Chap 13 NMR

Chap 13 NMR - Chapter 13 NMR Spectroscopy Recall that electrons have two spin states spin up(1/2 and spin down-1/2 Similarly nuclei have spin

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Chapter 13 NMR Spectroscopy
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Recall that electrons have two “spin states”: spin up (1/2) and spin down (-1/2). Similarly nuclei have spin quantum states…. Nuclei of interest. By coincidence, each of these has two states, ½ and – ½ . By the way, note that 14 N has three states: -1, 0, 1. They each differ by 1.
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The nuclear spin quantum number determines how many spin states there are Spectroscopy involves using energy to excite a system from one state (ground state) to another of higher energy (excited state). Nuclear spin quantum number Quantum numbers of spin states Number of spin states 0 0 1 1/2 - ½ , ½ 2 1 -1, 0, 1 3 3/2 -3/2, -1/2, 1/2, 3/2 4
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Normally, nuclei in different spin states have the same energy. Can not do spectroscopy. We need to have a ground state and excited state. In a magnetic field they have different energies. Now we can do spectroscopy…. We apply a magnetic field and create a ground state and a higher energy excited state (perhaps more than one).
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5249 CHEM 51. TR9A KUNJAPPU 148NE 5250 CHEM 51. TR9J HOWELL 133NE 5251 CHEM 51. TR9C FANG 133NE 5252 CHEM 51. TR9E KUNJAPPU 148NE 5253 CHEM 51. TR9G HANS 1310N 5255 CHEM 51. TR9B METLITSKY 1310N 5278 CHEM 51. TR9K LUSHTAK 148NE 5279 CHEM 51. TR9D MOLLICA 1310N Final Exam Schedule, Thursday December 18, 8AM
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Apply a strong external field…. . Both orientations have same energy if no magnetic field
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Figure 13.3, p.497
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Figure 13.4, p.499
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•Two kinds of hydrogens in methyl acetate: two peaks. (Peak at zero is tetramethyl silane to standardize the instrument. ) Chemical shift: where on horizontal axis the signal from a nucleus occurs. Question: What causes nuclei to appear with different chemical shift?? Answer: the sigma bonding electrons in a molecule will be set in motion to establish a magnetic field that opposes the external magnetic field. The nuclei are shielded . •The shielded nuclei experience less of a magnetic field, closer energy states. •The shielded nuclei require less energy to excite and their signal occurs to the right in the spectrum. More shielded, nuclei experience lesser magnetic field. Less energy to excite. Example of nmr spectrum: methyl acetate.
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p. 481
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Doing nmr spectroscopy: the magnetic field creates the energy difference between the spin states of the nucleus and Radio waves provide the energy needed to excite the nucleus from the lower energy state to the excited state. More Shielding Simplifying The energy supplied by the radio waves has to match the energy gap created by the magnetic field. We can vary either the magnetic field or the frequency of the radio waves to match the exciting radiation energy with the energy needed to reach the excited state.
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Hold the external magnetic field constant, vary radio frequency. Less energy needed to excite the nuclei when more shielded . Hold excitation energy (radio waves) constant,
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This note was uploaded on 12/05/2011 for the course C 341 taught by Professor Skagg during the Spring '11 term at Indiana.

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Chap 13 NMR - Chapter 13 NMR Spectroscopy Recall that electrons have two spin states spin up(1/2 and spin down-1/2 Similarly nuclei have spin

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