Unimolecular reactions polyatomic molecules

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

Nevertheless, predissociation in polyatomic molecules can be visualized as qualitatively similar to a unimolecular reaction in the Rice-Ramsperger-Kassel sense. With diatomic molecules possessing only one degree of vibrational freedom,... 

Desouter-Lecomte, M., Dehareng, D., Leyh-Nihant, B., Praet, M.Th., Lorquet, A.J., and Lorquet, J.C. (1985). Nonadiabatic unimolecular reactions of polyatomic molecules, J. Phys. Chem. 89, 214-222. 

In principle, one can induce and control unimolecular reactions directly in the electronic ground state via intense IR fields. Note that this resembles traditional thermal unimolecular reactions, in the sense that the dynamics is confined to the electronic ground state. High intensities are typically required in order to climb up the vibrational ladder and induce bond breaking (or isomerization). The dissociation probability is substantially enhanced when the frequency of the field is time dependent, i.e., the frequency must decrease as a function of time in order to accommodate the anharmonicity of the potential. Selective bond breaking in polyatomic molecules is, in addition, complicated by the fact that the dynamics in various bond-stretching coordinates is coupled due to anharmonic terms in the potential. 

A number of studies of unimolecular reactions induced by single-photon vibrational overtone excitation have appeared this year. The basic motivation behind such experiments has been the possibility of observing mode-selective photochemistry resulting from the apparently highly localized nature of overtones of X-H stretching vibrations (X = C, O, etc.) in polyatomic molecules. A series of studies by Jasinski, Frisoli, and Moore examined the isomeriza-... 

Most of the ingredients of the model described above have been formerly postulated in treatments of unimolecular reactions. In particular, the model for the barrier dynamics is inherent in the usual TST for unimolecular reactions involving polyatomic molecules, while taking the total molecular energy Ej as the important dynamic variable in the well is the underlying assumption in theories that use a master or a diffusion equation for Ej- as their starting point. 

Theoretical studies of unimolecular reaction kinetics of polyatomic molecules in a mass spectrometer requires the knowledge of the number of states of a molecule with internal energy E, fV(E). It has been shown by Rosenstock et... 

The concepts of laser chemistry are developed along the lines of unimolecular reactions, or, in other words, dissociative processes in the most common sense. The discussion evolves from the photodissociation of diatomic molecules through triatomic species up to larger polyatomic entities (Chapters 15 to 17). Suitable coverage is also given to multiphoton and photoionization processes, which involve the subtle inclusion of intermediate and continuum states (Chapter 18). The part on unimolecular reactions concludes with a discus-... 

The principal experimental methods for measuring bond dissociation energies in polyatomic molecules have been described in detail by Cottrell (35). Two methods in particular have proved fruitful in recent years, namely the toluene carrier gas technique introduced by Szwarc (167) to investigate the unimolecular kinetics of pyrolytic decomposition reactions, and the electron impact method, as applied by Stevenson (165) and widely used since by several investigators. These methods do not usually give very sharp D values, and the number of well-authenticated ( 2... 

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. 

Path Integral Methods Reaction Path Hamiltonian and its Use for Investigating Reaction Mechanisms Reactive Scattering of Polyatomic Molecules State to State Reactive Scattering Statistical Adiabatic Channel Models Time Correlation Functions Transition State Theory Unimolecular Reaction Dynamics. 

Most of the methodology of classical trajectory calculations has been developed for A + BC bimolecular reactions, for which extensive reviews exist.Much of this methodology is transferable to unimolecular studies of polyatomic molecules. However, several modifications pertaining to the selection of initial conditions, the analysis of final results, and the numerical integration of the classical equations of motion are required. These modifictions can be illustrated by considering the reaction given in equation (4). 

The problems in analyzing the results of photochemical (and chemical) activation experiments on unimolecular reactions (see Section 1.4) are closely related to those which arise in studies of fluorescence quenching. Excitation of a polyatomic molecule with monochromatic radiation can produce species with a well-defined energy (e) which exceeds the critical energy (e°) for some unimolecular chemical change. The usual goal in such experiments is to find the specific rate constant, k e defined by... 

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