Infrared
Spectroscopy (FT-IR)
Infrared spectroscopy is a
technique used to identify various functional groups in unknown substances
through the identification of different covalent bonds that are present
in the compound. By identifying the different covalent bonds that are
present in a compound, you can establish the types of functional groups
present. By comparing the absorptions seen in an experimental spectrum
to the literature absorptions of various functional groups, you can
determine a list of possible identities for the bonds present.
The web tutorial Infrared Spectroscopy and Organic Functional Groups has more information.
Apparatus

Procedure
CHEM 211 students may run IR spectra only during their regularly scheduled laboratory time.
1. If the software is not already running, double click on the Spectrum icon to start the acquisition program. When prompted, log in as chem212 with the password org212. The instrument is 1. Spectrum One. Do not activate IR assistant. Press Return or click OK.
2. The program will open and check the hardware. Then you will see a message, which is titled “Accessory Ready Check”. It should say “System Ready for Use”. You may click the Cancel button.
3. If you are not the first user and there is a spectrum already displayed, click on the Delete icon to clear the window for you and skip to step 4 below.
4. Run a background spectrum.
A. Make sure the sample area is clean and empty and DRY (from cleaning with ethanol).
B. Choose Scan from the Instrument menu drop down list.
C. A window titled Spectrum One Scan and Instrument Setup will open. It has several pages accessed by clicking on the tabs. Choose the Sample tab and type the name background for Name. Next click on the Scan tab and, under Options in the middle of the page, select Background as the Scan type. All other settings can be left with their default values.
D. Click the Apply button and then the Scan button. The window will refresh, and soon you will see your background scan as it is running. A bar in the lower left corner of the screen shows the progress of the scan. WAIT UNTIL THE SCAN FINISHES. When the scan is complete, you may be asked if you want to overwrite the old background scan. Answer: Overwrite.
E. Click the Delete icon to clear the spectrum window. The background scan is not lost, just stored!
5. Run a spectrum of your sample.
A. Place a small quantity of your sample on the center of the sample plate. Swing the pressure arm over the sample and adjust until it touches the sample. Do not apply pressure yet.

B. Choose Scan from the Instrument menu drop down list.
C. The Spectrum One Scan and Instrument Setup window will open. Choose the Sample tab and enter a filename for your sample in the Name line. Fill in the description and comments as you choose. Then click the Apply button.
D. If you have a liquid, go to E. For a solid, click on the Monitor icon (it looks like a fuel gauge) in the upper left corner of the window. Then choose the Sample icon (the middle of the three blue-ringed icons) and adjust the pressure by turning the knurled knob on the pressure arm. You will see a green bar appear in the Force Gauge area. Adjust the pressure until the green bar almost fills the window. You should have a reading of 90-100. Click the Stop button and then click the Scan button to start your scan.
E. For a liquid, click the Scan button to start your scan.
F. To label peaks, click on the Peaks icon to automatically label your peaks. Clicking a second time removes the labels. To label peaks that are still unlabeled, click on the vertical cursor icon, Vcursr, then drag the green line over the peak and double click. Double click on the green line to remove the line.
G. To add text to your spectrum, click on the Text (ABC) icon. You have control of the font, and you can drag the text to a new position after it is written.
6. Save your spectrum to your USB flash drive.
A. Under Edit, select Copy.
B. Open the Paint program (if it isn’t already open) and Paste in your spectrum.
C. Save your spectrum as a jpeg file on your USB drive.
(If you must print your spectrum, click on the Print icon to print a copy of your spectrum.)
7. CLEAN UP! Remove your liquid sample with KimWipes or use the vacuum to remove your solid sample from the sample area. Then, use damp ethanol KimWipes to thoroughly clean the sample area and pressure arm.
8. Click the Delete icon to clear the screen for the next user, or if nobody is waiting, please Exit the program.
The
following is an example data table which you should use to display
the data given in your infrared spectra.
Experimental
Frequencies (cm-1) |
Literature Frequencies
(cm-1)* |
Possible
Peak Assignments |
3320-3300 |
3500-3300
3600-3200 |
N-H stretch: 2o amine
OH stretch: alcohol,
phenol |
1730-1710 |
1730-1625 |
C=O stretch: carboxylic
acid, ketone, aldehyde |
(Literature IR
frequency absorptions were taken from Table 1 below)
Table 1:
Principal IR Absorptions for Certain Functional Groups above 1400
cm-1
Functional
Group Names
&
Example Compounds
|
Absorption
Ranges Frequency (cm--1)
[Look for a single
absorption in these regions unless stated otherwise.] |
Type
of Vibration
(causing
IR absorption)
|
Alkanes:
 |
3000-2800
(Note:
The absorptions can be seen a several distinct peaks in this
region.)
1500-1450 |
C-H Stretch
C-H Bend |
Alkenes:
|
3100-3000
1675-1600 |
=C-H Stretch
C=C Stretch |
Alkynes:

|
3300-3200
2200-2100 |

 |
Aromatic
Rings:
|
3100-3000
1600-1580
1500-1450 |
=C-H Stretch
C=C Stretch
C=C Stretch |
Alcohols,
Phenols:

|
3600-3100
(Note:
Phenols MUST have Aromatic Ring Absorptions too.)
(1300-1000)
|
H-bonded
O-H Stretch
(C - O Stretch) |
Ketones:

|
1750-1625 |
C=O Stretch |
Aldehydes:
 |
1750-1625
2850-2800
2750-2700 |
C=O Stretch
C-H Stretch
off C=O
C-H Stretch
off C=O |
Carboxylic
Acids:
|
3400-2400
(Note: This peak always covers the entire region with a VERY
BROAD peak.)
1730-1660 |
H-bonded O-H
Stretch
C=O Stretch |
Esters:

|
1735
(1300-1000) |
C=O Stretch
(C - O Stretch) |
Ethers:

|
(1300-1000)
|
(C - O Stretch)
|
Amines:
Primary
 |
3500-3200
(TWO PEAKS!)
1640-1560 |
N-H Stretch
N-H Bend |
Amines:
Secondary
|
3500-3200
(ONE
PEAK!)
1550-1450 |
N-H Stretch
N-H Bend |
Nitriles:

|
2300-2200 |
 |
Nitro Groups:

(Note:
Both peaks are < 200 cm-1 apart.) |
1600-1500
1400-1300
|
N=O Stretch
N=O Stretch |
Amides:

|
3500-3100
1670-1600
1640-1550 |
N-H Stretch
C=O Stretch
N-H Bend |
Printable Version of
FT-IR Literature Table |