Spin-Spin Splitting in ¹H NMR Spectroscopy

Every ¹H NMR spectrum we have seen so far has had single peaks for signals. However, in ¹H NMR, unlike ¹³C NMR, the signals do not always appear as single peaks. Often, ¹H signals are split into seemingly complex patterns. This experiment will be an introduction to the phenomenon called spin-spin splitting. You need to read section 13.11 and 13.12 in your McMurry textbook. Pay careful attention to rules on p. 462 in Table 13.4. We will be more concerned with the practical consequences and not so much the theory of spin-spin splitting.

General Instructions:

• We will look at a few spectra that demonstrate different simple splitting patterns.
• Each student will make up an NMR sample of one of the compounds below and run its spectrum.
• We will look at some examples of complex splitting patterns

For the spectrum that you obtained in lab, do the following:

1. Draw the skeletal structure of the compound.
2. Label the different H's on the compound using lower case letters.
3. Label each signal on the spectrum with the letter corresponding to the type of hydrogen.
4. Based on the structure you drew, discuss how the expected chemical shift and integration match what you observe on the spectrum.
5. Based on the structure you drew, predict how each signal should be split. Compare your predictions with the spectrum and account for any apparent discrepancies.

For 3-methylstyrene (structure shown below):

1. Do the same kind of analysis that you did for the spectrum you obtained in lab, described above.
2. The alkene proton region has been expanded and the coupling constants shown. Draw out a qualitative tree diagram similar to the one shown on p. 466 of the McMurry textbook.

For the three unknown compounds:

Propose plausible structures for each of the unknown compounds based on their NMR spectra and chemical formulas. The possible functional groups are shown on the inside cover of your McMurry text. After you have proposed a structure, you should analyze it as you did the previous spectra in order to check your answer. The chemical shift, integration, and splitting of the proposed structure should be consistent with the predicted values for your proposed structure.

Turn in all labeled spectra with your report.