CRITICAL TEST OF PM3 CALCULATED GAS-PHASE ACIDITIES

Gas-phase acidities have been calculated for 175 compounds using the PM3 semiempirical molecular orbital model. With some exceptions, PM3 seems to be a useful tool for the investigation of gas-phase acidities. The main problems encountered involve two rather different classes of acids: one which generates small anions (e.g., halide ions, hydride ion, etc.), in which the charge is localized on one atom, and, a second, represented by anions that contain bulky electron acceptor substituents characterized by an extensive negative charge delocalization. In some cases (anilines, amides, alcohols, and phenols) the average error in predicted gas-phase acidity can be significantly reduced by employing an empirically derived correction. Comparison with AM1 results shows that both methods are of roughly equal quality with the exception of hypervalent molecules where PM3 is better (averaged unsigned errors are 11.8 and 17.0 kcal/mol for PM3 and AM1, respectively).