Bridging the gap: viable reaction pathways from tetrahedrane to benzyne

Abstract

The addition of sp-carbon-containing molecules to polycyclic sp³ tetrahedrane (c-C₄H₄) results in the formation of both o-benzyne (c-C₆H₄) and benzene (c-C₆H₆). Since both c-C₆H₄ and c-C₆H₆ have been detected in the interstellar medium (ISM), providing additional pathways for their possible astrochemical formation mechanisms can lead to the discovery of other molecules, such as c-C₄H₄, benzvalyne, and vinylidene (:CCH₂). Addition of diatomic carbon (C₂), the ethynyl radical (C₂H), vinylidene, and acetylene (HCCH) to c-C₄H₄ is undertaken in individual pathways through high-level quantum chemical computations at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory. The resulting C₂ addition pathway proceeds barrierlessly through benzvalyne as an intermediate and reaches a true minimum at c-C₆H₄, but no leaving groups are produced which is required to dissipate excess energy within an interstellar chemical scheme. Similarly, the C₂H addition to c-C₄H₄ produces benzvalyne as well as its related isomers. This pathway allows for the loss of a hydrogen leaving group to dissipate the resulting energy. Lastly, the HCCH and :CCH₂ addition pathways follow through both benzvalene and benzvalyne in order to reach c-C₆H₆ (benzene) and c-C₆H₄ (o-benzyne) as well as H₂ as the required leaving group. Although there is a barrier to the HCCH addition, the :CCH₂ addition presents the contrary with only submerged barriers. These proposed mechanisms provide alternative possibilities for the formation of complex organic molecules in space.