Singlet and triplet cyclonona‐3,5,7‐trienylidenes and their α, ά‐halogenated derivatives at DFT

Singlet and triplet boat‐shaped cyclonona‐3,5,7‐trienylidene ( 1 H‐S and 1 H‐T ) and their corresponding α, ά‐tetrahalo derivatives ( 1 F‐S , 1 F‐T , 1 Cl‐S , 1 Cl‐T , 1 Br‐S , 1 Br‐T , 1 I‐S , and 1 I‐T ) are compared at M06‐2X/AUG‐cc‐pVDZ, B3LYP/AUG‐cc‐pVTZ, and B3LYP/6‐311++G** levels of theory. Apart from 1 H‐S which turns out as a transition state for having a negative force constant, the other structures appear as minima on their potential energy surfaces. 1 H , 1 Cl , and 1 Br carbenes have triplet ground states, while 1 F and 1 I which initially take on a singlet multiplicity and eventually transform to a more stable intramolecular [1 + 2] cycloaddition product, and trans alkene, respectively. Except for 1 H‐T and 1 F‐T carbenes, the other triplet states emerge as allene isomer after optimization. We conclude that halogen substitution increases stability based on absolute value of singlet‐triplet energy separation ( | Δ E S‐T | ), in the following order 1 Br < 1 H ≤ 1 Cl < 1 I < 1 F at B3LYP/AUG‐cc‐pVTZ, and 1 Br < 1 Cl < 1 H < 1 I < 1 F at M06‐2X/AUG‐cc‐pVDZ. Nucleophilicity of the carbenic center is decreased due to higher electronegativity of substituted halogens. The scrutinized carbenes 1 Cl‐S , 1 Br‐S , and 1 I‐S show higher nucleophilicity and higher electrophilicity than the synthesized common NHCs ( 2 ‐ 6 ). Hence, compared with 2 ‐ 6 carbenes, these halogenated carbenes exhibit ambiphilic character owing to the presence of a σ 2 ‐orbital and vacant 2p π ‐orbital that makes them theoretically more susceptible for attacking an electrophile or a nucleophile.