Do C—H Groups Form Hydrogen Bonds

1 Chloroform. The properties of chloroform dissolved in basic solvents furnish a most interesting example. There is a body of opinion on the side that the relatively weak interactions of chloroform should not be classified as H bonds but rather should be attributed to the less specific effects of a polar solvent (e.g., see 349, 347). With reference to the theory of H bonding the case is important because C—H compounds are not usually listed as H bonding acids. 

Proton magnetic resonance is extremely sensitive to H bond formation. Hence Huggins, Pimentel, and Shoolery turned to this technique as an aid in deciding whether chloroform interactions involve H bonding (982). For both acetone and triethylamine bases, the chloroform NMR spectra show the characteristics of H bond formation. 

Staveley, Tupman, and Hart found that volume-change as well as the thermodynamic effects, on mixing acetone and chloroform, show the large deviations characteristic of association (1940). Different types of data which have been offered sis evidence of CHCb-base association include solubility (438, 2236, 436, 1349), vapor pressure (628), PVT behavior (1182a), dielectric polarization (572, 777), diamagnetism 

In summary, the evidence in favor of association of chloroform with bases is conclusive. The evidence that this association is of the H bonding type is substantial, and it is consistent with the statement that chloroform forms H bonds, at least with such strong bases as pyridine and triethylamine. Thus it must be concluded that, in this instance, a G— group can serve as a H bonding acid. 

3 Acetylenes. Evidence for H bonding of acetylenic C—H bonds is good, but it is not voluminous. Stanford and Gordy found systematic frequency shifts of the acetylenic C— H stretching mode of phenyl acetylene dissolved in bases (1932). Another type of evidence concerns the solubilities of acetylenic compounds in many bases. McKinnis, for example, presumes that the solubility of acetylene is dominated by the H bond interaction, and is able to correlate the solubility with electronegativity of the base atom B (1308). (See also 563.) 

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