Linear carbon clusters, isomers

As has been pointed out in the past (e.g. concerning the linear-cyclic equilibrium in Ceand Cio carbon clusters (40)), Hartree-Fock underestimates the resonance stabilization of aromatic relative to non-aromatic systems (in the case at hand, between the N- and / -protonated isomers) and MP2 overcorrects. The structures are found to be nearly isoenergetic at the CCSD level inclusion of connected triple excitations favors the N-protonated ion. The direction of the effect of connected quadruples is somewhat unclear, and a CCSD(TQ) or CCSDT(Q) calculation impossible on systems this size, but the contribution will anyhow be much smaller in absolute magnitude than that of connected triple excitations, particularly for systems like these which are dominated by a single reference determinant. We may therefore infer that at the full Cl limit, the N-protonated species will be slightly more stable than its / -protonated counterpart. 

Aside from fuUerene research, carbon clusters have always attracted much interest among scientists. " Linear cumulene-like Ce (Pooh) 132 is well known from computational studies as well as experimentally. " Cyclic Cg, which will be considered as the extreme case of a dehydrobenzene here, has a long and varied history. " In particular, two cyclic isomers possessing D h and Dg/, symmetry (133 and 134) have been discussed. " According to the most recent calculations, 133 ( Aj) is lowest in energy 134 ( Ai ) and 132 ( S )) being less stable by 8 and 10 kcal/mol, respec-tively. 42-... 

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