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

The singlet and triplet states of cyclobutenylidene, cyclopenten-3-ylidene, cyclohexen-3-ylidene, and cyclohepten-3-ylidene and some of their isomers (cyclohexen-4-ylidene, cyclohepten-4-ylidene, and cyclohepten-5-ylidene) ha ve been studied computationally (using ab initio and DFT methods) in order to assess the effect of angle strain on the S-T gap of small- and medium-si ze cyclic alkenylidenes. Ground-state intramolecular rearrangements of the conjugated cycloalkelnylidenes have also been examined with an emphasis on the smaller four-membered ring, which is compared to its higher homologues. It is found that cyclobutenylidene has a singlet ground state and a signif icant singlet-triplet gap (25 kcal mol(-1)). This strong preference for the singlet state may be understood if cyclobutenylidene is also viewed as bic yclobut-1(2)-ene. As the size of the ring increases, the singlet state beco mes destabilized with respect to the triplet. The break-even point occurs w ith the six-membered ring where the triplet and singlet states are close in energy. The barrier for the 1,2 hydrogen shift in cyclobutenylidene (50.5 kcal mol(-1)) is found to be much higher compared to its higher homologues and to other alkylcarbenes. The ring contraction to form methylenecycloprop ene (1,2 carbon shift) is energetically more favorable, requiring 34.8 kcal mol(-1). However, the lowest isomerization path available for singlet cycl obutenylidene is the formation of vinylacetylene, which is predicted to hav e a barrier of around 9 kcal mol(-1). This small but significant barrier im plies that cyclobutenylidene should be observable.