MNDO/PM3 Study of the Reaction Mechanism between Methane and Complexes Generated in Br2·AlBr3 System

Potential energy surface (PES) of systems AlBr₅ and AlBr₅ + CH₄ ⁺ were investigated by MNDO/PM3 method. All the five donor-acceptor complexes Br₂·AlBr₃ with no barrier add to methane providing multiple adducts with various localization of interactions and with different conformations. However further transformations occur only with adducts of two complexes Br₂·AlBr₃ possessing considerable ionic character. On the reaction path resulting in CH₃Br and HBr as intermediates function bromonium type complexes CH₃BrH⁺·AlBr₄ ^- and the intermediates on the path leading to CH2Br2 and H₂ are the complexes with 3c-2e bond of H2 quasi-molecule with the C atom of bromomethyl cation H₂C(H-H)Br⁺·AlBr₄ ^-. Potential barriers on both reaction paths are about 30 kcal mol-1, and the transition states (TS) are analogous to the classical 3c-2e TS (Olah scheme) with an electrophile attack on a CH bond and to the recently suggested TS with an electrophile attack on an unshared electron pair of the carbon atom in the nonclassical methane H2C(H-H) respectively.     

Investigation of the reactions of pyrocatechol derivatives with AlBr3 by NMR spectroscopy

Alkylpyrocatechols are dealkylated by AlBr₃ in CH₂Br₂. The formation of stable carbenium ions (tert-butyl, methylcyclopentyl, and l-adamantyl) was detected by¹H NMR spectroscopy. Pyrogallol and 2,3-dihydroxynaphthalene are coordinated with AlBr₃ in the tautomeric keto form, and 2-mercapto-4-methylphenol forms a complex in the hydroxy form. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1158–1160, June, 1997.     

Mechanisms of the reactions of propane with CBr3+ cation and superelectrophilic complex CBr4+·AlBr3- containing a bromine-centered cationic center: a theoretical study

Fragments of the potential energy surfaces (PES) of the systems [C₃H₈ + CBr₃ ⁺] and [C₃H₈ + Br₂CBr⁺·Br₂AlBr₂ ^–] were simulated by the MNDO/PM3 method. Energy minima corresponding to weakly bound adducts of propane molecule with the CBr₃ ⁺ cation or neutral complex CBr₃ ⁺·AlBr₄ ^– were found on the PES's of both systems. These are adducts with the coordination of a H atom of the methylene group of the propane molecule to the electrophile at the Br atom carrying the largest positive charge. As the fragments of the adducts are brought close together, the coordinated H atom migrates to the C atom of the CBr₃ ⁺ fragment. The potential barriers of these migrations were found to be low for both systems. The reactions proceed without formation of cyclic intermediates or transition states typical of the Olah mechanism.     

Cationic polymerization of hydrocarbon monomers induced by complexes of acyl halides with Lewis acids

The polymerization of isobutylene inn-hexane at −78°C under the action of the superacid HBr·2A1Br₃ as well as acetyl complexes MeCOBr·AlBr₃ and MeCOBr·2AlBr₃ in the presence of HBr·AlBr₃ and HBr·2AlBr₃, respectively, was studied. Unlike the superacid providing a quantitative yield of polyisobutylene (PIB) due to protonogenic initiation, the acetyl complexes suppress the proton initiation. In the presence of a mixture of both complexes with the superacid, only macromolecules with the head acetyl fragments MeC(O) are formed, which is evidence for a carbocationic initiation. The data obtained are explained by trapping of protons by the carbonyl groups to form ionic structures of the type (where PIB is polyisobutylene) and to suppress the ionization of the superacids due to the common ion effect. For Part 7, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 974–978, May, 1997.     

反式2-甲基-2-丁烯酸甲酯与臭氧反应机理的计算研究

采用G3B3方法构建反式2-甲基-2-丁烯酸甲酯与O3反应体系以及后续Criegee自由基有、无水分子参与下异构化反应的势能面剖面.结果表明,反式2-甲基-2-丁烯酸甲酯与O3反应首先生成一个稳定的五元环中间体,此中间体按断键位置不同后续裂解反应存在两条路径,分别生成产物P1(CH3CHOO+CH3OC(O)C(CH3)O)和P2(CH3CHO+CH3OC(O)C(CH3)OO).利用经典过渡态理论(TST)并结合Wigner矫正模型计算了200-1200 K温度区间内标题反应的速率常数kTST/W.计算结果显示,294 K时,该反应速率常数为7.55×10-18cm3molecule-1s-1,与Bernard等对类似反应所测实验值非常接近.生成的Criegee自由基(CH3CHOO和CH3OC(O)C(CH3)OO)可分别与水分子发生α-加成及β-氢迁移反应,其中Criegee自由基与水的α-加成反应较其与水的β-氢迁移反应具有优势.另外与无水分子参与CH3CHOO和CH3OC(O)C(CH3)OO异构化反应相比,水分子的参与使得异构化反应较为容易进行.     

Unusually facile transformations of cyclopentane into cyclohexanes, decalins, and adamantanes

Cyclopentane is converted into a mixture of cyclohexanes, decalins, and adamantanes (and isomeric cycloalkanes) in overall yields of 18–31% (w/w) under the action of superelectrophilic complex CBr₄·2AlBr₃ either in CH₂X₂ (X=Br, Cl) or without a solvent at 20°C. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2304–2309, December, 1999.     

Monoammoniates of Aluminium Halides: The Crystal Structures of AlBr3 · NH3 and AlI3 · NH3

Single crystals of AlBr₃ · NH₃ and AlI₃ · NH₃ sufficient in size for X‐ray structure determinations were obtained by evaporation/ sublimation of the respective compound from its melt. The ammoniates were synthesized by the reaction of the pure halide with NH₃ at ‐78°C and following homogenization by slowly heating the reaction mixture up to the melting points of the ammoniates (124°C and 126°C, respectively). The X‐ray structure determinations for both monoammoniates were successfully carried out for the heavy atom positions (no hydrogen atoms): AlBr₃ · NH₃: Pbca, Z = 16, a = 11.529 (5) Å, b = 12.188 (2) Å, c = 19.701 (4) Å AlI₃ · NH₃: Pbca, Z = 8, a = 13.536 (5) Å, b = 8.759 (2) Å, c = 14.348 (4) Å The structures contain tetrahedral molecules Al(NH₃)X₃ with X = Br, I. They are not isotypic. The main difference is given for the coordination of NH₃ by X^‐ from neighbouring molecules. In Al(NH₃)Br₃ one of the two crystallographically independent NH₃ ligands has 6Br^‐ and the other 7Br^‐ as neighbours whereas in Al(NH)₃I₃ only 5I^‐ surround the one kind of NH₃.     

Cationic polymerization of hydrocarbon monomers induced by complexes of acyl halides with Lewis acids

Polymerization of isobutylene inn-hexane at −78°C initiated by MeCOBr·AlBr₃ was studied. The results obtained were compared with the corresponding data for RCOX·2AlBr₃ complexes (R=Me or Ph, X=Cl or Br). The main peculiarities of the polymerization mechanism under the action of MeCOBr·AlBr₃ were established. The rate constants of proton elimination and of chain termination and chain growth were determined experimentally. For Part 8, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1147–1151, June, 1997.     

Effects of substituents and n-donors on the energies of Si←N,Si←O,and Si=C bonds in hypervalent silenes

The effect of the nature of substituents at sp²-hybridized silicon atom in the R₂Si=CH₂ (R = SiH₃, H, Me, OH, Cl, F) molecules on the structure and energy characteristics of complexes of these molecules with ammonia, trimethylamine, and tetrahydrofuran was studied by the ab initio (MP4/6-311G(d)//MP2/6-31G(d)+ZPE) method. As the electronegativity, χ, of the substituent R increases, the coordination bond energies, D(Si← N(O)), increase from 4.7 to 25.9 kcal mol⁻¹ for the complexes of R₂Si=CH₂ with NH₃, from 10.6 to 37.1 kcal mol⁻¹ for the complexes with Me₃N, and from 5.0 to 22.2 kcal mol⁻¹ for the complexes with THF. The n-donor ability changes as follows: THF ≤ NH₃ < Me₃N. The calculated barrier to hindered internal rotation about the silicon—carbon double bond was used as a measure of the Si=C π-bond energy. As χ increases, the rotational barriers decrease from 18.9 to 5.2 kcal mol⁻¹ for the complexes with NH₃ and from 16.9 to 5.7 kcal mol⁻¹ for the complexes with Me₃N. The lowering of rotational barriers occurs in parallel to the decrease in D _π(Si=C) we have established earlier for free silenes. On the average, the D _π(Si=C) energy decreases by ∼25 kcal mol⁻¹ for NH₃· R₂Si=CH₂ and Me₃N·R₂Si=CH₂. The D(Si←N) values for the R₂Si=CH₂· 2Me₃N complexes are 11.4 (R = H) and 24.3 kcal mol⁻¹ (R = F). sp²-Hybridized silicon atom can form transannular coordination bonds in 1,1-bis[N-(dimethylamino)acetimidato]silene (6). The open form (I) of molecule 6 is 35.1 and 43.5 kcal mol⁻¹ less stable than the cyclic (II, one transannular Si←N bond) and bicyclic (III, two transannular Si←N bonds) forms of this molecule, respectively. The D(Si←N) energy for structure III was estimated at 21.8 kcal mol⁻¹. Dedicated to Academician N. S. Zefirov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1952–1961, September, 2005.     

Diastereoselective bromination of(R)-N-cinnamoyl-4-phenyloxazolidin-2-one in the presence of Lewis acids

The stereochemistry of addition of Br₂ toE-andZ-(R)-N-cinnamoyl-4-phenyloxazolidin-2-ones was studied. It was established that both theE-andZ-isomers give only two out of four possible diastereoisomers in the presence of Lewis acids (BPr₃ or AlBr₃). The absolute configurations of the diastereoisomers [(2S', 3R') and (2R', 3S') of the side chain] were established by X-ray structural analysis. The stereochemistry observed is a consequence of the stepwise bromination and the absence of bridging bromine atoms along the reaction coordinate. In the case of theZ-isomer, the diastereoselectivity of the reaction was high, whereas it is low in the case of theE-isomer. It was suggested that at the first stage of addition of Br₂ at the C=C bond, the attack of the Br⁺ cation occurs at the α position, and the second stage of transfer of Br⁻ occurs with the participation of boron or aluminum complexes in the intermediate state of the bromination reaction. This hypothesis as well as the results of calculations of the initial conformations of the substrates provide an explanation of the regularities observed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1022–1028, May, 1997.     

The Acidity of the HBr/AlBr3 System: Stabilization of Crystalline Protonated Arenes and Their Acidity in Bromoaluminate Ionic Liquids

Bulk protonated mesitylene, toluene, and benzene bromoaluminate salts were stabilized and characterized in the superacidic system HBr/n AlBr₃ with NMR spectroscopy and X‐ray analysis of [HC₆H₃(CH₃)₃]⁺[AlBr₄]^? ( 1 ), [HC₆H₅(CH₃)]⁺[AlBr₄]^? ( 2 ), and [C₆H₇]⁺[Al₂Br₇]^??C₆H₆ ( 3 ). Protonation attempts in bromoaluminate ILs led to a complete protonation of mesitylene, and a protonation degree of up to 15 % for toluene in the IL BMP⁺[Al₂Br₇]^?. Benzene could only be protonated in the more acidic IL BMP⁺[Al₃Br₁₀]^?, with a degree of 25 %. Protonation attempts on aromatics provide evidence that the bromoaluminate ILs tolerate superacidic environments. On the basis of the absolute Brønsted acidity scale, quantum chemical calculations confirmed the superacidic properties, and rank the acidities in ILs down to a pH_abs value of 164 with an error of less than one pH unit compared with experimental findings. The neat AlBr₃/HBr system even may reach acidities down to pH_abs 163.     

三接触弯曲构型的蓝移氢键CH3…Y的理论研究

运用量子化学从头算方法, 在MP2/6-311＋＋G(d,p), MP2/6-311＋＋G(2df,2p), MP2/6-311＋＋G(3df,3pd)和QCISD/6-311＋＋G(d,p)水平上, 研究了CH₃F, CH₃Cl和CH₃Br作为质子给体与Cl^－, Br^－作为质子接受体形成的氢键CH₃…Y. 计算结果表明: 6种复合物中C—H键收缩, 伸缩振动频率增大, 形成蓝移氢键. 分子中原子(Atoms in Molecules, AIM)分析表明, 这些复合物的电子密度拓扑性质与普通氢键有着本质的不同, 在Y…H之间不存在键临界点, 而在Y与C之间存在键临界点, 因此这些相互作用严格地不能称为氢键. 自然键轨道(Natural bond orbital, NBO)分析表明, 在这些复合物中弯曲的CH…Y的特殊结构使得分子间超共轭n(Y)®σ*(C—H)减小到可以忽略; 质子接受体的电子密度没有转移到σ*(C—H)上, 而是转移到了σ*(C—X) (X＝F, Cl, Br)上; 存在一定程度的重杂化; 分子内超共轭相互作用减小使得σ*(C—H)的电子密度减少. 这些因素共同导致C—H伸缩振动频率的蓝移.     

Ab initio study of the potential energy surface and product branching ratios for the reaction of O(D) with CH3CH2F

The potential energy surface of O(¹D) + CH₃CH₂F reaction has been studied using QCISD(T)/6-311++G(d,p)//MP2/6-311G(d,p) method. The calculations reveal an insertion–elimination reaction mechanism of the title reaction. The insertion process has two possibilities: one is the O(¹D) atom inserting into C–F bond of CH₃CH₂F produces one energy-rich intermediate CH₃CH₂OF and another is the O(¹D) atom inserting into one of the C–H bonds of CH₃CH₂F produces two energy-rich intermediates, IM1 and IM2. The three intermediates subsequently decompose to various products. The calculations of the branching ratios of various products formed though the three intermediates have been carried out using RRKM theory at the collision energies of 0, 5, 10, 15, 20, 25 and 30 kcal/mol. CH₃CH₂O is the main decomposition product of CH₃CH₂OF. HF and CH₃ are the main decomposition products for IM1; CH₂OH is the main decomposition product for IM2. Since IM1 is more stable and more likely to form than CH₃CH₂OF and IM2, HF and CH₃ are probably the main products of the O(¹D) + CH₃CH₂F reaction. Our computational results can give insight to reaction mechanism and provide probable explanations for future experiments.     

Chemistry of superacids

Superelectrophilic nitration of deactivated aromatics with NO₂Cl−3MX _n complexes in aprotic nonpolar solvents such as CH₂Cl₂ makes it possible to obtain the corresponding nitro derivatives in good to almost quantitative yields under mild conditions. For part 34, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 953–956, May, 1998.     

EPR spectra and structures of dinuclear copper(<Emphasis Type="SmallCaps">II</Emphasis>) complexes with acyldihydrazones of benzenedicarboxylic acids

Spacer-armed dinuclear copper(II) complexes with condensation products of isophthalic and terephthalic acid dihydrazides with salicylaldehyde and 2-hydroxyacetophenone were synthesized and studied by EPR and X-ray diffraction. The compositions and structures of most of the complexes were determined by elemental analysis, thermogravimetric analysis, and IR spectroscopy. The structure of the copper(II) complex with acyldihydrazone of salicylaldehyde and 1,3-benzenedicarboxylic acid (H₄L) with the composition [Cu₂L¹·2morph·MeOH] (morph is morpholine) was established by X-ray diffraction. The Cu^II atoms are spaced by 10.29 Å and are structurally nonequivalent. One copper cation has a square-planar coordination formed by donor atoms (2 N + O) of the doubly deprotonated acylhydrazine fragment and the N atom of the morpholine molecule. The second copper atom is additionally coordinated by a methanol molecule through the oxygen atom, so that this copper atom is in a tetragonal-pyramidal coordination with the oxygen atom in the axial position. The EPR spectra of liquid solutions of the complexes based on 1,4-benzenedicarboxylic acid acyldihydrazones and 1,3-benzenedicarboxylic acid bis(salicylidene)hydrazone at room temperature show a four-line hyperfine structure with the constant a _Cu = 54.4–67.0·10⁻⁴ cm⁻¹ (g = 2.105–2.147), which is indicative of the independent behavior of the paramagnetic centers. The EPR spectrum of a solution of the complex based on isophthalic acid and 2-hydroxyacetophenone shows the seven-line hyperfine structure corresponding to two equivalent copper nuclei (g = 2.11, a _Cu = 36.5·10⁻⁴ cm⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1898–1905, October, 2007.     

Carbon Dioxide Reforming of Methane over Ni Catalysts Supported on Al2O3 Modified with La2O3, MgO, and CaO

The preparation of synthesis gas from carbon dioxide reforming of methane (CDR) has attracted increasing attention. The present review mainly focuses on CDR to produce synthesis gas over Ni/MO_x/Al₂O₃ (X = La, Mg, Ca) catalysts. From the examination of various supported nickel catalysts, the promotional effects of La₂O₃, MgO, and CaO have been found. The addition of promoters to Al₂O₃-supported nickel catalysts enhances the catalytic activity as well as stability. The catalytic performance is strongly dependent on the loading amount of promoters. For example, the highest CH₄ and CO₂ conversion were obtained when the ratios of metal M to Al were in the range of 0.04–0.06. In the case of Ni/La₂O₃/Al₂O₃ catalyst, the highest CH₄ conversion (96%) and CO₂ conversion (97%) was achieved with the catalyst (La/Al = 0.05 (atom/atom)). For Ni/CaO/Al₂O₃ catalyst, the catalyst with Ca/Al = 0.04 (atom/atom) exhibited the highest CH₄ conversion (91%) and CO₂ conversion (92%) among the catalysts with various CaO content. Also, Ni/MgO/Al₂O₃ catalyst with Mg/Al = 0.06 (atom/atom) showed the highest CH₄ conversion (89%) and CO₂ conversion (90%) among the catalysts with various Mg/Al ratios. Thus it is most likely that the optimal ratios of M to Al for the highest activities of the catalysts are related to the highly dispersed metal species. In addition, the improved catalytic performance of Al₂O₃-supported nickel catalysts promoted with metal oxides is due to the strong interaction between Ni and metal oxide, the stabilization of metal oxide on Al₂O₃ and the basic property of metal oxide to prevent carbon formation.     

N2O4 complexes with electron donor molecules. An STO-3G study

The quantum chemical calculations for N₂O₄ complexes with some electron-donor molecules: CH₃NO₂, CH₃COOH, CH₃CN, C₆H₆ and with another molecule of N₂O₄ show that the donor molecule lies under the N₂O₄ plane and its electron-donor atom points to the center of N-N bond of N₂O₄ (or near this center). The electron charge is transferred from the donor to N₂O₄ molecule, mostly to the N-N bond of N₂O₄.     

Theoretical study on the reaction mechanisms of CH2SH + NO reaction

The mechanisms for the CH₂SH + NO reaction were investigated on both of the singlet and triplet PES at the BMC-CCSD//B3LYP/6-311+G(d,p) level. The results indicate that the singlet PES is much lower than the triplet PES energetically; therefore, the reaction occurs on the singlet PES dominantly. The most favorable channel on the singlet PES takes place by a barrierless addition of N atom to CH₂SH radical to form HSCH₂NO. Subsequently, the rearrangement of the initial adduct HSCH₂NO (IM1) to form another intermediate IM3 via a four-center transition state, followed by the C–O bond fission in IM3 leading to the major product CH₂S + HNO. Due to high barriers, other product including HC(N)SH + HO, HON + CH₂S, and HNO + CHSH could be negligible. The direct abstraction channel was also determined to yield CH₂S + HON. With high barrier (33.3 kcal/mol), it is not competitive with the addition channel, in which all stationary points are lower than reactant energetically. While on the triplet PES, with the lowest barrier height (18.8 kcal/mol), the direct N-abstracted channel to form CH₂S + HNO is dominant. However, it is not competitive with the channels on the singlet PES. Our results are in good accordance with experimental conclusions that the reaction proceeds via addition mechanism.     

Functionalization of one or two methyl groups in the [Cp*<Subscript>2</Subscript>RuBr]<Superscript>+</Superscript>Br<Superscript>−</Superscript> complex in the reaction with bromine

The reaction of [Cp*₂RuBr]⁺Br⁻ with bromine in CH₂Cl₂ (CD₂Cl₂) in an inert atmosphere at room temperature produces the complexes [Cp*Ru(Br)C₅Me₄CH₂Br]⁺Br₃ ⁻ (syn conformer), [Cp*Ru(Br)C₅Me₃(CH₂Br)₂]⁺ (syn and anti conformers), and [Ru(Br)(C₅Me₄CH₂Br)₂]⁺ (syn conformer). All complexes were characterized by ¹H and ¹³C NMR spectroscopy; the former complex, by elemental analysis. These complexes were also prepared by the reaction of [Cp*RuC₅Me₄CH₂]⁺BF₄ ⁻ with bromine in CH₂Cl₂. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2712–2718, December, 2005.