In chemistry, a C–H···O interaction is occasionally described as a special type of weak hydrogen bond. These interactions frequently occur in the structures of important biomolecules like

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s, sugars, DNA and RNA.

History

The C–H···O interaction was discovered in 1937 by Samuel Glasstone. Glasstone studied properties of mixtures of

acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscib ...

with different halogenated derivatives of

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

s and realized that dipole moments of these mixtures differ from dipole moments of pure substances. He explained this by establishing the concept of C–H···O interactions. The first crystallographic analysis of C-H ⋯O hydrogen bonds were published by June Sutor in 1962.

Properties

Similar to hydrogen bonds, a C–H···O interaction involves interactions of dipoles and therefore has directionality. The directionality of a C–H···O interaction is usually defined by the angle ''α'' between the С, Н and О atoms, and the distance ''d'' between the O and C atoms. In a С–Н···О interaction, the angle ''α'' is in the range between 90 and 180°, and the distance ''d'' is usually smaller than 3.2 Å. Bond strength is less than 1 kcal/mol. In the case of

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

C–H donors, C–H···O interactions are not linear due to influence of

aromatic ring In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

substituents near the interacting C-H group. If aromatic molecules involved in С–Н···О interaction belong to the group of

polycyclic aromatic hydrocarbons A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

, the strength of C–H···O interactions increases with the number of aromatic rings. C–H···O interactions can be important in

drug design Drug design, often referred to as rational drug design or simply rational design, is the inventive process of finding new medications based on the knowledge of a biological target. The drug is most commonly an organic small molecule that acti ...

, being present in structures of therapeutic proteins, and nucleic acids. O-H···C and N-H···C type interactions could also play a significant role and were first analyzed in 1993. M.A. Viswamitra, R. Radhakrishnan, J. Bandekar, G. R. Desiraju, "Evidence for O-H···C and N-H···C hydrogen bonding in crystalline alkynes, alkenes, and aromatics", J. Am. Chem. Soc. 1993, 115, 4868-4869.DOI:10.1021/ja00064a055

References

{{DEFAULTSORT:C-H O interaction Organic chemistry