The dicarbon bonding puzzle viewed with photoelectron imaging

B. A. Laws (), S. T. Gibson, B. R. Lewis and R. W. Field
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B. A. Laws: The Australian National University
S. T. Gibson: The Australian National University
B. R. Lewis: The Australian National University
R. W. Field: Massachusetts Institute of Technology

Nature Communications, 2019, vol. 10, issue 1, 1-8

Abstract: Abstract Bonding in the ground state of C$${}_{2}$$2 is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of $$2$$2, $$3$$3, or $$4$$4. Here we report on photoelectron spectra of the C$${}_{2}^{-}$$2− anion, measured at a range of wavelengths using a high-resolution photoelectron imaging spectrometer, which reveal both the ground $${X}^{1}{\Sigma}_{\mathrm{g}}^{+}$$X1Σg+ and first-excited $${a}^{3}{\Pi}_{{\mathrm{u}}}$$a3Πu electronic states. These measurements yield electron angular anisotropies that identify the character of two orbitals: the diffuse detachment orbital of the anion and the highest occupied molecular orbital of the neutral. This work indicates that electron detachment occurs from predominantly $$s$$s-like ($$3{\sigma}_{\mathrm{g}}$$3σg) and $$p$$p-like ($$1{\pi }_{{\mathrm{u}}}$$1πu) orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly $$sp$$sp-mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2$$\pi$$π bonds and no accompanying $$\sigma$$σ bond.

Date: 2019
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DOI: 10.1038/s41467-019-13039-y

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