Nature of the transition state in gas phase SN2 identity reactions: correlation between nucleophilicity and proton affinity

Abstract

The potential energy profiles of the cationic: A + CH₃A⁺→⁺ACH₃ + A (A = NH₃, H₂O, HF), and anionic: A^– + CH₃A→ACH₃ + A^– (A^– = NH₂^–, OH^–, F^–) S_N2 reactions were obtained using quantum chemical methods [MP2, G2, CCSD(T)]. A linear rate–energy relationship in the form of a correlation between the critical energy, ΔE ^diff, and the proton affinity of A was found within subgroups of similar nucleophiles (NH₃, H₂O, HF; NH₂^–, OH^–, F^– ; F^–, Cl^–, Br^–, I^–). The analysis shows that the different susceptibility for each class is due to properties of the transition structures alone, and not of the reactants.