9701 · 37.3
Common exam mistakes on 9701 Carbon-13 NMR spectroscopy. Learn what loses marks, then practise the topic with Examiner’s Ink.
When answering questions, always refer to your Data Booklet. A common question type involves providing a spectrum and asking you to match peaks to carbons in a given structure. Quote the chemical shift value from the spectrum and the expected range from the Data Booklet to justify your assignment.
The natural abundance of ¹³C is only 1.1%. This means the statistical probability of two ¹³C atoms being next to each other in a molecule is extremely low (about 0.011 x 0.011 = 0.0121%). Therefore, ¹³C-¹³C coupling is negligible and not observed in a standard spectrum.
No. Enantiomers are non-superimposable mirror images. They have identical connectivity and bond lengths/angles, so their carbon atoms exist in identical chemical environments. Therefore, enantiomers will produce identical ¹³C NMR spectra. Diastereomers, however, will have different spectra.
A ketone C=O carbon is typically more deshielded (190-220 ppm) than an ester C=O carbon (160-185 ppm). This is because the lone pair on the single-bonded oxygen in an ester participates in resonance, donating electron density towards the carbonyl carbon and increasing its shielding relative to a ketone.