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 Title: THE DICARBON BONDING PUZZLE Author(s): Laws, Benjamin A. Contributor(s): Gibson, Stephen T. Subject(s): Small molecules Abstract: \begin{wrapfigure} {r}{0pt} \includegraphics[scale=0.47]{C2VMI355nm.eps} \end{wrapfigure} At first glance dicarbon, C$_2$, would appear to be a very simple homonuclear diatomic molecule. However the bonding structure of C$_2$ has long been a topic of debate, with different qualitative theories predicting a C$-$C bond order of 2, 3, or even 4\footnote{R. M. Macrae, \emph{Sci. Prog.} {\bf99}, 1 (2016)}. Recent evidence for a quadruply bonded dicarbon has been provided by multiconfigurational \emph{ab-initio} calculations\footnote{S. Shaik, D. Danovich, W. Wu, P. Su, H. Rzepa, P. Hiberty, \emph{Nat. Chem.} {\bf 4}, 195 (2012)}. However, the interpretation of these calculations has caused debate, with some research preferring the notion of a double, triple, or quasi double-triple bond, while other studies note that there is not enough evidence to clearly define the bonding nature of C$_2$\footnote{R. Zhong, M.Zhang, H. Xu, Z. Su, \emph{Chem. Sci.} {\bf 7}, 1028 (2016)}. In this work, photoelectron spectra of the C$_2^-$ anion are measured using a high resolution photoelectron imaging (HR-PEI) spectrometer. The electron anisotropy of the detachment reveals the character of the parent anion orbital. Detachment to both the ground $\tilde{X} ^1\Sigma_g^+$ and first excited $\tilde{a} ^3\Pi_u$ electronic states is observed, identifying the character of two orbitals: the diffuse detachment orbital of the anion, and the HOMO of the neutral. The measurements show that electron detachment occurs from a pure s-like orbital ($3\sigma_g$) and a dominant p-like orbital (1$\pi_u$), that is inconsistent with the predictions of strongly mixed (50:50) sp orbitals required for the high bond order models, a result compatible only with the predictions of a C$=$C double bonding scheme. \makeatletter{\renewcommand*{\@makefnmark}{}\footnotetext{Research supported by the Australian Research Council Discovery Project Grant DP160102585.}\makeatother} Issue Date: 06/20/18 Publisher: International Symposium on Molecular Spectroscopy Citation Info: APS Genre: Conference Paper / Presentation Type: Text Language: English URI: http://hdl.handle.net/2142/100444 DOI: 10.15278/isms.2018.WH01 Other Identifier(s): WH01 Date Available in IDEALS: 2018-08-172018-12-12
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