Ionic hydrocarbon compounds that contain hypercarbon atoms, which bond to five or more atoms, are important intermediates in chemical synthesis and may also find applications in hydrogen storage. Extensive investigations have identified hydrocarbon compounds that contain a five- or six-coordinated hypercarbon atom, such as the pentagonal-pyramidal hexamethylbenzene, C6(CH3)62+, in which a hexacoordinate carbon atom is involved. It remains challenging to search for further higher-coordinated carbon in ionic hydrocarbon compounds, such as seven- and eight-coordinated carbon. Here, we report ab initio density functional calculations that show a stable 3D hexagonal-pyramidal configuration of tropylium trication, (C7H7)3+, in which a heptacoordinate carbon atom is involved. We show that this tropylium trication is stable against deprotonation, dissociation, and structural deformation. In contrast, the pyramidal configurations of ionic C8H8 compounds, which would contain an octacoordinate carbon atom, are unstable. These results provide insights for developing new molecular structures containing hypercarbon atoms, which may have potential applications in chemical synthesis and in hydrogen storage.
Heptacoordinate hypercarbon Density functional theory Ionic hydrocarbon compounds Tropylium trication Ab initio electronic structure calculations
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The authors would like to thank Michael Lee from the Oklahoma School of Science and Mathematics (now at the University of Oklahoma) for valuable discussions at the early stage of this work. The calculations have been performed using computational resources at the OU Supercomputing Center for Education & Research (OSCER) at the University of Oklahoma.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.