The dicarbon bonding puzzle viewed with photoelectron imaging

Abstract

Bonding in the ground state of C is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of , , or . Here we report on photoelectron spectra of the C anion, measured at a range of wavelengths using a high-resolution photoelectron imaging spectrometer, which reveal both the ground and first-excited 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 -like () and -like () orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly -mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2 bonds and no accompanying bond.

C2: An Asymmetric Specie ?

“…The calculated Mulliken and Löwdin bond orders are 2.43 and 2.84, respectively, in excellent agreement with the photoelectron spectroscopy experimental data obtained in 2019 by Laws et al [7], which concludes that "this result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2π bonds and no accompanying σ bond. "…”

“…Based on the results obtained here, as well as literature data [1][2][3][4][5][6][7][8][9], some conclusion can be pointed out:…”

“…Our present calculations, supported by such experimental data [7] of course contradicts the hypothesis of a fourth order bond.…”

“…As can also be verified from [7], which concludes that "this result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2π bonds and no accompanying σ bond. "…”

A Critical Look at Linus Pauling’s Influence on the Understanding of Chemical Bonding

“…The controversy was recently solved by an experimental study of the photoelectron spectra of the C 2 − anion. Using a high-resolution photoelectron imaging spectrometer, the results show that the dominant contribution to the dicarbon bonding in C 2 involves a double-bonded configuration, with 2π bonds and no accompanying σ bond [ 77 ].…”