Prepairing mechanism

Believing that a concerted mechanism was not possible, Sinniah et al. [33] considered alternatives. They were particularly concerned with a prepairing mechanism, in which H atoms are paired up on adjacent sites due to an (unspecified) attractive interaction. This would easily rationalize first-order kinetics because the desorption rate would depend on the density of correlated pairs of H atoms, rather than the probability for two uncorrelated atoms to be on adjacent sites. They performed an isotopic mixing experiment... 

Some effects that are left out of cluster calculations are illustrated in the work of Vittadini et al. [87]. They have suggested another prepairing mechanism, in which H atoms on the same side of adjacent singly occupied dimers ( interdimer airing) are more stable than independent singly occupied dimers. These are periodic slab calculations using a large unti eell and a local density functional (tests with the nonlocal Becke-Perdew functional do not... 

Calculations from five different groups have now appeared that conclude that the prepairing mechanism is consistent with experiment [99-103]. These groups have all used density functional theory to approximate the desorption energetics and activation energy from the doubly occupied dimer. This work... 

The only theoretical estimate of a prefactor for desorption is that of Pai and Doren [103], who used transition state theory in the harmonic approximation. Their predicted value for the prepairing mechanism was more than an order of magnitude below the measured value. Given the simplicity of the calculation and the uncertainty in experimental prefactors, it is premature to interpret the disagreement as evidence against the prepairing mechanism. 

Several groups have made initial attempts to calculate dynamics with models of the prepairing mechanism. Convincing agreement with experiment remains elusive, though these calculations include many approximations. 

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