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.

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

“…The absorptivity of NIR transitions gradually decreases toward the higher tones and higher order combinations. The co-existence of various bands (e.g., first, second, and third overtones Gonjo et al, 2011; Futami et al, 2012; Chen et al, 2014) within the same spectra is a key advantage here. Low band intensities enable systematic studies of the molecules in solution with a widely spread concentration range.…”

“…The computations [B3LYP/6-311++G(3df,3pd) and CCSD/aug-cc-pVQZ levels; IPCM solvent approximation] elucidated the potential and dipole moment function variations in response to the changing solvent's dielectric constant. The investigation of the solvent effect in NIR and IR was continued by Chen et al (2014); in this case, C=O stretching vibrations in acetone and 2-hexanone were examined.…”

Solvent-Independent Anharmonicity for Carbonyl Oscillators

“…This measurement is common in the gas phase by measuring overtone and combination bands but less common in condensed phases largely because these transitions are weak and often overwhelmed by modes from the solvent. 30 While the vibrational Stark spectrum of acetophenone and other carbonyl probes in a mixed frozen solvent where both H-bonded and non-H-bonded forms are present can be fit with a single $false|\mathrm{normal\Delta }{\overrightarrow{\mu }}_{\mathrm{normalC}=\mathrm{normalO}}false|f$, 7,8 2D IR can provide a more direct measurement of the anharmonicity in fluid solution because both the ν = 0 → 1 $false({\overline{\nu }}_{01}false)$ diagonal peak and the ν = 1 → 2 $false({\overline{\nu }}_{12}false)$ off-diagonal peak are readily observed. The anharmonic shift, Δ, is given by subtracting the frequencies of the ν = 0 → 1 and ν = 1 → 2 peaks, for example, $\mathrm{normal\Delta }={\overline{\nu }}_{01}-{\overline{\nu }}_{12}$.…”

Identification of molecular vibrations associated with tacticity in polypropylene: Density functional theory‐based simulations

“…Since calculations were performed in Gaussian, the simulation results obtained correspond to the gas phase. It should be mentioned that the use of solution models is possible in Gaussian, but there are reported small changes in vibration frequency at the IR level . It is expected that there would not be significant changes in the vibrational frequencies at low wavenumbers (for IR regime, which corresponds to the vibrations of entire molecular chain).…”