Structure Determination
32.1 Nuclear Magnetic Resonance (NMR)
32.1 nuclear magnetic resonance (NMR) spectroscopy
Spectroscopy:
- NMR is used to help determine the structure of 32.2 proton NMR
organic molecules. 32.3 interpreting proton, 1H NMR, spectra
- 13C gives a simpler spectrum than 1H.
- The larger the shielding around the nucleus, the smaller the magnetic field felt by the nucleus and the
lower frequency that it resonates at.
- The chemical shift is measured in parts per million, tetramethylsilane (TMS) is used as zero as a
standard.
- TMS is used as it is inert, non-toxic, and easy to remove from the sample
32.2 Proton NMR:
- The greater the electron density around a hydrogen the smaller the chemical shift.
- Shifts in proton NMR are much smaller than 13C.
- The further away a hydrogen atom is from an electronegative atom the smaller its chemical shift.
- In proton NMR the area under the peak is proportional to the number of hydrogen atoms in each
environment. This is shown by an integration trace.
32.3 Interpreting Proton, 1H, NMR Spectra:
- Spin-spin coupling is why the peaks can split into smaller peaks
- The amount of splitting depends on the number of hydrogen atoms on the neighbouring carbon atoms.
- The number of peaks is found by the n + 1 rule:
- N hydrogens on an adjacent carbon atom will split a peak into n + 1 smaller peaks
- If there are 2 peaks, they will split in a 1:1 ratio. This is called a duplet
- If there are 3 peaks, they will split in a 1:2:1 ratio. This is called a triplet
- If there are 4 peaks, they will split in a 1:3:3:1 ratio. This is called a quartet.
- The solvents used cannot contain any hydrogen as that would swamp the signals from the sample.
- A commonly used solvent is tetrachloromethane CCl4.
- Some solvents contain deuterium (2H) but this is expensive such as CDCl3, D2O, C6D6
32.1 Nuclear Magnetic Resonance (NMR)
32.1 nuclear magnetic resonance (NMR) spectroscopy
Spectroscopy:
- NMR is used to help determine the structure of 32.2 proton NMR
organic molecules. 32.3 interpreting proton, 1H NMR, spectra
- 13C gives a simpler spectrum than 1H.
- The larger the shielding around the nucleus, the smaller the magnetic field felt by the nucleus and the
lower frequency that it resonates at.
- The chemical shift is measured in parts per million, tetramethylsilane (TMS) is used as zero as a
standard.
- TMS is used as it is inert, non-toxic, and easy to remove from the sample
32.2 Proton NMR:
- The greater the electron density around a hydrogen the smaller the chemical shift.
- Shifts in proton NMR are much smaller than 13C.
- The further away a hydrogen atom is from an electronegative atom the smaller its chemical shift.
- In proton NMR the area under the peak is proportional to the number of hydrogen atoms in each
environment. This is shown by an integration trace.
32.3 Interpreting Proton, 1H, NMR Spectra:
- Spin-spin coupling is why the peaks can split into smaller peaks
- The amount of splitting depends on the number of hydrogen atoms on the neighbouring carbon atoms.
- The number of peaks is found by the n + 1 rule:
- N hydrogens on an adjacent carbon atom will split a peak into n + 1 smaller peaks
- If there are 2 peaks, they will split in a 1:1 ratio. This is called a duplet
- If there are 3 peaks, they will split in a 1:2:1 ratio. This is called a triplet
- If there are 4 peaks, they will split in a 1:3:3:1 ratio. This is called a quartet.
- The solvents used cannot contain any hydrogen as that would swamp the signals from the sample.
- A commonly used solvent is tetrachloromethane CCl4.
- Some solvents contain deuterium (2H) but this is expensive such as CDCl3, D2O, C6D6
