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Metal carbides are known to contain small carbon units similar to those found in the molecules of methane, acetylene, and allene. However, for numerous binary systems ab initio calculations predict the formation of unusual metal carbides with exotic polycarbon units, [C₆] rings, and graphitic carbon sheets at high pressure (HP). Here we report the synthesis and structural characterization of a HP-CaC₂ polymorph and a Ca₃C7 compound featuring deprotonated polyacene-like and para-poly(indenoindene)-like nanoribbons, respectively. We also demonstrate that carbides with infinite chains of fused [C6] rings can exist even at conditions of deep planetary interiors (~140 GPa and ~3300 K). Hydrolysismore » of high-pressure carbides may provide a possible abiotic route to polycyclic aromatic hydrocarbons in Universe.« less

In this work, the recently synthesized acetylide compound KSeCCH containing the main group element selenium within the novel and in crystalline form unprecedented [SeCCH]^– anion was successfully investigated in the gas phase by high-resolution velocity-map imaging (VMI) and magnetic-bottle (MB) photoelectron spectroscopy coupled with an electrospray ionization source. Both VMI and MB spectra exhibited identical electron affinities (EA, 2.517 ± 0.002 eV), spin–orbit coupling (SOC) splittings (1492 ± 20 cm^–1), and Se–C stretching frequencies (573 ± 20 cm^–1) of the corresponding neutral tetra-atomic radical [SeCCH]^• with the VMI spectrum possessing six times higher spectral resolution compared with the MB spectrum.more » These experimental values were well reproduced by calculations at the CCSD(T) level, in which both the isolated [SeCCH]^– anion and the [SeCCH]^• radical adopted linear geometries. The simulated spectra based on the calculated Franck–Condon factors, the SOC splitting, and the experimental line width matched well with the measured spectra. Furthermore, comparisons of the EA and SOC splitting values with the previously reported isolobal species [SeCN]^• are also made and discussed. The decrease in the EA and SOC splitting of [SeCCH]^• is ascribed to the differences in the electronegativities between C and N atoms as well as the electron density on the Se atom in its singly occupied molecular orbital (SOMO).« less