Structure–Property Relationship of Oxygen-Doped Two-Dimensional Gallium Selenide for Hydrogen Evolution Reaction Revealed from Density Functional Theory

Two-dimensional (2D) gallium selenide (GaSe) is known for its inert surface and wide bandgap, limiting its application as a photocatalytic material for the hydrogen evolution reaction (HER). Partial substitution of Se with O atoms can improve its catalytic efficiency. This work discovered that the surface activity of the substitutional O-doped single-layer GaSe surfaces (GaSe1–xOx, for x ≤ 22%) and their bandgap sizes are dependent on the detailed atomic configuration of the dopants, as revealed from density functional theory. For GaSe1–xOx at low O contents, where all O atoms are favorably separated by at least one -Ga-Se-Ga- unit, the surface activity for the HER is insignificantly improved by increasing dopant concentration. By contrast, when more O dopants are available and arranged in adjacent positions (O-Ga-O), the hydrogen adsorption efficiency of GaSe1–xOx increases and their bandgaps are reduced with increasing dopant concentration. These important features are attributed to weakening of the Ga–O covalent interaction in these more localized dopant arrangements, which in turn strengthens the O–H bonds. This weakened Ga–O covalent bond also descends the conduction band minimum toward the Fermi level, resulting in bandgap reduction and thus favoring visible-light absorption. Optimal atomic configurations (all having localized O-dopant arrangements) have been identified, and they exhibit almost thermoneutral hydrogen adsorption free energy ΔGH and small bandgaps (2.09–2.21 eV), making them promising materials to perform an efficient HER. Fine-tuning the Ga–O interaction by applying tensile strength TS parallel to the 2D surface of up to 1% further reduces their bandgaps to 1.95–2.05 eV. Our theoretical predictions suggest that controlling the atomic configuration of dopants provides opportunities for engineering single-layered GaSe1–xOx materials with surface reactivity and bandgaps that suit photocatalytic water splitting

Reductions of Oxygen, Carbon Dioxide, and Acetonitrile by the Magnesium(II)/Magnesium(I) Couple in Aqueous Media: Theoretical Insights from a Nano-Sized Water Droplet

Reductions of O₂, CO₂, and CH₃CN by the half-reaction of the Mg­(II)/Mg­(I) couple (Mg²⁺ + e^– → Mg^+•) confined in a nanosized water droplet ([Mg­(H₂O)₁₆]^•+) have been examined theoretically by means of density functional theory based molecular dynamics methods. The present works have revealed many intriguing aspects of the reaction dynamics of the water clusters within several picoseconds or even in subpicoseconds. The reduction of O₂ requires an overall doublet spin state of the system. The reductions of CO₂ and CH₃CN are facilitated by their bending vibrations and the electron-transfer processes complete within 0.5 ps. For all reactions studied, the radical anions, i.e., O₂^•–, CO₂^•–, and CH₃CN^•–, are initially formed on the cluster surface. O₂^•– and CO₂^•– can integrate into the clusters due to their high hydrophilicity. They are either solvated in the second solvation shell of Mg²⁺ as a solvent-separated ion pair (ssip) or directly coordinated to Mg²⁺ as a contact-ion pair (cip) having the ¹η-[MgO₂]^•+ and ¹η-[MgOCO]^•+ coordination modes. The ¹η-[MgO₂]^•+ core is more crowded than the ¹η-[MgOCO]^•+ core. The reaction enthalpies of the formation of ssip and cip of [Mg­(CO₂)­(H₂O)₁₆]^•+ are −36 ± 4 kJ mol^–1 and −30 ± 9 kJ mol^–1, respectively, which were estimated based on the average temperature changes during the ion–molecule reaction between CO₂ and [Mg­(H₂O)₁₆]^•+. The values for the formation of ssip and cip of [Mg­(O₂)­(H₂O)₁₆]^•+ are estimated to be −112 ± 18 kJ mol^–1 and −128 ± 28 kJ mol^–1, respectively. CH₃CN^•– undergoes protonation spontaneously to form the hydrophobic [CH₃CN, H]^•. Both CH₃CN and [CH₃CN, H]^• cannot efficiently penetrate into the clusters with activation barriers of 22 kJ mol^–1 and ∼40 kJ mol^–1, respectively. These results provide fundamental insights into the solvation dynamics of the Mg²⁺/Mg^•+ couple on the molecular level

Ab Initio Studies on Al⁺(H₂O)<i>_n</i>, HAlOH⁺(H₂O)<i>_n</i>_-₁, and the Size-Dependent H₂ Elimination Reaction

We report computational studies on Al+(H2O)n, n = 6−9, and HAlOH+(H2O)n-1, n = 6−14, by the density functional theory based ab initio molecular dynamics method, employing a planewave basis set with pseudopotentials, and also by conventional methods with Gaussian basis sets. The mechanism for the intracluster H2 elimination reaction is explored. First, a new size-dependent insertion reaction for the transformation of Al+(H2O)n into HAlOH+(H2O)n-1 is discovered for n ≥ 8. This is because of the presence of a fairly stable six-water-ring structure in Al+(H2O)n with 12 members, including the Al+. This structure promotes acidic dissociation and, for n ≥ 8, leads to the insertion reaction. Gaussian based BPW91 and MP2 calculations with 6-31G* and 6-31G** basis sets confirmed the existence of such structures and located the transition structures for the insertion reaction. The calculated transition barrier is 10.0 kcal/mol for n = 9 and 7.1 kcal/mol for n = 8 at the MP2/6-31G** level, with zero-point energy corrections. Second, the experimentally observed size-dependent H2 elimination reaction is related to the conformation of HAlOH+(H2O)n-1, instead of Al+(H2O)n. As n increases from 6 to 14, the structure of the HAlOH+(H2O)n-1 cluster changes into a caged structure, with the Al−H bond buried inside, and protons produced in acidic dissociation could then travel through the H2O network to the vicinity of the Al−H bond and react with the hydride H to produce H2. The structural transformation is completed at n = 13, coincident approximately with the onset of the H2 elimination reaction. From constrained ab initio MD simulations, we estimated the free energy barrier for the H2 elimination reaction to be 0.7 eV (16 kcal/mol) at n = 13, 1.5 eV (35 kcal/mol) at n = 12, and 4.5 eV (100 kcal/mol) at n = 8. The existence of transition structures for the H2 elimination has also been verified by ab initio calculations at the MP2/6-31G** level. Finally, the switch-off of the H2 elimination for n > 24 is explored and attributed to the diffusion of protons through enlarged hydrogen bonded H2O networks, which reduces the probability of finding a proton near the Al−H bond

How Large Is the [Fe^III(Protoporphyrin IX)]⁺ Ion (Hemin⁺) in the Gas Phase?

Comparison of the collision cross-section of the [FeIII−protoporphyrin IX]+ ion, hemin+, measured by means of ion-mobility experiments and the cross-sections calculated from theoretical structures based on density functional theory reveals that hemin+, in the gas phase, contains intramolecular hydrogen bonding between its two propionic acid side-chains

Masked Reactivity of Hydrated Clusters of Monovalent Manganese Ions: Water Insertion versus Nitrous Oxide Activation–A Density Functional Theory Investigation

Previous mass spectrometric (MS) studies demonstrated that singly charged hydration clusters of manganese ions [Mn(H2O)n]+ were, on one hand, highly reactive toward intracluster water insertion but, on the other hand, inert toward nitrous oxide activation. This contrast in reactivity has been rationalized by our present theoretical investigation for the interconversion between the pristine Mn(I) monovalent form as a monatomic ion in [MnI(H2O)n]+ and the oxidized Mn(III) trivalent form as a hydride–hydroxide in [HMnIIIOH(H2O)n−1], as well as their reactivity toward nitrous oxide activation. Our theoretical interpretations are supported with quantum chemical calculations based on density functional theory (DFT), performed systematically for the cluster-size range of n = 1 – 12. Our DFT results show that water insertion is kinetically and thermodynamically favorable for n ≥ 8, suggesting [HMnIIIOH(H2O)n−1]+ is the predominant form, as observed in previous MS experiments. While [MnI(H2O)n]+ is capable of N2O reduction, the process of which is highly exothermic, similar reactions are unfavorable with [HMnIIIOH(H2O)n−1]+, which can only form weakly bound adducts with N2O. This work demonstrates the masking effect of water molecules over the high reactivity of the hydrated Mn(I) center and sheds light on the potential roles of water in transition metal systems

Hydration Leads to Efficient Reactions of the Carbonate Radical Anion with Hydrogen Chloride in the Gas Phase

The carbonate radical anion CO₃^•– is a key intermediate in tropospheric anion chemistry. Despite its radical character, only a small number of reactions have been reported in the literature. Here we investigate the gas-phase reactions of CO₃^•– and CO₃^•–(H₂O) with HCl under ultrahigh vacuum conditions. Bare CO₃^•– forms OHCl^•– with a rate constant of 4.2 × 10^–12 cm³ s^–1, which corresponds to an efficiency of only 0.4%. Hydration accelerates the reaction, and ligand exchange of H₂O against HCl proceeds with a rate of 2.7 × 10^–10 cm³ s^–1. Quantum chemical calculations reveal that OHCl^•– is best described as an OH^• hydrogen bonded to Cl^–, while the ligand exchange product is Cl^–(HCO₃^•). Under tropospheric conditions, where CO₃^•–(H₂O) is the dominant species, Cl^–(HCO₃^•) is efficiently formed. These reactions must be included in models of tropospheric anion chemistry

Abundant Dipositively Charged Protonated a₂ and a₃ Ions from Diproline and Triproline

Abundant (a2 + H)2+ from diproline and (a3 + H)2+ from triproline were observed via collisionally activated charge disproportionation of [La(peptide)(CH3CN)1,2]3+. These small, dipositive ions with the charges formally located on the peptide backbone are stabilized by charge delocalization onto the pyrrolidine-derived rings, making their formation competitive against other monopositive ions. The (a2 + H)2+ and (a3 + H)2+ ions from diproline and triproline are major products in contrast with those derived from triglycine, whose unprecedented and surprising observations were recently reported (Shi et al. Angew. Chem., Int. Ed. 2008, 47, 8288−8291)

Optimization of Parameters Used in Algorithms of Ion-Mobility Calculation for Conformational Analyses

Structural information of gaseous ions can be obtained by comparing their collision cross sections as determined by ion-mobility experiments with those by theoretical modeling. Three theoretical models, the projection approximation (PA), the exact hard-sphere scattering (EHSS), and the trajectory (TJ) models, have been employed to determine the theoretical cross sections of candidate geometries. The accuracy of these models is largely dependent on the empirical parameters used for ion−buffer gas interactions. Optimal empirical parameters for each model have been determined by comparing the experimental cross sections of 20 calibrant ions with their theoretical cross sections obtained by using geometries sampled by density-functional-theory-based molecular dynamics simulations. The maximum absolute deviations of the cross sections of 15.5% (PA), 20.7% (EHSS), and 11.7% (TJ) obtained from the original parameters are reduced to 5.6% (PA), 4.6% (EHSS), and 3.4% (TJ) obtained from the new optimized parameters. The root-mean-square deviations of the predicted cross sections using the new parameters from the experimental values are also drastically reduced to 2.1% (PA), 1.9% (EHSS), and 1.6% (TJ). The new parameters are verified on protonated triglycine, protonated trialanine, and doubly protonated bradykinin

Bond Dissociation Energies of Solvated Silver(I)−Amide Complexes: Competitive Threshold Collision-Induced Dissociations and Calculations

Using competitive threshold collision-induced dissociation (TCID) measurements, experimental bond dissociation energies have been evaluated for the water, methanol, and acetonitrile adducts of silver(I)−amide complexes. The influence of the solvent molecules on the binding energy of silver(I) to acetamide, N-methylacetamide, and N,N-dimethylacetamide was investigated. Experimental results show that solvents decrease the amide binding energy by 4−6 kcal mol−1. Using density functional theory (DFT), binding energies were evaluated using nine functionals, after full geometry optimizations with the ECP28MWB basis set for silver and the 6-311++G(2df,2pd) basis set for the other atomic constituents of the ligands. In addition, calculations employing the DZVP basis set for Ag and DZVP2 for C, H, N, and O atoms at the B3LYP and MP2 levels of theory were used to investigate the influence of the basis set on the theoretical bond energies. A comparison of the experimental and theoretical silver(I)−ligand bond dissociation energies enables an assessment of the limitations in the basis sets and functionals in describing the energetics of the metal−solvent interaction and the metal−amide interaction. No single functional/basis set combination was found capable of predicting binding energies with a sufficiently high level of accuracy for the silver(I)−amide solvent complexes

Threshold Collision-Induced Dissociation Measurements Using a Ring Ion Guide as the Collision Cell in a Triple-Quadrupole Mass Spectrometer

A triple-quadrupole mass spectrometer has been modified for bond-dissociation energy measurements via threshold collision-induced dissociations (TCIDs) by replacing the conventional collision cell with a ring ion guide. Optimal operating conditions for the ring ion guide were determined or derived, and validated using a set of complexes for which bond dissociation energies are known. A comparison with reference data (within a range of 16−57 kcal/mol) indicates an accuracy approaching that of TCID determined on a guided ion-beam mass spectrometer. Complexes for which bond-dissociation energies were measured include metal ion complexes of simple ligands, amino acids and peptides, as well as of carbonic acid. There is excellent agreement between our experimental data and literature data, as well as theoretical data determined using a high-level computational method