Solvent Dependence of Absorption Intensities and Wavenumbers of the Fundamental and First Overtone of NH Stretching Vibration of Pyrrole Studied by Near-Infrared/Infrared Spectroscopy and DFT Calculations

Abstract

Near-infrared (NIR) and IR spectra were measured for pyrrole in CCl(4), CHCl(3), and CH(2)Cl(2) to study solvent dependence of absorption intensities and wavenumbers of the fundamental and first overtone of NH stretching vibration. It was found that the wavenumbers of the NH fundamental and its first overtone decrease in the order of CCl(4), CHCl(3), and CH(2)Cl(2), which is the increasing order for of the dielectric constant of the solvents. Their absorption intensities increase in the same order, and the intensity increase is more significant for the fundamental than the overtone. These results for the solvent dependence of the wavenumbers and absorption intensities of NH stretching bands of pyrrole are quite different from those due to the formation of hydrogen bonds. Quantum chemical calculations of the wavenumbers and absorption intensities of NH stretching bands by using the 1D Schrödinger equation based on the self-consistent reaction field (SCRF)/isodensity surface polarized continuum model (IPCM) suggest that the decreases in the wavenumbers of both the fundamental and the overtone of the NH stretching mode with the increase in the dielectric constant of the solvents arise from the anharmonicity of vibrational potential and their intensity increases come from the gradual increase in the slope of the dipole moment function.

Breakthrough Potential in Near-Infrared Spectroscopy: Spectra Simulation. A Review of Recent Developments

“…Gonjo et al concluded a “parity” in relative intensities over the sequences of transitions and they suggested that an intermolecular OH…Cl hydrogen bond between phenols and the solvent is responsible for that phenomenon (Gonjo et al, 2011). A similar problem was further investigated by Futami et al for pyrrole (Futami et al, 2011). An accurate modeling of single-mode anharmonicity also enabled studies on the influence of the solvent's dielectric constant on X-H stretching mode in the solution phase, as reported by Futami et al on the example of HF as a prototypic polar molecule (Futami et al, 2012).…”

“…A highly accurate determination of vibrational states and transition intensities was performed e.g., in systematic examinations of subtle spectral variations due to certain substituents (Takahashi and Yabushita, 2013) or solvent effects (Futami et al, 2011, 2012; Gonjo et al, 2011) in NIR and MIR. For example, Gonjo et al studied solvent effects in vibrational spectra of phenol and 2,6-dihalogenated derivatives (Gonjo et al, 2011).…”

Near-Infrared Spectroscopy—Its Versatility in Analytical Chemistry

“…[1][2][3][4][5][6][7][8][9][10] Recent rapid developments of NIR spectrometers, particularly FT-NIR spectrometers, and spectral-analysis methods, like chemometrics, [16][17][18][19][20] two-dimensional correlation analysis, 24,25 and quantum chemical calculations, [26][27][28] have stimulated novel studies on the overtones and combination modes, as well as their anharmonicities and molecular structure and interactions. [1][2][3][4][5] For the last five to ten years, much attention has been paid to NIR imaging, portable and hand-held spectrometers, on-line monitoring, PAT (process analysis technology), and medical diagnosis.…”

“…Moreover, recently, quantum chemical calculations, such as density function theory (DFT) calculations, are becoming popular, even in NIR spectral analysis. [26][27][28] One can calculate the intensities and frequencies of overtones bands using quantum chemistry calculations.…”

“…84 One good thing is rapid progress of quantum chemical calculations of overtones. [26][27][28] We suppose that quantum chemical calculations play very important roles in spectral analysis and investigations in NIR spectroscopy.…”