Nuclear Magnetic Resonance (NMR) Spectroscopy

1. Nuclear Magnetic Resonance (NMR) Spectroscopy Radio waves do the trick.

2. Bo With external magnetic field Spins aligned No external magnetic field Spin alignment random Nuclei can be thought of as tiny magnets.

3. DE energy DE DE energy Magnetic field strength When a nucleus occupying the α spin state is subjected to radio waves, an absorption can take place. β spin state magnetic field is applied α spin state

4. 1H NMR Spectrum An NMR spectrum is a plot of resonance frequency vs. the intensity of rf absorption by the sample.

5. Number of Signals homotopic enantiotopic diastereotopic

6. Exercise 1: Identify the number of signals expected in the 1H NMR spectrum of the following compounds. 1 signal 4 signals 4 signals 1 signal 5 signals 2 signals 2 signals 3 signals

7. Exercise 2: Determine whether the two protons shown in red are homotopic, enantiotopic, or diastereotopic. diastereotopic diastereotopic enantiotopic homotopic homotopic enantiotopic

8. Position of Signals The x-axis is a frequency scale but is normalized to be independent of the field strength. spectrometer operating at 300 MHz spectrometer operating at 60 MHz 2181 Hz larger than that of TMS 436 Hz larger than that of TMS

9. deshielded shielded

11. The different degree of shielding experienced by the protons is due to electron density.

15. The Shoolery’sadditivity rules: 0.6 + 2.5 + 0.3 = 3.4 exp = 3.505 ppm

17. Intensity of Signals

18. Spin-Spin Splitting (Coupling) If n is the number of neighboring protons, then the multiplicity will be n + 1.

19. What causes splitting?

20. When signal splitting occurs, the distance between the individual peaks of a signal is called the coupling constant, or J value. It is measured in hertz and is independent of field strength.