PROTON ABSTRACTION FROM CARBON ACIDS - AB-INITIO MOLECULAR-ORBITAL STUDY ON THE PROTON ABSTRACTION FROM ACETALDEHYDE BY NH2-, OH-, F-, SIH3-, PH2-, AND SH-

Proton abstraction from acetaldehyde by the first- and second-row hydr ide anions NH2-, OH-, F-, SiH3-, PH2-, and SH- have been investigated with ab initio quantum mechanical calculations up to the MP2/6-31+G*/ /HF/6-31+G and MP2/6-311+G**//MP2/6-31+G* level. Progress of energies , geometric variables, and charge distributions along the reaction coo rdinate have been investigated by employing the intrinsic reaction coo rdinate method. These computations revealed that proton abstractions o ccur in three essentially separate steps. The main difference between the first- and second-row hydrides is that the carbon, which donates p roton in the reaction, changes hybridization from sp(3) to sp(2) later in the reaction coordinate in the case of the first-row systems. Cons equently, the flow of negative charge from the CH2 to CHO end of the d eveloping enolate anion is retarded more in the case of the first-row hydrides. The results showed that the proton abstractions by the first -row hydride anions have more imbalanced transition states than the se cond-row hydride anions.