Substituent effects on the acidity of weak acids. 4. Anilinium ions

The gas phase acidity of anilinium ions has been calculated at a number of theoretical levels from B3LYP/6-311++G** to MP2 and CCSD/6-311++G(2dp, 2pd). The highest level calculations find anilinium ion and p-protonated aniline to have essentially the same energy, in agreement with experimental studies. They also give a proton affinity for aniline that is in very good agreement with the experimental value. The B3LYP and MP2 calculations are less successful, with B3LYP favoring p-protonation and MP2 favoring N-protonation for aniline. Despite this difficulty, the calculated effect of substituents on the proton affinities of p-substituted anilines that undergo N-protonation agreed well with the experimental data. The effect of substituents on the anilinium ions and on the anilines were examined separately using a series of group transfer reaction.