carbon-13 NMR

Proton NMR maps the hydrogens of a molecule, but what about the carbons themselves — the actual backbone? Carbon-13 NMR looks directly at the carbon skeleton. It answers a simple, powerful question that proton NMR cannot answer directly: how many chemically different carbons does the molecule have, and what kind is each one?

Why carbon-13 and not ordinary carbon? The common isotope, carbon-12, has no nuclear spin and is invisible to NMR; only the rare carbon-13, about one in a hundred carbon atoms, has spin and gives a signal. This makes carbon NMR less sensitive and slower than proton NMR, but modern instruments handle it routinely. The payoff is a spectrum spread over a much wider range, roughly 0 to 220 ppm, so signals rarely overlap. Each chemically distinct carbon usually shows up as a single clean line, so you can often simply count the lines to count the kinds of carbon. Carbons are commonly recorded with the hydrogen coupling removed, so each carbon appears as one sharp peak rather than a split cluster.

Chemical-shift ranges in carbon NMR are diagnostic too: saturated alkane carbons near 0–50 ppm, carbons bonded to oxygen near 50–90, alkene and aromatic carbons near 100–150, and the carbonyl carbon of a ketone or aldehyde far downfield near 190–220. A single peak around 200 ppm is a near-certain flag for a carbonyl carbon. Carbon NMR pairs naturally with proton NMR — protons tell you about hydrogens and their neighbors, carbons confirm the count and type of the skeleton itself.