AQA Chem4: N.M.R

• NMR stands for Nuclear Magnetic Resonance Spectroscopy
• there are two isotopes of carbon, 12 and 13, only 1% are 13 however modern instruments are sensitive enough to obtain a carbon 13 spectrum
• carbon atoms in different environments are affected differently  the magnetic field
• C atoms are surrounded by electrons in adjacent covalent bonds and the carbon nulei are shielded by these electrons
• the degree of shielding depends on the electron density surrounding the nucleus
• NMR produces a graph of energy absorbed vertically against chemical shift horizontally
• chemical shift is measured in parts per million (ppm) from a defined zero related to a compound called tetramethylsilane, TMS
• the number of peaks on the graph is the number of different carbon environments in the molecule 

H (proton) NMR gives us information on:

• the number of hydrogen atoms in the compound
• the position of the H atoms in the compound
• the number of different environments of the H atoms

• the sample is dissolved in a proton free solvent e.g. CCl4 or CDCl3 - this is so they do not produce any absorptions which interfere with the spectra
• the NMR spectra are recorded with the peaks giving information about the H atoms in the molecule

• chemical shift is measured relative to an internal standard
• this is tetramethylsilane (CH3)4Si, TMS
• it has 12 equivalent H atoms and so produces a single intense peak
• its peak is upfield from nearly all other H atom peaks
• it is non-toxic and inert and easily vapourised

Chemical Shift:

• chemical shift is a measure of the movements caused by shielding and deshielding, atoms with high shielding are moved upfield
• electronegative atoms cause deshielding so the peak moves downfield

Information about the peaks:

• the number of peaks shows the number of different H atom environments
• the area under the peaks shows the number of equivalent H atoms, it is quoted as a ratio or given as an intergration trace
• the position of the peaks shows information about the position of H atoms in the molecule using the H NMR shift data

High resolution H NMR involves peak splitting:

• splitting of peaks occurs when neighbouring non-equivalent hydrogen atoms are present
• non-equivalent adjacent H atoms interact (couple) with each other
• this results in the signal being split into several peaks
• splitting is only observed between nuclei with different chemical shifts
• only H atoms on adjacent C atoms should be considered
• O-H groups are non-splitting
• H atoms without non-equivalent neighbouring H atoms are not split so single peaks are produced
• the number of peaks is equivalent to the number of non-equivalent adjacent protons (n+1 rule)
• a single peak is a singlet, a double peak is a doublet, a triple peak is a triplet and a quadruple peak is a quartet
• example: a CH3-CH2 would create a triplet peak and a quartet peak