Oxidation of methylamine

A detailed chemical kinetic model for oxidation of methylamine has been developed, based on theoretical work and a critical evaluation of data from the literature. The rate coefficients for the reactions of CHNH + O CHNH / CHNH + HO, CHNH + H CH + NH, CHNH CHNH, and CHNH + O CHNH + HO were calculated from ab initio theory. The mechanism was validated against experimental results from batch reactors, flow reactors, shock tubes, and premixed flames. The model predicts satisfactorily explosion limits for CHNH and its oxidation in a flow reactor. However, oxidation in the presence of nitric oxide, which strongly promotes reaction at lower temperatures, is only described qualitatively. Furthermore, calculated flame speeds are higher than reported experimental values; the model does not capture the inhibiting effect of the NH group in CHNH compared to CH. More work is desirable to confirm the products of the CHNH + NO reaction and to look into possible pathways to NH in methylamine oxidation.     

Multiconformer transition state theory rate constants for the reaction between OH and -dimethoxyfluoropolyethers

In this work, we have calculated rate constants for the tropospheric reaction between the OH radical and -dimethoxyfluoropolyethers. The latter are a specific class of the hydrofluoropolyethers family with the general formula , from which we have selected three case studies: , , and . The calculations were performed by applying a cost-effective protocol developed for bimolecular hydrogen-abstraction reactions and based on multiconformer transition state theory relying on computationally accessible M08-HX/apcseg-2//M08-HX/pcseg-1 calculations. Within the protocol's uncertainties and approximations, the results show that (1) the calculated rate constants have the same order of magnitude and (2) if observed together with previous experimental and theoretical investigations, the chain length (that varies with q and p) is seen to have a small effect on the rate constant, which is consistent with the “no discernible effect” reported in the experimental work.     

Kinetics of two redox reactions in 2-butoxyethanol and water near its critical point of solution

The rate constants of two redox reactions and in the critical solution of 2-butoxyethanol and water have been measured by using the UV spectrophotometry at the initial reaction stage. It was found that the rate constants at various temperatures for two reactions were well described by the Arrhenius equation in the noncritical region. The critical slowing down effect was detected in the critical region. The critical slowing down exponents were determined to be 0.044 ± 0.004 and 0.046 ± 0.005 for reactions and , respectively. The values of the critical slowing down exponents showed that only dynamic critical slowing down effect, and no thermodynamic singularity could be observed for the two reactions.     

Thermal rate constant for the C(3P) + OH(X2Π) → CO(X1Σ) + H(2S) reaction using stochastic energy grained master equation method

In the present work, the kinetic mechanism of the reaction is studied. The rate constants were determined using the Master Equation Solver for Multi-Energy Well Reactions (MESMER). The master equation modeling was also employed to examine the pressure dependence for each pathway involved. The theoretical analysis shows that the overall rate coefficient is practically independent of pressure up to 100 Torr for the temperature range 125-500 K. The unusual dependence of the overall rate constant with temperature was fit with the d-Arrhenius expression , where cm³molecule⁻¹s⁻¹, , and  kJ·mol⁻¹, for 125⩽ T ⩽ 500 K. The thermal rate constant results are in relatively good agreement with other theoretical studies.     

Twenty-species and 15-species chemical kinetic mechanisms for cyclohexane using the local self-similarity tabulation method

The 1081 species cyclohexane-oxidation elementary reaction mechanism of Silke et al. (DOI: 10.1021/jp067592d ) is reduced in the number of species by a factor using the local self-similarity tabulation (LS2T) method. Reduced-species mechanisms of both 20 (R20) and 15 (R15) species are created in the high-pressure combustion regime typical of diesel engines. To evaluate the performance of R20 and R15 against the elementary kinetics, simulations are performed for cyclohexane/air mixtures at initial temperatures of 1150, 900, 750, and 680 K and constant pressures of 20 and 40 bar for a variety of equivalence ratios (, 1.0, and 2 for 1150 and 900 K; for 750 K; for 680 K). Very good agreement between R20 and R15 with the elementary kinetics mechanism is demonstrated at 1150 and 900 K for which the self-similarity is very well obeyed; however, only fair agreement is obtained at 750 and 680 K, a fact which is traced to the less faithful adherence to the self-similarity due to the one order of magnitude increase in ignition time over the range 750-680 K. These results are found to be quasi-independent of the tabulation grid. Future work is proposed to improve the reduction in the cold-ignition, high-pressure regime.     

Finding of remarkable synergistic effect on the aroxyl-radical-scavenging rates under the coexistence of α-tocopherol and catechins

Measurements of aroxyl radical (ArO^•)-scavenging rate constants () of antioxidants (AOHs) (α-tocopherol (α-TocH) and three catechins (CatHs) (ie, epicatechin (EC), epigallocatechin (EGC), and epigallocatechin gallate (EGCG)) were performed in ethanol solution, using stopped-flow spectrophotometry. values were measured not only for each AOH, but also for the mixtures of two AOHs (α-TocH and CatH). A notable synergistic effect that the value of α-TocH increases 1.29, 1.84, and 1.65 times under the coexistence of constant concentrations of EC, EGC, and EGCG, respectively, was observed for the solutions including α-TocH and CatH. Similarly, values of CatHs (EC, EGC, and EGCG) increased 1.72, 2.25, and 2.34 times under the coexistence of constant concentrations of α-TocH, respectively. UV-Vis absorption of α-tocopheroxyl radical (α-Toc^•) (λ_max = 428 nm), which had been produced by reaction of α-TocH with ArO^•, decreased remarkably under the coexistence of α-TocH and CatHs due to the fast α-TocH-regeneration reaction by CatHs. The result suggests that the prooxidant reaction due to α-Toc^• is suppressed by the coexistence of CatHs. By analyzing the formation and decay curves of α-Toc^•, it has been ascertained that one molecule of EGCG having three OH groups at B-ring may rapidly regenerate three molecules of α-Toc^• to α-TocH.     

Real-Space Interpretation of Interatomic Charge Transfer and Electron Exchange Effects by Combining Static and Kinetic Potentials and Associated Vector Fields**

Intricate behaviour of one-electron potentials from the Euler equation for electron density and corresponding gradient force fields in crystals was studied. Channels of locally enhanced kinetic potential and corresponding saddle Lagrange points were found between chemically bonded atoms. Superposition of electrostatic and kinetic potentials and electron density allowed partitioning any molecules and crystals into atomic - and potential-based -basins; -basins explicitly account for the electron exchange effect, which is missed for -ones. Phenomena of interatomic charge transfer and related electron exchange were explained in terms of space gaps between zero-flux surfaces of - and -basins. The gap between - and -basins represents the charge transfer, while the gap between - and -basins is a real-space manifestation of sharing the transferred electrons caused by the static exchange and kinetic effects as a response against the electron transfer. The regularity describing relative positions of -, -, and - basin boundaries between interacting atoms was proposed. The position of -boundary between - and -ones within an electron occupier atom determines the extent of transferred electron sharing. The stronger an H⋅⋅⋅O hydrogen bond is, the deeper hydrogen atom's -basin penetrates oxygen atom's -basin, while for covalent bonds a -boundary closely approaches a -one indicating almost complete sharing of the transferred electrons. In the case of ionic bonds, the same region corresponds to electron pairing within the -basin of an electron occupier atom.     

Fluorine Substitution of TCNQ Alters the Redox-Driven Catalytic Pathway for the Ferricyanide-Thiosulfate Reaction

Mechanistic variation in catalysis through substituent-based redox tuning is well established. Fluorination of TCNQ (TCNQ=tetracyanoquinodimethane) provides ~850 mV variation in the redox potentials of the and (n=0, 2, 4) processes. With , catalysis of the kinetically very slow ferrocyanide-thiosulfate redox reaction in aqueous solution occurs via a mechanism in which the catalyst is reduced to when reacting with which is oxidised to . Subsequently, reacts with to form and reform the catalyst, in another thermodynamically favoured process. An analogous mechanism applies with as a catalyst. In contrast, since the reaction of with is thermodynamically unfavourable, an alternative mechanism is required to explain the catalytic activity observed in this non-fluorinated system. Here, upon addition of , reduction of to occurs with concomitant oxidation of to , which then acts as the catalyst for oxidation. Thermodynamic data explain the observed differences in the catalytic mechanisms. (n=0, 4) also act as catalysts for the ferricyanide-thiosulfate reaction in aqueous solution. The present study shows that homogeneous pathways are available following addition of these dissolved materials. Previously, these (n=0, 4) coordination polymers have been regarded as insoluble in water and proposed as heterogeneous catalysts for the ferricyanide-thiosulfate reaction. Details and mechanistic differences were established using UV-visible spectrophotometry and cyclic voltammetry.     

Kinetic model for Ostwald's rule of stages with applications to Boc-diphenylalanine self-assembly

A kinetic model which describes Ostwald's rule of stages, during the process of crystal growth from solution, is reported here. Reaction equations for stages are given where the stages convert from one to another. The final stage reacts to release a portion of solute back into solution, while the remainder converts to the final equilibrium form. Additionally, a remnant of the solute that was not consumed by any of the transitional stages, ultimately is converted into the final product. This particular model was motivated by a recent report for Boc-diphenylalanine self-assembly where the dissolved peptide was observed to go through two polymorphic stages before reaching the equilibrium supramolecular assembly [A. Levin et al., Nat. Commun. 5, 5219, (2014)]. Kinetic data for the concentration of solute present during the process are listed in the above-mentioned report. We show here how the model, for , describes the time-dependent behavior of the solute decay during the growth process. After comparing the model to the experimental data, we are able to report values for all of the rate constants and propose a rule whereby the relative magnitudes of these constants can be used to predict whether a supersaturated substance will noticeably pass through transitional stages or simply convert from solute to the equilibrium solid form.     

The lift force on a charged sphere that translates and rotates in an electrolyte

The distortion of the charge cloud around a uniformly charged, dielectric, rigid sphere that translates and rotates in an unbounded binary, symmetric electrolyte at zero Reynolds number is examined. The zeta potential of the particle ζ is assumed small relative to the thermal voltage scale. It is assumed that the equilibrium structure of the cloud is slightly distorted, which requires that the Péclet numbers characterizing distortion due to particle translation, , and rotation, , are small compared to unity. Here, a is radius of the particle; D is the ionic diffusion coefficient; and , where U and Ω are the rectilinear and angular velocities of the particle, respectively. Perturbation expansions for small and are employed to calculate the nonequilibrium structure of the cloud, whence the force and torque on the particle are determined. In particular, we predict that the sphere experiences a force orthogonal to its directions of translation and rotation. This “lift” force arises from the nonlinear distortion of the cloud under the combined actions of particle translation and rotation. The lift force is given by . Here, ε is the permittivity of the electrolyte; is the Debye length; and is a negative function that decreases in magnitude with increasing . The lift force implies that an unconstrained particle would follow a curved path; an electrokinetic analog of the inertial Magnus effect. Finally, the implication of the lift force on cross‐streamline migration of an electrophoretic particle in shear flow is discussed.     

On the accurate estimation of free energies using the jarzynski equality

The Jarzynski equality is one of the most widely celebrated and scrutinized nonequilibrium work theorems, relating free energy to the external work performed in nonequilibrium transitions. In practice, the required ensemble average of the Boltzmann weights of infinite nonequilibrium transitions is estimated as a finite sample average, resulting in the so-called Jarzynski estimator, . Alternatively, the second-order approximation of the Jarzynski equality, though seldom invoked, is exact for Gaussian distributions and gives rise to the Fluctuation-Dissipation estimator . Here we derive the parametric maximum-likelihood estimator (MLE) of the free energy considering unidirectional work distributions belonging to Gaussian or Gamma families, and compare this estimator to . We further consider bidirectional work distributions belonging to the same families, and compare the corresponding bidirectional to the Bennett acceptance ratio () estimator. We show that, for Gaussian unidirectional work distributions, is in fact the parametric MLE of the free energy, and as such, the most efficient estimator for this statistical family. We observe that and perform better than and , for unidirectional and bidirectional distributions, respectively. These results illustrate that the characterization of the underlying work distribution permits an optimal use of the Jarzynski equality. © 2018 Wiley Periodicals, Inc.     

Is it possible to synthesize bulk salt compounds?

The existence and stability of bulk salt compounds are theoretically investigated in this study. This undertaking is carried out to address the following challenge: synthesizing a bulk salt compound containing a noble gas lighter than Kr. The reliability of theoretical calculations on systems is assessed by benchmark calculations of the well-known salt. In the benchmark calculations, a two-pronged evaluation strategy, including direct and indirect evaluation methods, is used to theoretically investigate the spectroscopic constants of cation and the existence and stability of the salt. The validity of the theoretical calculation methods in the benchmark calculations of salt allows us to adopt a similar methodology to effectively predict the existence and stability of salt compounds. Calculations based on the Born-Haber cycle using estimated lattice energies and some necessary ancillary thermochemical data show that salt compounds can be synthesized, and their upper-limit stable temperatures are estimated to be −237.589, −197.76, and −80.539°C. The salt compound is the most promising candidate. Calculations also show that the salt compounds cannot be stabilized.     

Temperature Dependence of 1H Paramagnetic Chemical Shifts in Actinide Complexes,Beyond Bleaney's Theory: The AnVIO22+–Dipicolinic Acid Complexes (An=Np,Pu) as an Example

Actinide +VI complexes ( = , and ) with dipicolinic acid derivatives were synthesized and characterized by powder XRD, SQUID magnetometry and NMR spectroscopy. In addition, and complexes were described by first principles CAS based and two-component spin-restricted DFT methods. The analysis of the ¹H paramagnetic NMR chemical shifts for all protons of the ligands according to the X-rays structures shows that the Fermi contact contribution is negligible in agreement with spin density determined by unrestricted DFT. The magnetic susceptibility tensor is determined by combining SQUID, pNMR shifts and Evans’ method. The SO-RASPT2 results fit well the experimental magnetic susceptibility and pNMR chemical shifts. The role of the counterions in the solid phase is pointed out; their presence impacts the magnetic properties of the complex. The temperature dependence of the pNMR chemical shifts has a strong contribution, contrarily to Bleaney's theory for lanthanide complexes. The fitting of the temperature dependence of the pNMR chemical shifts and SQUID magnetic susceptibility by a two-Kramers-doublet model for the complex and a non-Kramers-doublet model for the complex allows for the experimental evaluation of energy gaps and magnetic moments of the paramagnetic center.     

Photoionization Bands of Cyanogen: Multi-Mode Vibronic Coupling and Renner-Teller Effects

Multi-mode vibronic coupling in the , , and electronic states of Cyanogen radical cation (C N ) is investigated with the aid of ab initio quantum chemistry and first principles quantum dynamics methods. The electronic degenerate states of Π symmetry of C N undergo Renner-Teller (RT) splitting along degenerate vibrational modes of π symmetry. The RT split components form symmetry allowed conical intersections with those from nearby RT split states or with non-degenerate electronic states of Σ symmetry. A parameterized vibronic Hamiltonian is constructed using standard vibronic coupling theory in a diabatic electronic basis and symmetry rules. The parameters of the Hamiltonian are derived from ab initio calculated adiabatic electronic energies. The vibronic spectrum is calculated, assigned and compared with the available experimental data. The impact of various electronic coupling on the vibronic structure of the spectrum is discussed.     

Size Evolution of Photoabsorption Spectra of Small Clusters: A Computational Study

Photoabsorption spectra of clusters, N=5–9, have been calculated using a diatomics-in-molecules like electronic structure model and a path-integral Monte Carlo sampling method. A qualitative change in the calculated spectra has been observed at N=9, which has been interpreted in terms of a structural transformation in the clusters consisting in a transition from trimer-like ionic cores observed for N≤7 to dimer-like ionic cores prevailing in through an intermediate state (comparable abundances of both types of ionic cores) observed in . The calculated spectra have been thoroughly compared with an earlier calculation on , , and reported from our group and data available for the same cluster sizes from an experiment.     

Potential of AMnO3 (A=Ca,Sr, Ba,La) as Active Layer in Inorganic Perovskite Solar Cells

Inorganic perovskite CaMnO was proposed as a substitution for the TiO anatase in electron transport layers of solar cells containing the hybrid perovskite CH NH PbI based on increased mobility of electrons and better optical matching. Due to a suitable band gap concerning the absorption of sunlight, we investigate the potential of CaMnO and similar manganite perovskites, where Ca is replaced by either Sr, Ba or La, as an absorber layer in inorganic perovskite solar cells. In this study, we have used optical measurements on the synthesized AMnO (A=Ca, Sr, Ba, La) samples to aid density functional theory calculations (DFT) in order to accurately simulate the electronic and optical properties of AMnO compounds and gauge their potential for the role of absorber layer. Both experimental measurements and theoretical calculations show suitable band gap of 1.1-1.5 eV, depending on the compound, and absorption coefficients of the order of cm in the visible part of the spectrum.     

Transient electrophoresis of a charged porous particle

The starting electrophoretic motion of a porous, uniformly charged, spherical particle, which models a solvent-permeable and ion-penetrable polyelectrolyte coil or floc of nanoparticles, in an arbitrary electrolyte solution due to the sudden application of an electric field is studied for the first time. The unsteady Stokes/Brinkman equations with the electric force term governing the fluid velocity fields are solved by means of the Laplace transform. An analytical formula for the electrophoretic mobility of the porous sphere is obtained as a function of the dimensionless parameters , , , and , where a is the radius of the particle, κ is the Debye screening parameter, λ is the reciprocal of the square root of the fluid permeability in the particle, ρ_p and ρ are the mass densities of the particle and fluid, respectively, ν is the kinematic viscosity of the fluid, and t is the time. The electrophoretic mobility normalized by its steady-state value increases monotonically with increases in and , but decreases monotonically with an increase in , keeping the other parameters unchanged. In general, a porous particle with a high fluid permeability trails behind an identical porous particle with a lower permeability and a corresponding hard particle in the growth of the normalized electrophoretic mobility The normalized electrophoretic acceleration of the porous sphere decreases monotonically with an increase in the time and increases with an increase in from zero at .     

Fluxional bonds in quasi-planar and half-sandwich (M = K,Rb, and Cs)

Detailed molecular orbital and bonding analyses reveal the existence of both fluxional σ- and π-bonds in the global minima C_s ( 1 ) and C_s MB₁₈ ( 3 ) and transition states C_s ( 2 ) and C_s ( 4 ) of dianion and monoanions (M = K, Rb, and Cs). It is the fluxional bonds that facilitate the fluxional behaviors of the quasi-planar and half-sandwich which possess energy barriers smaller than the difference of the corresponding zero-point corrections. © 2019 Wiley Periodicals, Inc.