Ethylene addition to OsO3(CH2) - A theoretical study

Quantum chemical calculations at the B3LYP/TZVP level of theory have been carried out for the initial steps of the addition reaction of ethylene to OsO₃(CH₂). The calculations predict that there are two reaction channels with low activation barriers. The kinetically and thermodynamically most favored reaction is the [3+2]_O, C addition which has a barrier of only 2.3 kcal mol⁻¹. The [3+2]_O, O addition has a slightly higher barrier of 6.5 kcal mol⁻¹. Four other reactions of OsO₃(CH₂) with C₂H₄ have significantly larger activation barriers. The addition of ethylene to one oxo group with concomitant migration of one hydrogen atom from ethylene to the methylene ligand yields thermodynamically stable products but the activation energies for the reactions are 16.7 and 20.9 kcal mol⁻¹. Even higher barriers are calculated for the [2+2] addition to the OsO bond (32.6 kcal mol⁻¹) and for the addition to the oxygen atom yielding an oxiran complex (41.2 kcal mol⁻¹). The activation barriers for the rearrangement to the bisoxoosmaoxirane isomer (36.3 kcal mol⁻¹) and for the addition reactions of the latter with C₂H₄ are also quite high. The most favorable reactions of the cyclic isomer are the slightly exothermic [2+2] addition across the OsO bond which has an activation barrier of 46.6 kcal mol⁻¹ and the [3+2]_O, O addition which is an endothermic process with an activation barrier of 44.3 kcal mol⁻¹.     

A quantum chemical consideration of ligand exchange in palladium(ii) aqueous and chloride complexes

The behavior of potassium tetrachloropalladate(II) in media simulating biological fluids has been studied. In aqueous solutions of NaCl, the aquation rate is higher than the rate of chloro ligand introduction into the internal coordination sphere of palladium. In HCl solutions, on the contrary, the process of palladium chloro complex formation predominates. The latter is apparently due to protonation of water molecules composing aqua complexes. By means of the ZINDO/1 method, the substitution of ligands – water molecules and hydronium ion – in planar complexes of palladium(II) by chloride ion has been investigated. All complexes containing H₂O and H₃O⁺ ligands, other than [Pd(H₂O)₄]²⁺, have intramolecular hydrogen bonds. In [Pd(H₂O)₃(H₃O)]³⁺ and trans-[Pd(H₂O)₂(H₃O)Cl]²⁺, a “non-classic” symmetric hydrogen bond O ··· H ··· O is established (ZINDO/1, RHF/STO-6G*). By the first three steps the substitution of hydronium ion in the internal sphere of palladium atom is more favorable thermodynamically, compared to water molecules. Logarithms of stepwise stability constants of palladium(II) chloride complexes correlate linearly to enthalpies (ZINDO/1, PM3) of water substitution by chloride ion.     

Formation and Unimolecular Dehydrogenation of Gaseous Alkaline‐earth Metal Amidoboranes M(NH2BH3)2 (M = Be – Ba): Comparative Computational Study

Formation of alkaline‐earth metal amidoboranes M(NH₂BH₃)₂ (M = Be, Mg, Ca, Sr, Ba) and unimolecular dehydrogenation reactions were computationally studied at the B3LYP/def2‐TZVPPD level of theory. Formation of M(NH₂BH₃)₂ from ammonia borane and MH₂ is exergonic, but subsequent unimolecular dehydrogenation reactions are endergonic at room temperature. In contrast to alkali metal amidoboranes, for M(NH₂BH₃)₂ the nature of M significantly affects their reactivity. Activation energies for the dehydrogenation of first and second hydrogen molecules decrease from Be to Ba. In case of Be compounds, intramolecular M ··· H–B contacts play an important role, whereas for heavier analogs such contacts are much less pronounced.     

A DFT study of the [4+2] cycloadditions of conjugated ketenes (vinylketene, imidoylketene and formylketene) with formaldimine. The pericyclic or pseudopericyclic character from magnetic properties

A comprehensive B3LYP/6-31+G^∗ study of the nature of the [4+2] cycloadditions of conjugated ketenes, vinylketene, imidoylketene and formylketene, with formaldimine was conducted. For each reaction, the complete pathway was determined and changes in different magnetic properties (magnetic susceptibility, χ, magnetic susceptibility anisotropy, χ_anis, and the nucleus-independent chemical shift, NICS) were monitored along the reaction profile with a view to estimate the aromatization associated to the process. We have also applied the ACID (anisotropy of the current-induced density) method with the same purpose. The deep analysis of the results indicates the existence of both disrotatory and conrotatory pericyclic paths for the cyclization step of the cycloaddition of vinylketene with formaldimine and the pseudopericyclic character of reactions with imidoylketene and formylketene.     

Theoretical study of the walk rearrangement in perfluorotetramethyl (Dewar thiophene) exo-S-oxide

A theoretical study of the ‘walk’ rearrangement in bicyclo[2.1.0]pentene and perfluorotetramethyl (Dewar thiophene) exo-S-oxide has been carried out. Despite the differences between them, the results for both reactions show an enhancement of aromaticity in the transition state, which is consistent with a pericyclic behavior. NBO calculations show that the small activation energy for the second reaction can be interpreted in terms of a strong stabilization of the transition state by the exo-oxide substituent. So, the mechanism proposed in the past should be revised.     

气相中HRnCCH和X(X=H_2O,NH_3)反应机理的理论研究

在CCSD(T)//MP2/aug-cc-pVTZ-pp理论水平上,研究了HRnCCH与大气中H2O及NH3分子反应的机理,反应主要包括HRnCCH与HRnOH及HRnNH2之间的转化、H2O和NH3在HRnCCH中的碳碳三键上的加成反应以及HRnCCH与双分子水反应等.结果表明,HRnCCH与H2O反应生成HCCH和HRnOH及HRnCCH与NH3反应生成HCCH和HRnNH2的能垒分别为54.1和75.2 kJ/mol,而生成HRnCHC(OH)H,HRnC(OH)CH2,HRnCHC(NH2)H和HRnC(NH2)CH2的活化能分别为219.6,220.5,174.4和182.4kJ/mol,此结果表明HRnCCH反应性较弱且是稳态存在的.此外,在HRnCCH与H2O反应中加入单个水分子,仍然生成HRnCHC(OH)H,但反应活化能却降低了96.4 kJ/mol,说明水分子对该反应有明显的催化作用.     

A Theoretical Study of the Reductive Elimination of CH3EH3 from cis‐[Pt(CH3)(EH3)(PH3)2] (E = Si,Ge) in the Presence of Acetylene

The reaction mechanism of the elimination of CH₃EH₃ from the platinum complexes cis‐[Pt(CH₃) · (EH₃)(PH₃)₂] (E = Si, Ge) in the presence of acetylene has been studied using gradient‐corrected DFT calculations at the B3LYP level. The reaction proceeds in two steps. The first step is the formation of the acetylene complex [Pt(CH₃)(HCCH)(EH₃)(PH₃)] which occurs in a associative/dissociate pathway via the five‐coordinated intermediate [Pt(CH₃)(HCCH)(EH₃)(PH₃)₂]. The rate‐determining step is the elimination of CH₃EH₃ via a four‐coordinated transition state. The alternative mechanism via direct dissociation from the five‐coordinated intermediates has higher activation barriers. The calculated activation energies of the model reactions are in good agreement with experimental results. The silyl complex has a lower barrier for the elimination reaction than the germyl complex. The calculated transition states show that the reason for the lower barrier is the strength of the nascending C–Si bond, which is higher than the C–Ge bond. The results are in agreement with the postulated mechanism of Ozawa et al. (Organometallics, 1998 , 17, 1018).     

Polyisoprenylated benzophenone derivatives from the fruits of Garcinia cambogia and their absolute configuration by quantum chemical circular dichroism calculations

Three new tetracyclic polyisoprenylated xanthones, named oxy-guttiferones M, K2, and I, along with oxy-guttiferone K and guttiferone M, have been isolated from the fruits of Garcinia cambogia. Their structures were elucidated by MS and NMR spectroscopic experiments. The absolute configurations of oxy-guttiferone K, taken as a model of tetracyclic xanthones, and guttiferone M, as a model of polyisoprenylated benzophenones, have been determined by comparison of their experimentally measured circular dichroism (CD) curves with the TDDFT-predicted curves.     

A theoretical study of the influence of BF3 on the reaction path of the [4+2] cycloaddition of vinylketene with formaldimine

A comprehensive B3LYP/6-31+G^∗ study of the influence of BF₃ on the [4+2] cycloaddition of vinylketene with formaldimine was conducted. For this purpose, the complete pathway was determined and changes in different magnetic properties (magnetic susceptibility, χ, magnetic susceptibility anisotropy, χ_anis, and the nucleus-independent chemical shifts, NICS) were monitored along the reaction profile with a view to estimate the aromatization associated to the process. We have also applied the ACID (anisotropy of the current-induced density) method with the same intention.     

FTIR and quantum chemical study of LiBr solvation in acetonitrile solutions

FTIR spectroscopy and quantum chemical calculations at the RTF + MP2/6-311G** level of theory with solvation model density (SMD) corrections were used to study ion solvation and association in LiBr/acetonitrile solutions. The aim of this study was to establish the composition and geometry of the predominant ionic species solvated by acetonitrile molecules and to analyse their spectroscopic signatures. The results obtained make it possible to propose an equilibrium between Li⁺Br⁻(CH₃CN)₃, Li⁺(CH₃CN)₄, and anionic Br⁻(CH₃CN)_n complexes with an undetermined n value and bent coordination of the solvent molecules. The calculated wavenumbers and the geometric parameters of the solvated ionic species were found to be in excellent agreement with the experimental data.     

Derivatization of Tris(trimethylsilyl)heptaphosphane

Through SiP bond cleavage, the reaction of P₇(SiMe₃)₃ with one equivalent of KO^tBu or LiO^tBu afforded different isomers of the heptaphosphanide anion [P₇(SiMe₃)₂]⁻. With LiO^tBu, concomitant inversion at an equatorial (silylated) phosphorus atom occurred and the C_s symmetric isomer characterized by a mirror plane formed. With KO^tBu, inversion did not occur and the resulting asymmetric anion with C₁ symmetry formed. With NaO^tBu, a mixture of both isomers was obtained. The symmetries and structures of the anions were elucidated with ³¹P{¹H} and ²⁹Si{¹H} NMR spectroscopy, and relative stabilities were calculated employing the B3LYP/6-31+G* method.The reaction of KP₇(SiMe₃)₂ or LiP₇(SiMe₃)₂ with 1,2-dichlorotetramethyldisilane led to (SiMe₃)₂P₇SiMe₂SiMe₂P₇(SiMe₃)₂, a molecule composed of two P₇-cages connected by a disilane bridge. It can also be obtained through silyl exchange using P₇(SiMe₃)₃ and ClMe₂SiSiMe₂Cl. The compound was characterized with ³¹P and ²⁹Si-NMR spectroscopy and elemental analysis. Treatment of P₇(SiMe₃)₃ with HypCl (Hyp = hypersilyl = Si(SiMe₃)₃) in DME led to the quantitative formation of Hyp₂P₇SiMe₃. Single crystal X-ray diffraction as well as ³¹P and ²⁹Si-NMR spectroscopy proves the presence of a heteroleptically substituted heptaphosphane cage.Quantum chemical HF and B3LYP/6-31G* calculations of equilibrium structures for the two possible isomers of P₇(SiMe₃)₃ (sym and asym) reveal that asym is destabilized by about 30-40 kJ mol⁻¹, which explains why its formation could not be observed. The phosphorus inversion barrier for the sym → asym transition is calculated as 60-70 kJ mol⁻¹.     

Validation of ion chromatographic methods for the trace analysis of ions in pharmacopoeial grades of water

Ion chromatography was investigated as an alternative technique to conventional pharmacopoeial methods for the determination of inorganic anions and cations in pharmacopoeial grades of water. These methods were validated by generating data on parameters such as specificity, linearity, precision, accuracy, sensitivity, ruggedness and stability of solution. The validation data demonstrate that ion chromatography is a viable alternative technique to current pharmacopoeial methods which enables automation of pharmacopoeial water analyses.     

Structures of methyl halide dimers in supersonic jets by matrix-isolation infrared spectroscopy and quantum chemical calculations

The CH₃Cl and CH₃Br dimers produced by supersonic-jet expansion were directly deposited on a cold plate using a standard matrix-isolation technique. Dependence of the relative intensities of the observed infrared bands on the stagnation pressure was used to assign the dimer bands appearing near the monomer bands. By a comparison of the wavenumber shifts from the monomer bands with the corresponding values obtained by quantum chemical calculations, DFT/B3LYP/6-311++G(3pd,3df) and MP2/LanL2DZ+fdp, the structures of CH₃Cl and CH₃Br dimers were determined to be a head-to-tail isomer, which is common to the CH₃F and CH₃I dimers determined previously by the same method. The remaining dimer bands, which could not be assigned to the head-to-tail isomer, were tentatively assigned to a head-to-head isomer in analogy with CH₃I dimer.     

Electrochemical approach to monitoring metal exchange reaction of acetylacetonate complexes with cadmium, copper and zinc ions

Metal exchange reactions of acetylacetonate complexes with Cd(II), Cu(II) and Zn(II) ions were investigated by using cadmium and copper ion selective electrodes. Changes in the electrode potential and pH of the solutions were monitored upon adding the pertinent metal Zn(II) of the acetylacetonate (AA) complexes. In the reverse system in which a stable Cu-AA complex exists in the solution prior to adding a secondary metal ion (Cd(II) or Zn(II)), no Cu(II) was replaced by either ion. In the systems containing Cd(II) and Zn(II) as a complexed form with AA or as free ions, the exchange reactions were not explained by considering the equilibrium stability constants of the Cd-AA and Zn-AA complexes.     

Density functional study of the l-proline-catalyzed α-aminoxylation of aldehydes reaction: The reaction mechanism and selectivity

The reaction mechanism of the l-proline-catalyzed α-aminoxylation reaction between aldehyde and nitrosobenzene has been investigated using density functional theory (DFT) calculation. Our calculation results reveal following conclusions [1]. The first step that corresponds to the formation of C–O bond, is the stereocontrolling and rate-determining step [2]. Among four reaction channels, the syn-attack reaction channel is more favorable than that of the anti one, and the TS-ss channel dominates among the four channels for this reaction in the step of C–O bond formation [3]. The intermolecular hydrogen bond between the acidic hydrogen of l-proline and the N atom of the nitrosobenzene in an early stage of the process catalyzes very effectively the C–O bond formation by a large stabilization of the negative charge that is developing at the O atom along the electrophilic attack [4]. The effect of solvent decreases the activation energy, and also, the calculated energy barriers are decrease with the enhancement of dielectric constants for C–O bond formation step. These results are in good agreement with experiment, and allow us to explain the origin of the catalysis and stereoselectivity for l-proline-catalyzed α-aminoxylation of aldehyde reaction. The addition of H₂O to substituted imine proline, intermolecular proton-transfer steps, and the l-proline elimination process were also studied in this paper.     

Displacement Reactions of Phosphorus(V) Compounds and Their Pentacoordinate Intermediates

Reactions of phosphorus(v) compounds involving the mutual interconversion of tetra- and pentacoordinate species are discussed in a critical review emphasizing stereochemical implications of the reaction mechanism. This discussion includes the formation and decomposition of the stable oxyphosphoranes, the Michaelis-Arbusov, Perkov, and Wittig reactions, interconversions of phosphines and their oxides, and the nucleophilic displacements on phosphonium compounds. Reactions of phosphate esters and related compounds receive particular attention. All chemical arguments are derived by considering the effect of factors determining the relative stabilities of phosphorane derivatives, their rates of formation, decomposition and rearrangement by bond deformation or rupture and recombination processes, considerations which are uniformly applied on the basis of concepts developed in a preceding communication^[2]. It is shown that a comprehensive mechanistic interpretation of the foregoing reactions requires substantial addition to available conceptual foundations such that, in many cases, present concepts and mechanisms must be revised.     

Tautomeric and conformational properties of dimethyl malonate, CH3OC(O)-CH2-C(O)OCH3: electron diffraction and quantum chemical study

The geometric structure of dimethyl malonate, CH₃OC(O)-CH₂-C(O)OCH₃, was studied by gas electron diffraction (GED) and quantum chemical methods. We conclude that only diketo conformers exist in the gas phase. According to the GED refinement, a mixture of two diketo conformers is present: 69(10)% (ac,ac) conformer with C₂ symmetry (both CO bonds anticlinal relative to the opposite C-C bond) and 31(10)% (sp,ac) conformer with C₁ symmetry (one CO bond with synperiplanar, the other CO bond with anticlinal orientation). Ab initio calculations, however, predict a preference of the (sp,ac) conformer rather than the (ac,ac) form.     

Crystal structure and quantum chemical investigation of 4-(3,4-dichlorphenyl)-1,2,4-triazole

1,2,4-Triazole derivative substituted in the 4th position — 4-(3,4-dichlorphenyl)-1,2,4-tirazole — has been synthesized. Crystal and molecular structure of the compound were determined by single crystal X-ray diffraction. Molecular geometry optimization and effective charge calculations were performed by DFT methods.     

Water exchange dynamics of manganese(II), cobalt(II), and nickel(II) ions in aqueous solution

The first row transition metal ions Mn(2+), Co(2+), and Ni(2+) have been studied by classical umbrella sampling molecular dynamics simulations. The water exchange mechanisms, estimates of reaction rates, as well as structural changes during the activation process are discussed. Mn(2+) was found to react via an I(A) mechanism, whereas Co(2+) and Ni(2+) both proceed via I(D). Reaction rate constants are generally higher than those obtained by experiment but the simply constructed metal(II) ion-water potential reproduces the relative order quite well.     

A DFT study of the polar Diels-Alder reaction between 4-aza-6-nitrobenzofuroxan and cyclopentadiene

The polar Diels-Alder reaction between 4-aza-6-nitrobenzofuroxan (ANBF) and cyclopentadiene has been studied using DFT procedures at the B3LYP/6-31G* level. Only one highly asynchronous transition state structure associated to the formation of the [4+2] adduct 13 is found. A further [3,3] sigmatropic shift on the [4+2] cycloadduct 13 allows its conversion into the thermodynamically more stable [2+4] cycloadduct 14. The analysis of the global and local electrophilicities of the reagents correctly explain the behaviour of ANBF as a strong electrophile in polar cycloadditions.