Scanning tunneling microscopy investigation of a chirally pure molecule atthe liquid-solid interface: Unambiguous topographic markers

The self-assembly of mixtures of hexadecanoic acid with (R)-2-Br-hexadecano ic acid and hexadecanoic acid with (S)-2-Br-hexadecanoic acid have been inv estigated on graphite at the liquid-solid interface using scanning tunnelin g microscopy (STM). The STM study of a chirally pure species provides an un ambiguous assignment of geometric configurations for various functional gro ups in these molecules. Specifically, the absolute conformation of the brom ine atom in 2-Br-hexadecanoic acid has been determined to be pointing "up" away from the graphite surface. This assignment confirms an earlier conject ure that Br atoms appearing with high tunneling probability in STM images a re conformationally in the up position. Furthermore, this geometric designa tion reinforces an explanation for the spontaneous separation of racemic 2- Br-hexadecanoic acid into enantiomerically pure domains as a process induce d by energetically favorable conformations of 2-Br-hexadecanoic acid on a s urface.