Effect of angle strain in conjugated cycloalkenylidenes. Singlet-triplet splitting of cyclobutenylidene and its ground-state intramolecular rearrangements

Angle strain influences preference for a singlet ground state in small-ring vinylcarbenes, in contrast to the parent vinylcarbene. As the angle at the carbenic center is permitted to widen with expanding ring size, the triplet becomes the ground state. The six-membered ring for which singlet and triplet states are close in energy provides the break-even point. For five-membered and larger rings, the singlet states are expected to immediately isomerize to the corresponding olefins by a 1,2 H migration of the neighboring methylenic hydrogen. These were gleaned from the computational study of the singlet and triplet states of four carbenes and some of their isomers to assess the effect of angle strain on the S-T gap of cyclic alkenylidenes.