Nuclear Magnetic Resonance (NMR) Spectroscopy

1. Preparation of NMR Starting Material

1. Add ~10 mg starting material to a clean NMR tube.
2. Dissolve the starting material in ~0.7 mL deuterated solvent (example given CDCl₃). A suitable height of the solvent for a good spectrum is 4.5-5 cm.
3. Cap the NMR tube carefully and write the sample name on the cap.
4. Shake the sample gently to ensure that all material had dissolved. Take care to avoid contact between the solvent and the cap, which could lead to possible contamination of the sample.
5. Insert the NMR tube carefully into a spinner. The spinner will rotate once inserted into the magnet to ensure that the entire sample experiences a homogenous magnetic field. Clean the outside of the NMR tube and spinner with 2-propanol and lab tissues in order to remove fingerprints and dirt.
6. Place the spinner in a sample depth gauge to ensure that the bottom of the NMR tube is not inserted too far into the NMR probe as that could possibly damage the spectrometer. Different probes have different sample depths and the user should be aware of the specific depth gauge.
7. Place the sample in the NMR spectrometer. Here a Varian 400 MHz spectrometer, equipped with an autosampler, was used.
8. After completion of the NMR measurement, process the spectrum and assign the peaks in the spectrum.

2. Preparation of 3 M NaOH and Chalcone Synthesis

1. Add 60 mg NaOH to a 50-mL volumetric flask.
2. Dissolve the NaOH by adding deionized water to half of the flask. Dilute the solution further by adding more water until the mark is reached.
3. Add 10 mL ethanol to a 50-mL round-bottom flask equipped with a magnetic stir bar.
4. Thereafter, add 680.5 mg 4-methoxybenzaldehyde and 5 mL NaOH solution prepared in step 2.1 to the same flask.
5. Add 671 mg 4-methylacetophenone subsequently to the stirred solution and cap the flask and stir at room temperature.
6. Monitor the progress of the reaction by ¹H NMR at 30 min intervals (see step 3) until full consumption of starting materials.
7. Add 5 mL water when the reaction has reached completion (~3 h). Filter the resulting precipitate and wash it with 20 mL 1:2 ethanol/water. Let the precipitate air dry.
8. Calculate the yield of the obtained product. Prepare an NMR sample according to step 1.2.2.7. Check purity with ¹H NMR. If not pure, purify the product via recrystallization with ethanol.
9. Add approximately 3 drops of reaction mixture to an NMR tube using a Pasteur pipette and rinse the pipette with deuterated solvent.
10. Repeat steps 1.2–1.8.

3. Brief Interpretation of an NMR Spectrum

1. Process the spectrum with a suitable program (example given MestReNova).
2. Correlate the different peaks to the NMR shifts in Table 1. The chemical shifts gives a hint of what type of environment the protons exists in.
3. Integrate the peaks to give the number of hydrogens corresponding to each peak. Integration of all peaks gives a relative number of total protons.
4. Evaluate the splitting of the proton-peaks, which indicate the number of neighbors.
5. Measure the J-coupling to see how the protons are connected to each other.