Theoretical investigation into structures and magnetic properties of ...

Theor Chem Acc (2001) 106:352±363 DOI 10.1007/s002140100284

Regular article Theoretical investigation into structures and magnetic properties of smaller fullerenes and their heteroanalogues Zhongfang Chen1,2, Haijun Jiao1,3, Michael BuÈhl2, Andreas Hirsch1, Walter Thiel2 1

Institut fuÈr Organische Chemie, UniversitaÈt Erlangen-NuÈrnberg, Henkestrasse 42, 91054 Erlangen, Germany Max-Planck-Institut fuÈr Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 MuÈlheim an der Ruhr, Germany 3 Laboratoire de Chimie du Solide et Inorganique MoleÂculaire, UMR CNRS 6511, Universite de Rennes 1, 35042 Rennes Cedex, France 2

Received: 26 March 2001 / Accepted: 10 May 2001 / Published online: 11 October 2001 Ó Springer-Verlag 2001

Abstract. The smaller fullerenes, C20, C24, C28, C32, C36, C40 and C50, their hydrogenation products and selected B-, N- and P-doped analogues have been investigated systematically at the B3LYP/6-31G* density functional level of theory. The degree of spherical electron delocalization is evaluated by using the computed nucleusindependent chemical shifts ( NICS) at the cage center and the individual ring centers of interest. The calculated NMR chemical shifts and the NICS values at the cage center, which can be accessed by endohedral 3He chemical shifts, should provide a basis for further experimental characterization of these compounds. Key words: Smaller fullerenes ± Aromaticity ± Nucleus-independent chemical shifts ± Endohedral helium chemical shifts

1 Introduction Are there fullerenes smaller than C60 which are stable enough to be isolated? The ®rst such claim was made by Piskoti et al. [1], but the nature of their isolated compound formulated as C36 is still unclear. In the gas phase, a number of fullerenes Cn (60>n ³ 20 [2]) have been detected with mass spectroscopic methods. Numerous theoretical calculations have been performed for smaller fullerenes. For Cn clusters with n