How Different Are Aromatic π Interactions from Aliphatic π Interactions and Non-π Stacking Interactions?

We compare aromatic π interactions with aliphatic π interactions of double- and triple-bonded π systems and non-π stacking interactions of single-bonded σ systems. The model dimer systems of acetylene (C₂H₂)₂, ethylene (C₂H₄)₂, ethane (C₂H₆)₂, benzene (C₆H₆)₂, and cyclohexane (C₆H₁₂)₂ are investigated. The ethylene dimer has large dispersion energy, while the acetylene dimer has strong electrostatic energy. The aromatic π interactions are strong with particularly large dispersion and electrostatic energies, which would explain why aromatic compounds are frequently found in crystal packing and molecular self-engineering. It should be noted that the difference in binding energy between the benzene dimer (aromatic–aromatic interactions) and the cyclohexane dimer (aliphatic–aliphatic interactions) is not properly described in most density functionals

Anion Binding by Electron-Deficient Arenes Based on Complementary Geometry and Charge Distribution

Extended electron-deficient arenes are investigated as potential neutral receptors for polyanions. Anion binds via σ interaction with extended arenes, which are composed solely of C and N ring atoms and CN substituents. As a result, the positive charge on the aromatic C is enhanced, consequently maximizing binding strength. Selectivity is achieved because different charge distributions can be obtained for target anions of a particular geometry. The halides F^– and Cl^– form the most stable complex with <b>6</b>, while the linear N₃^– interacts most favorably with <b>7</b>. The trigonal NO₃^– and tetrahedral ClO₄^– fit the 3-fold rotational axis of <b>6</b> but do not form stable complexes with <b>5</b> and <b>7</b>. The Y-shaped HCOO^– forms complexes with <b>4</b>, <b>5</b>, and <b>7</b>, with the latter being the most stable. Thus, the anion complexes exhibit strong binding and the best geometrical fit between guest and host, reminiscent of Lego blocks

Intercalation of Transition Metals into Stacked Benzene Rings: A Model Study of the Intercalation of Transition Metals into Bilayered Graphene

Structures of neutral metal–dibenzene complexes, M(C₆H₆)₂ (M = Sc–Zn), are investigated by using Møller–Plesset second order perturbation theory (MP2). The benzene molecules change their conformation and shape upon complexation with the transition metals. We find two types of structures: (i) stacked forms for early transition metal complexes and (ii) distorted forms for late transition metal ones. The benzene molecules and the metal atom are bound together by δ bonds which originate from the interaction of π-MOs and d orbitals. The binding energy shows a maximum for Cr(C₆H₆)₂, which obeys the 18-electron rule. It is noticeable that Mn(C₆H₆)₂, a 19-electron complex, manages to have a stacked structure with an excess electron delocalized. For other late transition metal complexes having more than 19 electrons, the benzene molecules are bent or stray away from each other to reduce the electron density around a metal atom. For the early transition metals, the M(C₆H₆) complexes are found to be more weakly bound than M(C₆H₆)₂. This is because the M(C₆H₆) complexes do not have enough electrons to satisfy the 18-electron rule, and so the M(C₆H₆)₂ complexes generally tend to have tighter binding with a shorter benzene–metal length than the M(C₆H₆) complexes, which is quite unusual. The present results could provide a possible explanation of why on the Ni surface graphene tends to grow in a few layers, while on the Cu surface the weak interaction between the copper surface and graphene allows for the formation of a single layer of graphene, in agreement with chemical vapor deposition experiments

Novel Ionophores with 2<i>n</i>‑Crown‑<i>n</i> Topology: Anion Sensing via Pure Aliphatic C–H···Anion Hydrogen Bonding

A series of novel coronands having a 2<i>n</i>-crown-<i>n</i> topology based on trioxane (6-crown-3) derivatives are designed and characterized. These neutral hosts can sense anions through pure aliphatic C–H hydrogen bonding (HB) in condensed phases due to the unusual topology of 2<i>n</i>-crown-<i>n</i>. C–H bonds are strongly polarized by two adjacent oxygen atoms in this scaffold. These hosts provide a rare opportunity to modulate anion binding strength by changing the electronic nature of aliphatic C–H bonds and offer ease of synthesis