Broad Transferability of Substituent Effects in π-Stacking Interactions Provides New Insights into Their Origin.

Abstract

Substituent effects in model stacked homodimers and heterodimers of benzene, borazine, and 1,3,5-triazine have been examined computationally. We show that substituent effects in these dimers are strongly dependent on the identity of the unsubstituted ring, yet are independent of the ring bearing the substituent. This supports the local, direct interaction model [J. Am. Chem. Soc. 2011, 133, 10262], which maintains that substituent effects in π-stacking interactions are dominated by through-space interactions of the substituents with the proximal vertex of the unsubstituted ring. In addition to dimers in which the unsubstituted ring is held constant, substituent effects are correlated in many other stacked dimers, including those in which neither the substituted nor unsubstituted rings are conserved. Whether substituent effects in a pair of dimers will be correlated is shown to hinge on the electrostatic components of the interaction energies, and the correlations are explained in terms of the interaction of the local dipole moments associated with the substituents and the electric fields of the unsubstituted rings. Overall, substituent effects are similar in two stacked dimers as long as the electric fields above the unsubstituted rings are similar, providing a more sound physical justification for the local, direct interaction model.