Benoit Mignolet

Department of Chemistry, UR MOLSYS, University of Liège, Belgium

Video Recording

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Abstract

Upon light absorption, the 5-atom thioformaldehyde S-oxide sulfine (H₂CSO) molecule undergoes a rich and unexpected dynamics resulting in the formation of nine photoproducts(1). In this presentation, we will first shed light on the sulfine photochemistry using state of the art molecular modeling and then investigate its photoexcitation by attosecond and few-femtosecond laser pulses using the XFAIMS mixed-quantum classical method that we recently developed.
We shed light on the sulfine photochemistry by reproducing in silico the dynamics following the photoexcitation of the sulfine to its lowest excited state(2). The nonradiative decay to the ground state as well as the following dynamics occurring on the ground state have been modeled with respectively the ab initio multiple spawning(3) (AIMS) method carried out at the MS-CASPT2 level and the Born-Oppenheimer molecular dynamics carried out at the DFT/PBE0 level. Amongst the nine photoproducts observed experimentally, we retrieve them all but surprisingly most of them (8) are formed on the ground state on a sub-picosecond timescale. Therefore the dynamics occurring on the hot ground state cannot be described by statistical methods such as RRKM because of the highly non-statistical character stemming from the excited-state dynamics. This unexpectedly rich chemistry occurring on the hot ground state challenges our view of typical photochemical or photodecomposition reactions in which it is usually the variety of conical intersections that leads to a variety of photoproducts.
We also investigated the photoexcitation the sulfine by attosecond and few-femtosecond laser pulses. With the recent developments of these short pulses (4), new opportunities for the controlling and probing of the electronic motion in atoms and molecules emerge. Due to the short nature of the laser pulse, the bandwidth can reach several eV and a band of several electronic states can be accessed, which can lead to an ultrafast charge migration. Therefore it becomes paramount to model the interaction with laser pulses. In this goal, we developed the eXternal Field Ab-Initio Multiple Spawning (5) (XFAIMS) method to model both the photoexcitation and nonradiative relaxation in medium-sized molecules. It is based on the well-known AIMS method in which we added the interaction with the electric field and adapted the spawning algorithm to fully model experiments from the photoexcitation of the molecule by the laser pulse to the nonradiative relaxation and/or its dissociation.

Prof. Dr. Vera Krewald

Technical University of Darmstadt, Department of Chemistry, Theoretical Chemistry Alarich-Weiss-Str. 4, 64287 Darmstadt, Germany

Exchange coupling interactions between open-shell ions in polynuclear transition metal complexes define key magnetic and spectroscopic properties of these systems. The metal coordination environment, especially the bridging ligands, determine the nature and magnitude of the magnetic coupling. This lecture will introduce phenomenological models for the interpretation of experimental data and computational chemistry methods relevant to the prediction and analysis of magnetically coupled electronic structures. 

Density functional theory (DFT), in particular broken-symmetry DFT (BS-DFT), is used routinely to predict the sign, strength and origin of magnetic coupling in transition metal complexes. The advantages and intrinsic limitations of BS-DFT will be discussed. 

In contrast to BS-DFT, multireference quantum-chemical calculations are in principle capable of describing each individual spin state arising from magnetic coupling of open-shell ions. The use of density matrix renormalization group (DMRG) for the description of realistic systems with multiple centers and many unpaired electrons will be outlined. In addition, a simple analytic tool that permits the identification of exchange coupling pathways in polynuclear transition metal complexes from an entanglement analysis will be introduced.

Recording:

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References

(i) F. E. Mabbs, D. J. Machin. Magnetism and Transition Metal Complexes. London: Chapman & Hall, 1973.

(ii) J. P. Malrieu, R. Caballol, C. J. Calzado, C. de Graaf, N. Guihéry. ‘Magnetic Interactions in Molecules and Highly Correlated Materials: Physical Content, Analytical Derivation, and Rigorous Extraction of Magnetic Hamiltonians’. Chem. Rev. 114, 2014, 429–492. https://doi.org/10.1021/cr300500z

(iii) D. A. Pantazis, V. Krewald, M. Orio, F. Neese. ‘Theoretical Magnetochemistry of Dinuclear Manganese Complexes: Broken Symmetry Density Functional Theory Investigation on the Influence of Bridging Motifs on Structure and Magnetism’. Dalton Trans. 39, 2010, 4959–4967. https://doi.org/10.1039/c001286f

(iv) V. Krewald, F. Neese, D. A. Pantazis. ‘On the Magnetic and Spectroscopic Properties of High-Valent Mn3CaO4 Cubanes as Structural Units of Natural and Artificial Water-Oxidizing Catalysts’. J. Am. Chem. Soc. 135, 2013, 5726–5739. https://doi.org/10.1021/ja312552f

(v) M. Roemelt, V. Krewald, D. A. Pantazis. ‘Exchange Coupling Interactions from the Density Matrix Renormalization Group and N-Electron Valence Perturbation Theory: Application to a Biomimetic Mixed-Valence Manganese Complex’. J. Chem. Theory Comput. 14, 2018, 166–179. https://doi.org/10.1021/acs.jctc.7b01035

(vi) C. J. Stein, D. A. Pantazis, V. Krewald. ‘Orbital Entanglement Analysis of Exchange-Coupled Systems’. J. Phys. Chem. Lett. 10, 2019, 6762–6770. https://doi.org/10.1021/acs.jpclett.9b02417

Henrique C. S. Junior

Universidade Federal Fluminense (UFF) – Rio de Janeiro, Brazil

Video Recording

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Abstract

Molecular Magnetic Compounds can have several different features, from single molecules with long range couplings to complex polymeric chains with strong interactions. The field of Molecular Magnetism occupy itself in understanding how these systems interact and in chemically modulating magnetic couplings by selecting adequate ligands and paramagnetic centers. The task to devise how the infinity of magnetic systems couple with each other in a crystal structure is possible with a methodological First-priciples Bottom-up (FPBU) approach and the use of Computational Methods like the Density Functional Theory (DFT), allowing for fast and accurate results. In this presentation we show, using examples of weak and strong interactions, how to apply the FPBU approach to choose reasonable magnetic systems as relevant candidates for magnetic coupling studies, functionals and basis sets leading to better results and how to use the broken-symmetry approach to obtain magnetic couplings (J).