9826
J. Phys. Chem. B 1999, 103, 9826-9830
Pulse Electron Paramagnetic Resonance (EPR) and Electron-Nuclear Double Resonance (ENDOR) Investigation of N@C60 in Polycrystalline C60† N. Weiden, H. Ka1 ss, and K.-P. Dinse* Phys. Chem. III, TU Darmstadt, Petersenstr. 20, D-64287 Darmstadt, Germany ReceiVed: May 3, 1999
By performing high-resolution electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments on nitrogen atoms encapsulated in C60, the capability of the quartet spin system to sense small local fields at the site of the atom is demonstrated. Symmetry lowering induced by a phase transition in polycrystalline C60 at 258 K can easily be detected by the appearance of a zero-field splitting of axial symmetry. Freezing of cage rotation is observed via the magnetic dipole interaction with 13C nuclei of the carbon shell.
Introduction Currently, C60 is the most studied member of the class of all carbon molecules. Because of its unique, nearly spherical structure, it was expected that the orientational potential of C60 would be very soft and that the molecules would be spinning freely at room temperature. However, as pointed out by David et al.,1 the existence of 30 shorter electron-rich “double bonds” and 60 electron-poor “single bonds” gives rise to a nonspherical Coulomb potential leading to an order/disorder phase transition at To ) 258 K. The crystal structure in the high-temperature phase was identified as face-centered cubic (space group Fm3hm),2 and the system undergoes a first-order phase transition at 258 K leading to long-range orientational correlations in a simple cubic phase of four molecules per unit cell (space group Pa3h).3 Because of the weak orientational potential, the individual molecules are not locked permanently in a particular orientation below To, but are librating and performing large angle reorientations with a correlation time of the order of nanoseconds between their equilibrium positions, as was shown in particular by 13C NMR. On the time scale of the NMR experiment, which is determined by the spectral width resulting from the 13C chemical shift anisotropy, orientational freezing occurs for τc ) 100 µs, which is observed at ∼100 K, whereas motional averaging is seen close to To because of τc ≈ 10-9 s. Clearly, no change in the 13C NMR spectrum is expected and observed at the phase transition,4 although the decrease of τc by 3 orders of magnitude leads to a change of the spin relaxation rates. More formally, the carbon atoms occupy general positions in both space groups and therefore no symmetry-dictated change in the parameters of the spin Hamiltonian can be expected, which could otherwise be used for directly probing the phase transition. Recently, C60 molecules have been synthesized in which an additional nitrogen atom is positioned at the center of the carbon frame. These molecules, which are generated by ion bombardment, are stable at room temperature. In addition, it was shown that the encapsulated nitrogen atom is not bound to the carbon cage but rather is found in its quartet spin ground state (4S3/2), * Corresponding author. E-mail:
[email protected]. † Dedicated to Prof. Dr. H. Witte on the occasion of his 90th birthday.
characteristic for the free atom. Obviously, the “centered” paramagnetic atom, which is therefore placed at special positions in the C60 crystal, is an ideal “spy” to sense a change in site symmetry resulting from the phase transition. Furthermore, hyperfine interaction between the paramagnetic spin at the center and the nuclear spins in the shell can be used to detect the reorientational dynamics, which are not directly related to the phase transition. In this contribution, we report the observation of a nonvanishing zero-field splitting (zfs) in the low-temperature phase of crystalline C60 sensed via the quartet electron-spin state of nitrogen. This interaction is “seen” by atoms situated at the special sites of the crystal, irrespective of fast rotational tumbling of the carbon cage. Furthermore, anisotropic and isotropic 13C hyperfine interaction (hfi) is detected using electron-nuclear double resonance (ENDOR) in both crystalline phases, the collapse of the powder-like spectrum at higher temperatures being indicative of rotational melting. Although allowed by symmetry in principle, no quadrupolar interaction with the spin I ) 1 14N nucleus is observed in the low-temperature phase within a limit of a few kilohertz, indicating the much higher sensitivity of the electronic spin for small symmetry-breaking effects. Experimental Section (1) Sample Preparation. N@C60 was prepared by ion bombardment using a low-pressure discharge as described elsewhere.5 The raw product was dissolved in toluene and separated from colloidal particles by microfiltration (0.05 µm). Polycrystalline material was obtained by slowly evaporating excess solvent. The sample was finally sealed off on a highvacuum line. The relative concentration of N@C60 in C60 was estimated as 3‚10-5. (2) Electron Paramagnetic Resonance (EPR)/ENDOR Spectroscopy. All spectra were obtained with a pulse EPR spectrometer (BRUKER ELEXSYS E 580) with integrated pulse ENDOR facility (BRUKER E 560P). Commercial probe heads were used for Fourier transform EPR (FT-EPR) as well as for the ENDOR experiments, which were inserted in an OXFORD CF 935 cryostat. Signal analysis was performed with standard software (BRUKER Xepr).
10.1021/jp9914471 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/28/1999
EPR and ENDOR Study of N@C60
Figure 1. The FT-EPR spectra of N@C60 in polycrystalline C60. Below the phase transition temperature of 260 K, a characteristic powder pattern appears, which can be simulated by assuming an axially symmetric zfs tensor with principle value D ) 0.52 MHz. The insert shows the low-frequency hyperfine component with the simulation in an expended scale. Deviations in the absolute intensities are attributed to dead time artifacts.
Results and Discussion (1) EPR Spectra of N@C60 in Polycrystalline C60. In the high-temperature phase, the C60 molecules have their centerof-mass on a face-centered cubic (fcc) lattice (space group Fm3hm). Accordingly, nitrogen atoms occupying special positions (4a) in the crystal, will experience a site symmetry of Oh. This high symmetry is the reason for vanishing expectation values of all traceless second-rank tensor operators. Because the effective spin Hamiltonian of a S ) 3/2 spin system can be described by tensor operators up to rank two, the spectrum can be explained by invoking scalar terms only; that is, the observation of a solution-like spectrum is predicted in the hightemperature phase. Below Tc, long-range orientational order is established and the symmetry is lowered to simple cubic (sc) with four molecules per unit cell (space group Pa3h). As a result, the site symmetry at the center positions is lowered from Oh to S6. In this group, no higher than two-dimensional irreducible representations exist. The presence of nonvanishing elements of an axially symmetric zfs tensor is therefore allowed by symmetry. For the first-order phase transition at 258 K, this result implies that a typical powder spectrum of a quartet spin should be observable just below Tc in the EPR spectrum. Figure 1 shows EPR spectra measured a few degrees above and below Tc. In the low-temperature phase, the expected satellites are clearly visible at each hyperfine component (hfc). (Although the spectra were taken in the pulsed mode to avoid line broadening by field modulation and/or power broadening, it was found that the additional structure can also be observed by conventional continuous wave (c.w.) EPR, if very low modulation frequencies are used.) The insert shows one of the hfc under higher spectral resolution with a best fit assuming an axially symmetric zfs tensor with D ) 0.52 MHz, D being defined by H/h ) νeSz + D(Sz2 - 5/4). Lowering the temperature, the general feature persists, although the spectral resolution decreases. This result can be attributed to anisotropic 13C hfi being observable at lower temperature when the rapid spinning of the carbon shell is sufficiently slowed.
J. Phys. Chem. B, Vol. 103, No. 45, 1999 9827
Figure 2. Wide-sweep ENDOR spectra of N@C60 above and below the phase transition temperature. Spectra were obtained using a stimulated echo sequence with selective EPR excitation (τπ/2 ) 100 ns) of the low-field 14N hyperfine component.
(2) ENDOR Spectra of N@C60 in Polycrystalline C60. In Figure 2, ENDOR spectra taken at 300 and 250 K are depicted. These spectra, which were obtained using a stimulated echo sequence under conditions of medium spectral resolution (radio frequency (rf) pulse width trf ) 11 and 18 µs corresponding to ∆ν ) 90 and 55 kHz, respectively), are practically identical in both phases. In a quartet electronic spin system, first-order ENDOR transitions (∆ms ) 0, ∆mI ) (1) are expected at
νENDOR′ ) |νn ( Azz(β,γ)/2|
(1a)
νENDOR′′ ) |νn ( 3Azz(β,γ)/2|
(1b)
and at
for I ) 1/2 nuclei like 13C and 1H, which collapse into single transitions in the case of vanishing hfi. Here, νn denotes the nuclear Larmor frequency, the dipolar and quadrupolar coupling terms Azz and Qzz are defined below. For I ) 1 nuclei like 14N, quadrupole interaction (nqi) can lead to an additional splitting, resulting in ENDOR transitions positioned at
νENDOR′ ) |νn ( Azz(β,γ)/2 ( 3/2Qzz(β,γ)|
(1c)
as well as at
νENDOR′′ ) |νn ( 3Azz(β,γ)/2 ( 3/2Qzz(β,γ)|
(1d)
Both dipolar and quadrupolar terms depend on the orientation of the molecule with respect to the external field axis, which is denoted by the Eulerian angles β and γ. In Figure 2, the transitions at the free proton and 13C frequencies depicted can be attributed to “distant” 13C and 1H nuclei with hyperfine coupling constants (hfcc) less than ∼50 kHz. Interaction of the electronic spin with the “local” 14N nucleus leads to two doublets of lines separated by twice the nitrogen nuclear Zeeman frequency, centered at 1/2Azz and 3/2Azz, respectively. No additional splitting resulting from nqi could be detected within the limits of spectral resolution. The expressions just given for the ENDOR transitions are obtained from the eigenvalues of the spin Hamiltonian
H/p ) ωeSz - ωnIz + SAI + IQI truncated to
(2)
9828 J. Phys. Chem. B, Vol. 103, No. 45, 1999
H/p ) ωeSz - ωnIz + AzzSzIz + 3/2QzzIz2
Weiden et al.
(3)
which is appropriate for small hyperfine and quadrupole interactions. In eq (2), second rank electron/nuclear dipolar and quadrupolar coupling tensors are denoted by A and Q, respectively. The coupling constants Azz and Qzz are calculated by transforming the corresponding tensors that are diagonal in their local eigen frame into the laboratory frame resulting in
Qzz ) Qxx(local) (cos2 γ - cos2 β cos2 γ) + Qyy(local) (1 - cos2 γ + cos2 β cos2 γ - cos2 β) + Qzz(local) cos2 β (4) with a similar expression for Azz. In case of a traceless interaction of axial symmetry, this simplifies further to
1 Qzz ) Qzz(local) (3 cos2 β - 1) 2
(5)
in which β denotes the angle between the external field direction and the principal local field axis. The appearance of “solution-like” ENDOR spectra as depicted in Figure 2 implies either vanishing anisotropic terms in the spin Hamiltonian and/or isotropic averaging with sufficiently short correlation time. Spectra taken at 80 K under conditions of improved spectral resolution reveal an extremely narrow 14N transition (Figure 3), indicating a complete lack of nqi and anisotropic hfi within the line width of 4 kHz. This residual width in part is still determined by the Fourier width of the rf pulse (trf ) 370 µs), whose length was limited by the electron spin T1. In contrast, the shape of the 13C transition is determined by anisotropic hfi with the nuclei of the local shell and probably also with distant nuclei (Figure 4). For discussion of the 13C powder ENDOR spectrum, the pattern resulting from coupling to “local” nuclei at 60 equivalent positions in the carbon cage is considered first. Because all these nuclei have the same distance to the electronic spin at the center and carry the same spin density, they are contributing powder pattern of identical shape and width. This equality is a prerequisite for the observation of powder spectra in this multinuclei situation. Considering first ENDOR transitions within the |Ms| ) 1/2 electron spin sublevels, contributions from point dipole-dipole interaction with the spin density at the center lead to singular points in the spectrum at
ωp(1/2) ) (γIγSr-3p 1 ωs(1/2) ) ( γIγSr-3p 2
(6a)
and at
ωp(3/2) ) (3γIγSr-3p 3 ωs(3/2) ) ( γIγSr-3p 2
(6b)
for nuclear spin transitions within the |Ms| ) 3/2 sublevels (here the orientation of the external field B0 parallel and perpendicular to the principal hfi axis is indicated by “p” and “s”, respectively). Using r ) 3.5‚10-10 m, a frequency splitting ∆ωp(1/2)/2π ) 440 kHz and ∆ωp(3/2)/2π ) 1320 kHz is predicted for the most intense “perpendicular” peaks ωs.
Figure 3. Low-frequency component of the Ms ) 1/2 doublet of the 14 N ENDOR signals measured at 80 K with an rf pulse of 370 µs. The signal can be fitted with a Lorentzian of 4 kHz width (fwhm). Weak shoulders can presumably be attributed to the Fourier spectrum of the rf excitation pulse.
Figure 4. The 13C ENDOR line measured at 80 K with an rf pulse width of 60 µs. The low-field 14N hyperfine component of the EPR spectrum was excited either on-resonance or 500 kHz off-resonance with EPR pulses of ∼200 kHz Fourier width. Powder pattern features (*, **) visible under “off-resonance” conditions are attributed to nuclear spin transitions within the |Ms| ) 3/2 electron spin manifolds.
In addition to this contribution to A, which is controlled by the relative position of carbons and the nitrogen atom at the center, transferred spin density to the cage results in a small isotropic coupling as well as in an additional anisotropic coupling tensor, which is axially symmetric and collinear with the point dipole contribution by symmetry arguments. Whereas the isotropic hfc was measured independently by an analysis of the 13C ENDOR transition in the high-temperature phase as aiso ) 32(2) kHz (vide infra), a reliable estimate of the additional anisotropic term is not yet possible because the actual amount of transferred spin density is not known. For an assignment of the observed edges in the ENDOR spectrum either to the ω(3/2) or ω(1/2) manifold, selective EPR excitation was invoked. As is seen from Figure 1, the weak shoulders in the EPR spectrum can be attributed to EPR transitions involving the Ms ) 1/2 and 3/2 electron spin sublevels, whereas the strong center line is formed predominantly by transitions from Ms ) 1/2 to Ms ) -1/2. The 13C
EPR and ENDOR Study of N@C60 ENDOR signals were accumulated using a highly selective τπ(5 µs) - τrf(60 µs) - τπ/2(2.5 µs) - τ(15 µs) - τπ(5 µs) echo sequence at 0 and 500 kHz frequency offset with respect to the center line. Figure 4 shows that the prominent edges (*) in the spectrum are clearly seen only when using off-center excitation. From this observation we conclude that these structures must originate from nuclear spin transitions within the Ms ) 3/2 electron levels. With this assignment, the pattern resulting from the Ms ) 1/2 manifold being narrower by a factor-of-three can no longer be resolved and contributes only to the strong central peak in the ENDOR spectrum. Spectral intensity at the center is also expected when taking into account 13C nuclei in next-neighbor molecules. For these spins with an average distance of 10‚10-10 m, the pattern collapses by a factor of ∼20, and can no longer be resolved. Insufficient signal-to-noise ratio, despite 14 h of data accumulation, prevents a detailed fit of the spectrum. We therefore assign the outmost spectral ridges (**) to ( ωp(3/2), obtaining Azz ) 300(30) kHz, clearly at variance with the value predicted using a simple point dipole model. We conclude that there must be a significant contribution to Azz resulting from spin density residing on the carbon atoms and that this term must counteract the point dipole interaction with the central atom. This conclusion indicates that the sign of the principal element of this local hfi tensor is negative. As expected, we observed a collapse of the dipolar structure when measuring at elevated temperatures, of 300 and at 250 K above and below the phase transition, respectively. Because of fast isotropic reorientation, the 13C ENDOR spectrum should be determined by the isotropic hfsc only. Because of unsurpassed spectral resolution obtainable with ENDOR, we therefore investigated the sample at ambient temperature for a confirmation of the small isotropic 13C hfsc, which was previously determined by a line-shape analysis of a high-resolution EPR spectrum of N@C60 in solution.6 In this study, aiso(13C) was deduced as 36(2) kHz. To be able to resolve the expected line quintet (consisting of a strong line at the “free” 13C Larmor frequency and four equidistant lines), a rf frequency resolution of at least 10 kHz would be required, necessitating rf pulses of 100-µs length. The length of the entire microwave/rf pulse sequence used for ENDOR detection is limited, however, by the electronic spin relaxation time T1, which is ∼200 µs in polycrystalline C60 at room temperature. The spectrum depicted in Figure 5 was measured with a selective τπ(5 µs) - τrf(40 µs) - τπ/2(2.5 µs) - τ(12 µs) τπ(5 µs) - echo sequence, compromising frequency resolution for signal-to-noise. The spectrum can be fitted with a line quintet using aiso(13C) ) 32(1) kHz and a line width of 25 kHz, which is consistent with the length of the rf pulse. In this experiment there is no ambiguity in the assignment of “Ms ) (3/2” transitions to the resolved shoulders, the “Ms ) (1/2” transitions being only barely resolved from the central peak, which can be attributed to distant 13C nuclei. The small difference between the value measured by ENDOR for N@C60 in polycrystalline C60 from the value determined previously in solution is not considered as significant, and no matrix-induced change in spin density transfer from the nitrogen atom to the carbon cage is postulated. To rationalize the small value of the 13C hfsc and to relate its value to the amount of spin density transferred from the central atom to the cage, the structure of N@C60 was first optimized on a BLYP/STO-3G level followed by a single-point calculation using BLYP/3-21G. An isotropic hfsc of 20 kHz is predicted, along with a total transfer of spin density of 2%.
J. Phys. Chem. B, Vol. 103, No. 45, 1999 9829
Figure 5. The 13C ENDOR signal detected at 300 K. The signal can be fitted using an isotropic splitting constant aiso(13C) ) 32(1) kHz and line width ∆ν1/2 ) 25(1) kHz (fwhm).
Because the calculated coupling constant is within a factor of 2 of the observed value, the predicted small transfer of spin density is taken as additional evidence that the model of an isolated but confined nitrogen atom is essentially correct. Conclusion The exceptional spin relaxation properties of N@C60 facilitate performing high-resolution pulse ENDOR experiments. Originating from nearly perfect decoupling of the encased quartet spin from the carbon cage, even at room-temperature electronic spin lattice relaxation times in excess of several 100 µs are observed. Under these conditions, an rf pulse width-limited spectral resolution of 25 kHz at room temperature and of only 2.5 kHz at 80 K can be obtained for the ENDOR lines. Using this spectral resolution we could show that even in the low temperature Pa3h phase with its S6 local symmetry at the nitrogen site, no quadrupole splitting could be observed, although longrange order is clearly seen in the EPR spectrum indicated by nonvanishing zfs. Rotational melting in polycrystalline C60 could be detected by measuring the dipolar interaction with 13C nuclei on the fullerene shell. At a temperature of 80 K, at which the relative orientation of the C60 molecules in the crystal is fixed, a powder pattern is observed at the 13C frequency. In contrast, fast isotropic averaging with a correlation time smaller than the inverse dipolar interaction of ∼300 kHz occurs at 300 K and also below the phase transition at 250 K, leading to a “solutionlike” 13C ENDOR spectrum. The small observed isotropic 13C hfsc of 32(1) kHz is in agreement with predictions from quantum chemical calculations modeling the encased nitrogen as an isolated spin system with negligible spin transfer to the cage. Acknowledgment. Financial support from the Deutsche Forschungsgemeinschaft (Di 182/21) is gratefully acknowledged. We are also grateful for a temporary loan of a X-Band ENDOR cavity by BRUKER. References and Notes (1) David, W. I. F.; Ibberson, R. M.; Matthewman, J. C.; Prassides, K.; Dennis, T. J. S.; Hare, J. P.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Nature 1991, 353, 147.
9830 J. Phys. Chem. B, Vol. 103, No. 45, 1999 (2) Heiney, P. A.; Fischer, J. E.; McGhie, A. R.; Romanow, W. J.; Denenstein, A. M.; McCanley, J. P.; Smith, A. B., III; Cox, D. Phys. ReV. Lett. 1991, 66, 2911. (3) Bu¨rgi, H. B.; Blanc, E.; Schwarzenbach, D.; Lin, S.; Kappes, M. K.; Ibers, J. A. Angew. Chemie Intern. Ed. Engl. 1992, 31, 641.
Weiden et al. (4) Tycko, R.; Dabbagh, G.; Fleming, R. M.; Haddon, R. C.; Makhija, A. V.; Zahurak, S. M. Phys. ReV. Lett. 1991, 67, 1886. (5) Pietzak, B.; Waiblinger, M.; Almeida Murphy, T.; Weidinger, A.; Ho¨hne, M.; Dietel, E.; Hirsch, A. Chem. Phys. Lett. 1997, 279, 259. (6) Knapp, C.; Weiden, N.; Dinse, K.-P., unpublished results.
