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

The potential energy profiles of the cationic: A + CH(3)A(+)-->(+)ACH(3) + A (A = NH3, H2O, HF), and anionic: A(-) + CH(3)A-->ACH(3) + A(-)(A(-) = NH2-, OH-, F-) S(N)2 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, Delta E-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.