Heteroatom impacts on structure, stability and aromaticity of X n C 20n fullerenes: A theoretical prediction M.Z. Kassaee a, * , F. Buazar a,b , M. Koohi a a Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran b Danish Power Systems Aps, Raadhusvej 59, DK 2920, Charlottenlund, Denmark article info Article history: Received 30 April 2009 Received in revised form 28 September 2009 Accepted 1 October 2009 Available online 6 October 2009 Keywords: Fullerene C 20 Heteroatom Ab initio DFT NICS Heat of atomization abstract Thirteen heterofullerenes with (X) n C 20n formula are compared and constricted at MP2/6-311+G* and B3LYP/6-311+G* levels of theory (X = B, N, P; and n = 2, 4, 5, 6, 10). The heats of atomization energies, band gaps (DE HOMO–LUMO ), aromaticity, geometrical and conformational structures of these heterofulle- renes strongly depend on their number of heteroatoms, filling patterns, locations and ‘‘B-, N-site attach- ments”. Fullerenes P 10 C 10 and N 10 C 10 are not energetically favorable for showing at least one imaginary state. Also, owing to localized electron structures in N 4 C 16 and high electron deficiencies in B 10 C 10 , these isomers turn out as non- and anti-aromatic, respectively. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The dodecahedral fullerene C 20 is the smallest possible fullerene consisting solely of 12 pentagons with extreme curvature and reac- tivity. Owing to its fascinating structure, this obscure molecule has been the subject of many theoretical investigations [1,2]. The aggregates C 20 have been studied by the AM1 quantum-chemical method and at least three different local energy minima found [3]. The impact of temperature on the relative stabilities of dodeca- hedron-shaped and bowel-shaped structures of C 20 were studied by Slanina and Adamowicz [4]. Singly charged complexes of dode- cadedron-shaped C 20 with Li were studied by means of the MNDO semiempirical method and optimal structures, energies and har- monic vibrations were obtained for C 20 , C 20 2 , [C 20 Li] + and [C 20 Li]  , respectively [5]. It is found that C 20 interactions with the Li atom do not produce substantial changes in the cage geom- etry. However, the fullerene cage structure of C 20 is computed to be less stable than the ring- and bowl-shaped structures, although the relative energies between these three types of isomers depend on the sophistication of the theoretical methods employed [1]. Some investigations even favor bicyclic rings [6] and linear chains [7]. Nevertheless, theoretically the dodecahedral fullerene C 20 is of the lowest-energy among all the mathematically possible 20-ver- tex polyhedral cages [8]. Interestingly, due to Jahn–Teller distortion, dodecahedral fuller- ene C 20 does not adopt the perfect I h symmetry [9], whereas its lowest-energy form is still in dispute [2,10]. In total five possible dodecahedral C 20 structures (with C 2 , C 2h , C i , D 3d and D 2h symme- tries, respectively) have been computed to be its lowest-energy candidates, but again their relative stability depends on the theo- retical methods used. However, the recent work by Chen et al. [2] demonstrates that all these five isomers are isoenergetic (with- in 0.2 kcal/mol 1 at B3LYP/6-31G*, and within 0.5 kcal/mol 1 at MP2/6-31G*) and have essentially the same structural parameters at both B3LYP and MP2 levels. Despite its labile nature, successful production of the C 20 fuller- ene is accomplished via stable dodecahedrane (C 20 H 20 ) [11] in gas- phase on the microsecond-scale lifetime. Owing to high strain and unstability of fullerene C 20 [12], the search for its stable analogues, namely heterofullerene, becomes appealing [13]. Alder et al. [14] investigated the relative strain energies for a series of T h symmetry X 8 C 12 (X = P, N, CH) molecules by computing the energy of isodes- mic reaction, X 8 C 12 H 12 +C 20 H 12 ? X 8 C 12 +C 20 H 20 . The reaction is predicted to be highly exothermic (by 86.7 kcal/mol) for X = P, but highly endothermic (by 148.1 kcal/mol) for X = N. Thus, the strain energy of the P 8 C 12 heterofullerene is far lower than that of the T h symmetry dodecahedratetraene. Thus, the T h symmetry P 8 C 12 [more exactly P 8 (C@C) 6 ] cluster is predicted to be a stable, 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.10.002 * Corresponding author. Tel.: +98 912 1000392; fax: +98 21 8009730. E-mail address: kassaeem@modares.ac.ir (M.Z. Kassaee). Journal of Molecular Structure: THEOCHEM 940 (2010) 19–28 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem