Potassium bromide region

Further information was obtained from a study of the infrared spectra in the potassium bromide region. Trimethyltin compounds normally give rise to two Sn—C vibrations at about 500 and 550 cm, belonging to the symmetrical and the asymmetrical Sn—C stretching vibrations of tetrahedral molecules, respectively 103, 104). Only one band (near 550 cm ) is expected, however, for the planar (CH3)3Sn configuration. Thus absence of the 500 cm band in trimethyltin fluoride and acylates 103) has been used as evidence for the occurrence of planar (CH3)3Sn ions in these compounds. 

Traditional infrared spectrophotometers were constructed with mono-chromation being carried out using sodium chloride or potassium bromide prisms, but these had the disadvantage that the prisms are hygroscopic and the middle-infrared region normally necessitated the use of two different prisms in order to obtain adequate dispersion over the whole range. 

Infrared spectra for solid organic compounds are frequently obtained by mixing and grinding a small sample of the material with specially dry and pure potassi um bromide (the carrier), then compressing the powder in a special metal die under a pressure of 15 30 tonnes to produce a transparent potassium bromide disc. As the potassium bromide has virtually no absorption in the middle-infrared region, a very well-resolved spectrum of the organic compound is obtained when the disc is placed in the path of the infrared beam. 

The infrared spectrum of hydralazine hydrochloride base in a potassium bromide dispersion (Figure 2) was recorded from 400 to 4000 cm-1, and the 200 to 550 cm-1 region was obtained from a mineral oil dispersion supported on polyethylene film. The spectra of potassium bromide dispersions of the base are qualitatively identical to those of mineral oil dispersions. The assignment of absorption bands in the spectrum of the base is similar+to that of the hydrochloride except for the presence of N-H stretch absorption in the latter. A spectrum of the base has been published (6). 

The solids may be examined as alkali halide mixture and usually NaCl is used because it is transparent throughout the infra red region. Potassium bromide also serves the purpose well. The substance under examination should also be perfectly dry because water absorbs strongly at about 3710 cm 1 and also at 1630 cm-1. 

Normal glass will only transmit radiation between about 350 nm and 3 /rm and, as a result, its use is restricted to the visible and near infrared regions of the spectrum. Materials suitable for the ultraviolet region include quartz and fused silica (Figure 2.28). The choice of materials for use in the infrared region presents some problems and most are alkali metal halides or alkaline earth metal halides, which are soft and susceptible to attack by water, e.g. rock salt and potassium bromide. Samples are often dissolved in suitable organic solvents, e.g. carbon tetrachloride or carbon disulphide, but when this is not possible or convenient, a mixture of the solid sample with potassium bromide is prepared and pressed into a disc-shaped pellet which is placed in the light path. 

M. le Blanc gave the refractive indices of solii. of potassium and rubidium bromides as 1 5593 and 1 5533 respectively, when the densities are 2"738 and 3 314 respectively. Hence the refraction eq. of potassium bromide by Gladstone and Dale s formula is therefore 24 32 and by Lorentz and Lorenz s formula 14-05 the corresponding values for rubidium bromide are27"62 and 15"98. The mol. refractions of potassium bromide in soln. by the two formulae are respectively 25"11 and 14 70 and of rubidium bromide in soln., 27 85 and 16 33. The mol. refractions of these salts are therefore greater in soln. than in the solid form. Crystals of potassium bromide, says H. Marbuch, exhibit optical activity. A. S. Newcomer found that sodium chloride was the only salt relatively soluble and yet capable of emitting fluorescent rays in the mid-ultra-violet region of the spectrum under the influence of X-rays. 

The method used for sample preparation depends upon the nature of the sample. Liquids are easily examined as films formed when one drop of the liquid is squeezed between two flat sodium chloride plates, which arc transparent to IR radiation in the 4000-666 cm region. Solids can be examined as solutions, mulls in Nujol, or as potassium bromide discs. For solutions, a 5% solution of the solid is introduced into a sodium chloride cell, which is usually 1 mm thick. The solvent employed should be reasonably transparent to IR, and the background should be obtained with the cell containing the solvent only. 

Lithium fluoride, calcium fluoride and potassium bromide prisms are used to study with high resolution the absorption characteristics of compounds in specified regions (usually in conjunction with diffraction gratings), e.g. 4000-1700,4200-1300,1100-385 cm 1 respectively. 

The infrared spectrum of a liquid may conveniently be recorded as a thin film of the substance held in the infrared beam between two infrared-transparent discs without the need for a diluting solvent. It is customary to use polished plates of sodium chloride as the support material this material is adequately transparent in the region 2-15 /im. Spectra in the longer wavelength region (12-25 m) can be recorded using potassium bromide plates. Sealed cells (p. 267) should be used for volatile liquids. 

In the pressed disc technique a known weight of sample is intimately ground with pure, dry potassium bromide and the mixture inserted into a special die and subjected to pressure under vacuum. The concentration of sample in the disc is usually in the region of 1.0 per cent. The disc so produced may be mounted directly in the sample beam path of the spectrophotometer and the spectrum recorded. This method has the advantage that the spectrum so produced is entirely due to the sample since pure dry potassium bromide is infrared transparent in the 2-25 /xm region. To eliminate the possibility of impurities in the potassium bromide, however, a blank disc (no sample) can be made and mounted in the reference beam path of the spectrophotometer. Care should be taken to ensure that both discs are of equal thickness otherwise inverse peaks may occur if the potassium bromide is damp or impure, and this will be particularly noticeable if the reference disc is thicker than the sample disc. 

Potassium bromide was selected as the binder because it does not have specific absorption bands in the mid-infrared region although it does absorb water readily. Water masks the OH absorption at 3400 cm"1, making interpretation of this region difficult. 

In a variation of this method, the tiiin-layer adsorbent is placed in the bottom of a glass vessel togetiier with a tiiangular wick of compressed potassium bromide. Solvent is added and it rises up the wick and evaporates from the upper region. The compound is conveyed up the wick by the solvent and accumulates at the tip of the tiimgle which is then cut off, dried, and used to prepare a disk. About 10 )Xg of compound is required to produce a satisfactory spectrum. The advantage of this technique is that tiie low part of the potassiiun bromide wick acts as a filter removes finely divided adsorbent which can give rise to spurious peaks. 

Bromine is too reactive to exist as a free element in nature. Instead, it occurs in compounds, the most common of which are sodium bromide (NaBr) and potassium bromide (KBr). These compounds are found in seawater and underground salt beds. These salt beds were formed in regions where oceans once covered the land. When the oceans evaporated (dried up), salts were left behind—primarily sodium chloride (NaCl), potassium chloride (KCl), and sodium and potassium bromide. Later, movements of Earth s crust buried the salt deposits. Now they are buried miles underground. The salts are brought to the surface in much the same way that coal is mined. 

The infrared spectrum of diltiazem hydrochloride dispersed in potassium bromide was recorded at 4 cm 1 resolution on a Nicolet Model 740 FTIR (11). Figure 3 shows the diltiazem HCI vibrational features in the 4000 to 400 cm-1 region after spectral subtraction of the absorptions (3430 and 1629 cm 1) due to adsorbed water. Table I lists characteristic frequencies, relative intensities, and vibrational assignments for the primary diltiazem HCI absorption bands. 

The infrared spectrum of phenoxymethyl penicillin (free acid) is number 88 in Hayden s compilation of spectra measured on a Perkin-Elmer Model 21 sodium chloride prism spectrophotometer. Infrared spectra of fourteen penicillins in the 1580 to 1880 cm-1 region are discussed by Ovechkin4. Figure 1 is the spectrum of the Squibb Primary Reference Substance of potassium phenoxymethyl penicillin recorded as a potassium bromide... 

When measnring transmission for thin samples, for example films, there is a need to ensnre absorption of IR energy is not too strong in any part of the spectrum. For a colorant in a powdered form the traditional way to obtain an infrared spectrum is to dilute the colorant whilst creating a homogenous dispersion either in Nujol (a hquid paraffin) or an alkali-metal hahde (normally potassium bromide). Clearly, it is important that the diluent (e.g. Nujol or KBr) does not have absorption bands in the same region of the spectrum as the sample being analysed. 

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