X-ray crystal structure, infrared, Raman and density functional studies of 7-azaindole-3-carboxaldehyde

• 7AI3CA molecules are linked by dual linear NH⋯N hydrogen bonds, in crystal.
• CH bonds play important role in crystal packing via CH⋯O intermolecular interactions.
• Absorption bands due to NH and CO stretches are affected by multiple Fermi resonances.
• Geometry and spectra calculated for a trimer are in excellent agreement with experiment.

A single crystal X-ray analysis, FTIR and FT-Raman spectroscopic studies and the B3LYP/6–311++G (p,d) calculations were performed for 7-azaindole-3-carboxaldehyde, 1H-pyrrolo [2,3-b]pyridine-3-carbaldehyde (7AI3CA). This compound is an important building block of antibacterial, antifungal and anticancer agents. It crystallizes in the monoclinic system (space group P21/c) with a = 3.83610 (13), b = 18.0442 (6), c = 9.9572 (4) Å, β = 96.682 (3)°, V = 684.55 (4) (Å3) and Z = 4. In the crystal structure, the 7AI3CA molecules form a dimer linked by the moderately strong, dual and nearly linear NH⋯N intermolecular hydrogen bonds between the pyrrole and pyridine rings. In addition, each dimer is held together with the neighboring molecules via CH⋯O intermolecular interactions, which play an important role in stabilizing the crystal structure. The optimized geometry and the calculated vibrational spectra of a trimer linked by NH⋯N and CH⋯O intermolecular hydrogen bonds show very good agreement with the experiment. Detailed vibrational assignments of the experimental IR and Raman spectra have been made on the basis of the potential energy distribution (PED), calculated for a trimer of 7AI3CA.

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