How to Identify Molecular Fragments from NMR and IR Spectra - dummies

How to Identify Molecular Fragments from NMR and IR Spectra

By Arthur Winter

When trying to determine the structure of a compound based on its molecular formula, you can use NMR (nuclear magnetic resonance) and IR (infrared) spectroscopy to help you identify the fragments of the molecule. (Once you identify these fragments, you can identify the molecule’s structure.)

After you’ve determined the number of hydrogen atoms that each NMR peak represents, you can assign each peak to a fragment of the molecule. This table shows some common fragments.

image0.jpg
image1.jpg

As you determine the fragments in the molecule, you should write them all down on a sheet of scratch paper so they’ll be in front of you when you’re ready to solve the structure. For example, if you have three peaks — one that integrates for 1H, one that integrates for 2H, and one that integrates for 3H, you would write on your scrap paper CH, CH2, and CH3. After you determine all the fragments (including fragments you determined from the IR, if you’re given one), add up all the atoms in your fragments to make sure that they match the molecular formula and to be certain that you’re not missing any atoms.

Here’s an example: Given the IR and NMR spectra for compound C5H10O, identify the fragments. From the NMR (shown in the figure), you determine that the whole number relative ratio for the molecule is 2:3:2:3. From the table, the most likely fragments for 2H, 3H, 2H, and 3H are CH2, CH3, CH2, and CH3, respectively. Adding up all the atoms from these fragments to see if the number of atoms matches the molecular formula shows that one carbon and one oxygen atom are missing. Based on its molecular formula, you know that the C5H10O molecule has one degree of unsaturation; this unsaturation could be accounted for by the double bond in the carbonyl seen in the IR spectrum (the intense peak at 1,710 cm–1 in the figure).

Carbonyl groups are found in aldehydes, ketones, esters, and carboxylic acids. Here, however, esters and carboxylic acids can be eliminated because the molecular formula shows only a single oxygen. Because the NMR spectrum doesn’t contain an aldehyde peak (aldehyde protons are seen around

image3.png

and the IR spectrum doesn’t show an aldehyde C-H stretch at 2,700 cm–1, the carbonyl group is a ketone.

By identifying these fragments (shown in the next figure), you can narrow down the number of ways in which they can be pieced together to form the final structure.

The final fragments.
The final fragments.