Theoretical study of reactions between AlH(1Σ) and HF molecule

Reaction mechanisms between AlH (¹Σ) and HF molecule are theoretically investigated. Ab initio calculations demonstrate that there are two parallel reaction channels: one is an addition reaction to give H₂AlF via the three‐membered ring transition state (TS) and the other is a dehydrogenation reaction to give AlF+H₂ via the four‐membered ring TS. The addition reaction is thermodynamically favorable and the dehydrogenation reaction is kinetically favorable. Thermodynamics and Eyring transition state theory (TST) with the Wigner correction are also used to compute the thermodynamic functions, the equilibrium constants, A factors, and the rate constants of these reaction channels at 200–1000 K. From the thermodynamics and TST calculations, it is valuable to point out that consideration on the entropy and thermal enthalpy is quite important in the study of chemical reactions on the basis of ab initio method. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 417–424, 1999     

On the kinetic stability of the SH3X species with X=F,Cl

Portions of the [S, H₃, X] (X=F, Cl) potential energy surfaces (PESs) were explored using the RHF, MP2, and QCISD(T) methods with emphasis on H₂ and HX eliminations, SH₃X→SHX+H₂ and SH₃X→SH₂+HX, respectively. Upon the halogen X substitution, the most favorable decomposition pathway of SH₄ went over to HX elimination, proceeding with a very low activation barrier of 6.9 (X=F) and 1.3 (X=Cl) kcal/mol. Moreover, the transition states (TSs) for H₂ elimination from SH₃X resembled the less favorable homopolar TS of SH₄. Upon the X=F substitution, the barrier to H₂ loss of SH₄ was calculated to increase from 19.5 to 21.5 kcal/mol. For X=Cl, only the indirect H₂ elimination path via the SH₂+HCl→SHCl+H₂ exchange was found. The hydrogen‐exchange reaction SH₂+HX→SH₂+HX was predicted to occur through formation of the hydrogen‐bonded complex XHSH₂ and with a relatively high barrier of 43.5 (X=F) and 38.5 (X=Cl) kcal/mol. FHSH₂ and ClHSH₂ were found to be the lowest energy species on the [S, H₃, F] and [S, H₃, Cl] PESs, lying 53.4 and 44.7 kcal/mol below SH₃F and SH₃Cl, respectively. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 37–43, 1999     

Theoretical Study of the Mechanism of Cycloaddition Reaction between Silylene Silylene (H2SiSi:) and Acetone

The mechanism of the cycloaddition reaction between singlet silylene silylene (H₂Si?Si:) and acetone has been investigated with the CCSD (T)//MP2/6‐31G?? method. According to the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction channels. The present rule of this reaction is that the [2+2] cycloaddition reaction of the two π‐bonds in silylene silylene (H₂Si?Si:) and acetone leads to the formation of a four‐membered ring silylene (E3). Because of the unsaturated property of Si: atom in E3, it further reacts with acetone to form a silicic bis‐heterocyclic compound (P7). Simultaneously, the ring strain of the four‐membered ring silylene (E3) makes it isomerize to a twisted four‐membered ring product (P4).     

Ab initio Study of Mechanism of Cycloaddition Reaction between Germylene Silylene (H2GeSi:) and Acetone

The mechanism of the cycloaddition reaction between singlet germylene silylene (H₂GeSi:) and acetone has been investigated with CCSD(T)/6‐31G*//MP2/6‐31G* method. From the potential energy profile, we could predict that the reaction has two competitive dominant reaction channels. The present rule of this reaction is that the [2+2] cycloaddition reaction of the two (‐bonds in germylene silylene and acetone generates a four‐membered ring silylene with Ge. Because of the unsaturated property of Si atom in the four‐membered ring silylene with Ge, it could further react with acetone, resulting in the generation of a bis‐heterocyclic compound with Si and Ge. Simultaneously, the ring strain of the four‐membered ring silylene with Ge makes it isomerize to a twisted four‐membered ring product.     

Theoretical study of the dehydrogenate reaction of H2S by Sc+(1D)

The reaction Sc⁺(¹D)+H₂S→Sc⁺S+H₂ is theoretically investigated by ab initio MO methods. Two possible reaction channels on the singlet potential surface (PES) and the reaction mechanism are examined and discussed. Three regions of the potential surface were studied, the molecular complex, the S‐H insertion products and the transition states for the reaction. In addition the singlet and triplet PESs of this reaction system are compared in an investigation the chemistry of excited electronic state. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 60–64, 2001     

Direct dynamics study on mechanism and kinetics of the biradical self‐reaction of HOO

A theoretical study of the mechanism and the kinetics for the hydrogen abstraction reaction of the biradical hydroperoxy radical has been presented at the CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G(d,p) level of theory. Our theoretical calculations suppose a stepwise mechanism involving the formation of a postreactant complex in the triplet and singlet entrance channels. Four transition states of the six‐membered chain complexes (³TS1 and ¹TS1) and six‐membered ring complexes (³TS2 and ¹TS2) are located at the high dual level CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G(d,p) method. The rate constants of Path 1 ～ Path 4 at the CCSD(T)/6‐311++G(3d,2p)//CCSD/6‐31+G (d,p) level are calculated by means of the conventional transition state theory (TST) and canonical variational TST without and with small‐curvature tunneling (SCT) correction within the temperature range of 200–2,500 K. The calculated results show that the triplet channel is the dominating reaction channel and Path 2 is found to be the most favorable pathway. The rate constants of Path 2 are in good agreement with the experimental values at the experimentally measured temperatures. Moreover, the variational effect is not obvious in the low temperature range but is not neglectable in the high temperature range. The SCT plays an important role particularly in the low temperature range. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

“Increased-valence” formulae and metal-ion catalysed dehydrogenation reactions with molecular oxygen and azo compounds as oxidants

“Increased-valence” mechanisms are formulated for the generalized dehydrogenation reactions SH₂ + O₂ → S + H₂O₂ and SH₂ + RNNR → S + RNH-NHR, in which SH₂ is assumed to be coordinated to a transition metal ion. The role of the metal ion in these mechanisms is to assist with the homolytic breaking of the S-H and NN bonds     

Theoretical study on reaction mechanism of isocyanate radical NCO with ethene

The NCO + C₂H₄ reaction is simple and prototype for reaction of the NCO radical with unsaturated hydrocarbons, and it is considered to be important in fuel‐rich combustion. In this article, we for the first time perform detailed theoretical investigations for its reaction mechanism based on Gaussian‐3//B3LYP scheme covering various entrance and decomposition channels. The most favorable channel is firstly the NCO and C₂H₄ approach each other, forming a weakly‐bound complex L1 OCN···C₂H₄, followed by the formation of isomer L2 OCNCH₂CH₂ via a small barrier of 1.3 kcal/mol. Transition states of any decomposable or isomeric channels for L2 in energy are much higher than reactants, which indicate that adduct L2 has stabilization effect in this NCO + C₂H₄ reaction. The direct H‐abstraction channel leading to P1 HNCO + C₂H₃, might have an important contribution to the eventual products in high temperature. These results can well explain available kinetic experiment. Moreover, reaction mechanism for the title reaction is significantly different from the NCO + C₂H₂ reaction which proceeds on most favorably to generate the products HCN + HCCO and OCCHCN + H via a four‐membered ring intermediate. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Structure and Composition of the 200 K‐Superconducting Phase of H2S at Ultrahigh Pressure: The Perovskite (SH−)(H3S+)

At ultrahigh pressure (>110 GPa), H₂S is converted into a metallic phase that becomes superconducting with a record T_c of approximately 200 K. It has been proposed that the superconducting phase is body‐centered cubic H₃S (Im m, a=3.089 Å) resulting from the decomposition reaction 3 H₂S→2 H₃S+S. The analogy between H₂S and H₂O led us to a very different conclusion. The well‐known dissociation of water into H₃O⁺ and OH^? increases by orders of magnitude under pressure. H₂S is anticipated to behave similarly under pressure, with the dissociation process 2 H₂S→H₃S⁺+SH^? leading to the perovskite structure (SH^?)(H₃S⁺). This phase consists of corner‐sharing SH₆ octahedra with SH^? ions at each A site (the centers of the S₈ cubes). DFT calculations show that the perovskite (SH^?)(H₃S⁺) is thermodynamically more stable than the Im m structure of H₃S, and suggest that the A site hydrogen atoms are most likely fluxional even at T_c .     

Structures,Energies, and Fundamental Vibrations of the Sulfonium Ions H3Sn+ (n = 1–4)

Using ab initio MO calculations at the MP2/6‐311G(2df,2pd) level of theory the most stable structures of the following seven ions were determined: H₃S⁺ (C_3v), H₂S–SH⁺ (C_s), H₂S–S–SH⁺ (C₁), HS–S(H)–SH⁺ (C₁), H₂S–S–S–SH⁺ (C₁), HS–S(H)–S–SH⁺ (C₁) and S(SH)₃⁺ (C₃). In the case of the isomeric H₃S₃⁺ cations the species protonated at the terminal sulfur atom is most stable while in the case of the H₃S₄⁺ ions the protonation at the β sulfur atom is energetically preferred. However, the energy differences between isomeric cations are rather small. At the same level of theory the wavenumbers of the harmonic fundamental vibrations were calculated and compared to the available experimental data leading to a support for the existing assignments in certain cases but in some cases to revisions. The reaction enthalpies and Gibbs free energies of the proton transfer reactions H₂S_n + H₂S_n+1 → H₃S_n⁺ + HS_n+1^– were calculated by the G2 method. For n = 1–3 the enthalpies are found in the range 639–731 kJ mol^–1.     

Dissection of the Multichannel Reaction O(3P) + C2H2: Differential Cross-Sections and Product Energy Distributions

The O(³P) + C₂H₂ reaction plays an important role in hydrocarbon combustion. It has two primary competing channels: H + HCCO (ketenyl) and CO + CH₂ (triplet methylene). To further understand the microscopic dynamic mechanism of this reaction, we report here a detailed quasi-classical trajectory study of the O(³P) + C₂H₂ reaction on the recently developed full-dimensional potential energy surface (PES). The entrance barrier TS1 is the rate-limiting barrier in the reaction. The translation of reactants can greatly promote reactivity, due to strong coupling with the reaction coordinate at TS1. The O(³P) + C₂H₂ reaction progress through a complex-forming mechanism, in which the intermediate HCCHO lives at least through the duration of a rotational period. The energy redistribution takes place during the creation of the long-lived high vibrationally (and rotationally) excited HCCHO in the reaction. The product energy partitioning of the two channels and CO vibrational distributions agree with experimental data, and the vibrational state distributions of all modes of products present a Boltzmann-like distribution.     

The Iminoborane tBuB≡NtBu as a Dipolarphile in (2+3) Cycloadditions

The iminoborane tBuB≡NtBu and the diazomethane tBuCH=N₂ give the (2+3) cycloadduct [—HC(tBu)—N=N—N(tBu)=B(tBu)—] in a 1:1 reaction and the seven‐membered ring [—C(tBu)=N—NH—N(tBu)=B(tBu)—N(tBu)=B(tBu)—] in a 2:1 reaction. The (2+3) cycloadduct decomposes above 0 °C to give the seven‐membered ring, N₂, and HC(tBu)=N—N=CH(tBu) in the ratio 2:1:1. The borane tBuB≡NtBu and organic azides R″N₃ yield the (2+3) cycloadducts [—R″N—N=N—N(tBu)=B(tBu)—] (R″ = Me, Et, Pr, Bu, iBu, sBu, C₅H₁₁, c‐C₅H₉, c‐C₆H₁₁, Bzl, EtOOC).     

Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1‐Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(C₆F₅)₂ undergoes 1,1‐carboboration reactions with the Me₃Si‐substituted enynes to give ring‐enlarged functionalized C₃‐bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H⁺/[B]H^? products. One such product shows activity as a metal‐free catalyst for the hydrogenation of enamines or a bulky imine. The ring‐enlarged FLPs contain dienylborane functionalities that undergo “bora‐Nazarov”‐type ring‐closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C₆F₅ migration is very favorable. Furthermore, substituted 2,5‐dihydroborole products are derived from cyclization and C₆F₅ migration from the photolysis reaction. In the case of the six‐membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2‐addition reaction with a terminal alkyne with rearrangement.     

Density Functional Studies on Thromboxane Biosynthesis: Mechanism and Role of the Heme‐Thiolate System

Reaction mechanisms for the isomerization of prostaglandin H₂ to thromboxane A₂, and degradation to 12‐L‐hydroxy‐5,8,10‐heptadecatrienoic acid (HHT) and malondialdehyde (MDA), catalyzed by thromboxane synthase, were investigated using the unrestricted Becke‐three‐parameter plus Lee–Yang–Parr (UB3LYP) density functional level theory. In addition to the reaction pathway through Fe^IV‐porphyrin intermediates, a new reaction pathway through Fe^III‐porphyrin π‐cation radical intermediates was found. Both reactions proceed with the homolytic cleavage of endoperoxide O? O to give an alkoxy radical. This intermediate converts into an allyl radical intermediate by a C? C homolytic cleavage, followed by the formation of thromboxane A₂ having a 6‐membered ring through a one electron transfer, or the degradation into HHT and MDA. The proposed mechanism shows that an iron(III)‐containing system having electron acceptor ability is essential for the 6‐membered ring formation leading to thromboxane A₂. Our results suggest that the step of the endoperoxide O? O homolytic bond cleavage has the highest activation energy following the binding of prostaglandin H₂ to thromboxane synthase.     

Computational study on the kinetics and mechanisms for the reactions of HCO with HONO and HNOH

The kinetics and mechanisms of the HCO reactions with HONO and HNOH have been studied at the G2M level of theory based on the geometric parameters optimized at BH&HLYP/6‐311G(d,p). The rate constants in the temperature range 200–3000 K at different pressures have been predicted by microcanonical RRKM and/or variational transition state theory calculations with Eckart tunneling corrections. For the HCO + HONO reaction, hydrogen abstraction from trans‐HONO and cis‐HONO by HCO produces H₂CO + NO₂, with the latter being dominant. Two other channels involving cis‐HONO by the association/decomposition mechanism via the HC(O)N(O)OH intermediate, which could fragment to give H₂O + CO + NO at high temperatures, were also found to be important. For the HCO + HNOH reaction, three reaction channels were identified: one association reaction giving a stable intermediate, HC(O)N(H)OH (LM2), and two hydrogen abstraction channels producing H₂CO and H₂NOH. The dominant products were predicted to be the formation of LM2 at low temperatures and H₂NOH + CO at middle and high temperatures. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 178–187 2004     

Reaction of N‐Heterocyclic Silylenes with Thioketone: Formation of Silicon–Sulfur Three (Si‐C‐S)‐ and Five (Si‐C‐C‐C‐S)‐Membered Ring Systems

Three‐ and five‐membered rings that bear the (Si‐C‐S ) and (Si‐C‐C‐C‐S ) unit have been synthesized by the reactions of L SiCl ( 1 ; L =PhC(NtBu)₂) and L′ Si ( 2 ; L′ =CH{(C?CH₂)(CMe)(2,6‐iPr₂C₆H₃N)₂}) with the thioketone 4,4′‐bis(dimethylamino)thiobenzophenone. Treatment of 4,4′‐bis(dimethylamino)thiobenzophenone with L SiCl at room temperature furnished the [1+2]‐cycloaddition product silathiacyclopropane 3 . However, reaction of 4,4′‐bis(dimethylamino)thiobenzophenone with L′ Si at low temperature afforded a [1+4]‐cycloaddition to yield the five‐membered ring product 4 . Compounds 3 and 4 were characterized by NMR spectroscopy, EIMS, and elemental analysis. The molecular structures of 3 and 4 were unambiguously established by single‐crystal X‐ray structural analysis. The room‐temperature reaction of 4,4′‐bis(dimethylamino)thiobenzophenone with L′ Si resulted in products 4 and 5 , in which 4 is the dearomatized product and 5 is formed under the 1,3‐migration of a hydrogen atom from the aromatic phenyl ring to the carbon atom of the C? S unit. Furthermore, the optimized structures of probable products were investigated by using DFT calculations.     

High‐level ab initio study of the N+(3P)+SH2 reactions in the gas phase: Role of spin‐forbidden pathways

The singlet and triplet potential energy surfaces involved in N⁺+SH₂ reactions have been explored using high‐level ab initio techniques. The geometries of the stationary points were optimized at the QCISD/6‐311G(df,p) level. The final energies were obtained in CCSD(T)/6‐311+G(3df,2p) single‐point calculations. The results obtained show that, although the N⁺(¹D)+SH₂ entrance channel is higher in energy than the N⁺(³P)+SH₂ one, most of the [H₂, S, N]⁺ singlet state cations are lower in energy than the corresponding triplets, due to their different bonding characteristics. Both singlet and triplet potential energy surfaces are quite close each other, and crossover between them can occur. The minimum energy crossing points were located by means of CASSCF(6,5) calculations. The spin‐orbit couplings show that the transition probability from the triplet to the singlet potential energy surface is significantly large. One of the most important consequences is that some of the products of the reaction, such as SH⁺, can be formed in typical spin‐forbidden processes. Since all the relevant structures along these pathways are much lower in energy than the reactants, this mechanism should be accessible even at low impact energies and therefore could be important in processes taking place in interstellar media. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Reaktionen von NaAlH4 mit primären Silylphosphanen und ‐arsanen: Synthese und Kristallstruktur eines cyclischen Natriumphosphanylalanats und eines polycyclischen Natriumarsanylalanats

Reaction of NaAlH4 with Primer Silylphosphines and Silylarsines: Synthesis and Crystal Structure of a Cyclic Sodium Phosphanylalanate and a Polycyclic Sodium Arsanylalanate The reaction of sodium aluminium hydride with H₂PSiMe₃ in the molar ratio 1:4 yields the compound [H₂Al{P(SiMe₃)₂}₂Na(dme)₂] ( 1 ). Central structural motif of this compound in a four‐membered AlP₂Na ring. Surprisingly the phosphorus atoms in the ring wear two exocyclic silylgroups each. From the reaction of NaAlH₄ with the primer silylarsine H₂AsSiiPr₃ in THF the ionic compound 2 can be obtained. In this compound cyclic [(H₂Al)₃(AsSiiPr₃)₃]^3‐ anions coordinate the sodium counter‐ions by the hydride ligands as well as by the arsenic atoms.     

Sulfur Tetrahydride and Allied Superhydride Clusters: When Resonance Takes Precedence

Sulfur offers a variety of bonding surprises compared to the parent oxygen atom of the chalcogen family. In the present work, we employ standard quantum chemistry methods to characterize formation of previously unrecognized sulfur tetrahydride (C_4v-symmetric SH₄) from hydrogen sulfide (H₂S) and molecular hydrogen (H₂) on the ground state potential energy surface. The unusual intramolecular interactions of SH₄ defy Lewis-like bonding conceptions, exhibiting the dominance of resonance-type donor-acceptor delocalizations well beyond those of SF₄ (C_2v sawhorse geometry) and other known tetrahalides. The distressed character of SH₄ bonding also leads to exotic intermolecular structural motifs in clusters of pure (SH₄)_n and mixed (SH₄⋅⋅⋅H₂S)_n composition. We evaluate structural, spectroscopic, and electronic properties for various 2D/3D coordination patterns and discuss how (SH₄⋅⋅⋅H₂S)_n-type building blocks may relate to recent experimental studies of superconductivity in high-pressure materials of “SH₃” stoichiometry.     

A theoretical study of the sulphonium cation SH+3

Ab initio Hartree—Fock calculations with STO-3G functions have been performed to determine the structure (1.371 Å and 95.33°) of SH⁺₃ and the proton affinity (≈196 kcal/mol) of H₂S. Inclusion of a sulphur 3d function in the basis has been found essential to give a better geometry of SH⁺₃.