Many years ago a colleague wanted my help running an NMR spectrum. This would be
ID: 1007971 • Letter: M
Question
Many years ago a colleague wanted my help running an NMR spectrum. This would be on the old 250 MHz instrument. They were not what you would call an “NMR person”, so I agreed to do so. When we got the tube in the magnet I could not get a lock signal, and asked my colleague what solvent they had used. “Pentane” was the response. “Deuterated pentane?” I asked. “That had to be pretty expensive.” “Deuterated? No, just pentane.” It was one of those laugh hysterically and/or sob uncontrollably moments. Getting control of myself, I explained briefly why this was going to be a wee bit of a problem. If you needed to explain to someone why “pentane” is a poor choice of solvent for NMR, what would you say? Make sure you include not having a lock signal and its importance as part of your explanation.
Explanation / Answer
In NMR spectroscopy we use a large excess of solvent to dissolve very small amount of sample compound. If we to use a non-deuterated solvent, the 1H peaks from solvent would swamp the signals from the compound. To avoid that we use deuterated solvent wherein, deuterium absorbs at a totally different frequency and does not interfere in the sample analysis. All deuterated solvents contain some non-deuterated solvent due to incomplete deuterium exchange and thus we see 1H signal corresponding to the solvent used in the spectrum. Another important reason for using deuterated solvent is to stabilize the field strength and measure the deuterium absorption of the solvent and adjusts the field strength to keep the resonance frequency (field strength) constant. this is referred to as locking the field in NMR. For pentane we would expect to see 3 signals in NMR spectrum for its 1H signals. This would thus complicate the spectrum as they would come in the upfield 0-2 ppm shift region where the sample signals might also come and make the things complicated for analysis. In NMR the choise of deuterated solvent is such that it would give minimal signals in spectrum and thus minimal interference to the actual compound analysis. Thus pentane would not be a good choice of solvent for NMR.