August 12, 2010
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
Structure elucidation by NMR involves a deep understanding of various technical aspects behind data acquisition. Being aware of how the instrument works can facilitate the process and reduce the aggravation.
For today’s puzzle, an unknown compound is known to comprise of 7 carbon atoms and exhibit the following 13C NMR spectrum with 1H decoupling. How can the 7 atoms be accounted for in the spectrum below?
Perhaps a carbonyl peak is beyond 200 ppm. Or maybe at least 2 carbons are equivalent.
For sure, the peaks at 118 and 112 ppm accounted for two equivalent carbons each (because of the high relative intensity, frequent with aromatic carbon). Taking this into account, 8 carbons can be counted. Also, it is quite surprising, but not impossible, to find a carbon at exactly 100.00 ppm. I think what is most probable is that the spectrum has been referenced with this peak. Thus, one peak could be an impurity, but because of the bad reference, it’s not possible to say which of them.
Peaks 4 and 5 – two equivalent pairs of aromatic carbons. Peak 2 at 100 ppm may be carrier frequency (not digital acquisition). Peak 1 – carbon in methoxy group.