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NMR Spectroscopy

Topic: Molecular Structure And Absorption Spectra

NMR (nuclear magnetic resonance) spectroscopy is one of the most widely used methods to characterize organic compounds because it can provide information about the different proton environments and thus the molecular structure.

The principle behind NMR is that many nuclei have spin and all nuclei are electrically charged. Proton nuclei can be modeled as a spinning positive charge, which means they have a magnetic moment (according to the right-hand rule) that is perpendicular to that rotation, and this is noted as either spin “up” or “down”. When a nuclei is placed in a magnetic field (Bo), the nuclei’s magnetic moment can either align or oppose the applied magnetic field, so there is a low-energy state (α), and a high energy state (β), respectively, which can exist. If the appropriate energy is input (in NMR, radio wave frequencies), the α state can flip to the β state, and if this occurs, the proton is said to be “in resonance” with the external magnetic field, hence the name nuclear magnetic resonance spectroscopy. When the spin relaxes back down to the α state, each proton’s resonance energy is recorded by the NMR spectrometer as a frequency and a signal is plotted on the spectrum. The α-to-β energy difference is related to the frequency of the resonance transition by the equation: E = hv where h is Planck’s constant and v is the frequency, so B is directly proportional to E and v (see the figure below).

Spectral analysis is explained in more detail below:

1. The position of the signal can tell you about the electronic structure directly around the proton.
2. The size/integrated area under each signal can tell you about equivalent protons.
3. The multiplicity can tell you about nearby protons (spin-spin splitting).

The Position of the Signal
Electrons near a proton nucleus (from other atoms, like C, O, N) create their own induced magnetic field in the opposite direction of Bo, which decreases the magnitude of the effective magnetic field (Beff) actually experienced by the proton, Beff < Bo. This is an effect called shielding. The magnetic field, B, is directly proportional to the frequency, v, so if the magnetic field decreases, so does the frequency of that proton’s resonance. Since NMR signals are plotted as a function of this frequency (relative to a standard), changing the frequency of resonance changes the position, or chemical shift (ppm), of the proton signals in the final spectrum. Relatively shielded protons are bound to C (not very electronegative) and their signals have lower frequencies (upfield).

The opposite effect can also occur: deshielded protons, which may be bound to a carbon which is also bound to an electronegative atom (i.e. Cl, O), have chemical shifts at higher frequencies on the spectrum. The more deshielded (i.e. the more distorted away from the atom the electron cloud is), the further downfield (the to the left) in an NMR spectrum it will appear. Most protons in organic compounds have chemical shift values between 0-12 ppm. Note that the position of protons directly bound to heteroatoms (as in OH or NH2 functional groups) may appear at a wide range of chemical shift values depending on the sample solvent and H-bonding interactions.

Equivalent Protons
Protons in different environments give different signals in an NMR spectrum, but if there are protons in the same environment, they are considered equivalent and there will only be one signal for all equivalent protons. Integrating the area under a signal in the NMR spectrum can tell you how many equivalent protons that signal corresponds to. To determine if two protons are equivalent, it is important to consider symmetry and chirality. Generally, if protons are equivalent, you should be able to substitute X at any of their positions and end up with the same compound in each case.

Spin-Spin Splitting
The spin-spin splitting phenomenon occurs when nonequivalent hydrogens interact with each other. The signals in an NMR spectrum can be split due to interactions with magnetic fields produced by neighboring protons (these are protons on neighboring atoms, or 3 bonds away from a proton). The degree of splitting depends on the number of adjacent hydrogens, and a signal will be split into n+1 peaks (with n=number of neighboring/interacting protons), and the peak height ratios generally follow the pyramid scheme.

For the ethanol example below, the OH proton will interact via H-bonding with other ethanol and water molecules in the sample solution, so practically speaking it will show up as a broadened singlet and not be split by or cause splitting to neighboring protons (although in theory it should, this is often an effect seen for OH or NH groups). However, the three equivalent Ha protons will be split by the Hb protons (n+1=2+1) and therefore show up as a triplet, and the Hb protons will be split by the Ha protons (n+1=3+1) and therefore show up as a quartet.

n+1 Rule

Practice Questions

Khan Academy 

Synthesis of anti-tumor drug Combretastatin and its derivatives

 MCAT Official Prep (AAMC)

Key Points

• The principle behind NMR is that many nuclei have spin and all nuclei are electrically charged.

• Different protons have different resonance frequencies (different chemical shifts) and equivalent protons have the same resonance frequencies (same chemical shifts).

• A proton’s chemical shift (position on the spectrum) depends on what atoms are close to it and how much it is shielded or deshielded.

• A signal’s integration corresponds to the number of equivalent protons of that type.

• The spin-spin splitting phenomenon occurs when nonequivalent hydrogens interact with each other.

• A signal’s multiplicity (number of peaks) depends on the protons nearby/on adjacent carbons and corresponds to n+1 with n=number of neighboring protons.

Key Terms

NMR: Nuclear magnetic resonance

Resonance: when the input frequency (radio waves) match with the energy required to flip the spin of a proton nuclei, it is said to be in resonance. These resonances are detected by an NMR spectrometer and translated into the signals in the spectrum which appear at chemical shift values relative to their resonant frequencies.

Equivalence: protons are said to be chemically equivalent if they are in the exact same chemical environment. To test for equivalency, if you are able to substitute X at any of the proton positions and end up with the same compound in each case, those protons are equivalent. It is important to consider symmetry and chirality.

Shielding/deshielding: Effects caused by the different chemical environments around proton nuclei. In general, an electron-rich environment causes shielding and results in lower chemical shifts, whereas more electronegative atoms around cause deshielding and higher chemical shifts.

Multiplicity: the number of peaks in a signal of an NMR spectrum. This is equal to n+1, where n is the number of protons on neighboring atoms (within 3 bonds from the proton responsible for the signal).

Chemical shift: The resonant frequency of a nucleus relative to a standard in a magnetic field.

Spin-spin splitting: The source of signal splitting; a term that describes the magnetic interactions between neighboring, non-equivalent NMR-active nuclei.

Upfield/Downfield: Describe the relative location of peaks; upfield means to the right, downfield means to the left.

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