1H and 13C NMR study of 1‐substituted isatins and their corresponding 3‐(dicyanomethylidene)indol‐2‐ones

The assignments of the 1H and 13C NMR signals of 1‐substituted isatins and their corresponding 3‐(dicyanomethylidene)indol‐2‐ones were derived by analysis of the 13C1H spin coupling patterns and by inspection of the two‐dimensional C, H shift correlation spectra. Erroneous literature assignments have been corrected. A significant difference between the 5Δ‐deuterium isotope effects on the 13C chemical shifts of the CN groups cis and trans to the heterocyclic amide fragment (D)HNCO was observed in partially deuteriated 3‐(dicyanomethylidene)indol‐2‐one. This geometric dependence was used to assign the two CN groups that are otherwise difficult to assign.