NH₃–NO Coadsorption System on Pt(111). II. Intermolecular Interaction

Coadsorption of ammonia and nitric oxide on the (111) surface of platinum causes the mutual stabilization of the two adsorbed species, arranged in an ordered 2 × 2 mixed layer. Furthermore, their interaction leads also to stable, isolated triangular units, which we observe on the surface after annealing to 345 K. Having provided in the preceding article (10.1021/jp406068y) a detailed structural description of the NH₃–NO mixed layer, we focus here on the stabilizing intermolecular interactions. By combining scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we identify the isolated triangular units as formed by one NH₃ and three NO molecules, and we characterize them in terms of structure, energetics, and charge rearrangement. Eventually, we investigate the nature of the chemical bond between the coadsorbed NH₃ and NO both in the mixed layer and in the isolated triangular units, pointing out the essential role of the surface mediation in inducing attractive dipole–dipole interactions and the presence of hydrogen bonds