Organic 15: NMR

Finding the structures of molecules

A magnetic field is applied to a sample surrounded by a source of radio waves and a radio receiver This generates an energy change in the nuclei of atoms which can be detected Electromagnetic energy is emitted which can be interpreted by a computer

Carbon-13 NMR Proton NMR

Because carbon-13 has a nuclear spin and carbon-12 does not Not all carbon-13 atoms resonate at the exact same magnetic field strength, they feel it differently depending on the functional group they are part of

The greater the electron density around a carbon-13 atom, the smaller the magnetic field felt by the nucleus, and the lower the frequency at which it resonates

Parts per million (ppm) Varies depending on atomic environment

Less shielded functional groups = higher chemical shift (i.e. lower electron density)

A compound used as a standard to compare the chemical shift of other atoms All twelve hydrogen atoms have identical environments, so the chemical shift of each is zero TMS is a liquid and is added to samples before the NMR spectra is run to calibrate the spectrum Used as it is inert, non-toxic, and easy to remove from the sample

CDCl3 (deuterated chloroform) Used as it is relatively inert

Each hydrogen atom is attached to a different functional group, so feels the magnetic field differently The greater the electron density around the hydrogen atom, the smaller the chemical shift

Protons have a stronger response to magnetic fields than carbon atoms They are also more shielded if surrounded by electron-rich areas

The further away a hydrogen atom is from an electronegative atom, the smaller the chemical shift

It is proportional to the number of hydrogen atoms of each type

The area of the peak is related to the number of hydrogen atom producing it This can be difficult to evaluate by eye, so the instrument produces a line called an integration trace The relative heights of the steps of this trace give the relative number of each type of hydrogen

The magnetic field applied to samples is affected by the natural magnetic field of the hydrogen atoms on the neighbouring carbon atom, causing splitting

Tetrachloromethane (CCl4) as it contains no hydrogen atoms Others include deuterotrichloromethane (CDCl3), D2O, and C6D6, each of which contains deuterium

Because hydrogen atoms in the solvent would interfere with the signals from the other hydrogen atoms in the sample solution So deuterium is used instead