J. Phys. Chem. 1995, 99, 3365-3370
3365
Detection of Fulleroid Sites in Fullerene-60 by High-Resolution Solid-state 'H NMR Waclaw Kolodziejski,*tt Avelino Coma$ Jamie Barras? and Jacek Hinowskis Instituto de Tecnologia Quimica UPV-CSIC, Universidad PolitCcnica de Valencia, Avda. de Los Navanjos, s/n., 46022 Valencia, Spain, and Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, U.K. Received: May 12, 1994; In Final Form: September 21, 1994%
'H solid-state NMR with magic-angle spinning (MAS NMR) detects dilute H-containing species in chromatographically purified CM. Because of the weakness of the 'H-'H dipolar couplings, the spectral peaks are very sharp and the TI values are not averaged by spin diffusion. On the basis of chemical shifts and peak connectivities as revealed by COSY, we have identified various fulleroids, as well as toluene and dioctyl phthalate (DOP) impurities. The fulleroids contain HC=CHCHzCH3, HC=CHCH3, and HC=CH groups. DOP is probably complexed to c 6 0 via the aromatic moiety. Laboratory and rotating-frame spindiffusion experiments show that the H-containing species are so diluted by bulk c 6 0 that they cannot communicate by intermolecular spin diffusion. The intramolecular spin-diffusion paths suggest that the alkyl chains of DOP are folded back toward the aromatic ring. We have found that two-dimensional 'H NMR techniques work well in the solid state without a multiple-pulse homonuclear decoupling, provided that the species under study are sufficiently dilute by a nondipolar medium.
cubic (FCC) and hexagonal close-packing (HCP) lattices, and solvent molecules are probably trapped in the stacking fault^.^^^^ Our knowledge about fullerene-60 (CW),'-~the soccer-ballWe show that organic molecules as well as fulleroid species shaped allotrope of carbon, is growing rapidly. The material present in chromatographically purified C a can be conveniently is promising as a reagentlo and as the main component of highobserved using 'H MAS NMR. The system is unique because temperature s u p e r c o n d ~ c t o r s . ~ ~Both - ~ ~ of these applications protons are very dilute and thus the 'H-" dipolar interactions require very pure c60. are weak. It follows that MAS alone is capable of producing Since fullerene is supposed to contain only carbon, earlier very sharp lines so that multiple-pulse narrowing techniques NMR studies were exclusively limited to 13C.14-19At room are unnecessary. Under such conditions, two-dimensional (2D) temperature and in the absence of MAS, Cm gives a single, NMR methodszz such as spectral spin d i f f ~ s i o n ~and ~-~~ symmetric, and relatively narrow peak at 143 ppm (FWHM of COSYz6-28work very well. The common feature of the two 500 and 70 Hz at fields of 9.4 and 1.4 T, re~pectively),'~~'~ methods is that both rely on the homonuclear correlation of implying fast isotropic molecular reorientation. Below 100 K, chemical shifts; conclusions are drawn from the so-called crossthis motion slows down and a typical powder pattern is peaks. However, the spectra differ with respect to the type of o b ~ e r v e d . ~ ~The . ' ~ chemical shift tensor with the principal spin-spin interactions which give rise to the cross-peaks. Spin components of 220, 186, and 40 ppm at 77 K confirms the diffusion, which proceeds through flip-flop spin transitions, aromatic nature of the c 6 0 carbons.15 c 6 0 isotopically enriched relies on through-space dipolar interactions, COSY on electronto 6% in 13Cgives at 77 K two Pake doublets superimposed on mediated couplings via the chemical bonds. COSY is well a huge peak from isolated spins.16 The size of the dipolar suited to liquids, and we have reviewed its performance in s littings yields the carbon-carbon distances of 1.45 and 1.40 solids.2g Two-dimensional spectral spin diffusion in the laborafor 5-6 and 6-6 ring fusion bonds, respectively, in tory frame (LF), formally analogous to the NOESY experiment accordance with the truncated icosahedral structure of C~I).'~ in is well established in various areas of solid-state Details of the molecular motion in c 6 0 were obtained from ~hemistry.~'-~* A corresponding experiment in the rotating longitudinal 13Crelaxation studied at various magnetic fields17 frame (RF) has been p r o p ~ s e d . ~The ~ , rate ~ ~ of spin diffusion and as a function of temperature.18 At 283 K, the rotational depends very strongly on the internuclear separation r, being correlation time is 9.2 ps, only 3 times longer than in the gas proportional to 1/13 for a rigid crystal lattice and to l/$ for p h a ~ e . ' ~ . The ' ~ vacuum-sublimed material exhibits a phase species undergoing rapid isotropic motion.25 As a result, for transition at ca. 249 K, indicated by the sharp discontinuity in all practical purposes, spin diffusion occurs only between nuclei the plot of T1 vs the temperature.18 X-ray diffraction (XRD) in adjacent functional groups within the same molecule (the indicate^'^ that the structure transforms from simple cubic at intramolecular case) or between nuclei in neighboring molecules low temperatures to face-centered cubic at high temperatures.z0 mixed on a microscopic level (the intermolecular case). We In the low-temperature ("ratchet") phase, there is jump rotational elucidate the nature and distribution of the H-containing sites diffusion between symmetry-equivalent orientations with the in c 6 0 using advanced solid-state NMR techniques, including activation energy of 4.5 kcal mol-', and in the high-temperature the first 'H COSY in the solid state. ("rotator") phase, there is free rotation or rotational diffusion with the activation energy of 1.4 kcal m01-1.18919Crystals of Experimental Section chromatographically purified c 6 0 are a mixture of face-centered The sample of c60, prepared from arc-processed carbon7 and chromatographed using toluene/activatedcharcoal, was supplied t Universidad Polit6cnica de Valencia-CSIC. by Alpherantz Research, Cambridge, U.K., and characterized University of Cambridge. by X-ray diffraction (Figure 1). Abstract published in Advunce ACS Abstracts, February 15, 1995.
Introduction
H
@
0022-365419512099-3365$09.00/0
0 1995 American Chemical Society
3366 J. Phys. Chem., Vol. 99, No. IO, 1995
Kolodziejski et al.
2.
'2 P -c
=
o
10
20
30
40
60
2 e (degress )
Figure 1. X-ray diffraction pattern of chromatography purified Cm
(Cu Ka radiation) indexed to the FCC lattice with a0 = 14.16 A.
Solid-state 'H and 13C MAS NMR spectra were recorded at room temperature on a Varian VXR-400 WB spectrometer at 400.0 and 100.6 MHz, respectively, using a Doty 5-mm MAS probe with zirconia rotors driven by dry air. The magic angle was set precisely by observing the 79Brresonance from KBr. Instrumental spectral resolution dependent on shimming, magicangle setting, and spinning stability was better than 5 Hz, as determined from the 13C CP/MAS spectrum of adamantane. One-dimensional (1D) spectra were acquired after single-pulse excitation. The liquid 13C spectrum was acquired on a Chemagnetics CMX-400 spectrometer at 100.6 MHz. We have examined the perfomance of conventional 'H COSY and "longrange" (or "delayed") COSY27 using different t delays. The latter 2D experiment, using the pulse sequence (n/2)-tl -t(~c/2)-z-acquisition(f2), gave the highest cross-peak intensities with t = 20 ms. Two-dimensional 'H LF spin-diffusion experimentsused a standard NOESY pulse program for liquids30 of ( n 2 )-tl -( d 2 )-zm-( d 2 ) -acquisition(t2) and different mixing times tm. The mixng time of 1 s was the most appropriate for bringing out the cross-peaks. Two-dimensional 'H RF spindiffusion experiments used the (d2)-t1-SL-acquisition(t2) pulse sequence,39where SL stands for the spin-lock pulse. The spin-lock field of 7.3 G was the same for each 2D RF experiment, but its duration was varied. The SL time of 80 ms was the most appropriate for bringing out the cross-peaks. Conscious of the fact that spin diffusion must be studied using different mixing (spin-lock) times, we have examined the process very carefully. Since all the cross-peaks were present, albeit with different intensities, for the sake of brevity we show only the best spectra for each mixing time. Acquisition parameters for all the solid-state experiments are given in Table 1. The 2D data were processed in both spectral dimensions with sine-bell-shifted apodization and a magnitude calculation followed by symmetrization. Results and Discussion The 'H spectrum of c 6 0 contains a number of resonances (Figure 2), which will be interpreted on the basis of chemical shifts40 and site connectivities retrieved from the COSY and spin-diffusion experiments (Figures 4,5,7, and 8). The details, together with peak assignments, are given in Table 2. The dominant 'H peaks (Figure 2) result from the contaminants, since all match well those of dioctyl phthalate (DOP, see Scheme l), the plasticizer present in plastic containers in which large amounts of toluene are supplied. We know from the producer that toluene was involved in the chromatographic
TABLE 1: Parameters of the NMR ExDerirnenW 2D 'HNMR 1D NMR spin diffusion parameters 'H I3C COSY LF RF pulse angleb 5.8 5.8 n/4 XI4 8.V 197 38 acquisition time, ms 100 195 38 3 5 5 recycle delay, s 60 3 FZ spectral width, Wz 50 13.4 5.2 13.4 30 5.2 13.4 13.4 F1 spectral width, Wz F2 FID points, K 10 12 2 1 1 F I FID pointsc 512 512 512 F2 FT points: K 2 32 32 4 2 FIFT points! K 4 2 2 no. of scans 96 32 100 2500 16 5 5 MAS rate, Wz 9 6 5 LF and RF denote spin diffusion in the laboratory frame and the rotating frame, respectively. In rad for 1D NMR and ys for 2D NMR. Equal to the number of experiments for the F1 dimension. FID points plus zero-filling. n/2 pulse, spin-lock at the same power level. purification of crude Cm. We have detected DOP by mass spectroscopy in the oily residue left after the evaporation of toluene. In addition, a portion of the Cm sample was repeatedly washed with acetone, the acetone evaporated, and the oily residue examined by 13C NMR. The spectrum (Figure 3) consists of 12 lines, all of which can be unambiguously assigned to DOP by comparison with the spectrum of DOP dissolved in CDC13.41 This unambiguously identifies DOP as the major contaminant of our sample of C a . We note that the aryl carbon resonances of toluene at ca. 137.8 and 125.3 ppm are absent from the spectrum of the oily residue, although the sample of c 6 0 does contain a small amount of toluene (see below). This is due to the thoroughness of the evaporation procedure. Note also that fulleroid species would not be expected to be carried over into the solution, as they are insoluble in acetone. Thus, by comparison with the 'H spectrum of DOP dissolved in CDC13,41the peaks at 0.85, 1.25, 1.60,4.13, 7.30, and 7.58 pprn in our 'H spectrum with the relative intensity ratio ca. 12:16: 2:4:2:2 (Figure 2) have been assigned to CH3, alkyl-CHzalkyl, CH, OCH2, and two kinds of inequivalent aromatic protons, respectively. The assignment is consistent with peak connectivities revealed by COSY (see Figure 4 and Table 2). All DOP peaks are shifted in the same direction by the magnetic susceptibility effect. Note the appreciable differences between the chemical shifts of aromatic and OCHz protons of DOP in c 6 0 and in the CDC13 solution: 7.58, 7.30, and 4.13 ppm vs 7.70,7.50, and 4.30 ppm, respectively. Since the other peaks are less shifted, it follows that the aromatic ring of DOP and the neighbor ester groups are located close to the Cm cage and are subject to large ring-cun-enteffects. Such effects have been postulated for 'H NMR.l0 It is probable that DOP forms an electron donor-acceptor (EDA) complex with c60, which involves n-electrons of both species, thus affecting the chemical shifts through the redistribution of electron density. We have also considered the presence of toluene and found minor resonances, which are tentatively assigned to methyl (2.5 ppm) and aromatic protons (7.02 and 7.09 ppm) of the solvent (Figure 2). The aliphatidaromatic intensity ratio of ca. 3:5 fits well the liquid-state spectrum, although the methyl peak is shifted by 0.2 ppm to high frequency and the aromatic peaks by 0.1 ppm to low frequency. The pattern of two overlapping aromatic peaks is consistent with the deliberately broadened liquid-state spin multiplet of the toluene ring. Both aromatic peaks disappear from the COSY spectrum (Figure 5 ) , probably because the transverse relaxation times are very short. One cannot expect COSY cross-peaks between the methyl and the ortho protons of toluene, because the corresponding scalar
Detection of Fulleroid Sites in Fullerene-60
J. Phys. Chem., Vol. 99, No. 10, 1995 3367 ' H MAS NMR
7.58
7.30
7.6
7.4
7.2
6.8
7.0
4.13
7.0
6.0
5.0
4.0
3.0
2.0
1.o
0
pprn from TMS Figure 2. 'Hspectrum of C a (Lorentzian apodization with LB = 1 Hz). Peak numbers below the plot correspond to those in Table 2. Chemical shifts are given above the peaks. Peaks 9, 10 and 11, 12 are resolved in the COSY spectrum (Figure 5 ) .
coupling constant is less than 1 Hz. Note that the toluene peaks are very small in comparison with the DOP peaks, which indicates that mainly high-mass molecules of DOP are trapped in fullerene. The peaks at 2.13, 1.78, and 1.01 ppm can be interpreted together with the peaks in the 6.5-6.9 ppm region on the basis of chemical shifts and connectivities deduced from the COSY experiment (Figures 4 and 5 ) . In addition, one should bear in mind that scalar couplings through four and more chemical bonds are too weak to produce observable cross-peaks in a solidstate COSY spectrum. We note that the peaks at 1.78 and 2.13 pprn have cross-peaks with the peaks at 6.64 and 6.78 ppm, respectively (Figure 4). The former two peaks can come from alkyl or alkyne protons and the latter two from alkene or aromatic protons. However, the CECH proton (6 = 1.8 ppm) cannot have a three-bond scalar coupling with alkene or aromatic protons, so the peaks at 1.78 and 2.13 ppm must be assigned to alkyl protons. The chemical shift of an alkyl group bound to an aromatic system would not then be below 2.3 ppm, and the alkyl group would not be able to shift an aromatic peak below 7 ppm. It appears that the peaks at 6.64 and 6.78 pprn (Figure 2) come from alkene protons in the >C=CHR groups and that each such resonance corresponds to a different alkyl substituent R. We note that only in this geminal configuration is the alkene proton separated by three bonds from the protons of the alkyl group R, thus allowing the corresponding COSY cross-peaks to be observed. We next consider the two simplest cases in which R denotes CH3 and CH2CH3. The suitable model compounds are H2C=CHCH3 and H~C-CHCHZCH~.The chemical shifts of 1.78, 1.01, and 2.13 ppm in our system agree well with d(CH3) = 1.71 ppm in propene and d(CH3) = 1.00 and d(CH2) = 2.00 ppm in n-butene. Thus, considering their COSY-detected couplings to the adjacent alkyl protons, the peaks at 6.64 and 6.78 ppm should be assigned to the >C=CHCH3 and >C=CHCH2CH3 protons, respectively. The methyl peaks at 1.78 and 1.01 ppm are sharp (Figure 2), because the 'H-lH
dipolar interaction is partially averaged by fast group rotation. COSY cross-peaks between the peaks at 1.01 and 2.13 ppm are absent (Figure 4), probably because the latter is too broad in comparison with the scalar coupling, so that the broad antiphase cross-peak components undergo a destructive interference which degrades the overall cross-peak intensity. The >C=CHR peak positions of the model compounds cannot be directly compared with those from our sample, because the chemical shifts are strongly affected by substituents on the other side of the double bond. Again, the COSY spectrum assists the interpretation by revealing four alkene peaks (6.78, 6.73, 6.64, and 6.61 ppm) with two distinct cross-peaks (Figure 5 ) . As the spectrum is symmetric with respect to the diagonal, we consider only its lower part. Note that the peaks at 6.64 and 6.61 ppm overlap considerably, so one cross-peak can have coordinates (6.78, 6.64) or (6.78, 6.61) and the other (6.73, 6.64) or (6.73, 6.61). Since the peaks at 6.78 and 6.64 pprn have already been assigned to different species, they cannot have a common COSY crosspeak. It follows that the first cross-peak has coordinates (6.78, 6.61). Next, one has to bear in mind that if the second crosspeak had coordinates (6.73, 6.61), the peaks at 6.78, 6.73, and 6.61 ppm would have to come from the same species, such as H1H2C=CHCH2CH3 with two highlighted protons being structurally inequivalent. However, according to the rules of chemical shift additivity, all the alkene protons in such mangement would have chemical shifts between 5 and 6 ppm, which is not consistent with our spectrum. Therefore, the connectivities we are looking for are given by the cross-peaks with coordinates (6.78, 6.61) and (6.73, 6.64). Two such pairs of coupled alkene protons reside in the HC=CHCH2CH3 and HC=CHCH3 sites. We opt for the trans configuration, in which the scalar coupling between vicinal protons is stronger than in the cis isomer, thus rendering the cross-peaks formation more favorable. Moreover, according to the rules of chemical shift additivity, only in the trans isomers of (aryl)HC=CHCHzCH3 and (aryl)HC=CHCH3 groupings do both vicinal alkene protons
3368 J. Phys. Chem., Vol. 99, No. IO, 1995 TABLE 11: Details of the Main Peaks" connectivity spin diffusion no. Nppm Tl/msb COSY LF RF assignment 1 0.85 5 3 3 f 4 1-3 1-3 1-3 CH3 of DOP 1-8 1-8 1-17 1-17 1-18 1-18 (aryl)HC=CHCHnCHj 2 1.01 3 1.25 4 5 2 f 4 3-1 3-1 3-1 CHz of DOP 3-8 3-8 3-17 3-17 3-18 3-18 CH of DOP 4 1.60 4-8 5 1.78 5-10 5-10 5-10 (aryl)HC=CHCH3 6 2.13' 6-12 6-12 6-12 (aryl)HC=CHCHzCHs CH3 of toluene 7 2.5 OCHz of DOP 8 4.13 420% 10 8-4 8-1 8-1 8-3 8-3 9 6.61 9-12 (aryl)HC=CHCHzCH3 10 6.64 10-5 10-5 (aryl)HC=CHCH3 10-11 11 6.73 11-10 (aryl)HC=CHCHs 12 6.78 12-6 12-6 12-6 (aryl)HC=CHCHzCH3 12-9 13 6.81 (aryl)HC=CH(aryl) 14 6.86 (aryl)HC=CH(aryl) 15 7.02 aromatic of toluene 16 7.09 aromatic of toluene 17 7.30 650 f 10 17-18 17-18 17-18 aromatic of DOP 17-1 17-1 17-3 17-3 18 7.58 660f 10 19-18 18-17 18-17 aromaticofDOP 18-1 18-1 18-3 18-3
Kolodziejski et al. 13C NMR
10
200
- 50
0
ppm from TMS
sample of Cm with acetone and evaporating the solvent. ' H COSY
4.0
SCHEME 1
5.0 U
50
100
Figure 3. I3C spectrum of an oily residue obtained by washing our
LF and RF denote spin diffusion in the laboratory frame and in the rotating frame, respectively. DOP stands for dioctyl phthalate (Scheme 1). For assignment, see Results and Discussion. Saturation recovery, MAS at 5 kHz, error bars from the fitting routine. At the top of the complex signal.
0
150
\5
CH2CH3 I
6.0*
C-0C H2CHCH, C H2CH,CH3
Pf
7.0-
u
/'
C - 0 C H2C HCH2C HzCH2CH3 II
0
I
CH2CH3
have chemical shifts over 6 ppm. Unfortunately, ring current effects in fullerene can be appreciable,1° and magnetic susceptibility corrections to the chemical shift should be calculated. As a result, chemical shifts cannot be accurately predicted. We conclude that the peaks at 6.61 and 6.73 pprn should be assigned to the protons geminal to the aryl moiety in the trans isomers of the (aryl)HC=CHCH2CH3 and (aryl)HC=CHCH3 species, respectively. Looking at the structures identified above, one may inquire whether any (aryl)HC=CH(aryl) species contribute to the spectra. In such a case, for both cis and trans isomers, the rules of chemical shift additivity predict ca. 7 ppm as the highest chemical shifts possible for alkene protons. Since the peaks at 6.86 and 6.81 ppm (Figure 5 ) do not have any cross-peaks, it is reasonable to assign them to some symmetric (aryl)HC=CH(aryl) species. An alternative assignment would be to aromatic protons which can resonate in this spectral region. Finally, we attempt to put together all the pieces of the jigsaw puzzle and explain the nature of the (aryl)HC=CHCH2CH3,
ppm from TMS
6.6
0.8
6.6 1.2 7.0 1.6
7.2
7.4 2.0 7.6
7.6
7.4
7.2
7.0
6.6
2.0
6.6
1.6
1.2
0.8
Fl
Figure 4. 'H COSY spectrum of Cm. Top, full range; bottom, expanded regions. The square, triangle, and circle mark cross-peaks
with coordinates (1.60.4.13). (1.78,6.64),and (2.13,6.78),respectively. (aryl)HC=CHCH3, and (aryl)HC=CH(aryl) species. The postulated functional groups are anchored to some aryl moiety, the 'H peaks of which have not been detected. This implies a large
Detection of Fulleroid Sites in Fullerene-60
J. Phys. Chem., Vol. 99, No. 10, 1995 3369
6.86
/)
H LF spin diffusion
6.78
H COSY
0
01
n 1
6.5-
l4.0 'Ol 6.66.76.80
8.9
-
O 0
-
F2'
7.0
7:O
610
i.0
4:O
3:O
2:O
Q 0
1:O
0
pprn from TMS
7.1.
f.1
i 0
67
68
6:g
i.6
6:5
F2
ppm from TMS
Figure 5. 'H COSY spectrum of Cm in the alkene region with the projection at the top. Peak numbers below the projection plot correspond to those in Table 2. Chemical shifts are given over the peaks. Arrows denote specious cross-peakscreated by symmetrization. '3C MAS NMR 14392
7.6
7.4
7.2
7.0
6.8
6.6
Fl
Figure 7. 'H LF spin-diffusion spectrum of (260 (T,,,= 1 s). Top, full
range; bottom, expanded regions.
I448
1444
144.0
143.6
143.2
142.8
ppm from TMS
Figure 6. 13C spectrum of c 6 0 (Lorentzian apodization with LB = 5 Hz). The area of the minor peak at 144.6 ppm is 36 times smaller
than that of the Cm peak at 143.9 ppm. condensed aromatic system. We suggest that the species responsible are fulleroid molecules1° trapped in Cm. Our data -do not allow us to be more specific as to the precise nature of the fulleroid structures, and it needs to be determined at what stage of the c 6 0 preparation the fulleroids are produced. Consider now the 13C spectrum (Figure 6) and try to assign the minor peak at 144.6 ppm, the only one which could correspond to a protonated carbon. Although the DOP peaks dominate the 'H spectrum, this impurity was not sufficiently abundant to produce observable 13C peaks. It is therefore very unlikely that some other H-containing species would. It follows that the peak at 144.6 ppm must be assigned to a quaternary carbon atom. Concerning the chemical shift, which is very close to that of c60. the minor 13Cresonance should be assigned to a carbon atom in a condensed aromatic system. Higher fullerenes can be ruled out, because they give multiline Since crystals of chromatographicallypurified Cm constitute a mixture of FCC and HCP 1attices:l we speculate that the peak could come from c 6 0 in the stacking faults, which are thought to
contain trapped organic molecule^.^^^^ In our case, these could be DOP, toluene, or the fulleroid species. Despite using very long accumulation and high-power 'H decoupling, we failed to detect any 13Cpeaks from the H-containing species. From this fact and from the sharpness of the c60 peak at 143.9 ppm (FWHM < 4 Hz!), we conclude that the H-containing species are very rare and that our sample is pure and highly crystalline. These conclusions are supported by 'H NMR spectra which give unusually sharp peaks (Figure 2). Moreover, the T I values for the aliphatic protons of DOP are not averaged by spin diffusion (Table 2 ) . We assume that the aromatic protons of DOP have the same TI relaxation times because they are structurally and motionally similar. The 'H NMR spectral features suggest that 'H-lH dipolar interactions are weak, so that the H-containing species are well separated in space. The reduction of the intersite dipolar interactions can be assisted by molecular motion. Detailed interpretation of the spin-diffusion spectra (Figures 7 and 8) is difficult, because the intensity of cross-peaks depends not only on the through-space internuclear distance but also on the frequency separation of the isotropic chemical shifts, chemical shift anisotropy and relaxation of the nuclei involved, the mobility of the species, and the MAS Therefore, we shall tackle only the general features of spin diffusion in our system. Note that both LF and RF experiments reveal a similar connectivity network (Figures 7 and 8 and Table 2). The intermolecular cross-peaks involving chemically different species are absent from the spectra, because the H-containing species are substantially diluted by c 6 0 (see above). Spindiffusion cross-peaks which correspond to those detected by COSY (Figure 4)are apparent, indicating their intramolecular origin. There are two exceptions: the (4.13, 1.60) cross-peak
3370 J. Phys. Chem., Vol. 99, No. IO, 1995
Kolodziejski et al.
References and Notes
H RF spin diffusion 0 1.o
k
2.0 3.0 4.0 5.0 6.0
520
7.0
0
0
CIIe
F2
7.0
6.0
5.0
4:O
3.0
2:O
1.0
0
ppm from TMS
6.6 6.8
7.0 7.2 7.4 7.6
I
7.6
7.4
7.2
7.0
6.8
6.6
Figure 8. 'HRF spin-diffusion spectrum of Cm (SL = 80 ms). Top, full range; bottom, expanded regions.
is missing in both spin-diffusion spectra and the (6.64, 1.78) cross-peak from the RF spectrum. In the former case, we assume a degrading effect of a fast CH reorientation provoked by the motion of three substituents. In the latter case, the mixing time (80 ms) may have been too short, but we could not use longer high-power spin-lock pulses because they lead to an excessive duty-cycle length of the probe head. The alkene region of both spin-diffusion spectra is difficult to discuss, because the strong diagonal peaks obscure possible cross-peaks expected to be located close to the diagonal. Note that for DOP there are cross-peaks from intramolecular spin-diffusion between the CH3 and OCHz groups and between the CH3 groups and the aromatic protons, while spin diffusion between the OCHz groups and the aromatic protons has not been detected. This suggests that the DOP alkyl chains are folded back toward the aromatic ring, thus rendering the respective 'H-lH distances suitable for the spin diffusion. We believe that molecular reorientation of the H-containing species in C ~ isO anisotropic and appreciably slower than that of the Cm host, simply because of a disturbing effect of dangling atom chains. For a fast isotropic molecular reorientation, intramolecular spin diffusion in our system should be quenched, as in the classic case of adamantane.24
Acknowledgment. We are grateful to Dr. John Foulkes, Alpherantz Research, Cambridge, for supplying the samples of fullerene and to the Spanish Ministry of Education and to Comision Interministerial de Ciencia y Tecnologfa (MAT911152) for support.
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