Hi Neil,
Maybe you already got your answer. Here is my understanding or answer
to your question:
> I was asked an interesting question by an Organic Chemistry faculty
> member. In a chiral molecule an ethyl group, CH3-CH2, can have two
> different chemical shifts for the two geminal protons on the CH2
> group. But I've never seen a double-doublet for the methyl group,
> always a triplet. Why can't the two vicinal J values be different
> if the two protons on the CH2 are not equivalent?
>
It's because of the very fast rotation of the methyl group. This
rotation is so fast that under the usual experimental conditions, the
three protons experience an averaged vicinal couplings with both of
the two diastereotopic protons, and the averaged 3J are usually the
same. Hope my answer helps you and your colleague.
Best wishes,
Xiulan
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Dr. Xiulan Xie
NMR-Abteilung Fachbereich Chemie
Philipps-Universität Marburg
Hans-Meerwein-Strasse
35032 Marburg, Germany
Phone: +49-6421-2825520
Fax: +49-6421-2825422
Email: xie_at_staff.uni-marburg.de
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Received on Sat Nov 29 2014 - 10:07:02 MST