Resonance effects infrared, Fermi

Bond distances obtained in infrared and Raman studies are normally ro or r distances. Although data from the infrared are now, in general, precise enough to obtain distances, the large amounts of pure rare isotopic species required to obtain these distances preclude the substitution technique, and only a handful of partial r, structmes have been obtained by optical methods. In these tables, whenever both r and ro stractmes have been derived for a given molecule, only the r structure is listed. UtKertainties in r parameters are not easily estimated since for polyatomic molecttles the study of several vibration-rotation bands is required and quite often the data used come from several different laboratories and have been obtained over a period of years. The derivation of r parameters also often necessitates certain assunqttions regarding the effects of perturbations, especially Fermi resonances. The limitations of ro parameters have been detailed above in Section 1.4.1. 

The results of Fermi resonance are important for infrared and near-infrared spectroscopy. Fermi resonance causes the following effects on spectral bands ... 

We have seen that the effect of a full or partial deuteration of the cation not only leads to line shifts but also significantly changes the intensities and modifies the assignment of the infrared signatures of the different isotopologues. This is due to the soft, anharmonic, and coupled potential of the Zundel cation, where the dynamics and spectroscopy are strongly dominated by Fermi resonances between various coupled zeroth-order vibrations. The discussed quantum dynamical calculations represent an important milestone in our understanding of the spectroscopy and dynamics of protonated water clusters and on their dramatic isotope effects [41], and could only be achieved after a full-dimensional quantum dynamical treatment of the clusters. 

The C—C=C—C stretching frequency appears near 2260-2190 cm" in hydrocarbons, " but because of symmetry, when the substituents are similar in mass and inductive and mesomeric properties, the intensity may be very weak or zero in the infrared. This vibration is far better studied in the Raman effect where it shows up strongly. In the Raman spectra of dialkyl acetylenes, two bands appear near 2300 and 2235 cm", probably due to Fermi-resonance doubling. When the two substituents are sufficiently different in their properties, the bond is made more polar and a strong band may result in the infrared. 

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