Question

Explain the proper techniques for cleaning the NMR tubes. Explain NMR and IR spectroscopy and what can be gained from both individually to analyze an unknown sample. Explain and show work.

Answer 1

Techniques for cleaning NMR tubes:

There are two cases, simple and difficult cleaning:

- Simple Cleaning: Rinse the tube with distilled water or an organic solvent, one at a time. A final rinse with acetone is used to remove last organic contents. If the sample will be dissolved in water, the tube is previously rinsed with distilled water.

- Difficult Cleaning: Caused when the sample is left for a period of time in the tube, which promotes de adhesion of them to the tube. In that case, it will be necessary to rinse them frequently with concentrated or fuming Nitric Acid. Finally a rinse with distilled water is applied.

If the sample is a synthetic polymer, some solvents may soften it, and then a pipe cleaner should do it. Also, agitation in ultrasonic bath with an appropriate solvent may work.

After doing so in both cases, water has to be removed, to which it is recommended to use a vacuum oven. To remove protic content of water, it is recommended to use D₂O prior to the oven.

If water degrades the sample, water must be reacted with a hydride solution, and then rinsed with an organic solvent.

NMR Centre. University of Guelph. "How To: Clean NMR Tubes". Retrieved from: http://nmr.uoguelph.ca/NMR%20Tube%20Cleaning.pdf

NMR Sprectroscopy:

It is a method of structural analysis for organic compounds. It is also used in inorganic and biochemistry. It can be used to analyze mainly nuclei with a spin number of 1/2, which includes most of the common elements (hydrogen, carbon, fluorine and phosphorus). What we obtain from an NMR Spectroscopy are spectrum, or peaks, which represent the difference in ppm (parts per million) from a reference molecule (usually tetramethylsilane). Carbon has a typical value of 220, and Hydrogen a value of 12.

Royal Society of Chemistry. (2014). Introduction to NMR Spectroscopy. Retrieved from: http://www.rsc.org/learn-chemistry/wiki/Introduction_to_NMR_spectroscopy

IR spectroscopy:

Infrared spectroscopy is used to determine structural and purity details of a compound. They are quick and easy to run, and it works like this: infrared radiation is absorbed by organic molecules and converted into energy, which results in IR spectra shown in a graph as shown below:

The difference with NMR spectroscopy, is that these peaks are compared to a database of already established theoretical spectra for diverse compounds, which results in obtaining the identity and its purity, with certain error of course.

University of Colorado. (2015). Chapter 15. Infrared Spectroscopy: Theory. Retrived from: http://orgchem.colorado.edu/Spectroscopy/irtutor/IRtheory.pdf