29.2 - nuclear magnetic resonance (NMR) spectroscopy

• Nuclear magnetic spectroscopy


  An organic compound analysis technique that uses a combination of a very strong magnetic field and radio frequency radiation. With the right combination of magnetic field strength and frequency, the nuclei of some atoms absorbs this radiation. The energy for the absorption can be measure and recorded as an NMR spectrum.
• What is NMR used for?


  Analysis of organic compounds.
• Nucleons


  Neutrons and protons.
• What types of atoms show signals on NMR?


  Atoms with an odd number of nucleons (mass number).
• What do almost all organic molecules contain?

  Carbon and hydrogen, mostly as ¹H and ¹²C isotopes, with a small proportion (1.1%) of the ¹³C isotope.
• What isotopes of carbon and hydrogen is NMR relevant for?


  ¹H and ¹³C, the isotopes with an odd number of nucleons.
• How many spin states do nuclei have?


  Two and these have different energies.
• Resonance


  With the right combination of a strong magnetic field and radio frequency radiation, the nucleus can absorb energy and rapidly flips between the two spin states.
• How does the energy of radio frquency radiation compared to infrared radiation?


  Radio frequency radiation has much less energy than the infrared radiation used in IR spectroscopy.
• What is the radio frequency radiation, required for resonance, proportional to in NMR?


  The magnetic field strength.
• What is the only environment that the energy of radio frequency radiation can be detected?


  It is only in strong and uniform magnetic fields that this small quantity of energy can be detected. Typically a very strong super-conducting electromagnet is used, cooled to 4K by liquid helium.
• What radio frequencies do most NMR spectrometers operate at for organic chemistry?


  100, 200 or 400 MHz.
• Practical application of NMR spectroscopy


  They are found in hospitals as MRI (magnetic resonance imaging) body scanners, a technique that uses the same technology.
• Chemical shift

  In an organic molecule, every carbon and hydrogen atom is bonded to other atoms. All atoms have electrons surrounding the nucleus, which shifts the energy and radio frequency needed for NMR to take place. This frequency shift is measured on a scale called chemical shift δ, in units of parts per million (ppm).
• What compound is used as the standard reference chemical against which all chemical shifts are measured?


  Tetremethylsilane (TMS), (CH₃)₄Si.
• Chemical shift value of tetramethylsilane (TMS)


  0 ppm
• What is the amount of chemical shift determined by?


  The chemical environment, especially the presence of nearby electronegative atoms.
• How does the chemical environment of a compound affect NMR?


  Depending on the chemical environment, nuclear magnetic resonance requires different energy and frequency, producing absorption peaks at chemical shifts.
• How to run an NMR spectrometer?


  1. The sample is dissolved in a solvent and placed in a narrow NMR sample tube, together with a small amount of TMS.

  
  

  2. The tube is placed inside the NMR spectrometer, where it is spun to even out any imperfections in the magnetic field within the sample.

  
  

  3. The spectrometer is zeroed against the TMS standard and the sample is given a pulse of radiation containing a range of radio frequencies, whilst maintaining a constant magnetic field.

  
  

  4. Any absorptions of energy resulting from resonance are detected and displayed on a computer screen.
• How can a sample be recovered after NMR?


  By evaporation of the solvent.
• What will most common solvents produce in an NMR spectrum?


  Molecules of most common solvents contain carbon and hydrogen atoms, which will produce a signal in both ¹³C and ¹H NRM spectra.
• Deuterated solvents


  A solvent in which the ¹H atoms have been replaced by ²H atoms (deuterium, D).
• Why are deuterated solvents used in NMR?


  Deuterium produces no NMR signal in the frequency ranges used in ¹H and ¹³C NMR spectroscopy.
• Common solvent used in NMR


  Deuterated trichloromethane, CDCl₃.
• What does deuterated trichloromethane, CDCl₃, usually produce when used in NMR?


  A peak in the carbon-13 NMR spectrum. The computer usually filters out this peak before displaying the spectrum.