Unimolecular reactions computer simulation

These results are complemented by theoretical calculations and computer simulations [110, 111] ford = l,2and3 ofbimoleculartrapping/annihilation reaction A + A—>0, A + T — Ax and A + Ax —> T (T is an immobile trap making A particle to become immobile too) and unimolecular trapping/annihilation, A + A —> 0, A — Ay, A + Ax —> 0. It was found that the kinetics of trapped particles can be described by the mean-field theory for bimolecular but not for unimolecular reactions. The kinetics of free A s is described by mean-field theory at short times, but at long times and low trap concentrations the concentration of free A s decays as (2.1.106). 

In this model the unimolecular constants are relative to the turnover number and the bimolecular constants are chosen to yield equilibrium constants in units of millimolar. The model is primarily based on dead-end inhibition by CrATP, the Michaelis constant for ATP in the ATPase reaction, the isotope partitioning experiments of Rose et al. (65), and various binding and kinetic constants found in the literature. The final model was based on a computer simulation study attempting to discover what combination of rate constants would lit the isotope partition data and the observed kinetic and binding constants. 

Initiated by the chemical dynamics simulations of Bunker [37,38] for the unimolecular decomposition of model triatomic molecules, computational chemistry has had an enormous impact on the development of unimolecular rate theory. Some of the calculations have been exploratory, in that potential energy functions have been used which do not represent a specific molecule or molecules, but instead describe general properties of a broad class of molecules. Such calculations have provided fundamental information concerning the unimolecular dissociation dynamics of molecules. The goal of other chemical dynamics simulations has been to accurately describe the unimolecular decomposition of specific molecules and make direct comparisons with experiment. The microscopic chemical dynamics obtained from these simulations is the detailed information required to formulate an accurate theory of unimolecular reaction rates. The role of computational chemistry in unimolecular kinetics was aptly described by Bunker [37] when he wrote The usual approach to chemical kinetic theory has been to base one s decisions on the relevance of various features of molecular motion upon the outcome of laboratory experiments. There is, however, no reason (other than the arduous calculations involved) why the bridge between experimental and theoretical reality might not equally well start on the opposite side of the gap. In this paper... results are reported of the simulation of the motion of large numbers of triatomic molecules by... 

Assuming that the concept of a rate constant is valid, we might consider using a microscopic theory of unimolecular chemical reactions to predict what the reaction rate should be and then check to see whether the theory is in agreement with that obtained from the computer simulation. The theory most widely used for this purpose is the RRKM theory developed by Rice and Ramsperger,36 Kassel,and Marcus and co-workers. As has been discussed in detail elsewhere,RRKM theory contains the same essential dynamical assumptions contained in transition-state theory. We discuss these assumptions briefly in the next section. 

Classical Dynamics of Nonequilibrium Processes in Fluids Integrating the Classical Equations of Motion Control of Microworld Chemical and Physical Processes Mixed Quantum-Classical Methods Multiphoton Excitation Non-adiabatic Derivative Couplings Photochemistry Rates of Chemical Reactions Reactive Scattering of Polyatomic Molecules Spectroscopy Computational Methods State to State Reactive Scattering Statistical Adiabatic Channel Models Time-dependent Multiconfigurational Hartree Method Trajectory Simulations of Molecular Collisions Classical Treatment Transition State Theory Unimolecular Reaction Dynamics Valence Bond Curve Crossing Models Vibrational Energy Level Calculations Vibronic Dynamics in Polyatomic Molecules Wave Packets. 

The relative compute times required for different ab initio methods are compared in Table 1 for the Cl 4- CH3CI Sn2 reaction." This comparison illustrates the utility of the MP2 method. Though it gives more accurate structures and energies than does HF, the MP2 calculations do not require appreciably more compute time that is, only approximately a factor of 3 more is needed for Cl + CH3CI. At the present time, a very high-level electronic structure theory such as CCSD(T) is not feasible for direct dynamics. Multiconfiguration ab initio methods are practical for direct dynamics simulations, as illustrated by the use of CASSCF in a recent study of the unimolecular dynamics of the cyclopropyl radical. ... 

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