Substituent Effects in Cation−π 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−π interactions of the type C6H5X···Na+ is achieved by modeling C6H5–(Φ1)n–X···Na + (1), C6H5–(Φ2)n–X···Na + (2), C6H5–(Φ2⊥)n–X···Na + (2′), and C6H 6 ···HX···Na + (3), where Φ1 = −CH 2CH2 −, Φ2 = −CHCH–, Φ2⊥ 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, E02′, and E03 for n = 0, the C6H 5 X···Na + systems. E01 is made up of inductive (EI) and through-space (ETS) effects while the difference (E02 – E02′) is purely resonance (ER) and E03 is attributed to the TS contribution (ETS) of the X. The total interaction energy of C6H5X···Na+ is nearly equal to the sum of EI, ER, and ETS, 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.