nuclear magnetic resonance spectroscopy (3.3.15)
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what has the development of a range of analytical techniques enabled?
the structures of new compounds to be confirmed
what does nuclear magnetic resonance (NMR) do?
give information about the position of 13 C or 1 H atoms in a molecule
what is the difference between 13 C and 1 H NMR?
13 C NMR gives simpler spectra than 1 H NMR
what is the δ scaled used for?
recording chemical shift in parts per million (ppm)
what does chemical shift measure?
how far the frequency of the proton signal has shifted away from that of TMS
what does chemical shift depend on?
the molecular environment
what do integrated spectra indicate?
the relative numbers of 1 H atoms in different environments
how are 1 H NMR spectra obtained?
using samples dissolved in deuterated solvents eg. CDCl3 or CCl4
what are the features of the solvents used to obtain 1 H NMR spectra?
they are inert and don’t contain any 1 H atoms so they don’t give any peaks in the NMR spectrum
which solvents are used to obtain 13 C NMR spectra?
the same as for 1 H
how is an NMR spectrum calibrated?
using a small amount of tetramethylsilane/Si(CH3)4 (TMS) as a standard which is added to the sample
why is TMS used as a standard?
• only gives one signal • signal is away from all the other H signals • gives strong signal so only a small amount needed • non toxic • inert • low boiling point so easily removed from the sample
how can the standard be identified on an NMR spectrum?
it forms a tall single peak at 0
how do you know from a 1 H NMR spectrum the number of non-equivalent H atoms present?
each type of equivalent H atom produces a separate peak (look at number of peaks)
how do you know from a 1 H NMR spectrum the number of equivalent H atoms for each peak (integration value)?
the area under the peak (look at ratios of different peaks)
how do you know from a 1 H NMR spectrum the local environment of the H atoms?
look at the position of the peaks
what do you have to remember when identifying the number of sets of equivalent H atoms that a molecule has?
just because there are two of the same group eg. CH3, it doesn’t mean those H atoms are equivalent - it also depends what those groups are bonded to
what is the difference between a high-res NMR spectrum and a low-res NMR spectrum?
high-res NMR spectra include peak splitting which tells you about the number of H atoms on adjacent C atoms
what does a singlet peak indicate on an NMR spectrum?
adjacent to C atom with no H atoms attached
what does a doublet peak indicate on an NMR spectrum?
adjacent to C atom with one H atom attached
what does a triplet peak indicate on an NMR spectrum?
adjacent to C atom with 2 H atoms attached
what does a quartet peak indicate on an NMR spectrum?
adjacent to C atom with 3 H atoms attached
what does the n+1 rule refer to?
n = number of non-equivalent H atoms in neighbouring chemical environment (adjacent C atom) n+1 = number that peak is split into
what does the chemical shift of a signal give information about?
the chemical environment of the H atoms in the molecule (look at data sheet) - the closer the H atoms to very electronegative atoms or double bonds, the greater the chemical shift