DFT Studies on Cu-Catalyzed Cross-Coupling of Diazo Compounds with Trimethylsilylethyne and <i>tert</i>-Butylethyne: Formation of Alkynes for Trimethylsilylethyne while Allenes for <i>tert</i>-Butylethyne

The detailed reaction mechanism for the Cu­(I)-catalyzed cross-coupling of (diazomethyl)­benzene with trimethylsilylethyne and <i>tert</i>-butylethyne was studied with the aid of density functional theory calculations. For both reactions, two catalytic cycles were considered. In one catalytic cycle, the active species reacts first with trimethylsilylethyne or <i>tert</i>-butylethyne, whereas, in the other one, the active species reacts first with (diazomethyl)­benzene. In both catalytic cycles, the copper acetylide formation, copper carbene migratory insertion, and protonation steps are involved. The calculation results show that the protonation step is crucial for the product selectivity. In addition, the reaction of diazoethane with <i>tert</i>-butylethyne and the reaction of (diazomethyl)­benzene with phenylacetylene were also considered theoretically

Decomposition and Oxidation of Methanol on Ir(111): A First-Principles Study

The adsorption, decomposition, and oxidation of methanol on Ir(111) were studied based on periodic density functional calculations. Each elementary step in the methanol decomposition reaction on clean Ir(111) via O–H, C–H, and C–O bond scissions was considered. The formation mechanisms of CO, CO₂, H₂O, and CH_<i>x</i> (<i>x</i> = 1–3) were elucidated. The results show that the desorption and decomposition of methanol are competitive on a clean surface, and the presence of O or OH has a larger effect on some specific reaction steps. The surface-assisted decomposition of methanol mainly follows two competitive dehydrogenation pathways initialed with O–H and C–H bond scissions, respectively, i.e., CH₃OH → CH₃O → HCHO → CHO → CO and CH₃OH → CH₂OH → CHOH → CHO → CO. The predosed O enhances the dehydrogenation of CH₃OH into CH₃O, while the surface is slightly more active toward the C–H bond breaking of CH₃O than O and OH. HCHO would like to dehydrogenate into CHO assisted by the surface or OH, followed by OH-assisted dehydrogenation into CO. CO combines with O to yield CO₂. However, if the surface O coverage is higher, CO₂ could be formed via the oxidation pathway of HCHO, i.e., HCHO →+OH₂CO₂→or+OH HCO₂ → CO₂. The comparison between theoretical results and experimental observation was made

Mechanism of Ammonia Decomposition and Oxidation on Ir(100): A First-Principles Study

Density functional theory (DFT) calculations combined with microkinetic analysis were performed to study the behavior of ammonia on clean, oxygen- and hydroxyl-predosed Ir(100). It is shown that the predosed oxygen or hydroxyl promotes NH₃ and NH dehydrogenation steps, while NH₂ dehydrogenation is slightly inhibited relative to clean Ir(100). In both cases, the hydrogen transfer from NH_<i>x</i> species to predosed O or OH is favored over thermal decomposition of NH_<i>x</i>. Furthermore, the predosed O exhibits higher activity on NH₃ and NH dehydrogenation steps than OH, while the case is reversed for NH₂. On clean Ir(100), N + N pathway is the major N₂ formation pathway when TPD experiment starts from 200 K, and N + NH is also involved but less competitive; however, three pathways N + N, N + NH, and NH + NH are all possible with respect to TPD experiment starting from 410 K. On O- and OH-predosed Ir(100), N + N pathway is the predominant pathway and is enhanced by the predosed O or OH. The microkinetic analysis further confirms that N₂ is the resulting product at different temperatures and ratios of NH₃/O₂, and the formation of NO is unfavorable

Formaldehyde Decomposition and Coupling on V(100): A First-Principles Study

The decomposition of formaldehyde (HCHO) and possible pathways for the formation of C₂H₄ and CH₄ on clean and oxygen-predosed V(100) surfaces were studied by periodic density functional theory (DFT). It is shown that both C–H and C–O bond scissions of HCHO are thermodynamically and kinetically favorable on clean V(100). Three reaction pathways for the formation of C₂H₄ and two for the formation of CH₄ were determined. Our results suggest that the preferred pathway for C₂H₄ formation at low temperature is the coupling of two methylenes (CH₂) produced by an early CO dissociation step at lower O coverage; while as the increase of the on-surface O coverage, this path is suppressed whereas the direct coupling of HCHO to form intermediate OCH₂CH₂O is favored at high temperature. For the formation of CH₄, different mechanisms are also identified corresponding to the two reaction regions. The low-temperature reaction likely occurs via successive hydrogenation of CH₂, while the high-temperature reaction may proceed via the CH₃O intermediate formed by hydrogenation of HCHO first. The present calculations show that the oxygen deposited on the V(100) surface contributes to the shifting of the mechanisms in low- and high-temperature regions, in line with the experimental results [Shen, M.; Zaera, F. <i>J. Am. Chem. Soc</i>. <b>2009</b>, <i>131</i>, 8708]

NO Reduction by H₂ on the Rh(111) and Rh(221) Surfaces: A Mechanistic and Kinetic Study

Periodic density functional theory (DFT) was used to investigate the selective catalytic reduction of NO by H₂ (H₂ SCR) on Rh(111) and stepped Rh(221) surfaces. The stepped Rh(221) surface exhibits a higher reactivity for NO reduction than the Rh(111) surface. NO dissociation on the Rh(221) surface exhibits almost no effect in the presence of H₂, whereas predosed H atoms slightly inhibit NO dissociation on Rh(111). Microkinetic calculations further predicted the product selectivity for H₂ SCR at different temperatures and pressures. It was found that, under ultrahigh-vacuum (UHV) conditions, NH₃ is the only N-containing product on Rh(111), consistent with the experimental observations, whereas on the Rh(221) surface, N₂O formation is predominant at low temperatures, and N₂ becomes main product above 480 K. Under near-atmospheric-pressure conditions, the product selectivity on the Rh(111) surface exhibits almost no change, whereas N₂O is the dominant product on Rh(221) throughout the whole temperature range. The present study indicates that the NO dissociation activity and product selectivity are strongly dependent on both the Rh surface structure and the experimental conditions

Mechanism of the Gaseous Hydrolysis Reaction of SO₂: Effects of NH₃ versus H₂O

Effects of ammonia and water molecules on the hydrolysis of sulfur dioxide are investigated by theoretical calculations of two series of the molecular clusters SO₂-(H₂O)_<i>n</i> (<i>n</i> = 1–5) and SO₂-(H₂O)_<i>n</i>-NH₃ (<i>n</i> = 1–3). The reaction in pure water clusters is thermodynamically unfavorable. The additional water in the clusters reduces the energy barrier for the reaction, and the effect of each water decreases with the increasing number of water molecules in the clusters. There is a considerable energy barrier for reaction in SO₂-(H₂O)₅, 5.69 kcal/mol. With ammonia included in the cluster, SO₂-(H₂O)_<i>n</i>-NH₃, the energy barrier is dramatically reduced, to 1.89 kcal/mol with <i>n</i> = 3, and the corresponding product of hydrated ammonium bisulfate NH₄HSO₃-(H₂O)₂ is also stabilized thermodynamically. The present study shows that ammonia has larger kinetic and thermodynamic effects than water in promoting the hydrolysis reaction of SO₂ in small clusters favorable in the atmosphere

DFT Studies on the Reaction Mechanism of 1,3-Conjugated Dienes Isomerization Catalyzed by Ruthenium Hydride

The detailed reaction mechanism for the isomerization of 1,3-conjugated dienes catalyzed by the ruthenium hydride complex RuHCl­(CO)­(H₂IMes)­(PCy₃) has been studied with the aid of density functional theory (DFT) calculations. Both <i>cis</i> and <i>trans</i> isomers of a 1,3-conjugated diene were considered as the reactants. For each isomer, two catalytic cycles were calculated, which (respectively) generate a 1,3-hydride shift product or a 1,5-hydride shift product. Both catalytic cycles proceed via alkene migratory insertion into the Ru–H bond, σ-allyl ruthenium isomerization, and β-H elimination steps. Our computational study shows that the <i>cis</i> isomer of the model reactant reacts preferentially via the pathway leading to the 1,5-hydride shift product, consistent with the experimental results. The σ-allyl ruthenium isomerization step is found to be crucial for reaction regioselectivity. Strong binding of the CC bond to Ru is involved in the generation of the 1,5-hydride shift product. In addition, the steric effect of the bulky N-heterocyclic carbene ligand in ruthenium hydride RuHCl­(CO)­(H₂IMes)­(PCy₃) was considered theoretically

Hydrolysis of Sulfur Dioxide in Small Clusters of Sulfuric Acid: Mechanistic and Kinetic Study

The deposition and hydrolysis reaction of SO₂ + H₂O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO₂–(H₂SO₄)_<i>n</i>–(H₂O)_<i>m</i> (<i>m</i> = 1,2; <i>n</i> = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO₂ as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO₂ + H₂O + H₂SO₄ – H₂O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H₂SO₄–(H₂O)₂ forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO₂ + (H₂SO₄)–H₂O > SO₂ + (H₂SO₄)₂–H₂O > SO₂ + H₂SO₄–H₂O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO₂). Consequently, sulfuric acid promotes the hydrolysis of SO₂ both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO₂. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO₂, leading to the formation and growth of new particles

Decomposition of Methanol on Clean and Oxygen-Predosed V(100): A First-Principles Study

The decomposition of CH₃OH on clean and oxygen-predosed V(100) surfaces was studied on the basis of periodic density functional calculations and microkinetic modeling. The results indicate that the O–H bond scission of CH₃OH is thermodynamically and kinetically favorable on clean V(100) while the C–H and C–O bond scissions are unlikely to occur at low temperature, and as a result, CH₃O is the major intermediate in the decomposition process. The C–O bond scission of CH₃O to form CH₃ is much easier than the C–H bond scission to form HCHO. Hydrogenation of CH₃ by the surface hydrogen from dissociating CH₃OH and CH₃O is responsible for the desorption of CH₄ at low and high temperatures, respectively. HCHO further undergoes decomposition or/and coupling to form CO or/and C₂H₄. When oxygen is preadsorbed on the surface at low coverage, the O–H bond scission of CH₃OH is virtually not affected, while the cleavages of the C–O and C–H bonds from CH₃O are inhibited in different degrees, leading to the decrease in the ratio of CH₄ produced at the low temperature relative to that at the high temperature. All products are delayed in temperature. The results are in good agreement with experimental observations

Honeycomb Boron Allotropes with Dirac Cones: A True Analogue to Graphene

We propose a series of planar boron allotropes with honeycomb topology and demonstrate that their band structures exhibit Dirac cones at the K point, the same as graphene. In particular, the Dirac point of one honeycomb boron sheet locates precisely on the Fermi level, rendering it as a topologically equivalent material to graphene. Its Fermi velocity (<i>v</i>_f) is 6.05 × 10⁵ m/s, close to that of graphene. Although the freestanding honeycomb B allotropes are higher in energy than α-sheet, our calculations show that a metal substrate can greatly stabilize these new allotropes. They are actually more stable than α-sheet sheet on the Ag(111) surface. Furthermore, we find that the honeycomb borons form low-energy nanoribbons that may open gaps or exhibit strong ferromagnetism at the two edges in contrast to the antiferromagnetic coupling of the graphene nanoribbon edges