February 15, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
One of the hardest parts of interpreting an NMR dataset is separating peaks from a mixture of compounds. To illustrate this point, below are the aromatic regions of two 75 MHz 13C NMR run on a mixture of methylbenzene and ethylbenzene in CDCl3.
If the mixture exists at an approximate 1:1 ratio (top figure), then deciding what peak belongs to which compound becomes very difficult with the current 1D NMR spectrum. However, if the ratio of the compounds is different enough, e.g. something like a 1:2 ratio (bottom figure), the peak height provides a key to differentiating the peaks. That is, the smaller peaks belong to the CHs from the methylbenzene.
TIP: As a first pass at a 1D NMR spectrum, check for any relative differences in areas or heights of the peaks.
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.
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…