Solution and Solid State NMR Studies of the Structure and Dynamics of C₆₀ and C₇₀

Abstract

We have Investigated the structure and dynamics of C 60 and C 7 0 with 13 C NMR spectroscopy. In solution, high-resolution spectra reveal that Co 0 has a single resonance at 143 ppm, Indicating a strained, aromatic system with high symmetry. This is strong evidence for a Co 0 "soccer ball" geometry. A 2D NMR INADEQUATE experIment on 13 C-enriched C 70 reveals the bonding connectivity to be a linear string, in firm support of the proposed "rugby ball" structure with D5h symmetry, and furnishes resonance assignments. Solid state NMR spectra of Co 0 at ambient temperatures yield a narrow resonance, Indicative of rapid molecular reorientation. Variable temperature T 1 measurements show that the rotational correlation time is -10-9s at 230 K. At 77 K, this time Increases to more than I ms, and the 13 C NMR spectrum of Co 0 is a powder pattern due to chemical shift anisotropy (tensor components 220, 186, 40 ppm). At intermediate temperatures a narrow peak is superimposed on the powder pattern, suggesting a distribution of barriers to molecular motion in the sample, or the presence of an additional phase in the solid state. A Carr-Purcell dipolar experiment on Co 0 in the solid state allows the first precise determination of the C 60 bond lengths: 1.45 and 1.40A.The architect and visionary R. Buckminster Fuller developed the concept of synergy, where parts combine to form a system whose effects are greater than those of the mere sum of the pads. 2 He later applied these ideas to develop geodesic structures, which distribute strain efficiently, and hence possess great structural strength. It is therefore not surprising that these ideas found expression in chemistry. In 1985 Smalley, Kroto, and co-workers ascribed a closed-shell, "soccer ball" structure to an observed 60-atom pure-carbon cluster. 3 The great stability of this structure was established five years later by the production of significant amounts of fullerenes. 4-7 The proposed structures 3 , 8 for C 60 and C 70 shown in Figure 1 are carbon shells consisting of hexagons and pentagons; the 12 pentagons in each structure produce torsional strain, creating curvature that leads to closure. Figure 1. The struc_b tures of C 6 o (a) and C 70 (b). All carbons in Co 0 are chemically equivalent, but C 70 consists of five kinds of carbons, labelled a-e in the figure.

Orbital Interactions between a C60 Molecule and Cu(111) Surface

“…Optimized lengths of the 6−6 and 5−6 bonds are, respectively, 1.41 and 1.48 Å, consistent with results of an NMR (1.40 and 1.46 Å, respectively) and an X-ray (1.40 and 1.45 Å, respectively) experiments. The geometry of the C 60 part is fixed in this I h structure in geometry optimizations of the five adsorption models .…”

Statistical incorporation of carbon-13 13C2 units into C60 (buckminsterfullerene)

“…Natural-abundance carbon rods were cored, packed with 99% l3C powder, and converted to C60 in a Smalley-type contact arc *"(3,5) = || -d"(3,5) -t"(3,5) -<?" (3,5) .. ( vaporization apparatus.3 ***Mass spectral analysis of the indicated 5.0 mol % 13C. This material was converted to C60-(Os04)(4-ierf-butylpyridine)27 and analyzed by l3C NMR.…”

“…Carbon types 3 and 5 each couple with three adjacent nonequivalent carbons, and carbon types 4 and 17 each couple with two adjacent nonequivalent carbons. Assuming random 13C distributions, sm dm tm and q" are given by eqs [2][3][4][5] for carbon types 3 and 5 and by eqs 6-9 for carbon types 4 and 17 (x = 2 for type 17, and x = 4 for type 4). The satellite to central peak ratios calculated for random l3C distribution within the carbon clusters using eqs 1-9 and the mass spectrum10 agree very well with the experimental values (Table I).…”