NMR: Difference between revisions
From Pumping Station One
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* By exposing atoms to a magnetic field and RF of a given frequency, we can put them into '''resonance'''. This can be measured, and is the basis for the technique. | * By exposing atoms to a magnetic field and RF of a given frequency, we can put them into '''resonance'''. This can be measured, and is the basis for the technique. | ||
While the nucleus plays a | While the nucleus plays a fundamental role in this technique working at all, the configuration of the electrons of an atom in relation to other atoms does play a subtle but very important role in NMR -- [https://www.youtube.com/watch?v=TJhVotrZt9I an atom's configuration of electrons does influence how much an atom is diamagnetically shielded from the effects of RF.]. We use this to our advantage because different electron configurations will resonate at slightly different frequencies. This allows us to discern between different ways a nucleus can exist within a sample. | ||
So, in essence | So, in essence and in the most simple of cases with NMR: | ||
* Ultimately | * First one selects something NMR active to work with (Hydrogen-1 / Proton NMR being the most common). | ||
* The height on the Y-Axis predictably represents how much of something there is. | * Ultimately the goal is to produce a spectrum. | ||
* The X-Axis represents resonance at different frequencies. | ** The height on the Y-Axis predictably represents how much of something there is. | ||
** The X-Axis represents resonance at different frequencies (called chemical shift). | |||
* If you are performing the typical Hydrogen-1 (Proton NMR) analysis of a sample, the peaks on the X-Axis represents different ways the electrons of a given Hydrogen within a compound are configured. For a simple compound where there is hydrogen in just one configuration (H2O / Water), you'd expect one peak. For Ethanol, CH3-CH2-OH, [https://www.youtube.com/watch?v=CjII0Cg882U we'd expect three peaks]. | |||
One thing to note at this point is that the resonant frequency for a given NMR active nucleus is related to the strength of your NMR's magnetic field | One thing to note at this point is that the resonant frequency for a given NMR active nucleus is related to the strength of your NMR's magnetic field -- the stronger magnetic field, the greater resolution (sharper peaks) one can achieve. | ||
You will often hear of people referring to the strength of their NMR in terms of MHz (as opposed to the strength of the magnetic field in Tesla). When this is the case, they are typically referring to the frequency of Hydrogen-1 at a given field strength. So, in our case, at 1.4 Tesla, you could say we have a 60MHz NMR. The implication, again, is that it operates at 60MHz when using the Hydrogen-1 probe. The resonant frequency is different if you are analyzing a different nucleus. | You will often hear of people referring to the strength of their NMR in terms of MHz (as opposed to the strength of the magnetic field in Tesla). When this is the case, they are typically referring to the frequency of Hydrogen-1 at a given field strength. So, in our case, at 1.4 Tesla, you could say we have a 60MHz NMR. The implication, again, is that it operates at 60MHz when using the Hydrogen-1 probe. The resonant frequency is different if you are analyzing a different nucleus. | ||
Because people have instruments of various field strengths, but would like to be able to compare data to eachother, the x-axis is not reported in frequency of Hz, but rather in a unit called ppm, which represents the chemical shift. PPM is simply the chemical shift in Hz divided by the NMR frequency and multiplied by 1,000,000. This standardizes the position of the resulting peaks across instruments, so something like water or vinegar will always shows peaks in the same ppm values regardless of the capabilities of an instrument. | |||
== Terminology == | == Terminology == | ||
