Approach Of Reactants Research Articles

Effect of Precursor Solution on Sedimentation of Alkaline-earth Phosphates at Different Temperatures

Kinetic study of sedimentation of alkaline-earth phosphates-magnesium phosphate (MP), calcium phosphate (CP), strontium phosphate (SP) and barium phosphate (BP) precipitates was carried out by varying reactant concentration approach involving turn-wise variation of precursor metal ions (Mg2+, Ca2+, Sr2+ and Ba2+) and phosphate ion (PO43-) concentrations, and monitoring the settling of the precipitate formed in a precipitate sedimentation measurement apparatus. A pseudo-rate order equation, associated with the approach of varying reactant concentration, was linearized, and the dependence of precipitate sedimentation on precursor concentration, established therewith. Whereas MP precipitate gave positive order-sedimentation increased with increase in concentrations of precursor solutions, orders of CP, SP and BP precipitates were negative-sedimentation decreased with increase in concentrations of precursor solutions. Inter-particle attraction and repulsion associated with high solid concentration, volume of precipitate particles formed as well as chemical interactions of precipitate with the mother solution might have caused these effects.

Effect of 4-Hydroxy-l-proline-containing podands on the stereoselectivity of Biginelli reaction according to molecular dynamics

The synthesis and theoretical study of the stereoselectivity of Biginelli reaction with chiral inductors based on 4-hydroxy-l-proline podands with four oxyethylene moieties have been performed. The predominant formation of the iminium intermediate during the interaction of the podand salt form with benzaldehyde was experimentally shown. Metadynamic simulation of reaction mixture with 4-hydroxy-l-proline-containing podand in salt and base forms allowed us to compare the probability of reactants approach from re- and Si-sides on the diffusion stage. We modeled the arrangement of reactants in the pre-reactional stage of alternative steps that could have a key effect on enantiomeric excess of 1,4-dihydropyrimidinone. Results showed that the interaction of a podand-acetoacetate adduct with N-benzylideneurea was preferred, when the podand was in a base form, while for a salt form, the intereaction with benzaldehyde was more favorable, which led to higher stereoselectivity. The effect of intermolecular interactions on the stabilization of prereaction states was analyzed.

Кинетика и механизм реакций ацильного переноса. Часть 16. Квантово-химическое моделирование механизма сульфонилирования N-метиланилина в водном 1,4-диоксане

The RHF/6-31G(d) quantum chemical simulation of the mechanism of the secondary fatty aromatic amine N-methylaniline interaction with benzenesulfonyl chloride under conditions of N-methylaniline specific solvation by one water molecule and one 1,4-dioxane molecule, and under conditions of N-methylaniline specific solvation by two water molecules and one 1,4-dioxane molecule. Three-dimensional potential energy surfaces of the processes pointed out are computed. It is shown that in the both cases a single route of the reactions is realized, starting as an axial nucleophilic attack, which goes further with decreasing of the attack angle as reagent molecules approach each other. It was established that both simulated reactions proceed in accordance with bimolecular concerted mechanism of nucleophilic substitution SN2, which implies the formation of a single transition state in the reaction path. It was found that geometrical configuration of the reaction center in the transition states of the reactions is medium between the trigonal-bipyramidal and tetragonal-pyramidal, which is associated with the change in the angle of N-methylaniline attack as the reactant molecules approach each other. In the benzenesulfonyl chloride reaction with N-methylaniline, solvated by one water molecule and one 1,4-dioxane molecule, the transition state is solvated only by 1,4-dioxane molecule, while water molecule moves away from the reaction center, whereas in the benzenesulfonyl chloride reaction with N-methylaniline, solvated by two water molecule and one 1,4-dioxane molecule the transition state is solvated by 1,4-dioxane molecule and one water molecule that forms hydrogen bond with chlorine atom and promote the S–Cl-bond loosening. The activation energies of the reactions were calculated; it is shown that specific solvation increases the reactions energetic barrier as compared with the reaction in gaseous phase, that is caused by the partial dehydratation of N-methylaniline molecule before the transition state formation. A decrease of the activation energy of the reaction with participation of N-methylaniline, solvated by two water molecule and one 1,4-dioxane molecule as compared with the cases of non-specific solvation of the reactants and N-methyl-aniline solvation by one water molecule and one 1,4-dioxane molecule is caused by the existence of the second water molecule in the system, forming a bond with amine group and facilitating N–H bond break.

Revisiting the Determination of Percent Aspirin Lab: Using a Limiting Reactant Approach for Students To Also Determine the Amount of Iron(III) Chloride

The spectrophotometric determination of the percent composition of an aspirin tablet is a common first-year, general chemistry undergraduate laboratory procedure. The experiment requires that a known amount of pure acetylsalicylic acid be quantitatively converted into the soluble disodium salicylic acid that is further reacted with excess iron(III) chloride solution to form a deep blue tetraaqua-octahedral complex. This complex, through serial dilutions, is used to construct a Beer’s law plot that may be used to determine the concentration of aspirin. In a novel twist, this paper describes how the same Beer’s law plot can be used to determine the concentration of iron in the iron(III) complex, which is then used to determine the concentration of the original iron(III) chloride solution used to make the aspirin–iron(III) complex. The method has been adapted by high school students in determining iron concentrations in stream waters.

Designed Synthesis and Structure–Property Relationships of Kinetically Stable [(PbSe)1+δ]m(VSe2)1 (m = 1, 2, 3, 4) Heterostructures

Understanding structure–function relationships is essential to guide the designed synthesis of novel materials with emergent properties. In this work, we targeted the metastable heterostructures [(PbSe)1+δ]m(VSe2)1, where m = 1–4, to test if the charge density wave (CDW) transition temperature increases as the layer thickness separating the VSe2 monolayers increases, as was observed when SnSe was the separating layer. The modulated elemental reactant approach was used to make the targeted products. This approach involves depositing elemental layers in which the number of atoms of each element per square angstrom in Pb|Se and V|Se bilayers equals the number calculated for a rock salt-structured PbSe bilayer and a CdI2-structured VSe2 slab, respectively. Layered elemental precursors with the correct composition and nanoarchitecture for each of the targeted compounds were prepared by repeatedly depositing a single V|Se bilayer followed by m Pb|Se bilayers. Precursors close to the targeted number of atoms per unit area were determined via X-ray fluorescence and the correct nanoarchitecture self-assembled to the targeted compounds during a low-temperature anneal. Resistivity measurements show that the number of PbSe layers per repeat unit (m) does not change the charge density transition onset temperature as previously reported for the analogous [(SnSe)1+δ]m(VSe2)1 compounds. The temperature dependence and absolute values of the resistivity of the m = 3 and 4 heterostructures scale as expected for composite behavior. The difference in the thickness dependence of the CDW transition between the PbSe- and SnSe-containing compounds highlights that the identity of the intervening rock salt layer plays a more important role in modifying the CDW onset temperature than the separation of the VSe2 layers.

A density functional theory study on the [3 + 2] cycloaddition of N-(p-methylphenacyl)benzothiazolium ylide and 1-nitro-2-(p-methoxyphenyl) ethene: the formation of two diastereomeric adducts via two different mechanisms

The [3 + 2] cycloaddition reaction of N-(p-methylphenacyl)benzothiazolium ylide (NBY) and 1-nitro-2-(p-methoxyphenyl) ethane (NME), experimentally investigated by Yan et al., was studied theoretically. They reported that the reaction proceeds via a stepwise mechanism and produces two diastereomeric adducts in a 4:1 ratio. For the study of the reaction, two regioselective attacks were considered between the reagents and their theoretical parameters were calculated. In excellent agreement with the experimental outcomes, the Fukui and Parr functions reactivity indices analysis as well as the energetic results indicated that among the two studied regioselective attacks, one which leads to the formation of two experimentally reported adducts, is more favorable than the other. The molecular mechanism of the studied reactions was characterized using the IRC, QTAIM and Wiberg bond indices analyses and the results suggested that two diastereomeric adducts are generated via two different mechanisms. The major adduct is produced via a two-stage one-step mechanism without the formation of any stable intermediate, whereas the minor one is generated through a stepwise mechanism along with the formation of a stable zwitterionic intermediate. The analysis of global electron density transfer showed that the reactions are polar and electron density fluxes from NBY toward NME. It was found from molecular electrostatic potential map that at the more favorable transition state, approach of reactants locates the oppositely charged regions over each other resulting in attractive forces between the two fragments. Analysis of the frontier molecular orbitals indicated that the HOMO orbital of NBY is also a frontier effective-for-reaction molecular orbital.

Кинетика и механизм реакций ацильного переноса. Часть 15. Квантово-химическое моделирование механизмов реакций сульфонилирования N-этиланилина

The RHF/6-31G(d) quantum chemical simulation of the mechanism of the interaction of the secondary fatty aromatic amine N-ethylaniline with benzenesulfonyl chloride under conditions of non-specific water solvation, using the continuum model of the solvent, as well as of sulfonylation reactions of N-ethylaniline solvation complexes containing one water molecule, modeled specific solvation of N-ethylaniline with water, and one molecule of water and one of dioxane, which simulate the solvation of the amine with aqueous dioxane. Three-dimensional potential energy surface of these processes is calculated. It is shown that in the case of a reaction proceeding under conditions of non-specific solvation of reagents, the route with axial attack of the N-ethylaniline molecule to the sulfonyl reaction center is realized, in the two other cases the reactions proceed along a single route, starting as an axial attack of the nucleophile, which goes further with decreasing of the attack angle as reagent molecules approach each other. It was established that all the simulated reactions proceed in accordance with bimolecular coordinated mechanism of nucleophilic substitution SN2, which implies the formation of a single transition state in the reaction path. It was found that geometrical configuration of the reaction center in the transition state of N-ethylaniline reaction with benzenesulfonyl chloride under non-specific solvation by water is close to trigonal-bipyramidal, which is determined by the axial direction of the nucleophilic attack, in the two other cases it is medium between the trigonal-bipyramidal and tetragonal-pyramidal, which is associated with the change in the angle of N-ethylaniline attack as the reactant molecules approach each other. In a reaction involving N-ethylaniline monohydrate, a water molecule forms a 6-membered cyclic structure with reagent molecules in the transition state, in which the transfer of a proton from N-ethylaniline amino group to a hydrogen chloride molecule occurs via a relay mechanism involving the water molecule. The activation energy values of the studied processes were calculated; it is shown that both specific and universal solvation significantly lower the energy barrier of the reaction compared to the reaction occurring in gas phase, which is consistent with the data obtained earlier for related processes.

Enhanced Cross-Plane Thermoelectric Transport of Rotationally Disordered SnSe2 via Se-Vapor Annealing.

We report cross-plane thermoelectric measurements of SnSe and SnSe2 films grown by the modulated element reactant (MER) approach. These materials exhibit ultralow cross-plane thermal conductivities, which are advantageous for thermoelectric energy conversion. The initially grown SnSe films have relatively low cross-plane Seebeck coefficients (-38.6 μV/K) due to significant unintentional doping originating from Se vacancies when annealed in nitrogen, as a result of the relatively high vapor pressure of Se. By performing postgrowth annealing at a fixed Se partial pressure (300 °C for 30 min using SnSe2 as the Se source in a sealed tube), a transition from SnSe to SnSe2 is induced, which is evidenced by clear changes in the X-ray diffraction patterns of the films. This results in a 16-fold increase in the cross-plane Seebeck coefficient (from -38.6 to -631 μV/K) after Se annealing due to both the SnSe-to-SnSe2 transition and the mitigation of unintentional doping by Se vacancies. We also observe a corresponding 6-fold drop in the electrical conductivity (from 3 to 0.5 S/m) after Se annealing, which is consistent with both a drop in the carrier concentration and an increase in band gap. The power factor S2σ increased by 44× (from 4.5 nW/m·K2 to 0.2 μW/m·K2) after Se annealing. We believe that these results demonstrate a robust method for mitigating unintentional doping in a promising class of materials for thermoelectric applications.

Accurate Quantum Wave Packet Study of the Deep Well D+ + HD Reaction: Product Ro-vibrational State-Resolved Integral and Differential Cross Sections.

The H+ + H2 reaction and its isotopic variants as the simplest triatomic ion-molecule reactive system have been attracting much interests, however there are few studies on the titled reaction at state-to-state level until recent years. In this work, accurate state-to-state quantum dynamics studies of the titled reaction have been carried out by a reactant Jacobi coordinate-based time-dependent wave packet approach on diabatic potential energy surfaces constructed by Kamisaka et al. Product ro-vibrational state-resolved information has been calculated for collision energies up to 0.2 eV with maximal total angular momentum J = 40. The necessity of including all K-component for accounting the Coriolis coupling for the reaction has been illuminated. Competitions between the two product channels, (D+ + HD' → D'+ + HD and D+ + HD' → H+ + DD') were investigated. Total integral cross sections suggest that resonances enhance the reactivity of channel D+ + HD'→ H+ + DD', however, resonances depress the reactivity of the another channel D+ + HD' → D'+ + HD. The structures of the differential cross sections are complicated and depend strongly on collision energies of the two channels and also on the product rotational states. All of the product ro-vibrational state-resolved differential cross sections for this reaction do not exhibit rigorous backward-forward symmetry which may indicate that the lifetimes of the intermediate resonance complexes should not be that long. The dynamical observables of this deuterated isotopic reaction are quite different from the reaction of H+ + H2 → H2 + H+ reported previously.

Acceleration of convective dissolution by an instantaneous chemical reaction: A comparison of experimental and numerical results

The effect of an instantaneous chemical reaction on convective dissolution in a Hele-Shaw cell is analyzed experimentally and numerically. We experimentally show that the non-dimensional onset time and growth rate of instability is determined only by the reactant concentration-ratio. Also, nonlinear numerical simulations are conducted to obtain the temporal evolution of the dissolution flux. The experimental and numerical results reveal a sharp increase of up to 300% in the dissolution flux, accompanied by a decrease in the convection onset time, when the reactant concentration-ratio approaches the stoichiometric value. These results have important implications for the optimization of carbon-dioxide storage in porous saline aquifers in the Earth’s subsurface.

Understanding structural adaptability: a reactant informatics approach to experiment design

The structural and electronic adaptability of a vanadium selenite framework is determined using cheminformatics data and machine learning algorithms.

Stereoselectivity in Autoionization Reactions of Hydrogenated Molecules by Metastable Noble Gas Atoms: The Role of Electronic Couplings.

Focus in the present paper is on the analysis of total and partial ionization cross sections, measured in absolute value as a function of the collision energy, representative of the probability of ionic product formation in selected electronic states in Ne*-H2 O, H2 S, and NH3 collisions. In order to characterize the imaginary part of the optical potential, related to electronic couplings, we generalize a methodology to obtain direct information on the opacity function of these reactions. Such a methodology has been recently exploited to test the real part of the optical potential (S. Falcinelli et al., Chem. Eur. J., 2016, 22, 764-771). Depending on the balance of noncovalent contributions, the real part controls the approach of neutral reactants, the removal of ionic products, and the structure of the transition state. Strength, range, and stereoselectivity of electronic couplings, triggering these and many other reactions, are directly obtained from the present investigation.

Theoretical study of mechanism and dynamics on reaction of (CH3)2NH with CH3.

The mechanism and dynamics for the bimolecular reaction of (CH3)2NH with CH3 have been investigated based on the G3//MP2/6-311G(d,p) level of theory. Our calculations show that when the two reactants approach each other, three prereaction complexes, RC1, RC2, and RC3, can be formed through van der Waals force or hydrogen bonding. From RC1, RC2, and RC3, six routes have been established. Among the six routes, the two routes (R1 and R2) from van der Waals prereaction complex RC1 are the main routes for the title reaction. R1 and R2 are hydrogen abstractions routes associated with HN and HCα atoms in DMA, respectively. The calculated energy barriers for R1 and R2 are 12.3 and 13.7 kcal/mol, respectively. Both the potential energy surfaces of R1 and R2 locate a "reactant-like" transition state, as well as van der Waals complexes before and after the transition state. The slight preference of R1 over R2 might be related to the higher similarity between the structures of RC1 and the transition state for R1 (TS1), namely, the structure of TS1 is more "reactant-like". The rate constants of the two favorable H abstraction reaction routes, R1 and R2, are evaluated over a wide temperature range of 200-3000 K by the variational transition state theory (VTST) methods, which can be expressed as kR1 = 5.30 × 10(-13)(T/1000)(3.0) exp(-2883/T) cm(3) molecule(-1) s(-1) and kR2 = 8.34 × 10(-13)(T/1000)(4.5) exp(-3100/T) cm(3) molecule(-1) s(-1), respectively. The predicted rate constant of the HN abstraction (route R1) is in good agreement with the available experimental data.

Dinuclear copper intermediates in copper(I)-catalyzed azide-alkyne cycloaddition directly observed by electrospray ionization mass spectrometry.

The mechanism of the CuAAC reaction has been investigated by electrospray ionization mass spectrometry (ESI-MS) using a combination of the neutral reactant approach and the ion-tagging strategy. Under these conditions, for the first time, putative dinuclear copper intermediates were fished out and characterized by ESI(+)-MS/MS. New insight into the CuAAC reaction mechanisms is provided and a catalytic cycle is proposed.

Dinuclear Copper Intermediates in Copper(I)‐Catalyzed Azide–Alkyne Cycloaddition Directly Observed by Electrospray Ionization Mass Spectrometry

AbstractThe mechanism of the CuAAC reaction has been investigated by electrospray ionization mass spectrometry (ESI‐MS) using a combination of the neutral reactant approach and the ion‐tagging strategy. Under these conditions, for the first time, putative dinuclear copper intermediates were fished out and characterized by ESI(+)‐MS/MS. New insight into the CuAAC reaction mechanisms is provided and a catalytic cycle is proposed.

Dynamics of gas phase Ne* + NH3 and Ne* + ND3 Penning ionisation at low temperatures

Two isotopic chemical reactions, Ne(*) + NH3, and Ne(*) + ND3, have been studied at low collision energies by means of a merged beams technique. Partial cross sections have been recorded for the two reactive channels, namely, Ne(*) + NH3 → Ne + NH3(+) + e(-), and Ne(*) + NH3 → Ne + NH2(+) + H + e(-), by detecting the NH3(+) and NH2(+) product ions, respectively. The cross sections for both reactions were found to increase with decreasing collision energy, Ecoll, in the range 8 μeV < Ecoll < 20 meV. The measured rate constant exhibits a curvature in a log(k)-log(Ecoll) plot from which it is concluded that the Langevin capture model does not properly describe the Ne(*) + NH3 reaction in the entire range of collision energies covered here. Calculations based on multichannel quantum defect theory were performed to reproduce and interpret the experimental results. Good agreement was obtained by including long range van der Waals interactions combined with a 6-12 Lennard-Jones potential. The branching ratio between the two reactive channels, Γ = [NH2(+)]/[NH2(+)] + [NH3(+)], is relatively constant, Γ ≈ 0.3, in the entire collision energy range studied here. Possible reasons for this observation are discussed and rationalized in terms of relative time scales of the reactant approach and the molecular rotation. Isotopic differences between the Ne(*) + NH3 and Ne(*) + ND3 reactions are small, as suggested by nearly equal branching ratios and cross sections for the two reactions.

Reaction Dynamics and Mechanism of the Cl + HD(v = 1) Reaction: A Quantum Mechanical Study

Time-independent quantum mechanical calculations have been performed in order to characterize the dynamics and stereodynamics of Cl + HD reactive collisions. Calculations have been carried out at two different total energy values and for various initial states using the adiabatic potential energy surface by Bian and Werner [J. Chem. Phys. 2000, 112, 220]. Special attention has been paid to the reaction with HD(v = 1) for which integral and differential cross-sections have been calculated and the effect of vibrational vs translational energy on the reactivity has been examined. In addition, the reactant polarization parameters and polarization-dependent differential cross-sections have been determined. From these results, the spatial preferences of the reaction and the extent of the control of the cross sections achievable through a suitable preparation of the reactants have been also studied. The directional requirements are tighter for the HCl channel than for the DCl one. Formation of the products takes place preferentially when the rotational angular momentum of the HD molecule is perpendicular to the reactants approach direction. Cross-sections and polarization moments computed from the scattering calculations have been compared with experimental results by Kandel et al. [J. Chem. Phys. 2000, 112, 670] for the reaction with HD(v = 1) produced by stimulated Raman pumping. The agreement so obtained is good, and it improves the accordance found in previous calculations with other methodologies and potential energy surfaces.

Thermal and Sc(OTf)3 catalyzed 1,3-dipolar cycloaddition of open-chain nitrones to α,β-unsaturated lactones: combined experimental and computational studies

The stereoselectivity of 1,3-dipolar cycloaddition reactions of C-phenyl open-chain nitrones and α,β-unsaturated γ- and δ-lactones was investigated under thermal and catalytic conditions. It was found that under thermal conditions, the endo approach of the reactants was preferred leading to the thermodynamic product. In the presence of Sc(OTf)3 the exo adduct was obtained in high yield and selectivity. The energies of the cycloaddition reactions were investigated by means of molecular orbital calculations at the B3LYP/6-31+G(d,p) and MP3/6-31+G(d,p) theory level. Different reaction channels and reactant approaches, fitting the individual regio- and stereochemical preferences, are discussed. The computational results were compared with the corresponding experimental data and found to be in good agreement.

From Exothermic to Endothermic Dehydrogenation – Interaction of Monoammoniate of Magnesium Amidoborane and Metal Hydrides

Metal amidoborane ammoniate such as Mg(NH2BH3)2·NH3 and Ca(NH2BH3)2·nNH3 with n = 1, 2, release NH3 predominantly and endothermically at low temperatures in an open-system. However, a strong exothermic reaction occurs when these ammoniates were dehydrogenated in a closed-system, where the adducted NH3 take part in the reaction. Our approach in tailoring the thermodynamic properties of Mg(NH2BH3)2·NH3, by replacing its adducted NH3 with amide was successful, yielding a composite consisting of bimetallic amidoborane and Mg(NH2)2. Crystal structures of bimetallic amidoboranes, i.e., Na2Mg(NH2BH3)4 and K2Mg(NH2BH3)4 were identified and solved. Significant improvement in the dehydrogenation thermodynamic was observed in the composite system as compared to the pristine Mg(NH2BH3)2·NH3, i.e., the dehydrogenation enthalpies were altered from an exothermic to an endothermic one. In addition, the detection of bimetallic amidoboranes in the composites urges detailed investigation on pristine bimetallic amidoborane, to which later we found that Na2Mg(NH2BH3)4 also dehydrogenated endothermically at the identical temperature range (ca. 150–170 °C) with that of composite systems. Similar activation barriers were observed in Na2Mg(NH2BH3)4 and composite systems, suggesting that metal hydride mediation may be the internal barrier that dominates the kinetic barrier of the composite system. First-principles calculations also showed that the thermodynamic stability of metal amidoborane (MNH2BH3, MAB) increases with decreasing Pauling electronegativity of the metal. Based on the calculated results, a reactant stabilization approach was proposed, which suggests that forming a stable reactant is an effective way of reducing the exothermicity of the dehydrogenation of metal amidoborane.

ChemInform Abstract: Synthesis of Iminosugars via 1,3‐Dipolar Cycloaddition Reactions of Nitrones to α,β‐Unsaturated Sugar Aldonolactones

AbstractReview: 230 refs.