Effect of potential shape on isomerization rate constants for the BGK model

As a model for isomerization reactions in liquids, we consider the motion, in a one-dimensional bistable potential, of a particle which suffers BGK collisions. We investigate how the relaxation rate constant as a function of collision frequency depends upon the shape of the potential. We discuss our results in relation to recent high-pressure NMR experiments involving cyclohexane isomerization. For a potential with narrow wells and a wide barrier, we find qualitative agreement between theory and experiment.     

Extrapolating H-atom transfer rate constants via thermochemical kinetics: The effect of tunneling

One of the important applications of chemical kinetics is the attempt to understand complex processes through kinetic modeling. This process frequently requires that rate constants be obtained by extrapolation of data either to higher or lower temperatures than the experimental, or by estimation or correlation with such data. Thermochemical kinetics combined with conventional transition state theory forms a framework from which this may be done. However, rate data for H transfer reactions may have a significant contribution from tunneling. In this work, a one dimensional approach to tunneling, consistent with conventional transition state theory, is taken to show how tunneling affects the extrapolation and correlation of rate constants in thermochemical kinetics. It is concluded that extrapolation and correlation are both quite good even when tunneling comprises 80% of the reaction. However, this is not without limitations, which are discussed. © 1993 John Wiley & Sons, Inc.     

Polychronous kinetics with nonstationary rate constants. Effect of a medium

Characteristic features of the kinetics of solid-state cage reactions with distributed parameters of the relaxing matrix were considered. Depending on the ratio of the constants of the reaction rate and relaxation of environment, the kinetics of chemical conversions can be either exponential or nonexponential. Plausible reasons for the unsteady-state character of the kinetics of the processes of two types,viz., the reactions of alkyl radicals in amorphous alcohol matrices and conversions in biological systems, were discussed. The main reason for the unsteady-state character of the reactions of the first type is a dispersion of the equilibrium distances between the reagents. Kinetics of the reactions of the second type, such as rebinding of the ligands in the heme-containing proteins (e.g., in myoglobin), is determined by the distances in the pairs of reagents and the relaxation transitions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 469–476, March, 1997.     

Theoretical calculation of rate constants for the thermal isomerization from 1,2-butadiene to 1,3-butadiene

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The lowest-lying electronic singlet and triplet potential energy surfaces for the HNO-NOH system: energetics, unimolecular rate constants, tunneling and kinetic isotope effects for the isomerization and dissociation reactions

The lowest-lying electronic singlet and triplet potential energy surfaces (PES) for the HNO-NOH system have been investigated employing high level ab initio quantum chemical methods. The reaction energies and barriers have been predicted for two isomerization and four dissociation reactions. Total energies are extrapolated to the complete basis set limit applying focal point analyses. Anharmonic zero-point vibrational energies, diagonal Born-Oppenheimer corrections, relativistic effects, and core correlation corrections are also taken into account. On the singlet PES, the (1)HNO → (1)NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol(-1). For the barrierless decomposition of (1)HNO to H + NO, the dissociation energy is estimated to be 47.48 ± 0.2 kcal mol(-1). For (1)NOH → H + NO, the reaction endothermicity and barrier are 5.25 ± 0.2 and 7.88 ± 0.2 kcal mol(-1). On the triplet PES the reaction energy and barrier including all corrections are predicted to be 7.73 ± 0.2 and 39.31 ± 0.2 kcal mol(-1) for the isomerization reaction (3)HNO → (3)NOH. For the triplet dissociation reaction (to H + NO) the corresponding results are 29.03 ± 0.2 and 32.41 ± 0.2 kcal mol(-1). Analogous results are 21.30 ± 0.2 and 33.67 ± 0.2 kcal mol(-1) for the dissociation reaction of (3)NOH (to H + NO). Unimolecular rate constants for the isomerization and dissociation reactions were obtained utilizing kinetic modeling methods. The tunneling and kinetic isotope effects are also investigated for these reactions. The adiabatic singlet-triplet energy splittings are predicted to be 18.45 ± 0.2 and 16.05 ± 0.2 kcal mol(-1) for HNO and NOH, respectively. Kinetic analyses based on solution of simultaneous first-order ordinary-differential rate equations demonstrate that the singlet NOH molecule will be difficult to prepare at room temperature, while the triplet NOH molecule is viable with respect to isomerization and dissociation reactions up to 400 K. Hence, our theoretical findings clearly explain why (1)NOH has not yet been observed experimentally.     

Determination of rate constants for heterogeneous mass transfer kinetics in liquid chromatography

The heterogeneity of the stationary phase surface leads to undesired peak broadening and tailing in chromatography. In order to properly characterize the interactions between the solute molecules and the surface of the stationary phase, the determination of the equilibrium isotherm is usually necessary by overloading the column. In this study we show that measurements made in the linear range of the isotherm can also be utilized to estimate the heterogeneity of the stationary phase. When one studies the peak shape obtained after injecting a small (analytical) amount of sample, the peak shape parameters can be correlated with the kinetic rate constants. The peak of a mildly polar solute, phenol, was measured in reversed-phase HPLC under various conditions and kinetic information was obtained regarding the heterogeneity of interactions.     

CIS-TRANS isomerization of styrenes via the triplet pathway:The perpendicular triplet state observed by laser flash photolysis

The kinetics of the cis-trans photoisomerization of 1-phenylcyclohexene via the triplet state, studied by either nanosecond pulse radiolysis or laser flash photolysis in the presence of sensitizers, reveal that the triplet species involved in the isomerization mechanism has a lifetime of 55 ns in fluid solution at room temperature. A transient absorption decaying with the same 55 ns lifetime, and therefore assigned to this triplet species, was abserved in the 320–345 nm region. Quite similar triplet-triplet absorptions were observed with 1-phenylcycloheptene, 1-phenylpropene and styrene itself From the experimental results and from considerations of the energy surfaces of the excited states of styrene, the observed triplet species is identified as the perpendicular (or “phantom”) triplet state of the styrene moiety.     

A theoretical study of the photochemical isomerization reactions of furans from the triplet state

The mechanisms of photoisomerization reactions were investigated theoretically using a model system of 2-methylfuran with the CASSCF (10-electron/8-orbital active space) and MP2-CAS methods and the 6-311(d,p) basis set. After 2-methylfuran molecules are produced in the T(1) state by photoexcitation at 254 nm, intersystem crossing to the S(0) surface is the most probable pathway for deactivation. Relaxing to the S(0) state, the 2-methylfuran molecules can dissociate into 3-methylcyclopropene and carbon monoxide products. Otherwise, they may revert to singlet 2-methylfuran or undergo photorearrangement to produce 3-methylfuran. These stepwise mechanisms are consistent with the available experimental observations.     

Determination of rate constants for radical telomerization chain growth and transfer from the molecular-mass distribution of oligomer

The ratio of rate constant for growth and transfer X(n), as a function of the chain length n has been found from the measured molecular-mass distributions of the products of tetrafluoroethylene telomerization in acetone, ethyl acetate, chloroform, and carbon tetrachloride. For all these telogens, the function increases by a factor of 1.5–2.5 in the range of n from 2 to 5, is almost constant for n of 6 to 10, and increases by a factor of 7–10 in the range of n from 12 to 20. This behavior of the function X(n) has been explained in terms of the model of diffusion-controlled propagation and kinetic chain transfer. The model takes into account the change in the diffusion nature of oligomers in the form of rigid rods with an increase in their length. A sharp increase in X(n) occurs when the oligomers that accumulate in the environment of growing macroradical sterically restrict the withdrawal of the forming oligomer to the bulk by an effective solid angle, which decreases with the increasing oligomer length and becomes minimal in the region of formation of colloidal particles.     

Determination of rate constants of benzoyl peroxide and azobisisobutyronitrile dissociation via polymerization kinetics

A linear expression is derived from Tobolsky's equation related to the dead-end polymerization method to determine the rate constant for the initiator dissociation. This novel graphical method applies remarkably well to the kinetic data collected by dilatometry from the polymerization of methyl methacrylate initiated by 2,2′-azobisisobutyronitrile and benzoyl peroxide in toluene at 60°C. Results obtained for these two initiators are consistent with those published in the literature. Applicability of the method is confined to at least 5% and at most 13% decomposition of initiator. However, these limiting values are sensitive to the experimental techniques employed. The effects of induced decomposition of benzoyl peroxide and thermal polymerization of methyl methacrylate are shown to be negligible in the present investigations.     

Activation enthalpies and entropies for the microscopic rate constants of acetate-catalyzed isomerization of 5-androstene-3,17-dione

Both acetic acid and acetate catalyze the isomerization of 5-androstene-3,17-dione (1) to its conjugated isomer, 4-androstene-3,17-dione (3), through a dienol(ate) intermediate. The temperature dependence of the overall isomerization rate constants and of the microscopic rate constants for this isomerization was determined, and the Arrhenius plots give the activation enthalpy and entropy for each step. The source of the activation energy for the overall isomerization and for each of the individual steps is predominantly enthalpic, with a moderate to low entropic penalty. Additionally, the entropy and enthalpy for the keto-enol equilibrium of 1 and dienol were determined; this equilibrium is entirely controlled by enthalpy with no entropic contribution. The relevance of these results to the mechanism of the isomerization of 1 catalyzed by the enzyme 3-oxo-Delta(5)-steroid isomerase is discussed.     

Highly efficient separation of lithium chloride from seawater

A complexing reagent composed of two bipyridine moieties enabled the efficient separation of lithium chloride through liquid membrane from seawater, in which 0.005% lithium chloride is contained (more than 99% metal chlorides are NaCl, KCl, MgCl2, and CaCl2). That is, two separations by our liquid membrane changed the molar ratio of LiCl from 0.005% to 80%. The striking characteristic of this compound is that the lithium ion is separated efficiently from alkali and alkaline earth metal ions without the lipophilic anion. Thus this new membrane system contructed by us offers a low-energy, low-cost, and environmentally friendly method to enable the routine use of lithium chloride separation from seawater.     

Highly efficient Pd-catalyzed synthesis of nitriles from aldoximes

An expedient method of Pd-catalyzed conversion of aldoxime into nitrile was developed. The reaction was carried out under the influence of Pd(OAc)₂/PPh₃ in refluxing CH₃CN to provide good to high yields. The use of Cs₂CO₃ (0.1-0.5 equiv) was crucial in some cases.     

Highly efficient synthesis of terminal alkenes from ketones

[reaction: see text] The rhodium(I)-catalyzed methylenation of ketones using trimethylsilyldiazomethane proceeds to give the corresponding alkenes in good yields (60-97%). The use of an excess of 2-propanol and 1,4-dioxane as a solvent were instrumental to obtain the desired alkenes in high yields. Superior results were achieved with the rhodium(I)-catalyzed methylenation in comparison with the standard Wittig reaction.     

Including anharmonicity in the calculation of rate constants. 1. The HCN/HNC isomerization reaction

A method for calculating anharmonic vibrational energy levels in asymmetric top and linear systems that is based on second-order perturbation theory in curvilinear coordinates is extended to the bound generalized normal modes at nonstationary points along a reaction path. Explicit formulas for the anharmonicity coefficients, x(ij), and the constant term, E0, are presented, and the necessary modifications for resonance cases are considered. The method is combined with variational transition state theory with semiclassical multidimensional tunneling approximations to calculate thermal rate constants for the HCN/HNC isomerization reaction. Although the results for this system are not very sensitive to the choice of coordinates, we find that the inclusion of anharmonicity leads to a substantial improvement in the vibrational energy levels. We also present detailed comparisons of rate constants computed with and without anharmonicity, with various approximations for incorporating tunneling along the reaction path, and with a more practical approach to calculating the vibrational partition functions needed for larger systems.     

An efficient route to thermal rate constants in reduced dimensional quantum scattering simulations: applications to the abstraction of hydrogen from alkanes

We present an efficient approach to the determination of two-dimensional potential energy surfaces for use in quantum reactive scattering simulations. Our method involves first determining the minimum energy path (MEP) for the reaction by means of an ab initio intrinsic reaction coordinate calculation. This one-dimensional potential is then corrected to take into account the zero point energies of the spectator modes. These are determined from Hessians in curvilinear coordinates after projecting out the modes to be explicitly treated in quantum scattering calculations. The final (1+1)-dimensional potential is constructed by harmonic expansion about each point along the MEP before transforming the whole surface to hyperspherical coordinates for use in the two-dimensional scattering simulations. This new method is applied to H-atom abstraction from methane, ethane and propane. For the latter, both reactive channels (producing i-C(3)H(7) or n-C(3)H(7)) are investigated. For all reactions, electronic structure calculations are performed using an efficient, explicitly correlated, coupled cluster methodology (CCSD(T)-F12). Calculated thermal rate constants are compared to experimental and previous theoretical results.     

Monomer exchange kinetics, radial diffusion, and hydrocarbon chain isomerization of sodium dodecylsulfate micelles in water

Molecular relaxation properties of sodium dodecylsulfate in aqueous solutions with surfactant concentrations between 0.009 and 0.4 mol/L have been studied using broadband ultrasonic spectrometry in the frequency range 0.1-2000 MHz. Ultrasonic excess attenuation characteristics were found that could be well represented by a sum of three relaxation terms, each one characterized by a discrete relaxation time. The low-frequency term with concentration-dependent relaxation time, tau1, between 0.06 and 3.5 micros is discussed in terms of the surfactant monomer exchange. The noticeable effect from the incomplete dissociation of the surfactant counter ions and the variation of the monomer concentration following thereby is discussed. The second relaxation term (0.9 相似文献