SUPERACIDITY OF NEUTRAL BRONSTED ACIDS IN GAS-PHASE

Quantum chemical calculations of potentially superacidic neutral Brons ted acids were carried out using the PM3 method. It was shown that the PM3 method can be used to predict the gas phase acidities of acidic c ompounds only if empirical corrections are made. A strong acidifying e ffect is predicted for a new family of compounds in which an sp(2) oxy gen is substituted by an =NSO2CF3 group. So, for example, such replace ment is expected to result in acid strengthening by 47.5 kcal/mol in t he case of CH3CHO and by 22.7 kcal/mol in the ease of CF3SO2OH. The ac idities of such compounds are predicted to be increased further (nonad ditively) by stepwise replacements of =O by =NSO2CF3. The geometries o f known superacidic systems were reproduced quite well by PM3 method. The geometries of several superacidic systems were analyzed. (C) 1996 by John Wiley & Sons, Inc.