Internal late 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,... 

This simple model would lead one to conclude that H2 dissociation on transition metals, where the unfilled d-states produce a low and early barrier (or even zero barrier), will show no vibrational enhancement, whereas dissociation on simple and noble metals, for which the barrier is high and late, will have vibrationally enhanced dissociation. This appears to be borne out in molecular beam experiments there is no observable increase in dissociation with internal state temperature for H2 on Ni(l 1 1), Ni(l 1 0), Pt(l 1 1) or Fe(l 1 0) [16-19], whereas dissociation on all surfaces of Cu shows an... 

These results demand a reassessment of our basic ideas on sorption kinetics and the role of intracrystalhne diffusion in zeohte-based processes. It seems clear that intracrystalline diffusion can be reliably measured only by microscopic or mesoscopic techniques. In ideal crystals these values should correspond with the values derived from macroscopic measurements of sorption rates, but, since the majority of crystals that have been studied appear to be far from ideal, such a correspondence should not be assumed a priori. Conversely, the role of true intracrystalline diffusion in determining the rates of sorption and catalytic processes may be minimal and we may be forced to conclude that the rates of most large-scale processes are in fact largely influenced or even controlled by surface and internal barriers imre-lated to the ideal zeohte structure. 

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