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Homework Help: Proton NMR - Spin-Spin Splitting and Multiplicity

  1. Jul 7, 2007 #1
    1. What will be the multiplicity due to spin-spin splitting of the highlighted protons in the molecule: (C6H5)-CH2-CH2-CH2-OH?

    2. Relevant equations

    3. The attempt at a solution
    I know that the ones surrounding the highlighted CH2 group are not equivalent. If they were, the multiplicity would be 5. When I make a branching diagram, I get 6 to a multiplet. In the text book, there was an example of a similar case where you could "assume" that they were equal, even though they were not. How do I know if I can "assume" in this case? My answer choices are a) doublet b) triplet c) quartet d) quintet e) hextet. I'm stuck between d) a quintet, if I assume that they are equal and e) the minimum, probably, but couldn't a multiplet of more than 6 form also? Any help? Thank you.
    Last edited: Jul 7, 2007
  2. jcsd
  3. Jul 9, 2007 #2


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    The assumption you are asked to make... is this first order coupling? Generally first order coupling is assumed for [tex]\Delta v\ /J > 20[/tex], where [tex]\Delta v\ [/tex] is the frequency difference from the centroid of the multiplets. Sometimes first order coupling is apparent where [tex]\Delta v\ /J =10[/tex]. The spin-spin coupling constants for methylene-methylene are usually about 5-7 Hz.

    Remember that J coupling is independent of magnetic field strength whereas the absorption frequency is a function of field strength.
    Last edited: Jul 9, 2007
  4. Jul 9, 2007 #3
    Thank you for your help. The problem never said that I could assume it was first-order coupling, but my professor also cleared up the situation. Thanks again.
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