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

Abstract

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.

Vibrational Stark shift spectroscopy of catalysts under the influence of electric fields at electrode–solution interfaces

“…Here, we briefly describe the VSFG experimental set-up from the Lian lab, shown in = 1 The SFG signal is emitted in the phase matching direction with its frequency equal to the sum of the frequencies of the two input pulses. (b) A representative VSFG spectra of the nitrile-stretch of 4-mercaptobenzonitrile (4-MBN) on gold as a function of potential, adapted from a publication by Sarkar et al 63 (c) The electrochemical VSFG set-up used in the Lian lab. Figures (a) and (c) are adapted from a publication by Ge et al 64 Stark shift measurements have been successfully reported for a range of catalysts attached to various metal and semiconductor surfaces with suitable anchoring groups.…”

“…Dawlaty and coworkers have carried out several studies focusing on the measurement of interfacial fields using 4-mercaptobenzonitrile (4-MBN) as the Stark effect reporter. 35,63,[67][68][69] These studies utilize covalent attachment of 4-MBN onto a gold surface and measure the stretching frequency of the nitrile group (CN) as a function of different variables to obtain information ranging from the interfacial solvation fields to the correlation between the electrochemical polarization and the Hammett parameters. The (CN) stretching frequency varies with applied potential ( ) and ionic-strength concentration ( ) of the electrolyte solution (KCl), exhibiting a shift that increases linearly with for all .…”

“…For > 0 an electron donating functional group, the opposite situation takes place resulting An < 0. electrode can also polarize the molecules in a similar way and thereby, there is a correspondence between electrochemical polarization and the inductive polarization effect of substituents that determine the Hammett parameter values. To establish this connection, Sarkar et al 63 measured the CN stretching frequency in FTIR spectra for a series of 4-R-substituted benzonitrile molecules, where the R substituents were chosen to span the Hammett parameter range .…”

“…An electrode can also polarize the molecules in a similar way and thereby, there is a correspondence between electrochemical polarization and the inductive polarization effect of substituents that determine the Hammett parameter values. To establish this connection, Sarkar et al 63 measured the CN stretching frequency in FTIR spectra for a series of 4-R-substituted benzonitrile molecules, where the R substituents were chosen to span the Hammett parameter range −0.83 ≤ σ ≤ + 1.11. The polarizing effect (electron donating or withdrawing) of the functional groups directly influence the CN stretching frequencies.…”

Data enhanced Hammett-equation: reaction barriers in chemical space

“…46 Similarly, r has shown to be hardly transferable and even exhibit an inconsistent temperature dependence. 3 Interestingly, Hammett parameters can be inferred from experiments: either by OH vibrational frequencies related to the electron density at the point of bonding, 47 by assessing NMR shis [48][49][50][51] or quadrupole resonance, 52,53 by relation to electron binding energies, 54,55 IR spectroscopy, 56 electrochemical polarization, 57 or charge transfer. 58 Extensive comparison to experiment however, uncovered special cases in which Hammett's model struggles to adequately model reality, partially leading to the introduction of several s values for the same functional group to be used in different molecular environments.…”