Spectroscopy notes for uni: NMR

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I am only publishing these as convenient access points for me. Some of this can be used at A Level, I think, but I'd hesitate before relying on them. 

NMR (Nuclear magnetic resonance)

Context behind NMR

  • NMR transitions originate from the radiowave section of the EM spectrum, with wavelengths ranging from about 1-100m. 
  • NMR takes advantage of the inherent nuclear spin (I) of a nucleus. 
  • This spin produces a nuclear magnetic moment (m) which causes the nucleus to engage in magnetic interactions. Both the spin and magnetic moment are quantised variables. 
  • Only certain nuclei will be NMR active - they will have a half-integer spin, where the number of protons is odd and the number of neutrons is even, or vice versa.
  • And when we take the nuclei and apply a magnetic field to them, they can align along or opposing the direction of the magnetic field. Which I kind of discussed a while ago.
  • The total number of values m can take is 2I + 1; for a nuclear spin of 1/2, you'll have two possible magnetic moments: -1/2 and +1/2. 
  • NMR works by utilising the energy difference between the different spin states in the magnetic field. Generating NMR spectra occurs by sending successive pulses which repeatedly excite and relax the nuclei, whilst maximising the signal sent relative to any background noise. 
  • This signal varies depending on where the nucleus is in the molecule. 
What NMR spectra look like
  • We can therefore have chemically equivalent or inequivalent nuclei in a molecule depending on the environment of the nucleus. This is related to the symmetry of a molecule's structure. 
  • Each associated resonance frequency (related to the applied magnetic field strength) of the NMR signal depends on the nucleus and its chemical environment. The variation of the resonance frequency with the chemical environment is known as the chemical shift, and is due to the interaction of electrons with the applied magnetic field.
  • This chemical shift is also affected by shielding, where if you have a high electrondensity around the nucleus, you'll have greater shielding effects around the nucleus and vice versa. I've written about this in a previous post as well.
  • One factor that affects this shielding is electronegativity - more electronegative atoms will attract electrons towards themselves, which will shield nearby nuclei.
  • Chemical shift is measured in parts per million (ppm); the greater the chemical shift, the signal will be further towards the left. In proton-NMR, the most common chemical shifts range from around 0 to 12ppm. Contrast this with C-13 NMR, where the most common chemical shifts will range from about 0 to 220ppm.

Coupling and splitting

  • We can also determine how many nuclei are responsible for a given peak in an NMR spectrum. We can do this by measuring the area under each absorption - this is known as the integration of each signal. We can use this for various spectra, such as proton and F-19 NMR, but we cannot do this for C-13 NMR.
  • The total number of lines that result in a splitting pattern has a similar equation to that of the total number of magnetic moments. The number of lines is equal to 2nI+1, where n is the number of coupling nuclei. For I = 1/2, we end up with the number of lines equal to n+1; this is the origin of the n+1 splitting rule.
  • To give an example, let's look at proton NMR. If you have a single peak, known as a singlet, this mean that any adjacent carbon bonded to this original proton is bonded to a C atom which itself has no protons bonded to it. If you have two peaks of equal intensity, you have a doublet, where the adjacent carbon bond is bonded to one hydrogen. The same applies for triplets, quartets, and so on. 
  • Another importance concept in NMR is spin-spin coupling. This is where neighbouring nuclei interact with each other and influence the effective magnetic field. This results in any observed peaks in the spectrum to split.
  • This effect can go beyond chemical bonds, and can occur between nuclei so long as they are, at most, three bonds apart from each other. 
  • Any chemical shift that is recorded will be in the middle of the two peaks. 
  • When coupling occurs you may notice a distance between the two peaks that have split. This distance is determined by the coupling constant J. Coupled nuclei will have an equal value of J to each other. 
  • Generally speaking, 1J > 2J > 3J in magnitude. The distance between peaks in 3J coupling will be greater than that in 2J, though.We can sometimes have more complex splitting patterns; as an example, take the molecule PHF2.
  • If we run an F 19 NMR, spectrum, we would notice that the F that with interact the P would be 1J to each other. However the fluorine atoms would also experience 2J coupling with the hydrogen atom, since they are two bonds apart from each other. So we will have two F atoms interacting with the P atom which will result in a doublet. But we also have two F atoms interacting with the H atom, which also results in a doublet. So you'll have a doublet of doublets. We've got 1J coupling along with 2J coupling, so we end up with a doublet of doublets.

Relaxation techniques

  • There are two alternative relaxation mechanisms we can use in NMR:
    • Spin-lattice relaxation, where relaxation energy is lost to surrounding molecules. 
    • Spin-spin relaxation, where this energy is lost to nearby spin-active nuclei. 

Epiloue 

If there any weird spelling errors or odd words used in this book post please let me know my post was dictated because my keyboard randomly stopped working midway through writing the post thanks! I mih o ack and edi hem when i he moe ime in he uue, u ih now I can' e asked o use his keyoad een moe han I'm oced o use i ih now.

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