Ch 9

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mckenzielarmstrong
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192399
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Ch 9
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2013-01-12 13:57:53
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Nuclear Magnetic Resonance Spectroscopy
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Ch 9
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  1. A spectroscopic method for measuring the absorption of radio frequency radiation by certain nuclei when the nuclei are in a strong magnetic field. The most common are 1H NMR spectra and 13C NMR spectra, which provide structural information about the carbon framework of a molecule, and the number and environment of hydrogen atoms attached to each carbon atom
    Nuclear Magnetic Resonance Spectroscopy
  2.  The position of a signal along the horizontal axis of a NMR spectrum at which a nucleus absorbs relative to a reference compound such as tetramethylsilane (TMS), and measured as delta values
    Chemical Shift
  3. An inert, volatile liquid that is added to samples as an internal standard for 1H NMR and 13C NMR. (CH3)4 Si has 12 homotopic hydrogens that are highly shielded, and 4 chemically equivalent carbons; therefore, only one large peak occurs higher upfield than most organic compounds, and it's chemical shift is defined as 0.000 ppm.
    Tetramethylsilane
  4.  Horizontal scale in NMR that shows the location of signals relative to the internal tetramethylsilane (TMS) standard, which is set at 0. The delta value (in ppm) is found by dividing the chemical shift (in Hz) by the oscillator frequency (in Hz) and multiplying by 1,000,000.
    Delta-scale
  5. The left side of a NMR spectrum where deshielded nuclei absorb, at larger delta values, and at higher radio frequencies (or lower external magnetic fields, which is "down the field").
    Downfield
  6. The right side of a NMR spectrum where shielded nuclei absorb, at smaller delta values, and at lower radio frequencies (or higher external magnetic fields, which is "up the field").
    Upfield
  7. The areas under the peaks in a 1H NMR spectrum that are calculated and plotted as a stepped trace that appears on the spectrum.
    Integration curve
  8. In 1H NMR, the ratio of the heights of the integration steps is multiplied by a whole number to give a ratio of integers, which is the relative number of hydrogen atoms represented by each signal.
    Relative ratio of integrated peaks
  9. In 1H NMR, the ratio that the sum of the integers in the relative ratio of integrated peaks gives the actual number of hydrogens present in the molecule.
    Absolute ration of integrated peaks
  10. The 1H NMR signal may be split into a multiplet by the magnetic effect of nonequivalent hydrogen neighbors separated by 2 or 3 bonds
    Signal splitting
  11. A group of peaks resulting from signal splitting; a singlet is a signal with only one peak; a doublet is a signal with two peaks; a triplet is a signal with three peaks; a quartet is a signal with four peaks; a quintet is a signal with five peaks; a sextet is a signal with six peaks; a septet is a signal with seven peaks; etc
    Multiplet
  12. The 1H NMR signal may be split into a multiplet with (n + 1) peaks, when it has n number of nonequivalent hydrogen neighbors that are separated by 2 or 3 covalent bonds.
    n+1 rule
  13.  In 1H NMR, the circulation of sigma and pi electrons within a chemical bond produces an internal electric field that induces an internal magnetic field, Bi , about the nuclei, which acts to oppose the applied magnetic field, B0 , and causes the effective magnetic field, Be , at the nuclei to be smaller, according to the formula: Be= B0 - Bi . Protons in electron dense environments are partially shielded and sense a smaller effective magnetic field, Be , so they absorb upfield (to the right) at smaller delta values, and at lower radio frequencies (or higher external magnetic fields).
    Magnetic shielding
  14. The induced electromotive force (EMF) in any closed circuit is proportional to the rate of change of the magnetic flux through the circuit
    Faraday's law of induction
  15. The current induced in a circuit will flow in a direction that produces a magnetic field opposing the change in flux that produces the current.
    Lenz's law
  16. Effects observed in 1H NMR caused by different levels of magnetic shielding from the circulation of electrons within a chemical bond. Shielding causes signals to appear upfield (to the right) at smaller delta values, and at lower radio frequencies (or higher external magnetic fields). Deshielding causes signals to appear downfield (to the left) at larger delta values, and at higher radio frequencies (or lower external magnetic fields).
    Shielding & deshielding of protons
  17.  In 1H NMR, protons that are in exactly the same chemical environment will behave the same in chemical reactions, will have identical chemical shifts, and give only one 1H NMR signal.
    Chemically equivalent protons
  18. When replacement of each of two hydrogen atoms by the same group yields compounds that are identical, then the two hydrogen atoms are chemically equivalent, have identical chemical shifts, and give only one 1H NMR signal.
    Homotropic hydrogen atoms
  19. Hydrogen atoms that have different molecular connectivities are chemically nonequivalent, have different chemical shifts, and give different 1H NMR signals, although some peaks may coincidentally overlap
    Heterotropic hydrogen atoms
  20. When replacement of each of two hydrogen atoms by the same group yields compounds that are enantiomers, then the two hydrogen atoms have the same chemical shift, and give only one 1H NMR signal
    Enantiotopic hydrogen atoms
  21. When replacement of each of two hydrogen atoms by the same group yields compounds that are diastereomers, then the two hydrogen atoms have different chemical shifts, and give different 1H NMR signals, although some peaks may be very close together or they may coincidentally overlap
    Diastereotopic hydrogen atoms
  22. The magnetic effect that produces signal splitting in 1H NMR spectra. Vicinal coupling occurs between heterotopic hydrogen atoms that are bound to adjacent atoms and are separated by three sigma bonds. Geminal coupling occurs between diastereotopic hydrogen atoms that are bound to the same atom and are separated by two sigma bonds.
    Spin-spin coupling
  23. An effect observed in NMR spectra that results in a signal appearing as a multiplet (i.e., doublet, triplet, quartet, etc.), which is caused by magnetic couplings of the nucleus being observed with the nuclei of nearby atoms.
    Spin-spin splitting
  24. The separation of the peaks of a multiplet, measured in hertz, which is caused by spin-spin coupling between the atoms labeled a and b
    Coupling constant (Jab)
  25. The angle that is formed whenever two planes meet.
    Dihedral angle
  26. An effect that causes spin-spin splitting not to be observed in 1H NMR spectra.
    Spin decoupling
  27. The hydrogens (protons) attached to oxygen or nitrogen can be rapidly exchanged between molecules. Addition of heavy water (D2O, deuterium oxide) will allow exchange between these hydrogens and deuterium, and the 1H NMR signal will disappear, or be reduced in height, because deuterium does not produce a signal in 1H NMR.
    Exchangable protons
  28. A technique in 13C NMR spectroscopy that removes the splitting of 13C signals caused by coupling of 13C and 1H nuclei, which causes all of the 13C signals to appear as singlets
    Broadband proton decoupling
  29. An electronic technique used in 13C NMR spectroscopy that allows decoupling of spin-spin interactions between 13C nuclei and 1H nuclei. In spectra obtained in this mode of operation all carbon resonances appear as singlets.
    Proton decoupling
  30. An electronic technique used in 13C NMR spectroscopy that allows one-bond couplings between 13C nuclei and 1H nuclei. In spectra obtained in this mode of operation, CH3 groups appear as quartets, CH2 groups appear as triplets, CH groups appear as doublets, and carbon atoms with no attached hydrogen atoms appear as singlets.
    Proton off-resonance decoupling
  31. An electronic technique used in 13C NMR spectroscopy that allows one-bond couplings between 13C nuclei and 1H nuclei. In spectra obtained in this mode of operation, CH3 groups appear as quartets, CH2 groups appear as triplets, CH groups appear as doublets, and carbon atoms with no attached hydrogen atoms appear as singlets.
    DEPT
  32. A technique based on NMR spectroscopy that is used in medicine
    Magnetic resonance imaging (MRI)
  33. A 2D NMR technique that correlates the chemical shifts of spin-coupled nuclei. Also called correlation spectroscopy.
    COSY
  34. A 2D NMR technique that correlates the 1H chemical shift of a proton to the 13C chemical shift of the carbon to which it is attached. Also called heteronuclear correlation spectroscopy
    HECTOR

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