Figure 6 . Gibbs free energy in gas phase obtained with
different theory levels versus CBS extrapolation.
This picture does not change when including solvents effects. The
reactivity order is maintained but, in some cases
(OH-, CH3O-,
C2H3NH-,
F-,
C2H3O- and
NH2-) the M06-2X functional
overestimates the energy values. As show in Figure 1, the enthalpy shows
a linear correlation with the Gibbs free energy and the error associated
with the functional is almost the same.
Most reactions do not show a transition state either in the gas phase or
in the solvent. When a transition state exists, the activation energy
calculated with MP2/aug-cc-pvtz underestimate the CBS values (on
average) by 0.5 kcal mol-1 and 0.6 kcal
mol-1 for the direct and the reverse reactions,
respectively. The values computed with the DFT functionals do not
diverge too significantly, although the M06-2X functional shows two
values that diverge from the CBS values
(PH2-_to and
H-_thf). When we remove these two cases, the average
deviation is 0.9 kcal mol-1 for the direct reaction.
The differences for the reverse reaction are larger because, as already
mentioned for the calculation of the reaction energy, the M06-2X
functional overestimates the Gibbs free energy in some cases. Therefore,
the activation energy for the reverse reaction is also affected. The
B3LYP-D3 results are close to the CBS values in both directions, with
maximum difference values of 3.1 kcal mol-1(SiH3-_w) for the direct reaction.