Results and discussion
In the first step, we have computed the enthalpy and Gibbs free energy
changes for the reaction of each anion with CO2 in the
gas phase. Our discussion is based on the values computed with the
MP2/aug-cc-pvtz method. The MP2 results are also compared to the
B3LYP-D3 and M06-2X results from Sucarrat and
Varandas.21 Additional data such as cartesian
coordinates of the optimized species, electronic energies, zero point
energies and all reaction profiles are given in the supporting
information.
The general trends here observed with the MP2 results are similar to
those found previously.21 In the gas phase most anions
react spontaneously and without any apparent activation barrier. As it
will be shown below, changes in the Gibbs free energy are about 8 kcal
mol-1 less negative than changes in the enthalpy. This
is a consequence of reducing the degrees of freedom from the reactants
(CO2 + anion) to the products (adduct). In this process,
the entropy decreases, with a consequent effect in the Gibbs free
energy. The solvent effect is more pronounced for solvents with higher
dielectric constants.