Calibration infrared frequencies

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 first frequency measurement of the 15 — 25 resonance made use of a transportable ClU-stabilized HeNe infrared frequency standard at 88 THz [24], built at the Institute of Laser Physics in Novosibirsk/Russia. For calibration it was transported repeatedly to the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig/Germany where it could be compared with a Cs atomic clock using the PTB frequency chain [25]. 

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... 

In order to derive structural information from infrared frequencies, input is required from quantum chemical calculations at computational levels which match the experimental resolution. Experimentally, gas-phase conditions imply extremely low sample densities, requiring special techniques in order to acquire infrared data. Some of those techniques involve double resonance approaches which provide unique opportunities for isomer selective IR spectroscopy. This facet is among the advantages of gas-phase experiments, making it possible to follow certain properties, such as excited state dynamics, as a function of molecular structure. At the same time, the availability of gas-phase data provides opportunities to calibrate computational methods, force fields, and functionals. 

Infrared spectra were measured on a Perkin-Elmer model 225 spectrometer in the range 1200-200 cm-1. The calibration of the instrument was checked by measuring the frequencies of C02 vibrational bands. The infrared spectra of zeolite samples were measured as pressed pellets containing approximately 3 mg of zeolite in 300 mg of Csl. 

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). 

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). 

Ford et al, (1969) have described a new technique for accurately calibrating the temperature of low-temperature infrared cells. A liquid of known melting point is introduced into the cell as a capillary film between NaCl or AgCl windows. The cell is assembled with the thermocouple in the usual position, and the temperature lowered until the liquid freezes. The monochromator is set at the frequency at which the... 

Alben and Caughey (1968) have studied the binding of carbon monoxide to hemes by high-resolution infrared difference spectroscopy. The spectrophotometer was adjusted to resolve water vapor bands at 1792.65 and 1790.95 cm . All carbonyl stretching frequencies were calibrated to the water vapor band at 1942.6 cm . The NH-stretching frequencies of metal-free porphyrins were calibrated from the 3447.20 cm water band. A standard slit-width program, which was just insufficient to resolve water-vapor bands at 1910.1 and 1908.2 cm was used, along with a five-fold expanded ordinate. 

In this procedmre, the phospholipids are first adsorbed on silicic acid from chloroform-acetone solution, and the nonadsorbed lipids are measured in carbon tetrachloride solution with a high-resolution infrared spectrophotometer (Perkin-Elmer Model 421). Absorbance readings are taken at stationary instrument settings (frequencies or wavelengths) corresponding to the carbonyl absorption peaks of triglycerides and cholesteryl esters. From suitable calibration data, concentrations of these two components can be calculated. 

The main problem of Far-Infrared ground based interferometers is the atmospheric attenuation at these frequencies. This effect can only be removed if the interferometer is space-based. However, water lines can be useful as a first instrument approach to calibrate the test-bed. Figure3.4 (left) shows the atmospheric attenuation and the Winston cone filtering in the 0-40 cm wavenumber range. 

Products were characterized by a number of different spectroscopic methods. Infrared spectra were recorded on a Perkin-Hmer Model 298 or 580 B spectrophotometer. Samples were prepared as 1% dispersions in Csl or Nujol mulls. nmr spectra were recorded on a Varian Model FT-80 spectrometer solutions of 5 mg of complex in 0.6 mL CDCI3 were used,with tetramethylsilane as internal reference. Electronic absoiption spectra were recorded on a Perkin-Elmer Model 330 spectrometer, using 5 x lO mol/L dichloromethane solutions. Raman spectra were recorded on samples cooled by liquid nitrogen, on a DILOR Model RTI 30 spectrometer, using an excitation wavelength of 514.5 nm. ESR spectra of the powders in MgS04 were recorded at X-band frequency with a Varian Model E-4 spectrometer. Signal g values and intensities were calibrated with a standard diphenylpicrylhydrazyl (DPPH) sample (g = 2.0036). 

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