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19F NMR and EPR study of fluorinated buckminsterfullerene C60F58
Journal of Physics and Chemistry of Solids 63 (2002) 483±489
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19
F NMR and EPR study of ¯uorinated buckminsterfullerene C60F58 A.M. Panich a,*, A.I. Shames a, H. Selig b a
Department of Physics, Ben-Gurion University of the Negev, PO Box 653, Be'er-Sheva 84105, Israel b Department of Inorganic & Analytical Chemistry, The Hebrew University, Jerusalem 91904, Israel Received 4 April 2001; revised 28 June 2001; accepted 28 June 2001
Abstract Solid state 19F NMR in the temperature range from 96 to 366 K and room temperature EPR studies of ¯uorinated buckminsterfullerene C60F58 have been carried out. The temperature dependence of the line width and the spin±lattice relaxation time show hindered molecular motion with the activation energy of DEa 1.9 kcal/mol. Neither phase transition nor random rotation of C60F58 have been obtained. The spin±lattice relaxation rate is strongly affected by the presence of paramagnetic centers, namely, dangling C±C bonds yielding localized unpaired electrons. Such broken bonds are caused by C±C bond rupture in a cage-opened structure of hyper¯uorinated species. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Fullerenes; D. Electron paramagnetic resonance (EPR); D. Nuclear magnetic resonance (NMR)
1. Introduction Buckminsterfullerene, C60, has recently attracted much interest because of its unusual structural and electronic properties and since it is expected to be an important host material for new functionality materials [1] with band gaps controlled by a dopant. Alkali-metal-doped C60 (MxC60: M K, Rb) are superconductors with relatively high TC and their band gaps decrease with the amount of dopants. In contrast to that, Kawasaki et al. [2] showed that the band gap of C60Fx increases with increasing ¯uorine content. The increase in the ¯uorine content also yields a reduction of conduction p-electrons due to the formation of covalent C±F bonds. Synchrotron-radiation photoemission measurements showed that C60F46 is a wide-band-gap insulator [3]. A powder X-ray diffraction study of Kniaz et al. [4] showed a `fuzzyball' structure of ¯uorinated fullerenes with ¯uorines externally attached to the fullerene skeleton by means of covalent C±F bonds, forming a face-centered Ê cubic ( fcc) lattice with a lattice constant of a0 16.68 A Ê . Okino et al. and an average C±F bond length of 1.49 A Ê [5,6]. Nakajima reported the fcc structure with a0 17.1 A Ê [7]. et al. reported the values of a0 from 17.1 to 17.6 A The latest room temperature powder X-ray and electron * Corresponding author. Tel.: 1972-8-647-2458; fax: 1972-8647-2903. E-mail address: [email protected] (A.M. Panich).
diffraction experiments of high purity C60F36 and C60F48 Ê yield base-centered cubic (bcc) lattice with a0 13.02 A for C60F36 and base-centered tetragonal (bct) lattice for Ê , c0 17.91 A Ê ) [8]. The structure of C60F48 (a0 11.85 A ¯uorinated fullerene [9] is given in Fig. 1. At T~353 K, the bct to fcc phase transition in C60F48 was observed by X-ray diffraction measurements [8]; the high Ê at temperature phase shows a lattice constant a0 17.2 A 463 K. Recent X-ray experiments on C60F48 also showed a phase transition at 358 K and some anomalous changes at about 310 K [10]. Heat capacity measurements showed a phase transition in C60F48 at 315±345 K [11]. However, no phase transition was observed in C60F36 [8,10,11]. Okino et al. [6] regarded the C60Fx molecules as two inner spherical shells of carbon and an outer shell of ¯uorine. They suggested the non-existence of C60Fx with x . 48 with an intact cage structure, since electronegative ¯uorine cannot attack C60F48 (i) because of the dense coverage of the sphere by 48 ¯uorine atoms and also (ii) because of the sought after carbon p-electrons should reside mainly inside the cage. The sphere needs to be broken as the 49th carbon atom is attacked by ¯uorine. Calculations also show that over-¯uorinated fullerenes are relatively unstable due to the unfavorable stereochemistry about each carbon atom and to the short F±F distances [12±14]. An increase in stability is achieved if some of the ¯uorine atoms are transferred from the exo positions outside the carbon cluster to the endo positions inside the carbon cluster [12,13]. Such a
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(01)00185-8
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A.M. Panich et al. / Journal of Physics and Chemistry of Solids 63 (2002) 483±489
47.9 (B0 1.192 T) and 53.05 (B0 1.3246 T) MHz in the temperature range from 96 to 366 K have been recorded with a Tecmag pulse NMR spectrometer using Fourier transformation of the phase cycled solid echo. The spin± lattice relaxation time in the laboratory frame T1 was measured using inversion recovery p± t ±p/2 sequence. 19 F chemical shifts are given relative to CFCl3. In our measurements, we used liquid C6F6 (d 2163 ppm relative to CFCl3) as a secondary reference. The negative sign shows the high ®eld shift relative to the standard. Room temperature EPR spectra were recorded with a Bruker EMX-220 digital X-band (n 9.4 GHz) EPR spectrometer. For the measurements, powdered sample was placed into an EPR silent Wilmad 5 mm o.d. quartz tube centered into the rectangular cavity. Calibrations of the g-factor and intensities were done by measuring EPR signals of 10 23 M TEMPOL water solution (g 2.0059). Spectra processing and parameters calculations were done using Bruker WIN-EPR Software.
Fig. 1. Molecular structure of ¯uorinated fullerene C60F48. Carbon and ¯uorine atoms are shown by shaded and open circles, respectively. The double bonds are indicated by bold lines (reproduced from Ref. [9]).
structure, however, was not observed experimentally. In contrast to the calculations, experiments show that further ¯uorination breaks the cage by C±C bond rupture [15] and forms a cage-opened structure. Hyper¯uorinated cageopened species with molecular formulae up to C60F78 have been detected [15±19]. The goal of our paper is to study molecular motion, structure and possible phase transition in ¯uorinated buckminsterfullerene C60F58 by means of solid state 19F NMR spectroscopy. We expected to obtain molecular motion from the temperature dependencies of the line width and spin±lattice relaxation time in the range from 96 to 366 K. Temperature dependent NMR measurements would also allow us to observe a possible phase transition (if it exists). We note that the vast majority of 19F NMR measurements of C60Fx reported up to now have been made with samples dissolved in different solvents such as deuterated acetone or tetrahydrofuran (THF), but not in the solid state. The nature of paramagnetic centers is expected to be derived from the EPR spectra.
2. Experimental Powder C60F58 compound has been prepared by ¯uorination of C60 with F2 at room temperature. The ¯uorination process lasted for about 3 months. The sample was a lightbrown ®ne powder. The composition was determined from the weight uptake and ¯uorine analysis. The 19F NMR spectra at frequencies 28.0 (B0 0.7 T),
3. Results and discussion 3.1. EPR spectra Room temperature EPR spectra of C60Fx powder shows intensive narrow signals within the region of g~2.0. The relative intensity of these signals depends on the incident microwave power. Thus, at low power levels (#200 mW) the spectrum consists of two overlapping singlet Lorentzian lines (Fig. 2): intensive line with giso 2.0035 ^ 0.0001, DHpp 0.115 ^ 0.005 mT and a weaker line with giso 2.0025 ^ 0.0001, DHpp 0.112 ^ 0.01 mT. Deconvolution of this spectrum into two Lorentzian lines showed the ratio of relative integrated intensities of 10:1. The concentration of paramagnetic centers (PCs) was estimated of the order of 10 18 spin/g. Both lines are typical for free radicals and paramagnetic structural defects. Parameters of the second, weaker line allow attributing it to well studied air/ 1 light induced C60 centers, which usually appear when the pristine fullerene powder was not intentionally kept under vacuum and dark conditions (see, for example, Refs. [20,21]). One can suggest that the ¯uorination is somewhat inhomogeneous, giving up some amount of non-¯uorinated 1 C60 molecular centers. The signal of these centers is easily saturated: at higher microwave power EPR spectra show only singlet line with g 2.0035. Such a behavior points out the difference in relaxation parameters of two types of 1 paramagnetic centers observed. C60 centers seem to be weakly bound with the lattice and are characterized by the long relaxation time values. In contrast, the signal of paramagnetic centers with giso 2.0035 shows shorter relaxation times which points out the stronger spin±phonon interaction in that subsystem. The origin of these centers may be unpaired electrons localized on a dangling C±C bond at ruptures of the fullerene cage caused by ¯uorination.
A.M. Panich et al. / Journal of Physics and Chemistry of Solids 63 (2002) 483±489
485
Fig. 2. Room temperature EPR spectra of C60F58. Spectra recorded at low incident power P 200 mW (upper spectrum, multiplied by four) and at high incident power P 50 mW (lower spectrum).
We note that our spectrum is different from that of C60F40 reported by Belaish et al. [22], which consists of three doublets and two singlets. This difference is evidently due to the different structure of the compound under study and that studied in Ref. [22]. In our case no hyper®ne splitting was observed for both 1 signals. The origin of a structureless C60 EPR line was studied elsewhere [20]. The absence of the hyper®ne structure of the second, more intensive Lorentzian line with g 2.0035 is likely due to the exchange interaction between unpaired electrons. It is known that the exchange coupling of the order of hyper®ne parameter (`intermediate' exchange) smoothes out the hyper®ne structure and yields a collapse of the hyper®ne splitting and vanishing of the g-factor anisotropy of EPR line (in our case, of the line with g 2.0035). Such an exchange starts to be active at the concentration of paramagnetic centers found (~10 18 spin/g). Another reason for the appearance of the observed exchange may be nonuniform distribution of PCs, yielding some regions where the effective local concentration of the unpaired spins may be higher than 10 18 spin/g. 3.2. 19F NMR spectra. 19 F NMR spectra of C60F58 in the temperature range 96 to 366 K are broad Gaussian-like resonances (Fig. 3). The line width Dn is ®eld independent and varies from 28.7 to 20.6 kHz when the sample is heated from 96 to 366 K (Fig. 4). Because of the low natural abundance of 13C isotope (1.1%), the line width is mainly caused by dipole± dipole interaction among ¯uorine spins. No asymmetry of
the line shape, along with the ®eld independence of Dn , means that the contributions to the line width occurred due to the electron±nuclear interactions (such as chemical shielding anisotropy and interaction of nuclei with the PCs) are small in comparison to that of nuclear dipolar interaction. The contribution of the susceptibility of PCs was also calculated from their amount determined in Section 3.1 and found to be negligibly small in comparison to that resulting from the dipolar F±F coupling. The chemical shift of the center of gravity is d 2105 ^ 10 ppm. For comparison, 19F shifts of C60Fx with 2 , x , 48, obtained in solutions, fall into the range of 2130 to 2170 ppm [4,18,23±28], though weak additional peaks at 267.9 and 269.7 ppm attributed to CF3-containing fullerenes have also been obtained [27]. As mentioned above, experiments show that hyper¯uorinated species form a cage-opened structure. In such a structure, besides the common C±F bonds, ¯uorine atoms might form some amount of CF2 and CF3 groups with the carbon atoms at ruptures of fullerene cage broken by over-¯uorination. Such groups have been obtained by means of XPS and NMR measurements in ¯uorine±graphite intercalation compounds [29,30], ¯uorocarbons [31] and ¯uorinated fullerenes [32]. Recently, they were observed experimentally [19] and predicted theoretically [13] in hyper¯uorinated fullerene species. While the range of 19F chemical shift characteristic for C±F bonds is around 2160 to 2240 ppm, CF3 and CF2 groups show chemical shift from 250 to 2100 and from 270 to 2140 ppm, respectively [33±35]. Though the resonances of each group are not resolved in our experiment, one can suggest that some amount of CF3 and CF2 groups (besides
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Fig. 3. Room temperature 19F NMR spectrum of C60F58 at 53.05 MHz. Narrow line is the signal of the liquid reference C6F6. Gaussian ®t is shown by dashed line.
C±F ones) exists and moves the 'average' 19F resonance from 2130 to 2170 ppm (observed in C60Fx with 2 , x , 48) to 2105 ppm in C60F58. One could suggest an order±disorder phase transition from the `rigid' lattice to the `rotator phase' such as obtained in solid C60, where the molecules show random rotation in the `rotator' phase and undergo a phase transition into a phase in which they reorient by performing rigid 608 jumps between two symmetry-equivalent positions being energetically preferred [36,37]. However, the temperature
dependence of the 19F line width (Fig. 4) is smooth and does not show a phase transition. We note that the line width Dn and the second moment S2 in the compound under study are caused by dipole±dipole interactions among 19F spins. Reduction of these parameters with increasing temperature is attributed to a molecular mobility that averages dipole±dipole interactions. In the case of the random rotation of buckyballs, the intra-molecular contribution to the second moment S2 is averaged out, while the inter-molecular contribution may be well estimated by
Fig. 4. Temperature dependence of 19F NMR line width Dn in the temperature range from 96 to 366 K.
A.M. Panich et al. / Journal of Physics and Chemistry of Solids 63 (2002) 483±489
487
Fig. 5. Temperature dependence of 19F NMR spin±lattice relaxation time T1 in the temperature range from 96 to 366 K.
placing all ¯uorine spins into the centers of the appropriate molecules [38]. For the face-centered cubic lattice with the Ê , the distance between the lattice constant a0 17.1 A Ê , yielding centers of two neighboring molecules is 12.1 A the value of the second moment S2 1.21 kHz and, since the second moment of a Gaussian line is S2 0.1803 £ (Dn 1/2) 2, the Gaussian line width of 2.59 kHz. This is too far from that observed in the experiment, which shows a variation of the line width from 28.7 to 20.6 kHz when the sample is heated from 96 to 366 K (Fig. 4). It means that the random rotation of ¯uorobuckyballs is excluded, while a hindered anisotropic rotational motion of them, which yields only a partial average of the intra-molecular contribution to the second moment S2, is likely. This mobility is practically frozen at ~96 K, where the line width reaches a plateau corresponding to `rigid' C60F58 molecules. 3.3. 19F NMR spin±lattice relaxation The temperature dependence of the 19F NMR spin± lattice relaxation time T1 measured at the frequency 48.9 MHz in the temperature range from 96 to 358 K, is given in Fig. 5. The temperature dependence of T1 shows a pronounced relaxation minimum at T~300 K, which is a typical Bloembergen±Purcell±Pound (BPP) minimum [39] characteristic for a ¯uctuation of dipole±dipole interaction among ¯uorines caused by molecular motion mentioned above. The activation energy of this process, calculated from the T1(1/T ) curve, is 1.9 kcal/mol. The molecular correlation time t calculated from the minimum condition v 0t ~1 is of the order of 20 ns. The value of the
activation energy DEa 1.9 kcal/mol is comparable with that of pristine C60, both in the high-temperature (free rotator) and low-temperature (ratchet) phases (1.4 and 4.2 kcal/mol, respectively [37]). We note that the measurements of 19F spin±lattice relaxation in our sample show very short values of T1, around several milliseconds (Fig. 5), which are shorter by two orders of magnitude in comparison to those in solid ¯uorocarbons (e.g. Te¯on shows T1 210 ms at ambient temperature [40]). Such fast spin±lattice relaxation is usually caused by the presence of localized paramagnetic centers, the large amount of which in C60F58 is detected by EPR. As known, the coupling between nuclear spins and unpaired electron spins (even in small concentrations) produces an effective channel for nuclear spin±lattice relaxation, and the relaxation rate (T1) 2 1 in such a case depends linearly on the concentration of paramagnetic centers [41]. For all the temperature range under study, the magnetization decay is well described by a stretched exponent M t M 01 2 2 £ exp 2 t=T1 a
1
with a ~0.5 (Fig. 6). This is the classic behavior characteristic for the nuclear relaxation via paramagnetic centers [42]. Deviation from the BBP curve and a plateau in T1(T ) dependence at low temperature is also a characteristic for the in¯uence of PCs. The spin±lattice relaxation time caused by molecular motion strongly elongates with decreasing temperature according to the BPP theory [39], while the relaxation via paramagnetic centers is short and almost temperature independent and becomes a dominant
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Fig. 6. Logarithmic plot of the 19F magnetization decay versus time at 267 K.
mechanism at low temperature. This conclusion is also supported by no temperature dependence of the coef®cient a . 4. Summary Analysis of the temperature dependence of 19F line width and the spin±lattice relaxation time of C60F58 in the temperature range from 96 to 366 K shows a hindered rotation of ¯uorobuckyballs with an activation energy DE 1.9 kcal/mol. Random rotation, characteristic for a rotator phase of C60, is not observed. The temperature dependence of line width is rather smooth and does not show a phase transition. Spin±lattice relaxation rate is strongly affected by paramagnetic centers. These paramagnetic centers result from the unpaired electrons localized on the broken, dangling C±C bonds at ruptures of fullerene cage caused by ¯uorination. CF2- and CF3- groups are likely formed on the edges of the fullerene rupture.
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[5] F. Okino, H. Touhara, K. Seki, R. Mitsumoto, K. Shigematsu, Y. Achiba, Fullerene Sci. Technol. 1 (1993) 425. [6] F. Okino, S. Kawasaki, Y. Fukushima, M. Kimura, T. Nakajima, H. Touhara, Fullerene Sci. Technol. 4 (1996) 873. [7] T. Nakajima, Y. Matsuo, S. Kasamatsu, K. Nakanishi, Carbon 32 (1994) 1177. [8] S. Kawasaki, H. Touhara, F. Okino, O.V. Boltanina, I.V. Gol'dt, S.I. Troyanov, R. Tailor, J. Phys. Chem. B 103 (1999) 1223. [9] L.G. Bulusheva, A.V. Okotrub, O.V. Boltalina, J. Phys. Chem. A 103 (1999) 9921. [10] S. Kawasaki, F. Okino, H. Touhara, Mol. Cryst. Liq. Cryst. 340 (2000) 629. [11] A.I. Druzhinina, N.A. Galeva, R.M. Varushchenko, O.V. Boltanina, L.N. Sidorov, J. Chem. Thermodynamics 31 (1999) 1469. [12] B.W. Clare, D.L. Kepert, J. Molec. Struct. (Theorchem) 367 (1996) 1. [13] B.W. Clare, D.L. Kepert, J. Phys. Chem. Solids 58 (1997) 1815. [14] V. Nolting, W.S. Verwoerd, J. Phys. Chem. Solids 58 (1997) 1907. [15] A.A. Tuinman, A.A. Gakh, J.L. Adcock, R.N. Compton, J. Am. Chem. Soc. 115 (1993) 5885. [16] S.K. Choudhury, S. Cameron, D.M. Cox, K. Kniaz, R.A Strongin, M.A. Cichy, J.E. Fischer, A.B. Smith, Org. Molec. Spectr. 28 (1993) 860. [17] H. Selig, K. Kniaz, G.B.M. Vaughan, J.E. Fischer, A.B. Smith, Macromol. Symp. 82 (1994) 89. [18] O.V. Boltalina, A.K. Abdul-Sada, R. Taylor, J. Chem. Soc. Perkin Trans. 2 (1995) 981. [19] A.A. Gakh, A.A. Tuinman, J. Phys. Chem. A 104 (2000) 5888. [20] A. Shames, E.A. Katz, S. Goren, D. Faiman, S. Shtutina, Mater. Sci. Eng. B 45 (1997) 134. [21] W. Kempinski, L. Piekara-Sady, E.A. Katz, Solid State Commun. 114 (2000) 173.
A.M. Panich et al. / Journal of Physics and Chemistry of Solids 63 (2002) 483±489 [22] I. Belaish, D. Davidov, H.E. Selig, J.E. Fischer, Advanced Materials 4 (1992) 411. [23] J.H. Holloway, E.G. Hope, R. Taylor, J. Langley, A.G. Avent, T.J. Dennis, J.P. Hare, H.W. Kroto, D.R.M. Walton, J. Chem. Soc., Chem. Commun. (1991) 966. [24] A.A. Gakh, A.A. Tuinman, J.L. Adcock, J. Am. Chem. Soc. 116 (1994) 819. [25] O.V. Boltalina, V.Yu. Markov, R. Taylor, M.P. Waugh, J. Chem. Soc., Chem. Commun. (1996) 2549. [26] O.V. Boltalina, J.M. Street, R. Taylor, J. Chem. Soc. Perkin Trans. 2 (1998) 649. [27] A.G. Avent, O.V. Boltalina, A.Yu. Lukonin, J.M. Street, R. Taylor, J. Chem. Soc. Perkin Trans. 2 (2000) 1359. [28] O.V. Boltalina, A.Yu. Lukonin, J.M. Street, R. Taylor, J. Chem. Soc., Chem. Commun. (2000) 1601. [29] A.M. Panich, T. Nakajima, H.-M. Vieth, A. Privalov, S. Goren, J. Phys.: Condens. Matter 10 (1998) 7633. [30] A.M. Panich, Synthetic Metals 100 (1999) 169. [31] E.W. Hagaman, D.K. Murray, G.D. Del Cul, Energy and Fuel 12 (1998) 399. [32] D.M. Cox, S.D. Cameron, A. Tuinman, A. Gakh, J.L. Adcock,
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R.N. Compton, E.W. Hamagan, K. Kniaz, J.E. Fischer, R.M. Strongin, M.A. Cichy, A.B. Smith III, J. Am. Chem. Soc. 116 (1994) 1115. H. Gunther, NMR Spectroscopy, John Wiley & Sons, Chichester, New York, 1980. E.F. Mooney, An Introduction to 19F NMR Spectroscopy, Heyden & Son Ltd, London, 1970. P. SohaÂr, Nuclear Magnetic Resonance Spectroscopy, CRC Press, Boca Raton, FL, 1983. R. Tycko, G. Dabbagh, R.M. Fleming, R.C. Haddon, A.V. Makhija, S.M. Zahurak, Phys. Rev. Lett. 67 (1991) 1886. R.D. Johnson, C.S. Yannoni, H.C. Dorn, J.R. Salem, D.S. Bethune, Science 225 (1992) 1235. E.R. Andrew, R.G. Eads, Proc. Roy. Soc. A 216 (1953) 398. N. Bloembergen, E.M. Purcell, R.V. Pound, Phys. Rev. 73 (1948) 679. G.B. Furman, A.M. Panich, S.D. Goren, Solid State Nuclear Magnetic Resonance 11 (1998) 225. A. Abragam, The Principles of Nuclear Magnetism, Clarendon Press, Oxford, 1961. W.E. Blumberg, Phys. Rev. 119 (1960) 79.
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19
F NMR and EPR study of ¯uorinated buckminsterfullerene C60F58 A.M. Panich a,*, A.I. Shames a, H. Selig b a
Department of Physics, Ben-Gurion University of the Negev, PO Box 653, Be'er-Sheva 84105, Israel b Department of Inorganic & Analytical Chemistry, The Hebrew University, Jerusalem 91904, Israel Received 4 April 2001; revised 28 June 2001; accepted 28 June 2001
Abstract Solid state 19F NMR in the temperature range from 96 to 366 K and room temperature EPR studies of ¯uorinated buckminsterfullerene C60F58 have been carried out. The temperature dependence of the line width and the spin±lattice relaxation time show hindered molecular motion with the activation energy of DEa 1.9 kcal/mol. Neither phase transition nor random rotation of C60F58 have been obtained. The spin±lattice relaxation rate is strongly affected by the presence of paramagnetic centers, namely, dangling C±C bonds yielding localized unpaired electrons. Such broken bonds are caused by C±C bond rupture in a cage-opened structure of hyper¯uorinated species. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Fullerenes; D. Electron paramagnetic resonance (EPR); D. Nuclear magnetic resonance (NMR)
1. Introduction Buckminsterfullerene, C60, has recently attracted much interest because of its unusual structural and electronic properties and since it is expected to be an important host material for new functionality materials [1] with band gaps controlled by a dopant. Alkali-metal-doped C60 (MxC60: M K, Rb) are superconductors with relatively high TC and their band gaps decrease with the amount of dopants. In contrast to that, Kawasaki et al. [2] showed that the band gap of C60Fx increases with increasing ¯uorine content. The increase in the ¯uorine content also yields a reduction of conduction p-electrons due to the formation of covalent C±F bonds. Synchrotron-radiation photoemission measurements showed that C60F46 is a wide-band-gap insulator [3]. A powder X-ray diffraction study of Kniaz et al. [4] showed a `fuzzyball' structure of ¯uorinated fullerenes with ¯uorines externally attached to the fullerene skeleton by means of covalent C±F bonds, forming a face-centered Ê cubic ( fcc) lattice with a lattice constant of a0 16.68 A Ê . Okino et al. and an average C±F bond length of 1.49 A Ê [5,6]. Nakajima reported the fcc structure with a0 17.1 A Ê [7]. et al. reported the values of a0 from 17.1 to 17.6 A The latest room temperature powder X-ray and electron * Corresponding author. Tel.: 1972-8-647-2458; fax: 1972-8647-2903. E-mail address: [email protected] (A.M. Panich).
diffraction experiments of high purity C60F36 and C60F48 Ê yield base-centered cubic (bcc) lattice with a0 13.02 A for C60F36 and base-centered tetragonal (bct) lattice for Ê , c0 17.91 A Ê ) [8]. The structure of C60F48 (a0 11.85 A ¯uorinated fullerene [9] is given in Fig. 1. At T~353 K, the bct to fcc phase transition in C60F48 was observed by X-ray diffraction measurements [8]; the high Ê at temperature phase shows a lattice constant a0 17.2 A 463 K. Recent X-ray experiments on C60F48 also showed a phase transition at 358 K and some anomalous changes at about 310 K [10]. Heat capacity measurements showed a phase transition in C60F48 at 315±345 K [11]. However, no phase transition was observed in C60F36 [8,10,11]. Okino et al. [6] regarded the C60Fx molecules as two inner spherical shells of carbon and an outer shell of ¯uorine. They suggested the non-existence of C60Fx with x . 48 with an intact cage structure, since electronegative ¯uorine cannot attack C60F48 (i) because of the dense coverage of the sphere by 48 ¯uorine atoms and also (ii) because of the sought after carbon p-electrons should reside mainly inside the cage. The sphere needs to be broken as the 49th carbon atom is attacked by ¯uorine. Calculations also show that over-¯uorinated fullerenes are relatively unstable due to the unfavorable stereochemistry about each carbon atom and to the short F±F distances [12±14]. An increase in stability is achieved if some of the ¯uorine atoms are transferred from the exo positions outside the carbon cluster to the endo positions inside the carbon cluster [12,13]. Such a
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(01)00185-8
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A.M. Panich et al. / Journal of Physics and Chemistry of Solids 63 (2002) 483±489
47.9 (B0 1.192 T) and 53.05 (B0 1.3246 T) MHz in the temperature range from 96 to 366 K have been recorded with a Tecmag pulse NMR spectrometer using Fourier transformation of the phase cycled solid echo. The spin± lattice relaxation time in the laboratory frame T1 was measured using inversion recovery p± t ±p/2 sequence. 19 F chemical shifts are given relative to CFCl3. In our measurements, we used liquid C6F6 (d 2163 ppm relative to CFCl3) as a secondary reference. The negative sign shows the high ®eld shift relative to the standard. Room temperature EPR spectra were recorded with a Bruker EMX-220 digital X-band (n 9.4 GHz) EPR spectrometer. For the measurements, powdered sample was placed into an EPR silent Wilmad 5 mm o.d. quartz tube centered into the rectangular cavity. Calibrations of the g-factor and intensities were done by measuring EPR signals of 10 23 M TEMPOL water solution (g 2.0059). Spectra processing and parameters calculations were done using Bruker WIN-EPR Software.
Fig. 1. Molecular structure of ¯uorinated fullerene C60F48. Carbon and ¯uorine atoms are shown by shaded and open circles, respectively. The double bonds are indicated by bold lines (reproduced from Ref. [9]).
structure, however, was not observed experimentally. In contrast to the calculations, experiments show that further ¯uorination breaks the cage by C±C bond rupture [15] and forms a cage-opened structure. Hyper¯uorinated cageopened species with molecular formulae up to C60F78 have been detected [15±19]. The goal of our paper is to study molecular motion, structure and possible phase transition in ¯uorinated buckminsterfullerene C60F58 by means of solid state 19F NMR spectroscopy. We expected to obtain molecular motion from the temperature dependencies of the line width and spin±lattice relaxation time in the range from 96 to 366 K. Temperature dependent NMR measurements would also allow us to observe a possible phase transition (if it exists). We note that the vast majority of 19F NMR measurements of C60Fx reported up to now have been made with samples dissolved in different solvents such as deuterated acetone or tetrahydrofuran (THF), but not in the solid state. The nature of paramagnetic centers is expected to be derived from the EPR spectra.
2. Experimental Powder C60F58 compound has been prepared by ¯uorination of C60 with F2 at room temperature. The ¯uorination process lasted for about 3 months. The sample was a lightbrown ®ne powder. The composition was determined from the weight uptake and ¯uorine analysis. The 19F NMR spectra at frequencies 28.0 (B0 0.7 T),
3. Results and discussion 3.1. EPR spectra Room temperature EPR spectra of C60Fx powder shows intensive narrow signals within the region of g~2.0. The relative intensity of these signals depends on the incident microwave power. Thus, at low power levels (#200 mW) the spectrum consists of two overlapping singlet Lorentzian lines (Fig. 2): intensive line with giso 2.0035 ^ 0.0001, DHpp 0.115 ^ 0.005 mT and a weaker line with giso 2.0025 ^ 0.0001, DHpp 0.112 ^ 0.01 mT. Deconvolution of this spectrum into two Lorentzian lines showed the ratio of relative integrated intensities of 10:1. The concentration of paramagnetic centers (PCs) was estimated of the order of 10 18 spin/g. Both lines are typical for free radicals and paramagnetic structural defects. Parameters of the second, weaker line allow attributing it to well studied air/ 1 light induced C60 centers, which usually appear when the pristine fullerene powder was not intentionally kept under vacuum and dark conditions (see, for example, Refs. [20,21]). One can suggest that the ¯uorination is somewhat inhomogeneous, giving up some amount of non-¯uorinated 1 C60 molecular centers. The signal of these centers is easily saturated: at higher microwave power EPR spectra show only singlet line with g 2.0035. Such a behavior points out the difference in relaxation parameters of two types of 1 paramagnetic centers observed. C60 centers seem to be weakly bound with the lattice and are characterized by the long relaxation time values. In contrast, the signal of paramagnetic centers with giso 2.0035 shows shorter relaxation times which points out the stronger spin±phonon interaction in that subsystem. The origin of these centers may be unpaired electrons localized on a dangling C±C bond at ruptures of the fullerene cage caused by ¯uorination.
A.M. Panich et al. / Journal of Physics and Chemistry of Solids 63 (2002) 483±489
485
Fig. 2. Room temperature EPR spectra of C60F58. Spectra recorded at low incident power P 200 mW (upper spectrum, multiplied by four) and at high incident power P 50 mW (lower spectrum).
We note that our spectrum is different from that of C60F40 reported by Belaish et al. [22], which consists of three doublets and two singlets. This difference is evidently due to the different structure of the compound under study and that studied in Ref. [22]. In our case no hyper®ne splitting was observed for both 1 signals. The origin of a structureless C60 EPR line was studied elsewhere [20]. The absence of the hyper®ne structure of the second, more intensive Lorentzian line with g 2.0035 is likely due to the exchange interaction between unpaired electrons. It is known that the exchange coupling of the order of hyper®ne parameter (`intermediate' exchange) smoothes out the hyper®ne structure and yields a collapse of the hyper®ne splitting and vanishing of the g-factor anisotropy of EPR line (in our case, of the line with g 2.0035). Such an exchange starts to be active at the concentration of paramagnetic centers found (~10 18 spin/g). Another reason for the appearance of the observed exchange may be nonuniform distribution of PCs, yielding some regions where the effective local concentration of the unpaired spins may be higher than 10 18 spin/g. 3.2. 19F NMR spectra. 19 F NMR spectra of C60F58 in the temperature range 96 to 366 K are broad Gaussian-like resonances (Fig. 3). The line width Dn is ®eld independent and varies from 28.7 to 20.6 kHz when the sample is heated from 96 to 366 K (Fig. 4). Because of the low natural abundance of 13C isotope (1.1%), the line width is mainly caused by dipole± dipole interaction among ¯uorine spins. No asymmetry of
the line shape, along with the ®eld independence of Dn , means that the contributions to the line width occurred due to the electron±nuclear interactions (such as chemical shielding anisotropy and interaction of nuclei with the PCs) are small in comparison to that of nuclear dipolar interaction. The contribution of the susceptibility of PCs was also calculated from their amount determined in Section 3.1 and found to be negligibly small in comparison to that resulting from the dipolar F±F coupling. The chemical shift of the center of gravity is d 2105 ^ 10 ppm. For comparison, 19F shifts of C60Fx with 2 , x , 48, obtained in solutions, fall into the range of 2130 to 2170 ppm [4,18,23±28], though weak additional peaks at 267.9 and 269.7 ppm attributed to CF3-containing fullerenes have also been obtained [27]. As mentioned above, experiments show that hyper¯uorinated species form a cage-opened structure. In such a structure, besides the common C±F bonds, ¯uorine atoms might form some amount of CF2 and CF3 groups with the carbon atoms at ruptures of fullerene cage broken by over-¯uorination. Such groups have been obtained by means of XPS and NMR measurements in ¯uorine±graphite intercalation compounds [29,30], ¯uorocarbons [31] and ¯uorinated fullerenes [32]. Recently, they were observed experimentally [19] and predicted theoretically [13] in hyper¯uorinated fullerene species. While the range of 19F chemical shift characteristic for C±F bonds is around 2160 to 2240 ppm, CF3 and CF2 groups show chemical shift from 250 to 2100 and from 270 to 2140 ppm, respectively [33±35]. Though the resonances of each group are not resolved in our experiment, one can suggest that some amount of CF3 and CF2 groups (besides
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Fig. 3. Room temperature 19F NMR spectrum of C60F58 at 53.05 MHz. Narrow line is the signal of the liquid reference C6F6. Gaussian ®t is shown by dashed line.
C±F ones) exists and moves the 'average' 19F resonance from 2130 to 2170 ppm (observed in C60Fx with 2 , x , 48) to 2105 ppm in C60F58. One could suggest an order±disorder phase transition from the `rigid' lattice to the `rotator phase' such as obtained in solid C60, where the molecules show random rotation in the `rotator' phase and undergo a phase transition into a phase in which they reorient by performing rigid 608 jumps between two symmetry-equivalent positions being energetically preferred [36,37]. However, the temperature
dependence of the 19F line width (Fig. 4) is smooth and does not show a phase transition. We note that the line width Dn and the second moment S2 in the compound under study are caused by dipole±dipole interactions among 19F spins. Reduction of these parameters with increasing temperature is attributed to a molecular mobility that averages dipole±dipole interactions. In the case of the random rotation of buckyballs, the intra-molecular contribution to the second moment S2 is averaged out, while the inter-molecular contribution may be well estimated by
Fig. 4. Temperature dependence of 19F NMR line width Dn in the temperature range from 96 to 366 K.
A.M. Panich et al. / Journal of Physics and Chemistry of Solids 63 (2002) 483±489
487
Fig. 5. Temperature dependence of 19F NMR spin±lattice relaxation time T1 in the temperature range from 96 to 366 K.
placing all ¯uorine spins into the centers of the appropriate molecules [38]. For the face-centered cubic lattice with the Ê , the distance between the lattice constant a0 17.1 A Ê , yielding centers of two neighboring molecules is 12.1 A the value of the second moment S2 1.21 kHz and, since the second moment of a Gaussian line is S2 0.1803 £ (Dn 1/2) 2, the Gaussian line width of 2.59 kHz. This is too far from that observed in the experiment, which shows a variation of the line width from 28.7 to 20.6 kHz when the sample is heated from 96 to 366 K (Fig. 4). It means that the random rotation of ¯uorobuckyballs is excluded, while a hindered anisotropic rotational motion of them, which yields only a partial average of the intra-molecular contribution to the second moment S2, is likely. This mobility is practically frozen at ~96 K, where the line width reaches a plateau corresponding to `rigid' C60F58 molecules. 3.3. 19F NMR spin±lattice relaxation The temperature dependence of the 19F NMR spin± lattice relaxation time T1 measured at the frequency 48.9 MHz in the temperature range from 96 to 358 K, is given in Fig. 5. The temperature dependence of T1 shows a pronounced relaxation minimum at T~300 K, which is a typical Bloembergen±Purcell±Pound (BPP) minimum [39] characteristic for a ¯uctuation of dipole±dipole interaction among ¯uorines caused by molecular motion mentioned above. The activation energy of this process, calculated from the T1(1/T ) curve, is 1.9 kcal/mol. The molecular correlation time t calculated from the minimum condition v 0t ~1 is of the order of 20 ns. The value of the
activation energy DEa 1.9 kcal/mol is comparable with that of pristine C60, both in the high-temperature (free rotator) and low-temperature (ratchet) phases (1.4 and 4.2 kcal/mol, respectively [37]). We note that the measurements of 19F spin±lattice relaxation in our sample show very short values of T1, around several milliseconds (Fig. 5), which are shorter by two orders of magnitude in comparison to those in solid ¯uorocarbons (e.g. Te¯on shows T1 210 ms at ambient temperature [40]). Such fast spin±lattice relaxation is usually caused by the presence of localized paramagnetic centers, the large amount of which in C60F58 is detected by EPR. As known, the coupling between nuclear spins and unpaired electron spins (even in small concentrations) produces an effective channel for nuclear spin±lattice relaxation, and the relaxation rate (T1) 2 1 in such a case depends linearly on the concentration of paramagnetic centers [41]. For all the temperature range under study, the magnetization decay is well described by a stretched exponent M t M 01 2 2 £ exp 2 t=T1 a
1
with a ~0.5 (Fig. 6). This is the classic behavior characteristic for the nuclear relaxation via paramagnetic centers [42]. Deviation from the BBP curve and a plateau in T1(T ) dependence at low temperature is also a characteristic for the in¯uence of PCs. The spin±lattice relaxation time caused by molecular motion strongly elongates with decreasing temperature according to the BPP theory [39], while the relaxation via paramagnetic centers is short and almost temperature independent and becomes a dominant
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Fig. 6. Logarithmic plot of the 19F magnetization decay versus time at 267 K.
mechanism at low temperature. This conclusion is also supported by no temperature dependence of the coef®cient a . 4. Summary Analysis of the temperature dependence of 19F line width and the spin±lattice relaxation time of C60F58 in the temperature range from 96 to 366 K shows a hindered rotation of ¯uorobuckyballs with an activation energy DE 1.9 kcal/mol. Random rotation, characteristic for a rotator phase of C60, is not observed. The temperature dependence of line width is rather smooth and does not show a phase transition. Spin±lattice relaxation rate is strongly affected by paramagnetic centers. These paramagnetic centers result from the unpaired electrons localized on the broken, dangling C±C bonds at ruptures of fullerene cage caused by ¯uorination. CF2- and CF3- groups are likely formed on the edges of the fullerene rupture.
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