Absorbers in the infrared region

Diatomic molecules such as HC1 have one vibrational mode, but it is important to note that symmetrical diatomic molecules, such as 02, N2, Cl2, F2, and H2, do not absorb in the infrared region of the spectrum. This is... 

The Guthion solutions were analyzed by measuring absorbances in the infrared region at 645 cm."1 and by the copper chelate method (I, 3). The latter method is the more specific and depends on basic hydrolysis of Guthion followed by formation of a colored Cu2+ chelate with the dimethyl dithiophosphoric acid formed. In instances where the Guthion concentrations were quite low, thin layer chromatographic techniques were employed. 

Figure 9. A cyanine borate absorbing in the infrared region [37],...

The energies associated with the vibrational modes of a chemical compound can be observed directly through their absorbance in the infrared region of... 

The most important absorbers in the infrared region of the spectrum are water vapour and CO2. Here the humidity of the atmosphere undergoes considerable fluctuations. The amount of water vapour is often described by the thickness w of a layer of water on the ground formed by the condensation of water vapour perpendicular to the ground. A typical value is w 20 mm. 

Applications of Quantitative Infrared Spectroscopy Infrared spectrophotometry offers the potential for determining an unusually large number of substances because nearly all molecular species absorb in the infrared region. Moreover, the uniqueness of an infrared spectrum provides a degree of specificity that is matched or exceeded by relatively few other analytical methods. This specificity has particular application to the analysis of mixtures of closely related organic compounds. 

Once in the atmosphere, methane does not absorb visible light at wavelengths longer than 146 nm and is thus photochemically inert in the lower atmosphere. Methane does absorb in the infrared region at 3.3 and 7.7 fxm and this gives rise to its role as a greenhouse gas. 

The dipole correlation time of the system is a valuable quantity to calculate as it is related to the sample s absorption spectrum. Liquids usually absorb in the infrared region of the electromagnetic spectrum, a typical spectrum being shown in Figure 7.12. As can be seen, the spectrum is very broad with none of the sharp peaks characteristic of a well-resolved spectrum for a species in the gas phase. This is because the overall dipole of a liquid does not change at a constant rate but, rather, there is a distribution of frequencies. The intensity of absorption at any frequency depends upon the relative contribution of that frequency to the overall distribution. If, on average, the overall dipole changes very rapidly (i.e. the relaxation time is short) then the maximum in the absorption spectrum will occur at a... 

Because most prism materials such as glass or quartz absorb in the infrared region, it is more convenient to use a Littrow grating (Sect. 4.1) as wavelength selector in this wavelength range. Figure 5.28 illustrates the line selection in... 

Silver Chloride Cells. Cells may be constructed of silver chloride. These plates may be used for liquid samples that contain small amounts of water, because silver chloride is water-insoluble. However, because water absorbs in the infrared region, as much water as possible should be removed, even when using silver chloride. Silver chloride plates must be stored in the dark. They darken when exposed to light, and they cannot be used with compounds that have an amino functional group. Amines react with silver chloride. 

Symmetrically substituted bonds, as in 2,3-dimethyl-2-butene, do not absorb in the infrared region (no dipole change). Highly substituted double bonds are often vanishingly weak in absorption. 

Finally, to close the section on micro-IR, the subject of dyes needs to be mentioned. A look at any of the dye structures shown in this text reveals that many possess functional groups that absorb in the infrared region. Note that dyes are present in much lower concentrations than the polymer substrate, but... 

In ketenimines, the cumulative double bond system absorbs in the infrared region at 2000-2050 cm . In cycloaddition reactions, ketenimines are less reactive than ketenes, and A-arylketenimines react faster than A-alkylketenimines. These reactions most likely proceed via a linear ionic intermediate, which can cyclize to give four-membered ring or six-membered ring cycloadducts, or react with a second mole of ketenimine to give a [2-I-2-I-2] cycloadduct. 

Determination of complex mixtures is one of the most common problems in infrared analysis. Hyphenated techniques such as chromatography provides a useful way of separating mixtures into individual compounds. Gas chromatography (GC) has been by far the best application of combining infrared spectroscopy and a separation technique. One of the principal reasons has been the use of carrier gases which do not absorb in the infrared region. 

The h in Equation 6-1 is Planck s constant and c is the speed of light. The wavelength of the photon absorbed in the infrared region is recorded in micrometers (microns, pm) or in terms of wavenumbers (1/A., in cm" ). In microwave Spectroscopy, the spectrum is recorded in terms of the frequency, V, of the radiation. The frequency of the radiation is determined as follows ... 

CO has a permanent dipole moment. The dipole changes on stretching, and so CO will also absorb in the infrared region. 

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