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.