NMR



Although I do not by any means consider myself an expert in organic chemistry, I felt that it would be wholly appropriate to start this series with what many would consider to be the King of all instrumental measuring techniques in chemistry.

Nuclear. Magnetic. Resonance.

The odds are actually pretty good for having encountered one of these spectroscopic machines in your lifetime. Take a populous that's afraid of all things nuclear and slap the word "Imaging" at the end of your acronym, and you're left with what's referred to as an MRI machine, frequently used in hospital settings for the diagnostic probing of the hydrogen atoms in both water and hydrocarbon structures.

Whether you're probing an individual molecule for spectroscopic responses, or an entire host of organic chemicals with the human body, the underlying quantum mechanical properties are the same.

The nucleus of every atom is made up of some collection of protons and neutrons. When these nuclei are placed into a magnetic field, their intrinsic "spin" states will either line up with the magnetic field, or against it (both positively-charged protons and non-charged neutrons are effected by magnetic fields because in reality, neither of them are fundamental particles: they're both comprised of what are called "quarks", up quark/up quark/down quark and up quark/down quark/down quark respectively, and it's these three entities that cause these sub-atomic particles to have a quantifiable angular momentum, or spin). Given the quantized nature of these spin states, the manner in which these particles (and by extension the atom, who's angular momentum can be considered a sum of the partials) align with the magnetic field can be analogous to a lowest-energy "ground state". When an array of radio frequencies are passed through the sample, a select set of frequencies will be absorbed by the atom, causing the spin state to be excited. When the atom relaxes from this excited state back to the ground state, it releases the excess energy in the form of a radio wave whos frequency can be detected.

Regarding spin states, things can get complicated fast for inorganic species (for example, a specific Zirconium isotope can have 6 possible spin state orientations). However, there is a simplicity to the analytical technique when your study focuses on atoms that are limited to a ground state and one excited state.

Coincidentally, there are two very common atoms that fit this parameter: 13C and 1H. Those lucky organic chemists.

Now, If NMR were purely a function of our individual, isolated atoms, there would be no differentiation between say, any given carbon center in your chemical. This is where the next piece to this puzzle, the electron, comes into play. From Maxwell's equations regarding electromagnetism, we know that moving charged particles will generate a corresponding magnetic field. Within the scope of NMR, that means that electron movement around a nucleus will generate a corresponding magnetic field. Induced by the external magnet from the NMR, these subsequent magnetic fields (corresponding to every electron that interacts with an atom we're interested in) coupled with the NMR's external magnetic field will generate a total magnetic field for the atom that is completely unique to said individual atom's environment. With these unique environments you get unique absorption wavelengths, and that's what you see when you observe an NMR spectra.


The last point I'm going to touch on in this post is a point that I feel is underappreciated in NMR studies, and that is the conversation revolving around the variable axes. So far in our discussion, nothing has been mentioned about correlating changes in energy states to some arbitrary unit of parts-per-million.


This arbitrary unit assignment has to do with the fact that NMR spectras are plotted on relative scales, not absolutes. Depending on your machine, differing mechanics could be applied to the mechanism of action, yielding differing applications of magnetic fields from one NMR device to the next. This, in it of itself, is a problem when it comes down to producing comparable data sets between researchers, because if we remember back to the beginning of this post, the terms of our ground state and excited state were functionalized by the external magnetic force. Changing the external magnetic force changes the frequency of absorption. 


This is where chemical shift comes into play. Taking the ratio of the absorption difference between your sample and a chemical standard (so far throughout instrumental analysis that standard has been tetramethylsilane, or TMS; see cover photo at ppm=0) with respect to the frequency of the external magnet, yields a dimensionless quantity that is constant for every NMR device. For example, Methane, or CHtheoretically absorbs for a 300MHz NMR device at 69Hz above tetramethylsilane. However, in a 60MHz NMR, your absorption frequency occurs 13.8Hz above TMS. Although they absorb at different frequencies, their chemical shift is the same.



Given that most NMR machines operate in the mega-hertz region (10Hz), reducing the exponential of our dimensionless result by a factor of 1/1,000,000 and representing such action with a "part per million" designation yields the final piece to the spectra.


I think that we'll conclude our discussion with that; it's enough to start making sense of 13C NMR, but there's still a lot more relevant material to NMR studies that I haven't discussed (like for example coupling in 1H, quantitative studies, et al), but getting into those specifics I feel is too large of an endeavor for the scope of this first post. For continued reading I recommend the following texts:

For a Physicist's Perspective (esp towards 
1H NMR):

Young, Hugh D., Roger A. Freedman, and A. Lewis Ford. "Chapter 43: Nuclear Physics." University Physics. Harlow, Essex: Pearson, 2012. Print.

For a Chemist's Perspective (also this manuscript is probably my personal favorite Organic Chemistry textbook):

Klein, David R. Organic Chemistry, Second Edition. "Chapter 16:Nuclear Magnetic Resonance Spectroscopy." New Jersey: John Wiley, 2012. Print.

And finally, I plan to move on to the next analytical technique but if you would really like me to talk more about NMR, leave a comment and we can see about the possibility of a Part II.


Thanks






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