- Email: [email protected]
EPR investigation of atoms in chemical traps
PERGAMON
Carbon 38 (2000) 1635–1640
EPR investigation of atoms in chemical traps ¨ C. Knapp, N. Weiden K.-P. Dinse*, H. Kaß, Phys. Chem. III, TU Darmstadt, Petersenstr. 20, D-64287 Darmstadt, Germany
Abstract By performing high-resolution EPR and ENDOR experiments on nitrogen atoms encapsulated in C 60 , the capability of the quartet spin system to sense small local fields at the site of the atom is demonstrated. Such symmetry lowering can either be induced by chemical modification of the cage or by a phase transition in polycrystalline C 60 . Additional line splittings in the EPR spectrum indicate the presence of a non-vanishing zero-field-splitting. Freezing of cage rotation can be observed via the magnetic dipole interaction with 13 C nuclei of the carbon shell resulting in broadening of ENDOR transitions. Fluctuating magnetic fields originating from additional paramagnetic species in solution can also be detected by their influence on the spin relaxation times. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Fullerene; B. Doping; C. EPR; D. Electronic structure
1. Introduction The study of particles in traps was always an attractive goal for physicists and chemists. Two features of traps contribute to this attractiveness: firstly, otherwise elusive particles can be stabilized, and secondly, in a suitable trap the interaction of a particle with its surroundings can be minimized thus enabling the study of intrinsic properties. Since the discovery of fullerenes with their appealing nearly spherical structure it was obvious that atoms or ions could in principle be encased by these all-carbon molecules. In the meantime, a significant number of elements of the periodic table has been encapsulated in fullerenes. Initially, group III elements like scandium, yttrium, and lanthanum were encapsulated [1,2]. However, it was noticed early that significant charge transfer from the encased atom to the carbon shell occurs combined with localization of the ion at specific positions at the inside of the carbon cage, i.e. a rather strong mixed ionic / covalent bond is formed. Because of an odd number of electrons in the uncharged compounds, their presence could be detected in trace quantities by EPR, which was of considerable importance in the early days of their investigation. It
*Corresponding author. Tel.: 149-6151-162-607; fax: 1496151-164-347. E-mail address: [email protected] (K.-P. Dinse).
required much more effort to prove that group II elements can also be encapsulated. One of the reasons for this slow progress was given by the simple fact that closed shell molecules are formed and so one is lacking the sensitive screening technique of EPR for an optimization of synthesis parameters. Because of a very low yield of endofullerenes (EF) ¨ using the Kratschmer / Huffman synthesis method, several groups attempted to find a more direct method for sample preparation. In particular it seemed attractive to use just one kind of fullerene as substrate and try to penetrate the carbon shell with a well-defined particle. This idea was first realized when Saunders succeeded in incorporating helium atoms in C 60 by applying pressure and elevated temperatures [3]. Although only up to one in 10 3 ‘cages’ could be filled using this procedure, the capability of NMR for a selective detection of 3 He isotopes gave clear evidence that noble gases can be encapsulated in fullerenes [4]. Instead of using heat and pressure, encapsulation by molecule / atom (ion) collisions has the advantage that the ‘synthesis’ mechanism can be studied in detail, in particular the energy dependence of the reaction cross section can be compared with predictions from molecular dynamics calculations. Early experiments by Schwarz [5] showed that optimal collision energies are in the range of 20 to 40 eV, large enough to allow penetration and not too high to allow for reformation of the temporarily disrupted carbon cage.
0008-6223 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00092-0
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The idea of ion bombardment for atom encapsulation was used by Weidinger [6] and also by Campbell [7] for a production of macroscopic quantities of encased atoms. In the case of Campbell, alkali ions (in particular lithium) were used. Mass spectroscopy and IR as well as Raman experiments gave evidence for the existence of stable Li@C 60 . Unfortunately, no EPR signals of the paramagnetic species of group I elements could as yet be detected, therefore no conclusive evidence for the existence of monomeric species in solution exists up to now. In contrast, fullerenes with enclosed group V elements like nitrogen and phosphorus, could be studied as ‘monomeric’ particles. They were characterized and identified by EPR, which gave clear proof of the quartet spin ground state of the encapsulated atom [6,8–10]. Such a high multiplicity spin state can only be realized if these compounds show vanishing electron and / or spin transfer to the carbon shell. In this respect they behave like the noble gas / fullerene systems. Because of their surprisingly narrow EPR lines, even trace quantities in the range of 1 ppm could be detected. The group V EF therefore qualify as particles in ‘chemical traps’, whereas the more abundant group III EF must be described as ‘internal salts’, in which the properties of the encased particles are completely different from those of the free atoms. Because of their topology, fullerenes come close to the ideal of a perfect cage, but they are by no means the only known examples of chemical traps. A few years ago, Sasamori et al. [11] reported about the observation of hydrogen atoms encased permanently in a silicon / oxygen cage. With respect to hydrogen trapping, this system is superior to fullerenes, in which up to now it could not be encapsulated for an extended period of time. Using a paramagnetic atom like nitrogen or phosphorus with a spin multiplicity of four, one has a method to detect deviations from cubic symmetry with high sensitivity. For instance, the paramagnetic atom, which because of its position in the center of the cage is placed at special positions in the C 60 crystal, can be an ideal ‘spy’ to sense a change in site symmetry resulting from phase transitions. Even more drastic effects can be observed if the cage is deformed by chemically modifying the cage or by using a cage like C 70 with less than spherical symmetry. Furthermore, hyperfine interactions between the paramagnetic spin at the center and the nuclear spins in the shell can also be used to detect the re-orientational dynamics of the cage, which is not directly related to the phase transition. In addition to the exploitation of the sensitivity of the electronic quartet spin for non-isotropic electric charge distributions, the magnetic moment of the spin can also be utilized to detect fluctuating magnetic fields originating from additional paramagnetic species in the neighborhood which might not be directly detectable because of fast relaxation processes.
2. Experimental N@C 60 as well as N@C 70 were prepared by ion bombardment of slowly subliming fullerene films using a low pressure nitrogen discharge as described elsewhere [12]. The raw product was dissolved in toluene and separated from colloidal particles by micro-filtration (0.05 mm). Polycrystalline material was obtained by evaporating the solvent. The sample containing either the polycrystalline material or a solution in toluene or CS 2 was finally sealed off on a high-vacuum line. The relative concentration of ‘filled’ cages within the parent fullerene was estimated as 3310 25 . All spectra were obtained with a pulse EPR spectrometer (Bruker ELEXSYS E580) with integrated pulse ENDOR facility (Bruker E560P). Commercial probe heads which were temperature controlled with an Oxford CF935 cryostat were used for FT-EPR as well as for the ENDOR experiments. Signal analysis was performed using the Bruker XEPR software package.
3. Results and discussion
3.1. EPR spectra of N@ C60 in polycrystalline C60 In the high-temperature phase nitrogen atoms occupy special positions (4a) in the face-centered cubic (fcc) ¯ lattice (space group Fm3m) [13]. Accordingly, they experience an exceptionally high O h site symmetry. This high symmetry is the reason for vanishing expectation values of all traceless second-rank tensor operators. Below T c , long range orientational order is established and the symmetry is lowered to simple cubic (sc) with four ¯ [14]. As a result, molecules per unit cell (space group Pa3) the site symmetry at the center position is lowered from O h to S6 . A non-vanishing axially symmetric ZFS tensor can therefore be expected. For the first-order phase transition at 258 K this implies that a typical EPR powder spectrum of a quartet spin should occur just below T c . The principal element D of the ZFS tensor being defined by an effective spin Hamiltonian as given in Eq. (1) can then be obtained by a fit of the spectrum H / " 5 ve Sz 1 D(S 2z 2 S 2 / 3).
(1)
Fig. 1 shows EPR spectra measured above and below T c . In the low temperature phase (but still at temperatures at which rapid cage rotation with a correlation time of shorter than 1 ns occurs) the expected additional spectral features are clearly visible. At lower temperatures additional broadening by anisotropic 13 C hfi is observed because of rotational freezing. The observed powder spectrum can be fitted by invoking an axially symmetric ZFS with a principal coupling element D/ 2p 5 0.52 (2) MHz as is shown in Fig. 2. This
K.-P. Dinse et al. / Carbon 38 (2000) 1635 – 1640
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value is much smaller than the value measured for the mono adduct N@C 61 (COOEt) 2 [12], for which the deformation of the cage is a local phenomenon. In contrast, the small value of D in the C 60 crystal results from long range order of the quickly reorienting C 60 cages, the order being defined by a preferred average orientation in the crystal.
3.2. ENDOR spectra of N@ C60 in polycrystalline C60
Fig. 1. FT-EPR spectra of N@C 60 in polycrystalline C 60 showing the 14 N low-frequency hyperfine component. Below the phase transition temperature of 258 K, a powder pattern characteristic for a quartet spin system appears.
In Fig. 3, the low frequency part of an ENDOR spectrum taken at 80 K is depicted. This spectrum, which was obtained using an inversion recovery echo sequence under conditions of medium spectral resolution (rf pulse width t rf 515 ms corresponding to Dn (FWHM)(65 kHz), already shows incipient line broadening for the 13 C transition by anisotropic hfi, expected under conditions of frozen cage rotation. In a quartet electronic spin system, ENDOR transitions (Dm s 50, Dm I 561) can be excited in the um s u51 / 2 and um s u53 / 2 electron spin sublevels. The transition frequencies are calculated from the eigenvalues of the spin system, which is described sufficiently accurate by additional nuclear spin dependent terms assuming quantization of S along the external field Hn / " 5 2 vn Iz 1 IASz 1 IQI.
(2a)
In case of small anisotropic terms in A, and Q being a small perturbation, one obtains
Fig. 2. Fit of the observed powder pattern assuming an axially symmetric ZFS tensor with D/ 2p 5 0.52 MHz. Deviations from the calculated spectrum might result from dead time-related truncation of broad structures in the FT-EPR spectrum.
Fig. 3. Pulse ENDOR spectrum of N@C 60 showing the 13 C and H transitions as well as 14 N transitions originating from um su 5 1 / 2 electronic spin sublevels.
1
K.-P. Dinse et al. / Carbon 38 (2000) 1635 – 1640
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U
U
1 1 n 9ENDOR 5 nn 6] A zz ( b, g ) / 26](2Iz 2 1)Q zz ( b, g ) 2p 2p (2b) as well as
U
1 1 n 99ENDOR 5 nn 63] A zz ( b, g ) / 26](2Iz 2p 2p
U
2 1)Q zz ( b, g )
(2c)
in which A zz and Q zz denote the (z, z) matrix element of A and Q after transformation into the laboratory frame. Both, dipolar and quadrupolar terms depend on the orientation of the molecule with respect to the external field axis, denoted by the Eulerian angles b and g. Unless determined by symmetry at the nuclear site, both tensors A and Q need not be collinear. In Fig. 3, the transitions at the free proton can be attributed to ‘distant’ 1 H nuclei with hyperfine coupling constants (hfcc) less than |50 kHz. Accordingly, transitions within the um s u51 / 2 and um s u53 / 2 electron spin multiplets can no longer be resolved. These weakly coupled protons indicate the presence of residual solvent molecules. Interaction of the electronic spin with the strongly coupled ‘local’ 14 N nucleus, however, leads to two doublets of lines, centered at A / 4p and 3A / 4p, respectively, separated by twice the nitrogen nuclear Zeeman frequency, the former doublet being depicted in Fig. 3. Spectra taken at the same temperature under conditions of improved spectral resolution reveal that this 14 N doublet consists of extremely narrow ENDOR lines, indicating a complete lack of nuclear quadrupole interaction and anisotropic hfi compared with the line width of 4 kHz [15]. Performing ENDOR experiments under the same experimental conditions (allowing a frequency resolution of at least 3 kHz), a powder-like spectrum is detected at the 13 C nuclear Zeeman frequency. For a discussion of the 13 C powder ENDOR spectrum which is depicted in Fig. 4, the pattern resulting from coupling to 13 C nuclei occupying one of the 60 equivalent ‘local’ positions in the carbon cage can be assumed to dominate the spectrum. Because all these nuclei have the same distance to the electronic spin at the center and carry the same isotropic spin density, they are contributing a powder pattern of identical shape and width. This is a prerequisite for the observation of powder spectra in this multi-nuclei situation. Contributions from point dipole–dipole interaction with the spin density at the center would lead to singular points in the spectrum for transitions within the um s u51 / 2 electron spin sublevels at
v (p1 / 2 ) 5 6gIgS r 23 " 1 v (s1 / 2 ) 5 6]gIgS r 23 " 2
Fig. 4. 13 C ENDOR lines detected at 80 K and 250 K. The low temperature transition can be simulated by invoking anisotropic dipole / dipole coupling of the ‘local’ nuclei of the cage giving rise to a powder-like spectrum in addition to coupling to distant carbon nuclei giving rise to the central peak at the 13 C Zeeman frequency.
and at
v (p3 / 2 ) 5 63gIgS r 23 " 3 v (s3 / 2 ) 5 6]gIgS r 23 " 2
(3b)
for nuclear spin transitions within the um s u53 / 2 sublevels. Using r 5 3.5 3 10 210 m, a frequency splitting Dv (p1 / 2 ) / 2p 5 440 kHz and Dv (p3 / 2 ) / 2p 5 1320 kHz is predicted for the most intense ‘perpendicular’ peaks. Using selective excitation of electron spin transitions, however, a value for A zz 5300(30) kHz was obtained from an analysis of the ENDOR spectrum, clearly at variance with the value predicted using a simple point dipole model. As is also seen in Fig. 4, one observes a collapse of the dipolar structure when measuring at elevated temperatures (250 K), but still below the phase transition. Because of fast isotropic reorientation, the 13 C ENDOR spectrum is determined by the isotropic hfcc only, leading to a somewhat broadened, non-Lorentzian line shape. This broadening can be quantitatively accounted for by introducing an isotropic 13 C hfcc of 32(2) kHz [16].
3.3. EPR spectra of N@ C70 in polycrystalline C70 (3a)
Using the same approach as with C 60 , we attempted to detect phase transitions in C 70 via changes in the line
K.-P. Dinse et al. / Carbon 38 (2000) 1635 – 1640
width of the EPR transitions. However, in contrast to C 60 , an intra-molecular contribution to the ZFS interaction is expected, thus possibly masking the crystal field-induced terms. Furthermore, a clear sequence of phase transitions in C 70 crystals has not yet been established, because apparently this might depend critically on the amount of remaining trace C 60 and solvent impurities. In Fig. 5, FT-EPR spectra of N@C 70 in solid C 70 are shown, measured at different temperatures. Apart from spectral broadening, no distinct new spectral features could be detected. Although for future studies the contamination with C 60 can be reduced by HPLC, no method is available to remove solvent molecules completely, because vacuum sublimation is impossible because of thermal release of nitrogen.
3.4. Sensing paramagnetic impurities via spin relaxation of N@ C60 An established method for the detection of paramagnetic species in solution, which do not lead to observable EPR spectra, relies on the fact that their presence can be indirectly seen by measuring the change of spin relaxation rates of ‘probe’ species. This can be done either directly by determining the relaxation rates T 2 and T 1 using echo techniques, or more easily by observing T 2 changes indirectly via an increase of the homogeneous EPR line width. For instance dissolved paramagnetic oxygen in biological relevant concentrations is detected invoking EPR line broadening of nitroxide spin labels. Concentrations as low as 20 mM can be determined quantitatively, the sensitivity being limited by the residual line width of the spin probe [17]. We anticipated that much lower concentrations of paramagnetic impurities should be detectable using N@C 60 instead, because its line width is more than one order of magnitude less. Surprisingly, the
Fig. 5. FT-EPR spectra of N@C 70 in polycrystalline C 70 measured at 80 K and at 300 K (narrow lines).
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line width of the N@C 60 sample did not change noticeably when saturating the solvent with air or even pure oxygen. It was necessary to determine the spin relaxation rates directly with 2 and 3 pulse echo experiments. As is seen in Fig. 6, a calibration of the change in relaxation rates using the stable nitroxide radical TEMPO indicates a linear increase of DT 21 (i 5 1, 2) with coni centration of paramagnetic species. Here, DT i21 is defined as 21
21
?
21
DT i 5 T i (X ) 2 T i (0)
(4a)
21 i
the reference values T (0) being given by the relaxation rates of the pure sample, T i21 (X ? ) denoting the rates in the ? presence of the paramagnetic species X . Writing DT 21 5 c[X ? ] i
(4b) 5
3
we found c 5 5 3 10 Hz / mol / dm . The small value of the proportionality constant can be rationalized only by excluding Heisenberg exchange completely because this process would lead to a value for c of the order of 3310 9 Hz / mol / dm 3 for a low viscosity solvent [17]. Instead we assume that only fluctuating dipolar interactions contribute to spin relaxation of the encased nitrogen spin. This assumption would be consistent with the observation that negligible spin transfer occurs to the carbon cage, thus preventing overlap of the electronic wave functions during collision with the ‘external’ diffusing spins which otherwise would lead to Heisenberg exchange. The contribution of different paramagnetic species via dipolar relaxation can be compared using Dg 2x " 2 S (x) (S (x) 1 1) ? ]]]]] T 21 ~ fX g 1 v 2d 5
(5)
Fig. 6. Dependence of the increase of spin relaxation rates of N@C 60 as a function of the concentration of TEMPO radicals in a solution of toluene. The relaxation rates were measured at room temperature.
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K.-P. Dinse et al. / Carbon 38 (2000) 1635 – 1640
in which d denotes the distance of closest approach, D gives the relative diffusion constant, gx is the magnetogyric ratio of X ? with spin S (x) , and [X ? ] gives the concentration of the ‘relaxer’ X ? [18]. The expression given in (5) is valid in the slow tumbling limit v 2t 2 4 1, which is not too well obeyed at v / 2p 5 9.5 GHz, the linear dependence on the concentration being valid over the full range of t, however. Eq. (5) indicates that molecular oxygen should be more efficient as relaxing agent than the spin label, because firstly S(S 1 1) is 2 instead of 3 / 4, secondly the relative diffusion constant will be larger, and furthermore the close contact distance d will be reduced for sterical reasons. As seen in Fig. 5, the increase in relaxation rates is about a factor of 10, well in line with this qualitative model.
4. Conclusion The exceptional spin relaxation properties of N@C 60 facilitate to perform high-resolution pulse ENDOR experiments. Originating from nearly perfect de-coupling of the encased quartet spin from the carbon cage, even at room temperature electronic spin lattice relaxation times in excess of several hundred ms are observed. Under these conditions, an rf pulse width-limited spectral resolution of 25 kHz (room temperature) and of 2.5 kHz (80 K) can be obtained. With this excellent spectral resolution we could show that even in the low temperature Pa3¯ phase with its S6 local symmetry at the nitrogen site, no quadrupole splitting could be observed, although long-range order is clearly seen in the EPR spectrum indicated by non-vanishing ZFS. Rotational melting in polycrystalline C 60 could be detected by measuring the dipolar interaction with 13 C nuclei on the fullerene shell. At 80 K, at which the relative orientation of the C 60 molecules in the crystal is fixed, a powder-like ENDOR spectrum is observed. Fast isotropic averaging on the time scale given by the dipolar interaction of |300 kHz occurs at 300 K and also already below the phase transition at 250 K, leading to a ‘solution-like’ 13 C ENDOR spectrum. In low viscosity solutions the effect of fluctuating magnetic fields, originating from additional paramagnetic species, is detectable only with EPR pulse techniques. The relatively small effect induced by spins up to a concentration of 10 21 M is clear proof that Heisenberg exchange during collisions can be neglected. This unique feature results from perfect shielding of the wave function of the encapsulated nitrogen atom. This observation is in accord with a very small value of the isotropic 13 C hfcc, which only recently has been measured as 32 kHz [16].
Acknowledgements Financial support from the Deutsche Forschungsgemeinschaft (Di 182 / 21) is gratefully acknowledged. We are also grateful for a temporary loan of an X-Band ENDOR cavity by Bruker. This investigation is part of a continuing collaboration with B. Pietzak, M. Waiblinger, and A. Weidinger (Hahn-Meitner-Institut, Berlin) who provided part of the N@C 60 samples studied.
References [1] Johnson RD, de Vries MS, Salem J, Bethune DS, Yannoni CS. Nature 1992;355:239. [2] Shinohara H, Sato H, Ohkohchi M, Ando Y, Kodama T, Shida T, Kato T, Saito Y. Nature 1992;357:52. [3] Saunders M, Jimenez-Vazquez HA, Cross RJ, Poreda RJ. Science 1993;259:1428. [4] Cross RJ, Jimenez-Vazquez HA, Lu Q, Saunders M, Schuster DI, Wilson SR, Zhao H. J Am Chem Soc 1996;118:11454. ¨ ¨ [5] Weiske T, Bohme DK, Hrusak J, Kratschmer W, Schwarz H. Angew Chem Int Ed Engl 1991;30:884. ¨ [6] Almeida Murphy T, Pawlik T, Weidinger A, Hohne M, Alcala R, Spaeth J-M. Phys Rev Lett 1996;77:1075. [7] Tellgmann R, Krawez N, Lin S-H, Hertel IV, Campbell EEB. Nature 1996;382:407. [8] Knapp C, Dinse K-P, Pietzak B, Waiblinger M, Weidinger A. Chem Phys Lett 1997;272:433. [9] Knapp C, Weiden N, Dinse K-P. Appl Phys A 1998;66:249. ¨ H, Dinse K-P, Pietzak B, Waibling[10] Knapp C, Weiden N, Kaß er M, Weidinger A. Mol Phys 1998;95:999. [11] Sasamori R, Okaue Y, Isobe T, Matsuda Y. Science 1994;265:1691. [12] Pietzak B, Waiblinger M, Almeida Murphy T, Weidinger A, ¨ Hohne M, Dietel E, Hirsch A. Chem Phys Lett 1997;279:259. [13] Heiney PA, Fischer JE, McGhie AR, Romanow WJ, Denenstein AM, McCanley JP, Smith III AB, Cox D. Phys Rev Lett 1991;66:2911. ¨ [14] Burgi HB, Blanc E, Schwarzenbach D, Lin S, Kappes MM, Ibers JA. Angew Chem Int Ed Engl 1992;31:641. ¨ [15] Weiden N, Kaß H, Dinse K-P. J Phys Chem B 1999;103:9826. [16] Knapp C, Weiden N, Dinse K-P. In preparation. [17] Swartz HM, Glockner JF. In: Hoff AJ, editor, Advanced EPR, applications in biology and biochemistry, Amsterdam: Elsevier, 1989. [18] Abragam A. The principles of nuclear magnetism, Oxford: Oxford University Press, 1961.
Carbon 38 (2000) 1635–1640
EPR investigation of atoms in chemical traps ¨ C. Knapp, N. Weiden K.-P. Dinse*, H. Kaß, Phys. Chem. III, TU Darmstadt, Petersenstr. 20, D-64287 Darmstadt, Germany
Abstract By performing high-resolution EPR and ENDOR experiments on nitrogen atoms encapsulated in C 60 , the capability of the quartet spin system to sense small local fields at the site of the atom is demonstrated. Such symmetry lowering can either be induced by chemical modification of the cage or by a phase transition in polycrystalline C 60 . Additional line splittings in the EPR spectrum indicate the presence of a non-vanishing zero-field-splitting. Freezing of cage rotation can be observed via the magnetic dipole interaction with 13 C nuclei of the carbon shell resulting in broadening of ENDOR transitions. Fluctuating magnetic fields originating from additional paramagnetic species in solution can also be detected by their influence on the spin relaxation times. 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Fullerene; B. Doping; C. EPR; D. Electronic structure
1. Introduction The study of particles in traps was always an attractive goal for physicists and chemists. Two features of traps contribute to this attractiveness: firstly, otherwise elusive particles can be stabilized, and secondly, in a suitable trap the interaction of a particle with its surroundings can be minimized thus enabling the study of intrinsic properties. Since the discovery of fullerenes with their appealing nearly spherical structure it was obvious that atoms or ions could in principle be encased by these all-carbon molecules. In the meantime, a significant number of elements of the periodic table has been encapsulated in fullerenes. Initially, group III elements like scandium, yttrium, and lanthanum were encapsulated [1,2]. However, it was noticed early that significant charge transfer from the encased atom to the carbon shell occurs combined with localization of the ion at specific positions at the inside of the carbon cage, i.e. a rather strong mixed ionic / covalent bond is formed. Because of an odd number of electrons in the uncharged compounds, their presence could be detected in trace quantities by EPR, which was of considerable importance in the early days of their investigation. It
*Corresponding author. Tel.: 149-6151-162-607; fax: 1496151-164-347. E-mail address: [email protected] (K.-P. Dinse).
required much more effort to prove that group II elements can also be encapsulated. One of the reasons for this slow progress was given by the simple fact that closed shell molecules are formed and so one is lacking the sensitive screening technique of EPR for an optimization of synthesis parameters. Because of a very low yield of endofullerenes (EF) ¨ using the Kratschmer / Huffman synthesis method, several groups attempted to find a more direct method for sample preparation. In particular it seemed attractive to use just one kind of fullerene as substrate and try to penetrate the carbon shell with a well-defined particle. This idea was first realized when Saunders succeeded in incorporating helium atoms in C 60 by applying pressure and elevated temperatures [3]. Although only up to one in 10 3 ‘cages’ could be filled using this procedure, the capability of NMR for a selective detection of 3 He isotopes gave clear evidence that noble gases can be encapsulated in fullerenes [4]. Instead of using heat and pressure, encapsulation by molecule / atom (ion) collisions has the advantage that the ‘synthesis’ mechanism can be studied in detail, in particular the energy dependence of the reaction cross section can be compared with predictions from molecular dynamics calculations. Early experiments by Schwarz [5] showed that optimal collision energies are in the range of 20 to 40 eV, large enough to allow penetration and not too high to allow for reformation of the temporarily disrupted carbon cage.
0008-6223 / 00 / $ – see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00092-0
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The idea of ion bombardment for atom encapsulation was used by Weidinger [6] and also by Campbell [7] for a production of macroscopic quantities of encased atoms. In the case of Campbell, alkali ions (in particular lithium) were used. Mass spectroscopy and IR as well as Raman experiments gave evidence for the existence of stable Li@C 60 . Unfortunately, no EPR signals of the paramagnetic species of group I elements could as yet be detected, therefore no conclusive evidence for the existence of monomeric species in solution exists up to now. In contrast, fullerenes with enclosed group V elements like nitrogen and phosphorus, could be studied as ‘monomeric’ particles. They were characterized and identified by EPR, which gave clear proof of the quartet spin ground state of the encapsulated atom [6,8–10]. Such a high multiplicity spin state can only be realized if these compounds show vanishing electron and / or spin transfer to the carbon shell. In this respect they behave like the noble gas / fullerene systems. Because of their surprisingly narrow EPR lines, even trace quantities in the range of 1 ppm could be detected. The group V EF therefore qualify as particles in ‘chemical traps’, whereas the more abundant group III EF must be described as ‘internal salts’, in which the properties of the encased particles are completely different from those of the free atoms. Because of their topology, fullerenes come close to the ideal of a perfect cage, but they are by no means the only known examples of chemical traps. A few years ago, Sasamori et al. [11] reported about the observation of hydrogen atoms encased permanently in a silicon / oxygen cage. With respect to hydrogen trapping, this system is superior to fullerenes, in which up to now it could not be encapsulated for an extended period of time. Using a paramagnetic atom like nitrogen or phosphorus with a spin multiplicity of four, one has a method to detect deviations from cubic symmetry with high sensitivity. For instance, the paramagnetic atom, which because of its position in the center of the cage is placed at special positions in the C 60 crystal, can be an ideal ‘spy’ to sense a change in site symmetry resulting from phase transitions. Even more drastic effects can be observed if the cage is deformed by chemically modifying the cage or by using a cage like C 70 with less than spherical symmetry. Furthermore, hyperfine interactions between the paramagnetic spin at the center and the nuclear spins in the shell can also be used to detect the re-orientational dynamics of the cage, which is not directly related to the phase transition. In addition to the exploitation of the sensitivity of the electronic quartet spin for non-isotropic electric charge distributions, the magnetic moment of the spin can also be utilized to detect fluctuating magnetic fields originating from additional paramagnetic species in the neighborhood which might not be directly detectable because of fast relaxation processes.
2. Experimental N@C 60 as well as N@C 70 were prepared by ion bombardment of slowly subliming fullerene films using a low pressure nitrogen discharge as described elsewhere [12]. The raw product was dissolved in toluene and separated from colloidal particles by micro-filtration (0.05 mm). Polycrystalline material was obtained by evaporating the solvent. The sample containing either the polycrystalline material or a solution in toluene or CS 2 was finally sealed off on a high-vacuum line. The relative concentration of ‘filled’ cages within the parent fullerene was estimated as 3310 25 . All spectra were obtained with a pulse EPR spectrometer (Bruker ELEXSYS E580) with integrated pulse ENDOR facility (Bruker E560P). Commercial probe heads which were temperature controlled with an Oxford CF935 cryostat were used for FT-EPR as well as for the ENDOR experiments. Signal analysis was performed using the Bruker XEPR software package.
3. Results and discussion
3.1. EPR spectra of N@ C60 in polycrystalline C60 In the high-temperature phase nitrogen atoms occupy special positions (4a) in the face-centered cubic (fcc) ¯ lattice (space group Fm3m) [13]. Accordingly, they experience an exceptionally high O h site symmetry. This high symmetry is the reason for vanishing expectation values of all traceless second-rank tensor operators. Below T c , long range orientational order is established and the symmetry is lowered to simple cubic (sc) with four ¯ [14]. As a result, molecules per unit cell (space group Pa3) the site symmetry at the center position is lowered from O h to S6 . A non-vanishing axially symmetric ZFS tensor can therefore be expected. For the first-order phase transition at 258 K this implies that a typical EPR powder spectrum of a quartet spin should occur just below T c . The principal element D of the ZFS tensor being defined by an effective spin Hamiltonian as given in Eq. (1) can then be obtained by a fit of the spectrum H / " 5 ve Sz 1 D(S 2z 2 S 2 / 3).
(1)
Fig. 1 shows EPR spectra measured above and below T c . In the low temperature phase (but still at temperatures at which rapid cage rotation with a correlation time of shorter than 1 ns occurs) the expected additional spectral features are clearly visible. At lower temperatures additional broadening by anisotropic 13 C hfi is observed because of rotational freezing. The observed powder spectrum can be fitted by invoking an axially symmetric ZFS with a principal coupling element D/ 2p 5 0.52 (2) MHz as is shown in Fig. 2. This
K.-P. Dinse et al. / Carbon 38 (2000) 1635 – 1640
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value is much smaller than the value measured for the mono adduct N@C 61 (COOEt) 2 [12], for which the deformation of the cage is a local phenomenon. In contrast, the small value of D in the C 60 crystal results from long range order of the quickly reorienting C 60 cages, the order being defined by a preferred average orientation in the crystal.
3.2. ENDOR spectra of N@ C60 in polycrystalline C60
Fig. 1. FT-EPR spectra of N@C 60 in polycrystalline C 60 showing the 14 N low-frequency hyperfine component. Below the phase transition temperature of 258 K, a powder pattern characteristic for a quartet spin system appears.
In Fig. 3, the low frequency part of an ENDOR spectrum taken at 80 K is depicted. This spectrum, which was obtained using an inversion recovery echo sequence under conditions of medium spectral resolution (rf pulse width t rf 515 ms corresponding to Dn (FWHM)(65 kHz), already shows incipient line broadening for the 13 C transition by anisotropic hfi, expected under conditions of frozen cage rotation. In a quartet electronic spin system, ENDOR transitions (Dm s 50, Dm I 561) can be excited in the um s u51 / 2 and um s u53 / 2 electron spin sublevels. The transition frequencies are calculated from the eigenvalues of the spin system, which is described sufficiently accurate by additional nuclear spin dependent terms assuming quantization of S along the external field Hn / " 5 2 vn Iz 1 IASz 1 IQI.
(2a)
In case of small anisotropic terms in A, and Q being a small perturbation, one obtains
Fig. 2. Fit of the observed powder pattern assuming an axially symmetric ZFS tensor with D/ 2p 5 0.52 MHz. Deviations from the calculated spectrum might result from dead time-related truncation of broad structures in the FT-EPR spectrum.
Fig. 3. Pulse ENDOR spectrum of N@C 60 showing the 13 C and H transitions as well as 14 N transitions originating from um su 5 1 / 2 electronic spin sublevels.
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K.-P. Dinse et al. / Carbon 38 (2000) 1635 – 1640
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U
U
1 1 n 9ENDOR 5 nn 6] A zz ( b, g ) / 26](2Iz 2 1)Q zz ( b, g ) 2p 2p (2b) as well as
U
1 1 n 99ENDOR 5 nn 63] A zz ( b, g ) / 26](2Iz 2p 2p
U
2 1)Q zz ( b, g )
(2c)
in which A zz and Q zz denote the (z, z) matrix element of A and Q after transformation into the laboratory frame. Both, dipolar and quadrupolar terms depend on the orientation of the molecule with respect to the external field axis, denoted by the Eulerian angles b and g. Unless determined by symmetry at the nuclear site, both tensors A and Q need not be collinear. In Fig. 3, the transitions at the free proton can be attributed to ‘distant’ 1 H nuclei with hyperfine coupling constants (hfcc) less than |50 kHz. Accordingly, transitions within the um s u51 / 2 and um s u53 / 2 electron spin multiplets can no longer be resolved. These weakly coupled protons indicate the presence of residual solvent molecules. Interaction of the electronic spin with the strongly coupled ‘local’ 14 N nucleus, however, leads to two doublets of lines, centered at A / 4p and 3A / 4p, respectively, separated by twice the nitrogen nuclear Zeeman frequency, the former doublet being depicted in Fig. 3. Spectra taken at the same temperature under conditions of improved spectral resolution reveal that this 14 N doublet consists of extremely narrow ENDOR lines, indicating a complete lack of nuclear quadrupole interaction and anisotropic hfi compared with the line width of 4 kHz [15]. Performing ENDOR experiments under the same experimental conditions (allowing a frequency resolution of at least 3 kHz), a powder-like spectrum is detected at the 13 C nuclear Zeeman frequency. For a discussion of the 13 C powder ENDOR spectrum which is depicted in Fig. 4, the pattern resulting from coupling to 13 C nuclei occupying one of the 60 equivalent ‘local’ positions in the carbon cage can be assumed to dominate the spectrum. Because all these nuclei have the same distance to the electronic spin at the center and carry the same isotropic spin density, they are contributing a powder pattern of identical shape and width. This is a prerequisite for the observation of powder spectra in this multi-nuclei situation. Contributions from point dipole–dipole interaction with the spin density at the center would lead to singular points in the spectrum for transitions within the um s u51 / 2 electron spin sublevels at
v (p1 / 2 ) 5 6gIgS r 23 " 1 v (s1 / 2 ) 5 6]gIgS r 23 " 2
Fig. 4. 13 C ENDOR lines detected at 80 K and 250 K. The low temperature transition can be simulated by invoking anisotropic dipole / dipole coupling of the ‘local’ nuclei of the cage giving rise to a powder-like spectrum in addition to coupling to distant carbon nuclei giving rise to the central peak at the 13 C Zeeman frequency.
and at
v (p3 / 2 ) 5 63gIgS r 23 " 3 v (s3 / 2 ) 5 6]gIgS r 23 " 2
(3b)
for nuclear spin transitions within the um s u53 / 2 sublevels. Using r 5 3.5 3 10 210 m, a frequency splitting Dv (p1 / 2 ) / 2p 5 440 kHz and Dv (p3 / 2 ) / 2p 5 1320 kHz is predicted for the most intense ‘perpendicular’ peaks. Using selective excitation of electron spin transitions, however, a value for A zz 5300(30) kHz was obtained from an analysis of the ENDOR spectrum, clearly at variance with the value predicted using a simple point dipole model. As is also seen in Fig. 4, one observes a collapse of the dipolar structure when measuring at elevated temperatures (250 K), but still below the phase transition. Because of fast isotropic reorientation, the 13 C ENDOR spectrum is determined by the isotropic hfcc only, leading to a somewhat broadened, non-Lorentzian line shape. This broadening can be quantitatively accounted for by introducing an isotropic 13 C hfcc of 32(2) kHz [16].
3.3. EPR spectra of N@ C70 in polycrystalline C70 (3a)
Using the same approach as with C 60 , we attempted to detect phase transitions in C 70 via changes in the line
K.-P. Dinse et al. / Carbon 38 (2000) 1635 – 1640
width of the EPR transitions. However, in contrast to C 60 , an intra-molecular contribution to the ZFS interaction is expected, thus possibly masking the crystal field-induced terms. Furthermore, a clear sequence of phase transitions in C 70 crystals has not yet been established, because apparently this might depend critically on the amount of remaining trace C 60 and solvent impurities. In Fig. 5, FT-EPR spectra of N@C 70 in solid C 70 are shown, measured at different temperatures. Apart from spectral broadening, no distinct new spectral features could be detected. Although for future studies the contamination with C 60 can be reduced by HPLC, no method is available to remove solvent molecules completely, because vacuum sublimation is impossible because of thermal release of nitrogen.
3.4. Sensing paramagnetic impurities via spin relaxation of N@ C60 An established method for the detection of paramagnetic species in solution, which do not lead to observable EPR spectra, relies on the fact that their presence can be indirectly seen by measuring the change of spin relaxation rates of ‘probe’ species. This can be done either directly by determining the relaxation rates T 2 and T 1 using echo techniques, or more easily by observing T 2 changes indirectly via an increase of the homogeneous EPR line width. For instance dissolved paramagnetic oxygen in biological relevant concentrations is detected invoking EPR line broadening of nitroxide spin labels. Concentrations as low as 20 mM can be determined quantitatively, the sensitivity being limited by the residual line width of the spin probe [17]. We anticipated that much lower concentrations of paramagnetic impurities should be detectable using N@C 60 instead, because its line width is more than one order of magnitude less. Surprisingly, the
Fig. 5. FT-EPR spectra of N@C 70 in polycrystalline C 70 measured at 80 K and at 300 K (narrow lines).
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line width of the N@C 60 sample did not change noticeably when saturating the solvent with air or even pure oxygen. It was necessary to determine the spin relaxation rates directly with 2 and 3 pulse echo experiments. As is seen in Fig. 6, a calibration of the change in relaxation rates using the stable nitroxide radical TEMPO indicates a linear increase of DT 21 (i 5 1, 2) with coni centration of paramagnetic species. Here, DT i21 is defined as 21
21
?
21
DT i 5 T i (X ) 2 T i (0)
(4a)
21 i
the reference values T (0) being given by the relaxation rates of the pure sample, T i21 (X ? ) denoting the rates in the ? presence of the paramagnetic species X . Writing DT 21 5 c[X ? ] i
(4b) 5
3
we found c 5 5 3 10 Hz / mol / dm . The small value of the proportionality constant can be rationalized only by excluding Heisenberg exchange completely because this process would lead to a value for c of the order of 3310 9 Hz / mol / dm 3 for a low viscosity solvent [17]. Instead we assume that only fluctuating dipolar interactions contribute to spin relaxation of the encased nitrogen spin. This assumption would be consistent with the observation that negligible spin transfer occurs to the carbon cage, thus preventing overlap of the electronic wave functions during collision with the ‘external’ diffusing spins which otherwise would lead to Heisenberg exchange. The contribution of different paramagnetic species via dipolar relaxation can be compared using Dg 2x " 2 S (x) (S (x) 1 1) ? ]]]]] T 21 ~ fX g 1 v 2d 5
(5)
Fig. 6. Dependence of the increase of spin relaxation rates of N@C 60 as a function of the concentration of TEMPO radicals in a solution of toluene. The relaxation rates were measured at room temperature.
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K.-P. Dinse et al. / Carbon 38 (2000) 1635 – 1640
in which d denotes the distance of closest approach, D gives the relative diffusion constant, gx is the magnetogyric ratio of X ? with spin S (x) , and [X ? ] gives the concentration of the ‘relaxer’ X ? [18]. The expression given in (5) is valid in the slow tumbling limit v 2t 2 4 1, which is not too well obeyed at v / 2p 5 9.5 GHz, the linear dependence on the concentration being valid over the full range of t, however. Eq. (5) indicates that molecular oxygen should be more efficient as relaxing agent than the spin label, because firstly S(S 1 1) is 2 instead of 3 / 4, secondly the relative diffusion constant will be larger, and furthermore the close contact distance d will be reduced for sterical reasons. As seen in Fig. 5, the increase in relaxation rates is about a factor of 10, well in line with this qualitative model.
4. Conclusion The exceptional spin relaxation properties of N@C 60 facilitate to perform high-resolution pulse ENDOR experiments. Originating from nearly perfect de-coupling of the encased quartet spin from the carbon cage, even at room temperature electronic spin lattice relaxation times in excess of several hundred ms are observed. Under these conditions, an rf pulse width-limited spectral resolution of 25 kHz (room temperature) and of 2.5 kHz (80 K) can be obtained. With this excellent spectral resolution we could show that even in the low temperature Pa3¯ phase with its S6 local symmetry at the nitrogen site, no quadrupole splitting could be observed, although long-range order is clearly seen in the EPR spectrum indicated by non-vanishing ZFS. Rotational melting in polycrystalline C 60 could be detected by measuring the dipolar interaction with 13 C nuclei on the fullerene shell. At 80 K, at which the relative orientation of the C 60 molecules in the crystal is fixed, a powder-like ENDOR spectrum is observed. Fast isotropic averaging on the time scale given by the dipolar interaction of |300 kHz occurs at 300 K and also already below the phase transition at 250 K, leading to a ‘solution-like’ 13 C ENDOR spectrum. In low viscosity solutions the effect of fluctuating magnetic fields, originating from additional paramagnetic species, is detectable only with EPR pulse techniques. The relatively small effect induced by spins up to a concentration of 10 21 M is clear proof that Heisenberg exchange during collisions can be neglected. This unique feature results from perfect shielding of the wave function of the encapsulated nitrogen atom. This observation is in accord with a very small value of the isotropic 13 C hfcc, which only recently has been measured as 32 kHz [16].
Acknowledgements Financial support from the Deutsche Forschungsgemeinschaft (Di 182 / 21) is gratefully acknowledged. We are also grateful for a temporary loan of an X-Band ENDOR cavity by Bruker. This investigation is part of a continuing collaboration with B. Pietzak, M. Waiblinger, and A. Weidinger (Hahn-Meitner-Institut, Berlin) who provided part of the N@C 60 samples studied.
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