EFFECT OF SUBSTITUENTS ON THE GAS-PHASE ACIDITY OF SILANOLS

Eighteen siloxide ions have been prepared by the gas-phase reactions of various silanes with hydroxide. These siloxide ions have then been selected by using tandem flowing afterglow-selected ion flow tube (FA-SIFT) techniques and used to determine the gas-phase acidities (DELTA-G-degrees(acid)) of the corresponding silanols. Such determinations for the simple silanols studied herein are impossible in solution because of facile condensation reactions. A variety of substituents on silicon have been studied, including alkyl, hydrogen, phenyl, methoxy, fluoro, and chloro substituents. Alkyl groups decrease silanol acidity, in contrast to alkyl group effects in alcohols. Phenyl, methoxy, fluoro, and chloro substituents all lead to increased acidity for silanols. The gas-phase acidity of trichlorosilanol is comparable to that of HBr and HNO3. Polarizabilities and inductive effects are used to explain the substituent effects in these silanols. In addition, the heat of formation of trifluorosilanol has been estimated.