NMR - CH 19: NMR Spectroscopy HW: 19-7, 19-11, 19-31 Proton...

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CH 19: NMR Spectroscopy HW: 19-7, 19-11, 19-31 Proton NMR Spectrum
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NMR Spectroscopy-The Physical Principles N S Based upon nuclear spin Spinning charges produce magnetic fields S N 1 H, 13 C, 19 F, and 31 P have two possible quantum mechanical spin states, +1/2 and -1/2 *Key Concepts
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NMR Spectroscopy-The Physical Principles In the absence of an applied magnetic field (B 0 ) the spin states have the same E. When a magnetic field is applied the states split in E. This opens up transitions (absorptions) that can be induced with radiofrequencies. These are detected during an NMR experiment and are called resonances .
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General Instrument Schematic • Spins are aligned with the large magnets. • The magnetic component of the RF input at the right frequency causes some spins to flip. • Spin flip is detected by the RF output coils.
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NMR Spectroscopy-The Physical Principles The energy gap is very small, about 0.1 cal/mol for an applied field of 20 Tesla. Compare this to a typical IR transition (1-10 kcal/mol) and electronic transition (1000 kcal/mol).
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Magnetic Fields Magnetic field is usually given in units of gauss or Tesla. These are units of energy. 1 T = = N/qv The magnetic field is the force acting on a charged particle (q) moving with velocity (v) through a magnetic field. 1 G = 10 -4 T Newton (coulomb)(meter/sec)
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NMR Spectroscopy-The Physical Principles The frequency of the transition is ν 0 = γ B 0 2 π where B 0 is the applied magnetic field and γ is the magnetogyric ratio. γ has a different value for each type of nucleus. Example 19-1 Many proton NMR instruments employ a magnet that provides a field strength of 4.69 T. At what frequency would the hydrogen nucleus absorb in such a field? ν 0 = γ B 0 2 π = (2.68 x 10 8 T -1 s -1 )(4.69 T)/2 π = 2.00 x 10 8 s -1 = 200 MHz Resonance or Larmor frequency
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NMR Spectroscopy-The Physical Principles The dependence of the splitting energy on magnetic field presents a small problem that we will address later: If two researchers want to compare their data using magnets of different strengths, they have to adjust for that difference.
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NMR Spectroscopy The magnetic field strength can be varied at constant RF frequency or vice versa.
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NMR Spectroscopy-The Physical Principles Relaxation Processes in NMR If the nuclei that become aligned against the magnetic field do not “relax” back, then the signal will decrease or “saturate”. Relaxation processes should be very fast to avoid saturation. Unfortunately, the faster the relaxation, the more NMR lines are broadened.
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NMR Spectroscopy-The Physical Principles Line Broadening We examined several mechanisms of line broadening in CH 8 (pp. 220): Doppler and Pressure Broadening. *Key Concept: The Heisenberg Uncertainty Principle is
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This note was uploaded on 02/23/2011 for the course CHEM 4181 at Colorado.

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NMR - CH 19: NMR Spectroscopy HW: 19-7, 19-11, 19-31 Proton...

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