The coupling mechanism of two propyne molecules on the Cu{111} surface has
been studied by means of a DFT cluster model approach. The gas-phase dimeri
zation is highly unfavored because of the energy cost to activate propyne b
y promoting molecules to the triplet state. However, on the surface, propyn
e is adsorbed with a geometry very close to that of gas-phase propyne in th
e triplet state and, therefore, activation of the reacting molecules does n
ot incur any additional energy cost. Moreover, isomerization to vinylcarben
e is necessary to allow head-to-tail or head-to-head coupling resulting in
1,4- and 1,3-cyclohexadiene intermediates. Vinylcarbene biradicals are pres
ent at the surface because the isomerization process proceeds at practicall
y no (thermodynamic) cost. Both head-to-tail and head-to-head interactions
suggested by experiment are possible. Both cyclohexadiene intermediates can
dehydrogenate to yield benzene and H-2 with a moderate energy cost. An alt
ernative head-to-head interaction, without interacting tails, yields two C-
6 noncyclic intermediates which, upon H-2 addition, can be regarded as bein
g responsible for the 82 amu product observed in the reaction.