CH14-lecture

CH14-lecture - Nuclear Magnetic Resonance Spectroscopy 1H...

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1 Nuclear Magnetic Resonance Spectroscopy 1 H NMR is used to determine the type and number of H atoms in a molecule. 13 C NMR is used to determine the type of carbon atoms in the molecule. NMR uses radio waves which have long wavelengths, and thus low energy and frequency (~10 -6 kcal/mol) to excite nuclear spins .
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2 When a charged particle such as a proton spins on its axis, it creates a magnetic field . Thus, the nucleus can be considered to be a tiny bar magnet. In the presence of a magnetic field B 0 the normally random ordering of the nuclei orient so that more nuclei are oriented with the applied field than against because this arrangement is lower in energy.
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3 A second magnetic field can be applied to FLIP some of the nuclei spins to the higher energy state (in opposition to the first magnetic field). However, this “flipping” will only happen when the second energy source (magnetic field) matches the energy difference ( Δ E) between these two states.
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4 The larger the magnetic field B 0 , the larger the energy gap.
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5 The Experimental Setup
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6 Are nuclei bare? Electron density shield nuclei from “feeling” the magnetic field B 0 . This changes the energy gap for spin flipping and hence the radio frequency for the second magnetic field. The frequency at which a particular proton absorbs is determined by its electronic environment .
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7 The less shielded the nucleus (the less electron density around the nucleus), the more of the magnetic field B 0 it feels. A deshielded nucleus needs a higher frequency (more energy) to achieve resonance. Higher frequency is to the left in an NMR spectrum, toward higher chemical shift—so deshielding shifts an absorption downfield. Protons near electronegative atoms are deshielded, so they absorb downfield.
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8
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9 1 H, 13 C, 19 F, 31 P, 2 H, and 14 N give rise to NMR signals. 12 C does not.
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An NMR spectrum is a plot of the intensity of a peak against its chemical shift, measured in parts per million (ppm) . 1
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This note was uploaded on 03/08/2012 for the course CHEM 351 taught by Professor Staff during the Fall '08 term at BYU.

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CH14-lecture - Nuclear Magnetic Resonance Spectroscopy 1H...

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