Experimental observation of structures with subtle balance between strong hydrogen bond and weak n → π(*) interaction: Gas phase laser spectroscopy of 7-azaindole⋯fluorosubstituted pyridines.

In this study, interplay between a strong hydrogen bond and a very weak n → π() interaction has been probed through experiment for the first time. We have used resonant 2-photon ionization, Infrared-ultraviolet double resonance spectroscopy, and quantum chemistry calculation to determine the structures of 7-azaindole⋯2,6-difluoropyridine and 7-azaindole⋯2,3,5,6-tetrafluororpyridine complexes, which are stabilized by both hydrogen bonding and n → π() interaction. The structures of the complexes studied in the present work have been compared with the double hydrogen bonded (N-H⋯N and C-H⋯N) planar structure of 7-azaindole⋯2-fluoropyridine. It has been found that the strength of the N-H⋯N hydrogen bond in the 7-azaindole⋯2,6-substituted fluoropyridines is affected due to several factors. The main reason for huge reduction in the strength of this N-H⋯N hydrogen bond in these complexes is due to loss of the C-H⋯N hydrogen bond, through substitution of fluorine atoms in 2 and 6 positions, which induces major structural changes by bending the hydrogen bond and introducing the n → π() interaction. Effect of fluorination as well as presence of the n → π() interaction in these complexes also contributes to the reduction of the strength of the N-H⋯N interaction. Although it is difficult to quantify the role of the n → π() interaction to affect the strength of the hydrogen bond, observation of the structures, where a strong hydrogen bond and a weak n → π() interaction co-exist, is confirmed.