Direct excitation of higher excited state and kinetics of photoreactions (original) (raw)
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Chemical Physics, 2000
A new linear method for the determination of the rate coecients of complex ®rst-order (or pseudo-®rst-order) mechanisms is presented and applied to simulated data. The errors associated with parameter recovery are compared with those of the traditional nonlinear least-squares method. Nonlinear methods based on convolution kinetics are also developed, and general convolution equations are obtained. Special attention is paid in both cases to excited state kinetics, where concentrations are usually known only up to a constant factor. The monomer±excimer kinetics is discussed in detail, explicit relations for parameter correlation being obtained. The in¯uence of transient eects is also quantitatively discussed. Ó
Introduction to Chemical Kinetics
Springer eBooks, 2017
After introducing the basis of the Arrhenius equation and its relationship to transition-state theory, forms of global chemical kinetic models are summarized, including shrinking core and pseudo nth-order reactions; sigmoidal reactions such as sequential, random scission, autocatalytic, logistic, and nucleation-growth model; and distributed reactivity models, including continuous and discrete activation energy distribution models. Isoconversional and model fitting methods for deriving chemical kinetic models are described, including how to use simple kinetic analyses to derive initial guesses for nonlinear regression of complex models. Common errors that lead to erroneous Arrhenius parameters are outlined.
Basic Concepts in Chemical Kinetics: Molecular Interpretations of Kinetic Phenomena 4.0 INTRODUCTION
In this chapter we treat the descriptions of the molecular events that lead to the kinetic phenomena that one observes in the laboratory. These events are referred to as the mechanism of the reaction. We begin with definitions of the various terms that are basic to the concept of reaction mechanisms, indicate how elementary events may be combined to yield a description that is consistent with observed macroscopic phenomena, and discuss some of the techniques that may be used to elucidate the mechanism of a reaction. Finally, two basic molecular theories of chemical kinetics are discussed: the kinetic theory of gases and transition-state theory. The determination of a reaction mechanism is a much more complex problem than that of obtaining an accurate rate expression, and the well-educated chemical engineer should have a knowledge of and an appreciation for some of the techniques used in such studies. There are at least two levels of sophistication at which one may approach the problem of providing a molecular description of the phenomena that occur during the course of a chemical reaction. At the first level the sequence of molecular events is described in terms of the number and type of molecules and molecular fragments that come together and react in the various steps. The second level of description contains all of the elements of the first but goes beyond it to treat the geometric and electronic configurations of the various species during the different stages of the reaction sequence. For this book, the first level of description is adequate. The second level is more appropriate for study in courses involving physical organic chemistry or advanced physical chemistry.
Chemical Kinetic Modeling: Reaction Principles
There are fundamental concepts or understandings involving chemical reactions which would be obvious to any scientist or engineer dealing with the subject on a routine basis. This may not be the case for the engineer which is assigned the task of developing a chemical reactor model including reactions for the first time. For this reason, these concepts dealing with energy, homogeneous and heterogeneous reactions, as well as the various types of possible reactions between compounds will be presented here without much mathematical foundations but illustrated with diagrams or example. Knowledge of the common reactions found in chemistry could be most useful in assembling elementary reactions for the detailed reaction model.
Chemical Physics Letters, 2011
The classical Pseudophase Model (a two state model) describes well the reactivity in the presence of receptors. This model assumes that the exchange between free and bound reactants is at equilibrium in spite of the reaction, a condition that usually cannot be maintained if the reactants are excited. However, the equation of the Pseudophase Model holds, at least formally, in this case. The formalism developed to clarify this question indicates that the parameters acquire a somewhat different meaning in the case of photochemical reactions. The experiments carried out in the presence of DNA and b-cyclodextrin support the formalism developed.
Calculations of Activation Entropies of Chemical Reactions in Solution
The elucidation of the role of entropic effects in enzyme catalysis is a problem of practical and fundamental interest. In order to address this problem it is essential to develop simulation methods capable of evaluating the entropic contribution, (∆S q)′, of the reacting fragments to the total activation entropy, ∆S q. In fact, the general ability to evaluate activation entropies of chemical reactions in solution has long been a challenge to computational chemists. The present work develops and examines a method for evaluation of (∆S q)′ and ∆S q. This method introduces a thermodynamic cycle that considers the transformation between the reactants state (RS) and the transition state (TS) in two paths. In the first path the reacting fragments are constrained to move along a single reaction coordinate while in the second path they are allowed to move also in the subspace perpendicular to the reaction coordinate. The difference between the activation barriers that correspond to the two paths provides the desired-T(∆S q)′. The cycle also involves two steps where a Cartesian restraint that fixes the reacting fragments in the RS and TS is released. The free energies, ∆G′'s, associated with these restraint release steps are used to complete the thermodynamic cycle and to provide the actual estimate of (∆S q)′. This estimate is optimized by using different initial conditions and by selecting the smallest value of |∆G′|. The solvent contributions to the activation entropy are evaluated using a newly developed version of the Langevin dipole model. The potential surfaces used in the present work are obtained by the empirical valence bond (EVB) method. This method provides analytical yet reliable potential surfaces that reflect properly the motions of the reacting fragments and the coupling between these motions and the solvent polarization. The analytical representation of the EVB surfaces allows us to perform the extensive sampling necessary in order to obtain converging results. Our method is examined by evaluating the activation entropy for the hydrolysis of formamide in water. It is found that the calculated activation entropies reach convergence in a reasonable computer time and that the agreement between the calculated and observed results is reasonable. This method can be easily implemented in studies of enzymatic reactions and helps in assessing the importance of entropic effects in enzyme catalysis.
KINETICS: A computer program to analyze chemical reaction data. Revision 2
1994
This is an informal report intended primarily for internal or limited external distribution. The opinionsand conclusionsstated are thoseof the author andmay or may not be those of the Laboratory. This work was performed under the auspices of the Department of Energy by the Lawrence Livennore National Laboratory under Contract W-7405-Eng-48. \ DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, ream-men&dtion, or favoring by the United States Government or any agency thereof. ?he views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.
ReactionKinetics—A Mathematica package with applications
Chemical Engineering Science, 2012
Requirements are formulated for a reaction kinetics program package to be useful for an as wide as possible circle of users and they are illustrated with examples using ReactionKinetics, a Mathematica based package currently being developed by the authors. Treating a realistic problem in any field of reaction kinetics raises a series of problems, both mathematical and computational: we illustrate a number of these also with examples using our package.
The impact of the rate coefficient over the reaction mechanism
Applied Nanoscience, 2020
Mathematically, complex chemical reactions can be simplified by "model reduction," that is, the rigorous way of approximating and representing a complex model in simplified form. Furthermore, to reduce the dimension of the reaction mechanism, there are different available model reduction techniques (MRT). Without losing the essential information of the system, we have reduced the higher-dimension manifold to low-dimension manifold using spectral quasi equilibrium manifold (SQEM). Near the equilibrium point for different values of rate constant, the solution trajectories of reacting species are examined. The possible approaches to their solution and key problems are based on the Horiuti's rules and their slow invariants. The characteristic properties of nodes and trees of reaction mechanisms are addressed with the graph theory. The impact of the rate coefficient over the equilibrium of the system has been measured. The numerical approximations have been measured using SQEM with MATLAB and the model is analyzed graphically.