Theoretical Investigation of the Reaction of Mn⁺ with Ethylene Oxide

The potential energy surfaces of Mn⁺ reaction with ethylene oxide in both the septet and quintet states are investigated at the B3LYP/DZVP level of theory. The reaction paths leading to the products of MnO⁺, MnO, MnCH₂⁺, MnCH₃, and MnH⁺ are described in detail. Two types of encounter complexes of Mn⁺ with ethylene oxide are formed because of attachments of the metal at different sites of ethylene oxide, i.e., the O atom and the CC bond. Mn⁺ would insert into a C–O bond or the C–C bond of ethylene oxide to form two different intermediates prior to forming various products. MnO⁺/MnO and MnH⁺ are formed in the C–O activation mechanism, while both C–O and C–C activations account for the MnCH₂⁺/MnCH₃ formation. Products MnO⁺, MnCH₂⁺, and MnH⁺ could be formed adiabatically on the quintet surface, while formation of MnO and MnCH₃ is endothermic on the PESs with both spins. In agreement with the experimental observations, the excited state a⁵D is calculated to be more reactive than the ground state a⁷S. This theoretical work sheds new light on the experimental observations and provides fundamental understanding of the reaction mechanism of ethylene oxide with transition metal cations

Theoretical Investigation of the Methanol Decomposition by Fe⁺ and Fe(C₂H₄)⁺: A π‑Type Ligand Effect

Density functional theory has been used to probe the mechanism of gas-phase methanol decomposition by bare Fe⁺ and ligated Fe­(C₂H₄)⁺ in both quartet and sextet states. For the Fe⁺/methanol system, Fe⁺ could directly attach to the O and methyl-H atoms of methanol, respectively, forming two encounter isomers. The methanol reaction with Fe⁺ prefers initial C–O bond activation to yield methyl with slight endothermicity, whereas CH₄ elimination is hindered by the strong endothermicity and high-energy barrier of hydroxyl-H shift. For the Fe­(C₂H₄)⁺/methanol system, the major product of H₂O is formed through six elementary steps: encounter complexation, C–O bond activation, C–C coupling, β-H shift, hydride H shift, and nonreactive dissociation. Both ligand exchange and initial C–O bond activation mechanisms could account for ethylene elimination with the ion products Fe­(CH₃OH)⁺ and (CH₃)­Fe­(OH)⁺, respectively. With the assistance of a π-type C₂H₄ ligand, the metal center in the Fe­(C₂H₄)⁺/CH₃OH system avoids formation of unfavorable multi-σ-type bonding and thus greatly enhances the reactivity compared to that of bare Fe⁺

Analysis of Petroleum Aromatics by Laser-Induced Acoustic Desorption/Tunable Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry

Laser-induced acoustic desorption coupled with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (LIAD/SVUVPI-MS) is employed to analyze aromatics prepared under different conditions from Lungu atmospheric residue (LGAR), i.e., the primary aromatics separated directly from LGAR, and the secondary aromatics after hydrogenation of LGAR and its resins. The mass spectra of the primary aromatics present a bimodal normal distribution in the range of 200–900 Da, in which the relative intensity of the two peaks changes significantly with the SVUV photon energies (9.0, 11.0, and 14.0 eV), indicating that at least two categories of compounds with different ionization energies (IEs) are included, i.e., polycyclic aromatics (IEs < 10.0 eV) in the mass range of 400–900 Da, and aliphatic and alicyclic compounds (IEs close to 11.0 eV) in 200–400 Da. Also detected in the aromatics are metalloporphyrins. Furthermore, the mass spectra of the secondary aromatics separated from LGAR and its resins at different hydrogenation temperatures (390, 400, 410, and 420 °C) are also recorded. The results indicate that the hydrogenation process, especially at higher temperatures, results in removal of alkyl-side and bridge chains in the aromatics, and the secondary aromatics from LGAR resins contain more alkyl side and bridge chains and metal compounds than those from LGAR

Dithiafulvenyl Unit as a New Donor for High-Efficiency Dye-Sensitized Solar Cells: Synthesis and Demonstration of a Family of Metal-Free Organic Sensitizers

This work identifies the dithiafulvenyl unit as an excellent electron donor for constructing D−π–A-type metal-free organic sensitizers of dye-sensitized solar cells (DSCs). Synthesized and tested are three sensitizers all with this donor and a cyanoacrylic acid acceptor but differing in the phenyl (<b>DTF-C1</b>), biphenyl (<b>DTF-C2</b>), and phenyl–thiopheneyl–phenyl π-bridges (<b>DTF-C3</b>). Devices based on these dyes exhibit a dramatically improved performance with the increasing π-bridge length, culminating with DTF-C3 in η = 8.29% under standard global AM 1.5 illumination

New Paradigm for Allosteric Regulation of Escherichia coli Aspartate Transcarbamoylase

For nearly 60 years, the ATP activation and the CTP inhibition of Escherichia coli aspartate trans­carbamoylase (ATCase) has been the textbook example of allosteric regulation. We present kinetic data and five X-ray structures determined in the absence and presence of a Mg²⁺ concentration within the physiological range. In the presence of 2 mM divalent cations (Mg²⁺, Ca²⁺, Zn²⁺), CTP does not significantly inhibit the enzyme, while the allosteric activation by ATP is enhanced. The data suggest that the actual allosteric inhibitor of ATCase in vivo is the combination of CTP, UTP, and a divalent cation, and the actual allosteric activator is a divalent cation with ATP or ATP and GTP. The structural data reveals that two NTPs can bind to each allosteric site with a divalent cation acting as a bridge between the triphosphates. Thus, the regulation of ATCase is far more complex than previously believed and calls many previous studies into question. The X-ray structures reveal that the catalytic chains undergo essentially no alternations; however, several regions of the regulatory chains undergo significant structural changes. Most significant is that the N-terminal region of the regulatory chains exists in different conformations in the allosterically activated and inhibited forms of the enzyme. Here, a new model of allosteric regulation is proposed

Metallic Iron–Nickel Sulfide Ultrathin Nanosheets As a Highly Active Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media

We report on the synthesis of iron-nickel sulfide (INS) ultrathin nanosheets by topotactic conversion from a hydroxide precursor. The INS nanosheets exhibit excellent activity and stability in strong acidic solutions as a hydrogen evolution reaction (HER) catalyst, lending an attractive alternative to the Pt catalyst. The metallic α-INS nanosheets show an even lower overpotential of 105 mV at 10 mA/cm² and a smaller Tafel slope of 40 mV/dec. With the help of DFT calculations, the high specific surface area, facile ion transport and charge transfer, abundant electrochemical active sites, suitable H⁺ adsorption, and H₂ formation kinetics and energetics are proposed to contribute to the high activity of the INS ultrathin nanosheets toward HER

Density Functional Theory Study of the Adsorption and Desulfurization of Thiophene and Its Hydrogenated Derivatives on Pt(111): Implication for the Mechanism of Hydrodesulfurization over Noble Metal Catalysts

Desulfurization of thiophene and its hydrogenated derivatives on Pt(111) are studied using self-consistent periodic density functional theory (DFT), and the hydrodesulfurization network is mapped out. On Pt(111), thiophene has two types of adsorption configurations (parallel cross-bridge and partially tilted bridge-hollow), and for its hydrogenated derivates, the molecule is gradually lifted up from the surface with the addition of hydrogen atoms. In all the adsorbed thiophenic compounds, the S atom is always sp³ hybridized; the C atom in the methylene group is always sp³ hybridized, whereas it is either sp² or sp³ hybridized in the methyne group, depending on how the group interacts with the surface Pt atoms. On the basis of the thermodynamic and kinetic analysis of the elementary steps, a direct desulfurization pathway is proposed for the hydrodesulfurization of thiophene on Pt(111). In contrast to the common thought that hydrogenation toward aromatic organosulfur compounds would make desulfurization easier, the present work clearly demonstrates that hydrogenations of thiophene on Pt(111) do not reduce the energy barrier for the C–S bond cleavage

Unraveling the Mechanism of the Zn-Improved Catalytic Activity of Pd-Based Catalysts for Water–Gas Shift Reaction

The water–gas shift (WGS) reaction plays a key role in hydrogen economy. Owing to the exothermic nature of the reaction, low-temperature WGS catalysts are highly desired. Zn-modified Pd-based catalysts are promising candidates for low-temperature WGS. Herein, the effect of Zn addition on the WGS catalysis is systematically studied by using the Pd(111) and PdZn(111) surface as models. Owing to the addition of Zn, the electron-accepting ability of the catalyst is weakened, while the electron-donating ability is increased. As a result, the adsorptions of electron-donor adsorbates, including H₂O, CO, H, <i>cis</i>-COOH, <i>trans</i>-COOH, and H₂, are weakened, while the adsorptions of electron-acceptor adsorbates, including O and OH, are strengthened. The same most favorable reaction path is found on Pd(111) and PdZn(111), which is the associative mechanism with the carboxyl dehydrogenation assisted by adsorbed OH. Although the most favorable path is the same, the weakening of CO adsorption makes the rate-determining step change from the association of CO and OH forming <i>cis</i>-COOH on Pd(111) to the dissociation of H₂O on PdZn(111). The rate-determining step on PdZn(111) has an energy barrier lower than the rate-determining step on Pd(111). The promotion mechanism of the PdZn alloy for WGS is therefore attributed to the fact that the addition of Zn weakens the adsorption of CO and thereby alters the rate-determining step

Theoretical Survey of the Thiophene Hydrodesulfurization Mechanism on Clean and Single-Sulfur-Atom-Modified MoP(001)

Molybdenum phosphide (MoP) has been extensively experimentally shown to possess high and surprisingly increasing hydrodesulfurization (HDS) activities during the HDS process. In order to understand the HDS mechanism, we investigate the HDS of thiophene on clean and single-sulfur-atom-modified MoP(001) using self-consistent periodic density functional theory (DFT). Thiophene strongly prefers <i>flat</i> adsorption, which is slightly weakened in the presence of a surface S atom. Thermodynamic and kinetic analyses of the elementary steps show that the HDS of thiophene takes place along the direct desulfurization (DDS) pathway on both clean and S-modified MoP(001), because of the very low C–S bond activation barriers as well as very high exothermicities involved. More importantly, the surface S atom does not elevate the C–S bond activation barriers but opens a new concerted pathway for the simultaneous rupture of both C–S bonds in thiophene. These results indicate that the presence of a surface S atom could be helpful for thiophene desulfurization. For comparison, we also investigate the influence of a surface S atom on the HDS of thiophene on Pt(111). The results show clearly a negative effect of the surface S atom, in accordance with the lower sulfur resistance of noble metals