Communication: On the first ionization threshold of the C2H radical

The elusive phenylethynyl radical and its cation: synthesis, electronic structure, and reactivity

Alkynyl radicals and cations are crucial reactive intermediates in chemistry, but often evade direct detection. Herein, we report the direct observation of the phenylethynyl radical (C6H5CC˙) and its cation (C6H5CC+), which are two of the most reactive intermediates in organic chemistry. The radical is generated via pyrolysis of (bromoethynyl)benzene at temperatures above 1500 K and is characterized by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). Photoionization of the phenylethynyl radical yields the phenylethynyl cation, which has never been synthesized due to its extreme electrophilicity. Vibrationally-resolved ms-TPES assisted by ab initio calculations unveiled the complex electronic structure of the phenylethynyl cation, which appears at an adiabatic ionization energy (AIE) of 8.90 ± 0.05 eV and exhibits an uncommon triplet (3B1) ground state, while the closed-shell singlet (1A1) state lies just 2.8 kcal mol-1 (0.12 eV) higher in energy. The reactive phenylethynyl radical abstracts hydrogen to form ethynylbenzene (C6H5CCH) but also isomerizes via H-shift to the o-, m-, and p-ethynylphenyl isomers (C6H4CCH). These radicals are very reactive and undergo ring-opening followed by H-loss to form a mixture of C8H4 triynes, along with low yields of cyclic 3- and 4-ethynylbenzynes (C6H3CCH). At higher temperatures, dehydrogenation from the unbranched C8H4 triynes forms the linear tetraacetylene (C8H2), an astrochemically relevant polyyne.

Photoelectron Photoion Coincidence Spectroscopy of Biradicals

The electronic structure of biradicals is characterized by the presence of two unpaired electrons in degenerate or near-degenerate molecular orbitals. In particular, some of the most relevant species are highly reactive, difficult to generate cleanly and can only be studied in the gas phase or in matrices. Unveiling their electronic structure is, however, of paramount interest to understand their chemistry. Photoelectron photoion coincidence (PEPICO) spectroscopy is an excellent approach to explore the electronic states of biradicals, because it enables a direct correlation between the detected ions and electrons. This permits to extract unique vibrationally resolved photoion mass-selected threshold photoelectron spectra (ms-TPES) to obtain insight in the electronic structure of both the neutral and the cation. In this review we highlight most recent advances on the spectroscopy of biradicals and biradicaloids, utilizing PEPICO spectroscopy and vacuum ultraviolet (VUV) synchrotron radiation.

Photoelectron spectroscopy in molecular physical chemistry

Photoelectron spectroscopy has long been a powerful method in the toolbox of experimental physical chemistry and molecular physics. Recent improvements in coincidence methods, charged-particle imaging, and electron energy resolution have greatly expanded the variety of environments in which photoelectron spectroscopy can be applied, as well as the range of questions that can now be addressed. In this Perspectives Article, we focus on selected recent studies that highlight these advances and research areas. The topics include reactive intermediates and new thermochemical data, high-resolution comparisons of experiment and theory using methods based on pulsed-field ionisation (PFI), and the application of photoelectron spectroscopy as an analytical tool to monitor chemical reactions in complex environments, like model flames, catalytic or high-temperature reactors.

Photoelectron spectroscopy of boron-containing reactive intermediates using synchrotron radiation: BH2, BH, and BF

Mass selected slow photoelectron spectra (SPES) of three boron-containing reactive species, BH₂, BH, and BF were recorded by double imaging photoion-photoelectron coincidence spectroscopy (i²PEPICO) using synchrotron radiation. All species were generated in a flow reactor from the H-abstraction of B₂H₆ by F atoms created in a F₂ microwave discharge. The spectrum of BH₂⁺ exhibits a long bending mode progression with a 970 cm^-1 spacing due to the large geometry change from bent to linear upon ionization. Its ionization energy was determined as 8.12 ± 0.02 eV. For BH, photoionisation from both X¹Σ⁺ singlet and a³Π triplet state was observed, permitting the experimental determination of the singlet/triplet gap (ΔE_ST) from the observed IE's of 9.82 eV and 8.48 eV. In addition, a threshold photoelectron spectrum of BF was recorded, which leads to an IE of 11.11 eV and an improved value for ν_BF⁺ of 1690 cm^-1. All spectra were simulated by calculating Franck-Condon factors from optimised structures based on quantum chemical calculations.

Photoionization studies of reactive intermediates using synchrotron radiation

Photoionization with synchrotron radiation enables sensitive and selective monitoring of reactive intermediates in environments such as flames and plasmas.

Disentangling the complex spectrum of the ethynyl cation

The ethynyl cation, C2H+, is of great importance in astrophysical media and in combustion. It is involved in the formation of larger organic compounds and in their decomposition mechanisms. Here, we investigate the low-lying electronic states of this cation using pure ab initio methodologies. The evolution of its potential energy surfaces along the stretching and bending coordinates reveals a high density of electronic states that favours mutual interactions and the mixing of wavefunctions. The ground state is of 3Π space symmetry and the lowest singlet state (1Π) is found to be a quasi-linear-quasi-linear Renner-Teller system. Our work suggests that the (spin-)rovibronic spectrum of such a molecular system is complicated, because of the contributions of multiple couplings, including Renner-Teller, vibronic and spin-orbit. We also deduced the adiabatic ionization energy of the ethynyl radical, in good agreement with recent measurements. In summary, our work shows that the ethynyl cation, in spite of its small size, still represents a challenging molecular problem to be solved.