Nuclear Magnetic Resonance (NMR) Spectroscopy
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Transcript of Nuclear Magnetic Resonance (NMR) Spectroscopy
1Nuclear Magnetic Resonance (NMR) SpectroscopyRadio waves do the trick.
2Nuclei can be thought of as tiny magnets.
No external magnetic fieldSpin alignment random
With external magnetic fieldSpins alignedBo3When a nucleus occupying the spin state is subjected to radio waves, an absorption can take place.
energyDE spin state spin statemagnetic fieldis appliedenergyMagnetic field strengthDEDE
41H NMR SpectrumAn NMR spectrum is a plot of resonance frequency vs. the intensity of rf absorption by the sample.
5Number of Signals
enantiotopic6Exercise 1:Identify the number of signals expected in the 1H NMR spectrum of the following compounds.
1 signal2 signals
2 signals4 signals4 signals5 signals3 signals7Exercise 2:Determine whether the two protons shown in red are homotopic, enantiotopic, or diastereotopic.
homotopicenantiotopicdiastereotopicdiastereotopicenantiotopichomotopic8The x-axis is a frequency scale but is normalized to be independent of the field strength.Position of Signals
spectrometer operating at 300 MHz2181 Hz larger than that of TMS
spectrometer operating at 60 MHz436 Hz larger than that of TMS
11The different degree of shielding experienced by the protons is due to electron density.
15The Shoolerys additivity rules:
0.6 + 2.5 + 0.3 = 3.4 exp = 3.505 ppm16
Intensity of Signals18If n is the number of neighboring protons, then the multiplicity will be n + 1.
Spin-Spin Splitting (Coupling)19What 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.