057_Ho_16_RG_13_07 - Chem 057 J. Walcott CENTER FOR...

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Chem 057 CENTER FOR LEARNING AND TEACHING Handout #16 J. Walcott LEARNING STRATEGIES CENTER Fall 2007 Review Guide #13 Lectures: 11/19/07 – 12/3/07: Chapters 12 and 13 I. Nuclear Magnetic Resonance (NMR) Spectroscopy A. When certain nuclei are placed in a strong magnetic field, their spins cause them to behave like tiny magnets (magnetic dipoles) which orient either with or against the field. The nuclei with magnetic dipoles aligned parallel to the applied field are lower in energy, and those with magnetic dipoles aligned antiparallel to the applied field are higher in energy. If the sample is subjected to electromagnetic radiation equal to this energy difference, energy can be absorbed by nucei in the lower energy state causing the nuclei to invert or “flip” their spins. The energy difference, ± E, lies in the radio wave region of the electromagnetic spectrum, and depends on the strength of the applied magnetic field. When radiation having this energy is applied, a resonant condition is established which causes absorption of the radiation that can be measured. As the applied magnetic field increases, ± E becomes larger. B. The absorption frequency for a nucleus is called its chemical shift . The chemical shift of a nucleus depends upon its chemical environment. C. In 1 H NMR spectroscopy, the magnetic field experienced by a particular proton is influenced by the electrons nearby. 1. If the electron density near the proton is high, the proton is shielded from the applied magnetic field and chemical shifts are found "upfield" (right side) on the spectrum . 2. If the electron density near the proton is low, the proton is deshielded from the applied magnetic field and chemical shifts are found “downfield” (left side) on the spectrum. Protons are deshielded if there are electronegative (electron withdrawing) groups nearby in the molecule which attract electron density to themselves and away from the protons. 3.
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057_Ho_16_RG_13_07 - Chem 057 J. Walcott CENTER FOR...

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