and
Fachbereich Biologie, Chemie, Pharmazie, Institut für Chemie-Anorganische und Analytische Chemie, Freie Universität Berlin, Fabeckstrasse 34-36, D-14195 Berlin, Germany, Fachbereich Biologie, Chemie, Pharmazie, Institut für Chemie-Kristallographie, Freie Universität Berlin, Takustrasse 6, D-14195 Berlin, Germany, and Institut für Physikalische und Theoretische Chemie, Universität Tübingen, Auf der Morgenstelle 8, D-72076 Tübingen, Germany
Received: February 23, 2000
In Final Form: May 9, 2000
Abstract:
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, 
=
102.280(10)
, V = 475.7(5) Å3, R1 = 0.0429, wR2 = 0.1179. The asymmetric unit consists of two molecules
with C1 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: 
1 = 107.2(2), 2 = - 4.8(2) for molecule
A and 1 = -103.9(2), 2 = 8.9(2) for molecule B. The GED analysis results in a mixture of two
conformers, 72(12)% possessing C1 symmetry (1 = 131(4), 2 = 43(5)) and 28(12)% possessing C2
symmetry (1 = 2 = 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.