Quantum-Mechanical Evaluation of pi-pi versus Substituent-pi Interactions in pi Stacking: Direct evidence for the Wheeler-Houk Picture

Robert M. Parrish and C. David Sherrill J. Am. Chem. Soc., 2014, 136 (50), pp 17386–17389

DOI: 10.1021/ja5101245

In this recent paper, the Sherrill group provides evidence from SAPT (symmetry-adapted perturbation theory) that supports the Wheeler-Houk model of substituent effects on pi-pi stacking.  The 

Wheeler-Houk model posits that substituent effects on binding energies between substituted aromatics arise almost entirely from direct interactions with substituents (and substituent-C bonds).  Although this paper provides evidence that pi-polarization (the Hunter-Sanders model) does contribute, it is found that direct interactions usually dominate.