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

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 C6H5X center dot center dot center dot Na+ is achieved by modeling C6H5-(Phi(1))(n)-X center dot center dot center dot Na (+) (1), C6H5-(Phi(2))(n)-X center dot center dot center dot Na (+) (2), C6H5-(Phi(2 perpendicular to))(n)-X center dot center dot center dot Na (+) (2'), and C6H6 center dot center dot center dot HX center dot center dot center dot Na (+) (3), where Phi(1) = -CH2CH2-, Phi(2) = -CHCH-, Phi(2 perpendicular to) indicates that Phi(2) is perpendicular to the plane of C6H5, 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 C6H (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 C6H5X 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.