Structure of C₆₀F₃₆: A Gas-Phase Electron Diffraction and Quantum Chemical Computational Study of a Remarkably...

The equilibrium molecular structure of the gaseous fluorofullerene C₆₀F₃₆ 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 C₁ symmetry, 81(4)%, with a small amount of the isomer of C₃ symmetry, in good accordance with HPLC-MS (atmospheric pressure photoionization), HPLC-UV/vis, and NMR spectroscopic data. The presence of the isomer of T symmetry, up to 5%, cannot be completely excluded. Theoretical structural parameters of the C₆₀F₃₆ molecule were compared with those of the C₆₀F₄₈ molecule. Relative to C₆₀, the C₆₀F₃₆ 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_π(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.