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Electronic transition in organic compounds

  1. May 31, 2017 #1
    Hello everybody :)
    I have a problem understanding the different types of pi -> pi * transition of organic compounds. I can't understand what's the difference between K-band, B- band, and E-bands, and I cannot find any explanation on the internet. Also I don't understand why benzene has two E bands ( E1 and E2), and Toluene only one; I also tried to see the pi molecular orbitals of different aromatic compounds, to see what pi -> pi* transitions are possible, but I found the MO's only of benzene. If anyone can explain to me the difference between the three types of band, maybe with a couple of examples, I would be very grateful.
    Thanks in advance
     
  2. jcsd
  3. May 31, 2017 #2

    TeethWhitener

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    As far as I can tell, these are just really antiquated ways of talking about ##\pi \rightarrow \pi^*## and ##n \rightarrow \pi^*## transitions of various conjugated and aromatic systems.
     
  4. May 31, 2017 #3
    Really? I didn' t know that, but I guess I'll have to try to understand it anyway for the exam :sorry:
    Thanks for the response :)
     
  5. May 31, 2017 #4

    TeethWhitener

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    It looks like R-bands are "radical" bands, or ##n \rightarrow \pi^*##. K bands are ##\pi \rightarrow \pi^*## bands in conjugated, non-cyclic systems like ethylene or 1,3-butadiene. B and E bands refer to the group representation conventions from the character table for the ##D_{6h}## point group that benzene is a member of. B bands are transitions from the ##A_{1g}## ground state into singly degenerate ##B_{1u}## and ##B_{2u}## excited states, and E bands are transitions into the doubly degenerate ##E_{1u}## state.

    (Note: the B and E bands are the only ones I'm sure of. These are still standard. The R and K bands were just what I could glean from various old papers.)
     
  6. Jun 1, 2017 #5
    Oh well, now it makes sense! It also makes sense why when the benzene has functional groups that make him lost its symmetry, it loses as well the E- bands. Thank you very much, that was very helpful!
     
  7. Jun 2, 2017 #6

    DrDu

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  8. Jun 2, 2017 #7
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