MNDO calculations of proton and methyl-and ethyl-cation affinities of neutral carbon,nitrogen, and oxygen bases

Proton affinities (PA) of 80 neutral bases were calculated using the semiempirical molecular orbital procedure MNDO. These were compared with the corresponding experimental and, where available, ab initio STO-3G and 4-31G data. For the 12 bases studied which led to ions which were not hyperconjugatively stabilized, the mean absolute error between the calculated and experimental values was 7.2 kcal mol^?1. However, a plot of these data revealed a clear tendency of MNDO to underestimate the PAs of the more basic molecules. Where hyperconjugative stabilization of the ion was possible, the calculated PAS were underestimated by a further 7–10 kcal mol^?1 for each attached alkyl group. Calculated and observed methyl-and ethyl-cation affinities were compared for 18 bases with qualitatively similar results.