Although it is now well established that the photoelectron angular distributions indicate the character of the relevant molecular anion orbitals 27, electric dipole selection rules also influence the electron anisotropy via the character of the final neutral states, providing insight to understand the nature of the bonding between carbon atoms in C 2. In this work, the photoelectron spectrum of C 2 − is revisited using a high-resolution photoelectron imaging (HR-PEI) spectrometer. This study provided the first experimental anisotropy measurement of photodetachment from the C 2 − anion. 26 probed the low-lying excited states of C 2, in a single wavelength measurement at 264 nm (as part of a larger study, employing time-resolved photoelectron spectroscopy to examine transitions from excited anion states). This source produced hot anions, with multiple hot bands present in the spectrum, and defined an accurate value for the electron affinity of C 2 of 3.269(6) eV 24. 24, on C 2 − anions produced in an O −/HCCH afterglow ion source. The first dicarbon photoelectron experiment was performed by Ervin et al. Likewise, a measured C–C bond dissociation energy of 602 kJ mol −1, as well as a calculated bond restoring force of 12 N 14, also lie between double and triple bond limits 23.Īs the dicarbon anion C 2 − is stable, photoelectron spectroscopy may be used to probe the reactive C 2 neutral molecule 24, 25. These studies support a bond order between 2 and 3, with a measured C–C bond length of 1.243 Å, longer then a typical alkyne triple bond, but shorter then a typical alkene double bond 19. Due to the highly reactive nature of C 2, most experimental studies involve flame-emission 17– 20 or plasma-discharge 21, 22 spectroscopy. However, not all of the recent studies are in agreement, with some research preferring the notion of a double 12, triple 3, or quasi double–triple 13 bond, whereas other studies note that the theoretical approaches are not sufficient to definitively discern between the different bonding models 14– 16.ĭespite advances in spectroscopic techniques, experimental studies have not been able to confirm any of the suggested bonding structures, with the majority of the debate currently driven by the results of ab-initio calculations 2.
This high bond-order model is further supported by a subsequent quantum chemistry calculation, which found higher magnetic shielding in C 2 compared with C 2H 2, supporting a bulkier C–C bond 11. The strength of this inverted bond between the s p 1 orbitals has since been calculated at various levels of theory and is estimated to contribute ~50–80 k mol −1 1, 5, 9, 10.
#Ion bonding puzzle plus#
This included contributions from a σ bond, 2 × π bonds, plus an interaction between the outward pointing s p 1 hybrid orbitals 1.
#Ion bonding puzzle full#
A recent high-level full configuration–interaction calculation, combined with valence bond (VB) theory, identified four distinct contributions to the bonding in C 2 1. However, if molecular orbital (MO) theory is used, a ground-state valence electron configuration for C 2 of KK ( σ 2 s ) 2 ( σ 2 s * ) 2 ( π 2 p ) 4 is predicted, yielding a bond order of 2, with the unusual situation of a π double bond with no accompanying σ bond.įrom an ab-initio approach, standard Hartree–Fock-based calculations support a dicarbon double bond however, more advanced theoretical studies have suggested that the C–C bond may be better described by a higher bond order 5– 8. In contrast, a hybrid-orbital (HO) style approach invokes s p 1 hybridisation to predict a triple bond between the carbons. However, this bonding assignment seems unlikely, with stable quadruple bonds typically only found between transition metal elements that have partially filled d-orbitals 4. From a simple Lewis electron-pair repulsion description, the 8 valence electrons in C 2 are predicted to form a quadruple bond. Standard qualitative theories predict different values for the dicarbon bond order 2, 3. This discussion has been driven by recent advances in computational methods, with various studies suggesting the carbon–carbon bond may have a bond order of 2, 3, or even 4, with the latter from ab-initio studies 1. Despite the relative simplicity of homonuclear diatomic molecules, the bonding nature of dicarbon, C 2, has long been a topic of debate.
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