3.15 Nuclear magnetic resonance spectroscopy
Chemists use a variety of techniques to deduce the structure of compounds. In this section, nuclear magnetic resonance spectroscopy is added to mass spectrometry and infrared spectroscopy as an analytical technique.
The emphasis is on the use of analytical data to solve problems rather than on spectroscopic theory.
Nuclear magnetic resonance (NMR) spectroscopy
NMR spectroscopy gives information about the position of 13C or 1H atoms in a molecule.
13C NMR gives simpler spectra than 1H NMR.
The use of the δ scale for recording chemical shift.
Chemical shift depends on the molecular environment.
Integrated spectra indicate the relative numbers of 1H atoms in different environments.
1H NMR spectra are obtained using samples dissolved in proton-free solvents (e.g. deuterated solvents or CCl4)
Tetramethylsilane (TMS) is used as a standard.
Students should be able to:
• explain why TMS is a suitable substance to use as a standard
• use 1H NMR and 13C NMR spectra and chemical shift data from the Chemistry Data Booklet to suggest possible structures or part structures for molecules
• use integration data from 1H NMR spectra to determine the relative numbers of equivalent protons in the molecule
• use the n+1 rule to deduce the spin–spin splitting patterns of adjacent, non-equivalent protons, limited to doublet, triplet and quartet formation in simple aliphatic compounds.
Assessment and practical opportunities
Students should be able to use data in the Chemistry Data Booklet to suggest possible structures for molecules.