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The Dicarbon Bonding Puzzle Viewed with Photoelectron Imaging

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The Dicarbon Bonding Puzzle Viewed with Photoelectron Imaging The dicarbon bonding puzzle viewed with photoelectron imaging The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Laws, B.A., et al., "The dicarbon bonding puzzle viewed with photoelectron imaging." Nature Communications 10 (Nov. 2019): no. 5199 doi 10.1038/s41467-019-13039-y ©2019 Author(s) As Published 10.1038/s41467-019-13039-y Publisher Springer Science and Business Media LLC Version Final published version Citable link https://hdl.handle.net/1721.1/125929 Terms of Use Creative Commons Attribution 4.0 International license Detailed Terms https://creativecommons.org/licenses/by/4.0/ ARTICLE https://doi.org/10.1038/s41467-019-13039-y OPEN The dicarbon bonding puzzle viewed with photoelectron imaging B.A. Laws 1*, S.T. Gibson 1, B.R. Lewis1 & R.W. Field 2 Bonding in the ground state of C2 is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of 2, 3,or4. Here we report on photoelectron spectra À of the C2 anion, measured at a range of wavelengths using a high-resolution photoelectron 1Σþ fi 3Π 1234567890():,; imaging spectrometer, which reveal both the ground X g and rst-excited a u 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 pre- σ π dominantly s-like (3 g) and p-like (1 u) orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly sp-mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2π bonds and no accompanying σ bond. 1 Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia. 2 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. *email: [email protected] NATURE COMMUNICATIONS | (2019) 10:5199 | https://doi.org/10.1038/s41467-019-13039-y | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-13039-y espite the relative simplicity of homonuclear diatomic distributions indicate the character of the relevant molecular anion 27 fl Dmolecules, the bonding nature of dicarbon, C2, has long orbitals , electric dipole selection rules also in uence the electron been a topic of debate. This discussion has been driven by anisotropy via the character of the final neutral states, providing recent advances in computational methods, with various studies insight to understand the nature of the bonding between carbon – suggesting the carbon carbon bond may have a bond order of 2, atoms in C2. Here we report on photoelectron angular distribu- 3, or even 4, with the latter from ab-initio studies1. Standard tions measured at a range (290 to 355 nm) of wavelengths, to probe qualitative theories predict different values for the dicarbon bond the character of the C2 orbitals. This reveals detachment occurs 2,3 σ π order . From a simple Lewis electron-pair repulsion description, from pure s-like (3 g)andp-like (1 u) orbitals, suggesting that the the 8 valence electrons in C2 are predicted to form a quadruple dominant contribution to the dicarbon bonding involves a double- bond. However, this bonding assignment seems unlikely, with bonded configuration, with 2π bonds and no accompanying stable quadruple bonds typically only found between transition σ bond. metal elements that have partially filled d-orbitals4. In contrast, a hybrid-orbital (HO) style approach invokes sp1 hybridisation to Results and Discussion predict a triple bond between the carbons. However, if molecular Theoretical discrepancies. The simple idea of a bond order, orbital (MO) theory is used, a ground-state valence electron defined as fi ðσ Þ2ðσà Þ2ðπ Þ4 con guration for C2 of KK 2s 2s 2p is predicted, yielding π Bonding electrons À Antibonding electrons a bond order of 2, with the unusual situation of a double bond ð1Þ with no accompanying σ bond. 2 From an ab-initio approach, standard Hartree–Fock-based calculations support a dicarbon double bond; however, more is often a useful way for chemists to describe the bonding nature fi advanced theoretical studies have suggested that the C–C bond in molecules. However, this can be an oversimpli cation in sys- may be better described by a higher bond order5–8. A recent high- tems where the contribution from different bonding/anti-bonding level full configuration–interaction calculation, combined with electrons are unequal. Furthermore, the concept breaks down valence bond (VB) theory, identified four distinct contributions to entirely in systems, which are intrinsically multi-reference in 1 σ nature. Thus, when discussing the bonding in a molecule as subtle the bonding in C2 . This included contributions from a bond, 2 ´ π bonds, plus an interaction between the outward pointing sp1 as C2, it is important to keep in mind that there are multiple fi hybrid orbitals1. The strength of this inverted bond between the con gurations all contributing to the overall state. sp1 orbitals has since been calculated at various levels of theory Ab-initio descriptions of the bonding are also complicated by − fi and is estimated to contribute ~50–80 k mol 11,5,9,10. This high the multi-reference nature of C2. Speci cally, it is the quasi- σà π σ bond-order model is further supported by a subsequent quantum degeneracy of the 2 u,1 u and 3 g molecular orbitals, responsible chemistry calculation, which found higher magnetic shielding in for the numerous low-lying excited electronic states of C2,which C compared with C H , supporting a bulkier C–C bond11. muddle the bonding picture. Much of this uncertainty is influenced 2 2 2 σà However, not all of the recent studies are in agreement, with some by the nature of the 2 u orbital, which is predicted to be a very research preferring the notion of a double12, triple3, or quasi weakly anti-bonding orbital1,28. To account for this behaviour, an double–triple13 bond, whereas other studies note that the theo- alternative orbital scheme has been proposed, involving hybridisa- fi fi σÃ= σ 1 retical approaches are not suf cient to de nitively discern tion of the 2 u 3 g molecular orbitals, forming sp -like singly between the different bonding models14–16. σà þ σ σà À σ 5 occupied hybrid orbitals (2 u 3 g) and (2 u 3 g) ,asdepicted Despite advances in spectroscopic techniques, experimental fi in Fig. 1. studies have not been able to con rm any of the suggested To visualise the differences between these possible bonding bonding structures, with the majority of the debate currently 2 pictures, ab-initio calculations were performed in this work using driven by the results of ab-initio calculations . Due to the highly NWChem29 and Q-Chem30 software. Most standard methods for reactive nature of C , most experimental studies involve flame- – 2 approximately solving the Schrödinger equation for molecules are emission17 20 or plasma-discharge21,22 spectroscopy. These stu- – fi – based on Hartree Fock [or self-consistent eld (SCF)] theory, dies support a bond order between 2 and 3, with a measured C C utilising a mean-field approximation. However, for molecules bond length of 1.243 Å, longer then a typical alkyne triple bond, 19 with quasi-degenerate or low-lying excited states, a multi- but shorter then a typical alkene double bond . Likewise, a configurational complete active space (CASSCF) approach may measured C–C bond dissociation energy of 602 kJ mol−1, as well 14 be required. This approach accounts for states that are a linear as a calculated bond restoring force of 12 N , also lie between combination of several quasi-degenerate configurations, allowing double and triple bond limits23. À for non-integer orbital occupation numbers. As the dicarbon anion C2 is stable, photoelectron spectroscopy 24,25 The molecular orbitals for C2 were calculated with both SCF and may be used to probe the reactive C2 neutral molecule . The CASSCF methods, using a cc-pVTZ Dunning basis set31,asshown first dicarbon photoelectron experiment was performed by Ervin À in Fig. 2. The key difference between the two approaches is the 24 − à et al. ,onC2 anions produced in an O /HCCH afterglow ion σ fi ordering of the 2 u orbital, with the multicon gurational calcula- source. This source produced hot anions, with multiple hot bands tion increasing the orbital energy due to possible mixing between present in the spectrum, and defined an accurate value for the σÃ= σà σ = σ fi 24 the 2 u 3 u and 2 g 3 g orbitals. This mixing is also illustrated in electron af nity of C2 of 3.269(6) eV . A later study by Bragg 26 the occupation numbers, with a substantial (0:4) occupation et al. probed the low-lying excited states of C2, in a single σ wavelength measurement at 264 nm (as part of a larger study, found in the 3 g orbital. As the dominant weight in this mixed σà employing time-resolved photoelectron spectroscopy to examine orbital lies with the anti-bonding 2 u, these occupation numbers transitions from excited anion states). This study provided the suggest a bond order of three, consistent with a pure hybrid-orbital 6 σÃ= σà σ = σ first experimental anisotropy measurement of photodetachment theory argument . However, if this 2 u 3 u and 2 g 3 g mixing is À φ ¼ from the C2 anion. allowed to occur before the calculation, using hybrid orbitals L À σ þ σ φ ¼ σ À σ fi In this work, the photoelectron spectrum of C2 is revisited using 2 u 3 g and R 2 u 3 g, the predominant con guration is a high-resolution photoelectron imaging (HR-PEI) spectrometer.
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