8.21 : Carbon-13 (¹³C) NMR: Overview

Carbon-13 is a naturally occurring NMR-active isotope of carbon with a low natural abundance of 1.1%. In contrast, carbon-12 is the most abundant isotope of carbon with zero nuclear spin. Therefore, it is NMR inactive. The gyromagnetic ratio of carbon-13 is smaller than that of protons. As a result, carbon-13 resonance is about 6000 times weaker than proton resonance. For a given magnetic field strength, the resonance frequency of carbon-13 is about one-fourth of the resonance frequency for protons.

The chemical shift range for carbon-13 is between 0‑220 ppm, a much larger range than in protons (0‑12ppm). Because of the broader chemical shift range, the non-equivalent carbons have distinct and well-resolved signals that do not overlap. Like proton chemical shifts, carbon-13 chemical shifts are affected by hybridization and magnetic anisotropy. However, the effect of electronegative substituents on carbon-13 chemical shifts is stronger than on proton chemical shifts because carbon-13 nuclei are directly attached to the electronegative substituents. In contrast, protons are separated from electron-withdrawing substituents by two bonds.

In the carbon-13 chemical shift scale, saturated carbons appear upfield near the TMS signal at zero ppm (0‑50 ppm), followed by alkynes and those attached to electronegative substituents (50‑100 ppm). Unsaturated and aromatic carbons appear further downfield (100‑150 ppm), followed by highly deshielded carbonyl carbons (150-220 ppm).