A mass spectrometry and DFT study of pyrithione complexes with transition metals in the gas phase

2‐Mercaptopyridine N ‐oxide (pyrithione, PTOH) along with several transition metal ions forms coordination compounds displaying notable biological activities. Gas‐phase complexes formed between pyrithione and manganese (II), cobalt (II), nickel (II), copper (II), and zinc (II) were investigated by infusion in the electrospray source of a quadrupole‐time of flight mass spectrometer. Remarkably, positive ion mode spectra displayed the singly charged metal adduct ion [C₁₀H₈MN₂O₂S₂]²⁺ ([M(PTO)₂]^+• or [M(DPTO)]^+•), where DPTO is dipyrithione, 2,2′‐dithiobis(pyridine N ‐oxide), among the most abundant peaks, implying a change in the oxidation state of whether the metal ion or the ligands. In addition, doubly charged ions were recognized as metal adduct ions containing DPTO ligands, [M(DPTO)_n]²⁺. Generation of [M(PTO)₂]^+• / [M(DPTO)]^+• could be traced by CID of [M(DPTO)₂]²⁺, by observation of the sequential losses of a charged (PTO⁺) and a radical (PTO^•) deprotonated pyrithione ligand. The fragmentation pathways of [M(PTO)₂]^+• / [M(DPTO)]^+• were compared among the different metal ions, and some common features were noticed. Density functional theory (DFT) calculations were employed to study the structures of the observed adduct ions, and especially, to decide in the adduct ion [M(PTO)₂]^+• / [M(DPTO)]^+• whether the ligands are 2 deprotonated pyrithiones or a single dipyrithione as well as the oxidation state of the metal ion in the complex. Characterization of gas‐phase pyrithione metal ion complexes becomes important, especially taking into account the presence of a redox‐active ligand in the complexes, because redox state changes that produce new species can have a marked effect on the overall toxicological/biological response elicited by the metal system.     

In Situ Investigation of Methane Dry Reforming on Metal/Ceria(111) Surfaces: Metal–Support Interactions and C−H Bond Activation at Low Temperature

Studies with a series of metal/ceria(111) (metal=Co, Ni, Cu; ceria=CeO₂) surfaces indicate that metal–oxide interactions can play a very important role for the activation of methane and its reforming with CO₂ at relatively low temperatures (600–700 K). Among the systems examined, Co/CeO₂(111) exhibits the best performance and Cu/CeO₂(111) has negligible activity. Experiments using ambient pressure X‐ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO₂(111) at temperatures as low as 300 K—generating CH_x and CO_x species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co²⁺/CeO₂(111), and to only 0.05 eV on Co⁰/CeO_2−x (111). At 700 K, under methane dry reforming conditions, CO₂ dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CH_x formed on the Co⁰/CeO_2−x (111) catalyst recombines to yield ethane or ethylene.     

Exploring the Interactions of Atomic Oxygen on Silver Clusters with Hydrogen

The interactions between Ag_nO^-(n=1-8) and H₂ (or D₂) were explored by combination of the mass spectroscopy experiments and density function theory (DFT) calculations. The experiments found that all oxygen atoms in Ag_nO^-(n=1-8) are inert in the interactions with H₂ or D₂ at the low temperature of 150 K, which is in contrast to their high reactivity with CO under the same condition. These observations are parallel with the preferential oxidation (PROX) of CO in excess hydrogen catalyzed by dispersed silver species in the condensed phase. Possible reaction paths between Ag_nO^-(n=1-8) and H₂ were explored using DFT calculations. The results indicated that adsorption of H₂ on any site of Ag_nO^-(n=1-8) is extremely weak, and oxidation of H₂ by any kind of oxygen in Ag_nO^-(n=1-8) has an apparent barrier strongly dependent on the adsorption style of the "O". These experiments and theoretical results about cluster reactions provided molecule-level insights into the activity of atomic oxygen on real silver catalysts.     

Light‐Controlled Release and Uptake of Zinc Ions in Solution by a Photochromic Terthiazole‐Based Ligand

We have synthesized and fully characterized a photochromic zinc complex with a terphenylthiazole‐based ligand with a salen‐like cavity. The solution stability of the complex was found to be greatly dependent on the state of the photochromic ligand and an interesting photo‐triggered release and uptake of zinc ions was found as well as monitored by its fluorescence. The contrasting stability difference of the zinc complex between its two isomeric states was rationalized by DFT calculations.     

Coupling Reactions of Alkynyl Indoles and CO2 by Bicyclic Guanidine: Origin of Catalytic Activity?

Density functional theory calculations were used to investigate the three possible modes of activation for the coupling of CO₂ with alkynyl indoles in the presence of a guanidine base. The first of these mechanisms, involving electrophilic activation, was originally proposed by Skrydstrup et al. (Angew. Chem. Int. Ed . 2015 , 54 , 6682). The second mechanism involves the nucleophilic activation of CO₂. Both of these electrophilic and nucleophilic activation processes involve the formation of a guanidine‐CO₂ zwitterion adduct. We have proposed a third mechanism involving the bifunctional activation of the bicyclic guanidine catalyst, allowing for the simultaneous activation of the indole and CO₂ by the catalyst. We demonstrated that a second molecule of catalyst is required to facilitate the final cyclization step. Based on the calculated turnover frequencies, our newly proposed bifunctional activation mechanism is the most plausible pathway for this reaction under these experimental conditions. Furthermore, we have shown that this bifunctional mode of activation is consistent with the experimental results. Thus, this guanidine‐catalyzed reaction favors a specific‐base catalyzed mechanism rather than the CO₂ activation mechanism. We therefore believe that this bifunctional mechanism for the activation of bicyclic guanidine is typical of most CO₂ coupling reactions.     

Error estimates for space–time discontinuous Galerkin formulation based on proper orthogonal decomposition

In this study, proper orthogonal decomposition (POD) method is applied to diffusion–convection–reaction equation, which is discretized using space–time discontinuous Galerkin (dG) method. We provide estimates for POD truncation error in dG-energy norm, dG-elliptic projection, and space–time projection. Using these new estimates, we analyze the error between the dG and the POD solution, and the error between the exact and the POD solution. Numerical results, which are consistent with theoretical convergence rates, are presented.     

Existence and multiplicity of solutions for nonlocal systems involving fractional Laplacian with non-differentiable terms

In this paper, we devote to the study of the existence and multiplicity of solutions of nonlocal systems involving fractional Laplacian with non-differentiable terms using some extended critical point theorems for locally Lipschitz function on product spaces.     

On the stability of Timoshenko-type systems with internal frictional dampings and discrete time delays

In this paper, we consider a vibrating system of Timoshenko-type in a bounded one-dimensional domain under Dirichlet–Dirichlet or Dirichlet–Neumann boundary conditions with one or two discrete time delays and one or two internal frictional dampings. First, we show that the system is well posed in the sens of semigroup theory. Second, we prove the exponential stability regardless to the speeds of wave propagation of the system if the weights of the time delays are smaller than the ones of the corresponding dampings, respectively. However, when the weight of one time delay is not smaller than the one of the corresponding damping, we prove the exponential stability in case of equal-speed wave propagation, and the polynomial stability in the opposite case.