Substituent effects in cation/pi interactions and electrostatic potentials above the centers of substituted benzenes are due primarily to through-space effects of the substituents

Abstract

Substituent effects in cation/pi interactions have been examined using the M05-2X DFT functional and CCSD(T) paired with triple-zeta-quality basis sets. In contrast to popular, intuitive models, trends in substituent effects are explained primarily in terms of direct through-space interactions with the substituents. While there is some scatter in the data, which is attributed to pi polarization, the trend in substituent effects in cation/pi interactions is captured by an additive model in which the substituent is isolated from the aryl ring. Similarly, changes in the electrostatic potential at a point above the center of a substituted benzene arise largely from through-space effects of the substituents; pi polarization is not the dominant underlying cause.

Figure 1

(a) M05-2X/6-311+G(2df,2p) (gray) and CCSD(T)/AVTZ (red) interaction energies of Na⁺ with C₆H₅X versus a simple additive model (kcal mol⁻¹). Least squares fit applied only to the M05-2X data; (b) M05-2X/6-311+G(2df,2p) ESPs evaluated at a single point above the center of the ring of substituted benzenes versus the ESP at that point from a simple additive model (kcal mol⁻¹). All quantities evaluated at the equilibrium C₆H₅X^⋯Na⁺ geometries.