The CH/π hydrogen bond is an attractive molecular force occurring between a soft acid and a soft base. Contribution from the dispersion energy is important in typical cases where aliphatic or aromatic CH groups are involved. Coulombic energy is of minor importance as compared to the other weak hydrogen bonds. The hydrogen bond nature of this force, however, has been confirmed by AIM analyses. The dual characteristic of the CH/π hydrogen bond is the basis for ubiquitous existence of this force in various fields of chemistry. A salient feature is that the CH/π hydrogen bond works cooperatively. Another significant point is that it works in nonpolar as well as polar, protic solvents such as water. The interaction energy depends on the nature of the molecular fragments, CH as well as π-groups: the stronger the proton donating ability of the CH group, the larger the stabilizing effect. This Perspective focuses on the consequence of this molecular force in the conformation of organic compounds and supramolecular chemistry. Implication of the CH/π hydrogen bond extends to the specificity of molecular recognition or selectivity in organic reactions, polymer science, surface phenomena and interactions involving proteins. Many problems, unsettled to date, will become clearer in the light of the CH/π paradigm.

The CH/π hydrogen bond in chemistry. Conformation, supramolecules, optical resolution and interactions involving carbohydrates

C-h⋯π-interactions in proteins

The CH/π interaction: Significance in molecular recognition

This treatise is an update to a preceding highlight (CH/π hydrogen bonds in crystals) published in this journal 5 years ago (M. Nishio, CrystEngComm, 2004, 6, 130–156). After the introductory part (sections 1 and 2), we survey recent results (mostly since 2004) relevant to the CH/π hydrogen bond: crystal conformation, packing and host/guest chemistry (section 3). Section 4 summarizes the results obtained by crystallographic database (CSD and PDB) analyses. In section 5, several topics in related fields (selectivity in organic reactions, surface chemistry, structural biology, drug design and high-level ab initio calculations of protein/substrate complexes and natural organic compounds) are introduced, and in the final part we comment on the prospects of this emerging field of chemistry.

CH/π hydrogen bonds in organic and organometallic chemistry

High-level ab initio calculations were carried out to evaluate the interaction between the π face of benzene and hydrocarbon molecules (methane, ethane, ethylene, and acetylene). Intermolecular interaction energies were calculated from extrapolated MP2 interaction energies at the basis set limit and CCSD(T) correction terms. The calculated benzene−methane interaction energy (−1.45 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The benzene−methane complex prefers a geometry in which the C−H bond points toward the benzene ring. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the complex (0.85 kcal/mol) is repulsive. The large gain of the attraction energy (−2.30 kcal/mol) by electron correlation correction indicates that dispersion interaction is the major source of the attraction. Although the electrostatic energy (−0.25 kcal/mol) is small, a highly ori...

The Magnitude of the CH/π Interaction between Benzene and Some Model Hydrocarbons

Intermolecular interactions in CH/π systems have been studied by PCILO and ab initio MO methods to elucidate the origin of the attractive interactions. In the PCILO calculations, the van der Waals molecules of CH4+ C2Hn(n= 2 or 4) were more stable than that of CH4+ C2H6, and the most stable arrangement was such that the carbon atom in methane is shifted from the C2 axis perpendicular to the C–C–H plane of ethene. Ab initio MO results supported this conclusion and showed also that the main contributions to attractive CH/π interactions come from electrostatic and charge-transfer effects. These results were supported by electrostatic-potential maps of C2H4 and C2H6.

Computational studies on CH/π interactions

A novel treatment for the estimation of intermolecular interactions is proposed, incorporating intermolecular parameters into the semiempirical MNDO MO method. These parameters vary depending on the atomic species, but are constants to be reduced to the original MNDO formalism when the atomic pair belongs to a same molecule. The core–core repulsion and the two electron integral terms were modified using these parameters. The parameters introduced have been determined so as to reproduce well the interaction energies of the different model systems of molecular interaction. The resulting parameters permit calculations of large systems with sufficient accuracy within reasonably short CPU time.

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Sanshiro Matsuo

Papers

Computational studies on CH/π interactions

A novel development for the simulater of intermolecular potentials

Computational Studies on CH/π Interactions