Clusters unimolecular reaction kinetics

We have explored in this chapter how quantum mechanical energy flow in moderate-sized to large molecules influences kinetics of unimolecular reactions and thermal conduction. In the first part of this chapter we addressed vibrational energy flow in moderate-sized molecules, and we also discussed its influence on kinetics of conformational isomerization. In the second part we examined the dynamics of vibrational energy flow through clusters of water molecules and through proteins, and we computed thermal transport coefficients for these objects. 

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. 

In more detail, our approach can be briefly summarized as follows gas-phase reactions, surface structures, and gas-surface reactions are treated at an ab initio level, using either cluster or periodic (plane-wave) calculations for surface structures, when appropriate. The results of these calculations are used to calculate reaction rate constants within the transition state (TS) or Rice-Ramsperger-Kassel-Marcus (RRKM) theory for bimolecular gas-phase reactions or unimolecular and surface reactions, respectively. The structure and energy characteristics of various surface groups can also be extracted from the results of ab initio calculations. Based on these results, a chemical mechanism can be constructed for both gas-phase reactions and surface growth. The film growth process is modeled within the kinetic Monte Carlo (KMC) approach, which provides an effective separation of fast and slow processes on an atomistic scale. The results of Monte Carlo (MC) simulations can be used in kinetic modeling based on formal chemical kinetics. 

Hypoiodous acid and iodine nitrate have been investigated with coupled cluster theory and the results extrapolated to the complete basis set limit. Together with revised thermochemistry for several ancillary molecules, the enthalpy changes of working reactions yields new thermochemistry for HOI and IONO2. The latter data, employed in unimolecular rate theory, appear to be consistent with kinetic measurements on the lO -l- NO2 reaction to within the uncertainties of the kinetic analysis. 

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