Introduction to NMR Spectroscopy

best-of-chemistry-lNMR is an acronym of Nuclear Magnetic Resonance. You may be familiar with the concept due to MRI, Magnetic resonance imagining. NMR Spectroscopy is used in modern chemistry to determine the structure of unknown species. It can describe the situations in which particular type of atom in a molecule is thus their structure.

Theory behind it.

  • Nuclei have spin like electrons
    • 1/2 – H1, C13, N15, F19, Si29
    • 0 – C12, O16
    • 1 – H2, N14
    • 3/2 – B11, Cl35
  • Any charged particle, i.e. the nuclei with spin, will generate a magnetic field. When an external magnetic field is applied to the system, the charged particles may align along the magnetic field or against it.
  • Alignment along the field (I=+1/2, a hydrogen) will be favored and will be lower in energy. But, Alignment against the field (I=-1/2 b hydrogen) will also exist despite the excess of a hydrogen. Their energy difference is very small; thus, excess will not be big
  • When radio wave is given, thus energy, the alignment will change direction so that b hydrogen becomes excess. When giving wave stops, the spins will orientate themselves back to equilibrium and release energy in forms of heat.
  • This phenomenon of nuclei absorbing energy from a particular frequency of wave is called resonance.
  • To yield a greater difference between the energy when aligned and when not aligned, the strength of the magnetic field has to be used. V=γB/2π thus E=h γB/2π. Since γ varies for all NMR active nuclei, the interference of a different nucleus in a particular NMR is very rare.
  • Superconducting magnets
  • Short pulse of radio frequency shot at the sample, as it is cooling to its equilibrium, the computer takes the data as intensity vs. time. Fourier Transform à Intensity vs. resonance frequency

Taking data from and Analyzing NMR graphs

  • Dissolve about 10mg of the compound in about 1mL of deuterated solvent or known ppm value such as water or DMSO and add 0.05% TMS.
  • Integral
    • Indicates the number of protons there are in a molecule that share the same environment.
  • Chemical shift
    • When a particular magnetic field (B) is given to a charged object, the charged objects too will create induced magnetic field (Bi). Thus, Bnet of each hydrogen will be different for hydrogens at different position for they will induce different fields. Also, since Bnet felt by most hydrogens are greater than the applied B due to electromagnetic surroundings, the molecules are deshielded and lies to the left of TMS.
    • TMS or tetra methyl silane are given 0ppm.
      • Alkanes
        • They usually appear around 1 because no electronegative group Deshields the molecule.
      • Electronegative group
        • Electron withdrawing groups will lessen the electron density, deshielding the molecule. This will lie further downfield
      • Allylic hydrogens
        • Carbon carbon double or triple bonds are electron withdrawing group, which Deshields the molecule
      • Vinylic hydrogens
        • Hydrogens attached directly to double bond is more deshielded
      • Delocalization
        • Since carbonylic acid groups and aldehyde group delocalizes the electron density of H, H is deshielded
      • Aromaticity
        • They always lie between 6.5 and 8 ppm
        • Ring current through the ring is created. Since the outside hydrogens receive magnetic field of both aromatic ring and Bo it feels more deshielded.
      • Attached to Oxygen or Nitrogen
        • They vary greatly on the nature of the solvent such as pH.
        • If pH is low, the proton in alcohol group will fall off.
      • Spin-spin coupling and Coupling Constant (J)
        • Hydrogens too have magnetic fields. Thus, Bnet experienced by a hydrogen will be affected by the hydrogen next to it. There are a lot of orientations of spin that the adjacent hydrogens may have. Let’s take an example of ethyl iodide. The hydrogens attached to the carbon with iodine will have four possibilities of different electron configurations: up, up, up, down, down, up, down, down arrangements. However, since up down and down up configurations are the same thing. Three peaks in total will appear with the integral ratio of 1:2:1 which in total will equal 2. Let’s look at the methyl group. There are 8 possible arrangements of spin. However, since six of the eight divide into three and three, only 4 peaks will appear with the integral ratio being 1:3:3:1 where the total will equal 3. As you may have noticed, the total number of peaks are proportional to the number of adjacent hydrogens. This is called the n+1 rule. Also, the integral ratios are given in Example in pg 729
        • Coupling constant accounts for the distance between the peaks. When peaks form from coupling, the distance between the two peaks are J Hz. This frequency value can be divided by the carrier frequency to obtain the ppm value. The coupling constants are varied by the bond distance between the affecting hydrogens and the affected hydrogen. For example, if the two hydrogens are three bond length away, meaning they are on adjacent carbons, the J value is between 6 to 8. Since J value for long range coupling is negligible, and the chance of two different protons being attached to a single carbon is highly improbable, only the coupling done over adjacent carbon will affect the NMR readings. The coupling constant also varies according to the angle that two hydrogens make. Dihedral angle is the angle between the two hydrogens when looked through newman projection. Karplus curve shows that J is highest when theta is 0 and 180 and lowest when theta is 90.
        • Decoupling is a technique which reduces the unnecessary coupling of hydrogens. If a C=C double bond exists, the vinylic hydrogen can couple with additional groups attached on the other end of the vinyl via long range coupling. Thus, NMR spectrometer apply two radio frequency wave, one which follows the routine and one which has the same frequency as the resonance frequency of the excess hydrogens. This allows rapid transition between spin states cancelling the coupling effect.

C13 NMR

C13 also has a nuclei spin of +_1/2. However, since C13 isotope rarely exists, it is not visible in the H1 NMR.

The second technique is called DEPT (distortionless enhancement with polarization transfer) measures the number of CH, CH2, CH3, and CR4 groups.

Today, NMR spectroscopy has advanced so much that now, using MRIs, (Magnetic Resonance imaging), images of nuclear magnetic resonance can be seen rather than peaks of resonance in a particular molecule. Yet, despite the advanced technology of spectrometer and structure determination, we have to be able to intuitively figure out the structure of experimentally obtained molecules by their physical and chemical properties.

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