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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.

sample image

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