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Tuesday, 2 June 2015

Couple of examples for judging 13C chemical shifts


Here are a couple of examples for judging 13C chemical shifts:

Remember that 13C shifts generally follow the pattern of 1H shifts, but are much larger.
Thus, in (a)

  • The peak at lowest field is the caarbon with the OH attached
  • The peak at highest field is the CH3 farthest from the OH
  • Generally in both 13C and 1H spectra, CH2 groups fall to lower field than CH3, which assigns the remaining peaks.
For (b):

  • The peak at lowest field is the C=O carbon, shifted by electronegativity and "diamagnetic anisotropy"
  • Assignment of the other two peaks follows from the CH2 lower than CH3 rule.
Now try predicting the splitting pattern to be found if the decoupler is turned off. 








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