Dynamics and Unimolecular Reactions

Classical dynamics tends to be more accurate for direct short-time dynamical processes, such as the FI - - H2 and F -f H2 reactions, than for processes requiring longer times.The latter may occur if there are potential wells on the potential energy surface giving rise to reaction intermediates. At long times 

The accuracy of classical mechanics for simulating classical reaction dynamics may be summarized by considering reaction [53]. At energies in 

Since classical mechanics allows C2H4F dissociation to occur without zero-point energy in the TS s vibrational modes, the energy distribution of the C2Fi3F + FI products is expected to agree with experiment only at the high-energy limit. If it is correct to assume the unimolecular decomposition 

The above description assumes that an intermediate is formed with statistical classical dynamics and pooling of zero-point energy. If the dynamics of the intermediate is nonstatistical (i.e. as for Cl ---CH3C1 °), the intermediate s lifetime and product energy distribution may agree with experiment. A discussion of the applicability of classical mechanics for studying the central barrier dynamics of the [C1---CH3---C1] moiety is given below. 

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Beil A, Luckhaus D, Quack M and Stohner J 1997 Intramolecular vibrational redistribution and unimolecular reactions concepts and new results on the femtosecond dynamics and statistics in CHBrCIF Ber. Bunsenges. Phys. Chem. 101 311-28... 

He Y, Pochert J, Quack M, Ranz R and Seyfang G 1995 Dynamics of unimolecular reactions induced by monochromatic infrared radiation experiment and theory for C F XI—> C F X + I probed with hyperfine-, Doppler- and uncertainty limited time resolution of iodine atom infrared absorption J. Chem. Soc. Faraday Discuss. 102 275-300... 

Though statistical models are important, they may not provide a complete picture of the microscopic reaction dynamics. There are several basic questions associated with the microscopic dynamics of gas-phase SN2 nucleophilic substitution that are important to the development of accurate theoretical models for bimolecular and unimolecular reactions.1 Collisional association of X" with RY to form the X-—RY... 

The second part (sections H and I) is devoted to a detailed discussion of the dynamics of unimolecular reactions in the presence and the absence of a potential barrier. Section H presents a critical examination of the Kramers approach. It is stressed that the expressions of the reaction rates in the low-, intermediate-, and high-friction limits are subjected to restrictive conditions, namely, the high barrier case and the quasi-stationary regime. The dynamics related to one-dimensional diffusion in a bistable potential is analyzed, and the exactness of the time dependence of the reaction rate is emphasized. The essential results of the non-Markovian theory extending the Kramers conclusions are also discussed. The final section investigates in detail the time evolution of an unimolecular reaction in the absence of a potential barrier. The formal treatment makes evident a two-time-scale description of the dynamics. 

Elements of classical dynamics of unimolecular reactions in particular, the Slater theory for indirect reactions, where the molecule is modeled as a set of uncoupled harmonic oscillators. Reaction is defined to occur when a particular bond length attains a critical value, and the rate constant is given as the frequency with which this occurs. 

Throughout this chapter we have been concerned with statistical approaches applied to isolated molecules. Since most (unimolecular) reactions occur in an environment comprised of other molecules, it is important to examine the effects of molecule-molecule interaction on the kinetics and dynamics of unimolecular reactions. Take the difference between quantum and classical transport as an example. Based on recent studies of quantum-classical... 

This minimally dynamic approach has been applied to both bimolecular and unimolecular reactions a typical result for the latter case is shown in Fig. 6. In this case we consider the dissociation of CCH on two different potential surfaces due to Wolf and Hase.36 These authors classified the first surface (their case IIC) as yielding RRKM dissociation, whereas their surface IIA yielded non-RRKM dynamics. The exact trajectory results for translational, vibrational, and rotational distributions for these two cases are shown as solid histograms in Fig. 6. The minimally dynamic construction, which requires only short-lived trajectory calculations, are shown as dashed histograms in the same figure and are seen to be in excellent agreement with the exact results. 

He Y, Pochert J, Quack M, Ranz R and Seyfang G 1995 Dynamics of unimolecular reactions... 

Intramolecular Dynamics Statistical Theories In sections A—7 we review theoretical and experimental work related to the internal dynamics in unimolecular reactions as opposed to collisional energy transfer. 

Photoinduced unimolecular decomposition reactions are among the simplest reactions which can be studied experimentally and theoretically. One such reaction which has received considerable attention is the vibrational predissociation of small isolated van der Waals (vdW) clusters for which one molecule is a chromophore and the other is a small "solvent" molecule. Two dynamical events may transpire in such a system following the initial photoexcitation to Si vibronic levels vibrational energy may be redistributed to modes other than the optically accessed zero order chromophore states and at sufficient energies the cluster may dissociate. The fundamental theoretical understanding of these two kinetic processes should be accessible in terms of Fermi s golden rulel and unimolecular reaction rate2 concepts. 

This work led to the understanding of intrinsic RRKM and non-RRKM dynamics for unimolecular reactions. ... 

The research presented in this chapter addresses the fundamental atomistic dynamics of unimolecular reactions and their relationships to the RRKM theoretical model. The presentation follows previous reviews of this topic. 

In the preceding sections we discussed cases in which the molecule is first electronically excited to an upper-state PES before it dissociates, in the second step, on this PES. Dissociation can, of course, also take place directly on the ground-state PES, without detour via an excited state. This process is known as unimolecular dissociation and plays an important role in combustion processes or atmospherical chemistry (see Rates of Chemical Reactions and Unimolecular Reaction Dynamics). Experimentally, unimolecular dissociations can be directly measured either by overtone excitation (OH stretching vibration, for example) or by. stimulated emission pumping from an excited electronic state. On the theoretical side, all... 

Steps (5) and (6) are discussed in Rates of Chemical Reactions Transition State Theory and Unimolecular Reaction Dynamics. Here, the focus will be on the first four steps of the procedure outlined above. 

The experiments that seem to show the most promise in studying unimolecular dynamics are the ones in which a molecule is vibration-ally excited by one photon absorption. In very elegant experiments Berry and coworkers have studied the intramolecular and unimolecular dynamics of benzene,methyl isocyanide,and allyl isocyanide containing 4-7 quanta of C-H stretch excitation. For the isomerization of allyl isocyanide nonstatistical state-selected effects were observed. These one photon absorption experiments suggest that state-selected behavior may be prevalent in many unimolecular reactions. However, it is apparent that the sensitivity of the experiments should be improved so that excitation and unimolecular reaction occur in a collision-free environment, thus ensuring the elimination of intermolecular effects that may wash out some of the state-selective characteristics. 

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