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68 Publications visible to you, out of a total of 68
Metal-Free Molecular Catalysts for the Oxygen Reduction Reaction: Electron Affinity as an Activity Descriptor

Abstract (Expand)

Heteroatom-doped polyaromatic hydrocarbons (or nanographenes) are promising molecular electrocatalysts for the oxygen reduction reaction (ORR). Here, we use density functional theory to investigate the … first step of the ORR pathway (chemisorption) for a set of molecules with experimentally determined catalytic activities. Weak chemisorption is found for only negatively charged catalysts, and a strong correlation is observed between the computed electron affinities and experimental catalytic activities for a range of B- and B,N-doped polyaromatic hydrocarbons. The electron affinity is put forward as a simple activity descriptor of charged (activated) catalysts on an electrode.

Authors: Christopher Ehlert, Anna Piras, Juliette Schleicher, Ganna Gryn’ova

Date Published: 19th Jan 2023

Publication Type: Journal

Locating Guest Molecules inside Metal–Organic Framework Pores with a Multilevel Computational Approach

Abstract (Expand)

Molecular docking has traditionally mostly been employed in the field of protein–ligand binding. Here, we extend this method, in combination with DFT-level geometry optimizations, to locate guest … molecules inside the pores of metal–organic frameworks. The position and nature of the guest molecules tune the physicochemical properties of the host–guest systems. Therefore, it is essential to be able to reliably locate them to rationally enhance the performance of the known metal–organic frameworks and facilitate new material discovery. The results obtained with this approach are compared to experimental data. We show that the presented method can, in general, accurately locate adsorption sites and structures of the host–guest complexes. We therefore propose our approach as a computational alternative when no experimental structures of guest-loaded MOFs are available. Additional information on the adsorption strength in the studied host–guest systems emerges from the computed interaction energies. Our findings provide the basis for other computational studies on MOF–guest systems and contribute to a better understanding of the structure–interaction–property interplay associated with them.

Authors: Michelle Ernst, Tomasz Poręba, Lars Gnägi, Ganna Gryn’ova

Date Published: 12th Jan 2023

Publication Type: Journal

Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy

Abstract (Expand)

The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically … excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220–250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.

Authors: D. Mayer, F. Lever, D. Picconi, J. Metje, S. Alisauskas, F. Calegari, S. Düsterer, C. Ehlert, R. Feifel, M. Niebuhr, B. Manschwetus, M. Kuhlmann, T. Mazza, M. S. Robinson, R. J. Squibb, A. Trabattoni, M. Wallner, P. Saalfrank, T. J. A. Wolf, M. Gühr

Date Published: 1st Dec 2022

Publication Type: Journal

Marchettiite, (NH 4 )C 5 H 3 N 4 O 3 , a new organic mineral from Mount Cervandone, Devero Valley, Western–Central Alps, Italy

Abstract (Expand)

The new mineral marchettiite (IMA2017-066) is the natural equivalent of ammonium hydrogen urate. It has a simple molecular formula C5H7N5O3 and can be alternatively written as (NH4)C5H3N4O3. Marchettiite … was found in a cleft at Mount Cervandone, Devero Valley, Piedmont, Italy, where it occurs as aggregates of opaque pale pink to white, platy prismatic crystals. This mineral has a white streak, dull and opaque lustre, it is not fluorescent and has a hardness of 2–2.5 (Mohs’ scale). The tenacity is brittle and crystals have a good cleavage parallel to {001}. The calculated density is 1.69 g/cm3. Marchettiite is biaxial (–) with 2V of 47.24°; the optical properties of marchettiite were determined by periodic-DFT methods providing the following values: α = 1.372, β = 1.681 and γ = 1.768. No twinning was observed. Electron microprobe analyses gave the following chemical formula: C4.99H6.97N4.91O3.00. Although the small crystal size did not allow refinement of structural data by single-crystal diffraction, we were able to refine the structure by powder micro X-ray diffraction. Marchettiite has space group P and the following unit-cell parameters: a = 3.6533(2) Å, b = 10.2046(7) Å, c = 10.5837(7) Å, α = 113.809(5)°, β = 91.313(8)°, γ = 92.44(1)° and V = 360.312 Å3. The strongest lines in the powder diffraction pattern [d in Å (I)(hkl)] are: 9.784(50)(001); 8.663(80)(01); 5.659(100)(011); 3.443(100)(10); 3.241(70)(003) and 3.158(100)(1. Marchettiite is named after Gianfranco Marchetti, the mineral collector who found this mineral.

Authors: Alessandro Guastoni, Fabrizio Nestola, Federico Zorzi, Arianna Lanza, Michelle Ernst, Paolo Gentile, Sergio Andò, Alessandra Lorenzetti

Date Published: 1st Dec 2022

Publication Type: Journal

Pitfalls in the location of guest molecules in metal-organic frameworks

Abstract

Not specified

Authors: Tomasz Poręba, Piero Macchi, Michelle Ernst

Date Published: 1st Dec 2022

Publication Type: Journal

Why Ultrafast Photoinduced CO Desorption Dominates over Oxidation on Ru(0001)

Abstract (Expand)

CO oxidation on Ru(0001) is a long-standing example of a reaction that, being thermally forbidden in ultrahigh vacuum, can be activated by femtosecond laser pulses. In spite of its relevance, the precise … dynamics of the photoinduced oxidation process as well as the reasons behind the dominant role of the competing CO photodesorption remain unclear. Here we use ab initio molecular dynamics with electronic friction that account for the highly excited and nonequilibrated system created by the laser to investigate both reactions. Our simulations successfully reproduce the main experimental findings: the existence of photoinduced oxidation and desorption, the large desorption to oxidation branching ratio, and the changes in the O K-edge X-ray absorption spectra attributed to the initial stage of the oxidation process. Now, we are able to monitor in detail the ultrafast CO desorption and CO oxidation occurring in the highly excited system and to disentangle what causes the unexpected inertness to the otherwise energetically favored oxidation.

Authors: Auguste Tetenoire, Christopher Ehlert, J. I. Juaristi, Peter Saalfrank, M. Alducin

Date Published: 15th Sep 2022

Publication Type: Journal

Stereoinversion of tetrahedral p -block element hydrides

Abstract (Expand)

The potential energy surfaces of 15 tetrahedral p-block element hydrides were screened on the multireference level. It was addressed whether stereoinversion competes against other reactions, such as … reductive H2-elimination or hydride loss, and if so, along which pathway the stereomutation occurs. Importantly, stereoinversion transition structures for the ammonium cation (C4v) and the tetrahydridoborate anion (Cs) were identified for the first time. Revisiting methane’s Cs symmetric inversion transition structure with the mHEAT+ protocol revealed an activation enthalpy for stereoinversion, in contrast to all earlier studies, which is 5 kJ mol−1 below the C–H bond dissociation enthalpy. Square planar structures were identified lowest in energy only for the inversion of AlH4−, but a novel stepwise Cs-inversion was discovered for SiH4 or PH4+. Overall, the present contribution delineates essentials of the potential energy surfaces of p-block element hydrides, while structure–energy relations offer design principles for the synthetically emerging field of structurally constrained compounds.

Authors: Lukas M. Sigmund, Christopher Ehlert, Ganna Gryn’ova, Lutz Greb

Date Published: 21st May 2022

Publication Type: Journal

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