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) an d silanols (X(n)SiH(3-n)OH) (X = F, Cl, Br) has been calculated approx imately using an ab initio molecular orbital calculation at the HF lev el first with the 3-21G basis set. The sequences obtained were interpr eted in terms of a competition between polarizability and electronegat ivity effects. The chlorinated compounds were found to be slightly mor e acidic than the corresponding brominated compounds in accordance wit h the higher electronegativity of chlorine as compared to bromine. On the other hand, this acidity decreases largely upon fluorination resul ting from a much smaller polarizability, charge capacity, or softness of fluorine. The calculated acidity sequences show a positive correlat ion with the charge on the acidic proton, indicating that this propert y, which is an approximation to local hardness, could be a valuable re activity 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 b asis set. Density functional theory based descriptors are also discuss ed, in particular the local softness which was found to fail as an ind icator of acidity.