It is shown that, at a temperature relevant for observation, the cyclic ground state of C,(g), proved by a recent advanced ab initio study, represents about 3% of Cg while more than 95% of it is of a linear structure. A careful treatment of the isomeric interplay shows that the difference between theoretical and observed value of the equilibrium constant of the C, system formation is about-S%.
Recently, Raghavachari et al.
[2] performed accurate ab initio calculations on the C6 system with the inclusion of electron correlation. Thus, experimental information [ 31 on the thermodynamic stability of the system can now be treated in a considerably more sophisticated way than in the previous theoretical (MIND0/2) study [ 41. C, is just one of the carbon clusters which have recently been studied intensively, see, e.g., refs. [ 5-91.
Geometrical optimization was carried out [ 21 in the 3-21G and 6-31G* basis sets. By vibrational analysis in the 3-2 1 G basis, four of the six stationary points found on the hypersurface were identified as energy minima -i.e. four isomers (see table ). Relative energies within the isomeric set were evaluated using complete fourth-order Msller-Plesset perturbation theory with the 6-3 1 G* basis set in connection with an evaluation of higher-order effects, The sophisticated calculations [ 21 show that the lowest potential energy (corrected for zero-point vibrations) is exhibited by the D,, cyclic structure (also labelled 4 [2]) followed by a linear structure (1) of Dmh symmetry and 32; electronic state, a second linear structure (2) of Da,, symmetry and 'CT state, * Part XXX of the series Multimolecular Clusters and Their Isomerism; for part XXIX, see ref.
[ 11.