Entrance channel barrier

The first photochemical study of this reaction was carried out in 1969 by Oldershaw and Porter [104], who photolyzed static N2O/HI samples at different wavelengths, and used final product analyses to deduce reaction probability versus photolysis wavelength. This provided clear evidence of a substantial entrance channel barrier (i.e., 4400cm ) for the highly exoergic reaction (4a), which was later confirmed and quantified by Marshall et al. [40,41], who carried out experimental rate constant versus temperature measurements as well as ab initio calculations of the stationary points on the potential surface. Oldershaw and Porter were also able to discern the appearance of reaction (4c) with an apparent threshold of 13,500 1400cm, in accord with the thermochemistry, as well as our observations, as discussed below. 

Next, we consider a LEPS PES for the FIj/NiflOO) system (Kara and DePristo, 1989), which is based upon recent ab initio calculations (Siegbahn et al. 1988) along with dynamically adjusted Sato parameters. Since the H—Ni binding energy is 2.7 eV while D h is 1.8 eV, the two-body terms do not dominate for this system. Contour plots in Fig. 25 demonstrate that there are small activation barriers of a 0.035 eV and 0.045 eV in the entrance channel of the bridge - center and atop center PES, respectively. The dependence of the entrance channel barrier on the orientation of H2 has not been determined quantitatively, but the dependence on r is quite weak. The variation with position in the unit cell is also weak. In addition, for the atop -> center dissociation there is also a barrier in the exit channel. The exit channel barrier obviously has a strong dependence upon bond length. Thus, the PES is quite complex. 

The simplest dynamical model of associative desorption is based upon a one-dimensional PES with, of course, a single barrier in the exit channel for desorption (Van Wiligen 1968). (An exit channel barrier for desorption is equivalent to an entrance channel barrier for dissociative chemisorption.) Assuming an equilibrium distribution of adsorbates at the surface temperature T, the model predicts a Boltzmann distribution of velocities with the Z-component of velocity centered around v = (2Fq/M) where Vq is the barrier height. The angular distribution of desorbed molecules is then... 

B3LYP predicts a negative overall barrier if X = Y = Cl (i.e. a barrier between the entry and exit ion-molecule complexes that lies below the entrance channel). Adamo and Barone [79] demonstrated that their new mPWlPW91 (modified Perdew-Wang) functional at least yields the correct sign for this problem. 

The 7-shifting method depends on our ability to identify a unique bottleneck geometry and is particularly well suited to reactions that have a barrier in the entrance channel. For cases where there is no barrier to reaction in the potential energy surface, a capture model [149,150,152] approach has been developed. In this approach the energy of the centrifugal barrier in an effective onedimensional potential is used to define the energy shift needed in Eq. (4.41). For the case of Ai = 0, we define the one-dimensional effective potential as (see Ref. 150 for the case of AT > 0)... 

A barrier that occurs in the entrance channel while the reactants are approaching each other is denoted as an early barrier, whereas a late barrier occurs in the exit channel as the products are separating. 

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