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

Abstract

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