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Combining density functional theory and high-resolution NC-AFM experiments, we study the on-surface reaction mechanisms responsible for the covalent dimerization of 4-iodobenzoic acid (IBA) organic molecules on the calcite (10.4) insulating surface. When annealed at 580K the molecules assemble in one-dimensional chains of covalently bound dimers. The chains have a unique orientation and are the result of a complex set of processes, including a nominally rather costly double dehalogenation reaction followed by dimerization. First, focusing on the latter two processes and using the Nudged Elastic Band method, we analyze a number of possible mechanisms involving one and two molecules and we isolate the key aspects facilitating the reaction on calcite. Second, we show that the insulating surface plays an active role as a catalyst by identifying two relevant processes: one exhibiting an intermediate state of chemisorbed molecules after independent de-halogenations and a second, highly non-trivial exothermic reaction channel where two iodine atoms “cooperate” to minimize the cost of their individual detachment from the molecules. Both processes have a drastically reduced energy barrier if compared to all other mechanisms analyzed. Knowledge of the formation mechanisms of a covalent assembly on insulators represents an important step towards the realization and control of structures that combine the robustness of covalent architectures with their electronic decoupling from the insulating substrate. This step has potentially important technological applications in nano- and molecular electronics

Mechanisms of Covalent Dimerization on a Bulk Insulating Surface