THEORETICAL-STUDY OF SUBSTITUENT-EFFECT ON THE STABILITY OF ENOLATE IONS

In this paper we describe the results obtained in an ab initio study of the effects of second- and third-period substituents on the stabilities of enolate ions H2C--C(=O)X with X=CH3, NH2, OH, F, SiH3, PH2, SH, Cl. Accurate stabilization energies have been computed in all cases using an SCF/MP2 treatment at the extended 6-31 +G* level. To rationalize the effect of the various substituents we have decomposed the stabilization energies into various contributions: (i) correlation energy effects; (ii) 3d orbital effects; (iii) effects associated with the pi orbital interactions (pi effect). It has been found that the pi effect is responsible for the trend of stabilization along a group of the Periodic Table (third-period substituents more stabilizing than the corresponding second-period. substituents). This effect also dictates the trend along the second period, while in determining the trend along the third period both the pi effect and the 3d orbital effect are important.