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.