The Infrared Spectrum of a Compound

Infrared spectra are normally recorded on chart paper as a graph of intensity (absorbance or percent transmittance) versus position 

Not all matter is capable of producing an infrared spectrum (e.g., metals do not). To interact with infrared radiation the molecule must have a permanent dipole moment and must vibrate about a bond (changing the bond length or angle), or rotate about an axis perpendicular to the bond. It is the interaction of radiation with these vibrations and rotations that gives rise to the absorption bands appearing on the spectrogram. 

The spectra of most compounds show a number of bands, the wavelengths or frequencies at which they appear supplying the primary data used to identify the compound. It is frequently assumed that this band-frequency information is all that is required for compound identification, but experience shows that intensity and band shape must also be considered. Lists of absorption frequencies or wavelengths normally do not furnish sufficient information for positive identification. However, the information available from the abscissa is the most important in qualitative analysis. The proper use of this information is discussed in Chapter 5. 

While the data from the ordinate have some use in qualitative analysis, it is primarily used in quantitative work. The abscissa data are absolute within the accuracy of the instrument, but ordinate data are relative, and depend on the instrument used, the operating conditions, the sample composition, the sample path length, and the concentration. The use of these data in quantitative analysis is discussed in Chapter 6. 

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Figures 2 through 9 are infrared spectra of fractions collected from partition columns, gas chromatography, thin-layer chromatography, or a combination of these separation techniques. Figure 10 is the infrared spectrum of a compound isolated by gas chromatography after hydrolysis of a pyrethrum concentrate. In this case the compound is a long-chain ester. All the infrared spectra were made with a Perkin-Elmer Model 221 instrument. The following operating parameters were used. A liquid demountable cell with a 0.01-mm path length was employed.

The infrared spectrum of a compound shows absorptions at 1700 cm 1 and 3600 cm 1. A possible structure is ... 

Bands corresponding to the binary overtones and combinations of the fundamental CO-stretching frequencies have been recorded in the infrared spectra of many carbonyl com )ouiids. These binary combination data must be consistent with a proposed assignment of the CO-stretching fundamentals if the assignment is to be considered acceptable. Thus, the infrared spectrum of a compound in the 4000 cm region serves as an excellent check on the assignment of the fundamentals. Certain factors, however, limit the use of the binary combination spectra and these will be noted first. 

It is not always possible by examination of the infrared spectrum of a compound alone to identily it unequivocally. It is normal to use infrared spectroscopy in conjunction with other techniques, such as chromatographic methods, mass spectrometry, NMR spectroscopy and various other spectroscopic techniques. 

The infrared spectrum of a compound is essentially the superposition of absorption bands of specific functional groups even small interactions with the surrounding atoms of the molecule impose the stamp of individuality on the spectrum of each compound. For qualitative analysis, one of the best features of an infrared spectrum is that the absorption or the lack of absorption in specific frequency regions can be correlated with specific stretching and bending motions and, in some cases, with the relationship of these groups to the rest of the molecule. 

Treatment of 2 4 6 tn tert butylphenol with bromine in cold acetic acid gives the compound CigH29BrO in quantitative yield The infrared spectrum of this compound contains absorptions at 1630 and 1655 cm Its H NMR spectrum shows only three peaks (all singlets) at 8 1 2 13 and 6 9 in the ratio 9 18 2 What is a reasonable structure for the compound" ... 

Hydroxyisoquinolines have been considered to exist as such rather than as 30 on the basis of chemical evidence, and, indeed, the infrared spectrum of this compound was reported to show a v OH band at 2.93/j(, (3413 cm ). These results have been questioned, the iso-... 

The tautoraerism of certain difunctional derivatives of l-thia-3,4-diazole has received considerable attention. Pala assigned structure 156 to 2,5-dimercapto-l-thia-3,4-diazole on the basis of infrared spectral data, and Thorn" reached the same conclusion by comparing its ultraviolet spectrum (measured in ethanol) with those of the four possible methylated derivatives. However, the infrared spectrum of a chloroform solution of the parent compound showed bands at 2600-2550 cm indicating an SH group and the probable occurrence of form 157 under these conditions, and this conclusion is supported by the occurrence of SH bands in solid state spectra obtained by Swiss investigators. For a summary of earlier work on these compounds, see reference 187. 

After washing the combined extracts with ammonium chloride solution and water and working up in the usual way a white solid (IV) is obtained which after one recrystalli2ation from aqueous methanol has MP 242° to 243°C. The infrared spectrum of this compound indi-... 

If the wavenumber of a C-H stretching band in the infrared spectrum of a certain compound is 2960 cm-1, calculate the wavenumber of the corresponding C-D stretching band in the deuterated homologue. 

Infrared spectrum of a compound gives more information than is normally available from the electronic spectra. 

Pentacarbonyl(l-oxacyclopent-2-ylidene)chromium(0) is an air-stable, bright yellow compound that dissolves readily in most organic solvents. The infrared spectrum of a heptane solution shows bands at 2066 (s), 1983 (m), 1958 (s), and 1944 (s) cm"1. The proton NMR spectrum in CS2 shows a two-proton triplet (7 = 8 Hz) at 6 4.90, a two-proton triplet (7=8 Hz) at 6 3.67, and a two-proton quintet (7=8 Hz) at 6 1.96 all relative to internal tetramethylsilane. 

The infrared spectrum (especially the near-infrared) has assumed great importance in chemical and biological research because of the highly specific absorption of chemical compounds at these wavelengths. The infrared absorption of a given organic compound may be used to characterize that particular compound. The infrared spectrum of a mixture of several compounds among which there is no interaction, does not lie between the spectra of the individual compounds, but consists of a direct superposition of the spectra of the individual compounds... 

The compound [N(PPh3)2][Ru3H(CO)10(SiEt3)2] is a red, crystalline material. The crystals are only slightly air sensitive and decompose above 120 °C over a broad temperature range. They dissolve in polar solvents such as THF, dichloromethane, trichloromethane, acetonitrile, methyl alcohol, or acetone. The red solutions are much more sensitive to oxygen than the solid. The infrared spectrum of a THF solution displays characteristic absorptions at 2070(w), 2019(m), 1992(m), 1984(vs), 1975(w), 1965(m), and 1925(sh) cm-1 (Nicolet MX-1 Spectrometer). The structure of the compound, tautomerism of the CO groups, and electrochemical characteristics have been reported.9... 

Here kb is the force constant or bond strength and r0 is the ideal or unstrained bond length. A first approximation to the force constant can be calculated from the fundamental vibration frequency, v, of the X-Y bond, taken from the infrared spectrum of a representative compound by using Eq. 15.2,... 

On the basis of the comparison of the infrared spectrum of synthetic compounds with sphingosine from naturally occurring sources, a trans configuration was assigned to the C-4 double-bond system. 

A student has just added ammonia to hexanoic acid and has begun to heat the mixture when he is called away to the telephone. After a long telephone conversation, he returns to find that the mixture has overheated and turned black. He distills the volatile components and recrystallizes the solid residue. Among the components he isolates are compounds A (a liquid molecular formula CgHnN) and B (a solid molecular formula C6H13NO). The infrared spectrum of A shows a strong, sharp absorption at 2247 cm-1. The infrared spectrum of B shows absorptions at 3390, 3200, and 1665 cm-1. Determine the structures of compounds A and B. 

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