Photoelectron Photoion Coincidence Spectroscopy of Biradicals

Abstract

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 spectroscopic study of 2-naphthylnitrene and its thermal rearrangement to cyanoindenes

“…Thermochemical data like heat of formation and bond strengths in neutral molecules and ions are important for a mechanistic understanding of astrochemical environments. 36,37…”

Thermal Decomposition of 2- and 4-Iodobenzyl Iodide Yields Fulvenallene and Ethynylcyclopentadienes: A Joint Threshold Photoelectron and Matrix Isolation Spectroscopic Study

“…The determined AIEs reported in this article have an uncertainty of 0.01 eV caused by the selected step-size of 10 meV for the recorded spectra. The PEPICO technique coupled to FVP has been used to detect and spectroscopically characterize many fundamental reactive intermediates, in particular, biradicals, as recently reviewed …”

“…The PEPICO technique coupled to FVP has been used to detect and spectroscopically characterize many fundamental reactive intermediates, in particular, biradicals, as recently reviewed. 47 Matrix isolation experiments were performed by standard techniques 48 using two-staged closed-cycle helium refrigerator systems (4 K). Precursors are sublimed and codeposited along with an excess of Ar at a flow rate of 1 sccm onto a CsI window held at 4 K. In FVP experiments, the gas mixture is passed through a hot quartz tube (8 mm diameter and an 80 mm heating zone) kept at 400−1200 K, followed by trapping and IR spectroscopic characterization of the FVP products in the matrix.…”

“…The PEPICO technique coupled to FVP has been used to detect and spectroscopically characterize many fundamental reactive intermediates, in particular, biradicals, as recently reviewed. 47 …”

Lutidyl Radical Photoelectron Spectra Reveal Additive Substituent Effects on Benzyl Derivatives’ Ionization Energy

“…Here, we investigate three lutidyl radicals, namely 2,4-, 2,6-, and 3,5-lutidyl (Scheme 1), as further representatives of the ten possible isomers. Photoelectron photoion coincidence (PEPICO) spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation enables us to obtain spectroscopic insights into reactive intermediates [27] by recording photoion mass-selected threshold photoelectron (ms-TPE) spectra. 2,4-, 2,6-, and 3,5-lutidyl radicals are produced by flash vacuum pyrolysis [28] from 2aminomethyl-4-methylpyridine (2AM4MP), 2-aminomethyl-6methylpyridine (2AM6MP), and 3-aminomethyl-5-methylpyridine (3AM5MP) by deamination, respectively (Scheme 1), and are sampled in a molecular beam.…”