AVS 51st International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM2
The Hammett Equation in Single Molecule Chemistry: Probing a Linear Free Energy Relationship One Molecule at a Time

Monday, November 15, 2004, 8:40 am, Room 210B

Session: Electronic Structure and Excitations
Presenter: L. Bartels, University of California, Riverside
Authors: B.V. Rao, University of California, Riverside
K.-Y. Kwon, University of California, Riverside
A. Liu, University of California, Riverside
L. Bartels, University of California, Riverside
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Selective addressal of specific chemical bonds inside a molecule is at the heart of modern chemistry. With the advent of low-temperature scanning tunneling microscopy (STM), specific bonds of individual molecules became accessible in a controlled manner. Here we present how this technique may be applied to the study of a fundamental pillar of physical organic chemistry, the Hammett Equation, one molecule at a time. Thiophenol and its halo/alkyl-substituted derivates adsorb on Cu(111) at 15K in a horizontal fashion leaving the thiol (-SH) group intact. Excitation by electrons of several hundred meV energy from an STM tip can induce selective dissociation of the SH-bond. This process is confirmed by STM-based vibrational spectroscopy of the S-H stretch mode. We measured the rate of electron-stimulated hydrogen abstraction for p-Fluoro-, p-Chloro-, p-Bromo-, p- Methyl-, m-Fluoro-, and m-Chloro-Thiophenol as well as for the unsubstituted species. The observed dehydrogenation rates follow the @sigma@-values of the Hammett equation, which were derived by pKs measurements on solutions of substituted benzoic acids 70 years ago. The positive @rho@ value of our measurements of 1.4 corresponds qualitatively to the solution phase value for thiophenols suggesting that a negatively-charged transition state is at the core of the STM-based hydrogen abstraction mechanism.