Cyclic carbon clusters, isomers

High-resolution IR spectra were reported for the clusters Hen.C02349 and Hen.OCS350 (both with n up to about 20). Values of vasC02 in the FTIR spectra of gaseous X. COz (X = Cl, Br, I) were all at lower wavenumbers than for free C02.351 There is matrix-IR evidence for the formation of a cyclic carbon trioxide isomer, (5) (X3A3) by addition of O atoms to C02.352... 

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-... 

In many cases electronic properties calculated at the Hartree-Fock level do not have the accuracy sufficient to make them useful in chemical predictions. For example, as revealed in a recent study,the stability of the cage isomer of the C20 carbon cluster relative to that of the cyclic isomer is underestimated at the Flartree-Fock level by as much as 200 kcal/mol. In such systems, the electron correlation effects have to be taken into account in quantum chemical calculations through application of approximate methods. One such approximate electron correlation methods that has gained a widespread popularity is the second-order Moller-Plesset perturbation theory (MP2). Until recently calculations involving the MP2 approach have used a traditional formulation in which the MP2 energy is evaluated as the sum... 

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