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One of the most ambitious as well as rewarding intellectual and experimental challenges in fullerene chemistry (1) and carbon chemistry in general is the elucidation of the mechanisms for fullerene formation (2). Understanding the mechanisms of fullerene synthesis may well lead to the discovery of other, from a classical synthesis viewpoint unconventional, preparative techniques and to the production of entire classes of new compounds. Isotopic scrambling observed in fullerenes generated by arc heating from mixtures of sup 12 C-and sup 13 C-graphite indicated that the reactant carbon source must undergo vaporization into atomic C or small carbon fragments, such as C sub 2 (3, 4), which contradicts the originally proposed sequential graphitic shell closure mechanism (5-7). This evidence, along with the early observation that fullerenes fragment through loss of C sub 2 (8), has resulted in models (4, 6) in which fullerene growth occurs upon assembly of small carbon species. More recently, fullerene growth has been modeled by sequential addition of larger polycyclic ring systems (C sub 10 to C sub 20 ) (9) and the implication of linear, monocyclic, and polycyclic carbon clusters in fullerene synthesis has been suggested (10-12).
As a part of our ongoing efforts to synthesize molecular forms of carbon from structurally and chemically well-defined precursors (13, 14), we prepared the carbon oxides C sub n (CO) sub n/3 (1 to 3) with n = 18, 24, and 30, as precursors to the cyclocarbons cyclo-C sub 18 , -C sub 24 , and -C sub 30 , respectively (15). Initial laser desorption mass spectrometry studies on the carbon oxides provided intriguing but ambiguous results (15). These experiments showed that the carbon oxides undergo loss of CO molecules to form the desired cyclocarbon ions. Large carbon cluster ions, (Character omitted) (x > 50) were also observed for all even x but with little or no dependence of the (Character omitted) distribution on the size of the precursor molecule. The production of similarly wide (Character omitted) (even x > 50) distributions by laser vaporization of various carbon-containing materials (such as polymers and polycyclic aromatic hydrocarbons) (16) suggests that the carbon oxides may undergo decomposition prior to forming large (Character omitted). A possible requirement for such decomposition prior to the growth of large (Character...