J. Phys. Chem. 1996, 100, 9579-9581
Motion of Scandium Ions in Sc2C84 Observed by
45Sc
9579
Solution NMR
Yoko Miyake, Shinzo Suzuki, Yasuhiko Kojima, Koichi Kikuchi, Kaoru Kobayashi, Shigeru Nagase, Masatsune Kainosho, and Yohji Achiba* Department of Chemistry, Tokyo Metropolitan UniVersity, Tokyo 192-03, Japan
Yutaka Maniwa Department of Physics, Tokyo Metropolitan UniVersity, Tokyo 192-03, Japan
Keith Fisher School of Chemistry, UniVersity of New South Wales, Sydney 2052, Australia ReceiVed: February 8, 1996; In Final Form: April 19, 1996X
We have used 45Sc solution NMR spectroscopy to show for the first time the internal motion of the scandium ions in two Sc2C84 isomers. In one isomer the two scandium ions are equivalent over the temperature range 238-433 K, whereas in the second isomer there are two inequivalent scandium ions below 383 K. At 383 K coalescence occurs, and above that temperature the scandium ions become equivalent. The processes are reversible, and we believe this is the first time such phenomena have been observed.
Introduction With the successful isolation of air-stable, solvent-extractable metallofullerenes1-5 such as MC82 (M ) La, Y, Sc, and some of the lanthanides) there have been many kind of experiments used to shed light on the molecular and electronic structures of these molecule. These experiments which include EXAFS,6 STM,7 TEM8 and X-ray diffraction9 have shown no direct evidence of the metal encapsulation inside the carbon cage, except for a recent X-ray powder diffraction study of YC82 by Shinohara et al.10 Their results clearly indicate that the Y ion is encapsulated inside the cage. Some of the metallofullerenes which have electron spin have been studied by use of EPR spectroscopy. Sc3C82 11-13 and LaC82 14-17 have been studied by this method to obtain information about the spin distribution, molecular reorientation, and the local fluctuation of the metal ions. NMR spectroscopy is one of the best methods to study the electronic and dynamic behavior of metal atoms inside a fullerene cage. Diamagnetic metallofullerenes have been isolated only for M2Cn series, and only one metal NMR spectrum has been obtained. The 45Sc solution NMR spectrum of a series of Sc2Cn species in the crude extract18,19 shows a very broad signal spanning about 200 ppm. The difficulty of separation and purification of this series has hampered further investigations. We have now isolated and separated two isomers of Sc2C84 in sufficient abundance to carry out a 45Sc solution NMR of both isomers. Experimental Section The preparation and purification of the Sc2C84 samples was essentially the same as previously described.1,2 Briefly, the dc arc discharge of Sc-doped carbon rods produced a soot which was extracted with CS2 or 1,2,4-trichlorobenzene. (Shinohara et al.20 had previously named the isomers I and III with regard to their retention time using toluene on a buckyclucher column, we will use the same nomenclature.) The Sc2C84 isomers were separated and purified by a combination of various chromatoX
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00421-2 CCC: $12.00
graphic procedures and the purity of the isomers checked by LD-TOF mass spectroscopy. There was no evidence in the mass spectra of contamination by any other scandium fullerene species. Solutions of isomers I and III were prepared in CS2 and o-dichlorobenzene-d4, degassed, and placed under argon atmosphere before the NMR measurements. 45Sc solution NMR spectra were obtained at 121.5 and 72.9 MHz using Varian Unity plus-500 and Unity-300 spectrometers, respectively. CS2 was used to obtain spectra in the temperature range 238-308 K and o-dichlorobenzene used in the temperature range 293-433 K. The 45Sc chemical shift scale was calibrated using 1 M Sc2O3/ HCl (H2O) as external reference (0 ppm). The T1 relaxation time of isomer III of Sc2C84 was measured by the inversion recovery methods. Results and Discussion Figure 1 shows the 45Sc NMR spectra, at 121.5 MHz, of isomer I at temperatures varying from 298 to 433 K (a-h). The figure also shows the simulated line shapes calculated using the Bloch equation (i-p). Figure 2 shows the spectra of isomer III at temperatures varying from 238 to 433K. The signals shown in both sets of spectra were the only signals obtained in the chemical shift range -200 to 800 ppm. The spectra of isomer I showed that at 363 K and lower temperatures two distinct signals of equal intensity were observed, indicating the presence of two magnetic environments for the scandium ions. The difference in chemical shift of the two signals was 50 ppm. At 383 K the signals coalesced and above this temperature the line width narrowed and the scandium ions became equivalent with an intermediate chemical shift compared to the two signals at room temperature. We attribute this change to increasing motion of the scandium ions from fixed different positions at lower temperatures. The rates of this process were determined at various temperatures and the barrier for this motion was estimated to be 0.8 eV (80 kJ mol-1), from the slope of the ln k vs 1/T (shown in Figure 3). The rate was estimated to be 104 s-1 at the coalescence temperature (383 K) and between 1 and 100 s -1 at room temperature. Since the © 1996 American Chemical Society
9580 J. Phys. Chem., Vol. 100, No. 23, 1996
Letters
Figure 3. Plot of the ln k (with error bar) vs 1/T from the line shape analysis of NMR signals of Sc2C84 isomer I.
Figure 1. 45Sc solution NMR spectra of Sc2C84 isomer I in CS2 (a, b), and o-dichlorobenzene (c-h) at 121.5 MHz. The exchange rate k at each temperature was estimated by the simulating the experimental line shape using the Bloch equation. Each simulated spectrum is shown in (i-p) when k ) (i) 10 s-1, (j) 26 s-1, (k) 240 s-1, (l) 1000 s-1, (m) 2500 s-1, (n) 8500 s-1, (o) 25000 s-1, (p) 130000 s-1.
negatively charged carbon cage. As the temperature is raised, motion of the ions occurs and above 383 K the rapid motion makes the two ions equivalent. This kind of motion of metal ions inside a fullerene cage has not been previously reported. In contrast to isomer I, 45Sc NMR spectra of isomer III showed only one signal over the temperature range 238-433 K. Comparing the line widths at 121.5 and 72.9 MHz indicated that the rather broad lines were not affected by the magnetic field strength. This also indicated there was no overlap of two resonances with almost similar chemical shifts. Thus the two scandium ions were equivalent and located in similar magnetic environments. Since the T1 relaxation time (measured to be 8 × 10-5 s at room temperature in case of isomer III) and the T2 relaxation time (estimated from the line width of 4500 Hz at room temperature) are the same, the “extreme narrowing condition” is satisfied in our system. Therefore, the line width (Lw) is linearly proportional to the correlation time (τc) and the main contribution to the line width is considered to be due to the relaxation through the nuclear quadrupole coupling (the nuclear spin (I) of 45Sc is 7/2). This relationship is expressed in the following formula:
2πLw )
Figure 2. 45Sc solution NMR spectra of Sc2C84 isomer III in CS2 (ac), and o-dichlorobenzene (d-i) at 121.5 MHz.
chemical shifts of the scandium ions observed here are in a similar range for those reported for Sc(III) compounds (from -50 to 250 ppm21), we think it likely that we are observing Sc3+ ions. The activation energy calculated for the change from inequivalent scandium ions to equivalent scandium ions (80 kJ mol-1) is not as large as normal transition metal carbon bonds (160-350 kJ mol-1)22 but much larger than hydrogen bonding (4-32 kJ mol-1). The scandium ions are probably at opposite ends of the molecule due to charge repulsion, and there is an electrostatic attraction between the positive ions and the
( )(
1 1 3π2 2I + 3 e2qQ ) ) T1 T2 10 I2(2I - 1) h
2
)
1 + β2 τc 3
(1)
where Q, q, β, and τc denote the quadrupole moment of the nucleus, the principle value of the electric field gradient tensor, its asymmetry parameter, and the correlation time, respectively. Usually the correlation time can be expressed by the DebyeStokes-Einstein formula, i.e., τc ) 4πR3η/kT (where R and η denote the hydrodynamic radius and the viscosity coefficient of the solvent, respectively), and the line width depends on both η and T. However, the experimental results show that the line width of Sc2C84 (isomer III) does not depend on η of the solvent. Figure 4a shows the plot for the line width of the signal of Sc2C84 (isomer III) vs 1/T in CS2 and in o-dichlorobenzene, each of which has a different viscosity coefficient. It is confirmed that despite the η difference (about 4 times larger for o-dichlorobenzene than for CS2 at the same temperature23), this plot fits one unique exponential curve very well. Assuming that τc is expressed in the formula, τc ) τ0 exp(Ea/kT), where τ0 is constant and Ea denotes some “activation energy” which determines the behavior of the correlation time, Ea was estimated to be 0.088 eV from the fit in Figure 4a. As shown in Figure 4b, a similar activation energy was estimated for isomer I from the temperature dependence of the line widths observed below the coalescence temperature (about 0.1 eV). One plausible explanation of the “activation energy” 0.1 eV is that the scandium ions in Sc2C84 are rigidly attached to the
Letters
J. Phys. Chem., Vol. 100, No. 23, 1996 9581 We express out thank to Prof. Y. Maruyama for providing us the Sc2O3 powder and to Prof. T. Nogami for the use of arcburning apparatus. We also thank Prof. H. Shinohara for sending us the submitted paper for 13C NMR study of Sc2C84 (isomer III) and Prof. K.-P. Dinse for fruitful discussion. Thanks are also due to Dr. H. C. Dorn and Prof. D. S. Bethune, who kindly sent us their 45Sc NMR data prior to publication. This work was partly supported by the Grants of the Ministry of Education, Science, Sports and Culture of Japan. References and Notes
Figure 4. Plot of the line widths of Sc2C84 isomer III (a) and isomer I (b) vs reciprocal temperature.In Sc2C84 isomer III (a), the line widths (b) were observed in CS2 solvent and (O) were observed in odichlorobenzene solvent. In Sc2C84 isomer I (b), the linewidths (2) were observed in CS2 and (4) were observed in o-dichlorobenzene at the temperature below the coalescence temperature, and they are plotted in the figure with error bars (++), which are estimated by the lineshape simulation, consistant with two Lorentzian lines.
cage, and the reorientation of the whole molecule in solution determines the correlation time. Actually, for empty fullerenes, such as C60, it has been shown by a picosecond transient experiment, that the correlation time due to the rotational reorientation cannot be described by a simple hydrodynamic Debye theory.24,25 Additionally, a preliminary investigation of the correlation time for empty fullerenes by use of 13C NMR has shown that τc does not depend linearly on the viscosity coefficient of the solvent.26 Since the correlation time due to the reorientational motion of Sc2C84 might be similar to that of LaC82, which was estimated to be ca. (1-2) × 10-11 s at room temperature,14,15 the quadrupole coupling constant (e2qQ/h) can be calculated to be 60 to 30 MHz in the limit of a vanishing β. This value is similar to that of ScO, 71 MHz,27 suggesting that the scandium ions are strongly orientated to the carbon cage. Conclusion In summary, our 45Sc solution NMR study for two isomer of Sc2C84 indicates that the two scandium ions in Sc2C84 are separated from each other inside the fullerene cage, and these two scandium ions are located at inequivalent positions for isomer I at room temperature. Above 383 K, however, the two scandium ions move and rapidly change their positions, resulting in an equivalent NMR line. The fast exchange motion would take place from one site to another, and thus the line width heavily reflects the quadrupolar interaction around the metal positions. Such a novel type motion of metal atoms inside the fullerene cage would directly lead to a potential use of the metallofullerene as “molecular devices” in future. Acknowledgment. We thank Toyo Tanso Co. Ltd. for making scandium-carbon composite rods for the arc burning.
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