Table 1 gives the ΔG results obtained with the MP2 and CBS scheme for the reactions in the gas phase, as well as the gas-phase basicity (GPB) of the anions (organized by decreasing values, based on the MP2/aug-cc-pvtz results). For comparison we also include in Table 1 the previous results obtained with the M06-2X and B3LYP-D3 functionals.21The trends found for changes in the Gibbs free energy and enthalpy are the same. This is indicated by the high correlation between these properties, as shown in Figure 1. The average difference between the Gibbs free energy and the enthalpy is around 8 kcal mol-1 at 298 K. As noted above, the entropy change is negative because the reaction leads to a more organized system, thence with reduction in the number of degrees of freedom.
Figure 1. Gibbs free energy versus enthalpy for A- + CO2 reactions with different methods.
The B3LYP-D3, MP2 and CBS results are close to each other, with differences smaller than 3.4 kcal mol-1. On the other hand, the Gibbs free energies computed with the M06-2X are more negative than those obtained with the MP2 method, particularly for OH-, CH3O-, C2H3NH-, F-, C2H3O- and NH2-, where it is on average 7 kcal mol-1 smaller. As expected, the base strength, quantified by the gas-phase basicity, shows a high correlation with the Gibbs free energy for the A- + CO2reactions (Figure 2). The more negative Gibbs free energies are found for stronger bases, which can donate electron density more easily.15
Table 1. Gibbs Free energies in the gas phase for the A- + CO2 reactions (eq. 8) and gas-phase basicity (GPB) of the anions, calculated with the MP2/aug-cc-pvtz method. All values are in kcal mol-1