Ab initio study of the carbon dioxide dimer and the carbon dioxide ion complexes [(CO2)2+ and (CO2)3+

Abstract

Ab initio calculations were performed at several levels for neutral CO/sub 2/, CO/sub 2/ dimer, and the cations (CO/sub 2/)/sub 2//sup +/ and (CO/sub 2/)/sub 3//sup +/. At the highest level, a CO/sub 2/ neutral dimer of C/sub 2h/ symmetry (staggered side-by-side) is bound by 1.3 kcal/mol, which is 0.2 kcal/mol more stable than a T-shaped dimer (C/sub 2v/ symmetry). The dissociation energy of a (CO/sub 2/)/sub 2//sup +/ complex of C/sub 2h/ symmetry is calculated to be 16.2 kcal/mol which is 4.4 kcal/mol greater than the dissociation energy of the T-shaped (CO/sub 2/)/sub 2//sup +/ complex. Due to a smaller amount of spin contamination (spin polarization), the stability of this complex which is characterized by a partial bond between oxygens is overestimated at the (MP4SDQ/6-31G*) level relative to the T-shaped ion complex and CO/sub 2//sup +/ (plus CO/sub 2/) which have significant spin contamination. Projecting out the largest spin contaminant at the PMP2/6-31G* level and at the PMP4/3-21G level leads to an estimated decrease in the energy separation between the C/sub 2h/ symmetry complex and the T-shaped complex of 6.3 kcal/mol (relative to the (MP4SDQ/6-31G*) method). Two trimers of (CO/sub 2/)/sub 3//sup +/ were considered, a cyclic complex (C/submore » 3h/) and a cross-shaped complex (D/sub 2h/). The more stable (CO/sub 2/)/sub 3//sup +/ complex (D/sub 2h/) is bound by 16.0 kcal/mol with respect to monomers. Vibrational frequencies were used to compute entropies of reaction for (CO/sub 2/)/sub 2//sup +/ ..-->.. CO/sub 2/ + CO/sub 2//sup +/ and (CO/sub 2/)/sub 3//sup +/ ..-->.. 2CO/sub 2/ + CO/sub 2//sup +/.« less