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COSY and TOCSY spectras

  1. Apr 2, 2013 #1
    Hi.

    I was just curious to how I make these spectras ?

    In some of my assignments I have to give a sketch of how a COSY and TOCSY spectra would look like from a given molecule structuere I have and their NMR spectras.

    So if I'm not mistaken, in a COSY spectra, I draw the H NMR spectrum on each eaxis, and then draw a dot at the places where there is a direct coupling between hydrogen atoms, right ?
    Like if I have:

    CH3 - CH2 - CH3,

    then CH2 would couple with both CH3, but the CH3's will not couple with each other ? And therefor there is no point/dot for the coordinates where CH3 and CH3 meet ?
    Or am I missing something for COSY at least ?

    Now, for TOCSY, the CH3's do connect in some way, but my question here is, how far along does hydrogens connect in this type of spectra ?
    Again, if I have:

    CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - CH3

    Does all the H's connect in the TOCSY spectra, or is it just the first 3 or 4 (or something else) within reach ?

    So basically, my question is, if I have understood how to do the COSY at least ? And how much coupling there is in TOCSY, and of course, have I totally misunderstood any of it ? :)


    Thanks in advance...
     
  2. jcsd
  3. Jan 28, 2017 #2

    TeethWhitener

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    Gold Member

    This is basically the right idea, with some of the details wrong. COSY spectra give correlation peaks for hydrogens on adjacent carbons, so in the case of propane above, you'd get correlation peaks between the CH2 and CH3 protons. However, since the two methyl groups are equivalent, their peaks are at the same chemical shift. Since the peaks are located at the same position in the spectrum, it's meaningless to talk about a cross correlation peak. Instead, consider a molecule like n-pentane:
    CH3 - CH2 - CH2 - CH2 - CH3
    The methyl group protons will give cross peaks with the protons from the CH2 group directly adjacent to it, and that group will give cross peaks with both the methyl end group and the middle CH2 group. The middle CH2 group will give cross peaks with the adjacent CH2 groups (which are magnetically equivalent), but not the end CH3 groups, because COSY doesn't couple protons on non-adjacent carbons.
    TOCSY couples longer chains (but it's not particularly simple). In this case, yes, the TOCSY spectrum will probably couple all the protons with all the other protons.
     
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