Spectrum, infrared calibration frequencies

Run the infrared spectrum of an unknown carbonyl compound obtained from the laboratory instructor. Be particularly careful that all apparatus and solvents are completely free of water, which will damage the sodium chloride cell plates. The spectrum can be calibrated by positioning the spectrometer pen at a wavelength of about 6.2 p without disturbing the paper, and rerunning the spectrum in the region from 6.2 to 6.4 p while holding the polystyrene calibration film in the sample beam. This will superimpose a sharp calibration peak at 6.246 p (1601 cm ) and a less intense peak at 6.317 p (1583 cm ) on the spectrum. Determine the frequency of the carbonyl peak and list the possible types of compounds that could correspond to this frequency (Table 2). 

Many characteristic molecular vibrations occur at frequencies in the infrared portion of the electromagnetic spectrum. We routinely analyze polymers by measuring the infrared frequencies that are absorbed by these molecular vibrations. Given a suitable calibration method we can obtain both qualitative and quantitative information regarding copolymer composition from an infrared spectrum. We can often identify unknown polymers by comparing their infrared spectra with electronic libraries containing spectra of known materials. 

The necessary derivations with respect to the small displacements can be performed either numerically, or, more recently, also analytically. These analytical methods have developed very rapidly in the past few years, allowing complete ab initio calculation of the spectra (frequencies and intensities) of medium sized molecules, such as furan, pyrrole, and thiophene (Simandiras et al., 1988) however, with this approach the method has reached its present limit. Similar calculations are obviously possible at the semi-empirical level and can be applied to larger systems. Different comparative studies have shown that the precise calculation of infrared and Raman intensities makes it necessary to consider a large number of excited states (Voisin et al., 1992). The complete quantum chemical calculation of a spectrum will therefore remain an exercise which can only be perfomied for relatively small molecule. For larger systems, the classical electro-optical parameters or polar tensors which are calibrated by quantum chemical methods applied to small molecules, will remain an attractive alternative. For intensity calculations the local density method is also increasing their capabilities and yield accurate results with comparatively reduced computer performance (Dobbs and Dixon, 1994). 

When infrared spectra are obtained, it is important to establish that the wavenumber values have been accurately recorded. Successful interpretation of the data often depends on very small shifts in these values. Calibration of the frequency scale is usually accomplished by obtaining the spectrum of a reference compound, such as polystyrene film. To save time, record absorption peaks only in the region of particular interest (this applies only to dispersive instrument derived data). 

Figure 12.2 is an infrared spectrum of 3-methyl-2-butanone. The horizontal axis at the bottom of the chart paper is calibrated in frequency (wavenumbers, cm ) that at the top is calibrated in wavelength (micrometers, ju,m). The frequency scale is often... 

Consider infrared spectroscopy in which absorbance is plotted as a function of a wavenumber. Most users take for granted that if a peak appears at 1700 cm on a screen or a printout, an actual absorbance of electromagnetic radiation with a frequency of 1700 cm occurred. That assumption, however, rests on another that the instrument has a reliable wavelength and frequency calibration. What if there were problems with the instrument, and a peak at 1700 cm really represented the sample absorbing light at 1600 cm The only way to detect such a problem would be to obtain a spectrum with known and reliable absorbances of a material in which the frequencies of absorbance are certified. NIST provides... 

For monitoring the pressure in anvil cells we use the frequncy shift of internal, chemically inert pressure calibrants. For Raman spectroscopic measurements, the most commonly used method is based on the pressure-induced frequency shift of the fluorescence line of a small piece of ruby that is placed in the sample compartment of the cell, next to the sample [1]. For infrared spectroscopic measurements, we have developed a quartz pressure scale [9], a BaSO pressure scale [10], and an HOD pressure scale [11], In the case of the first two techniques, a small amount of powdered quartz or BaSO powder are placed in the sample hole on the gasket, together with the sample under investigation. The infrared spectra of quartz or BaSO, which are relatively simple, are recorded simultaneously with the spectrum of the sample and the pressure on the sample b then determined from the frequency shift of the infrared bands of quartz or BaSO. The HOD pressure scale was developed specifically for aqueous solutions. In this case, the pressure in solution is determined from the frequency shift of the uncoupled O-H stretching band of residual HOD in DjO solutions, or from the uncoupled O-D stretching band of residual HOD in HjO solutions [11]. 

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