Proton affinity and protonation sites of aniline. Energetic behavior and density functional reactivity indices

High-level theoretical methods based on both density functional and Hartree -Fock and Moller-Plesset theories have been employed in order to give bette r insight in the longstanding question concerning the protonation site of g aseous aniline. From the thermodynamic point of view, nitrogen and para rin g carbon appear to be the preferred protonation sites. Density functional a nd MP4 computations show that the protonation on para ring carbon atom is t he slightly favored process. The orbital Fukui indices analysis does not re solve unequivocally the dilemma of preferred protonation site attributing t o para ring carbon and nitrogen atoms the same probability to work as nucle ophilic centers. The absolute hardness values of protonated forms of anilin e do not follow the maximum hardness principle. Proton affinity values clos er to the experimental counterparts are obtained when the G2(MP2), B3LYP, B P, and MP4 levels of theory are used.