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2444-091211 - FT-NMR Time and Frequency Domains Fourier...

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a short blast of radio waves is delivered to the sample and then re-emitted radiation by the sample is monitored over time. Because frequency and time are related by the Heisenberg Uncertainty Principle (like energy and position are), if we know the duration of the radiation pulse precisely we will have many different frequencies present at the same time. All of the nuclei are excited by the pulse, but then they began to "relax" and emit radio waves of an energy that matches their ! E. The result is a free induction decay (FID). The complex FID pattern contains information on all the nuclei that were excited by our radiation pulse, and we can convert theses oscillations in time back to each nucleus' frequency by using a mathematical process called a Fourier transform. http://chemlab.truman.edu/CHEM121Labs/Electronegativity.htm ww.chemie.uni-erlangen.de/ bauer/music3.htm Fourier transform FT-NMR Time and Frequency Domains: Fourier Transform
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ppm 1 0 2 3 4 5 6 7 8 High Field Low Field (downfield) (upfield) Typical proton NMR spectrum Irradiation of the sample which it is in the magnet will lead to absorption by protons, if ! E and H o are in resonance, as described above. For practical reasons, we hold ! E constant (60 MHz, 200 MHz, 600 MHz, etc.) and change H o . As ! E is changed incrementally, each proton will resonate (absorb ! E) for its particular value of H o and we generate a series of absorption peaks for each different proton . This means that each peak will represent a signal for a different proton . The instrument will monitor each absorption and plot it as ! E against H o . If ! E is constant, the change in magnetic field will be very small because each proton generates only a tiny H H . In fact, the change will be measured in Hertz (Hz) Low Field & High Field: The Proton NMR Spectrum
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ppm 1 0 2 3 4 5 6 7 8 High Field Low Field (downfield) (upfield) Typical proton NMR spectrum The change will be measured in Hertz (Hz) while ! E is measured in megahertz (MHz). This means that the signal in Hz is in millionths relative to ! E. We must establish a zero point. We use tetramethylsilane [(Me 3 ) 4 Si; TMS] as an internal standard. This molecule gives rise to one peak for the methyl groups and we measure everything relative to it.
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