Chapter13 - NMR Spectroscopy Used to determine relative...

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1 1 NMR Spectroscopy ± Used to determine relative location of atoms that have a nuclear spin within a molecule ± Most helpful spectroscopic technique in organic chemistry ± Related to MRI in medicine (Magnetic Resonance Imaging) ± Maps carbon-hydrogen framework of molecules ± Depends on very strong magnetic fields 2 Magnetic nuclei 1. If the number of neutrons and the number of protons are both even, then the nucleus has NO spin and is nonmagnetic. 2. If the number of neutrons plus the number of protons is odd, then the nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2) 1 H ½ , 13 C ½ , 19 F ½ , 31 P ½, 3. If the number of neutrons and the number of protons are both odd, then the nucleus has an integer spin (i.e. 1, 2, 3) 2 H 1, 14 N 1 3 M 13.1 Nuclear Magnetic Resonance Spectroscopy ± 1 H or 13 C nucleus spins and the internal magnetic field aligns parallel to or against an aligned external magnetic field ± Parallel orientation is lower in energy making this spin state more populated ± Radio energy of exactly correct frequency (resonance) causes nuclei to flip into anti- parallel state NMR experiment 7 Tesla superconducting magnet of a 300 MHz NMR spectrometer
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2 5 Radio frequency needed for resonance is proportional to the field strength ± Radio energy of exactly correct frequency (resonance) causes nuclei to flip into anti-parallel state ± Energy needed is related to type of nucleus ( 13 C has smaller γ than 1 H) and its environment 1 H resonates at 200 MHz in a 4.7 Tesla field 1 H, at 400 MHz in a 9.4 Tesla field M 13.2 The Nature of NMR Absorptions ± The magnetic field caused by circulating electrons shield the nucleus from the strong magnet’s field ± Different signals appear for nuclei in different environments 13 C NMR spectrum of methyl acetate TMS reference compound: SiMe 4 Si H C H H H C H H H C H H H
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Chapter13 - NMR Spectroscopy Used to determine relative...

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