Spherical Aromaticity: Recent Work on Fullerenes, Polyhedral Boranes, and Related Structures†

Recently, the presence of fullerenes in the interstellar medium (ISM) has been confirmed and new findings suggest that these fullerenes may possibly form from polycyclic aromatic hydrocarbons (PAHs) in the ISM. Moreover, the first confirmed identification of two strong diffuse interstellar bands (DIBs) with the fullerene, C 60 + , connects the long standing suggestion that various fullerenes could be DIB carriers. These new discoveries justify reassessing the overall importance of interstellar fullerene compounds, including fullerenes of various sizes with endohedral or exohedral inclusions and heterofullerenes (EEHFs). The phenomenology of fullerene compounds is complex. In addition to fullerene formation in grain shattering, fullerene formation from fully dehydrogenated PAHs in diffuse interstellar clouds could perhaps transform a significant percentage of the tail of low-mass PAH distribution into fullerenes including EEHFs. But many uncertain processes make it extremely difficult to assess their expected abundance, composition and size distribution, except for the substantial abundance measured for C 60 + . EEHFs share many properties with pure fullerenes, such as C 60 , as regards stability, formation/destruction and chemical processes, as well as many basic spectral features. Because DIBs are ubiquitous in all lines of sight in the ISM, we address several questions about the interstellar importance of various EEHFs, especially as possible carriers of diffuse interstellar bands. Specifically, we discuss basic interstellar properties and the likely contributions of fullerenes of various sizes and their charged counterparts such as C 60 + , and then in turn: 1) metallofullerenes; 2) heterofullerenes; 3) fulleranes; 4) fullerene-PAH compounds; 5) H 2 @C 60 . From this reassessment of the literature and from combining it with known DIB line identifications, we conclude that the general landscape of interstellar fullerene compounds is probably much richer than heretofore realized. EEHFs, together with pure fullerenes of various sizes, have many properties necessary to be suitably carriers of DIBs: carbonaceous nature; stability and resilience in the harsh conditions of the ISM; existing with various heteroatoms and ionization states; relatively easy formation; few stable isomers; spectral lines in the right spectral range; various and complex energy internal conversion; rich Jahn-Teller fine structure. This is supported by the first identification of a DIB carrier as C 60 + . Unfortunately, the lack of any precise information about the complex optical spectra of EEHFs and most pure fullerenes other than C 60 and about their interstellar abundances still precludes definitive assessment of the importance of fullerene compounds as DIB carriers. Their compounds could significantly contribute to DIBs, but it still seems difficult that they are the only important DIB carriers. Regardless, DIBs appear as the most promising way of tracing the interstellar abundances of various fullerene compounds if the breakthrough in identifying C 60 + as a DIB carrier can be extended to more spectral features through systematic studies of their laboratory gas-phase spectroscopy.

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Interstellar fullerene compounds and diffuse interstellar bands

This article presents the results of systematic ab initio quantum chemical study of charged and neutral analogues of fullerene molecules: C59X[XSi, Ge, Sn], C59X− [XB, Al, Ga, In], and C59X+ [XN, P, As, Sb]. Hartree–Fock (HF) and density functional theory (DFT) levels of theory with Stuttgart–Dresden basis set were used to investigate the structure and properties of substituted fullerene molecules. A replacement of fullerene carbon atom with a heteroatom results in a unique chemical site on the fullerene surface, which may be used as a reactive center or to modify the electronic properties. We show the possibility of utilization of substituted fullerenes as atom-like building units. Heteroatom substitution allows the tuning of the physical and chemical properties of original molecule for different material science and nanotechnology applications. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Ab initio quantum chemical studies of fullerene molecules with substitutes C59X [XSi, Ge, Sn], C59X− [XB, Al, Ga, In], and C59X [XN, P, As, Sb]

The stability of all 23 C58N2 and C58B2 heterofullerenes in the singlet and triplet states was determined at the B3LYP/6-31G** level. In equilibrium mixture the achiral (1,4) C58N2 isomer would be ...

On stability, chirality measures, and theoretical VCD spectra of the chiral C58X2 fullerenes (X = N, B).

Systematic investigation of the molecular behaviors of heterofullerenes C48X2 (X=B, N)

The heterofullerenes C59N and C69N and their dimers are examined by the intermediate neglect of differential overlap (INDO/1 and INDO/S) models. The results confirm the stability of the (C59N)2 and (C69)2 (C2h) isomers with a 6–6 closure fusion link between the two monomers. The nearest neighbor C of nitrogen, at the hexagon–hexagon fusion in C59N and in C69N where the N replaces one C in the pentagon of the fullerene has the largest electronic spin density, and this is where the two radicals of C59N or C69N link to form dimers. Also the dimers readily dissociate to free radical monomers to react regioselectively with other reagents. The electronic spectra of the radicals are similar to those of the dimers, which suggest essentially identical electronic structures. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 422–436, 2000

Theoretical investigation of the heterofullerenes C59N and C69N and their dimers

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Theoretical investigation of the heterofullerenes C59N and C69N and their dimers