Molecular Structure of Free Molecules of the Fullerene C 70 from Gas-Phase Electron Diffraction

Electron-diffraction patterns of the fullerene C 70 in the gaseous state at 810−835 °C have been recorded in experiments similar to those recently described for C 60 . The radial distribution curve calculated from the scattered intensity is entirely consistent with a molecule of D 5 h symmetry. With assumption of this symmetry, 12 parameters are required to specify the structure. Reliable values are thus much more difficult to obtain for these parameters than for C 60 whose structure is completely defined by only two parameters. Six different models were found that give excellent fits to the diffraction data. The models may be divided into two types characterized either by a shorter (1.4+ Å) or a longer (1.5+ Å) equatorial bond. Despite this difference, however, the average length of the eight bonds is similar in all models (1.434 Å; average deviation 0.006 Å). Since no model could be favored on the basis of the electron-diffraction data alone, a best model was selected from considerations of theoretical energies (BP86/TZP level of density functional theory) and by comparison of computed 13 C NMR chemical shifts (gauge-including atomic orbitals, GIAO-SCF/TZP) with those from experiment. This model is in good agreement with structures determined in the crystal by neutron and X-ray diffraction, and with ab initio calculated structures (BP86/TZP), with one important difference: the equatorial bond is some 0.06 Å longer. Based on assumed D 5 h symmetry, and designating the five circles of atoms from the top (capping) pentagon to the equator as a, b, c, d, and e, the bond lengths ( r a /Å) are as follows: r (a−a) = 1.461(8), r (a−b) = 1.388(16), r (b−c) = 1.453(11), r (c−c) = 1.386(25), r (c−d) = 1.468(11), r (d−d) = 1.425(14), r (d−e) = 1.405(13), r (e−e) = 1.538(19). The equatorial diameter of the ellipsoid is 7.178(50) Å, and the distance between the polar pentagons is 7.906(64) Å; quantities in parentheses are 2 esd.