Carbonyl stretching vibrational frequencies

As an indication of the importance of charge on the energy levels of molecules, we look at the variation in carbonyl stretching vibrational frequencies in the series Mn(CO)6, Cr(CO)6, and V(CO)6. All arc low spin isoelectronic molecules. The orbital details of the attachment of the carbon mono.xidc ligand to a transition metal are reserved for Section 15.1, but an important part is played by the acceptor behavior of the carbonyl 77 0 (8.12). The more important donation from... 

For meta- and para-substituted aromatic carbonyl compounds, a linear relationship exists between the earbonyl absorption frequency and the Hammett reactivity constant. - - - A relationship between the carbonyl stretching vibration frequency of aromatic carbonyl compound and the pAT " of the corresponding aromatic carboxylic acid has been demonstrated. Correlations with other parameters, such as electronegativities,"" ionization potentials, Taft a values, half-wave potentials, etc., have also been made." For aromatic compounds with ortho- substituents, a combination of factors may be important, such as chelation, steric effects, and field effects (dipole interactions through space). 

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

The IR spectrum of the pseudo-geminal 46) 4-acetyl-13-bromo[2.2]para-cyclophane (18) shows a band for the carbonyl stretching vibration at 1663 cm-1. This frequency lies outside the range of frequencies (1666—1668 cm-1) found for the absorption of other isomers and has been attributed by Reich and Cram to transannular Br...C=0 interactions. 

The additional structure C, which cannot be drawn for an unconjugated carbonyl derivative, implies that the carbonyl band in an enone has more single bond character and is therefore weaker. The involvement of a carbonyl group in hydrogen bonding reduces the frequency of the carbonyl stretching vibration by about 10 cm-. This can be rationalised in a manner analogous to that proposed above for free and H-bonded 0-H vibrations. 

Specific interactions between PCL and PVC are clearly indicated. In the solid state (Figure 5.9a) the spectrum of neat PCL indicates the presence of crystalline (1724 cm 1) and amorphous (1737 cm"1) bands. At mole ratios up to 2 1 of PVC to PCL, the spectra indicate that in the solid state the blends consist of crystalline and amorphous phases. As the PVC concentration increases, a parallel increase of the intensity of the amorphous band is observed. Moreover, the frequency shifts observed for both the crystalline and amorphous bands as a function of the composition of the blend suggests that specific interactions between the two polymers occur. No shift is observed in the carbonyl stretching vibration of PPL/PVC blends, in the molten state or in the solid state over the entire range of compositions and the two polymers are incompatible [28]. 

The VCD features for a number of larger peptide models have been calculated in the course of our efforts to define their solution conformation. These calculations proceeded exactly in the same manner as the ones described for small oligonucleotides. Cartesian coordinates from X-ray experiments, or from the program MacroModel [21), were used, along with a vibrational frequency for an unperturbed, single amide I or amide I vibration. The dipole transition moment for the amide I vibration was taken somewhat lower than that of the nucleotide base carbonyl stretching vibration, in agreement with observed data and literature values. Details of these calculations will also be provided in Section 4. 

The mesoionic 1,3-dithiolones of type (2) show substituent-dependent IR carbonyl stretching vibrations between 1612 and 1558 cm-1 (Table 7). These low frequencies are characteristic for this class of compound and are in agreement with the mesoionic structure. Most of the mesoionic 1,3-dithiolones containing p-substituted phenyl groups show split carbonyl absorption bands, presumably as the result of Fermi resonances (76CB740). 

The infrared spectrum of isatin shows two strong bands at 1740 and 1620 cm 1 corresponding to the carbonyl stretching vibrations. A broad band occurs at 3190 cm 1 due to the A-H stretching, and it is accompanied by many sub-bands, all of which are moved to a lower frequency on deuteration, which also affects several bands in the region of 1400-1100 cm, associated with A-H in-plane bending . Although the vC=0 values are not modified... 

In general, the position, shape, and intensity of an infrared-active absorption band are solvent-sensitive. In particular, carbonyl stretching vibrations of metal carbonyl complexes exhibit large frequency shifts 10, 11, 21, 21, 28, 37, 64, 69, 12, 115, 120) and sizable broadening of bands... 

The halocarbonyl derivatives Pd2X4(CO)2, Pt2X4(CO)2, PtX2(CO)2, and AuX(CO) are characterized by carbonyl stretching vibrations at high frequencies, sometimes higher than CO itself (vco = 2143 cm ) (see Table 5)... 

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