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66 Publications visible to you, out of a total of 66
New mechanism for autoxidation of polyolefins: kinetic Monte Carlo modelling of the role of short-chain branches, molecular oxygen and unsaturated moieties

Abstract (Expand)

In this work, a bivariate kinetic Monte Carlo (kMC) model is constructed to study autoxidation, which is the degradation of polymers in the presence of oxygen. The use of computational methods for the … determination of rate coefficients as input for the model is illustrated. Focus is on the presence of short-chain branches (SCB) and unsaturated moieties and their role in the fate of alkyl, alkoxyl and alkylperoxyl radicals in the autoxidation mechanism. The autoxidation kinetics are studied for three model polymers, namely poly(ethylene) (reference case), poly(butadiene) (presence of allylic hydrogens), and poly(isobutylene) (presence of quaternary carbon atoms). Using the kMC model, reaction path analysis shows that the autoxidation mechanism for each of the polymer types follows a chain reaction mechanism, but that the presence of branches/unsaturated moieties influences the dominant reaction pathway in the autoxidation mechanism, and thus also the autoxidation rate. It is shown that the influence of varying oxygen concentration and initiation rate coefficient (e.g. to simulate variable ultraviolet (UV) light intensity) on the dominant pathway is small as their role is mainly situated in the first steps of the chain mechanism.

Authors: Lies De Keer, Paul Van Steenberge, Marie-Françoise Reyniers, Ganna Gryn'ova, Heather M. Aitken, Michelle L. Coote

Date Published: 2022

Publication Type: Journal

Dioxygen Activation and Pyrrole α‐Cleavage with Calix[4]pyrrolato Aluminates: Enzyme Model by Structural Constraint

Abstract (Expand)

The present work describes the reaction of triplet dioxygen with the porphyrinogenic calix[4]pyrrolato aluminates to alkylperoxido aluminates in high selectivity. Multiconfigurational quantum chemical … computations disclose the mechanism for this spin-forbidden process. Despite a negligible spin–orbit coupling constant, the intersystem crossing (ISC) is facilitated by singlet and triplet state degeneracy and spin–vibronic coupling. The formed peroxides are stable toward external substrates but undergo an unprecedented oxidative pyrrole α-cleavage by ligand aromatization/dearomatization-initiated O−O σ-bond scission. A detailed comparison of the calix[4]pyrrolato aluminates with dioxygen-related enzymology provides insights into the ISC of metal- or cofactor-free enzymes. It substantiates the importance of structural constraint and element–ligand cooperativity for the functions of aerobic life.

Authors: Lukas Maximilian Sigmund, Christopher Ehlert, Markus Enders, Jürgen Graf, Ganna Gryn'ova, Lutz Greb

Date Published: 5th Jul 2021

Publication Type: Journal

The Law of Attraction: Relating Computed Energetics of Physisorption with Performance of Graphene-Based Sensors for Nitroaromatic Contaminants

Abstract (Expand)

The ability to detect persistent nitroaromatic contaminants, e.g. DNT and TNT, with high sensitivity and selectivity is central to environmental science and medicinal diagnostics. Graphene-based materials … rise to this challenge, offering supreme performance, biocompatibility, and low toxicity at a reasonable cost. In the first step of the electrochemical sensing process, these substrates establish non-covalent interactions with the analytes, which we show to be indicative of their respective detection limits. Employing a combination of semiempirical tight binding quantum chemistry, meta- dynamics, density functional theory, and symmetry-adapted perturbation theory in conjunction with curated data from experimental literature, we investigate the physisorption of DNT and TNT on a series of functionalised graphene derivatives. In agreement with experimental observations, systems with greater planarity and positively charged substrates afford stronger non-covalent interactions than their highly oxidised distorted counterparts. Despite the highly polar nature of the investigated species, their non-covalent interactions are largely driven by dispersion forces. To harness these design principles, we considered a series of boron and nitrogen (co)doped two-dimensional materials. One of these systems featuring a chain of B–N–C units was found to adsorb nitroaromatic molecules stronger than the pristine graphene itself. These findings form the basis for the design principles of sensing materials and illustrate the utility of relatively low cost in silico procedures for testing the viability of designed graphene-based sensors for a plethora of analytes.

Authors: Anna Piras, Ganna Gryn'ova

Date Published: 5th Apr 2021

Publication Type: Unpublished

Sensing and sensitivity: Computational chemistry of graphene‐based sensors

Abstract (Expand)

Highly efficient, tunable, biocompatible, and environmentally friendly electrochemical sensors featuring graphene‐based materials pose a formidable challenge for computational chemistry. In silico … rationalization, optimization and, ultimately, prediction of their performance requires exploring a vast structural space of potential surface‐analyte complexes, further complicated by the presence of various defects and functionalities within the infinite graphene lattice. This immense number of systems and their periodic nature greatly limit the choice of computational tools applicable at a reasonable cost. An alternative approach using finite nanoflake models opens the doors to many more advanced and accurate electronic structure methods, while sacrificing the realism of representation. Locating the surface‐analyte complex is followed by an in‐depth in silico analysis of its energetic and electronic properties using, for example, energy decomposition schemes, as well as simulation of the signal, for example, a zero‐bias transmission spectra or a current–voltage curve, by means of the nonequilibrium Green's function method. These and other properties are examined in the context of a sensor's selectivity, sensitivity, and limit of detection with an aim to establish design principles for future devices. Herein, we analyze the advantages and limitations of diverse computational chemistry methods used at each of these steps in simulating graphene‐based electrochemical sensors. We present outstanding challenges toward predictive models and sketch possible solutions involving such contemporary techniques as multiscale simulations and high‐throughput screening.

Authors: Anna Piras, Christopher Ehlert, Ganna Gryn'ova

Date Published: 3rd Mar 2021

Publication Type: Journal

Ultrafast dynamics of 2-thiouracil investigated by time-resolved Auger spectroscopy

Abstract (Expand)

We present time-resolved ultraviolet-pump x-ray probe Auger spectra of 2-thiouracil. An ultraviolet induced shift towards higher kinetic energies is observed in the sulfur 2p Auger decay. The difference … Auger spectra of pumped and unpumped molecules exhibit ultrafast dynamics in the shift amplitude, in which three phases can be recognized. In the first 100 fs, a shift towards higher kinetic energies is observed, followed by a 400 fs shift back to lower kinetic energies and a 1 ps shift again to higher kinetic energies. We use a simple Coulomb-model, aided by quantum chemical calculations of potential energy states, to deduce a C–S bond expansion within the first 100 fs. The bond elongation triggers internal conversion from the photoexcited S2 to the S1 state. Based on timescales, the subsequent dynamics can be interpreted in terms of S1 nuclear relaxation and S1-triplet internal conversion.

Authors: F Lever, D Mayer, 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: 17th Dec 2020

Publication Type: Journal

PSIXAS : A Psi4 plugin for efficient simulations of X‐ray absorption spectra based on the transition‐potential and Δ‐Kohn–Sham method

Abstract (Expand)

Near edge X‐ray absorption fine structure (NEXAFS) spectra and their pump‐probe extension (PP‐NEXAFS) offer insights into valence‐ and core‐excited states. We present PSIXAS, a recent implementation … for simulating NEXAFS and PP‐NEXAFS spectra by means of the transition‐potential and the Δ‐Kohn–Sham method. The approach is implemented in form of a software plugin for the Psi4 code, which provides access to a wide selection of basis sets as well as density functionals. We briefly outline the theoretical foundation and the key aspects of the plugin. Then, we use the plugin to simulate PP‐NEXAFS spectra of thymine, a system already investigated by others and us. It is found that larger, extended basis sets are needed to obtain more accurate absolute resonance positions. We further demonstrate that, in contrast to ordinary NEXAFS simulations, where the choice of the density functional plays a minor role for the shape of the spectrum, for PP‐NEXAFS simulations the choice of the density functional is important. Especially hybrid functionals (which could not be used straightforwardly before to simulate PP‐NEXAFS spectra) and their amount of “Hartree‐Fock like” exact exchange affects relative resonance positions in the spectrum.

Authors: Christopher Ehlert, Tillmann Klamroth

Date Published: 15th Jul 2020

Publication Type: Journal

Au 21 cage structures and their magic number tri‐cations

Abstract (Expand)

We examine the stability and properties of three Au21 cage structures, one with D3 symmetry and denoted as Au21 (D3), which is novel, and the other two with C2v symmetry. One, denoted as Au21 (C2v-1), … has been previously reported but the other, denoted as Au21 (C2v-2), is novel. As reference Au21 structures, we also examine a sheet isomer and a compact isomer, Au21 (Cs-Tetra), formed by adsorbing an Au atom on Au20 (Td). For all structures, we consider charge ranging from −1 to +4. For the Au21 cage structures, a primary property of interest is their spherical aromaticity, as measured by their nucleus independent chemical shift. Our focus is on charge +3 since each gold atom is assumed to contribute one (6s) valence electron, and 18 is a magic number for shell closing. We find that, although Au21 (D3)+3 has the largest aromaticity, it is not the most stable urn:x-wiley:00207608:media:qua26191:qua26191-math-0001 cage species. Surprisingly, Au21 (C2v-2), which is not even stable as a neutral cage species, is the most stable tri-cation cage species. We also examine the stability of (neutral) Au21Xn cage structures relative to Au21Xn structures derived from Au21 (Cs-Tetra) (with X = F, Cl, Br, I and n = 1, 3, 5). We find that, although Au21F3 derived from Au21 (D3) has the largest aromaticity, it is not the most stable Au21F3 cage structure. Nonetheless, all cage structures are stabilized relative to Au21 (Cs-Tetra) and, remarkably, for the trichlorinated, tribrominated, and triiodineated clusters, at least one cage structure is more stable than its Au21 (Cs-Tetra) counterpart.

Authors: Christopher Ehlert, Xiaojing J. Liu, Ian P. Hamilton

Date Published: 5th Jun 2020

Publication Type: Journal

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