Energy maxima in anionic nucleophilic additions to carbonyl groups: The location of a saddle point for the addition of hydride to formaldehyde

The reaction profile of the anionic nucleophilic addition of hydride to formaldehyde giving methanolate was re-investigated using theoretical ab initio methods. A variety of basis sets (including 4-31G and 6-31G with and without polarization and diffuse functions) were used, and electron correlation calculations were performed at the MP2, MP3, and MP4 levels with geometry optimizations at the MP2 level. Upon the addition of diffuse functions to the basis set, a local energy maximum was introduced into the SCF reaction profile, a major change in the character of the profile compared to the entirely attractive potential energy curve found in previous studies. On the reactants side of the barrier a local minimum 4.56 kcal/mol lower in energy was found, corresponding to an ion–dipole complex. With the inclusion of electron correlation, the local barrier almost disappeared, with the energy difference between the ion–dipole complex and the barrier maximum decreasing to less than 1 kcal/mol. Furthermore, the location of the energy maximum along the reaction coordinate changed by 0.4 Å. These results are used to critically re-evaluate some prevalent viewpoints on the nature of the reaction profiles of this type of anionic nucleophilic addition.