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Cross-Polarization of C,, and C7, in the Presence of Organic Impurities John V. Hama* and Michael A. Wilson CSIRO Division of Coal and Energy Technology. P.O. Box 136, North Ryde, NSW 21 13, Australia (Received: April 23, 1992; In Final Form: June IO, 1992)
It is demonstrated that the IH-W cross-polarization of Csoand C7,,fullerenes is possible, with the protonated species facilitating this process being the toluene and aliphatic impurities residing within crude preparations. Both "C cross-polarization,magic angle spinning and 'H combined rotational and multiple-pulse spectra techniques prove definitive in the characterization of these protonated species. The weak average dipolar couplings extending t o the rapidly moving fullertnes through space yield very unusual magic angle spinning frequency-dependent cross-polarization dynamics. The substantial TCHdifferences measured for each of the inequivalent Ca and CT0sites are discussed in terms of the relative frequency and anisotropy of motion for each molecule.
Introduction Crude fullerene mixtures consisting of approximately 90% CsoI and 10%C,,,and other higher fullerenes are routinely prepared from the soot formed from arcing graphite'-3 or coal-derived material4in inert atmospheres. The soot is extracted with toluene, and the crude Cbois isolated by evaporation. However, it is not *To whom correspondence should be addressed.
generally appreciated that toluene is strongly absorbed by C , and, not being easily removed by drying at elevated temperatures, is thus consistently present in crude preparations. We have also observed that small amounts of aliphatic materials may also be a constituent of these crude preparations. These trace quantities of toluene and aliphatic impurities will not be detected in solid-state singlepulse "C NMR spectra because the absence of high-power dipolar IH decoupling renders the I3C line widths of such species
0022-365419212096-65 18$03.00/0 0 1992 American Chemical Society
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too broad to be observed. We now report that I3C cross-polarization: magic angle spinning6 (CPMAS) spectra of Cso and C70 can be obtained from dipolar contact with trace amounts of toluene and/or aliphatic impurities and that these spectra also contain resonances from these impurity species; it is also shown that high-resolution 'H combined rotational and multiple-pulse spectra (CRAMPS)7of these impurities, obtained with use of the BR-24 sequence,8 are extremely useful in characterizing the protonated species facilitating cross-polarization, Moreover, "C CPMAS studies of the crass-polarization dynamics throw light on the nature of the molecular motion of Cb0and C70.
Experimental Section 13C CPMAS and IH CRAMPS spectra were obtained on a Bruker MSL 400 spectrometer operating at the 13Cand 'H frequencies of 100.625 and 400.13 MHz, respectively. For the 13C CPMAS variable &tact studies, a recycle delay of 3 s and a 'H 90° pulse length of 3.5 M were used on a 7-mm doubleair-bearing CPMAS probe spinning at rotational frequencies up to 4.3 kHz. A total of 5000 transients were accumulated at each variable contact period, which were comprised of 2K data points collected over an acquisition period of 20 ms. Other experimental conditions are listed in the captions of Figure 1. Spin temperature alternation9 and quadrature phase cycliigl0 of the CPMAS experiment were used to eliminate spectral artifacts and the detection of directly produced I3C magnetization. Nevertheless, it was necessary to establish that the observed I3C signal actually emanated from the cross-polarization phenomenon and not from carbon magnetization directly generated during the Hartmann-Hahn contact period. This was verified by disabling the proton decoupler during the contact period, after which no signal acquisition resulted. 'H CRAMPS spectra were obtained using the BR-24 sequence employing a 'H 90° pulse length of 1.5 ps, a dead time interval of 2.5 ps, and a magic angle spinning frequency of 2.5 kHz which were achieved using a 7-mm double-air-bearing CRAMPS probe. A repetition rate of 10 s was used for all CRAMPS studies allowing for full IH relaxation and probe recovery, with at least 256 transients being averaged for each free induction decay which comprised 128 data points. 'H and I3C chemical shifts were both externally referenced to tetramethylsilane. Results and Discussion The static solid-state single-pulse I3C NMR spectrum of the fullerene Cm at 15 MHz (Le., without CPMAS) consists of one sharp line due to its unique Zh symmetry's2 and rapid molecular rotation, exhibiting motional correlation times of
