NaCl prism

IR spectrometers have the same components as UY/visible, except the materials need to be specially selected for their transmission properties in the IR (e.g., NaCl prisms for the monochromators). The radiation source is simply an inert substance heated to about 1500 °C (e.g., the Nernst glower, which uses a cylinder composed of rare earth oxides). Detection is usually by a thermal detector, such as a simple thermocouple, or some similar device. Two-beam system instruments often work on the null principle, in which the power of the reference beam is mechanically attenuated by the gradual insertion of a wedge-shaped absorber inserted into the beam, until it matches the power in the sample beam. In a simple ( flatbed ) system with a chart recorder, the movement of the mechanical attenuator is directly linked to the chart recorder. The output spectrum is essentially a record of the degree of... 

Differential thermal analysis was performed with the DuPont 900 differential thermal analyzer the heating rate was usually 10°C. per minute. To determine heats of reaction, the calorimeter attachment to the Du Pont instrument was employed. Planimeter determinations of peak areas were converted to heat values by using standard calibration curves. For the infrared spectra either a Beckman IR5A instrument or a Perkin Elmer 521 spectrophotometer with a Barnes Engineering temperature-controlled chamber, maintained dry, was used. Specimens for infrared were examined, respectively, as Nujol mulls on a NaCl prism or as finely divided powders, sandwiched between two AgCl plates. For x-ray diffraction studies, the acid-soap samples were enclosed in a fine capillary. Exposures were 1.5 hours in standard Norelco equipment with Cu Ko radiation. For powder patterns the specimen-to-film distance was 57.3 mm. and, for long-spacing determinations, 156 mm. 

The primary dimensional requirement on a polymer sample is that it be sufficiently thin. (It is possible to obtain reflection spectra of polymers [Robinson and Price (187, 188)], in which case thin specimens are not required, but the use of this technique has thus far not proven to be as fruitful as transmission spectra, and we will not consider it here.) In the NaCl prism region (roughly 650 to 3500 cm-1) specimens as thin as 0.002 mm may be required in order to avoid essentially 100% absorption at some band peaks. The average thickness required in this region for most bands is usually about 0.02 mm. Thicknesses about ten times larger are optimum for frequencies above 3500 cm 1 (the overtone and combination region) and below 650 cm-1 (the far infrared region). Samples areas down to 1 by 3 mm are usable [Wood (247)], and even smaller if a microspectrometer is employed [Blout (76)]. 

Figure 29 shows the 4-8-m spectrum of CO chemisorbed on platinum in a KBr disk. This spectrum was obtained with a NaCl prism. Spectrum A of Fig. 30 shows the 14.5-16.5-a region and B shows the 20.0 22.0- region. Figure 30 was obtained with a KBr prism. 

Fig. 4. Infrared-spectrum of m i (C=C) 1566 cm. The infrared band at 2105 cm"1 arises from the C=C bond of (CoHs MC CCoHs present in small amount as impurity 235).

Figure 2. Polarized IR spectra of sections of human dental enamel. Data collected with xlO magnification Schwarzschild-type reflecting microscope, NaCl prism spectrometer and selenium polarizer (Ford et al. 1958). [Figure 4.7 from Elliott (1994), reprinted with permission from Elsevier Science.]...

Fig. 5. Infrared spectrum of alI-cir-perhydro-9b-boraphenalene (XXV). Solid line d = O.OS mm (NaCl prism) dasbed line d = 0.025 mm (NaCl prism).

Fig. 7. Infrared spectrum of B-(3 -methyl-F-but-3 -enyl)-3-methylborolane. Solid line d = 0.05 mm (NaCl prism) dashed line ca. 10% in cyclohexane.

Fig. 11. Infrared spectrum of bis(3-methylborolane) from 3-methylborolane and B-deutero-3-methylborolane d = 0.05 mm (NaCl prism). Neat, except for the darkened areas, which were run in ca. 15% solution.

This collection of infrared spectra was run on an NaCl prism spectrometer. It does not pretend to be a complete library of spectra by any means. Its purpose is to provide one or two examples of most of the important functional group frequencies used in qualitative analysis. These are labeled directly on the spectra. 

Fig. 2. Dispersion data for IRS spectrophotometer with NaCl prism.

Fig. 3. Dispersion data for IR4 spectrophotometer with NaCl prisms.

Fig. 4. Dispersion data for IR7 spectrophotometer with NaCl prism and 75-lines/mm grating.

Figure 2-18. Resolution of a polystyrene film sample on an NaCl prism instrument.

The spectrum of the molecule NH3 is of interest because the fine-line structure of some of the absorption bands can be seen even with an NaCl prism. An additional feature of interest is the presence of inversion doubling. 

NaCl Prism Cell Sompit n Blank uor nt Entroy by Sliuttari... 

Stretching vibrations should appear, and in the range of the NaCl prism some typical frequencies of CsH, with only slight shifts. Of these the absorptions of the free C=C bonds should be especially observable, although they might have lower intensity in the metal derivatives because the enhancement of intensity by eonjugation should be lessened by electron removal towards the metal. 

When observed with NaCl prism resolution, this region is generally recorded as a single symmetric band (Figure 5.3a). 

Fig. 53 The C—H stretching region of IR spectra of thiophenol with (a) NaCl prism resolution and (b) grating resolution.

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