11. Proton NMR - 2010 Department of Chemistry, The...

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© 2010 Department of Chemistry, The University of Western Ontario 11.1 11. Proton NMR (text 12.5 – 12.11) A. Proton Signals Like 13 C, 1 H atoms have spins of ±½ and are NMR-active. Non-equivalent protons in a molecule will absorb a different energy of radiofrequency energy. Absorptions are plotted as chemical shifts that typically range from 0 – 13 ppm. For example, methyl acetate contains two 1 H environments. In methyl acetate, the peak at 3.7 ppm is due to the three protons of the methoxy group, while the 2.0 ppm peak is due to the three protons of the acyl CH 3 group. O O H H H H H H See textbook “How To 12.2” for more information on how to determine the number of proton signals.
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11.2 Chemical shifts are determined by the type, hybridization, and electronegativity of the atom that the H is bonded to. 1 H chemical shifts give us the same structural information for protons that 13 C shifts do for carbon atoms. However, 1 H NMR has two additional features that make increase its usefulness for the elucidation of chemical structures: signal area and signal splitting .
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11.3 A. Signal Area The area (integration) under a signal peak is proportional to the number of protons contributing to that peak. NMR instruments have an automatic integrator that displays the area of a peak as a vertical line superimposed on the spectrum. The amount of vertical displacement is proportional to the area. o Above, the relative areas of a : b are 23:67, or 1:3. o If we know that the formula of the compound is C 6 H 12 O 2 , a must correspond to 3 H, and b to 9H. By combining the NMR data with the formula and the IR spectrum, which would indicate the presence of an ester, we conclude that the molecule must be tert -butyl acetate.
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This note was uploaded on 05/01/2011 for the course CHEMISTRY Chemistry taught by Professor Dr.felixlee during the Fall '11 term at UWO.

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11. Proton NMR - 2010 Department of Chemistry, The...

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