Theoretical investigation of in-plane hydrogen-bonded complexes of ammonia with partially substituted fluorobenzenes

Abstract

Systematic investigation of in-plane hydrogen-bonded complexes of ammonia with partially substituted fluorobenzenes has revealed that fluorobenzene, difluorobenzene, and trifluorobenzene favor formation of cyclic complexes with a C-H center dot center dot center dot N-H center dot center dot center dot F-C binding motif. On the other hand, tetrafluorobenzene and pentafluorobenzene favor formation of linear C-H center dot center dot center dot N hydrogen-bonded complexes. The complete absence of exclusively linear N-H center dot center dot center dot F hydrogen-bonded complexes for the entire series indicates that C-F bond in fluorobenzenes is a reluctant hydrogen-bond acceptor. However, fluorine does hydrogen bond when cooperatively stabilized with C-H center dot center dot center dot N hydrogen bonds for the lower fluoro analogues. The propensity of fluorobenzenes to adapt to the C-H center dot center dot center dot N-H center dot center dot center dot F-C binding motif decreases with the progressive fluorination of the benzene ring and disappears completely when benzene ring is substituted with five or more fluorine atoms.