13 Handouts - Chapter 13 Nuclear Magnetic Resonance...

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Chapter 13 Nuclear Magnetic Resonance Spectroscopy
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Chapter 13 2 Introduction NMR is the most powerful tool available for organic structure determination. Originally discovered in the 1940s by two physicists – Purcell and Bloch (Nobel Prize for Physics, 1952) It is used to study a wide variety of nuclei: ¾ 1 H ¾ 13 C ¾ 15 N ¾ 19 F ¾ 31 P =>
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Chapter 13 3 Nuclear Spin • A nucleus with an odd atomic number or an odd mass number has a nuclear spin. • The spinning charged nucleus generates a magnetic field. =>
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Chapter 13 4 External Magnetic Field When placed in an external field, spinning protons act like bar magnets. =>
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Chapter 13 5 Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field. A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. =>
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Chapter 13 6 Δ E and Magnet Strength • Energy difference is proportional to the (applied) magnetic field strength (B 0 ). Δ E = h ν = γ h B 0 2 π • Gyromagnetic ratio, γ , is a constant for each nucleus (26,753 s -1 gauss -1 for H). • From above: = γ B 0 2 π • In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. • Low energy, radio frequency. It is operationally easier to keep this constant and vary the magnetic field strength. =>
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Chapter 13 7 Magnetic Shielding • If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. • But protons are surrounded by electrons that shield them from the external field. • Circulating electrons create an induced magnetic field that opposes the external magnetic field. =>
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Chapter 13 8 Shielded Protons Magnetic field strength must be increased for a shielded proton to flip at the same frequency. =>
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Chapter 13 9 Protons in a Molecule Depending on their chemical environment, protons in a molecule are shielded by different amounts. =>
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Chapter 13 10 NMR Signals •T h e number of signals shows how many different kinds of protons are present. h e location of the signals shows how shielded or deshielded the proton is. h e intensity of the signal shows the number of protons of that type. • Signal splitting shows the number of protons on adjacent atoms (discussed later). =>
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Chapter 13 11 The NMR Spectrometer =>
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Chapter 13 12 The NMR Graph => What’s this?
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This note was uploaded on 04/08/2008 for the course CHEM 224 taught by Professor Pearson during the Spring '08 term at Case Western.

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13 Handouts - Chapter 13 Nuclear Magnetic Resonance...

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