STRUCTURES AND CHARGE-DISTRIBUTIONS IN ALKOXIDE IONS

The structures of methoxide ion, ethoxide ion, tert-butoxide ion, fluoromethoxide ion, and trifluoromethoxide ion were obtained via geometry optimizations with the 6-31G* and 6-311++G** basis sets. In all cases, the carbon-oxygen bond shortened about 0.1 Å on going from the alcohol to the alkoxide ion, whereas the bonds attached to the alcohol carbon increased in length. The origin of the changes in bond lengths was studied with use of Bader's theory of atoms in molecules. Two types of interactions were found. With C-H or C-C bonds, the oxygen in the alkoxide ions was found to repel charge from the adjacent carbon, and onto the groups attached to this carbon. This results in a larger volume element for the ion and a decrease in the classical electrostatic energy. The coulombic attraction between the alkoxy oxygen and the carbon leads to a shorter bond. With C-F bonds, the charge polarization is so large that little further polarization can occur on going to the alkoxide ion. Here, charge transfer from the oxygen to the electron-deficient carbon was found. The energy changes in the ionization of the alcohols and of acetic acid were studied. The change in energy of the hydroxy proton on ionization accounts for half of the difference in ionization energy of ethanol and acetic acid. © 1990, American Chemical Society. All rights reserved.