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« The intricacies of solving for an unknown structure using solution NMR data | Main | Why collect NMR or MS^n data? »

February 15, 2008


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It may be safe to make an assumption in a comparison of methylbenzene and ethylbenzene. And, in theory, if you are always comparing similar molecules you may be lucky with this approach. However, when you take into account the complexities of relaxation times on quaternary carbons it is very easy to get confused in terms of the quantitative approach you are suggesting. Been there done that...been confused. 2D NMR is the way out of the problem.

Gary Martin

Mixtures of alkyl benzenes are fine. Not only are there considerations of relaxation times for quats as Chemspiderman notes, there is also the aspect of preferred axes of roation that can have a significant impact on protonated carbon relaxation times. IMHO, after 30+ years of doing 13C NMR, this is an approach that must be used with very considerable caution when dealing with unknowns even if they're simple alkyl benzene analogs. When it comes to anything more complex, don't go there...

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