High-Level Ab initio Computations of Structures and Relative Energies of Two Isomers of the CO₂ Trimer

The two-body interaction energies of the cyclic (C3h) and noncyclic (C2) trimers at the basis set limit (15.55 and 16.55 kJ/mol) were estimated by means of second-order Moller-Plesset perturbation theory including terms linear in the interelectronic distance (MP2-R12 approach). Coupled cluster calculations with single and double substitutions and with noniterative triple excitations [CCSD(T)] were carried out to correct the MP2-R12 two-body interaction energies for the effect of higher excitations, and also for the evaluation of the three-body interaction energies. The calculated bonding energies of the trimers using this model calculation (16.82 and 17.20 kJ/mol) suggest that the two isomers are nearly isoenergetic. The three-body terms are small (only of the order of 1 kJ/mol), stabilizing the cyclic trimer, but destabilizing the noncyclic trimer.

Fig. 1