Computational prediction of a global planar penta-coordinate carbon structure CAl4Ga+

Abstract

Ever since the proposal of the exotic concept “planar tetra-coordinate carbon” (ptC), continuous endeavors have been hotly devoted to the design and characterization of molecules with planar tetra- or hyper-coordinate atomic centers. In sharp contrast to the big ptC family, examples with the planar penta-coordinate carbon (ppC) have remained very scarce, only including CAl5+, CAl4Be, CAl3Be2, CAl2Be32 and CBe5E (E=Al, Ga). To enrich the member of ppC, in this work, we report our global isomeric characterization of CAl4X+ (X=Al, Ga, In and Tl) at the CCSD(T)/def2-TZVPP//B3LYP/def2-TZVP level with zero-point energy correction, among which, to our knowledge, systems with X=Ga, In and Tl have not been considered previously. For CAl5+, besides confirming previous computational study that ppC is the ground structure, we find a new isomer (4th) among the former seven species here. Promisingly, by means of the structural and orbital analysis, CAl4Ga+ is a novel ppC structure though the penta-coordinate ligands of C are not average due to the Al→Ga substitution. For the heavier CAl4X+ (X=In and Tl), there does not exist a ppC structure and the global isomer can formally be considered as an X+ adsorbed to the neutral Td-like CAl4. In the order of X=Al, Ga, In and Tl, the preference of ppC over the Td-C decreases for CAl4X+, which could be interpreted by the decreased covalent ability. The newly disclosed CAl4Ga+ awaits future mass spectrometry characterization.