News-Archiv

24.05.2023

Another Torture Track for Quantum Chemistry: Reinvestigation of the Benzaldehyde Amidation by Nitrogen-Atom Transfer from Platinum(II) and Palladium(II) Metallonitrenes

Veröffentlichung in "Israel Journal of Chemistry"

H. Verplancke, M. Diefenbach, J. N. Lienert, M. Ugandi, M.-P. Kitsaras, M. Roemelt, S. Stopkowicz and M. C. Holthausen

We showcase here a dramatic failure of CCSD(T) theory that originates from the pronounced multi-reference character of a key intermediate formed in the benzaldehyde amidation by N-atom transfer from Pd(II) and Pt(II) metallonitrenes studied recently in combined experimental and theoretical work. For detailed analysis we devised a minimal model system, for which we established reliable reference energies based on approximate full configuration interaction theory, to assess the performance of single-reference coupled-cluster theory up to the CCSDTQ(P) excitation level. While RHF-based CCSD(T) theory suffered dramatic errors, in one case exceeding 220 kcal mol⁻¹, we show that the use of broken-symmetry (BS) or Kohn-Sham (KS) orbital references yields substantially improved CCSD(T) results. Further, the EOM-SF-CCSD(T)(a)* approach met the reference data with excellent accuracy. We applied the KS-CCSD(T*)-F12b variant as high-level part of an ONIOM(KS-CC:DFT) scheme to reinvestigate the reactivity of the full Pt(II) and Pd(II) metallonitrenes. The revised reaction pathway energetics provide a detailed mechanistic rationale for the experimental observations.

Vollständiger Artikel

14.01.2023

A DZ white dwarf with a 30 MG magnetic field

Veröffentlichung in "Monthly Notices of the Royal Astronomical Society"

M. A. Hollands, S. Stopkowicz, M.-P. Kitsaras, F. Hampe, S. Blaschke and J.J. Hermes

Magnetic white dwarfs with field strengths below 10 MG are easy to recognise since the Zeeman splitting of spectral lines appears proportional to the magnetic field strength. For fields ≳ 100 MG, however, transition wavelengths become chaotic, requiring quantum-chemical predictions of wavelengths and oscillator strengths with a non-perturbative treatment of the magnetic field. While highly accurate calculations have previously been performed for hydrogen and helium, the variational techniques employed become computationally intractable for systems with more than three to four electrons. Modern computational techniques, such as finite-field coupled-cluster theory, allow the calculation of many-electron systems in arbitrarily strong magnetic fields. Because around 25 percent of white dwarfs have metal lines in their spectra, and some of those are also magnetic, the possibility arises for some metals to be observed in very strong magnetic fields, resulting in unrecognisable spectra. We have identified SDSS J114333.48+661531.83 as a magnetic DZ white dwarf, with a spectrum exhibiting many unusually shaped lines at unknown wavelengths. Using atomic data calculated from computational finite-field coupled-cluster methods, we have identified some of these lines arising from Na, Mg, and Ca. Surprisingly, we find a relatively low field strength of 30 MG, where the large number of overlapping lines from different elements make the spectrum challenging to interpret at a much lower field strength than for DAs and DBs. Finally we model the field structure of SDSS J1143+6615 finding the data are consistent with an offset dipole.

Vollständiger Artikel

31.10.2022

Trendbericht Theoretische Chemie 2022: Quantenchemie für Atome und Moleküle in starken Magnetfeldern

Veröffentlichung in "Nachrichten aus der Chemie"

Maschinelles Lernen eignet sich, um Photochemie und somit elektronisch angeregte Zustände zu beschreiben; klassische Molekulardynamiktechniken erlauben, bestimmte Aspekte der nuklearen Quanteneffekte in Probleme der physikalischen Chemie einzubeziehen, und was Finite-Feld-Methoden mit alten Sternen zu tun haben.

Vollständiger Artikel