NMR: Nuclear magnetic resonance spectroscopy to analyse organic compounds. Nuclei of atoms
absorbs magnetic field and radio frequency radiation, absorption measured by NMR. Frequency
required for resonance proportional to magnetic field strength.
NMR used in magnetic resonance imaging to provide diagnostic information.
Nuclear Spin: Electrons spin. Nucleus also has nuclear spin, which is significant if there is an odd
number of nucleons (protons and neutrons).
Isotopes: Organic compounds contain C and H so NMR used to detect 1H and 13C isotopes with
odd number of nucleons. 1H is a proton so proton NMR. NMR can also be used to detect other
elements.
Resonance: An electron has two different spin states. Nucleus also has two different spin states
and these have different energies. With both magnetic field and radio frequency radiation,
nucleus absorbs energy and flips between two different spin states- called resonance.
Chemical Shift: Chemical shift is the frequency shift measured in units parts per million (ppm).
Shift happens due to chemical environment, producing absorption peak. Scale increases from
right to left.
Factors such as solvent and concentration may move a peak outside chemical shift ranges on
data sheet.
TMS: Tetramethylsilane TMS (CH3)4Si used as standard reference for chemical shifts
measurements. TMS is 0 ppm for chemical shift. Produces single peak. Easily removed.
Carrying Out NMR: Sample dissolved in solvent with some TMS. Sample spun to even out
magnetic field. Given range of radio frequencies with constant magnetic field. Absorptions of
energy resulting from resonance detected.
Deuterated Solvents: Most solvents contain C and H which produce signal in NMR. Deuterated
solvent used where 1H atoms replaced by 2H atoms (deuterium D). They produce no chemical
shift as contain no H. Deuterated trichloromethane, CDCl3, a common solvent. However this
produces peak in C- 13 NMR, so computer filters this out.