Chapter 6 - IR and Raman Spectra-Structure Correlations: Characteristic Group Frequencies

Abstract

This chapter describes the generalized infrared and Raman spectra of selected regions, which illustrate the generalized appearance of bands for selected characteristic group frequencies. The upper spectrum shows the transmission infrared spectra with bands pointing downward and the lower spectrum shows the Raman spectrum with bands pointing up. The large differences in X–H stretching frequencies are mostly due to differences in bonding and therefore the force constants. The OH stretch is particularly weak in the Raman but strong in the IR. Mechanical coupling of the stretching vibrations of bonds attached to olefinic groups is the critical variable in determining C=C stretching frequencies in cyclic olefin species. Raman spectra can be used to characterize aromatic substitution types. Useful group frequencies include CH stretch bands, ring stretch bands, ring bend bands, CH rock and CH wag bands, and aromatic summation bands. The carbonyl stretching frequencies are dependent upon mass, mechanical, and force constant effects. The mass effect is typically the least important factor in understanding carbonyl frequency dependence upon structure. The C–F stretch of organic fluorine compounds results in strong IR bands but weak to moderately intense Raman bands. Both IR and Raman spectroscopy provide important structural information on a multitude of ionic inorganic compounds. Both techniques are sensitive to the crystalline form of the inorganic species.