A Theoretical Investigation of the Effects of Electronegative Substitution on the Strength of C−H···N Hydrogen Bonds

The effects of electronegative substitution (as modeled using fluoro substituents) on the strength of C−H···N interactions, and how these effects change with hybridization or with acidity of the proton donor, are examined through the use of quantum chemistry. The binding energies (Do) in complexes between fluorinated derivatives of acetylene, ethylene, ethane and methane (the donors) and ammonia (the acceptor) are considered. We find that fluoro substitution leads to a strengthening of the C−H···N hydrogen bonds in all cases. The effect of replacing a beta hydrogen by fluorine increases as the hybridization of the proton donor goes from sp < sp2 < sp3. This trend is the opposite of the propensity of the unsubstituted C−H donors to participate in a hydrogen bond. The magnitude of the effect of an alpha fluorine is significantly greater than that of a beta fluorine for the ethylene−ammonia complex, but the difference is much smaller for the ethane−ammonia complex. In general, the increase in the hydrogen-bond strength upon fluoro substitution of the proton donor qualitatively parallels an increase in the acidity of the donor. We find that the strength of even the most weakly bound systems (i.e., those with sp2- and sp3-hybridized proton donors) can be made comparable to, or larger than, that of the acetylene−ammonia complex through electronegative (fluoro) substitution.