Polymorphism and History of 2-Dimethylsufuranylidene-1,3-indanedione (YLID)

A second, centrosymmetric and monoclinic, form of 2-dimethylsufuranylidene-l,3-indanedione (YLID) has been synthesized and structurally characterized. Theoretical computations suggest that the monoclinic polymorph is ∼2.77 kcal/mol less stable than the noncentrosymmetric orthorhombic form. The molecular parameters of the monoclinic form, orthorhombic polymorph, and theoretically (DFT) optimized geometry of YLID in gas phase are in excellent agreement with each other. The two polymorphs are indefinitely stable in the 100−298 K temperature range, but their molecular packings have little in common. We do not believe an irreversible phase transition takes place upon cooling of the orthorhombic polymorph from 298 to 100 K. Spherically ground crystals of the orthorhombic form of YLID have been used as test crystals for diffractometer alignment and instrument testing by a major commercial diffractometry manufacturer since 1969. A statistical analysis of 223 complete room temperature data sets on orthorhombic YLID yields average cell constants of a = 5.961(4) Å, b = 9.038(4) Å, c = 18.390(9) Å, V = 990.8(13) Å 3 . The average a: b: c axial ratio is 1:1.5161(7):3.0850(15), while the realistic precision of the unit cell lengths determined from complete CCD area-detector data sets is ∼5 parts in 10 4 . Temperature dependence of the unit cell parameters of both polymorphs of YLID is presented. The b axis of the monoclinic polymorph has a negative thermal expansion coefficient.