Boundary conditions for the Swain-Schaad relationship as a criterion for hydrogen tunneling

Hydrogen quantum mechanical tunneling has been suggested to play a role in a wide variety of hydrogen-transfer reactions in chemistry and enzymology. An important experimental criterion for tunneling is based on the breakdown of the semiclassical prediction for the relationship among the rates of the three isotopes of hydrogen (hydrogen -H, deuterium -D, and tritium -T). This is denoted the Swain-Schaad relationship. This study examines the breakdown of the Swain-Schaad relationship as criterion for tunneling. The semiclassical (no tunneling) limit used hereto (e.g., 3.34, for H/T to D/T kinetic isotope effects), was based on simple theoretical considerations of a diatomic cleavage of a stable covalent bond, for example, a C-H bond. Yet, most experimental evidence for a tunneling contribution has come from breakdown of those relationship for a secondary hydrogen, that is, not the hydrogen whose bond is being cleaved but its geminal neighbor. Furthermore, many of the reported experiments have been mixed-labeling experiments, in which a secondary H/T kinetic isotope effect was measured for C-H cleavage, while the D/T secondary effect accompanied C-D cleavage. In experiments of this type, the breakdown of the Swain-Schaad relationship indicates both tunneling and the degree of coupled motion between the primary and secondary hydrogens. We found a new semiclassical limit (e.g., 4.8 for H/T to D/T kinetic isotope effects), whose breakdown can serve as a more reliable experimental evidence for tunneling in this common mixed-labeling experiment. We study the tunneling contribution to C-H bond activation, for which many relevant experimental and theoretical data are available. However, these studies can be applied to any hydrogen-transfer reaction. First, an extension of the original approach was applied, and then vibrational analysis studies were carried out for a model system (the enzyme alcohol dehydrogenase). Finally, the effect of complex kinetics on the observed Swain-Schaad relationship was examined. All three methods yield a new semiclassical limit (4.8), above which tunneling must be considered. Yet, it was found that for many cases the original, localized limit (3.34), holds fairly well. For experimental results that are between the original and new limits (within statistical errors), several methods are suggested that can support or exclude tunneling. These new and clearer criteria provide a basis for future applications of the Swain-Schaad relationship to demonstrate tunneling in complex systems.     

Algorithm for error-compensated kinetic determinations without prior knowledge of reaction order or rate constant

The development and evaluation of a new algorithm for error-compensating predictive kinetic determinations are described. With the new algorithm it is possible to fit kinetic data without prior knowledge of the rate constant, reaction order or initial and final values of the detector signal. A curve-fitting method is used to obtain values of these parameters that give the best fit of the model to the data. Although intended to be used primarily to compute signal changes between t=0 and ∞, the method can also be used to determine reaction orders. Although the algorithm fails for reaction orders of zero and unity, it works well for orders between these values and orders greater than unity. Simulated data are used to evaluate the effects of reaction order, signal noise and data range on computed values of signal change and, to a lesser extent, reaction order.     

Comparison of algorithms for error-compensated kinetic determinations without prior knowledge of reaction orders or rate constants

This paper describes the evaluation of algorithms written for error-compensated kinetic determinations, that do not need prior knowledge of reaction orders or rate constants. Results are reported for the quantification of acetoacetate in aqueous solution and urine. In addition to examination by nonlinear and linearized versions of the proposed new algorithms, the kinetic data were processed by a first-order model for comparison purposes. All the models yielded linear relationships between computed absorbance change and concentration; the new algorithms yield virtually identical results that represent better fits to the kinetic data than those obtained with the first-order model. The ability of the new algorithms to detect errors in the models is briefly discussed.     

The role of long range tunneling in electron transfer processes in condensed media

Experimental evidence is given for the importance of long range tunneling in electron transfer reactions in condensed media: the unusually weak effect of electrostatic repulsion on the rate of some electron transfer and spin exchange processes; electron transfer between distant (up to ≈ 30 A) species in solids at a rate considerably exceeding that of thermal diffusion; the unusual concentration dependence of radiation yields in the presence of scavengers, etc. The concept of long range tunneling is shown to permit quantitative explanation and correlation of experimental data on electron transfer in quite different fields.The factors determining the efficiency of tunneling, as well as some peculiar features of tunneling kinetics are considered.The role of long range tunneling in various chemical processes involving electron transfer in condensed media are discussed (ion reactions in solutions, photochemistry, radiation chemistry, reactions with polymers, some biochemical reactions).     

The role of local motions in the kinetic peculiarities of atomic tunneling in solids

The kinetic data on the simplest reactions occurring at low temperatures in solids are considered. All reactions are stimulated by molecular motions whose frequency substantially exceeds that of the diffusion displacements of particles. In all of the cases the reaction zone radius is about 10 Å. A model of the elementary chemical act in the solid, particularly the tunneling one, used to account for experiment is discussed.     

A simple method for extending the range of pseudo-order kinetic measurements

Data are presented demonstrating the validity of a new method of measuring pseudo-order rate constants, based on an iterative computational procedure long in use for kinetic conditions where the “infinity” value is not experimentally accessible. This new method requires that the kinetic order be known for the limiting reagent, but is otherwise general. It allows measurement of the pseudo-order rate constant even at a ratio of reactants of 1:1 by maximizing the fit of three half-lives of data to the appropriate pseudo-order kinetic equation, treating the “infinity” value as a variable.     

Confidence range for the determination of kinetic constants obtained from steady-state measurements

The problem of the confidence range for the determination of kinetic constants obtained from steady-state measurements is studied. The range of constants every point of which reproduces the results with equal accuracy is selected. As an illustration, the steady-state kinetic model of butadiene cyclooligomerization in the presence of low-valent Ni complexes is discussed.

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Air-stable racemization catalysts for the dynamic kinetic resolution of secondary alcohols

The substitution of a carbonyl ligand with PPh(3) in cyclopentadienylruthenium dicarbonyl complexes produces a new class of recyclable alcohol racemization catalysts. The catalysts are active at room temperature under aerobic conditions in the presence of silver oxide. Furthermore, the catalysts are compatible with the use of a lipase and isopropenyl acetate for the dynamic kinetic resolution (DKR) of secondary alcohols under ambient conditions.     

The influence of microwave irradiation on lipase-catalyzed kinetic resolution of racemic secondary alcohols

The influence of microwave irradiation on the Novozyme 435^® (Candida antarctica lipase) catalyzed kinetic resolution of secondary alcohols with different functional groups was studied in comparison to the use of conventional heating at 60 °C. p-Chlorophenyl acetate was used as an acyl donor and toluene as the solvent. (±)-1-Phenyl-1-propanol 1, (±)-1-(4-bromophenyl)-propan-1-ol 3, (±)-1-phenylbut-3-en-1-ol 5 and (±)-3-bromo-2-(2-hydroxypropyl)-1,4-dimethoxynaphthalene 7 were successfully resolved into their (S)-alcohols and (R)-esters, respectively, in good enantiomeric excess. Resolution of (±)-ethyl-5-(4-methoxybenyloxy)-3-hydroxypentanoate 9 afforded its (R)-alcohol and (S)-ester using this method. In addition, microwave-assisted lipase transesterification of meso-symmetric diol 11 effected desymmetrization to ester 12 with high enantiomeric excess. In all cases studied, the conversion value for the microwave-assisted lipase kinetic resolution of secondary alcohols was higher than that obtained using conventional heating.     

Electron tunneling in solid phase redox reactions. Experimental values of parameters characterizing the rate of tunneling and theoretical models for long range electron transfer

The problem of the quantitative analysis of the kinetic data for the tunnel electron transfer reactions in solids is discussed. Various procedures are considered with the help of which one can determine from the experimental data the parameters v and a in the equation w = v exp (?2r/a) describing the dependence of the electron tunneling frequency w on the distance r between the reacting particles. For numerous tunnel reactions studied in the literature these parameters are calculated and their values are analyzed in terms of theoretical models of electron tunneling. The physical background of these models is discussed in detail to make it more understandable for experimentalists. The limits of applications for these models are discussed. A novel model for nuclear motion in the process of electron tunneling is proposed to account for the unusually high values of the frequency factor found for some of the reactions.     

Air-stable racemization catalyst for dynamic kinetic resolution of secondary alcohols at room temperature

[reaction: see text] A novel racemization catalyst was synthesized for the dynamic kinetic resolution (DKR) of alcohols with a lipase at room temperature in the air. Furthermore, a polymer-supported derivative was also synthesized and tested as a recyclable catalyst for the aerobic DKR of alcohols.     

The effect of conjugation on the magnitude of secondary alpha deuterium kinetic isotope effects

The unexpectedly small secondary alpha deuterium KIE in the 4-methoxybenzyl chloride-thiophenoxide ion reaction is attributed to the increased conjugation between the aryl group and the alpha carbon in the S_N2 transition state.     

Palladium catalysts for aerobic oxidative kinetic resolution of secondary alcohols based on mechanistic insight

The oxidative kinetic resolution of secondary alcohols has been accomplished using 1:1 complexes of PdCl(2) and N-heterocyclic carbenes. In these reactions, both achiral and chiral carbene ligands are used in conjunction with the chiral base (-)-sparteine. A general synthesis of 1:1 PdCl(2)-carbene complexes has been developed and is amenable to a wide range of carbene ligands. The potential of these complexes in aerobic oxidations is highlighted by the use of a chiral Pd(II) complex and the chiral base (-)-sparteine to enhance the kinetic resolution of a racemic alcohol. [reaction--see text]     

A polymer-supported proline-based diamine catalyst for the kinetic resolution of racemic secondary alcohols.

The preparation of polymer-supported proline-based diamine catalyst 12 for the kinetic resolution of racemic mixtures of secondary alcohols is described. Not only is the catalyst effective for the resolution of a host of different alcohols, it can also be recovered and reused several times without loss of either activity or selectivity. The catalyst has been used in conjunction with a polymer-supported sequestration strategy, giving rise to an essentially pure mixture of resolved products that can be separated using flash chromatography.     

The high mass range time-of-flight secondary ion mass spectra of different polyarene films on silver

 Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to investigate thin films of poly(styrene), poly(vinyl napthalene), and poly(4-chlorostyrene) on clean silver substrate surfaces. The mass spectra were taken in the high molecular mass range (m/z>1000 amu). The different fragmentation patterns found are discussed in detail. Obviously, the fragmentation mechanisms are influenced by the electron density of the aryl rings stabilizing or destabilizing the formed cations. Received: 10 April 1996 / Revised: 12 June 1996/Accepted: 14 June 1996     

The exponents method for calculating equilibrium concentrations of complex species in solution

The exponents method for calculating the concentrations of species in multimetal-multiligand systems is introduced. This method uses the Newton-Raphson method with restricted step iteration, which guarantees a monotonically decreasing objective function. Variable transformation and scaling are performed to avoid underflows and overflows during the calculations. A special linear solver using the eigenvalues and eigenvectors of the Jacobian matrix is implemented for overcoming disastrous singularity of this matrix, and the singular value decomposition method is applied for setting the initial guess. In addition, polynomial extrapolation is used for improving the performance when simulating a diagram of concentrations of species. The method was tested with 14 systems of different sizes over the whole pH range and presented robust and efficient behavior. © 1995 by John Wiley & Sons, Inc.     

New air-stable iron catalyst for efficient dynamic kinetic resolution of secondary benzylic and aliphatic alcohols

We herein report a catalyst system for the dynamic kinetic resolution of secondary alcohols by combining the enzymatic resolution with an iron-catalyzed racemization. A new air-stable tricarbonyl (cyclopentadienone)iron complex is identified as the active racemization catalyst for this transformation without any additive. Various substrates including benzylic, heteroaromatic, aliphatic alcohols can be used and afford the corresponding esters in good yields and with excellent enantioselectivities.     

Catalytic kinetic methods for photometric or fluorometric determination of heavy metal ions

An overview is presented on catalytic kinetic methods for heavy metal determination by photometry or fluorometry reported in recent years. The article summarizes the analytical procedures, the kinetics and reaction mechanisms of some typical methods, and their applications to real sample determination.     

Combined kinetic analysis of solid-state reactions: a powerful tool for the simultaneous determination of kinetic parameters and the kinetic model without previous assumptions on the reaction mechanism

The combined kinetic analysis implies a simultaneous analysis of experimental data representative of the forward solid-state reaction obtained under any experimental conditions. The analysis is based on the fact that when a solid-state reaction is described by a single activation energy, preexponetial factor and kinetic model, every experimental T-alpha-dalpha/dt triplet should fit the general differential equation independently of the experimental conditions used for recording such a triplet. Thus, only the correct kinetic model would fit all of the experimental data yielding a unique activation energy and preexponential factor. Nevertheless, a limitation of the method should be considered; thus, the proposed solid-state kinetic models have been derived by supposing ideal conditions, such as unique particle size and morphology. In real systems, deviations from such ideal conditions are expected, and therefore, experimental data might deviate from ideal equations. In this paper, we propose a modification in the combined kinetic analysis by using an empirical equation that fits every f(alpha) of the ideal kinetic models most extensively used in the literature and even their deviations produced by particle size distributions or heterogeneities in particle morphologies. The procedure here proposed allows the combined kinetic analysis of data obtained under any experimental conditions without any previous assumption about the kinetic model followed by the reaction. The procedure has been verified with simulated and experimental data.