EPR Investigation of Atoms in Fullerene Traps

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EPR INVESTIGATION OF ATOMS IN FULLERENE TRAPS
K.-P. Dinse, H. Käß, N. Weiden, and C. Knapp
Phys. Chem. III, TU Darmstadt, Petersenstr. 20, D-64287 Darmstadt, Germany
e-mail: dinse@pc07.pc.chemie.tu-darmstadt.de
ABSTRACT
By performing high-resolution EPR and 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 additional line
splittings indicating the presence of Zero-Field-Splitting of axial
symmetry. Freezing of cage rotation is observed via the magnetic
dipole interaction with 13C nuclei of the carbon shell. Fluctuating
magnetic fields originating from additional paramagnetic species in
solution can also be detected by their influence on the spin relaxation
times.
INTRODUCTION
C60 is currently 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 be very soft and that the molecules would be spinning freely at room
temperature. The crystal structure in the high temperature phase was identified as face-
centered cubic (space group
m3Fm
) /1/ and the system undergoes a first-order transition
at Tc = 258(2) K leading to long-range orientational correlation in a simple cubic phase of
four molecules per unit cell (space group
3Pa
) /2/.
Because of the weak orientational potential, the individual molecules are not locked
permanently in a particular orientation below Tc, 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 /3/.
Recently, C60 molecules could be synthesized in which an additional Nitrogen atom
is positioned at the center of the carbon frame /4/ as is shown in Fig. 1. These molecules,
which are generated by ion bombardment /5/ are stable at room temperature and 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), characteristic for the free atom /6-8/.
Obviously, the paramagnetic atom, which is placed at special positions in the C60 crystal
is an ideal „spy“ to sense a change in site symmetry resulting from the phase transitions.

Furthermore, hyperfine interaction between the paramagnetic spin at the center and the
nuclear spins in the shell can also 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 non-vanishing 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 ENDOR in both
crystalline phases, the collapse of the powder-like spectrum at higher temperatures being
indicative of rotational melting.
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.
EXPERIMENTAL
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 micro-filtration (0.05 µ m). Polycrystalline material was obtained
by slowly evaporating excess solvent. The sample containing either the polycrystalline
material or a solution in toluene or CS2 was finally sealed off on a high-vacuum line. The
relative concentration of N@C60 in C60 was estimated as 3∙10-5. 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 FT-EPR as
well as for the ENDOR experiments, which were inserted in an OXFORD CF935
cryostat. Signal analysis was performed with the BRUKER Xepr program.
RESULTS AND DISCUSSION
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
m3Fm
). Accordingly, they experience an
exceptionally high Oh site symmetry. This high symmetry is the reason for vanishing
expectation values of all traceless second-rank tensor operators. 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
3
Pa
). As a result, the site symmetry at the
center positions is lowered from Oh to S6. The presence of non-vanishing elements of an
axially symmetric ZFS tensor therefore is allowed by symmetry. For the first-order phase
transition at 258 K this implies that a typical powder spectrum of a quartet spin should be
observable just below Tc in the EPR spectrum. This spectrum can be calculated using an

effective Hamiltonian (assuming axial symmetry for the ZFS)
)
3
1
(/ 2
2SH −
+= zz
eS
DS
ω
h
(1)
Fig. 1 shows EPR spectra measured above and below Tc. In the low temperature
phase (but still at elevated temperatures) the expected additional spectral features are
clearly visible. (Although the spectra were taken in the pulsed mode in order to avoid line
broadening by field modulation and/or power broadening, it should be noted that the
additional structure can also be observed by conventional continuous wave (c. w.). EPR,
if very low modulation frequencies are used.) At lower temperatures additional
broadening by anisotropic 13C hfi is observed because of rotational freezing.
-20 -18 -16 -14 -12
140 K
265 K
255 K
microwave frequency offset ( MHz )
Figure 1 FT-EPR spectra of N@C60 in polycrystalline C60 showing the 14N low-
frequency hyperfine component. Below the phase transition temperature of 258 K, a
powder pattern characteristic for a quartet spin system appears.
The observed powder spectrum can be fitted by invoking an axially symmetric ZFS with

a principal coupling element D/2π = 0.52 (2) MHz as is shown in Fig. 2. This value is
much smaller than the value measured for the mono adduct N@C61(COOEt)2 /5/, for
which the deformation of the cage is a local phenomenon. Here, the finite D results from
long range order of the quickly reorienting C60 cages, the order being defined by a
preferred average orientation in the crystal.
-2 -1 0 1 2
simulated
experimental
frequency offset ( MHz )
FT-EPR signal amplitude ( arb. units )
Figure 2 Fit of the observed powder pattern assuming an axially symmetric ZFS
tensor with D/2π = 0.52 MHz. Deviations from the calculated spectrum might result from
dead time related truncation of broad structures in the FT-ERP spectrum.
2) ENDOR spectra of N@C60 in polycrystalline C60
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 trf = 15 µs corresponding to ∆n
≅ 65 kHz), already shows incipient line broadening for the 13C transition, expected under
conditions of frozen cage rotation because of anisotropic hfi.

In a quartet electronic spin system, first order ENDOR transitions (∆ms = 0, ∆mI =
±1) are expected at
),
(Q
2
/3
2/
)
,(
A'
zz
zznENDOR
γβ
±γ
β
±n=n
(2a)
as well as at
),(Q2/32/),(A3'' zz
zznENDOR γβ±γβ±n=n
(2b)
Both, dipolar and quadrupolar terms depend on the orientation of the molecule with
respect to the external field axis, which are denoted by the Eulerian angles
β
and
γ
.
In Fig. 3, the transitions at the free proton can be attributed to "distant" 1H nuclei
with hyperfine coupling constants (hfcc) less than approximately 50 kHz. These protons
indicate the presence of residual solvent molecules. Interaction of the electronic spin with
the "local" 14N nucleus leads to two doublets of lines, centered at Azz/2 and 3Azz/2,
510 15
80 K 1
H
14N
13C
frequency ( MHz )
Figure 3 Pulse ENDOR spectrum of N@C60 showing the 13C and 1H transitions as
well as 14N transitions originating from Ms = 1/2 electronic spin sublevels.

respectively, separated by twice the Nitrogen nuclear Zeeman frequency, the former
doublet being visible in Fig. 3.
Spectra taken at the same temperature under conditions of improved spectral
resolution reveal that this doublet consists of extremely narrow 14N ENDOR lines,
indicating a complete lack of nuclear quadrupol interaction and anisotropic hfi within the
line width of 4 kHz /9/. In contrast, the shape of the 13C transition is determined by
anisotropic hfi with the nuclei of the local shell as well as probably with distant nuclei.
Using the same experimental conditions allowing ENDOR frequency resolution of at
least 3 kHz, a powder-like spectrum is detected at the 13C nuclear Zeeman frequency. For
a discussion of the 13C powder ENDOR spectrum as depicted in Fig. 4, the pattern
3.0 3.5 4.0 4.5
250 K
80 K
frequency ( MHz )
Figure 4 13C 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 13C Zeeman frequency.

resulting from coupling to 13C 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 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 |Ms| = 1/2 electron
spin sublevels at
h
h
3
SI
)2/1(
s
3
SI
)2/1(
p
r
2
1
r
−
−
γγ±=ω
γγ±=ω
(3a)
and at
h
h
3
S
I
)2
/3(
s
3
S
I
)2/3(
p
r
2
3
r
3
−
−
γγ
±=ω
γ
γ±=
ω
(3b)
for nuclear spin transitions within the |Ms| = 3/2 sublevels. 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. Using selective excitation of electron spin
transitions, however, a value for Azz = 300(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 13C 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 13C hfcc of 32 (2) kHz /9/.
3) 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 T2 and T1 using echo
techniques, or more easily by observing T2 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 µM can be determined quantitatively, the sensitivity
being limited by the residual line width of the spin probe /10/. We anticipated that much
lower concentrations of paramagnetic impurities should be detectable using N@C60
instead, because its line width is more than one order of magnitude less. Surprisingly, the

line width of the N@C60 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.
102
103
104
105
0.001 0.01 0.1
x : ∆T
2
-1
o : ∆T
1-1
calibration with TEMPO
solution saturated
with pure oxygen
solution saturated
with air
concentration of paramagnetic species (mol/lit)
∆T
1
-1
, ∆T
2
-1
(s
-1
)
Figure 5 Dependence of the increase of spin relaxation rates of N@C60 as function
of the concentration of TEMPO radicals in a solution of Toluene. The relaxation rates
were measured at room temperature.
As is seen in Fig. 5, a calibration of the change in relaxation rates using the stable
Nitroxide radical TEMPO indicates a linear increase of ∆Ti-1 (i = 1, 2) with concentration
of paramagnetic species. Here, ∆Ti-1 is defined as
)0()(
111 −
•
−−
−=∆
iii
TXT
T
(4a)
the reference values Ti-1(0) being given by the relaxation rates of the pure sample, Ti-1(X.)
denoting the rates in the presence of the relaxer X.. Writing
][
1•
−
⋅=∆ XcT
i
(4b)
we found c = 5.105 Hz/Mol. 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 in the order of 3.109 Hz/Mol for a low viscosity solvent /10/.
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
[ ]
.
52
)x()x
(2
2
x
1
1X
d
D)1S(
S
Tω
⋅+γ
∝
−h
(5)
in which d denotes the distance of closest approach, D gives the relative diffusion
constant, γx is the magneto-gyric ratio of X with spin S(x), and [X.] gives the
concentration of the relaxer X. /11/. The expression given in (5) is valid in the slow
tumbling limit
ω2τ2 >> 1, which is not too well obeyed at ω/2π = 9.5 GHz, the linear dependence on the
concentration being valid over the full range of τ, however. Eq (5) indicates that
molecular Oxygen should be more efficient as relaxer than the spin label, because first
S(S+1) is 2 instead of 3/4, second the relative diffusion constant will be larger, and
furthermore the close contact distance d will be reduced for steric reasons. As seen in Fig.
5, the increase in relaxation rates is about a factor of 10, well in line with this qualitative
model.
CONCLUSION
The exceptional spin relaxation properties of N@C60 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 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. Using this spectral resolution we could show that even
in the low temperature
3Pa
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 C60 could be detected by measuring the dipolar
interaction with 13C nuclei on the fullerene shell. At 80 K, at which the relative
orientation of the C60 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
approximately 300 kHz occurs at 300 K and also already below the phase transition at
250 K, leading to a "solution-like" 13C 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 10-1 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 13C hfcc, which only
recently has been measured as 32 kHz /9/.
ACKNOWLEDGEMENT
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. 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@C60 samples studied.
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