Orthorhombic polymorph of (2E)-2-(2,3-dihydro-1H-inden-1-ylidene)-2,3-di-hydro-1H-inden-1-one

The title compound, C18H14O, is polymorphic at 123 K. The orthorhombic form reported herein has two independent molecules in the asymmetric unit, with molecular volume 313.5 angstrom(3). The previously reported triclinic (P (1) over bar) form [Raston & Scott (2000). Green Chem., 2, 49-52] has molecular volume 309.6 angstrom(3) at the same temperature. All three molecules deviate significantly and systematically from the putative Cs symmetry (delta(r.m.s). = 0.0265, 0.0256, 0.0497 angstrom). Comparison of the two molecules in the orthorhombic polymorph shows that 16 of the 19 equivalent pairs of framework atoms have a mirror-image pattern of deviations (above/below plane), suggesting that the two are quasi-enantiomorphs. The pattern of deviations in the triclinic form is nearly the same (13 of 19 atom pairs) as the orthorhombic form.