NMR spectroscopy

• NMR 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 absorb this radiation
  • The energy for the absorption can be measured and recorded as an NMR spectrum
• NMR stands for nuclear magnetic resonance
• The nucleus also has a nuclear spin, that is significant if there is an odd number of nucleons (protons and neutrons)
• NMR is relevant for H-1 and C-13, the isotopes with an odd number of nucleons
• NMR spectroscopy can be used to detect isotopes of other elements with odd numbers of nucleons
• C-13 and especially H NMR spectroscopy are the most common forms of analysis used
• As a H-1 nucleus consists of just a proton, 'H NMR is usually referred to as proton NMR
• The nucleus has 2 different spin states with different energies
• Why is this spectroscopy named 'nuclear magnetic resonance'?

  • strong magnetic field
  • radio frequency radiation
  • resonance: nucleus absorbs energy and rapidly flips between the two spin states
• Radio frequency radiation has much less energy than the infrared radiation used in IR spectroscopy
• • The frequency required for resonance is proportional to the magnetic field strength
  • 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.
• A typical NMR spectrometer is a large cylinder which houses the electromagnet, cooled by liquid helium.
• Most routine spectrometers operate at radio frequencies of 100, 200, or 400 MHz
• MRI body scanners uses the same technology
• All atoms have electrons surrounding the nucleus, which shifts the energy and radio frequency needed for NMR
• Tetramethylsilane (TMS), (CH3)4Si, is used as the standard reference chemical against which all chemical shifts are measured.
• TMS is given a chemical shift value of 0 ppm
• The amount of chemical shift is determined by chemical environment, especially the presence of nearby electronegative atoms
• Pi bonds like C=O and C=C have a higher chemical shift than electronegative atoms like C-O. Lower down is C-C and the lowest C-Si has a value of 0 ppm.
• Depending on the chemical environment, NMR requires a different energy and frequency, producing different absorption peaks at chemical shifts
• NMR allows the carbon and hydrogen arrangement in a molecule to be mapped out without needing to carry out chemical tests and destroying the organic compound under test
• In an NMR spectrometer, the sample is dissolved in a solvent and placed in a narrow NMR sample tube, together with a small amount of TMS
• 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
• Any absorptions of energy resulting from resonance are detected and displayed on a computer
• After analysis, the sample can be recovered by evaporation of the solvent
• A deuterated solvent is usually used in which the 1H atoms have been replaced by 2H atoms (deuterium, D)
• Deuterium produces no NMR signal in the frequency ranges used in C-13 and H NMR spectroscopy
• Deuterated trichloromethane, CDCl3, is commonly used as a solvent in NMR spectroscopy, but this will still produce a peak in a C-13 NMR spectrum. The computer usually filters out this peak before displaying the spectrum.
• The frequency shift is measured on a scale called chemical shift, in units of parts per million (ppm)