Frequencies and absorption intensities of fundamentals and overtones of NH stretching vibrations of pyrrole and pyrrole–pyridine complex studied by near-infrared/infrared spectroscopy and density-functional-theory calculations

Near-infrared (NIR)/IR spectra were measured for pyrrole, pyridine and pyrrole–pyridine complex in CCl4 solutions. The first-overtone of the NH stretching vibration of free pyrrole was observed at 6856 cm−1, but that of pyrrole–pyridine complex was missing or barely detectable. Theoretical calculations of the molecular vibrational potentials and the dipole moment functions of the NH stretching modes of free pyrrole and pyrrole–pyridine complex revealed that the transition dipole moment is significantly decreased upon formation of the complex, resulting in the remarkably weak intensity of the overtone mode of the hydrogen-bonded NH group.