Theoretical Models for Mechanism and Catalysis in Carbonyl Addition

Exploration of potential energy curves, calculated by ab initio procedures at the STO-3G and 4-31G levels, for the systems H2O + CH2O, HO- + CH2O, and H2O + CH2OH+ produces models for enforced concertedness of proton transfer and heavy-atom reorganization (in the H2O + CH2O reaction) and specific-acid-base catalysis (in the two ion-molecule reactions). Thus, in the former case, approach of water to formaldehyde along such directions as to allow formation of the zwitterionic intermediate compound H2O+CH2O- gives rise to a completely repulsive interaction, so that this structure does not exist as a bound species. A transition state for four-center, concerted addition does, however, exist, and reaction through this state is enforced by nonexistence of the intermediate required along the alternative stepwise route. In the ion-molecule reactions, prior protonation of formaldehyde or prior deprotonation of water leads to formation of the corresponding ionic adducts (H20+CH20H and H0CH20”), with no barrier to reaction, simulating specific-acid and -base catalysis, respectively.