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

doi:10.1073/pnas.0406223101

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

Department of Chemistry, University of California, Riverside, CA 92521

Edited by Gerhard Ertl, Max Planck Society for the Advancement of Science, Berlin, Germany, and approved October 15, 2004 (received for review August 24, 2004)

Abstract

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.

Footnotes

↵ * To whom correspondence should be addressed. E-mail: ludwig.bartels{at}ucr.edu.
Author contributions: K.-Y.K. and L.B. designed research; B.V.R. and L.B. performed research; B.V.R., K.-Y.K., and L.B. analyzed data; L.B. wrote the paper; B.V.R. and L.B. acquired data; and A.L. and L.B. developed the instrumentation.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: STM, scanning tunneling microscope; TP, thiophenol.
↵ † The Hammett equation (11) is part of any introductory course in organic chemistry (12).
↵ ‡ The data set obtained in this study comprises several thousand molecular excitations performed during ≈4 months of measurements with >12 sample preparations separated each by bake-out of the home-built STM system, extensive sample cleaning, and tip reforming. The behavior described has been found in all instances and does not reflect observations limited to a small subset of experiments only.
↵ § Our experiments are complementary to gas phase measurements in as much as the substrate provides for nonradiative deexcitation of reactants, facile momentum conservation, and stabilization of the abstracted hydrogen.