lecture 13 - 13. Structure Determination: Nuclear Magnetic...

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13. Structure Determination: Nuclear Magnetic Resonance Spectroscopy Based on McMurry’s Organic Chemistry , 7 th edition
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2 The Use of NMR Spectroscopy Used to determine relative location of atoms within a molecule Most helpful spectroscopic technique in organic chemistry Maps carbon-hydrogen framework of molecules Depends on very strong magnetic fields
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3 Why This Chapter? NMR is the most valuable spectroscopic technique used for structure determination More advanced NMR techniques are used in biological chemistry to study protein structure and folding
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4 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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5 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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6 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 Can be used to measure rates and activation energies of very fast processes
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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
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This note was uploaded on 09/06/2009 for the course CHEM chem 12 AB taught by Professor Adamczeski during the Spring '09 term at San Jose City College.

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lecture 13 - 13. Structure Determination: Nuclear Magnetic...

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