NMR Spectroscopy

• Since its invention about 70 years ago, nuclear magnetic resonance spectroscopy has revolutionised the analysis of organic compounds.
• The technique used 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 spectrum
• Electrons have a property called spin
• The nucleus also has a nuclear spin, that is significant is there is an odd number of nucleons (protons and neutrons).
• Almost all organic molecules contain carbon and hydrogen, mostly as the 1H and 12C isotopes, with a small proportion (1.1%) of the 13C isotope
• For organic chemists, NMR is relevant for 1H and 13C, the isotopes with an add number of nucleons
• NMR spectroscopy can be used to detect isotopes of other elements with odd numbers of nucleons, such as 19F and 31P. However 13C and 1H NMR are the most common forms of analysis used.
• A 1H nucleus consists of just 1 proton - 1H NMR is usually referred to as proton NMR
• Resonance: an electron has two different spin states. The nucleus also has two different spin states and these have different energies
• 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 - this is called resonance and the whole process gives the name 'nuclear magnetic resonance'
• The NMR spectrometer: radio frequency radiation had much less energy than the infrared radiation used in IR spectroscopy.
• The frequency required for resonance is proportional to the magnetic field and 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 large cylinder houses the electromagnet, cooled by liquid helium.
• For organic chemistry, most routine spectrometers operate at frequencies of 100, 200 or 400 MHz. They are found in hospitals as MRI (magnetic resonance imaging) body scanners, a technique that uses the same technology
• Chemical shift and TMS: 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 nuclear magnetic resonance to take place. The frequency shift is measured on a scale called chemical shift (delta), in units of parts per million (ppm)
• Tetramethylsilane (TMS), (CH3)4Si is used as the standard reference chemical against which all chemical shifts are measured. TMS is gen a chemical shift value of 0ppm
• The amount of chemical shift is determined by chemical environment, especially the presence of nearby electronegative atoms
• So depending on the chemical environment, nuclear magnetic resonance requires a different energy and frequency, producing absorption peaks at chemical shifts. This means that the carbon and hydrogen arrangement in a molecule can be mapped out without needing to carry out conventional chemical tests and without destroying the organic compound under test
• Running the spectrum:

  • 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
  • tube is placed inside the NMR spectrometer, where it is spun to even out any imperfections in the magnetic field within the sample
  • 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 absorption of energy resulting from resonance are detected and displayed on a computer screen
  • after analysis - sample can be recovered by evaporation of the solvent
• Deuterated solvents:

  • molecules of most common solvents contain carbon and hydrogen atoms which will produce a signal in both 13C and 1H NMR spectra
  • a deuterated solvent is usually used in with the 1H atoms have been replaced 2H atoms (deuterium, D)
  • Deuterium produces no NMR signal in the frequency ranges used in 1H and 13C spectroscopy
• Deuterated trichloromethane, CDCl3 is commonly used as a solvent in NMR spectroscopy, but this will produce a peak in a carbon-13 NMR spectrum. the computer usually filters out this peak before displaying the spectrum