CRITICAL TEST OF PM3 CALCULATED GAS-PHASE ACIDITIES

Gas-phase acidities have been calculated for 175 compounds using the P M3 semiempirical molecular orbital model. With some exceptions, PM3 se ems 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 io n, etc.), in which the charge is localized on one atom, and, a second, represented by anions that contain bulky electron acceptor substituen ts characterized by an extensive negative charge delocalization. In so me cases (anilines, amides, alcohols, and phenols) the average error i n predicted gas-phase acidity can be significantly reduced by employin g 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 ar e 11.8 and 17.0 kcal/mol for PM3 and AM1, respectively).