Strong acidifying effect of the substituents Y and Z is required for enolization.
Enolization energies (see Table 4), calculated at the highest applied level of Theory--B3LYP/6-311 + [G.
Fair correlation was found between our calculated enolization reaction energies and experimental acidities  of corresponding substituted methanes (see Fig.
2], where the DFT results predict less enolization than the HF method.
66) of the regression line clearly indicate systematic overestimation of the enolization of the studied phosphoryl compounds by the PM3 method.
Closer examination of data reveals that this discrepancy stems from the overestimation of the enolization energies for all compounds containing the sulphuryl group.
So, the MNDO/d method seems to be the preferable semiempirical method for the calculation of enolization energies for larger systems that do not contain hypervalent sulphur.