CHE202A - Chapter 13

CHE202A - Chapter 13 - Chapter 13 Nuclear Magnetic...

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Chapter 13 Nuclear Magnetic Resonance (NMR) Spectroscopy
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13-1 Introduction 2 • NMR is the most powerful tool available for organic structure determination. • It is used to study a wide variety of nuclei: 1 H 13 C 15 N 19 F 31 P
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13-2 Theory of Nuclear Magnetic Resonance 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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Which nuclei have a “spin”? If mass # and atomic # are both even, I = 0 and the nucleus has no spin . ( I = spin quantum #) e.g. Carbon-12, Oxygen-16 For each nucleus with a spin, the # of allowed spin states can be quantized: For a nucleus with I, there are 2I + 1 allowed spin states. 1 H, 13 C, 19 F, 31 P all have I = 1/2
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5 External Magnetic Field • When placed in an external field, spinning protons act like bar magnets.
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When a Nucleus aligned with a Magnetic field, B 0 , absorbs radiation frequency (R f ), it can change spin orientation to a higher energy spin state. By relaxing back to the parallel (+1/2) spin state, the nucleus is said to be in Resonance . Hence, NMR
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7 • 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. Two Energy States H 0 lower energy, MORE STABLE! higher energy, LESS STABLE! H 0 α -spin state β -spin state H 0 placed in an external field NO FIELD MAGNETIC FIELD Δ E lower energy higher energy
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8 Δ E and Magnet Strength • Energy difference is proportional to the magnetic field strength. Δ E = γ ( h/2 π )B 0 • Gyromagnetic ratio, γ , is a constant for each nucleus (26,753 s -1 gauss -1 for H). • In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. • Low energy, radio frequency.
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Energy Difference ( Δ E) Between Two Different Spin States of a Nucleus With I=1/2 +1/2 E 400 MHz 300 MHz 200 MHz 100 MHz 23,500 47,000 70,500 104,000 parallel inc. magnetic field strength, Gauss B 0 9
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13-3 Magnetic Shielding by Electrons 10 • 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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11 H 0 S N induced current H induced H 0 H induced induced curculation of electrons
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12 Magnetic field strength must be increased for a shielded proton to flip at the same frequency. Shielded Protons H 0 = 14,092 gauss, "nacked" proton absorbs H 0 H induced = 0.3 gauss H 0 effective field is 14,091.7 gauss H 0 = 14,092 gauss, shielded proton d o e s n o t a b s o r b H induced = 0.3 gauss H 0 = 14,092.3 gauss, shielded proton absorbs 60 MHz 60 MHz 60 MHz
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13 • Depending on their chemical environment, protons in a molecule are shielded by different amounts.
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