The phenomenon where 13 C-isotopomers at natural abundance overcomes the complications from strong coupling between protons bound to different carbons was first reported over 60 years ago. Figure shows a simulation of an ABX H spin system with all protons bound to 12 C nuclei. HA and HB are strongly coupled with a difference in frequency Dn !"# = n $ -n % of 16 Hz and a coupling JHAHB of 12 Hz, while HX is weakly coupled to HA and HB. The near-degeneracy of HA and HB causes their multiplets to coalesce, and a frequency selective pulse would not be able to invert 'purely' the HA or HB spins. Moreover, the HA-HX and HB-HX splittings observed on the HA and HB multiplets (and on the HX multiplet, not shown) are found to be ca. 8.5 Hz and 4.5 Hz, whereas the input JAX and JBX couplings in the simulation were 9.0 and 4.0 Hz. 2DJ spectra would display similar discrepancies between line splittings and J-couplings. 12 Figures 1b and 1c illustrate how the presence of a single 13 C nucleus bound to either HA or HB fully overcomes all these complications by introducing a 1 JCH coupling (145 Hz). At natural 13 C isotope abundance (1.1%), it is safe to neglect responses from isotopomers featuring a second 13 C nucleus at a specific position. For simplicity, the simulation ignores small JCH couplings and 13 C isotope shifts. Since the 13 C-bound proton's multiplet shifts either left or right depending on the 13 C spin isomer (a or b), and the 12 C-bound proton stays put, the effective frequency difference between HA and HB increases significantly compared to the all-12 C isotopomer. For the 13 CA isotopomer, these become