Barrierless reaction path

Molecule/surface systems are typically divided into two main groups, activated and non-activated systems. Activated systems are those in which the molecule needs a minimum energy to dissociate on the surface, i.e., those which present a minimum reaction barrier. On the other hand, systems with at least one barrierless reaction path are called non-activated. In the following, we show some representative quantum dynamics studies for prototypical activated and non-activated systems. 

Ab initio electronic-structure calculations using RHF, MP2, and CCSD(T) levels were employed to study the hydrosilylation reaction catalysed by divalent Ti (modelled by TiH2 and TiCl2). All levels of theory predict a barrierless reaction path compared to a barrier of at least 55kcalmor, obtained at the MP2 level for the analogous uncatalysed reactions. " ... 

For definiteness, consider barrierless adsorption-desorption reaction (2). In the variational TST, the position qf of transition complex TCad in the reaction path (see Figures 9.1 and 9.2) corresponds to a maximum of the Helmholtz free energy cf(qr) of the trial transition complex TCad( r) considered as a function of the reaction coordinate. 

In the present chapter we will describe some examples in which barrierless minimum energy reaction paths along the potential hypersurfaces of several systems will be shown to connect initially excited singlet states with the triplet manifold. Those examples include the isoalloxazine molecule and different DNA nucleobases. 

Once it has been established that a reaction is barrierless, the next question is how to estimate the rate parameters. Conventional transition state theory cannot be applied to reactions without a well-defined energetic barrier along the reaction path. Variational transition state theory, which varies the structure of the transition state, or the position of the dividing... 

This variational version of TST can be used to calculate temperature-dependent rate constants for the barrierless reactions [3-6]. In VTST, the TS state is located at the free energy maximum along the reaction path, and the association rate constant is given by... 

Thus, at first (in the first phase of an elementary act), a barrierless (with respect to q) process occurs as a result of the solvent reorganization. In this case, the system in the final state is in equilibrium with respect to the solvent and is far from equilibrium with respect to the coordinate of chlorine atom. Then a rapid relaxation to the equilibrium value of R takes place, the stretched adsorption bond is contracted, and the excess energy AU is dissipated. Thus, strictly speaking, this process is not barrierless, since a barrier does exist in the reaction path. However, it exists only along the coordinate of discharging particle and is independent of potential. 

Figure 4 Opening of a fast radiationless decay channel via conical intersection for (a) a barrier controlled reaction, (b) a barrierless path, and (c) an uphill path without transition state (sloped conical intersection). M" is an excited state intermediate and FC is a Franck-Condon point.

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