CH Donor Hydrogen Bonds

8 Desiraju, G. R., The C-H---0 hydrogen bond structural implications and supramolecular design , Acc. Chem. Res. 1996,29, 441-449 Nishio, M., CH/Jt hydrogen bonds in crystals , CrystEngComm 2004, 6, 130-158. 

While the most prevalent, C—H O hydrogen bonds are not the only form of weak hydrogen bond with crystal engineering potential. Carbon acid interactions of type C—H N, C—H Cl and C—H ras well as interactions to weak acceptors such as O—H andN—H tt, have all been observed. A report by Roger Hunter of the University of Cape Town, South Africa, describes the observation of a sandwich-like pair of C—H tv interactions in which two molecules of Ph3PMe  

CH---7t bonds work cooperatively. The weak directionality of CH---7t interactions and the large surfaces offered by n systems means that many aromatic compounds exhibit multiple such interactions. While they are individually weak their combined, cooperative effect can be very significant. The existence of CH---7t interactions is manifest in the commonly higher melting points and facile crystallisation of aromatic compounds compared to aliphatic compounds of similar molecular weight. 

The CH---7t bond is statistically likely. The commonplace occnrrence of relatively acidic CH groups and 7t-systems means that there are many possible interactions in a given compound or crystal and hence a high probability they will form. 

The CH- -7t bond is effective in water. The CH - tt interaction depends mostly on dispersion forces and hence, unlike electrostatic interactions, it is not damped by polar solvents. As a result the interaction is a significant contributor to the hydrophobic effect. 

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