KBr Pellet Method

A common routine method for the control of solid-phase reactions is the use of FT-IR spectra from potassium bromide samples. For this, potassium bromide is mixed with resin beads and sintered under high pressure. The sample contains approximately 2% of the polymer support, and can be measured in any conventional IR-spectrometer. 

The IR-spectroscopic control of a multi-step organic synthesis is illustrated on the example of the SPOS of an alkaloid compound. A polymer-bound benzyloxycarbonyl anchor group on polystyrene resin is used as the starting material for the synthesis (Fig. 16.1). Each reaction step of the seven-step synthesis of 1,2,3,4-tetrahydro-P-carbo-lines [26] can be verified by IR-spectroscopy (Fig. 16.2). 

The PS-benzyloxycarbonyl resin (Fig. 16.2) shows at 1774 cm-1 the typical carbonyl absorption which disappears by loading the resin with amino acid fluorenylmethyl esters. This can be detected by a new carbonyl absorption at 1726 cm-1 and the characteristic C—C absorption band of the fluorene at 740 cm - which disappears by treating the resin with piperidine. The successful reaction to the Pfp-ester can be detected by the additional carbonyl absorption band at 1794 cm -1. The following reduction step to the amino alcohol is proofed by the disappearance of this band. The oxidation to the aldehyde and the formation of the imine is shown by the characteristic absorptions of new functional groups. [v(C—H, aldehyde = 2720 cm-1), imine (v(C=N) = 1670 cm-1].The successful Pictet-Spengler cyclization is proofed by the disappearance of the imine band. 

Besides the monitoring of reactions, which is possible through the appearance or disappearance of characteristic IR absorptions, the study of reaction kinetics (see Sections 16.2.2 and 16.2.3) is also possible with the KBr pellet method. The disadvantage of this method is the time-consuming sample preparation, each sample requiring about 10 minutes. 

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What advantage do the KBr pellet method and the reflectance methods have over the solution and mineral oil mull methods for solids ... 

The infrared absorption spectrum of EDTA shown in Figure 3 was obtained using the KBr pellet method on a Perkin-Elmer infrared spectrophotometer. The principal peaks observed were at 3480, 3380, 2998, 1603 cm-1. 

Pantoprazole sodium may be identified as such on the basis of its characteristic infrared absorption spectrum (KBr pellet method), with the spectrum shown in Figure 6 serving as the reference. 

Figure 12 shows the spectra of slurries of humic acid and Cu +-humate in H 0. The main difference between the spectrum of sodium humate (Fig. i 1C pH 8.0) and that of humic acid (Fig. I2A) is the occurrence of the band centered at —1700 cm in the latter. Upon neutralization, this band decreases in intensity considerably and new bands appear at —1570 and -1395 cm consistent with the ionization of carboxylic acid groups. The fact that the —1700 cm band has practically disappeared at pH 8.0 indicates that this band is due almost exclusively to carboxyl groups with little contribution from aldehyde and ketone groups. This is consistent with what has been reported by other workers (Wagner and Stevenson, 1965 Theng et al.. 1967) using the KBr pellet method on dried samples. 

Fig. 17.6. Infrared spectra of CPMPS with various values of m. In this measurement, the KBr pellet method is used.

Figure 16.5 shows the comparison of the IR-spectrum of an aryl ether immobilized on polystyrene resin using the KBr pellet method with a spectrum taken with the ATR method [29]. With regard to the resolution of the spectrometer, the absolute positions of the absorptions are identical. 

The portion of the reflected IR beam caused by diffuse reflectance is examined. This is the part of the radiation which interacted with the sample. The optics collect this scattered IR light and direct it to the detector. The required amount of sample material, quality of the spectra, time for measurement and the costs for the equipment are comparable with the KBr pellet method. 

FIGURE 1.9 Infrared spectra of an activated carbon (KBr pellet method). The Z , c, d, and e are scale expansion spectra. (From Friedel, R.A. and Carlson, G.L., Fuel, 51, 194, 1972. With permission.)... 

Polyphenylene sulfide is a partially crystalline polymer featuring an aromatic ring bridged by sulfur atoms in the 1,4-positions, as shown below. The form presented has the trade name of Ryton. Sample preparation of this material can be difficult, and the traditional KBr pellet method produces a poor-quality spectrum with Christiansen-type distortion (see Fig. 44b). A Csl pellet may be used instead to obtain an improved IR spectrum, suitable for quantitative analysis (see Fig. 44a). This polymer is usually used in conjunction with structural reinforcement additives, such as glass fibers or fillers such as PTFE (Teflon), and caution is necessary when attempting... 

FTIR spectra for the monomers and polymers were recorded on a Perkin-Elmer 2000 FTIR spectrophotometer using KBr pellet method. NMR and C NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. Dimethyl-sulfoxide (DMSO-ds) was used as the NMR. 

The mull technique is a second way of taking the spectra of solids in transmission. It works on powders or anything that can be ground into a powder, and competes directly with the KBr pellet method. Mulls are made by first grinding the sample to reduce particle size. This is to avoid light scattering and sloped... 

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