated by spectroscopic and DSC meas urements. The two highest temperature phases are orien tationally disordered with phase I being face-centred cubic (a = 8.5469(1) Å at 240 K) and phase II primitive cubic (a = 10.60558(4) Å at 194 K). Phase III, which appears over a narrow temperature range on warming, is monoclinic, space group P21/a, Z' = 4, with a = 21.858(4) Å, b = 10.964(2) Å, c = 9.333(1) Å β = 90.491(8)° at 185 K. Phase IV, which occurs below 175 K, is monoclinic, space group C2/c, Z' = 3, with a = 13.6372(2) Å, b = 10.2593(1) Å, c = 24.0466(3) Å, β = 102.5212(4)° at 50 K. The structures of phases III and IV are ordered and have been solved from the powder diffraction data by direct-space methods, then refined by the Rietveld method, restraining the molecules to the known structure, which has m2m (C2v) symmetry.