chapter13

chapter13 - 13-13-1Organic Organic...

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Unformatted text preview: 13-13-1Organic Organic ChemistryChemistryWilliam H. BrownWilliam H. BrownChristopher S. FooteChristopher S. FooteBrent L. IversonBrent L. Iverson13-13-2Nuclear Magnetic Nuclear Magnetic ResonanceResonanceChapter 13Chapter 1313-13-3Molecular SpectroscopyMolecular SpectroscopyNuclear magnetic resonance (NMR) Nuclear magnetic resonance (NMR) spectroscopyspectroscopy::a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of hydrogen atoms using 1H-NMR spectroscopycarbon atoms using 13C-NMR spectroscopyphosphorus atoms using 31P-NMR spectroscopy13-13-4Nuclear Spin StatesNuclear Spin StatesAn electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2 this spinning charge creates an associated magnetic fieldin effect, an electron behaves as if it is a tiny bar magnet and has what is called a magnetic momentThe same effect holds for certain atomic nucleiany atomic nucleus that has an odd mass number, an odd atomic number, or both also has a spin and a resulting nuclear magnetic momentthe allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus13-13-5Nuclear Spin StatesNuclear Spin Statesa nucleus with spin quantum number Ihas 2I+ 1+ 1spin states; if I= 1/2, there are two allowed spin statesTable 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds1H2H12C13C14N16O31P32SElementNuclear spinquantum number (I )Number ofspin states1/211/212312311/22113-13-6Nuclear Spins in BNuclear Spins in Bwithin a collection of 1H and 13C atoms, nuclear spins are completely random in orientationwhen placed in a strong external magnetic field of strength B, however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed 13-13-7Nuclear Spins in BNuclear Spins in Bfor 1H and 13C, only two orientations are allowed13-13-8Nuclear Spins in BNuclear Spins in BIn an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for 1H is approximately 0.120 J (0.0286 cal)/mol, which corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz)13C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz)13-13-9Nuclear Spin in BNuclear Spin in Bthe energy difference between allowed spin states increases linearly with applied field strength values shown here are for 1H nuclei13-13-1010Nuclear Magnetic ResonanceNuclear Magnetic Resonancewhen nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins are aligned with the applied field in the lower energy statethe nucleus begins to precess and traces out a cone-...
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This note was uploaded on 03/21/2008 for the course CHEM 261 taught by Professor Austell during the Spring '08 term at UNC.

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chapter13 - 13-13-1Organic Organic...

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