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Flashcards

1H nmr

Casey Champion Chemistry English 38 cards

1H nmr

Cards in this deck (38)

  1. 1What 3 key pieces of information does 1H NMR provide?1. Chemical shift – chemical environment of protons
    2. Spin coupling – info on nearby protons
    3. Peak integration – relative number of protons in the signal
  2. 2What do the precise chemical shift values of C and H depend upon within their ranges and how do they differ?Depend upon electronic effects.
    - Electron donating groups lower Ꟙ.
    - Electron withdrawing groups increase Ꟙ.
  3. 3What’s the range for 1H NMR?0-12 ppm
  4. 4whats the general range for aliphatic H0-2 ppm
  5. 5whats the general range for aliphatic H near strong EWG2-4.5 ppm
  6. 6whats the general range for olefinic H5-6 ppm
  7. 7whats the general range for aromatic H6.5-8.5 ppm
  8. 8whats the general range for CHO9-11 ppm
  9. 9whats the general range for COOH9-12 ppm
  10. 10What are the chemical shifts of olefinic and aromatic protons on carbons subject to and how does this affect their chemical shift values?The protons are subject to resonance effects.
    - Moved to higher chemical shift values by EWGs.
    - Moved to lower chemical shift values by EDGs.
  11. 11On methoxybenzene, why are protons chemical shift values ortho and para to the EDG lowered compared to unsubstituted benzene?The lone pair electrons are pushed from the EDG (O atom) through to the ortho and para positions causing shielding and lower chemical shifts.
  12. 12On methyl benzoic acid, why protons chemical shift values ortho and para to the EWG increased compared to unsubstituted benzene?The electrons from the delocalised ring are pushed toward the EWG and deshielding occurs, resulting in higher chemical shifts.
  13. 13How are meta protons affected by the addition of an EDG or EWG to a benzene?They are largely unaffected as their electron density is not changed to a significant degree.
  14. 14What happens when molecules containing π-systems are placed in the magnetic field of an NMR spectrometer?The magnetic field causes charged particles to move so the electrons circulate within the π-orbitals. The circulating electrons themselves generate a secondary magnetic field which can influence the local magnetic field experienced by the 1H and 13C nuclei.
  15. 15In a region of a molecule where the induced magnetic field is oriented with the direction of the external magnetic field, what effect is had on the chemical shift?The chemical shift of the nuclei of this region increases.
  16. 16In a region of the molecule where the induced magnetic field is opposing the direction of the external magnetic field, what effect is had on the chemical shift?The chemical shift of the nuclei of this region decreases.
  17. 17What is the name of the effect that results differing chemical shift values due to direction the induced and external and magnetic fields?Anisotropic effects.
  18. 18How do anisotropic effects explain the chemical shifts of alkene protons?Alkene protons have a high chemical shift (5-6 ppm) even in the absence of EWGs:
    In proximity to the protons the induced magnetic field is in the same direction as the external magnetic field = increased chemical shift.    How do anisotropic effects explain the chemical shifts of alkene protons?
  19. 19How do anisotropic effects explain the chemical shifts of alkyne protons?Alkyne protons have a relatively low chemical shift (2-3 ppm):
    In proximity to the protons the induced magnetic field is in the opposite direction to the external magnetic field = decreased chemical shift.    How do anisotropic effects explain the chemical shifts of alkyne protons?
  20. 20protons on heteroatoms whats the chemical shift range of RNH0.5-4.5 ppm
  21. 21protons on heteroatoms whats the chemical shift range of ROH0.5-5 pmm
  22. 22protons on heteroatoms whats the chemical shift range of PhOH5-11 ppm
  23. 23protons on heteroatoms whats the chemical shift range of COOH9-12 ppm
  24. 24What are chemical shift values dependent on?Concentration
  25. 25What does H-bonding cause in NMR spectra?Signal broadening
  26. 26What happens to the signals from protons on heteroatoms when D2O is added to a sample and why?The signals disappear because the protons exchange with deuterium which doesn’t give a signal in the chemical shift range.
  27. 27What are the three electronic effects that influence the chemical shifts of protons?1. Inductive effects
    2. Resonance effects
    3. Anisotropic effects
  28. 28What is the cause of splitting observed in 1H NMR?1H-1H coupling, mainly due to presence of 1H’s on adjacent carbon atoms.
  29. 29Why does coupling arise?NMR active nuclei interact with one another by altering the local magnetic field.
  30. 30In principle, what are the two mechanisms of how nuclei can couple to each other?- Dipolar coupling
    - Scalar coupling
  31. 31What is dipolar coupling?It occurs through space. It’s seen when protons are held in fixed orientation with respect to each other e.g. on double bonds and some ring systems. In all other circumstances it averages out to zero as the positions of the nuclei move through rotation of the bonds between them.
  32. 32What is scalar coupling?It’s mediated by electrons in bonds between nuclei. As electrons can spin, they can transmit coupling between nuclei. This doesn’t average to zero through bond rotation and Is the most common type of coupling.
  33. 33The amount by which a signal is split is called what?the coupling constant
  34. 34What’s the equation for calculating the coupling constant?J (Hz) = ∆Ꟙ (ppm) × νres (MHz)
  35. 35What is the coupling constant independent of and why?The external magnetic field strength, as it is a property of the nuclei themselves.
  36. 36What must the 1H’s be to couple to each other?1. Non-equivalent
    2. Close enough in the molecule for coupling to be efficiently transmitted by electrons in the bonds between them.
  37. 37What happens to the scalar coupling constant with the increase of bonds between the 1H’s?J decreases very rapidly.
  38. 38What is peak integration on an 1H NMR spectrum and what is it proportional to?Peak integration is a measurement of the area under the peak of an NMR signal and is proportional to the relative number of protons in the signal.

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Chemistry (sas) Flashcards on 1H nmr, created by Casey Champion on 23/04/2023.
Created by
Casey Champion · Apr 2023
Updated
Apr 23, 2023
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