Showing papers by "Christof Hättig published in 2021"
TL;DR: Results and DFT calculations revealed that Pd2+ species interacting with pyridine‐like N atoms significantly enhance the selective hydrogenolysis of the C−OH bond in the presence of FA due to their high ability for the activation of FA and the stabilization of H−.
Abstract: Biomass-derived 5-hydroxymethylfurfural (HMF) is regarded as one of the most promising platform chemicals to produce 2,5-dimethylfuran (DMF) as a potential liquid transportation fuel. Pd nanoparticles supported on N-containing and N-free mesoporous carbon materials were prepared, characterized, and applied in the hydrogenolysis of HMF to DMF under mild reaction conditions. Quantitative conversion of HMF to DMF was achieved in the presence of formic acid (FA) and H2 over Pd/NMC within 2 h. The reaction mechanism, especially the multiple roles of FA, was explored through a detailed comparative study by varying hydrogen source, additive, and substrate as well as by applying in situ ATR-IR spectroscopy. The major role of FA is to shift the dominant reaction pathway from the hydrogenation of the aldehyde group to the hydrogenolysis of the hydroxymethyl group via the protonation by FA at the C-OH group, lowering the activation barrier of the C-O bond cleavage and thus significantly enhancing the reaction rate. XPS results and DFT calculations revealed that Pd2+ species interacting with pyridine-like N atoms significantly enhance the selective hydrogenolysis of the C-OH bond in the presence of FA due to their high ability for the activation of FA and the stabilization of H- .
48 citations
TL;DR: In this article, the process of hydrogen spillover on metal nanoparticle decorated carbon surfaces has been discussed for many years due to its importance for heterogeneous catalysis and hydrogen storage.
Abstract: The process of hydrogen spillover on metal nanoparticle decorated carbon surfaces has been discussed for many years due to its importance for heterogeneous catalysis and hydrogen storage. The prese...
14 citations
12 citations
TL;DR: Using density functional theory with on-site Hubbard U cor... as mentioned in this paper, nanocrystals are efficient and competitive anode materials in the field of electrochemical water splitting using CoFe2O4 (CFO).
Abstract: Cobalt ferrite, CoFe2O4 (CFO), nanocrystals are efficient and competitive anode materials in the field of electrochemical water splitting. Using density functional theory with on-site Hubbard U cor...
9 citations
TL;DR: In this paper, the authors simulated the UV-vis absorption and magnetic circular dichroism spectra of naphthalene and some of its derivatives at the Coupled Cluster Singles and Approximate Doubles (CC2) level of theory, and at the Time-Dependent Density Functional Theory (TD-DFT) level using the B3LYP and CAM-B3lyP functionals.
Abstract: The UV-vis absorption and magnetic circular dichroism spectra of naphthalene and some of its derivatives have been simulated at the Coupled Cluster Singles and Approximate Doubles (CC2) level of theory, and at the Time-Dependent Density Functional Theory (TD-DFT) level using the B3LYP and CAM-B3LYP functionals. DFT and CC2 predict in general opposite energetic ordering of the Lb and La transitions (in gas phase), as previously observed in adenine. The CC2 simulations of UV and MCD spectra show the best agreement with the experimental data. Analysis of the Cartesian components of the electric dipole transition strengths and the magnetic dipole transition moment between the excited states have been considered in the interpretation of the electronic transitions and the Faraday B term inversion among the naphthalene derivatives.
6 citations
TL;DR: In this paper, the ultraviolet and X-ray absorption spectra of pyridazine in aqueous solution, as well as those in the gas phase, were calculated with models based on the algebraic-diagrammatic construction through second order [ADC(2)] with the resolution-of-identity (RI) approximation and compared with the spectra obtained in experiments.
Abstract: Electrostatic interaction of the solvent with the solute and fluctuations of the solvent configurations may make excitation energies of the solute different from those in the gas phase. These effects may dominate photoinduced or chemical reaction dynamics in solution systems and can be observed as shifts or broadening of peaks in absorption spectra. In this work, the nitrogen K-edge X-ray absorption spectra were measured for pyridazine in the gas phase and in aqueous solution. The ultraviolet and X-ray absorption spectra of pyridazine in aqueous solution, as well as those in the gas phase, were then calculated with models based on the algebraic-diagrammatic construction through second order [ADC(2)] with the resolution-of-identity (RI) approximation and compared with the spectra obtained in experiments. For aqueous solution, explicit local solvation structures were extracted from an ab initio molecular dynamics (AIMD) trajectory of pyridazine in bulk water, and RI-ADC(2) was combined with the conductor-like screening model (COSMO). The experimental absorption spectra of pyridazine in aqueous solution were reproduced with good accuracy by theoretical treatment of an ensemble containing the explicit local solvation structures of pyridazine with relevant water molecules combined with the COSMO solvation model of water for long-range solvation.
5 citations
TL;DR: An implementation of a complex solver for the solution of the linear equations required to compute the complex response functions of damped response theory is presented for the resolution-of-identity (RI) coupled cluster singles and approximate doubles (CC2) method.
Abstract: An implementation of a complex solver for the solution of the linear equations required to compute the complex response functions of damped response theory is presented for the resolution-of-identity (RI) coupled cluster singles and approximate doubles (CC2) method. The implementation uses a partitioned formulation that avoids the storage of double excitation amplitudes to make it applicable to large molecules. The solver is the keystone element for the development of the damped coupled cluster response formalism for linear and nonlinear effects in resonant frequency regions at the RI-CC2 level of theory. Illustrative results are reported for the one-photon absorption cross section of C60, the electronic circular dichroism of n-helicenes (n = 5, 6, 7), and the C6 dispersion coefficients of a set of selected organic molecules and fullerenes.
5 citations
TL;DR: An implementation of a complex solver for the solution of the response equations required to compute the complex response functions of damped response theory is presented for the resolution-of-identity (RI) coupled-cluster singles and approximate doubles CC2 method in this article.
Abstract: An implementation of a complex solver for the solution of the response equations required to compute the complex response functions of damped response theory is presented for the resolution-of-identity (RI) coupled-cluster singles and approximate doubles CC2 method. The implementation uses a partitioned formulation that avoids the storage of double excitation amplitudes to make it applicable to large molecules. The solver is the keystone element for the development of the damped coupled-cluster response formalism for linear and nonlinear effects in resonant frequency regions at the RI-CC2 level of theory. Illustrative results are reported for the one-photon absorption cross section of C60, the electronic circular dichroism of $n$-helicenes ($n$ = 5, 6, 7), and the $C_6$ dispersion coefficients of a set of selected organic molecules and fullerenes.
4 citations
TL;DR: In this paper, the authors used density functional theory and the hybrid Perdew-Burke-Ernzerhof exchange-correlation functional (PBE0) within the periodic electrostatic embedded cluster model.
Abstract: Dioxygen activation pathways on the (001) surfaces of cobalt ferrite, CoFe2 O4 , were investigated computationally using density functional theory and the hybrid Perdew-Burke-Ernzerhof exchange-correlation functional (PBE0) within the periodic electrostatic embedded cluster model. We considered two terminations: the A-layer exposing Fe2+ and Co2+ metal sites in tetrahedral and octahedral positions, respectively, and the B-layer exposing octahedrally coordinated Co3+ . On the A-layer, molecular oxygen is chemisorbed as a superoxide on the Fe monocenter or bridging a Fe-Co cation pair, whereas on the B-layer it is adsorbed at the most stable anionic vacancy. Activation is promoted by transfer of electrons provided by the d metal centers onto the adsorbed oxygen. The subsequent dissociation of dioxygen into monoatomic species and surface reoxidation have been identified as the most critical steps that may limit the rate of the oxidation processes. Of the reactive metal-O species, [FeIII -O]2+ is thermodynamically most stable, while the oxygen of the Co-O species may easily migrate across the A-layer with barriers smaller than the associative desorption.
3 citations
TL;DR: In this article, an automatized workflow is presented to identify reaction events, filter them, and prepare them for accurate quantum chemical calculations using Density Functional Theory (DFT) or Coupled Cluster methods.
Abstract: We present an automatized workflow which, starting from molecular dynamics simulations, identifies reaction events, filters them, and prepares them for accurate quantum chemical calculations using, for example, Density Functional Theory (DFT) or Coupled Cluster methods. The capabilities of the automatized workflow are demonstrated by the example of simulations for the combustion of some polycyclic aromatic hydrocarbons (PAHs). It is shown how key elementary reaction candidates are filtered out of a much larger set of redundant reactions and refined further. The molecular species in question are optimized using DFT and reaction energies, barrier heights, and reaction rates are calculated. The setup is general enough to include at this stage configurational sampling, which can be exploited in the future. Using the introduced machinery, we investigate how the observed reaction types depend on the gas atmosphere used in the molecular dynamics simulation. For the re-optimization on the DFT level, we show how the additional information needed to switch from reactive force-field to electronic structure calculations can be filled in and study how well ReaxFF and DFT agree with each other and shine light on the perspective of using more accurate semi-empirical methods in the MD simulation.
3 citations
TL;DR: In this article, the authors investigated how the absorption and fluorescence of halogenated imidazolium compounds in acetonitrile solution is influenced by the presence of counterions and the ability to act as halogen-bond donors.
Abstract: We investigate how the absorption and fluorescence of halogenated imidazolium compounds in acetonitrile solution is influenced by the presence of counterions and the ability to act as halogen-bond donors. Experimental measurements and quantum chemical calculations with correlated wavefunction methods are applied to study three monodentate halogen-bond complexes of iodo-imidazolium, iodo-benzimidazolium and bromo-benzimidazolium cations with triflate counterions, and a bidentate complex of bis(iodo-benzimidazolium) dications with chloride as counterion. The three monodentate complexes with triflate counterions relax after photoexcitation to minima on the S1 potential energy surface where the C–I bond and the I⋯O halogen bond are partially broken. For the bidentate complex with the smaller chloride counterion the halogen-bond interaction stays intact in the S1 minimum that is reached by relaxation from the Franck–Condon point. In a complementing experimental approach, stationary absorption and emission as well as transient fluorescence spectra are recorded for iodo- and bromo-benzimidazolium in acetonitrile. Variation of the counterion, substitution of the iodine by bromine, hydrogen, or methyl, and the comparison to theory allows the identification of spectroscopic signatures and photoinduced dynamics associated with ion-pairing.