9701 · 37.3
Carbon-13 NMR spectroscopy — FAQ
Frequently asked questions for 9701 Carbon-13 NMR spectroscopy. Direct answers first, then deeper explanation — then practise with marking.
Why don't we see splitting from adjacent ¹³C atoms?
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.
Can ¹³C NMR distinguish between enantiomers?
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.
What is the difference in chemical shift between a C=O in a ketone and a C=O in an ester?
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.