Electrostatics does not dictate the slip-stacked arrangement of aromatic π–π interactions

Abstract

<p>According to the Hunter–Sanders model, geometries in π–π systems arise from competition between quadrupolar electrostatics (favoring an edge-to-face geometry) and London dispersion (favoring stacking), but this model misrepresents the molecular physics.</p>

Reinterpreting π-stacking

“…[9][10][11][12][13][14][15][16][17][18][19][20][21] This conventional wisdom persists despite considerable evidence that charge penetration effects, which nullify or at least complicate classical electrostatic arguments, are significant at typical π-stacking dis- tances. [22][23][24][25][26][27][28][29] Benzene dimer is the archetypal π-stacked system and its conformational preferences are traditionally discussed in terms of several geometric isomers that are depicted in Fig. 1.…”

“…14,15 We have recently provided a clear and concise demonstration that the importance of electrostatics in π-π interactions has been misconstrued, and that the Hunter-Sanders model does not simply "overemphasize" electrostatics, 1 but is in fact qualitatively wrong and represents a fundamentally flawed framework for understanding π-π interactions. 29 Rather than being dictated by quadrupolar electrostatics, conformational preferences in systems such as (C 6 H 6 ) 2 and (C 6 H 6 )• • • (C 6 F 6 ) are instead driven by van der Waals (vdW) interactions, by which we mean a combination of dispersion and Pauli repulsion. The vdW model provides a unified explanation for the emergence of a slip-stacked geometry in both cofacial (C 6 H 6 ) 2 , where the quadrupolar interaction is re-pulsive, but also in (C 6 H 6 )• • • (C 6 F 6 ), were the polarity of the C-F bonds reverses the sign of the C 6 F 6 quadrupole moment, relative to that of C 6 H 6 .…”

“…5,6 The problem with the classical quadrupole model is that it fail to account for charge penetration at short range. 24,27,29,42 Note that charge penetration is a fundamentally different concept than intermolecular charge transfer. 21 The latter describes a particular form of polarization, whose definition can be quite sensitive to the choice of orbitals and basis set, 43 but which is rather small for the systems considered here.…”

Reinterpreting π-Stacking

“…Parallel-offset geometries in cofacial PAHs minimize exchange repulsion, allowing for a slight decrease in intermolecular separation, e.g., 3.8 Å for the cofacial benzene dimer saddle point (Fig.1a) versus 3.4 Å for the paralleloffset minimum (Fig.1b). 29,32 This maximizes stabilization from charge penetration and dispersion. 29 In contrast, stacked PSHs exhibit a single low-energy conformation characterized by interlocking C-H moieties on opposite monomers.…”

“…14,15 We have recently provided a clear and concise demonstration that the importance of electrostatics in π-π interactions has been misconstrued, and that the Hunter-Sanders model does not simply "overemphasize" electrostatics, 1 but is in fact qualitatively wrong and represents a fundamentally flawed framework for understanding π-π interactions. 29 Rather than being dictated by quadrupolar electrostatics, conformational preferences in systems such as (C 6 H 6 ) 2 and (C 6 H 6 )• • • (C 6 F 6 ) are instead driven by van der Waals (vdW) interactions, by which we mean a combination of dispersion and Pauli repulsion. The vdW model provides a unified explanation for the emergence of a slip-stacked geometry in both cofacial (C 6 H 6 ) 2 , where the quadrupolar interaction is re-pulsive, but also in (C 6 H 6 )• • • (C 6 F 6 ), were the polarity of the C-F bonds reverses the sign of the C 6 F 6 quadrupole moment, relative to that of C 6 H 6 .…”