Self-Assembled Trimer Structures Highlight the Competitive Roles of Intermolecular and Adsorbate-Substrate Interactions: PVBA Trimer on Pd(111)

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We observed the orientation of 4-trans-2-(pyrid-4-yl-vinyl)benzoic acid (PVBA) trimers on Pd(111) using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV). The image showed three different types of trimers, one of which does not follow predicted dimer orientations. This type of trimer displays 10° rotations of each molecule in clockwise or counterclockwise directions. Calculations of adsorbate–substrate energy and hydrogen bonding energy revealed that the rotations are a result of competition between adsorbate–adsorbate and adsorbate–substrate interactions.