The Joumal of
Physical Chemistry ~~
Q Copyright 1993 by the American Chemical Society
VOLUME 97, NUMBER 51, DECEMBER 23,1993
LETTERS ESR Study of the Electronic Structures of Metallofullerenes: A Comparison between hac82 and s c cS2 Tstsuhisa Kato’ Institute for Molecular Science, Myodaiji. Okazaki 444, Japan
Shinzo Suzuki, Koichi Kikuchi, and Yohji Achiba’ Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan Received: June 8. 1993; In Final Form: October 4, 1993’
The line widths of the hyperfine components of ESR spectra of La@Csz and SC@CSZin CSZand toluene solutions have been measured as a function of temperature. The extent of line broadening due not only to the insufficient rotational averaging of g and hyperfine tensors but also to the spin-rotation coupling interaction have been determined. The values of anisotropy for the g and hyperfine tensors are deduced by analysis according to Kivelson’s formalism for these line broadening mechanisms. Although the two spectra for L a @ & and Sc@Cez in solution at room temperature look similar to each other, the different electronic structure for each endohedral metal is reflected in their differing values for the anisotropy of the g and hyperfine tensors. The electronic structures of La@Csz and SC@CSZ are discussed comparatively in terms of a theoretical interpretation for the anisotropic correction of the g factor and the hyperfine coupling constant. The results exhibit different electronic structures for La@Csz and Sc@Csz; the radical electron is assigned to a ?r orbital of the CSZcage for La@Csz but to a d orbital of the metal for s C @ c S Z .
Introduction
Since the successful laboratory synthesis of Cm and other carbon cluster yfullerenes”, there have been many papers concerning these clusters and their chemical derivatives reported in the past few years. For fullerenes associated with a metal center, M @ C82 (M = Sc, Y,and La) are the only reported metallofullerenes to have been macroscopically prepared. The high stabilityof M @CSZ under 1 atm pressure suggests the encapsulation of the metal inside ,a C82 cage, and this peculiarity of Csz as a cage molecule proposes a point for investigation concerning the formation mechanism of fullerenes using the arc-discharge method of synthesis. From this point of view a study of the electronic structure of these endohedral complexes is expected to be very significant. Here we report an investigation into the electronic Abstract published in Adounce ACS Absrrucrs, December 1, 1993.
structure of M a c 8 2 by analysis of the ESR spectral change with temperature. The detection of 3A group metal atoms encapsulated in Caz, La@Cs2,1-3Y @ C82,4,*and Sc@CSz6s7 has been reported recently. The discussion of the electronic state of these metallofullerenes has been based upon the results of ESR measurements in solution at ambient temperature, and theelectronicstate wassubsequently described as Me3+@C823-. This feature of a +3 oxidation state for the metal with a -3 anion radical of the C ~cage Z was deduced from extraordinarily small isotropic hyperfine coupling (hfc) constants measured for the metal nucleus and a g factor similar to that of a Cm- radical anion reported before. Theseexperimental results, however, are not direct evidenceof a M3+@C8Z3-electronic state, nor can they exclude the possibility that a radical electron occupies the d orbital of the metal center. Recent laser desorption and thermal desorption mass spectrometric measurements of
0022-3654/93/2097-13425304.00/0 Q 1993 American Chemical Society
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13426 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
primary metallofullerene soots were interpreted as showing the presence of the +2 oxidation state for the electronic structure of the metallofullerene other than the +3 state.* In order to experimentally characterize the electronic structure of these metallofullerenes, anisotropic components of hfc and g tensors need to be measured. The values of thegfactor and hfc constant for radicals, including the anisotropic components, are given by an analysis of the ESR spect7al change with temperature in solution. The line widths of the ESR spectra of vanadyI acetylacetonate in liquid toluene have been measured as a function of temperature by Kivelson et al.,9 and two line broadening mechanisms which are due to the anisotropic g and hfc tension components, and the spin-rotation interaction, were demonstrated. The line widths of thevanadium hyperfine components were fitted to a function with two components: a cubic polynomial of the nuclear magnetic spin quantum number of vanadium and a residual line width. The anisotropic gand hfc tensors could then be deduced from Kivelson's formula.
SCQC,,
17 OK
3355
3350
l
l
l
3355
3350
3365
3360
l
3360
3370
3380
217K
l
3365
3375
3370
I
I
3375
3380
290K
I
Experimental Section The metallofullerene containing soot was produced by a dc arc discharge of a composite rod of graphite and metal oxide (La2O3, Sc2O3) under -200 Torr He pressure. The soot was extracted by carbon disulfide (CS2). CS2 solutions of the extracts were degassed using a freeze-pumpthaw cycle and sealed in quartz tubes. The ESR spectra were obtained with a Bruker ESR300E spectrometer in combination with a temperature control unit set for temperatures between 290 and 170 K.
,
,
I
3350
3355
3360
3365
I
I
3370
3375
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3380
Magnetic F i e l d / Gauss
Figure 1. ESR spectra of Sc@Cg2 in liquid CS2 measured at 290, 217, and 170 K.
ReSults
ESRspectraofSc@Cs2in CSlmeasuredat 290,217, and 170 K are shown in Figure 1 . The eight lines are attributed to the hyperfine structure of the Sc nuclear magnetic moment of 7/2, each peak having equal intensity due to their equal statistical weight, as seen in the spectrum at 290 K. The intensity of the hyperfine components at higher magnetic field in the spectra at 217 and 170 K appears reduced by a line broadening effect. This broadening effect depends on the quantum number, m1, of the Sc nuclear magnetic moment and increases with decreasing temperature. The line widths of the Sc@Cs2 spectrum at the various temperatures are plotted with the quantum number ml in Figure 2 . The plot showsa parabolicdependenceon ml, and the parabola becomes steeper at lower temperature. The line widths at maximum slope Aums1in the ESR spectra were fitted to an expression of the form AumsI= k,
+ k l m l + k,m12
where
+
ko = m c { ( 7 / 4 S ) ( A y B o ) 2 63b2/16)+ K
(1)
according to the theory of line broadening developed by Kivelson et aL9 Kivelson originally proposed a cubic polynomial formula, but our data show that the cubic term is too small to be determined experimentally; consequently, a quadratic formula is adopted here. The parameter BOis the magnetic field intensity, and Ay and b are the anisotropy amplitude of the g and hfc tensors and are expressed as
b = (2/3)(a, - (a, + ay)/2j The rotational correlation time
T~
is given by Debye theory as
rC= 4?raR3/3kT
in which 7 is theviscosity of the solvent and R is the hydrodynamic radius of the molecule. The coefficients kl and k2 plotted against v / T are crudely proportional to ( v / T ) , as shown in Figure 3 . Assuming a hydrodynamic radius of 5.8 A and taking the viscosity I) from the literature,I0 the amplitude of the anisotropy of the hfc tensors a, - (ax+ uy)/2can be estimated from the slope of the plot of k~ vs v / T . The constant term ko, however, contains two components, as shown in Figure 3. One is represented by the first term in formula 1 , which linearly depends on ( q / T ) , and the other corresponds to the residual term K,which increases with increasing temperature. This residual line width K has been attributed to a spin-rotation interaction and expressed by the form
K = ( 1 / ( 12d3?rR3))(Ag: + Ag:
+ Ag;)kT/q
(4) At higher temperature the residual term Kwill become dominant and the parameter Agz2+ Agx2+ Agy2can be obtained from the value of the line width a t high temperature. These parameters are tabulated in Table 1 along with the values of the isotropic g factors (giso) and hfc constants (aiso). The spectra of La@& in CS2 solvent (not given) show the same broadening effect as Sc@C82,and the line widthsareplotted against mI in Figure 4. Comparing these with the results of scQcs2, it is noteworthy that the line widths of La@Cs2 shows a linear dependence on m1 a t all temperatures. This prominent feature is due to the much smaller value of the parameter az (a, + ay)/2 for LaQCg2 than for Sc@Cg2. which is given as a maximum limiting value in Table 1. On the other hand, similar
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The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13427
/
lSc@Ce2
cs21
0
197K/
--
lo
7i2
512
312
112
-172
-372
-5'12
-7j2
Q u a n t u m Number MI
/ T ....x ....".....**' 18OK 238K
,]:l
217K
I
I
I
10
20
I
I
I
50
60x10'6
50
60x10"
0 0
3 q I T 40
4 x 1 0.'
Ki
V.S.
T/T
-2
X 0
10
20
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-
0.8
2
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-
0.2
0.0
-I 0
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I
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20
34,T
40
50
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Figure 3. Coefficients KO,KI, and K2 plotted against v / T .
TABLE I: Summary of the Values of Isotropic g Factors and Hyperfiie Coupling Constants Obtained by ESR Measurement. ~~
~
2.0012
I
1)2
F i p e 2. Line widths of the spectra for Sc@Cr~z at various temperatures plotted against quantum number m~of the Sc nuclear magnetic moment.
20 15
10
2.0002
Ag,~+Agxx2+Agyyz ( 1 6 . 4 f 0 . 8 ) X lW5 ( l l . l f O . 6 ) X lW5 a i (GI 1.2 3.8 3.1 f 0.2
