Analyzing an IR Spectrum

When analyzing an IR spectrum, the first step is to draw a line at 1500 cm. Focus on any signals to the left of this line (the diagnostic region). In doing so, it will be extremely helpful if you can identify the following regions  

The location of signals arising from several different X—H bonds. 

Remember that each signal appearing in the diagnostic region wiU have three characteristics (wavenumber, intensity, and shape). Make sure to analyTe all three characteristics. 

LEARN the skill A compound with molecular formula C6HiqO gives the following IR spectrum. 

Identify the structure below that is most consistent with the spectrum  

The following table is a summary of useful signals in the diagnostic region of an IR spectrum  

Structural Unit Wavenumber Icm I Structural Unit Wavenumber Icm I 

When looking for X—H bonds, draw a line at 3000 cm and look for signals that appear to the left of the line  

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When analyzing an IR spectrum, look for double bonds, triple bonds, and X—H bonds. 

As a result, bonds absorb in four predictable regions in an IR spectrum. These four regions, and the bonds that absorb there, are summarized in Figure 13.10. Remembering the information in this figure will help you analyze the spectra of unknown compounds. To help you remember it, keep in mind the following two points ... 

There are several types of methods to obtain an IR spectrum depending on the nature of the sample to be analyzed. Transmission spectroscopy is based on the absorption of IR radiation at specific wavelengths as the radiation passes through a sample. With this technique, it is possible to analyze liquids, solids, or gaseous samples. This is the most common method the samples can be placed in a cell in solution, dispersed in NaCl, KBr, Cap2 (for water-soluble samples), or CsBr. Liquid thin films can also be analyzed using a drop of the sample, which will be sandwiched between two IR KBr cells and placed in a holder. When samples are analyzed in a solvent solution, several factors must be considered the solvent has to dissolve the whole sample with a minimum of solvent-solute chemical interactions and it should not strongly absorb IR radiation. 

The flnal IR spectrum observed by a user for any sample (organic molecule or polymer) consists of a series of scans. An IR spectrum is composed generally of at least 20 scans. However, the combination of multiple scans with FT-IR provides a powerful real-time (RT) method for monitoring chemical changes, such as an in situ polymerization reaction. The rate of UV curing and photopolymerization, for example, can be easily calculated by analyzing the quantitative appearance or disappearance of a specific absorption band. 

A molecular vibration is infrared active (i.e., excitation of the vibrational mode can be measured via an IR spectrum absorption) only if it results in a change in the dipole moment of the molecule. The three vibrations of water (Table 4.12) can be analyzed in this way to determine their infrared behavior. 

Solution. A common technique for obtaining an IR spectrum of a solid is to analyze it in solution. The preferred solvents are chloroform and carbon disulfide, in that order. The solubility of the sample in the selected solvent must be in the range of 5-10 weight percent because solutions of such concentration generally provide suitable spectra in a fixed-thickness cell having a pathlength of 0.1 mm. 

Weigh the product and calculate the percent yield. Determine the boiling point, density, and refractive index (optional) and compare your values with those listed for o-xylene in the literature. Obtain an IR spectrum of the crude (dry) reaction product using the capillary film sampling technique and analyze it following the analysis given below. 

Fingerprint region (Section 12.3D) Vibrations in the region 1500 to 400 cm of an IR spectrum are complex and difficult to analyze but are characteristic for different molecules. 

In the previous section, we saw that the intensity of a signal in an IR spectrum is dependent on how efficiently a bond absorbs IR radiation. The intensity of the signal is also dependent on other factors, such as the concentration of the sample being analyzed. This fact is used by law enforcement officials to measure blood alcohol levels accurately. The Intoxilyzer is the most current and accurate device for measuring blood alcohol levels. It is essentially an IR spectrometer that is specifically tuned to measure the... 

This difference can be used to monitor the progress of the reaction. A drop of the reaction mixture can be removed from the reaction flask at periodic intervals and analyzed in an IR spectrum. As the product forms, the signal at 1650 cm should vanish. 

The expert system for structure elucidation spectral analysis supplies the molecular structures from the input of an IR spectrum. The system requires the molecular formula for analyzing certain spectral regions. 

Equation (4.2) can be used to determine the concentration of a compound in a solution, if the value of K is known for that compound. Chemical bonds such as C—O, O—H, and so on, absorb different amounts of infrared energy over various wavelengths. Absorption patterns vary from sharp to broad for different bonds. Peak IR absorption wavelength (wave number) is characteristic of a chemical bond. Absorption over the infrared spectrum is a fingerprint characteristic of an organic material. Qualitative identification can be achieved by obtaining and analyzing the IR spectrum of a material. 

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