Infrared spectroscopy frequencies table

It may seem there are too many numbers to memorize in infrared spectroscopy. Hundreds of characteristic absorptions for different kinds of compounds are listed in Appendix 2. Please glance at Appendix 2, and note that Appendix 2A is organized visually, while Appendix 2B is organized by functional groups. For everyday use, we can get by with only a few stretching frequencies, shown in Table 12-2. When using this table, remember... 

Different types of carbonyl groups give characteristic strong absorptions at different positions in the infrared spectrum. As a result, infrared spectroscopy is often the best method to detect and differentiate these carboxylic acid derivatives. Table 21-3 summarizes the characteristic IR absorptions of carbonyl functional groups. As in Chapter 12, we are using about 1710 cm-1 for simple ketones and acids as a standard for comparison. Appendix 2 gives a more complete table of characteristic IR frequencies. 

Section 13.19 Infrared spectroscopy probes molecular structure by examining transitions between vibrational energy levels using electromagnetic radiation in the 625-4000-cm range. The presence or absence of a peak at a characteristic frequency tells us whether a certain functional group is present. Table 13.4 lists IR absorption frequencies for common structural units. 

Figure 2.21 shows the seven normal modes of vibration of square-planar XY4 molecules. Vibrations V3, V6, and V7 are infrared-active, whereas Vi V2, and V4 are Raman-active. However, V5 is inactive both in infrared and Raman spectra. In the case of the [PdCl4] ion, the V5 was observed at 136 cm by inelastic neutron scattering (INS) spectroscopy [1088]. Table 2.6j lists the vibrational frequencies of some ions belonging to this group. Chen et al. [1088a] also reported the IR and Raman spectra of K2[MX4] (M = Pt or Pd and X = Cl or Br) and band assignments on the basis of normal coordinate analysis. 

Certain difficulties arise when using water as a solvent in infrared spectroscopy. The infrared modes of water are very intense and may overlap with the sample modes of interest. This problem may be overcome by substituting water with deuterium oxide (D2O), The infrared modes of D2O occur at different frequencies to those observed for water because of the mass dependence of the vibrational frequency. Table 3.2b lists the characteristic bands observed for both H2O and D2O. 

Infrared spectroscopy has been used in a large number of studies of proteins over a range of different environments. Some examples of the proteins that have been analysed are listed in Table 6.2c, which details the deconvolved amide I frequencies and secondary structure assignments that have been made for a series of proteins in D2O. These assignments are based on the fact that the secondary structures of these globular proteins have been very well characterised by X-ray crystallography. 

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