Scanning Tunneling Microscopy Investigation of a Chirally Pure Molecule at the Liquid−Solid Interface:  Unambiguous Topographic Markers

The self-assembly of mixtures of hexadecanoic acid with (R)-2-Br-hexadecanoic acid and hexadecanoic acid with (S)-2-Br-hexadecanoic acid have been investigated on graphite at the liquid−solid interface using scanning tunneling microscopy (STM). The STM study of a chirally pure species provides an unambiguous assignment of geometric configurations for various functional groups in these molecules. Specifically, the absolute conformation of the bromine atom in 2-Br-hexadecanoic acid has been determined to be pointing “up” away from the graphite surface. This assignment confirms an earlier conjecture that Br atoms appearing with high tunneling probability in STM images are conformationally in the up position. Furthermore, this geometric designation reinforces an explanation for the spontaneous separation of racemic 2-Br-hexadecanoic acid into enantiomerically pure domains as a process induced by energetically favorable conformations of 2-Br-hexadecanoic acid on a surface.