NMR Spectroscopy

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NMR Spectroscopy

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NMR Spectroscopy

  1. 1. Nuclear Magnetic Resonance Spectroscopy
  2. 2. Nuclear magnetic resonance, or NMR as it is abbreviated by scientists, is a phenomenon which occurs when the nuclei of certain atoms are immersed in a static magnetic field and exposed to an oscillating electromagnetic field. Some nuclei experience this phenomenon, and others do not, dependent upon whether they possess a property called spin. The versatility of NMR makes it pervasive in the sciences. NMR Spectroscopy
  3. 3. Nuclear magnetic resonance spectroscopy is the use of the NMR phenomenon to study physical, chemical, and biological properties of matter. As a consequence, NMR spectroscopy finds applications in several areas of science. NMR spectroscopy is routinely used by chemists to study chemical structure using simple one-dimensional techniques. Two-dimensional techniques are used to determine the structure of more complicated molecules. 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 NMR Spectroscopy
  4. 4. 1H or 13C 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  Energy needed is related to molecular environment (proportional to field strength) NMR Spectroscopy
  5. 5. 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 (imagine the strongest electromagnet you can and the imagine it on steroids) Use of NMR Spectroscopy
  6. 6. The spin state of a nucleus is affected by an applied magnetic field
  7. 7. The energy difference between the two spin states depends on the strength of the magnetic field (that the atom “feels”)
  8. 8. The electrons surrounding a nucleus affect the effective magnetic field sensed by the nucleus
  9. 9. Nature of NMR Absorptions Electrons in bonds shield nuclei from magnetic field Different signals appear for nuclei in different environments
  10. 10. 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
  11. 11. The NMR Measurement
  12. 12. 13C NMR Spectroscopy: Signal Averaging and FT-NMR Carbon-13: only carbon isotope with a nuclear spin Natural abundance 1.1% of C’s in molecules Sample is thus very dilute in this isotope Sample is measured using repeated accumulation of data and averaging of signals, incorporating pulse and the operation of Fourier transform (FT-NMR) All signals are obtained simultaneously using a broad pulse of energy and resonance recorded Frequent repeated pulses give many sets of data that are averaged to eliminate noise Fourier-transform of averaged pulsed data gives spectrum
  13. 13. Applications of NMR The microstucture of polymer chains Resonance assignement regioisomerism Stereochemical configuration Geometric isomerism Characterization in the Solid State Chain conformation in the solid state Solid-solid transitions Organization in the solid state In multi-phase polymers Orientation Imaging
  14. 14. Applications of NMR Dynamics of Polymers in the Solid State Semicristalline polymers Amorphous polymers Polymer Systems Polymer blends and miscibility Multiphase systems

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