Vibrational spectra carbonyl frequencies

The carbonyl stretching frequency of both the keto and enol tautomers can be recognized in the vibrational spectrum of pentane-2,4-dione. The enol has v(C=0) at 1618cm" , generally the dominant peak in the spectrum and more intense than the in- and out-of-phase v(C=0) stretching modes of the keto form, which are found at 1727 and 1707 cm" , respectively. These are identified by their Raman counterparts at 1719 cm" (polarized) and 1697 cm" (depolarized) (Ernstbrunner, 1970). The ratio of absorbances of the enol and the out-of-phase keto bands in the ir was used as an early method of analysis of the keto/enol equilibrium in different solvents (Le Fevre and Welsh, 1949). 

IR spectra of pyridazines and polypyridazines were studied with respect to interactions with the side chain. The IR spectral shift due to hydrogenbonding between methanol and pyridazine (and other azines) was determined the Av was compared with the pK . In connection with the structure of maleic hydrazide and its methyl derivatives, the Raman spectrum of the 1,2-dimethyl derivative was recorded and helped to explain the anomalously low reported IR carbonyl frequency. The laser Raman polarization spectrum of the lattice vibrational region of pyridazine was reported. ... 

The bands due to Fe(CO)4 are shown in Fig. 8. This spectrum (68) was particularly important because it showed that in the gas phase Fe(CO)4 had at least two vq—o vibrations. Although metal carbonyls have broad vC—o absorptions in the gas phase, much more overlapped than in solution or in a matrix, the presence of the two Vc—o bands of Fe(CO)4 was clear. These two bands show that in the gas phase Fe(CO)4 has a distorted non-tetrahedral structure. The frequencies of these bands were close to those of Fe(CO)4 isolated in a Ne matrix at 4 K (86). Previous matrix, isolation experiments (15) (see Section I,A) has shown that Fe(CO)4 in the matrix had a distorted C2v structure (Scheme 1) and a paramagnetic ground state. This conclusion has since been supported by both approximate (17,18) and ab initio (19) molecular orbital calculations for Fe(CO)4 with a 3B2 ground state. The observation of a distorted structure for Fe(CO)4 in the gas phase proved that the distortion of matrix-isolated Fe(CO)4 was not an artifact introduced by the solid state. 

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. 

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