The periodic table and the intrinsic barrier in SN2 reactions

The identity S(N)2 reactions on nitrogen (see eq 3) with nucleophiles having the general structure HnX- where X belongs to the group of nonmetallic elements which do not border the line separating them from the metallic elements (X = F, Cl, Br, I, O, S, Se, N, P, and C) were studied at the G2+ level. The results show that, similarly to the previously observed phenomenon for SN2 reaction on carbon (J. Am. Chem. Soc. 1999, 121, 7724), the Periodic Table, through the valence of the element X, controls the intrinsic barrier for the reaction. The average intrinsic barriers obtained for nitrogen substrates were 20, 27, 39, and 57 kcal/mol for the mono-, di-, tri-, and tetravalent X's, respectively. It is also concluded that the intrinsic barriers are similar for N- and C-based substrates and dimethyl substitution on both raises the intrinsic barrier by ca. 10 kcal/mol.