The formate anion: An ab initio study of its equilibrium structure, general harmonic force field and fundamental vibrational frequencies

Abstract

The general harmonic force field (GHFF) of the formate anion has been calculated by ab initio restricted Hartree—Fock theory using the minimal STO-3G and split valence 4–31G basis sets. This work was carried out in conjunction with a previously reported experimental study in which the ion's force field was determined from its vibrational frequencies in polycrystalline sodium formate by an improved refinement procedure. The ab initio results reported here proved valuable in that study as a guide to the true physical values of the harmonic force constants. The 4–31G basis gives better results than the STO-3G basis. The agreement between the 4–31G and the experimental results suggests that interactions in the crystal are of secondary importance in determining the experimental force field. However, there are discrepancies between the 4–31G values and the experimental values for the force constants which exceed the empirical guidelines for such calculations on neutral molecules. These deviations may be real and due to the different environments of the ion in the two studies or they may be due to experimental and theoretical errors. Conjugative delocalization of an oxygen lone pair in the molecular plane into an antibonding CH bond orbital provides a simple and consistent explanation for the weakness of CH bonds in HCOO − and related species. The inductive effect cannot account for this overall weakness, but it can account for the increase in CH bond strength from H 2CO and HCOO − to HCOOH and HCOOCH 3 and for its decrease from H 2CO to CH 3CHO.