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