Understanding NMR Spectroscopy Basics

1. NMR SPECTROSCOPY

2. 1H NMR SPECTROSCOPY

3. NUCLEAR SPIN • Protons and neutrons can be regarded as spinning about their axis. • In many atoms these spins are paired against each other and so the nucleus has no overall spin (e.g. 12C). • In some atoms (e.g. 1H and 13C) the nucleus has an overall spin. • A nucleus that spins generates a magnetic field. • The direction of the magnetic field depends which way the nucleus spins.

4. NUCLEAR SPIN • Usually the two possible spin states of the nucleus have the same amount of energy. • However, in a magnetic field, the two spin states have different energies. Energy Magnetic field opposed to applied field Applied magnetic field Energy gap corresponds to frequency of radiowaves Magnetic field in same direction as applied field

5. butanone

6. EQUIVALENT H's • In a spectrum, there is one signal for each set of equivalent H atoms i.e. those H atoms in the same environment • The intensity or area of each signal being proportional to the number of equivalent H atoms it represents.

7. To identify hydrogen environments we need to look at the molecule: Draw out the molecule in full. Circle the first environment in one colour Circle the next environment in another colour Continue this process till there are no more hydrogen environments Then work out the number of protons in each hydrogen environment to give you the peak ratio`s (peak areas).

8. 2 sets of equivalent H’s: ratio 6:2 (3:1) 4 sets of equivalent H’s: ratio 3:1:2:3 5 sets of equivalent H’s: ratio 3:1:2:2:3 4 sets of equivalent H’s: ratio 6:1:2:3

9. For each of the following compounds, predict the number of signals and the relative intensity of the signals. a) methylpropene • propene • 2-chloropropane • propanone • methylamine • ethyl propanoate • 1,2-dibromopropane • dimethylethyl propanoate • but-2-ene

10. 2 signals: ratio 6:2 (3:1) 1 signal 3 signals: ratio 2:1:3 2 signals: ratio 3:2 2 signals: ratio 6:1 4 signals: ratio 3:2:2:3

11. 3 signals: ratio 3:2:9 3 signals: ratio 2:1:3 2 signals: ratio 6:2 (3:1)

12. SOLVENTS & CALIBRATION • Samples are dissolved in solvents free of 1H atoms, e.g. CCl4, CDCl3. • A small amount of TMS (tetramethylsilane) is added to calibrate the spectrum. • It is used because: • its signal is away from all the others • it only gives one signal • it is non-toxic • it is inert • it has a low boiling point so is easy to remove

13. CHEMICAL SHIFT • Thed is a measure in parts per million (ppm) of how far the magnetic field required for absorption is shifted away from that for TMS. • The ddepends on what other atoms/groups are near the H – more electronegative groups gives a greater shift.

14. SPIN-SPIN COUPLING Coupling / Splitting / Multiplicity Number of H’s next door +1

15. Data from AQA datasheet

16. This is the table that should be used when doing questions on NMR

17. Example 1 2 1 3

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32. Example 2 3 4 1 2

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46. n+1 0 H next door singlet (s) 1 H next door doublet (d) 2 H next door triplet (t) 3 H next door quartet (q) more H next door multiplet (m)

47. SPIN-SPIN COUPLING