Functional groups, infrared absorption frequencies

Infrared Absorption Frequencies of Functional Groups Table 4.7, Saturated Compounds sp sharp, br = bfoad, (w) = weak, (s) = strong... 

Aldehydes Amides Carboxylic acid anhydrides Carboxylic acids Esters Ketones and functional groups, 56 infrared absorption frequencies, 519,... 

Table 1.1 Infrared absorption frequencies of common functional groups...

IR Infrared absorption frequencies ordered by functional groups or compound types. 

Polyatomic molecules vibrate in a very complicated way, but, expressed in temis of their normal coordinates, atoms or groups of atoms vibrate sinusoidally in phase, with the same frequency. Each mode of motion functions as an independent hamionic oscillator and, provided certain selection rules are satisfied, contributes a band to the vibrational spectr um. There will be at least as many bands as there are degrees of freedom, but the frequencies of the normal coordinates will dominate the vibrational spectrum for simple molecules. An example is water, which has a pair of infrared absorption maxima centered at about 3780 cm and a single peak at about 1580 cm (nist webbook). 

The functional groups almost exclusively involved in NIRS are those involving the hydrogen atom C-H, N-H, O-H (see Figure 5.1). These groups are the overtones and combinations of their fundamental frequencies in the mid-infrared and produce absorption bands of useful intensity in the NIR. Because the absorptivi-ties of vibrational overtone and combination bands are so much weaker, in NIRS the spectra of condensed phase, physically thick samples, can be measured without sample dilution or the need to resort to difficult short-path length sampling techniques. Thus conventional sample preparation is redundant, and fortunately so, because most PAT applications require direct measurement of the sample " either in situ, or after extraction of the sample from the process in a fast loop or bypass. 

The infrared absorption spectrum of cortisone acetate was obtained as a KBr pellet (approximately 2% loading) using a Perkin Elmer 1310IR spectrophotometer. The spectrum itself is shown in Figure 4. The functional groups of the molecule are correlated with the observed frequencies in Table 3. 

The role of specific interactions in the plasticization of PVC has been proposed from work on specific interactions of esters in solvents (eg, hydrogenated chlorocarbons) (13), work on blends of polyesters with PVC (14—19), and work on plasticized PVC itself (20—23). Modes of interaction between the carbonyl functionality of the plasticizer ester or polyester were proposed, mostly on the basis of results from Fourier transform infrared spectroscopy (ftir). Shifts in the absorption frequency of the carbonyl group of the plasticizer ester to lower wave number, indicative of a reduction in polarity (ie, some interaction between this functionality and the polymer) have been reported (20—22). Work performed with dibutyl phthalate (22) suggests an optimum concentration at which such interactions are maximized. Spectral shifts are in the range 3—8 cm-1. Similar shifts have also been reported in blends of PVC with polyesters (14—20), again showing a concentration dependence of the shift to lower wave number of the ester carbonyl absorption frequency. 

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... 

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