Infrared (IR) Spectroscopy

Transcription

1 Infrared (IR) Spectroscopy CSI always tells you WHO, but IR can tell you WHAT! Molecular Fingerprints and Functional Group Identification Infrared (IR) Spectroscopy IR spectra result from the absorption of energy in the infrared region of the electromagnetic spectrum. A spectrum is a plot of absorbance versus the frequency or wavelength of the radiation being absorbed. The normal range of frequencies is quoted in reciprocal centimeters (cm -1 ) and ranges from 400 to 4000 cm -1. 1

2 Molecular Fingerprints In the Who Has My Compound Experiment, you will spend considerable time to get three data points for comparison. With IR, you can get dozens of data points in minutes! Compounds with different structures will always give IR spectra that are different. Every compound will give a unique IR spectrum! Molecular Fingerprints Computers can compare the IR spectrum of an unknown to thousands of known structures in seconds and determine which known structures have similar spectra. One limitation to the use of IR for conclusive identification is that the sample must be pure - impurities would result in absorbances not found in the known spectra. Small amounts of impurities might be undetected, however. 2

3 Functional Group Identification A second important use of IR is the detection of specific functional groups. Absorbances in the higher energy range (above 1500 cm -1 ) of an IR spectrum result from absorption of energy by changes in the bond stretching frequencies of multiple bonds (alkene, alkyne, aromatic C=C, carbonyl, nitrile) and single bonds of hydrogen to C, N, and O. Functional Group Identification Functional Group Region < 1500 cm -1 Fingerprint Region 3

4 Theory of IR Spectroscopy Today you will make use of a computer tutorial on infrared spectroscopy, IR Tutor, to obtain an understanding of the interaction of molecules with infrared radiation. During lab, you must complete a set of questions that you will answer by working through the tutorial to find the answers. You will also complete a worksheet deducing structure from IR spectra. Make use of the tutorial to help you complete this worksheet. Questions and Worksheet Although you will work in pairs on the laptop computers, each of you must complete the set of questions and worksheet to submit as part of your lab report. Please note that you will need to get your TA to initial both handouts before you leave lab. 4

5 Use of IR Spectrometers In today s chemical experiment, you will use an infrared spectrometer to obtain the IR spectrum of one specific compound -- one of six possible compounds. You will identify the absorbances that are characteristic of the functional groups present in your compound and compare this spectrum with those of two other specific compounds that have been obtained by other students. This information will be recorded on a worksheet given to you in lab. Spectral comparisons In Part II, you will need to find students that obtained an IR spectrum of each of the three compounds you have not yet seen these compounds are indicated on the back of your worksheet. You will then enter the data for these compounds and answer the comparison questions on your worksheet before getting it signed by your TA. 5

6 Analyzing Spectra You may also want to look at spectra of related compounds (same functional group) using the computer tutorial. Remember this is the only opportunity you are going to get to learn what absorbances correspond to various functional groups. Obtaining Spectra You will obtain your IR spectrum by the method known as Attenuated Total Reflectance (ATR) on new FTIR spectrometers. We will NOT be using salt plates! There will be a student worker in the instrument room to assist you in obtaining a spectrum of your assigned compound. This experiment illustrates the simplicity of obtaining analytical data with modern instruments. 6

7 Obtaining Spectra by ATR To obtain a spectrum of a liquid with an ATR sampling plate, all we have to do is place a drop of the liquid on the crystal in the middle of the sampling plate. Then press the start button! The computer will scan the sample several times, analyze the data, and print the spectrum. Make sure you label the spectrum with your name when it comes off the printer. Sample Clean-up Use a cotton ball to absorb your liquid sample off the sampling plate. Discard it by placing it in the disposal beaker. Then use a little acetone on a second cotton ball to wipe the sample plate clean. This also goes into the disposal beaker. BE CAREFUL to NOT drip acetone on the spectrometer s plastic surfaces!! 7

8 Lab Schedule You will work in pairs on the laptop computers to do the IR tutorial. Start this when you arrive at the lab. During the lab period you will be called in small groups into the instrument room to perform your IR analysis. Remember that you are not finished until you have obtained data about the other compounds from other students. Reports You will have THREE different worksheets to complete in this lab session. 1. You will have the completed IR tutorial sheet that you used with the computers. 2. There is also a Interpreting Spectra worksheet of 4 spectra to be completed. 3. You will also have the worksheet analyzing the spectra obtained in lab today. 8