Substituent Effects in Cation-pi Interactions: A Unified View from Inductive, Resonance, and Through-Space Effects

Abstract

The quantification of inductive (I), resonance (R), and through-space (TS) effects of a variety of substituents (X) in cation-pi interactions of the type C(6)H(5)X center dot center dot center dot Na(+) is achieved by modeling C(6)H(5)-(Phi(1))(n)-X center dot center dot center dot Na (+) (1), C(6)H(5)-(Phi(2))(n)-X center dot center dot center dot Na (+) (2), C(6)H(5)-(Phi(2 perpendicular to))(n)-X center dot center dot center dot Na (+) (2'), and C(6)H(6)center dot center dot center dot HX center dot center dot center dot Na (+) (3), where Phi(1) = -CH(2)CH(2)-, Phi(2) = -CHCH-, Phi(2 perpendicular to) indicates that Phi(2) is perpendicular to the plane of C(6)H(5), and n = 1-5. The cation-pi interaction energies of 1, 2, 2', and 3, relative to X = H and fitted to polynomial equations in n have been used to extract the substituent effect E(0)(1), E (2)(0), E(0)(2'),and E(0)(3) for n = 0, the C(6)H (5) X center dot center dot center dot Na (+) systems. E(0)(1) is made up of inductive (E(I)) and through-space (E(TS)) effects while the difference (E(0)(2)-E(0)(2')) is purely resonance (E(R)) and E(0)(3) is attributed to the TS contribution (E(TS)) of the X. The total interaction energy of C(6)H(5)X center dot center dot center dot Na(+) is nearly equal to the sum of E(I), E(R), and E(TS), which brings out the unified view of cation-pi interaction in terms of I, R, and TS effects. The electron-withdrawing substituents contribute largely by TS effect, whereas the electron-donating substituents contribute mainly by resonance effect to the total cation-pi interaction energy.