Steric “attraction”: not by dispersion alone

Abstract

Non-covalent interactions between neutral, sterically hindered organic molecules generally involve a strong stabilizing contribution from dispersion forces that in many systems turns the ‘steric repulsion’ into a ‘steric attraction’. In addition to London dispersion, such systems benefit from electrostatic stabilization, which arises from a short-range effect of charge penetration and gets bigger with increasing steric bulk. In the present work, we quantify this contribution for a diverse set of molecular cores, ranging from unsubstituted benzene and cyclohexane to their derivatives carrying tert-butyl, phenyl, cyclohexyl and adamantyl substituents. While the importance of electrostatic interactions in the dimers of sp2-rich (e.g., π-conjugated) cores is well appreciated, less polarizable assemblies of sp3-rich systems with multiple short-range CH···HC contacts between the bulky cyclohexyl and adamantyl moieties are also significantly influenced by electrostatics. Charge penetration is drastically larger in absolute terms for the sp2-rich cores, but still has a non-negligible effect on the sp3-rich dimers, investigated herein, both in terms of their energetics and equilibrium interaction distances. These results emphasize the importance of this electrostatic effect, which has so far been less recognized in aliphatic systems compared to London dispersion, and are therefore likely to have implications for the development of force fields and methods for crystal structure prediction.

Short but Weak: The Z‐DNA Lone‐Pair⋅⋅⋅π Conundrum Challenges Standard Carbon Van der Waals Radii

“…At the minimum of the “total energy” (2.97 Å), Δ E disp is roughly compensated by the exchange–repulsion term (Δ E exch ) that augments below 3.5 Å as a result of overlapping fragment charge densities. In the same distance range, the electrostatic contribution (Δ E elect ) becomes more attractive due to stabilizing “charge‐penetration” effects . Negligible contributions near the energy minimum are provided by the induction (Δ E ind ) and charge‐transfer (Δ E CT ) terms (Supporting Information, Table S2).…”

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

“…between electrostatics and dispersion has since emerged as the principle paradigm for interpreting conformational preferences in aromatic systems, [13][14][15][16][17][18][19][20][21] and in particular is used to explain the emergence of the slip-stacked conguration in (C 6 H 6 ) 2 . 11 Since the seminal paper by Hunter and Sanders, 11 additional analysis has begun to erode the simple electrostatic picture proffered by this model.…”

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.…”