NMR "Finger Prints" of N-Heterocyclic Carbenes, DFT Analysis: Scopes and Limitations

The scopes and limitations of theoretical methods for estimating various NMR metrics of NHCs are considered on a wide range of model NHCs and their derivatives, using a number of computational approximations. On the whole, the most reliable, from the point of view of predictability and insusceptibility to additional effects, are ³¹P/¹³C NMR shifts of NHC-phosphinidene adducts and ¹³C CSs of carbenes themselves. The method based on the analysis of ⁷⁷Se CS NHC-selenoureas has some limitations since the observed NMR parameters can also be modulated by exchange effects due to their formation with non-classical hydrogen bonds. As for HEP, since the delicate balance of electron distribution between Pd and two carbon centers can nonlinearly affect relativistic spin-orbit effects, the accuracy of the estimation of this metric may still be limited. ¹³C CSs of NHC-azolium salts do not seem to be reliable, since the observed values are strongly influenced by the effects of the exchange between different forms with counterions, which are difficult to estimate correctly.