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).