C60(g), C70(g), saturated carbon vapour and increase of cluster populations with temperature: A combined AM1 quantum-chemical and statistical-mechanical study

Saturated vapour above graphite and the content of Cn(g) aggregates therein have been studied at various temperatures. The equilibrium constants of C(g) into Cn(g) association have been obtained for: (i) n = 1-5 from the available tabulated thermodynamical data, (ii) C60 and C70 from a quantum-chemical description in terms of the AM1 method as a source of molecular parameters for the construction of partition functions, and (iii) the other aggregates by linear inter- or extrapolation of the data sub (i) and (ii). The content of clusters with n ≥ 2 in the saturated vapour has been shown to increase steeply with temperature. This increase is due to small clusters (n ~ 2-5), whereas the recently proved particular structures C60 and C70 are - according to these calculations – negligibly populated in the saturated vapour at all temperatures studied. At the conditions of saturated vapour the population of C60 always exceeds that of C70 by several orders of magnitude. The relations of these calculations to the recent observations of distinct populations of C60 and C70 are discussed.