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Updated: Oct 17, 2025

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
Published on: April 19, 2019
σ0π2 Singlet Ground State Carbenes Undergo Least-Motion Reactions with H2 and Alkenes
Stefan F Clewing1, J Philipp Wagner1
1Institut für Organische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany.
Abstract:
Ground state singlet carbenes commonly feature σ2π0 orbital occupations and are known for their concerted σ-bond insertion and cycloaddition reactions. Despite the facility of these transformations, orbital symmetry conservation forces them into non-least-motion π-approach reaction pathways. This situation completely changes when the singlet σ0π2 electron configuration becomes the ground state, which we show here by means of high-level CCSD(T) geometry optimizations. Carbenes like the experimentally known 2H-imidazol-2-ylidene react with H2 and ethylene with negligible or no barrier in a σ-fashion, which effectively corresponds to a least-motion reaction trajectory.
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