Structure determination
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What is NMR spectroscopy
Nuclear magnetic resonance spectroscopy
Summarise how NMR works
• A magnetic field is applied to a sample, which is surrounded by a source of radio waves and a radio receiver. • This generates an energy change in the nuclei of atoms in the sample that can be detected. • Electromagnetic energy is emitted which can then be interpreted by a computer.
Which carbon isotope does carbon NMR use and why
• Carbon-13 because it has a nuclear spin whereas carbon-12 does not. • This is because carbon-12 has an even number of nucleons.
What does the NMR instrument produce
A graph of energy absorbed (from the radio signal) vertically against chemical shift horizontally
Why don’t all of the carbon-13 atoms in a molecule resonate at the same magnetic field strength
• Carbon atoms in different functional groups feel the magnetic field differently. • This is because all nuclei are shielded from the external magnetic field by the electrons that surround them. • Nuclei with more electrons around them are better shielded. • The greater the electron density around a carbon-13 atom, the smaller the magnetic field felt by the nucleus and the lower the frequency at which it resonates.
What are the units for chemical shift
Parts per million (ppm)
What compound do we use to calibrate/compare other NMR values to
Tetramethylsilane (TMS) which has a chemical shift that we define as 0
What is the key principle that CNMR works on
That carbon atoms in different environments will give different chemical shift values.
Why do the two carbons in ethanol have different chemical shift values
• They are in different environments. • The carbon that is bonded to the oxygen has a greater chemical shift value because the oxygen is electronegative and so draws electrons away from this carbon, causing it to become deshielded. • The other carbon is surrounded by more electrons and therefore is more shielded so has a smaller chemical shift value.
Why is it easier to get a HNMR spectrum than a CNMR spectrum
Nearly all hydrogen atoms are 1H whereas not many carbon atoms are 13C
What are the two types of NMR
• CNMR (carbon NMR) • HNMR (proton NMR)
How does HNMR work
• Hydrogen atoms attached to different functional groups feel the magnetic field differently. • this is because all nuclei are shielded from the external magnetic field by the electrons that surround them. • Nuclei with more electrons around them are better shielded. • The greater the electron density around a hydrogen atom, the smaller the chemical shift value.
What does the height of the peaks on a HNMR spectrum tell us
The relative number of each type of hydrogen.
Describe the relationship between how close the hydrogen atoms are to electronegative atoms and the chemical shift
The closer the H atoms are to very electronegative atoms or double C bonds, the greater the chemical shift
Why is tetramethylsilane used to compare chemical shift values
• All hydrogen atoms are in the same environment • All carbon atoms are in the same environment • it is inert • it is non-toxic • it is easy to remove from the sample
Why does spin-spin coupling occur
The applied magnetic field felt by the hydrogen atoms is affected by the magnetic field of the hydrogen atoms on the neighbouring carbon atoms.
What is the n+1 rule for HNMR
n hydrogens on an adjacent carbon atom will split a peak into n+1 smaller peaks
What solvents can be used for HNMR and why
-Tetrachloromethane CCl4 which is useful because it contains no hydrogen atoms. - Solvents containing deuterium are also used because whilst deuterium is an isotope of hydrogen, it does not produce an NMR signal in the same range as hydrogen.
What 4 things are involved in interpreting a HNMR spectra
1) number of signals 2) relative intensity of signals 3) Splitting pattern of signals 4) position of signals