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

Abstract

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) have 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-size 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 significant singlet-triplet gap (25 kcal mol-1). This strong preference for the singlet state may be understood ff cyclobutenylidene is also viewed as bicyclobut-1(2)-ene. As the size of the ring increases, the singlet state becomes destabilized with respect to the triplet. The break-even point occurs with 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 methylenecyclopropene (1,2 carbon shift) is energetically more favorable, requiring 34.8 kcal mol-1. However, the lowest isomerization path available for singlet cyclobutenylidene is the formation of vinylacetylene, which is predicted to have a barrier of around 9 kcal mol-1. This small but significant barrier implies that cyclobutenylidene should be observable.