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-π interactions of the type C₆H₅X···Na⁺ is achieved by modeling C₆H₅-(Φ₁)(n)-X···Na⁺ (1), C₆H₅-(Φ₂)(n)-X···Na⁺ (2), C₆H₅-(Φ(2perpendicular))(n)-X···Na⁺ (2'), and C₆H₆ ···HX···Na⁺ (3), where Φ₁ = -CH₂CH₂-, Φ₂ = -CHCH-, Φ(2perpendicular) indicates that Φ₂ is perpendicular to the plane of C₆H₅, and n = 1-5. The cation-π 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 E₀¹, E₀², E₀(2'), and E₀³ for n = 0, the C₆H₅X···Na⁺ systems. E₀¹ is made up of inductive (E(I)) and through-space (E(TS)) effects while the difference (E₀² - E₀(2')) is purely resonance (E(R)) and E₀³ is attributed to the TS contribution (E(TS)) of the X. The total interaction energy of C₆H₅X···Na⁺ is nearly equal to the sum of E(I), E(R), and E(TS), which brings out the unified view of cation-π 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-π interaction energy.