New clathrate phases of the Ni/NCS/24-methylpyridine/4 complex with nitrotoluene isomers

Selectivity of clathration, stoichiometry, structure and stability of clathrates of the Ni/NCS/₂/4-methylpyridine/₄ complex with nitrotoluene isomers was Found different from the properties of clathrates previously known.     

The B6H14 problem: Generation of a catalogue of conceivable isomers

The structure-generating feature of the computer program RAIN (reactions and intermediates networks) was used to produce the complete set of 48 conceivable B₆H₁₄ isomers with twofold constitutional symmetry. The constitutional isomers are built from BH units and contain up to two closed BBB three-center bonds.     

Stereoselective hydrogenation of (−)-menthone and (+)-isomenthone mixture using nickel catalysts

The catalytic hydrogenation of an optically active mixture of (−)-menthone and (+)-isomenthone was studied. The catalysts Raney nickel and Ni/SiO₂ were used for these hydrogenations. Catalysts modified with (2R,3R)-(+)-tartaric acid were tested as well. Various non-equilibrium mixtures of isomers were produced by means of these reactions, especially mixtures containing the less stable menthol isomers, (+)-neomenthol and (+)-neoisomenthol. (−)-Menthol was produced during these reactions to a limited extent, the production of (−)-isomenthol was barcly existent. Isomerization of (−)-menthone and (+)-isomenthone occurred during these reactions. Isomerizations among individual isomers were barely detectable. The catalysts used showed a very low activity under the given reaction conditions.     

Mechanism reduction via principal component analysis

Principal component analysis, an advanced technique of sensitivity analysis, has been used to determine reduced mechanisms that can model species and temperature profiles in Plug Flow Reactors (PFR), Premixed Laminar Flames (PLF), and Perfectly Stirred Reactors (PSR) for two H₂/air and two CH₄/air mechanisms over a range of input parameters including initial temperature, equivalence ratio, and residence time. The results show that principal component analysis can be used effectively to reduce a comprehensive mechanism that contains unimportant reactions to a reduced mechanism that contains necessary and sufficient reactions. The accuracy of a reduced mechanism determined from principal component analysis can be easily controlled by carefully selecting reduction criteria. For the conditions chosen here, namely the requirement that radical profiles computed with reduced and comprehensive mechanisms agree to within 5%, substantial reductions were not achieved. Principal component analysis is able to resolve the influence of stoichiometry, combustor type, and mechanism on mechanism reduction. The two H₂/air mechanisms were each reduced to mechanisms that can model all the cases considered, and the extent of reduction in each was very similar and modest. For H₂/air chemistry, equivalence ratio had little effect on reduction. Combustor type was slightly more influential with the number of required reactions decreasing from PFR to PLF to PSR combustion. Relative to the H₂/air system, principal component analysis of the CH₄/air system is more difficult because of mechanism size. For CH₄/air combustion, if we consider all equivalence ratios, PLFs require the most reactions, if individual equivalence ratios are examined, PFRs require the greatest number of reactions. Combustor type influences mechanism reduction substantially because of the different couplings between the fluid mechanics and chemistry. In H₂/air combustion rich combustion required the fewest reactions and in CH₄/air, it required the most. Reduction must be achieved by considering the entire mechanism since reactions interact in concert, for example, reactions unimportant in one CH₄ mechanism are often important in the other. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 393–414, 1997.     

Theoretical study of isomerism of compounds of N2 and N2H2 molecules with aluminum clusters Al13 and Al12Ti

Segments of the potential energy surfaces corresponding to successive elementary stages of multistep fragmentation of nitrogen and diimine molecules upon their reaction with the aluminum cluster Al₁₃ and its doped analogue Al₁₂Ti have been calculated by the density functional theory method. The minimum energy pathways of these reactions have been calculated for the stages of physisorption, chemisorption, and N₂ and N₂H₂ fragmentation with different ways of insertion of fragments into the Al₁₃ and Al₁₂Ti cages. Relative energies, structural characteristics, and vibrational frequencies of coordinated and fragment isomers have been calculated, the barriers separating these isomers have been evaluated, and molecular diagrams of the reactions have been constructed. The effect of doping on the relative energies of intermediates and activation barriers has been considered. A conclusion has been drawn that doping with titanium should facilitate the reactions of molecular nitrogen with aluminum clusters. Unusual isomers with a five- and six-coordinate nitrogen atom N* have been localized. The results are compared with the data of analogous previous calculations of the PES of isomers corresponding to stepwise fragmentation of an acetylene molecule in related Al₁₃C₂H₂ and Al₁₂TiC₂H₂ derivatives.     

On the nature of isomerism in oxalatecopper(II) diammine thermal decomposition of α-, β-, and γ-Cu(C2O4)(NH3)2

The mechanisms of the thermal decompositions of various isomers of Cu(C₂O₄)-(NH₃)₂ were studied in this work. Data were obtained by the DSC technique and Derivatograph as well as infrared spectra and chemical analysis. The destruction of the complexes was found to take place in three steps—the first two of them being endothermic, and the third one exothermic. The values of ΔH₁, apparent activation energy and stoichiometry of the first step of destruction suggest that certain differences in bonding of the oxalate groups in each isomer lead to quite significant differences in their thermal decomposition.     

Ab initio investigations of the C2F4S isomers and of their interconversions

The transition states and the activation energies for the unobserved isomerization reactions between the three possible C₂F₄S isomers with divalent sulfur, trifluorothioacetyl fluoride 1, tetrafluorothiirane 2, and trifluoroethenesulfenyl fluoride 3, have been determined by ab initio Hartree-Fock (HF), Møller-Plesset second-order perturbation (MP2) calculations and by Gaussian-3 theory. The results show that the unobserved isomerization reactions are feasible. Furthermore, all three isomers should exist as stable species, but the unknown isomer, 3, is considerably less stable than the known isomers, 1 and 2.     

Influence of the double bond position in combustion chemistry of methyl butene isomers: A shock tube and laser absorption study

Shock tube experiments have been carried out on 2-methyl-1-butene (2M1B), 2-methyl-2-butene (2M2B), and 3-methyl-1-butene (3M1B)—the three isomers of methyl butene compound. Carbon monoxide (CO) time-histories and ignition delay times are obtained behind reflected shockwaves over the temperature range of 1350-1630 K and pressures of 8.3-10.5 atm with stoichiometric mixtures of 0.075% fuel in O₂/Ar. Comparative ignition study reveals that 3M1B ignites significantly faster than the other two isomers, while 2M1B dissociates earlier but ignites later than 2M2B. Possible mechanisms for this behavior are discussed with ignition delay time sensitivity and reaction path analysis. In addition, time-resolved CO measurements are compared with three different reaction mechanisms from the literature. Sensitivity analyses have been carried out to identify important reactions that need attention to accurately predict the chemistry of these isomers. Further investigation into the rates of unimolecular fuel decomposition reactions and C₃H₃ + O₂ = CH₂CO + HCO reaction are suggested based on the current investigation.     

The kinetics and mechanisms of elementary NF2 reactions with O and N atoms

A combined EPR–LMR spectrometer with a fast-flow system has been used to investigate the kinetics and mechanisms of NF₂ reactions with O and N atoms at 298 K. The overall rate constants of these reactions are: k₀ = (2.8 ± 0.4) × 10^?11 cm³/s and k_N = (5.7 ± 0.8) × 10^?11 cm³/s. The stoichiometry of the reactions with respect to O, N, NF₂, F, and NO has been determined. The statistical theory of bimolecular reactions has been used for interpretation of the results obtained.     

Spectrophotometric studies of reduced molybdoantimonylphosphoric acid

Spectrophotometric studies have been used to determine the optimal formation conditions and stoichiometry of the reduced molybdoantimonylphosphoric acid. It was found that a [H⁺]/[MoO^2-₄] ratio of70± 10 was optimal for formation between 0.0008 and 0.01 M molybdenum. This corresponds to solution conditions that favor the existence of the dimeric form of molybdenum. The [H⁺]/[MoO^2-₄] criteria allows simple variation of experimental conditions. The stoichiometry of the reduced heteropoly acid was surmised to be PSb₂Mo₁₀O₄₀ from spectral studies and elemental analysis.     

Use of ion-molecule reaction equilibria to identify structural isomers

A new approach is described for identifying structural isomers which cannot be distin-guished on the basis of their electron-impact mass spectra. Using a pulsed ion cyclotron resonance mass spectrometer, equilibrium constsnts are measured for ion-molecule reactions involving various C₈H₁₀ isomers and C₇H₈O isomers. The equilibrium constants are shown to be highly sensitive to the structural features of the various isomers.     

Stereochime de reactions au niveau des liaisons silicium (ou germanium)— phosphore dans le systeme dimethyl-1,2 sila (ou germa)) cyclopentanique

The stereochemistry of nucleophilic displacement of diethylphosphino leaving group from geometrical isomers of 1,2-dimethy -1-diethylphosphino-1-sila (or germa) cyclopentane 1 (or 2 has been studied. Retention of configuration at Si (or Ge) atom was found in reduction with LiAlH₄(Et₂OorTHF), and inversion with bdLiBH₄. With reagents like n-butyllithium, allyllithium or allylmagnesium bromide, 1 (or 2) gives retention of configuration. Alcohols and acidic reagents react far more rapidly, but reactions are not (or weakly) stereospecific. PR₂; is a poor leaving group, neighbouring hydrogen atom in the Sommer-Corriu's scale.The stereochemistry of addition reactions of 1 (or 2) with various unsaturated derivatives (carbonyl and dissymetric ethylenic derivatives, ambident nucleophiles) has been also investigated. It was shown that these reactions are both regiospecitic and stercospecific, and proceed with retention at the silicon (or germanium) centre.The stereochemical results are consistent with recent conclusions that S_N-Si retention reactions involve frontier orbital interactions.Relative configurations of these geometrical isomers, and the stereochemistry of their reactions, were assigned by pmr and cmr spectroscopy.     

The use of FTIR and NMR spectroscopies to study prepolymerisation interactions in nitrogen heterocycles

A detailed investigation into the functional groups responsible for the formation of a noncovalent complex between 2-aminopyridine (template) and methacrylic acid (functional monomer) has been carried out using FTIR spectroscopy and confirmed by ¹H NMR spectroscopic data. The approach adopted to confirm the mechanism of interaction was the analysis of the template plus the structurally similar 2-methylaminopyridine and 2-dimethylaminopyridine. A 1:1 stoichiometry of complexation was determined by Job plot analysis following titration, with FTIR results complementing those of the ¹H NMR study. The strength of interaction between 2-aminopyridine and the functional monomer measured through band shifts by FTIR spectroscopy was compared with such interactions for the isomers 3- and 4-aminopyridine. This comparison identified a clear correlation between template pK _a, degree of interaction and subsequent nonspecific binding in the nonimprinted polymer. Using FTIR spectroscopy it was also possible to observe the effect of temperature on the prepolymerisation solution. IR spectra showed that lower temperatures led to more stabilized interactions of the hydrogen-bonded complex. The potential advantages of FTIR spectroscopy compared with ¹H NMR spectroscopy in studying prepolymerisation solutions have been identified.     

Chlorination reactions of ephedrines revisited. Stereochemistry and functional groups effect on the reaction mechanisms

The stereochemistry of the chlorination reactions with SOCl₂ of free ephedrine and pseudoephedrine and their hydrochlorides, oxamides and sulfonamides was analyzed. Chlorination of free and hydrochloride erythro isomers occurs with 100% inversion of configuration at C-1 (S_N2 mechanism). Chlorination of oxamides and sulfonamides of erythro isomers occurs with retention of the configuration at C-1, (S_Ni mechanism). Chlorination reactions in all threo isomers and derivatives hydrochlorides, oxamides or sulfonamides gave the same ratio of erythro (40%) and threo isomers (60%) (S_N1 mechanism). Treatment of the isomeric mixture of the chlorodeoxyephedrine and chlorodeoxypseudoephedrine hydrochloride in DMSO with HCl changes the isomeric ratio, increasing the erythro isomer content (65%). Using the erythro ethanolamines it is possible to arrive stereoselectively at the erythro chloroamines if the compound is previously tosylated or converted to the amide, or to the threo chloroamines if the compound is directly chlorinated with SOCl₂.     

Structure, stability and dissociation of silanitriles RSiN (R = H2B, H2N, H2P)

Structure, stability, and dissociation of H₂BSiN, H₂NSiN, H₂PSiN and their isomers H₂BNSi, H₂NNSi, H₂PNSi have been studied in detail using ab initio MP2 and CCSD(T) methods. After dissociation of H₂BNSi, H₂NNSi, H₂PNSi and their isomers, the fragmented atoms have been considered to be either in their ground state or in their valence excited state in various dissociation channels. Only allowed dissociations of these molecules are considered. Various dissociation channels of H₂BNSi, H₂NNSi, H₂PNSi and their isomers have been explored and interesting trends are observed for the dissociation of stable isomers H₂BNSi, H₂NNSi, H₂PNSi and less stable isomers H₂BSiN, H₂NSiN, H₂PSiN. The effect of substituents on their structural properties has been discussed. The potential energy surfaces for the RSiN ? RNSi isomerization reactions have been analyzed. The structural properties of these molecules agree well with the theoretical values wherever available.     

Thermoanalytical study of linkage isomerism in coordination compounds

The non-isothermal differential scanning calorimetry (DSC) experiments revealed that linkage isomerization of both pure trans-[Co(en)₂(ONO)₂]PF₆ (dinitrito isomer) and trans-[Co(en)₂(NO₂)₂]PF₆ (dinitro isomer) occurs in the solid state at elevated temperatures. The process was found to be exothermic for the dinitrito isomer and endothermic for dinitro isomer. The pure isomers could be considered to be in metastable states at ambient temperatures which would be converted to an equilibrium mixture of both isomers (stable state) upon heating. Since the isomerization of both isomers may be described as a two stages process, the corresponding DSC peaks have been resolved into two peaks by means of nonlinear curve fitting tools of PeakFit^® software. The resolution of the peaks helped us to determine enthalpy changes of stepwise isomerization of both isomers. A first-order initial rate and Kissinger methods have been employed to estimate kinetic parameters of the stepwise isomerization reactions. The Kissinger method provided more reliable kinetic results. The high activation energy and positive entropy changes of isomerization of both isomers are considered as indication of a dissociative mechanism in solid state.     

Density functional theory study of Te(CN)2, Te(CN)(NC), and Te(NC)2 and their isomerizations

The equilibrium structures of Te(CN)₂, Te(CN)(NC), and Te(NC)₂ and three isomerization reactions: Te(CN)₂ ? Te(CN)(NC), Te(CN)(NC) ? Te(NC)₂, and Te(CN)₂ ? Te(NC)₂ were studied in the gas-phase using density functional theory. Three functionals (B3LYP, BLYP, and BHLYP) were employed to characterize the low-lying electronic singlet and triplet TeC₂N₂ isomers. The basis sets for carbon and nitrogen used were of double-ζ plus polarization quality with additional s- and p-type diffuse functions, DZP++. For the tellurium atom, the LANL2DZ (ECP) basis set was used. The energetic ordering (kcal mol^?1) (B3LYP) including zero-point vibrational energy corrections for the singlet ground state isomers follows: Te(CN)₂ (0.0, global minimum) < Te(CN)(NC) (15.4) < Te(NC)₂ (29.8). Electrostatic potentials and average local ionization energies of the ground state Te(CN)₂, Te(CN)(NC), and Te(NC)₂ isomers provide some guidance as to sites for noncovalent and covalent interactions. Energetics such as the different forms of electron affinities, ionization energies, and singlet–triplet gaps were also reported. Further the theoretical rate constants for the isomerization reactions were evaluated using transition state theory. We predict that these isomers may crystallize in similar patterns, if stable, as does Se(CN)₂.     

Synthesis and X‐ray Structural Characterization of the Polymeric Cluster Compound [Hg3(O2SePh)(SePh)5]n

Two synthetic routes to prepare [Hg₃(O₂SePh)(SePh)₅]_n and its structural characterization are presented in this paper. This compound results either from the partial oxidation of mercury phenylselenolate clusters or from a mixture of its components with defined stoichiometry. This compound was observed in almost all of our reactions as a by‐product, during the development of the synthesis of new mercury selenolate clusters. It was now characterized by single‐crystal X‐ray diffractometry, elemental analysis, and infrared spectroscopy.     

Theoretical modeling of dissociative addition of an H<Subscript>2</Subscript> molecule to doped aluminum clusters FeAl<Subscript>12</Subscript> and CoAl<Subscript>12</Subscript>

The potential energy surfaces (PES) of the reactions FeAl₁₂ + Н₂ → FeН₂Al₁₂ (1) and CoAl₁₂ + Н₂ → CoН₂Al₁₂ (2) of dissociative addition of an H₂ molecule to Fe- and Co-doped aluminum clusters have been calculated by the density functional theory method. Local minima on the PES in the vicinity of low-lying isomers, intermediates, and transition states have been found, and their structural and spectroscopic characteristics and energies have been calculated. The energies of the successive stages of the catalytic cycle have been evaluated, and the channels corresponding to the minimum energy path of the reactions have been studied. Differences between the structural characteristics and energies of key structures in reactions (1) and (2) have been considered. The results are compared with previous calculations of the PES of hydrogenation reactions performed for related clusters doped with nickel and titanium atoms.     

Isomerizations of CH3SO2: Quantum Chemistry and Topological Study

The structures, stabilities and the isomerization reactions of CH₃SO₂ isomers in a doublet electronic state have been studied at B3LYP/6‐311+ +G (d,p), MP2/6‐311++G (d,p) and CCSD(T)/6‐311++G (d,p) levels. The three different levels of calculation give the similar results: thirteen minimum isomers were located and they were connected by eleven transition states. Among the thirteen isomers, cis‐CH₃OSO, trans‐CH₃OSO and CH₃SO₂ are the most stable species, and they should be detected easily in experiment. This is well consistent with the experimental result. These isomers could isomerize to each other by chemical bond vibration, chemical bond rotation and atom migration. The non‐planar ring structure transition state (STS), which was found in this paper, extended the concept of ring STS to the non‐planar systems.