ACIDITY OF HALOGENATED ALCOHOLS AND SILANOLS - COMPETITION OF ELECTRONEGATIVITY AND SOFTNESS IN 2ND AND HIGHER ROW ATOMS

The gas phase acidity of some halogenated methanols (X(n)CH(3-n)OH) and silanols (X(n)SiH(3-n)OH) (X = F, Cl, Br) has been calculated approximately using an ab initio molecular orbital calculation at the HF level first with the 3-21G basis set. The sequences obtained were interpreted in terms of a competition between polarizability and electronegativity effects. The chlorinated compounds were found to be slightly more acidic than the corresponding brominated compounds in accordance with the higher electronegativity of chlorine as compared to bromine. On the other hand, this acidity decreases largely upon fluorination resulting from a much smaller polarizability, charge capacity, or softness of fluorine. The calculated acidity sequences show a positive correlation with the charge on the acidic proton, indicating that this property, which is an approximation to local hardness, could be a valuable reactivity index. Some calculations were repeated with the extended 6-31+G* basis set, which includes both diffuse and polarization functions, for fluoro- and chloromethanols and silanols. (This basis set is not available for bromine.) Although some minor differences appear in some sequences, the latter results confirm our findings with the smaller basis set. Density functional theory based descriptors are also discussed, in particular the local softness which was found to fail as an indicator of acidity.