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Threshold Photoelectron Spectrum of Cyclobutadiene: Comparison with Time-Dependent Wavepacket Simulations
... When combined with high-level computations, photoelectron spectroscopy has shown to be an excellent tool to characterize the electronic structure of reactive molecules, 39 as shown for example in recent work on C 4 H 4 , the paradigm for antiaromaticity. 40 Note that the photoionization of the isomer isocyanic acid, HNCO has already been extensively investigated by our group. 41,42 II. ...
... The photoelectron spectrum of fulminic acid was simulated in a similar manner to that described previously for both cyclobutadiene 40,64 and phenol 65 from wavepacket dynamics simulations using the MCTDH method 66 implemented in Quantics 63 . In this case, however, thermal effects were found to be important and a thermalised density operator was propagated in place of the usual 0 K wavepacket (see SI for 0 K spectra and assignments). ...
We report a joint experimental and computational study of the photoelectron spectroscopy and the dissociative photoionization of fulminic acid, HCNO. The molecule is of interest to astrochemistry and astrobiology as a potential precursor of prebiotic molecules. Synchrotron radiation was used as the photon source. Dispersive photoelectron spectra were recorded from 10~eV to 22~eV, covering four band systems in the HCNO cation and an ionization energy of 10.83~eV was determined. Transitions into the Renner-Teller distorted $X^+{}^2\Pi$ state of the cation were simulated using wavepacket dynamics based on a vibronic coupling Hamiltonian. Very good agreement between experiment and theory is obtained. While the first excited state of the cation shows only a broad and unstructured spectrum, the next two higher states exhibit a well-resolved vibrational progression. Transitions into the excited electronic states of \cation{HCNO}{+} were not simulated, due to the large number of electronic states that contribute to these transitions. Nevertheless, a qualitative assignment is given, based on the character of the orbitals involved in the transitions. The dissociative photoionization was investigated by photoelectron-photoion coincidence spectroscopy. The breakdown diagram shows evidence for isomerization from \cation{HCNO}{+} to \cation{HNCO}{+} on the cationic potential energy surface. Zero Kelvin appearance energies for the daughter ions \cation{HCO}{+} and \cation{NCO}{+} have been derived.
... In recent work, time-dependent wavepacket dynamics was employed to simulate the photoelectron spectrum, using the MCTDH (multiconfiguration time-dependent Hartree) approach. [111] The model included vibronic coupling in both ion, and neutral, and considered eight vibrational modes in the cation. ...
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.
Photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) is a synchrotron-based, universal, sensitive, and multiplexed detection tool applied in the areas of catalysis, combustion, and gas-phase reactions. Isomer-selective vibrational fingerprints in the ms-TPES of stable and reactive intermediates allow for unequivocal assignment of spectral carriers. Case studies are presented on heterogeneous catalysis, revealing the role of ketenes in the methanol-to-olefins process, the catalytic pyrolysis mechanism of lignin model compounds, and the radical chemistry upon C-H activation in oxyhalogenation. These studies demonstrate the potential of ms-TPES as an analytical technique for elucidating complex reaction mechanisms. We examine the robustness of ms-TPES assignments and address sampling effects, especially the temperature dependence of ms-TPES due to rovibrational broadening. Data acquisition approaches and the Stark shift from the extraction field are also considered to arrive at general recommendations. Finally, the PhotoElectron PhotoIon Spectral Compendium (https://pepisco.psi.ch), a spectral database hosted at Paul Scherrer Institute to support assignment, is introduced.
Due to their unusual electronic structure, the biradical m-benzyne, C6H4, and its cation are of considerable interest in chemistry. Here, the photoion mass-selected threshold photoelectron spectrum of the m-benzyne biradical is presented. An adiabatic ionization energy of 8.65 ± 0.015 eV is derived, while a vibrational progression of 0.10 eV is assigned to the ν9+ ring breathing mode, in excellent agreement with computations. The experimental spectrum was reproduced well by Franck-Condon spectral modeling of the 2A1 ← X 1A1 transition, in which the cation retains a monocyclic C6 framework. The energetically close-lying bicyclic 2A2 cation state exhibits low Franck-Condon factors, due to the large change in geometry, and thus cannot be observed.
We provide compelling experimental and theoretical evidence for the transition state nature of the cyclopropyl cation. Synchrotron photoionization spectroscopy employing coincidence techniques together with a novel simulation based on high-accuracy ab initio calculations reveal that the cation is unstable via its allowed disrotatory ring-opening path. The ring strains of the cation and the radical are similar, but both ring opening paths for the radical are forbidden when the full electronic symmetries are considered. These findings are discussed in light of the early predictions by Longuet-Higgins alongside Woodward and Hoffman; we also propose a simple phase space explanation for the appearance of the cyclopropyl photoionization spectrum. The results of this work allow the refinement of the cyclopropane C-H bond dissociation energy, in addition to the cyclopropyl radical and cation cyclization energies, via the Active Thermochemical Tables approach.
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.
A Package of ab initio Programs
- H J Werner
- P J Knowles
- G Knizia
- F R Manby
- M Schütz
- M Abe
- Diradicals
Abe, M. Diradicals. Chem. Rev. 2013, 113, 7011−7088.
- J H D Eland
Eland, J. H. D. Photoelectron Spectroscopy; Butterworths, London, 2013.
- H Köppel
- W Domcke
- L S Cederbaum
Köppel, H.; Domcke, W.; Cederbaum, L. S. Adv. Chem. Phys. 1984, 57, 59.
- M J Frisch
Frisch, M. J.; et al. Gaussian 09, Rev. E.01; Gaussian Inc.:
Wallingford, CT, 2013.
- I Judy
- C Wu
- Y Mo
- F A Evangelista
Judy, I.; Wu, C.; Mo, Y.; Evangelista, F. A.; von Ragué Schleyer, P. Chem. Comm.
2012, 48, 8437-8439.
- T Bally
Bally, T. Angew. Chem. Int. Ed. 2006, 45, 6616-6619.
- A D Allen
- T T Tidwell
Allen, A. D.; Tidwell, T. T. Chem. Rev. 2001, 101, 1333-1348.
- T Stuyver
- B Chen
- T Zeng
- P Geerlings
- F De Proft
- R Hoffmann
Stuyver, T.; Chen, B.; Zeng, T.; Geerlings, P.; De Proft, F.; Hoffmann, R. Chem. Rev.
2019, 119, 11291-11351.
- S Masamune
- F A Souto-Bachiller
- T Machiguchi
- J E Bertie
Masamune, S.; Souto-Bachiller, F. A.; Machiguchi, T.; Bertie, J. E. J. Am. Chem. Soc.
1978, 100, 4889-4891.
- M Paterson
- M Bearpark
- M Robb
- L Blancafort
- G Worth
Paterson, M.; Bearpark, M.; Robb, M.; Blancafort, L.; Worth, G. Phys. Chem. Chem.
Phys. 2005, 7, 2100-2115.
- H J Wörner
- F Merkt
Wörner, H. J.; Merkt, F. J. Chem. Phys 2007, 127, 034303.
- J Kreile
- N Mnzel
- A Schweig
- H Specht
Kreile, J.; Mnzel, N.; Schweig, A.; Specht, H. Chem. Phys. Lett. 1986, 124, 140-146.
- D W Kohn
- P Chen
Kohn, D. W.; Chen, P. J. Am. Chem. Soc. 1993, 115, 2844-2848.