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Updated: May 7, 2026

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
Published on: April 19, 2019
Electronic and ligand properties of annelated normal and abnormal (mesoionic) N-heterocyclic carbenes: a theoretical
Ashwini K Phukan1, Ankur Kanti Guha, Satyajit Sarmah
1Department of Chemical Sciences, Tezpur University , Napaam 784028, Assam, India.
Abstract:
The effect of annelation and carbonylation on the electronic and ligating properties of N-heterocyclic carbenes (NHCs) has been studied quantum chemically. The thermodynamic and kinetic stability of these NHCs have been assessed on the basis of their singlet-triplet and HOMO-LUMO gaps respectively. Both annelation and carbonylation have been found to decrease the stability of NHCs. Compared to nonannelated carbenes, annelated and carbonylated carbenes are found to be weaker σ donors but better π acceptors. However, the effect of carbonylation is more pronounced than annelation toward increasing the π acidity of the NHCs. The reactivity of these carbenes has been discussed in terms of nucleophilicity and electrophilicity indices. The calculated values of the relative redox potential and (31)P NMR chemical shifts of corresponding carbene-phosphinidene adducts have been found to correlate well with the π acidity of the NHCs.
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