Utility of 1H/2H isotope shifts for the 13C NMR assignments of indole derivatives

The deuterium/hydrogen NMR isotope effects on the 13C chemical shift (DHIECS) of the cycloalkano[b]indoles 1,2,3,4‐tetrahydro‐9H‐carbazole, 3‐tert‐butyl‐1,2,3,4‐tetrahydro‐9H‐carbazole and 1,2,3,4‐tetrahydrocyclopent‐[b]indole, wherein the labile NH was partially exchanged to ND, were measured at a concentration of 0.83 M in CDCI3 and in DMSO‐d6 solutions. The DHIECS values show defined trends that allow definitive 13C NMR assignments. The isotope effects over three and four bonds for the sp3 hybridized carbons are linearly interdependent, even when one of these isotope effects becomes negative.