Quantum-Mechanical Evaluation of π–π versus Substituent−π Interactions in π Stacking: Direct Evidence for the Wheeler–Houk Picture

The influence of substituents on
π-stacking interactions
has previously been explained by two competing hypotheses: a nonlocal
effect in which tuning of the π density by the substituent alters
the interaction (the Hunter–Sanders picture) or a local effect
in which the direct interaction of the added substituent and changed
polarity of the phenyl-substituent σ bond alter the interaction
(the Wheeler–Houk picture). In this work, we applied the recently
developed functional-group partition of symmetry-adapted perturbation
theory (F-SAPT) to directly quantify these two effects in situ. The
results show that both pictures contribute to the change in interaction
energy but that the Wheeler–Houk picture is usually dominant.