C—H—n interactions

While the most prevalent, C—H - 0 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 rand N—H--- n, have all been observed. A report by Roger Hunter of the University of Cape Town, South Africa,97 describes the observation of a sandwich-like pair of C—H-- - n interactions in which two molecules of Ph3PMe+... 

Figure 10 The two types of edge-edge C-H N interactions (dashed lines) present between four layers of molecular pens in the structure (9)2(CH3-CCl3). These interactions all operate between opposite enantiomers of the host. Hydrogen atoms are omitted for clarity.

The substituents at C-3 and C-5 may also have stacking interactions in the crystal and thus packing forces can dictate the preferential conformation of the 1,2,4-trioxolane ring as the C-5 and peroxide envelope or even the half chair conformations are energetically close. For instance in the 6,7,8-trioxa-3-thiabicyclo[3.2.1]octane 21, centrosym-metrical pairs are formed by 71-71 stacking between two phenyl rings combined with two weak C-H- -n interactions <2000AXC1510>. 

A further hydrogen-bonding example which has also been demonstrated with pyridine derivatives is the aromatic C-H- -N interaction <2001T5791, 2002AXCo697>. These interactions are exemplified by the solid state of pyridine dicarboxylate esters, with the interactions supplemented by C-H- -0=C contacts <2001CE0170>, and pyridiny-lisoxazoles <2000TL5827>. 

Maciejewska" has published 13C CP/MAS NMR spectra of 2-methyl-4-(4-X-phenylazo)imidazoles. Proton transfer phenomena can occur in imidazoles. Depending on the nature and orientation of the substituent X it was possible to identify one tautomer for X = H, Br and N02 and two tautomers for X = OCH3. Solid 2-methyl-4-(4-X-phenylazo)imidazoles form hydrogen-bonded chains with N—H- N bonds and C—H- O or C—H- N interactions. 

Murrell and co-workers (65RTC1399) have compared the NMR spectra of azabiphenyls and have shown that coplanarity increases in the series biphenyl (116), 2-phenylpyridine (117), 2-phenylpyrimidine (118), and 2,2 -bipyridyl (119). It was deduced that the through space C—H - N interactions are weaker than the C—H H—C ones (Scheme 88). 

The linearity of equation 4 agrees with the predominance of the C—H N interaction, where aniline acts as a proton acceptor, even if the interaction involving the tr-electrons may also be operative in the complex formation, but the latter may be considered as a secondary interaction. 

Berkovitch-Yellin, Z. and Leiserowitz, L. (1984) The role played by C-H- - -O and C-H- - -N interactions in determining molecular packing and conformation, Acta Cryst. B40, 159-165. Parthasarathy, R., Fridey, S.M., and Srikrishnan, T. (1989) Conformation and hydrogen bonding of A-formylmethionyl peptides. II. Crystal and molecular structure of iV-formyl-L-methionyl-L-phenylalanine, Int. J. Peptide Protein Res. 33, 308-312. 

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