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3.3.15 NMR spectroscopy (¹H NMR (information about position of ¹H atoms in…
3.3.15 NMR spectroscopy
¹H NMR
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hydrogen atoms in different functional groups resonate differently; all nuclei are shielded from the external magnetic field by electrons surrounding them; more electrons = more shielding; greater electron density around carbon-13 atom, the lower the frequency it resonates
the further away a hydrogen atom is from an electronegative atom, the smaller its chemical shift
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n+1 rule: n hydrogens on an adjacent carbon atom in a different environment will split a peak into n+1 smaller peaks
e.g. if a hydrogen has an adjacent carbon attached to three hydrogens in a different environment to itself, the peak will be split into 4
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¹³C NMR
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carbon-12 does not have any nuclear spin, but carbon-13 does; only 1% of carbon atoms are carbon-13
carbon atoms in different functional groups resonate differently; all nuclei are shielded from the external magnetic field by electrons surrounding them; more electrons = more shielding; greater electron density around carbon-13 atom, the lower the frequency it resonates
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NMR
scientists have developed a range of analytical techniques which together enable the structures of new compounds to be confirmed
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a magnetic field is applied to a sample which is surrounded by a source of radio waves and a radio receiver; generated an energy change in nuclei of atoms; can be detected; electromagnetic energy is emitted which can be interpreted using a computer
chemical shift δ:
measured in parts per million, ppm
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Tetramethylsilane
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inert, non-toxic, easy to remove from sample
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run in solvent:
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e.g. tetrachloromethane, CCl₄
e.g. deuterium based solvents: CDCl₃, D₂O, C₆D₆
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