An ab initio insight into the Cu(111)-mediated Ullmann reaction

The coupling process of phenyl radicals-the important intermediates in the prototypal Ullmann reaction-on Cu(111) is addressed using density functional theory. Consistent with experiments, we prove that the fragments interact attractively already at relatively large distances. An intermediate state involving a popping-out'' surface atom is reached through a non-trivial surface diffusion path. The overall process of coupling to the final biphenyl state (with a barrier of 0.38 eV) is governed by a subtle electronic mechanism that reminds the concepts postulated by Hoffmann about the alignment of molecular frontier orbitals with the metallic Fermi level. Our results can be applied to more complex surface processes in the field of molecular self-assembly.