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Electrodes as Polarizing Functional Groups: Correlation between Hammett Parameters and Electrochemical Polarization

Sarkar, Sohini, Patrow, Joel G., Voegtle, Matthew J., Pennathur, Anuj K., Dawlaty, Jahan M.
Journal of physical chemistry 2019 v.123 no.8 pp. 4926-4937
electric field, electrochemistry, electrodes, gold, moieties, silver, silver chloride
Bridging the concepts of homogeneous and heterogeneous reactions is an important challenge in modern chemistry. Toward that end, here, we connect the homogeneous chemistry concept of the Hammett parameter, used by organic and organometallic chemists to quantify the electron-withdrawing capability of a functional group, to the electrochemical concept of polarization induced by a biased electrode. Because these two effects share similar origins, a theoretically motivated and experimentally verifiable link between them can be established. A convenient experiment that links the two is measuring the shift of vibrational frequency that is induced by these factors. To achieve this, first, we have measured the vibrational frequency of the nitrile stretch of 4-R-benzonitrile for a series of functional groups R spanning the Hammett parameter range −0.83 ≤ σₚ ≤ +1.11. Because the nitrile stretch is sensitive to molecular polarization, its frequency depends on the Hammett parameter of the polarizing functional groups. Second, we have measured the nitrile vibrational frequency of 4-mercaptobenzonitrile tethered on a gold electrode and polarized in an electrochemical cell as a function of potential from −1.4 to +0.6 V versus Ag/AgCl. Comparison of the nitrile-stretch frequency between the two experiments allows us to correlate the polarization caused by a functional group to that induced by the electrode. The data suggest equivalence between the Hammett parameter σₚ and the local electric field at the electrode interface, therefore allowing a polarizing electrode to be treated as a functional group. Computational work supports the experimental results and allows for a quantitative relation between the interfacial electric field and σₚ. We anticipate the benefits of this correlation, in particular, in linking concepts between homogeneous and heterogeneous reactions.