Nuclear Magnetic Resonance (NMR) Spectroscopy

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Nuclear Magnetic Resonance (NMR) Spectroscopy. Radio waves do the trick. B o. With external magnetic field Spins aligned. No external magnetic field Spin alignment random. Nuclei can be thought of as tiny magnets. D E. energy. D E. D E. energy. Magnetic field strength. - PowerPoint PPT Presentation

Transcript of Nuclear Magnetic Resonance (NMR) Spectroscopy

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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

homotopic

diastereotopic

enantiotopic6Exercise 1:Identify the number of signals expected in the 1H NMR spectrum of the following compounds.

1 signal

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

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deshieldedshielded10

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

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15The Shoolerys additivity rules:

0.6 + 2.5 + 0.3 = 3.4 exp = 3.505 ppm16

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Intensity of Signals18If n is the number of neighboring protons, then the multiplicity will be n + 1.

Spin-Spin Splitting (Coupling)19What causes splitting?

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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.