Vibrational spectroscopic studies of fullerenes and their derivatives
Vibrational spectroscopic (IR and Raman) studies of the fullerenes can provide the valuable information of the structure, bonding and internal dynamics of these molecules. However, to get the full advantages of these methods, detailed interpretation of the spectra is required, which can be now achieved with the help of high level DFT calculations. Combining experimental and theoretical studies, we have been able to conform molecular structures of several endohedral metallofullerenes and obtained information on the state of the endohedral cluster and on the cluster-cage bonding. For the derivatives of empty fullerenes, spectral patterns are very characteristic for each addition pattern, and hence vibrational spectroscopy can be powerful structural tool.
Vibrational spectroscopic studies of endohedral metallofullerenes
The figure shows experimental spectrum of the major isomer of Dy3N@C78 isolated in IFW Dresden in comparison to the computed IR spectra of two Y3N@C78 isomers, IPR D3h(5) and non-IPR C2(22010). The spectrum of the non-IPR isomer perfectly matches experimental data, while the IPR isomer exhibits poor agreement. Thus, comparison of DFT-computed vibrational spectra of possible isomers to the experimental spectra can be used to determine molecular structures of new endohedral metallofullerenes, especially when other techniques such as NMR and single-crystal X-ray diffraction are not available.
The figure shows the frequency of the metal-nitrogen antisymmetric stretching mode in a series of M3N@C80 clusterfullerenes as a function of the ionic radius of the metal. From Lu to Dy the frequency remains almost the same, but for Tb and Gd significant decrease of the vibrational frequency is found. These data correlates well with the pyramidalization of the M3N cluster inside the carbon cage known from single-crystal X-ray diffraction data: from Lu3N to Dy3N the cluster is almost planar, however for larger metal atoms the steric hindrance forces pyramidalization of the cluster. An inset shows DFT-optimized structure of Gd3N@C80 where pyramidalization of Gd3N can be clearly seen.
For details on synthesis and spectroscopic studies of EMFs in IFW Dresden, see Synthesis of Nanomaterials
Vibrational spectroscopic studies of the derivatives of empty fullerenes
Vibrational spectra of fullerene derivatives are very senstive to the addition pattern. The figure (left) shows Raman spectra of two isomers of C60F48 with D3 and S6 symmetry, whose Schlegel diagrams are shown on the right. The structures of the isomers are very similar, they are different only in position of three double bonds (highlighted in red and blue). However, even such small structural differences can be easily distinguished by the vibraional spectra. Comparison of experimental Raman spectrum to the calculated spectra of two isomers clearly shows that the D3 isomer is dominating in the product of synthesis.
The studies of the fullerene derivatives are performed in close collaboration with Strauss/Boltalina research group in Colorado State University (Fort Collins, USA)