Ir spectroscopy manual
By : James W Zubrick Email: j.zubrick@hvcc.edu Unlike the chromatographies, which physically separate materials, infrared ( IR) spectroscopy is a method of determining what you have after you’ve separated it. The IR spectrum is the name given to a band of frequencies between 4000 and 650 cm-1 beyond the red end of the visible spectrum. The units are called wave numbers or reciprocal centimeters (that’s what cm-1 means). This range is also expressed as wavelengths from 2.5 to 15 micrometers (j Um). With your sample in the sample beam, the instrument scans the IR spectrum. Specific functional groups absorb specific energies. And because the spectrum is laid out on a piece of paper, these specific energies become specific places on the chart. Look at Fig. 118. Here’s a fine example of a pair of alcohols if ever there was one. See the peak (some might call it trough) at about 3400 cm-1(2.9 [im)l That’s due to the OH group, specifically the stretch in the O— H bond, the OH stretch. Now, consider a couple of ketones, 2-butanone and cyclohexanone ( Fig. 119). There’s no OH peak about 3400 cm-1 (2.9/^m is there? Should there be? Of course not. Is there an OH in 2-butanone? Of course not. But there is a C=0, and where’s that? The peak about 1700 cm-1 (5.9/^m). It’s not there for the alcohols and it is there for the ketones. Right. You’ve just correlated or interpreted four IRs. Because the first two ( Fig. 118) have the characteristic OH stretch of alo-chols, they might just be alcohols. And the other two ( Fig. 119) might be ketones because of the characteristic C— O stretch at 1700 cm-1 (5.9/zm) in each. What about all the other peaks? You can ascribe some sort of meaning to each of them, but it can be very difficult. That’s why frequency correlation diagrams, or IR tables, exist ( Fig. 120). They identify regions of the IR spectrum where peaks for various functional groups show.
The FTIR / Vibrational spectroscopy exercise follows the format of a detective story involving solving a series of problems rather than the normal lab format. The experiment is an adaptation of Pollution Police by Profs. Jodye Selco and Janet Beery at the University of Redlands, which was presented at the Division of Chemical Education Regional ACS meeting in Ontario, CA 1999. Introduction The theory of FTIR spectrometer operation is discussed in SHN Chapters 16 and 17. The group theory and vibrational quantum mechanics are discussed in Mc Quarrie and Simon ( Chem 110 B text) Chapters 12 and 13. The experimental portion of the exercise is problem 10 and problem 11. The computer portion of the exercise is in problems 2, 7, 8, and 9, and is done separately on the computers in room. All four computers are set up to run Hyper Chem, Gaussian and Spartan. Figure 0: UCD FTIR instruments, both new and old. Track out the light path in the old instrument Setting the Scene You have been hired by an environmental testing company to monitor air pollutants. Some of these chemicals are released into the atmosphere from factories, cars, or cattle; others evaporate from agricultural fields. Since most of these chemicals absorb infrared light, we are able to detect them with an infrared spectrometer. A test sample is obtained by taking an evacuated cell to the target location, opening a valve, and allowing the ambient air to fill the cell. A FTIR – Fourier Transform Infrared Spectrometer will be used to take the IR spectra. The final output from the spectrometer, called an infrared spectrum, is a plot of the intensity of light reaching the detector divided by the initial intensity of light, as a function of frequency (% T= I/ Io vs. frequency in wavenumbers). The goal of this project is to gain a better understanding of group theory and to identify atmospheric pollutants from their infrared.