Journal or Book Title
Journal of the American Chemical Society
A comprehensive ab initio study of the adsorption of benzene on the silicon(100) surface is presented. Five potential candidates ([2+2] adduct, [4+2] adduct, two tetra-σ-bonded structures, and one radical-like structure) for the reaction product are examined to determine the lowest energy adsorption configuration. A [4+2] butterfly structure is determined to be the global minimum (−29.0 kcal/mol), although one of the two tetra-σ-bonded structures (−26.7 kcal/mol) is similar in energy to it. Multireference perturbation theory suggests that the [4+2] addition mechanism of benzene on Si(100) is very similar to the usual Diels−Alder reaction (i.e., small or zero activation barrier), even though benzene adsorption entails the loss of benzene aromaticity during the reaction. On the other hand, the [2+2] cycloaddition mechanism is shown 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 barrier relative to reactants is only 8.9 kcal/mol. Careful examination of the interconversion reactions among the reaction products indicates that the two tetra-σ-bonded structures (that are energetically comparable to the [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 to the tetra-σ-bonded structures entails huge barriers (>37.0 kcal/mol) and is unlikely to occur. This suggests that 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).
American Chemical Society
Jung, Yousung and Gordon, Mark S., "Cycloaddition of Benzene on Si(100) and Its Surface Conversions" (2005). Chemistry Publications. 454.