11_Spectroscopy_NMR

11_Spectroscopy_NMR - Chem 267 Basic Organic Chemistry II...

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry II 1
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2 frequency used in NMR spectroscopy
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Nuclear Magnetic Resonance (NMR) Spectroscopy “NMR spectroscopy tells the information about the different types of magnetically distinct atoms in a molecule” magnetic field lines shown by iron filings current produces magnetic field atomic magnetic dipole hydrogen atom has “nuclear spin”: a simple, tiny bar magnet 3
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A nucleus is a charged particle in motion 1) any moving charged particle develops a magnetic filed, which can be affected by external magnetic field ( H 0 ); 2) nuclei with an odd mass or odd atomic number have “nuclear spin”, e.g. 1 H, 13 C, 19 F; 3) nuclei with even mass or even atomic number do NOT have “nuclear spin”, e.g. 2 12 C; 4) possible nuclear spin states are quantized and determined by their nuclear spin quantum number I ; 5) for each nucleus, the number of allowed nuclear spin states are (2 I +1); for example, 1 H: I = ½, allowed spin states ( + ½, - ½); 6) if no external magnetic field presents (ground state), all allowed spin states are equal in energy (degenerate), which implies that all allowed spin states are equally populated with atoms; 4
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5 1) if no external magnetic field applies, the nuclear spins will be randomly oriented; 2) if an external magnetic field ( H 0 ) is applied, the orientation of the nuclear spins of 1 H atoms will be changed: a) aligned with H 0 (lower in energy, favoured); b) aligned against H 0 (higher in energy, disfavoured); 3) the number of both types of aligned atoms can be predicted by Boltzmann distribution; 4) there will always be a small excess of 1 H atoms aligned with the external magnetic field; In the simplest case, 1 H system. .. 12 degenerate spins No field With field N S N S H 0
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NMR Experiment Scheme 6
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7 The strength of magnetic field is proportional to the resonance frequency 1) without external magnetic field, all possible spin states are degenerate; 2) when external field applied, spin states begin to show different orientations; 3) The field strength and resonance frequency are linearly proportional; 4) chemical shift δ : a field-independent number to compare the positions of hydrogen absorption peaks;
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8 1-bromobutane in different external magnetic fields 60 MHz 400 MHz “400MHz NMR spectrometer provides better resolution, and keeps the same chemical shift values”
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9 An NMR spectrum is plot of the radio frequency applied against absorption easier to flip harder to flip
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10 Integrated NMR Spectra
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11 Chemical Shift: the position of an NMR absorption “the position of an NMR signal depends on the local electronic environment of the nucleus” if the hydrogen nucleus is here, it will be easier to resonate (deshielding) if the hydrogen nucleus is here, it will be harder to resonate (shielding)
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12 3.05 ppm CH Cl H H 7.27 ppm Cl Cl Cl 5.30 ppm Cl H Cl more deshielding (downfield) H 3 C Br H H H H Br CH 3 H 3 C H 1.88 ppm 3.39 ppm 1.03 ppm 4.21 ppm 1.73 ppm 1.73 ppm inductive effect contributes most to the chemical shift values The local electron density
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This note was uploaded on 07/07/2010 for the course CHEM 267 taught by Professor Fang during the Winter '09 term at Waterloo.

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11_Spectroscopy_NMR - Chem 267 Basic Organic Chemistry II...

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