Structure of CF: A Gas-Phase Electron Diffraction and Quantum Chemical Computational Study of a Remarkably Distorted Fluorofullerene.

The equilibrium molecular structure of the gaseous fluorofullerene CF has been determined for the first time by the electron diffraction method with the use of quantum chemical calculations up to the RI-MP2/def2-TZVPP level of theory. Vibrational amplitudes and quadratic and cubic force constants were calculated by density functional theory methods. It was found that the sample under study consists of the isomer of symmetry, 81(4)%, with a small amount of the isomer of symmetry, in good accordance with HPLC-MS (atmospheric pressure photoionization), HPLC-UV/vis, and NMR spectroscopic data. The presence of the isomer of symmetry, up to 5%, cannot be completely excluded. Theoretical structural parameters of the CF molecule were compared with those of the CF molecule. Relative to C, the CF molecule has a remarkably distorted carbon cage because of steric, electrostatic, and orbital interactions. This results in the longest carbon-carbon bond (1.671 Å) found in free molecules. In particular, about the longest FC-CF bond, the dihedral angle is only around 20°, which leads to the very short nonbonded distance between electronegative vicinal fluorine atoms (2.531 Å) that is much shorter than the sum of van der Waals radii of fluorine atoms (2.94 Å). A natural bond orbital analysis revealed that strong n(F) → σ*(FC-CF) interactions delocalize the lone pair of π-type at the fluorine atoms into the antibonding orbital of the FC-CF bond. This hyperconjugation results in additional elongation of FC-CF bonds.