[29.2] nuclear magnetic resonance (NMR) spectroscopy

interaction of nuclei with an external magnetic field and radiofrequency radiation

• protons and neutrons can be regarded as spinning about their axis • in many atoms, these spins are paired against each other so the nucleus has no overall spin (eg. 12 C) • in atoms with an odd number of p + n (eg. 13 C) the nucleus has an overall spin • a nucleus that spins generates a magnetic field

which way the nucleus spins

when the nuclei of atoms absorb energy and flip their spin state to a higher energy level at specific radio frequencies

the environment of each atom

• for a given radio frequency, each atom will need a slightly different magnetic field applied to it to bring it into the resonance condition depending on what exactly it is bonded to • the magnetic field needed is a useful guide to the carbon atom’s environment in the molecule

• surrounding electrons / bonds affect the energy and radio frequency for resonance to take place • therefore, atoms in different environments respond differently to the field • each different environment produces a signal in a different position

• scale which measures frequency shift • units = ppm, signified by δ • amount of chemical shift is determined by chemical environment

¹³C and ¹H

• peaks are measured relative to the standard reference compound, TMS, in parts per million (ppm) • TMS is given a chemical shift value of 0

• samples are dissolved in solvents free of ¹H atoms eg. CCl₄, CDCl₃ • TMS is added to calibrate the spectrum

• signal is away from all the others • only gives one signal • non-toxic • intert • low bp so easy to remove