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13C NMR shifts of phenyl groups

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chmjmhi:
Could someone please point me towards a fool-proof (read: viva voce-proof) theoretical explanation for the simple fact that unfunctionalised phenyl groups sometimes appear as three 13C signals rather than the four I would expect (ipso, ortho, meta and para).

Ψ×Ψ:
Could it just be an intensity problem?  The ipso signal probably appears much less intense than the others...

kiwi:

--- Quote from: ?*? on May 08, 2007, 10:40:13 AM ---Could it just be an intensity problem?  The ipso signal probably appears much less intense than the others...

--- End quote ---

they often do

Mitch:
Sounds like a signal problem. Can you see it if you really pack your nmr-tube?

kiwi:
i assume that you are familiar with the nuclear overhauser effect, which is why the ipso signal is lower. From a practical viewpoint, jamming a whole bunch of compound in the tube is a good way. but if you are limited in compound, you can encourage the peaks to appear by increasing d1, the relaxation delay. your experiment time will also increase however.

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