Assignment of Fundamental CO-Stretching Modes

In association with infrared data, Raman spectra are invaluable in assigning CO-stretching frequencies. Few Raman data for carbonyl compounds are available, however, because of the experimental difficulties involved. Solutions of the compounds at high concentrations are required to obtain acceptable Raman data unfortunately, many carbonyl compounds are insufficiently soluble in the appropriate solvents. There are a few examples, however, where this problem of solubility has been circumvented by employing another phase. Thus, Raman data have been reported for the compounds, M(CO)5 (M = Mo, Cr, or W) (8), M2(CO)io [M = Re (104, 122, 179, 220) or Mn (122)] -, Mn(CO)sBr (122), Re(CO)6l (179), and Re3(CO)i2H3 (277) in the solid state and Ni(CO)4 in the gas phase (39). Another limitation is that decomposition or isomerism of the compound may occur on irradiation. The fact that all colored compounds absorb the mercury excitation line at 4358 A poses an additional problem. However, it is possible to use the helium lines at 5876, 6678, and 7065 A to obtain Raman spectra, as has been done for the compounds Fe(CO)s (283), HFe(CO)4-, Fe(CO) -, Co(CO)4- (282), M[Co(CO)4]2 (M = Cd or Hg) (281), and Ni(CO)4 (280). Further, the use of laser sources in the measurement of Raman spectra overcomes many of these difficulties this technique is now being applied extensively (122, 179, 198). 

Certain other features must also be considered in interpreting the Raman spectrum of a carbonyl compound in the 2000 cm region. Thus, certain bands may be of very low intensity for example, the and the CO-stretching fundamentals were not observed in the Raman spectra of the compounds rans-Mo(CO)4[As(C2H5)3]2 (35) and Re2(CO)io (104, 220), respectively. Furthermore, the possibility of accidental degeneracy must not be overlooked. Solid state splitting, as has been observed in the Raman spectrum of the compound Mn2(CO)io in the 2000 cm region (122), is an additional limitation. It must also be emphasized that infrared and Raman data can only be compared provided spectra of the compound in the same phase are available. Thus, the 

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. 

There are three major approaches to the interpretation of the relative energies of the CO-stretching vibrations (i) the theory of local oscillating dipoles, (u) correlation curves, and (iii) simplified force constant calculations. These approaches will now be discussed with particular reference to their value in confirming band assignments. 

Trends in the frequency of the totally symmetric CO-stretching vibration for a series of compounds have also been exjjlained on the basis of local oscillating dipoles. For instance, the order Cl Br I and Mn Re is observed in the frequency of the high-energy fundamental of the compounds M(CO)sX and [M(00)4X]2 (M = Mn or Re X = Cl, Br, or I). This order can be correlated with a decrease in the coplanarity of the 

CO groups cis to the halogen atom with the central metal atom and a consequent decrease in the repulsion between the corresponding dipoles as the size of the halogen increases and that of the metal atom decreases 137, 138). 

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Corrosion inhibition, 22 244 Cosmic radiation formation of, 3 293-301 products from, 3 299-301 CO-stretching frequencies band assignment, 12 90, 91 in carbonyl complexes, 12 54, 55 assignment of fundamental modes, 12 69-107... 

The spectroscopy of FCO is reasonably well understood. The first infrared spectrum of FCO formed in a CO matrix was obtained by Milligan and co-workers [175]. Three peaks due to FCO were assigned to the three fundamentals. Jacox [176] reexamined the spectra of FCO using isotopic substitution to refine the vibrational assignment of FCO. The CO stretch mode for FCO occurs at 1861 cm" (Vj), the FCO bend is at 628 cm", and the CF stretch occurs at 1026 cm" Nagai and co-workers [177] used infrared diode laser spectroscopy to obtain a high-resolution spectra of the CO and CF stretching modes. There have been several studies of the ultra-... 

The fundamentals have been assigned for the M-H-M backbone of HM2(CO)io , M = Cr, Mo, and W. When it is observable, the asymmetric M-H-M stretch occurs around 1700 cm-1 in low-temperature ir spectra. One or possibly two deformation modes occur around 850 cm-1, in conjunction with overtones that are enhanced in intensity by Fermi resonance. The symmetric stretch, which involves predominantly metal motion, is expected below 150 cm-1. For the molybdenum and tungsten compounds, this band is obscured by other low frequency features. Vibrational spectroscopic evidence is presented for a bent Cr-H-Cr array in [PPN][(OC)5Cr-H-Cr(CO)5]. 

Hydroxygermylene, HGeOH, was first produced in Ar matrix at 15 K upon photoin-duc (340-300 nm) intramolecular insertion of Ge atom into the OH bond of H2O submolecule in a Ge OH2 complex, formed by co-deposition of Ge atoms and water with excess Ar . Three observed IR bands at 1741.3, 661.3 and 566.2 cm were assigned to Ge OH stretching, Ge—O stretching and torsion vibrational modes. Later, HGeOH was identified as one of the products of the photochemical reaction of GeELj with O3 in Ar matrices. AU the fundamental frequencies of this molecule [vi(OH) = 3652.0,... 

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