6. ELECTRONIC STATES AND STRUCTURES
6.2 ESR detection of structural isomers of metallofullerenes
Suzuki et al. (1992) reported the presence of structural isomers of M@C82
(M¼Sc, Y, and La) via ESR hfs measurements and also hfs to 13C in natural abundance on the fullerene cage. Figure 16 shows ESR spectra of Sc@C82, Y@C82, and La@C82 which exhibit13C hfs. Hoinkis et al. (1992)
similarly reported the presence of isomers of La@C82and Y@C82. Bandow et al. (1992) also found the structural isomers of La@C82 as well as the presence of other La-fullerenes such as La@C76and La@C84 by ESR hfs measurements only when anaerobic sampling of soot containing La-fullerenes was employed.
These structural isomers of M@C82have been separated and isolated by HPLC (Section 3). The minor isomer of La@C82, i.e., La@C82 (II), was isolated by Yamamoto et al. (1994b). Similar to the major isomer,
335.5 336.0 336.5
336.5
336.5 336.3 336.1
336.6 336.7
336.7 B (mT)
336.8 336.9
336.9
337.0
337.1 337.3 B (mT)
337.0 B (mT)
337.5 338.0 338.5 A
B
C
FIGURE 16 ESR spectra of (A) Sc@C82in CS2(9.4325 GHz, 0.1 mW); (B) Y@C82in toluene (9.4296 GHz, 0.1 mW); (C) La@C82in toluene (9.4307 GHz, 0.1 mW).
La@C82 (I), La@C82 (II) exhibits an equally spaced octet line but with a smaller hfs constant. Sc@C82also has two structural isomers (I and II) which have been separated and isolated (Inakuma et al., 1995). The two Sc@C82
isomers show equally spaced, eight narrow ESR hfs owing to the hfc to the scandium nucleus (Inakuma et al., 1995; Shinohara et al., 1992b; Yannoni et al., 1992). The hfs of Sc@C82is much bigger than that of La@C82.
Two isomers of Y@C82(I, II) have been separated and isolated by the two-stage HPLC method (Inakuma et al., 1995; Kikuchi et al., 1994b;
Shinohara et al., 1995a,b). Both isomers of Y@C82 show distinct ESR hyperfine doublets due to I¼1/2 yttrium nucleus. The overall ESR spectral patterns of Y@C82(I, II) in CS2solution at room temperature are similar, but the hfs values differ. Moreover, the appearance of the small satellite peaks due to13C, adjacent to the main doublets, is much less clear in Y@C82 (II). Obviously, the electronic structures of the two isomers are different. It has been found that isomer (II) is less stable in air and much more reactive toward various solvents than isomer (I). Yamamoto et al. (1997) produced and isolated three isomers of La@C90 (I–III) and observed the hfs. The observed hfc values are much smaller than those of La@C82(I, II).
Other mono-metallofullerenes are either simply ESR-silent or show fine structures (instead of hfs) only at low temperature. For example, the Gd@C82 metallofullerene exhibits a fine structure at low temperature (Kato et al., 1995b, 1996). Di-metallofullerenes, such as La2@C80, Y2@C82, and Sc2C2@C82, are known to be ESR-silent, indicating that these species are diamagnetic. Knapp et al. (1998) observed ESR signals of Lu@C82
arising from unresolved hyperfine interaction withI¼7/2 nuclear spin of175Lu, whereas no ESR signals were detected for Ho@C82. The unobser- vability of solution and frozen matrix ESR on Ho@C82can be attributed to a high spin state (a5I8ground state for Ho3þ) leading to strong spin lattice relaxation and rigid limit spectral broadening (Knapp et al., 1998). The general inability to observe well-resolved ESR hfs for the lanthanide metallofullerenes Ln@C82(Ln¼Ce,. . ., Lu) as in group 3 metallofuller- enes might also be due to this strong nuclear spin relaxation.
The tri-scandium fullerene, Sc3C2@C80(formerly assigned as Sc3@C82), has so far been the only ESR-active metallofullerene identified other than the mono-metallofullerenes of the typeM@C82(R¼Sc, Y, La). Figure 17 shows perfectly symmetric, equally spaced, 22 narrow ESR hfs, which is a manifestation of the isotropic hfc of three scandium nuclei withI¼7/2 in the C80cage (Shinohara et al., 1992b; Yannoni et al., 1992). The presence of the perfectly symmetric 22 hfs lines suggests the geometrical equivalence of the three scandium atoms in the C80cage (Anderson et al., 1997b; Kato et al., 1994, 1995a; Shinohara et al., 1992b, 1993c; Stevenson et al., 1994b;
van Loosdrecht et al., 1994b; Yannoni et al., 1992). Based on the appear- ance of the perfectly symmetric 22 hfs and the results of theoretical
calculations (Ungerer and Hughbanks, 1993), three scandium atoms form an equilateral Sc3trimer within the C80cage so as to retain a threefold axis as an entire Sc3C2@C80 molecule. The result is consistent with a recent X-ray structural study on Sc3C2@C80 as described in Section 4.3.6. The molecular structure of Sc3C2@C80is shown in Figure 13.
The temperature dependence of the 22 hfs lines can provide us with further structural information on Sc3C2@C80 (Kato et al., 1995b; van Loosdrecht et al., 1994b). The H(magnetic field) value has a minimum at 220 K above which the hfs linewidth increases as temperature increases. A similar temperature dependence has been reported for La@C82(Bandow et al., 1992; Kato et al., 1993). Even at this temperature, the hfs linewidth of Sc3C2@C80(Hpp¼0.4 G at 220 K) is about ten times as broad as that of Sc@C82(0.036 G at 300 K). Such a large linewidth could be due to incomplete motional averaging of local field variations due to strong magnetic anisotropy of the entire molecule (van Loosdrecht et al., 1994b). The intramolecular dynamics is the inherent nature of the Sc3 trimer encapsulated in the C80 cage. In addition to Sc3C2@C80described above, Suzuki et al. (1994) observed a series of ESR hfs due to non-equivalent Sc trimers encaged in fullerene cages other than C80.
The observed ESR hyperfine parameters for M@C82 (R¼Sc, Y, La), La@C90and Sc3C2@C80are summarized in Table 3.
3280 3300
Sc3@C82(isolated)
3320 3340 3360 3380 H (G)
3400 3420 3440 3460
FIGURE 17 ESR spectrum (X-band, 9.4360 GHz) for Sc3C2@C80in CS2solution at 220 K, showing well resolved, equally spaced and perfectly symmetric 22 hfs (g¼1.9985, hfs¼6.51,DHpp¼0.770 G).