Hydrogen Bonding in Chloro- and Hydroxy-7-Azaindoles: Insights from X-Ray, Vibrational Spectroscopy, and DFT Studies

The crystal structures and vibrational spectra of 5-chloro-7-azaindole (5Cl7AI), 4,5-dichloro-7-azaindole (4,5Cl7AI), and 5-hydroxy-7-azaindole (5OH7AI) were investigated to elucidate how ring substituents modulate intermolecular hydrogen bonding and molecular packing in the solid state. Density functional theory (DFT) calculations were employed to support the interpretation of the spectroscopic data, while Hirshfeld surface analysis provided additional insight into intermolecular contacts. Single-crystal X-ray diffraction revealed that the halogenated derivatives form nearly linear N-H···N hydrogen-bonded dimers or layered arrangements, whereas 5OH7AI adopts a three-dimensional network stabilized by N-H···O and O-H···N interactions. FT-IR and FT-Raman spectra showed that variations in hydrogen-bond topology strongly affect the N-H and O-H stretching regions: the halogenated derivatives exhibit broad, red-shifted bands (3300-2500 cm^-1) characteristic of N-H···N hydrogen bonds, while 5OH7AI displays smaller red shifts of the N-H stretching bands accompanied by some additional features from O-H stretching vibrations. DFT calculations at the B3LYP-D3 and ωB97X-D levels reproduced the experimental geometries and vibrational spectra very well, providing detailed insight into the relationship between hydrogen-bond linearity, network dimensionality, and vibrational behavior.