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Question on VO(acac)-piperidine

  1. Oct 5, 2009 #1
    I am curious of the reason why the V=O stretching here has a lower frequency than that in VO(acac)2. Is it related to resonance? I am not sure.
     
  2. jcsd
  3. Oct 5, 2009 #2

    alxm

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    Yeah, you can explain it in terms of resonance.

    Basically (ignoring the two existing bonds to vanadium) you have V=O <--> -V-O- where in the latter case, some ligand forms an additional bond to the vanadium atom, creating a cation somewhere else.

    In the case of VO(acac)2, an oxygen forms an additional bond, leaving a cation on the carbon it is bonded to. With piperedine, the nitrogen can form a second bond to vanadium, with its lone pair, making the nitrogen a cation.

    The nitrogen cation is more stable than the carbocation, so the V-O- resonance form is more stable with piperidine, so the V=O double-bond is weaker, so the V=O stretch has a lower frequency.

    (Updated: I did a calculation, it's a pretty small shift, about 20 cm-1? http://i38.tinypic.com/25z5pwg.gif")
     
    Last edited by a moderator: Apr 24, 2017
  4. Oct 5, 2009 #3
    about 40 cm^-1

    the V=O stretching vibration for VO(acac)2 at 995cm^-1 and for VO(acac)-piperidine it is at 957cm^-1

    I think there should be four bonds except the V=O ? also, one acac is substituted by piperidine, isn't it?

    Thanks.
     
  5. Oct 5, 2009 #4

    alxm

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    Well, close enough - well within the margin of error. :)

    Well, with VO(acac)2 you'll have four oxygens liganding, but with two bonds which you could view as resonating between the two acetylacetonate oxygen atoms. So, 'half' a bond to each oxygen would really be a better way of thinking of it. (and in the zwitterion-resonance form, you'd have two full bonds to one acac, and a carbocation on the same acac) With VO(acac) you'd replace one acetylacetonate with the piperidine (CH2)5N-.
     
  6. Oct 6, 2009 #5
    Thanks for your answer.
    and finally could you tell me how to calculate it ? It seems that I can not calculate it based on the information I knew in the IR( I don't learn the IR on metal in details.... But it looks pretty similar to IR but in more complex way...)
     
  7. Oct 12, 2009 #6
    Hello there,

    The stretching frequency of v(V=O) band is 957 cm^(-1), could you please tell me if it is your experimental value or literature value?

    Thanks
     
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