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

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

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

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

Number of Signals homotopic enantiotopic diastereotopic

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

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

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 2181 Hz larger than that of TMS spectrometer operating at 60 MHz 436 Hz larger than that of TMS

deshielded shielded

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

The Shoolery’s additivity rules: 0.6 + 2.5 + 0.3 = 3.4 exp = 3.505 ppm

Intensity of Signals

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

What causes splitting?

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.