Study of the reaction of MoOS2(S2CNR2)2 with PPh3 in 1,2-dichloroethane: Part II. Detection of reaction intermediate pentavalent molybdenum using ESR method at low temperature

Intermediate product of the reaction of MoOS₂(S₂CNR₂)₂ and PPh₃ in dichloroethane has been detected by ESR spectroscopy. Two ESR signals have been observed at low temperature in the reaction system which was stopped by quenching it in liquid nitrogen. The g values are 2.020 ± 0.001 and 1.972 ± 0.001 respectively. The signal at g = 2.020 is attributed to a reaction intermediate with pentavalent molybdenum. A reaction mechanism has been proposed which is consistent with the observation of pentavalent molybdenum as the intermediate in the process of reaction.     

Dediazoniations of arenediazonium ions. Part 26 Influence of Substituents on the Formation of Benzoic Acids in Reactions of Aryl-Cation-Type Intermediates with CO

The N₂-molecule-aryl-cation pair formed as the first intermediate in dediazoniations of arenediazonium ions can be trapped with CO in H₂O with formation of the corresponding arenecarboxylic acids. This reaction is considered as a model for the reverse of dediazoniation, since CO is isoelectronic with N₂. The evaluation of the yields of arenecarboxylic acids formed from substituted benzenediazonium ions using Taft's dual substituent parameter treatment demonstrates that the field reaction constant p_F and the resonance reaction constant p_R are positive and negative, respectively, as expected for a reaction which corresponds electronically to the addition of N₂ to aryl cations.     

MNDO Study of reaction pathways for SN2 reactions. Menschutkin reaction potential energy surfaces

MNDO molecular orbital calculations have been employed to investigate limited reaction pathways and potential energy surfaces for a series of S_N2 reactions. Model calculations for X^? + CH₃X (X = H, F, OH, OCH₃, and CN) indicate that the MNDO method gives qualitative agreement with ab initio studies except for the hydride–CH₄ exchange. Studies involving alkylation of pyridine (Menschutkin reaction) were also carried out. For the reaction of pyridine with CH₃Cl, which involves charge separation, our MNDO studies (which do not include solvation effects) do not produce a characteristic S_N2 pathway. For the reaction of pyridine with trimethyloxonium cation [(CH₃)₃O⁺] as the alkylating agent, a well defined S_N2 reaction pathway was obtained; this reaction involves charge transfer. A potential energy surface for the pyridine–trimethyloxonium cation reaction shows the presence of a saddle point transition state that resembles starting materials, in agreement with the Hammond postulate for this exothermic reaction.     

THE OVERALL KINETICS OF FREE RADICAL (CO)POLYMERIZATION WITH HIGHLY DIFFUSION CONTROLLED TERMINATION

The overall reaction rate kinetics of polymerization of diethyleneglycol dimethacrylate and copolymerization of it with styrene in bulk and in the presence of inert diluents were investigated. Theresults indicated that these reactions can be treated as free radical polymerization with highly diffu-sion controlled termination reaction in which the termination rate constant is an empirically derivedfunction of monomer conversion: K_t=K_(to)(1-c ln[M]/ [M_0])~(-1) in which K_(to) is the initial terminationrate constant and c is a factor related to the magnitude of diffusion co?re The following equationof monomer conversion as a function of time could then be derived: U=1-exp {1/c [1-(1+ckt/2)~2]}in which k=K_P(R_i/2K_(to))~(1/2) and t is the time of reaction. Excellent agreement between the theoreticaland experimental overall reaction kinetic curves was obtained. The equation is valid for crosslinkingand noncrosslinking free radical polymerizations in which the self-acceleration effcct is effective fromthe very beginning of the reaction. The equation can be expressed in a more generally applicableform: U=1--exp{1/e[1--(1+?t/n)~n] in which n≥0.     

CH3SOx (x＝0, 1)与三线态Oy (y＝1, 2, 3)反应机理的理论研究

采用B3LYP方法和6-311G(d, p)基组对CH₃S及其氧化后继物CH₃SO与O_y (y＝1, 2, 3)反应形成酸雨的微观机理进行了理论研究. 对反应势能面上的各驻点进行几何构型全优化. 振动分析和IRC计算证实了中间体和过渡态的真实性和相互连接关系. 找到了7条生成SO₂的反应途径, 其中CH₃S与O直接反应得到产物CH₃和SO最容易进行; CH₃S先与O₃反应, 其产物再与O₃反应得到CH₃SO₂, CH₃SO₂最后分解得到CH₃S和SO₂较容易进行, 其它的反应较难进行.     

Copper-Catalyzed Coupling Reaction of Secondary Alkyl 2-Pyridinesulfonates with Alkyl Lithium-Based Reagents

The Cu(OTf)₂-catalyzed alkyl–alkyl coupling reaction of a secondary substrate MeCH(OSO₂Py)CH₂CH₂C₆H₄(4-OMe) with a nBuLi-based reagent prepared by transmetalation with MgBr₂ ⋅ THF₃ in THF produced a coupling product in 74 % yield. The use of soluble MgBr₂ ⋅ THF₃ in THF was required for this reaction. This method was applied to sBuLi and Ph(CH₂)₄Li. In contrast, transmetalation of MeLi with soluble MgCl₂ ⋅ THF₂ in THF produced the Me reagent, which was reactive for the coupling reaction. The reaction proceeded with inversion of the stereogenic carbon. Furthermore, (S)-14-methyloctadecan-2-one, a sex pheromone produced by lichen moths, was synthesized.     

Allgemeine Basenkatalyse der Azokupplung von o-Diazophenolen. 19. Mitteilung zur Kenntnis der Azokupplungsreaktion

1. The kinetics and the mechanism of the diazo coupling reaction of 2-diazophenol-4-sulphonic acid with 1-naphthol-2-sulphonic acid have been investigated at 0° and ionic strength I=0.45. 2. The pK_a-value of the hydroxyl group in 2-diazophenol-4-sulphonic acid has been determined: pK_a=- 0.04 ± 0.10. It is the diazonium-phenolate anion which actually enters into the diazo coupling reaction. 3. The reaction is subject to general base catalysis. It is shown that no intermediate is enriched during the reaction at pH 11.3–11.6 which proceeds by a two-step mechanism with a steady state intermediate.     

Ab initio study on the mechanism of reaction HNCO+NH2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH₂+ HNCO?NH₃+NCO (1) and NH₂+HNCO?N₂H₃+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300–2700 K, transition theory rate constant for reaction (1) is 1.68×10¹¹–3.29×10¹¹ mL·mol^-1·s^-1, which is close to the experimental one of 5.0×10¹¹mL·mol^-1·s^-1or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes,cis andtrans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (forcis-mode) and 147.43 kJ/mol (fortrans-mode), respectively, which is much higher than reaction (1). So reaction (1) is the main channel for the titled reaction.     

Kinetics and mechanism of pyrogallol autoxidation: Calibration of the dynamic response of an oxygen electrode

The kinetics of the reaction between 1,2,3-trihydroxybenzene (pyrogallol) and O₂ (autoxidation) have been determined by monitoring the concentration of dissolved dioxygen with a polarographic oxygen electrode. The reaction is carried out in pseudo-first-order excess pyrogallol, 25°C, 0.08 M NaCl, and 0.04 M phosphate buffer in the pH range 6.9–10.5. Data collection precedes reaction initiation, but only the data recorded after the estimated 3.2 s dead time are used in kinetics calculations. Observed rate constants are corrected for incomplete mixing, which is treated as a first-order process that has an experimentally determined mixing rate constant of 4.0 s^?1. The rate law for the reaction is ?d[O₂]/dt=k_app[PYR]_tot[O₂], in which [PYR]_tot is the total stoichiometric pyrogallol concentration. A mechanism is presented which explains the increase in rate with increasing [OH^?] by postulating that H₂PYR^? (k₂) has greater reactivity with dissolved dioxygen than does H₃PYR (k₁). The data best fit the equation k_app=(k₁ + k₂K_H[OH^?])/(1 + K_H[OH^?]) when the value of the hydrolysis constant K_H (the quotient of the pyrogallol acid dissociation and water autoprotolysis constants) for this medium equals 3.1×10⁴ M^?1. The resulting values of k₁ and k₂, respectively, equal (0.13 + 0.01) M^?1 s^?1 and (3.5 plusmn; 0.1) M^?1 s^?1. This reaction is recommended as a test reaction for calibrating the dynamic response of an O₂-electrode. © 1993 John Wiley & Sons, Inc.     

Synthesis of Bis‐Heterocyclic 1H‐Imidazole 3‐Oxides from 3‐Oxido‐1H‐imidazole‐4‐carbohydrazides

The reaction of 1H‐imidazole‐4‐carbohydrazides 1 , which are conveniently accessible by treatment of the corresponding esters with NH₂NH₂?H₂O, with isothiocyanates in refluxing EtOH led to thiosemicarbazides (=hydrazinecarbothioamides) 4 in high yields (Scheme 2). Whereas 4 in boiling aqueous NaOH yielded 2,4‐dihydro‐3H‐1,2,4‐triazole‐3‐thiones 5 , the reaction in concentrated H₂SO₄ at room temperature gave 1,3,4‐thiadiazol‐2‐amines 6 . Similarly, the reaction of 1 with butyl isocyanate led to semicarbazides 7 , which, under basic conditions, undergo cyclization to give 2,4‐dihydro‐3H‐1,2,4‐triazol‐3‐ones 8 (Scheme 3). Treatment of 1 with Ac₂O yielded the diacylhydrazine derivatives 9 exclusively, and the alternative isomerization of 1 to imidazol‐2‐ones was not observed (Scheme 4). It is important to note that, in all these transformations, the imidazole N‐oxide residue is retained. Furthermore, it was shown that imidazole N‐oxides bearing a 1,2,4‐triazole‐3‐thione or 1,3,4‐thiadiazol‐2‐amine moiety undergo the S‐transfer reaction to give bis‐heterocyclic 1H‐imidazole‐2‐thiones 11 by treatment with 2,2,4,4‐tetramethylcyclobutane‐1,3‐dithione (Scheme 5).     

Catalytic Decomposition of H2O2over Supported ZnO

 Transition metal sulfates of Cu(II), Co(II), Ni(II), Cr(III), Mn(II), and Fe(III) supported on ZnO were prepared and characterized by SEM, EDX, and XRD. The kinetics of the heterogeneous decomposition of H₂O₂ over these supported catalysts was investigated. The reaction rate is correlated with both the amount of supported metal ion and its redox potential. The rate of reaction increases with increasing initial concentration of H₂O₂, attains a maximum, and decreases thereafter. It also increases with pH and reaches a maximum at high pH values. A reaction mechanism is proposed that implies the formation of a peroxo intermediate at the early stages of the reaction. A second intermediate is assumed to be formed at high [H₂O₂]_o which inhibits the progress of the reaction.     

An Ab Initio Study on the Mechanism of the Atmospheric Reaction NH2+O3→H2NO+O2

The atmospheric reaction NH₂+O₃→H₂NO+O₂ has been investigated theoretically by using MP2, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. At the MP2 level of theory, the hypersurface of the potential energy (HPES) shows a two step reaction mechanism. Therefore, the mechanism proceeds along two transition states (TS1 and TS2), separated by an intermediate designated as Int. However, when the single‐reference higher correlated QCISD and the multiconfigurational CASSCF methodologies have been employed, the minimum structure Int and TS2 are not found on the HPES, which thus confirms a direct reaction mechanism. Single‐reference high correlated and multiconfigurational methods consistently predict the barrier height of the reaction to be within the range of 3.9 to 6.6 kcal mol^?1, which is somewhat higher than the experimental value. ¹ The calculated reaction enthalpy is ?67.7 kcal mol^?1.     

Energy resonance and inverse population of the product vibrational states for the reaction F + H2(v = 0) → HF(v′) + H,LCAC‐SW theoretical quantum scattering study

LCAC‐SW (linear combination of arrangement channel‐scattering wavefunction) method was used to calculate collinear state‐to‐state reaction probabilities for the reaction F + H₂(v = 0) → HF(v′) + H on the 6SEC potential energy surface. The results show that reaction probabilities P₀₂ and P₀₃ [i. e., v′ = 2,3 for reaction F + H₂ (v = 0) + HF(v′) + H] are primary, the population of product vibrational states is inverse and the reaction probabilities are oscillatory with collision energies, i.e., there is energy resonance in this reaction, which agrees with a new experiment.     

Solid state interaction between V2O5 and MoO3: specific features related to minor vanadia transfer

Vanadia transport, which is a minor reaction flux in the solid state reaction between V₂O₅ and MoO₃, was studied using chemical and neutron activation analyses and electron spectroscopy for chemical analysis. It was found that negligible quantities of vanadia were transferred in a molybdena briquette during the reaction. Vanadia was presumably localized in thin external layers of molybdena grains. The reaction potential difference U _r across a Pt|MoO₃|V₂O₅|Pt cell was studied. It was shown that in this cell U _r was produced at the molybdena briquette and was due to vanadia transport. The U _r value changed with time in two stages. The reaction potential difference U _r was constant (or diminished slightly) at the first stage and dropped abruptly at the second stage. The duration of the first stage depended on the initial thickness of the MoO₃ briquette: the thicker the briquette, the longer the U _r value was nearly constant. Causes and probable mechanisms of U _r generation are discussed in different terms: chemical reaction, variation of a _{O 2} at the boundary between the reaction product and initialoxides, or surface spreading of the minor (V₂O₅ or V₉Mo₆O₄₀) diffusant. The last mechanism, which received the least study in the general case, was shown to be the most probable one for the reaction at hand. Electronic Publication     

Theoretical Investigation on the Topochemical Polymerization of Diacetylenes

Abstract

The solid-state polymerization of diacetylenes (MDA-PBT-PDA) is studied with a concerted reaction model and the calculation method of EHMO-ASED and EHCO-ASED, where MDA = crystalline molecular diacetylenes, PBT = polybutatrienes, and PDA = polydiacetylenes. As the reaction goes on, the symmetry of frontier orbitals inverts at state PBT, HOCO from C ₂-antisymmetry to C ₂-symmetry and LUCO from C ₂-symmetry to C ₂-antisymmetry, which means completion of the 1,4-addition. Two necessary conditions must be satisfied for the reaction to take place: 1) the geometric parameters must undergo a series of concerted changes to make the conformation suitable for the intermolecular 1,4-addition, which should overcome an energy barrier E_b ; 2) the symmetry match between the frontier crystal orbitals of the reactant and the product must be satisfied-electrons of the reactant should be excited from HOCO (C ₂-antisymmetry) into LUCO (C ₂-symmetry), which faces an energy gap E _g. At state MDA, there is E _g(MDA) ≈ 5.6 eV. If MDA and PDA are analyzed according to Woodward-Hoffmann's rules, this reaction would be considered photochemically allowed but thermochemically forbidden. It has been shown that the E _g gradually decreases along the reaction coordinate from state MDA to PBT. At state PBT there is E _g(PBT) ≤ 0.1 eV, and the electrons of the reactant can be easily excited there. Since E_b ≤ 1.0 eV is not very large and E_g (PBT) ≤ 0.1 eV is very small, the two necessary conditions mentioned above can be satisfied thermally. Therefore, thermal polymerization can take place smoothly. By this pathway the apparent activation energy of the reaction will be E_a ≤ 1.0 eV, which is consistent with the activation energies of the polymerizations of diacetylenes in the literature.     

Kinetics and Mechanism of Catalytic Reduction of U(VI) with Hydrazine on Platinum Catalysts in Nitric Acid Media

The kinetics of U(IV) produced by hydrazine reduction of U(VI) with platinum as a catalyst in nitric acid media was studied to reveal the reaction mechanism and optimize the reaction process. Electron spin resonance (ESR) was used to determine the influence of nitric acid oxidation. The effects of nitric acid, hydrazine, U(VI) concentration, catalyst dosage and temperature on the reaction rate were also studied. In addition, the simulation of the reaction process was performed using density functional theory. The results show that the influence of oxidation on the main reaction is limited when the concentration of nitric acid is below 0.5 mol/L. The reaction kinetics equation below the concentration of 0.5 mol/L is found as: -dc(UO₂²⁺)/dt)=kc^0.5323(UO₂²⁺)c^0.2074(N₂H₅⁺)c^-0.2009(H⁺). When the temperature is 50 ℃, and the solid/liquid ratio r is 0.0667 g/mL, the reaction kinetics constant is k=0.00199 (mol/L)^0.4712/min. Between 20 ℃ and 80 ℃, the reaction rate gradually increases with the increase of temperature, and changes from chemically controlled to diffusion-controlled. The simulations of density functional theory give further insight into the influence of various factors on the reaction process, with which the reaction mechanisms are determined according to the reaction kinetics and the simulation results.     

Ab initio study on the reaction mechanism of ozone with bromine atom

Ab initio calculations of the potential energy surface (PES) for the Br+O₃ reaction have been performed using the MP2, CCSD(T), and QCISD(T) methods with 6‐31G(d), 6‐311G(d), and 6‐311+G(3df). The reaction begins with a transition state (TS) when the Br atom attacks a terminal oxygen of ozone, producing an intermediate, the bromine trioxide (M), which immediately dissociates to BrO+O₂. The geometry optimizations of the reactants, products, and intermediate and transition states are carried out at the MP2/6‐31G(d) level. The reaction potential barrier is 3.09 kcal/mol at the CCSD(T)/6‐311+G(3df)//MP2 level, which shows that the bromine atom trends intensively to react with the ozone. The comparison of the Br+O₃ reaction with the F+O₃ and Cl+O₃ reactions indicates that the reactions of ozone with the halogen atoms have the similar reaction mechanism. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

New ethylzinc reagents with remarkable properties in palladium-catalyzed zinc-ene reactions

Pd-Catalyzed Zn-ene allylic olefinations with the new ethylzinc reagents Et? Zn? OSO₂CF₃ ( 4 ) and Et? Zn? OC(O)CF(MeO)CF₃ ( 5 ) in CH₂Cl₂ showed an unexpected trans-selectivity in the ring closure to cyclopentane derivatives (see Scheme 2 and Table 1). This strong trans-selectivity is in contrast with the corresponding known Zn-ene reaction using Et₂Zn in Et₂O which shows a high cis-selectivity (Table 1). The probable radical origin of the observed trans-selectivity is discussed. The Zn-ene reaction products of the type R? Zn? OSO₂CF₃ could be derivatized by the known protonation, iodination, and cyanation yielding 8–10 (Scheme 4 and Table 2), these derivatizations could furthermore be extended by allylation and oxidation reaction (→ 13, 15 , and 16 ; see Scheme 5).     

Verbindungen des Germaniums und Zinns. 3. Sterisch überladene Alkylarylstannane durch Übertragung und Isomerisierung von 2,4,6-Tri-tert-butylphenylgruppen

Compounds of Germanium and Tin. 3. Sterically Congested Alkylarylstannanes by Transfer and Isomerization of 2,4,6-Tri-tert-butylphenyl Groups Reaction of SnBr₄ and SnI₄ with 2,4,6-tri-tert-butylphenyllithium (ArLi) by rearrangement of two Ar-groups gives the stannanes ArR₂SnBr ( 3 ) and ArR₂SnI ( 4 ), R = 2-methyl-2(3,5-di-tert-butylphenyl)propyl, which by a further transalkylation reaction with methyl lithium yield ArR₂SnCH₃ ( 5 ). However, treatment of 3 and 4 with tert-butyl lithium exclusively leads to ArR₂SnH ( 6 ) which surprisingly is also obtained by reaction of ArRSnCl₂ with tert-butyl lithium, presumably by an intermolecular R-group transfer. The structures of 5 and 6 were confirmed by X-ray crystallography.     

Synthesis of an ansa-Zirconocene via a Novel S4-Symmetric Spirobis(silastannaindacene) Compound

A stereoselective reaction of Sn(NMe ₂ ) ₄ with the silyl-bridged bis(cyclopentadienyl) derivative 1 generates the novel spiro compound 2 , in which two C₂-symmetric six-membered rings of opposite configuration are joined at a tin center: One ring is R,R-, and the other S,S-configured. Subsequent reaction with two equivalents of ZrCl₄ affords, by stereoselective Sn/Zr exchange, exclusively the C₂-symmetric isomer of the ansa-zirconocene rac- 3 .