4.5 IR Data Table
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- Understand where certain group frequencies are
- Be able to read and understand the data table for IR spectroscopy
As stated in the previous section, similar bonds will show up grouped near the same frequency because they tell us about the presence or absence of specific functional groups in a sample. From there, a data table of approximate frequencies for different types of bonds has been created to use to help IR spectrum analysis.
Table of Common IR Absorptions.
| Approximate Frequency (cm-1) | Description | Bond Vibration | Notes |
| 3500 - 3200 | broad, round | O-H | much broader, lower frequency (3200-2500) if next to C=O |
| 3400-3300 | weak, triangular | N-H | stronger if next to C=O |
| 3300 | medium-strong | =C-H (sp C-H) | |
| 3100-3000 | weak-medium | =C-H (sp2 C-H) | can get bigger if lots of bonds present |
| 3000-2900 | weak-medium | -C-H (sp3 C-H) | can get bigger if lots of bonds present |
| 2800 and 2700 | medium | C-H in O=C-H | two peaks; "alligator jaws" |
| 2250 | medium | C≡N | |
| 2250-2100 | weak-medium | C≡C | stronger if near electronegative atoms |
| 1800-1600 | strong | C=O | lower frequency (1650-1550) if attached to O or N middle frequency if attached to C, H higher frequency (1800) if attached to Cl |
| 1650-1450 | weak-medium | C=C | lower frequency (1600-1450) if conjugated (i.e. C=C-C=C) often several if benzene present |
| 1450 | weak-medium | H-C-H bend | |
| 1300 - 1000 | medium-strong | C-O | higher frequency (1200-1300) if conjugated (i.e. O=C-O or C=C-O) |
| 1250-1000 | medium | C-N | |
| 1000-650 | strong | C=C-H bend | often several if benzene present |
Note: strong, medium, weak refers to the length of the peak (in the y axis direction).
Note: spectra taken by ATR method (used at CSB/SJU) have weaker peaks between 4000-2500 cm-1 compared to reference spectra taken by transmittance methods (typical on SDBS and other sites).
What wavenumber range would you predict the triple bond region to be?
- Answer
-
2200 - 2500 cm-1
How could you determine the presence of an aldehyde and rule out a ketone?
- Answer
-
The aldehyde would have peaks at 2700 and 2800 cm-1, whereas the ketone would lack these. The absorption is due to the the Csp2-H bond.
How can you tell the difference between and alcohol and a carboxylic acid?
- Answer
-
Here the main point of difference is either the presence of a carbonyl (C=O) or its absence. The carboxylic acid contains a C=O, so you would expect a peak somewhere between 1800-1600 cm-1, whereas an alcohol would not have a peak here.
