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

Abstract

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.

Understanding the n → π* non-covalent interaction using different experimental and theoretical approaches

“…18(a) and (b)). 96 In contrast, a red-shift of 107 and 69 cm À1 in the N-H stretching frequency was observed in the 7AIÁ Á Á2,6-DFP and 7AIÁ Á Á2,3,5,6 TTFP complexes, respectively, which are stabilized by N-HÁ Á ÁN and np* interactions (Fig. 18(c) and (d)).…”

“…96 The 7AIÁ Á Á2-FP complex is planar due to the N-HÁ Á ÁN and C-HÁ Á ÁN hydrogen bonds, while the N-HÁ Á ÁN hydrogen bond in the 7AIÁ Á Á2,6-DFP and 7AIÁ Á Á2,3,5,6-TTFP complexes is bent to accommodate the np* interaction. 96 The result shows the influence of the np* interaction in Fig. 17 governing the geometries of the complexes and the balance between the np* interaction and the N-HÁ Á ÁN hydrogen bond.…”

“…17 governing the geometries of the complexes and the balance between the np* interaction and the N-HÁ Á ÁN hydrogen bond. 96 The weakening of the strength of the N-HÁ Á ÁN hydrogen bond and thus the reduction in the IR red-shift in the N-H stretching frequency is affected by the presence of the np* interaction. 96 Later, Das and co-workers provided direct spectroscopic evidence of the np* interaction by probing the CQO stretching frequency of phenyl formate (PhOF) in the gasphase.…”

“…96 The weakening of the strength of the N-HÁ Á ÁN hydrogen bond and thus the reduction in the IR red-shift in the N-H stretching frequency is affected by the presence of the np* interaction. 96 Later, Das and co-workers provided direct spectroscopic evidence of the np* interaction by probing the CQO stretching frequency of phenyl formate (PhOF) in the gasphase. 56 Two conformers of PhOF were observed in the experiment.…”

Interconverting Hydrogen-Bonding and Weak n → π* Interactions in Aqueous Solution: A Direct Spectroscopic Evidence

“…Analogous to the H-bond interactions, where lone-pair electrons are delocalized into a σ* orbital of an acceptor moiety (n → σ*), the carbonyl (CO) group can participate in weak noncovalent interactions with the acceptor’s π* orbital (n → π*). n → π* interactions involving electron delocalization from an oxygen lone pair to the antibonding π* orbital of either another carbonyl (n → π* CO ) or an aromatic ring (n → π* ar ) have strong presence in nucleic acids, − proteins, , supramolecular assemblies, , and small molecules. − …”