The Structures of Difluorodiisocyanatomethane, CF2((NCO)2:  X-ray Crystallography, Gas Electron Diffraction, and Quantum Chemical Calculations

Difluorodiisocyanatomethane was prepared by reaction of difluoromalonyl chloride with trimethylsilyl azide. Its molecular structure was determined by X-ray crystallography at 141 K and by gas electron diffraction (GED), and quantum chemical calculations were performed at different levels of theory. Difluorodiisocyanatomethane crystallizes monoclinic, space group P21, a = 7.900(4), b = 4.890(3), c = 12.601(7) Å, β = 102.280(10)°, V = 475.7(5) Å3, R 1 = 0.0429, wR 2 = 0.1179. The asymmetric unit consists of two molecules with C 1 symmetry that are enantiomers. They differ only by the sign of related dihedral N−C−NC angles, which describe the orientations of the two NCO groups: Φ1 = 107.2(2)°, Φ2 = − 4.8(2)° for molecule A and Φ1 = −103.9(2)°, Φ2 = 8.9(2)° for molecule B. The GED analysis results in a mixture of two conformers, 72(12)% possessing C 1 symmetry (Φ1 = 131(4)°, Φ2 = 43(5)°) and 28(12)% possessing C 2 symmetry (Φ1 = Φ2 = 52(8)°). Bond lengths and bond angles in the solid state and in the gas phase are very similar, but dihedral angles differ by almost 50°. Quantum chemical calculations (HF, MP2, B3PW91, and B3LYP with different basis sets) reproduce the conformational composition, bond lengths, and bond angles very well. Predicted dihedral angles, however, depend strongly on the computational method and none of the calculations reproduces the experimental gas-phase values satisfactorily. They demonstrate, nevertheless, that the potential surface for internal rotation around the two C−N bonds is very flat.