Bommisetty V. Rao

2,5-dichlorothiophenol on Cu(111): Initial adsorption site and scanning tunnel microscope-based abstraction of hydrogen at high intramolecular selectivity

We investigated the adsorption of 2,5-di-chloro-thio-phenol (DCTP) on Cu(111) at 15 K and the formation of the thiolate upon electronic and thermal excitation. Initially, the sulfur atom of DCTP adsorbs at an on-top site and the molecule is able to rotate through six almost identical surface orientations. Attachment or removal of electrons from anywhere within the molecule at several hundred mV bias leads to the abstraction of the hydrogen atom from the thiol group in a nonthermal one-electron process with perfect selectivity. The resultant thiolate is locked into position on the surface.

Coverage and nearest-neighbor dependence of adsorbate diffusion

We present data on the coverage and nearest-neighbor dependences of the diffusion of CO on Cu(111) by time-lapsed scanning tunneling microscope (STM) imaging. Most notable is a maximum in diffusivity of CO at a local coverage of one molecule per 20 substrate atoms and a repulsion between CO molecules upon approach closer than three adsites, which in combination with a less pronounced increase in potential energy at the diffusion transition state, leads to rapid diffusion of CO molecules around one another. We propose a new method of evaluating STM-based diffusion data that provides all parameters necessary for the modeling of the dynamics of an adsorbate population.

Measurement of a linear free energy relationship one molecule at a time

A systematic study of the dehydrogenation of substituted thiophenols by controlled charge injection from the tip of a scanning tunneling microscope (STM) reveals a pronounced dependence of the reaction yield on the position and the chemical nature of the substituent. We evaluate the dehydrogenation rate of para-halo-substituted species within a linear free energy relationship, namely the Hammett equation. The resultant ρ value of 1.4 can faithfully predict the reaction rates of molecules that are meta-halo-substituted or para-methyl-substituted. The positive sign of ρ suggests a negatively charged transition state at the core of the STM-induced process, and the magnitude of the ρ value indicates that the presence of the substrate does not preclude substantial substituent effects. The applicability of the Hammett equation to single-molecule chemistry offers facile prediction of the rate of STM-based single-molecule chemistry in a field, which so far has been addressed by focusing on involved quantum-mechanical modeling of its underlying processes.