The stabilization of alpha-substituted oxy- and thiocarbanions

In this paper we report the results obtained in an ab initio study at an extended level of the effects of the oxy and thio substituents upon the stabilities of ?-substituted carbanions. To rationalize this problem, we have decomposed the stabilization energies into the component terms associated with the ? effects, the nonbonded interaction effects, the 3d orbitals effects, and the correlation energy effects. At all levels of computations used in this investigation it is found that the SH group stabilizes the anionic center more than the OH group and that this trend is determined by the ? effects. The present analysis has shown also that the effect of the nonbonded interaction favors OH over SH and that the contribution of the sulfur 3d orbitals is very small. More significant appears to be the contribution associated with the correlation energy effects, which favors SH over OH. © 1986 American Chemical Society.