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m i.e., they belong to point group Q and have no symmetry-imposed degeneracies, which implies that all levels (electronic, vibrational, rotational, etc.) are nondegenerate and every mode is both IR and Raman-active. Group theory predicts this symmetry lowering, but the intensities of IR and Raman lines do not change much upon addition of 13C isotopes. Optically-inactive modes before the isotopic symmetry-lowering effect remain mostly inactive, and the optically active modes still show strong intensity. The consequences of this symmetry lowering for the IR and Raman study of the normal mode vibrations are further discussed in §11.5.5.

The isotopic distribution has unique consequences with regard to the rotational levels of the C60 molecule and hence also with regard to the libra-tional states of the corresponding solid. This is discussed further in §11.3.3. Other experiments sensitive to the isotopic abundance include NMR measurements (see §16.1) and studies of the isotope effect regarding the superconducting transition temperature Tc (see §15.5).

Table 4.26

Character table for point group Clh.

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