Abstract. An ab initio study of the structural and physical properties of fullerene fragments based on corannulene shows a distinct de®ning point between bowl and tube-like character. Key words: Fullerene fragments ± Ab initio ± Density functional ± Corannulene ± Bucky tube The introduction of a pentagon to a tessellation of hexagons causes warping (bowling) of the surface, and, if 12 pentagons are cooperatively arranged, a closed surface is obtained [1]. Chemically, buckminsterfullerene (1) [2] and corannulene (2) [3, 4] express these extremes. When only six pentagons are added to the motif, it would seem logical that a hemisphere results, to which hexagons could be added forming a capped tube. Such structures represent the minimalist bucky tube motif [5, 6], a family of carbon-rich structures with technological promise [7±11]. Because curvature in graphitic networks alters chemical properties such as dipole moment, ionization potential, and metal binding, the question arises: when does a buckybowl [12, 13] become a buckytube? Despite the enormous power of chemical synthesis [14], corannulene-based fullerene fragments C 30 H 10 - C 50 H 10 (3±5) are presently not to be had. As well, these structures have proven a dicult computational chal- lenge; low level computational methods have proven to be ambiguous for quantitative information [15±17], and only corannulene has been treated at correlated levels with promising results [18±20]. Until recent advances in supercomputer hardware, ab initio prediction of the electronic structure of 2±5 at double-f plus polarization quality and beyond, SCF or correlated methods, would not have been feasible. Following the lead of Jan AlmloÈ f [21±25], we have capitalized on these newer technologies to tackle computational problems exceeding 1000 basis functions (1 kiloBoys or 1 Pople) [26]. Structural computations of the series 1±5 were per- formed at RHF/DZV(2d,p)[27] using direct [28] SCF methods within the T3E parallel version of GAMESS [29], with select density functional theory, B3PW91/ DZ(d,p), computations on 2±4, using GAUSSIAN94 [30], to uncover eects of dynamic electron correlation. From the fully optimized structures, chemical and physical properties such as ionization potential (Koop- mans Theorem) [31, 32] dipole moment, bond localiza- tion, and surface curvature (carbon pyramidalization) [33±35] were derived. More extensive computations on the smaller buckyfragments were performed to deter- mine eects of basis set and correlation. These include RHF/DZ(2df,2p), B3PW91/DZ(2d,p), and MP2[36]/cc- pVDZ[37] levels of theory. The hybrid DFT methods employ Becke's three parameter hybrid method [38] with the correlation functional of Perdew/Wang91[39] (B3PW91). The most striking feature in the series 2±5 is the in- creasing bowl depth and surface curvature (Figs. 1, 2). Using the POAV method of Haddon, [33±35, 40] one can assign a pyramidalization value at each carbon. The POAV values for a ¯at polynuclear aromatic hydrocar- bon (PAH) like coronene, the highly symmetrical buckminsterfullerene, the equatorial belt of C 70 , and a cylindrical benzenoid belt, are 90°, 102°, 99°, and 96°, respectively [35]. Unlike these standards, bowls and capped tubes have variable pyramidality at carbon de- pending on the position. Because of the conical c 5 v symmetry of 2±5, one can de®ne concentric rings of carbons with identical POAV angles. The progression of POAV angle value from the cap-ring (ring 1) to the rim- ring (ring n) is a characteristic of the topography (Ta- ble 1). In 2, the cap pyramidality is similar to the belt region of C 70 , but already in 3 the cap carbons are es- sentially as pyramidal as those in C 60 . By the time one gets to 5, a pattern of almost constant curvature appears for the lower ring numbers, with the rim ring and ring (n)1) more belt-like. Extension of hexagons onto 5 could continue without substantial change in pyramid- ality of these limiting carbons, thus pointing to an ob- vious de®nable transition between bowl and tube structure. Another structural feature is the angle be- Corannulene-based fullerene fragments C 20 H 10 -C 50 H 10 : when does a buckybowl become a buckytube? Kim K. Baldridge 1 , Jay S. Siegel 2 1 San Diego Supercomputer Center, P.O. Box 85608, San Diego, CA 92186-9784, USA 2 Department of Chemistry, University of California-San Diego, La Jolla, CA 92093-0358, USA Received: 14 February 1997 / Accepted: 6 May 1997 Theor Chem Acc (1997) 97:67±71 Correspondence to: K. Baldridge