Electron-Diffraction Investigation of the Fluorofullerene C_(60)F_(48)

摘要

The structure of the fullerene C_(60)F_(48) has been investigated in the gas phase by electron diffraction from a sample volatilized at 360 ℃. The analysis was carried out under two assumptions: (1) the molecules have either D_3 or S_6 symmetry as suggested by NMR spectroscopy and verified by an X-ray study of the crystal, and only one of these is present in the gas; (2) all carbon-fluorine bonds have the same length. With the named symmetries, the structure of the carbon skeleton may be defined by the positions of 10 atoms forming two pentagons, one near the top of the molecule and one near the equator, and the locations of the fluorine atoms obtained as the resultant of three vectors originating from carbons not involving a double bond. Simultaneous refinement of the large number of geometrical parameters (30 for the carbon skeleton and 17 for the fluorines) either failed to converge or yielded implausible values, but successive refinements of small groups of four or five parameters were successful. Dozens of groups were tested and all of the resulting models gave satisfactory fits to the observed diffraction patterns. Although values of individual parameters in these models might differ appreciably, the values obtained as averages from the many refinements have good precision. Some of these averaged results (r_a/A, ∠/deg) for the D_3/S_6 models, with estimated standard deviations, are the following: r(C-F) = 1.368(1)/1.368(1); r(C=C) = 1.327(3)/1.326(4); r(C_(sp)~2-C_(sp)~3) = 1.503(15)/1.500(11); r(C_(sp)~3-C_(sp)~3) = 1.585(44)/1.585(41); (C-C=C) = 113.7(4)/113.6(4) and (C-C-C) = 105.5(1)/105.5(2) within pentagons; and (C-C=C) = 124.2(3)/124.0(4) and (C-C-C) = 116.6(3)/116.5(3) within hexagons. The average distances from the center of the cage (spherical radii) are quite different for the three types of carbon atoms (those in a double bond, those adjacent to a double bond, and those not adjacent to a double bond) and quite different from the C_(60) value of 3.555A for all atoms. For symmetries D_3/S_6 these radii (R/A) are 3.937(23)/3.937(17) for sp~3 atoms not bonded to sp~2 ones and 3.781(18)/3.778(20) for sp~3 atoms bonded to sp~2 ones. The average radii to the sp~2 atoms are much shorter than those to the other atoms. These radii fall into two groups for each symmetry: for symmetry D_3 they are 3.018(14) and 3.190(15) A, and for S_6, 3.017(11) and 3.180(15) A. The surprising length of some of the carbon-carbon bonds and other features of the structures relative to the structure of C_(60) are discussed.