Figure 3. Effect of the nature of the atom that interacts with CO2 in the Gibbs free energy.
For the set CH3X- (X = NH, O, PH or S), the changes in the Gibbs-free energy are −45.8, −29.3, −19.1 and −10.2 kcal mol-1, respectively. For the set C2H3X- the corresponding changes are smaller, namely −25.4, −8.1, −7.3 and −2.1 kcal mol-1, respectively. Clearly, the conjugation with the vicinal double bond helps to stabilize the negative charge. However, this stabilization is much higher in the nitrogenated and oxygenated cases than in the anions containing phosphorous or sulfur atoms. In these anions, charge delocalization through the double bond is not so effective, therefore having a smaller effect. Although the methyl group can act either as electron donor or as electron acceptor,37 its insertion in NH2-, PH2- and in SH-makes the Gibbs free energy more negative than in the corresponding anion without the methyl group. Conversely, for oxygenated species, the insertion of a methyl group (OH- x CH3O-, for example) reduces the changes in the Gibbs free energy. We associated this fact with the stabilization of the adduct, which helps to decrease the charge localization. For example, CH3NH- and NH2- have essentially the same GPB (Table 1), although CH3NH- leads to a more spontaneous reaction. For CH3PH-and PH2- both the GPB and the Gibbs free energy are more negative for the methylated species.
In the reaction of the anions with CO2, in addition to the structural changes in the geometry of the CO2molecule, which goes from a linear to a bent structure, there is a strong charge transfer from the anions to CO2. This charge transfer leads to charge redistribution, with consequent stabilization of the whole system. Thus, the amount of charge transferred between the two reactants may have some correlation with the relative stability of the adduct. Table 2 gives the natural population (NPA) electronic charge on the CO2 moiety in the adduct (QCO2_adduct), the distance between the interacting atom and the carbon atom of CO2(dA_CO2) and the OCO angle. As the isolated CO2 molecule is neutral, the charge on this moiety in the adduct is a measure of the amount of charge transferred from the anion to CO2. Also, from the geometrical point of view, a larger OCO angle is expected for a stronger interaction between CO2 and the anions, because of the rehybridization that takes place in the carbon atom. These electronic and geometrical parameters have been reported to correlate with the anion/CO2 bond strength.38,39
Table 2 . Total NPA charge (in e-) on the CO2 moiety in the anion/CO2 adduct (QCO2_adduct), distance between the interacting atom and the carbon atom in the adduct (dA_CO2, in Å) and the OCO angle (AOCO_MP2, in degrees).