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Chapter13 - 13 Structure Determination Nuclear Magnetic...

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13. Structure Determination: Nuclear Magnetic Resonance Spectroscopy Based on McMurry’s Organic Chemistry , 6 th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto
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McMurry Organic Chemistry 6th edition Chapter 13 (c) 20 2 The Use of NMR Spectroscopy Used to determine relative location of atoms 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
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McMurry Organic Chemistry 6th edition Chapter 13 (c) 20 3 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 (See Figure 13.1) 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 Energy needed is related to molecular environment (proportional to field strength, B ) – see Figure 13.2
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McMurry Organic Chemistry 6th edition Chapter 13 (c) 20 4 13.2 The Nature of NMR Absorptions Electrons in bonds shield nuclei from magnetic field Different signals appear for nuclei in different environments
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McMurry Organic Chemistry 6th edition Chapter 13 (c) 20 5 The NMR Measurement The sample is dissolved in a solvent that does not have a signal itself and placed in a long thin tube The tube is placed within the gap of a magnet and spun Radiofrequency energy is transmitted and absorption is detected Species that interconvert give an averaged signal that can be analyzed to find the rate of conversion
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McMurry Organic Chemistry 6th edition Chapter 13 (c) 20 6 13.3 Chemical Shifts The relative energy of resonance of a particular nucleus resulting from its local environment is called chemical shift NMR spectra show applied field strength increasing from left to right Left part is downfield is upfield Nuclei that absorb on upfield side are strongly shielded. Chart calibrated versus a reference point, set as 0, tetramethylsilane [TMS]
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McMurry Organic Chemistry 6th edition Chapter 13 (c) 20 7 Measuring Chemical Shift Numeric value of chemical shift: difference between strength of
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