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Ion/molecule reactions of carbon cluster ions (C-n(.+), n = 10-20) in the gas phase with crotononitrile (CRN) were investigated by Fourier transform ion cyclotron resonance spectrometry (FT-ICR). The carbon cluster ions studied were generated by electron impact ionization of perchlorinated aromatic compounds and subsequent elimination of all Cl substituents from the molecular ion. Clear pseudo-first-order reactions were observed for the carbon cluster ions C-n(.+) thus formed without any sign of isomeric clusters of different reactivity. AM1 calculations of the stability of C10Clx, (x = 4, 6, 8) derived from octachloronaphthalene indicate that the most stable monocyclic C-n(.+) are formed by this method. The reactions of C-n(.+) with CRN at a low operating pressure (1.2 X 10(-8)-8.0 x 10(-7) mbar) include the formation of adduct ions stabilized by radiative emission or/and loss of a neutral species (H or HCN) from the adduct ion. Rate constants for the reaction with CRN are distinctly larger than for the cluster reactions with HCN and acrylonitrile, and an exceptionally large reaction efficiency is found for C-13(.+) and C-17(.+). This correlates inversely with the thermochemical stability of the carbon clusters. The nature of the reaction products depends on the C number of C-n(.+), Consecutive reactions of the primary product ions Cn+4H4N+, formed by loss of H from the adduct ion, were observed only for reactions of even numbered C-n(.+). On collision induced dissociation (CID) the adduct ions from C-n(.+) (n = 11, 13) redissociate to the original cluster ion while the adduct ions of C-17(.+) lose C-3 and C-6 (2C(3)). A reaction model explaining these observations is proposed.