The Late Barrier

In the entrance valley rAB is large and decreases towards the col. At the entrance to the exit valley rBC is large, and decreases towards the col. At the col rAB = rBC, and this holds for all points along the line, PQ, as drawn. On the profile the activated complex lies at a maximum where rAB = rBC- 

Other situations can occur where the activated complex lies in the entrance valley, an early barrier, or in the exit valley, a late barrier. 

The activated complex and PE barrier are in the entrance valley, where rAH rBC, 

The activated complex and PE barrier are in the exit valley, where rAB rBC, corresponding to a configuration A—-B------C for the activated complex. The 3-D 

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The gas phase results reflected the Evans-Polanyi rule very closely— primarily reactant translational energy needed for the early barrier and primarily vibrational energy for the late barrier, as shown in Figure 20. Li and Wilson ° also found that the gas phase and solution phase vibrational energies tracked each other almost perfectly over the 1 ps they studied, as shown in... 

The late barrier with the elongated H-H bond should, according to the formalism of Chapter 4, be qualified as a loose transition state, in which the molecule is even allowed to rotate in a plane parallel to the surface. Interestingly, we will also find a loose transition state for the dissociation of methane, to be discussed in Section 6.4.2. 

Figure 5.19 Trajectories showing the effect of energy distribution of the reactants on the reactivity. Translational energy of reactants is effective for crossing an early barrier (A), but ineffective for late barriers (D) on the PES. In contrast, the vibrational energy of the reactants helps cross the late barriers (C), but not the early ones (B).

Calculations of QCL trajectories have also been carried out on endoergic reactive systems to explore whether vibrational energy transfer is facilitated by this form of intermolecular potential. It was anticipated that collisions in which the line (/ AB/ e.AB) = ( Bc/ e,Bc) was crossed twice (or a larger number of even times) by trajectories which turned the corner on the surface but failed to surmount the late barrier, might be especially effective in transferring energy. This was confirmed, but these encounters were rare. 

An important further consequence of curvature of the interaction region and a late barrier is tliat molecules that fail to dissociate can return to the gas-phase in vibrational states different from the initial, as has been observed experunentally in the H2/CU system [53, ]. To undergo vibrational (de-)excitation, the molecules must round the elbow part way, but fail to go over the barrier, eitlier because it is too high, or because the combination of vibrational and translational motions is such that the molecule moves across rather than over the barrier. Such vibrational excitation and de-excitation constrains the PES in that we require the elbow to have high curvature. Dissociation is not necessary, however, for as we have pointed out, vibrational excitation is observed in the scattering of NO from Ag(l 11) [55]. 

In the late 1960s a new class of AN copolymers and multipolymers was introduced that contain >60% acrylonitrile. These are commonly known as barrier resins and have found thek greatest acceptance where excellent barrier properties toward gases (5), chemicals, and solvents are needed. They may be processed into bottles, sheets, films, and various laminates, and have found wide usage in the packaging industry (see Barrier polymers). 

With transdermal dosage forms being of great importance of late, it is advisable to test for compatibilities with ointment excipients and with polymers (e.g., ethylvinyl polymer, if that is the desired barrier). 

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

Water exchange proceeds through a trigonal bipyramidal reactive intermediate [Li(H20)5]+, reached via a late transition state. In accordance with the above-mentioned experimental observation of a very fast exchange process, the activation barrier... 

Nickel chloride has been shown to cross the foetomatemal barrier and enter the foetus [297-301]. After injection of nickel chloride, a marked uptake of nickel already in the 5- and 6-day embryo was obtained [299]. In late gestational stages, 63Ni concentrations in most foetuses increase and can even be higher in foetal organs than in maternal ones [298],... 

The thermoneutral reactions mostly follow the same behaviour as with the endothermic reactions. If there are late barriers, vibrational energy is most effective, otherwise the translational energy is relatively more important. 

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