Carbonyl, first overtone

Diels-Alder reactions have also been studied via the fundamental and the first overtone absorptions of carbonyl stretching modes. Absorbance spectra measured during the cycloaddition of 2 (with R = CCI3) and 3 in CH2CI2 solution at 1000 bar and 95 °C are shown in Fig. 6.7-18. 

For example, a fundamental carbonyl stretching vibration at 1750 cm1 or 5714 nm would have a first overtone at... 

An analysis of the solvent dependence of the A>c and /hh couplings combined with an analysis of the changes in the IR first overtone carbonyl bond intensities and theoretical calculations allowed Abraham and co-workers to determine directly the conformational equilibria in 2-bromocyclo-hexanone, a model compound, by the use of which the authors illustrated an improved method of conformational analysis of substituted cyclohexanones. 

Figure 8 shows an online infrared spectrum of HCl. The wide dynamic range afforded by FTIR allows both intense bands such as the first overtone of the H-Cl stretch of the bulk sample and weakly absorbing bands such as the carbonyl stretch of phosgene to be used. It was recognized that variations in gas pressure will affect the analysis result because of gas law effects and spectral line shape changes. Pressure... 

For example, a fundamental carbonyl stretching vibration at 1750 cm or 5714 nm would have a first overtone at approximately 3000 nm, a weaker second overtone at 2100 nm, and a third very weak overtone at 1650 nm. The fourth overtone, at about 1370 nm, would be so weak as to be analytically useless. (These values are based on a 5% anharmonicity constant.)... 

Bands corresponding to the binary overtones and combinations of the fundamental CO-stretching frequencies have been recorded in the infrared spectra of many carbonyl com )ouiids. These binary combination data must be consistent with a proposed assignment of the CO-stretching fundamentals if the assignment is to be considered acceptable. Thus, the infrared spectrum of a compound in the 4000 cm region serves as an excellent check on the assignment of the fundamentals. Certain factors, however, limit the use of the binary combination spectra and these will be noted first. 

I shall first discuss briefly the experimental techniques Involved. I shall then review the effects of bond dissociation anharmonlclty in a diatomic molecule. Next I shall introduce the idea of local modes with a simple classical model, and then extend this to a mathematically defined quantum mechanical model which I shall discuss in detail for the case of two symmetry related stretching vibrations, as in the water molecule. I shall then introduce the effects of Fermi resonance, and describe some of our recent work on the dlchloromethane molecule. I shall also describe similar fits to the overtones of carbonyl stretching vibrations in metal carbonyls. Finally I shall comment briefly on the implications of this work for intramolecular vibrational relaxation (IVR) and chemical dynamics. 

The weak combination and overtone absorptions that occur between 2000 and 1667 cm . This set of combination bands (see Figure 25.19B) may not be as useful as the first set given here because the spectral sample must be very concentrated for them to be visible. But they are often weak. In addition, a broad carbonyl absorption may overlap and obscure this region, rendering it useless. 

---