Hydrogen Bonding Effects on the Wavenumbers and Absorption Intensities of the OH Fundamental and the First, Second, and Third Overtones of Phenol and 2,6-Dihalogenated Phenols Studied by Visible/Near-Infrared/Infrared Spectroscopy

Visible, near-infrared (NIR) and IR spectra in the 15600–2500 cm–1 region were measured for phenol and 2,6-difluorophenol, 2,6-dichlorophenol, and 2,6-dibromophenol in n-hexane, CCl4, CHCl3 and CH2Cl2 to study hydrogen bonding effects and solvent dependences of wavenumbers and absorption intensities of the fundamental and the first, second, and third overtones of OH stretching vibrations. A band shift of the OH stretching vibrations from a gas state to a solution state (solvent shift) was plotted versus vibrational quantum number (v = 0, 1, 2 and 3), and it was found that there is a linear relation between the solvent shift and the vibrational quantum number. The slope of solvent shift decreases in the order of phenol, 2,6-difluorophenol and 2,6-dichlorophenol. For all of the solute molecules, the slope becomes larger with the increase in the dielectric constant of the solvents. The relative intensities of the OH stretching vibrations of phenol in CCl4, CHCl3, and CH2Cl2 against the intensity of the corresponding OH vibration in n-hexane increase in the fundamental and the second overtone but decrease in the first and third overtones; the relative intensities show so-called “parity”. The parity is more prominent for phenol that has an intermolecular hydrogen bonding than for 2,6-dihalogenated phenols that have an intramolecular hydrogen bond. These observations suggest that the intermolecular hydrogen bond between the OH group and the Cl atom plays a key role for the parity and that the intermolecular interaction between the solutes and the solvents (solvent effects) does not have a significant role in the parity

Combined IR/NIR and Density Functional Theory Calculations Analysis of the Solvent Effects on Frequencies and Intensities of the Fundamental and Overtones of the CO Stretching Vibrations of Acetone and 2‑Hexanone

Vibrational overtone studies primarily focus on XH stretching overtone transitions, where X is an atom like C, O, N, or S. In contrast, the studies on the CO stretching overtones are very scattered. To advance the research in this field, we measured the fundamental, first, and second overtones of the CO stretching vibration of acetone and 2-hexanone in n-hexane, CCl4, and CHCl3, as well as in the vapor phase using FT-IR/FT-NIR spectroscopy. Density functional theory (DFT) calculations have also been performed to help the assignment of the CO stretching bands and to guide interpretation of the experimental results. It was found that the wavenumbers, absorption intensities, and oscillator strengths of the CO stretching bands show marked solvent dependence. In the fundamental and the first overtone regions, the intensities of the CO stretching vibration were found to be pronouncedly more intense than those of the CH stretching vibration. In the second overtone region, the intensities of the CH stretching vibration are comparable to those of the CO stretching vibration. The theoretical and observed decrease in integrated intensity upon going from the fundamental to the first overtone of the CO stretching vibration is around 50, which is significantly larger than those of the OH, CH, and SH stretching vibration. Both the calculated and experimental results suggest that excessive weakness in the CO stretching overtone was shown to be a result of both a low anharmonicity and a substantial reduction in the oscillator strength. These results provide new insight into our understanding of the CO stretching vibration