| Abstract
| - The effects of electronegative substitution (as modeled using fluoro substituents) on the strength of C−H···Ninteractions, and how these effects change with hybridization or with acidity of the proton donor, are examinedthrough the use of quantum chemistry. The binding energies (Do) in complexes between fluorinated derivativesof acetylene, ethylene, ethane and methane (the donors) and ammonia (the acceptor) are considered. We findthat fluoro substitution leads to a strengthening of the C−H···N hydrogen bonds in all cases. The effect ofreplacing 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 hydrogenbond. The magnitude of the effect of an alpha fluorine is significantly greater than that of a beta fluorine forthe ethylene−ammonia complex, but the difference is much smaller for the ethane−ammonia complex. Ingeneral, the increase in the hydrogen-bond strength upon fluoro substitution of the proton donor qualitativelyparallels an increase in the acidity of the donor. We find that the strength of even the most weakly boundsystems (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.
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