| Abstract
| - A comprehensive ab initio study of the adsorption of benzene on the silicon(100) surface ispresented. Five potential candidates ([2+2] adduct, [4+2] adduct, two tetra-σ-bonded structures, and oneradical-like structure) for the reaction product are examined to determine the lowest energy adsorptionconfiguration. A [4+2] butterfly structure is determined to be the global minimum (−29.0 kcal/mol), althoughone of the two tetra-σ-bonded structures (−26.7 kcal/mol) is similar in energy to it. Multireference perturbationtheory suggests that the [4+2] addition mechanism of benzene on Si(100) is very similar to the usualDiels−Alder reaction (i.e., small or zero activation barrier), even though benzene adsorption entails theloss of benzene aromaticity during the reaction. On the other hand, the [2+2] cycloaddition mechanism isshown to require a relatively high activation barrier (17.8 kcal/mol), in which the initial step is to form a(relatively strongly bound) van der Waals complex (−8.9 kcal/mol). However, the net activation barrierrelative to reactants is only 8.9 kcal/mol. Careful examination of the interconversion reactions among thereaction products indicates that the two tetra-σ-bonded structures (that are energetically comparable tothe [4+2] product) can be derived from the [2+2] adduct with activation barriers of 15.5 and 21.4 kcal/mol.However, unlike the previous theoretical predictions, it is found that the conversion of the [4+2] product tothe tetra-σ-bonded structures entails huge barriers (>37.0 kcal/mol) and is unlikely to occur. This suggeststhat the [4+2] product is not only thermodynamically the most stable configuration (lowest energy product)but also kinetically very stable (large barriers with respect to the isomerization to other products).
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