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Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Combining Electrochemistry and Computational Chemistry to Understand Aryl-Radical Formation in Electrografting Processes Miriam C. Rodríguez Gonzaĺ ez,* Alberto Hernań dez Creus, and Pilar Carro* Á rea de Química Física, Departamento de Química, Facultad de Ciencias, Universidad de La Laguna (ULL), Instituto de Materiales y Nanotecnología (IMN), 38200 La Laguna, Tenerife, Spain

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S Supporting Information *

ABSTRACT: Surface modification has been considered as a fundamental item in upper-division undergraduate and master courses. Thus, the development of experiments regarding processes and characterization of surfacemodification phenomena is highly desired. In this article, we propose an electrochemical and computational study of widely used surfacefunctionalization reactions. Electrochemistry was used as a tool to favor the reactions on a pencil tip, which acted as the electrode. By means of cyclic voltammetry, the reduction of two different precursors of grafting species was characterized. The differences observed in the electrochemical signal for both precursors were rationalized through thermodynamics parameters for the reduction reaction calculated by density functional theory (DFT). A theoretical value can be found with an acceptable error (around 12%) when compared with the electrochemical experimental difference. Consequently, a simple, economic, and complete practical and theoretical exercise is presented as an introduction to the Electrochemistry, Surface Chemistry, and Theoretical Chemistry fields. KEYWORDS: Upper-Division Undergraduate, Graduate Education/Research, Physical Chemistry, Computational Chemistry, Electrochemistry, Hands-On Learning/Manipulatives, Laboratory Instruction

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PEDAGOGICAL GOALS Nanoscale surface modification has been explored as a simple process to control surface properties on a whim.1,2 A wide range of surfaces can be modified by electrografting, such as metals, semiconductors, and carbons, leading to the development of a huge number of methods to carry out their functionalization.3 Among these methods, electrochemistry is a very convenient approach because of its facility and control of the reaction conditions.4 That is the reason why these reactions are used in multiple applications, such as electronics, energy, catalysis, and others.5 In particular, electrografted films allow the easy and versatile modification of different surfaces, and they have been used as modifiers in applications such as sensors and molecular electronic devices and as coupling agents for nanomaterials and biomolecules on surfaces. In this laboratory experiment, the following concepts can be introduced to students: • the use of cyclic voltammetry as a tool to control electrochemical processes and reactions of organic molecules, • the suitability of theoretical calculations to predict electrochemical reactions and the behavior of organic molecules, • the possibility of using the generation of aryl radicals for the modification of a surface.

the most important methods,1,6,7 they can be introduced to the cyclic-voltammetry technique in the laboratory.8 The reduction of 4-nitrobenzene diazonium (NBD) and 1-iodo-4-nitrobenzene (INB) has been characterized. An economic working electrode consisting of the tip of a pencil was used.9 Once the reduction potentials for both molecules have been obtained, a theoretical study to understand the observed differences is carried out. At this point, by means of DFT calculations, the enthalpy and Gibbs-free-energy reduction reactions are evaluated. Thus, an estimation of electrochemical behavior for both molecules can be obtained. Therefore, the exercise gives a global vision of very common electrochemical reactions from an experimental and theoretical point of view.

EXPERIMENTAL OVERVIEW Once students have become familiar with electrochemicalsurface and electrode modification through an introduction to

Received: March 1, 2018 Revised: June 5, 2018

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EXPERIMENTAL SECTION All materials were commercially available and were used as received. The supporting electrolyte was 0.1 M tetrabutyl ammonium tetrafluoroborate (TBABF4, Sigma-Aldrich, 98%), and acetonitrile (Sigma-Aldrich, anhydrous) was used as the solvent. The electroactive species were 10 mM 4-nitrobenzene diazonium tetrafluoroborate (NBD, Sigma-Aldrich, 97%) and 10 mM 1-iodo-4-nitrobenzene (INB, Sigma-Aldrich, 98%). The chemical structure for NBD and INB is shown in Figure 1.

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© XXXX American Chemical Society and Division of Chemical Education, Inc.

A

DOI: 10.1021/acs.jchemed.8b00147 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

reaction’s thermodynamic data for the species implied in the bond cleavage of the precursor following the general scheme ArX → Ar* + X−

(1) −

where ArX, Ar*, and X stand for aryl derivate, the aryl radical, and the fragment (N2 for NBD or I− for INB), respectively (Figure 1). The geometry optimization and frequency calculations were based on density functional theory (DFT)11−13 Accurate energies were computed using the hybrid method proposed by Becke14 at the unrestricted UB3LYP/6-31G(d)15 level of theory for H, C, N, and O and LANL2DZ for I with the same effective core potentials (ECPs) on iodine.16 Additionally, vibrational frequencies were also calculated at the same level of theory used in the optimization procedure in order to correct the electronic-energy values by the inclusion of zero-point energies (ZPE) as well as translational, rotational, and vibrational contributions to the enthalpy at T = 298.15 K.17 The enthalpy and Gibbs free energy of reaction 1 were computed as the differences in the enthalpies and Gibbs free energy for the products and the reactant. Figure 1. Precursors of the aryl radical used in this work: 1-iodo-4nitrobenzene (INB, left) and 4-nitrobenzene diazonium tetrafluoroborate (NBD, right).

ΔHr = H(Ar*) + H(X−) − H(ArX)

(2)

ΔGr = G(Ar*) + G(X−) − G(ArX)

(3)

The solvent effect was introduced into the calculation by considering the electrostatic influence by means of a selfconsistent-reaction-field (SCRF) method. The solvent (ACN) was defined by a continuous medium characterized by its dielectric constant, ε = 35.688, following the conductor-likepolarized-continuum model (CPCM).18−20 Some theoretical tips about the basis of the calculations and the specific steps used to obtain the DFT results are included in the Supporting Information.

In the laboratory, the students used a simple glass cell (Scheme 1). A cyclic-voltammetry experiment was performed in a three-electrode setup: one saturated-calomel reference electrode (SCE), to whose scale all the potentials are referred; a platinum electrode used as the counter electrode; and the working electrode. The working electrode consisted of a carboncontaining material. Specifically, we employed a graphite pencil (Staendler, 8B) as the working electrode,9 which permitted us to obtain cheap, new, and clean electrode surfaces easily with a pencil sharpener. The preparation of the pencil tip is commented upon in the Supporting Information. Theoretical calculations were performed by means of Gaussian 03W10 software and the visualization package GaussView. The exercise involved the calculation of the

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HAZARDS Acetonitrile is highly flammable, an irritant if skin adsorption occurs, and harmful if ingested. TBABF4 is an irritant. Appropriate personal-protection equipment (laboratory coat,

Scheme 1. Aryl-Radical Formation on the Pencil-Tip Electrode (Graphite) through the Electrochemical Reduction of the Precursorsa

a

The inset shows the setup for the electrochemical experiments. WE: working electrode, RE: reference electrode, CE: counter electrode. B




prior to 3D growth, which implies the continuous attack in the meta position of the aryl radical species generated at the electrode−solution interface on the species already grafted on the surface. Although both precursors originated from the same grafted species, the surface modification in each case was different. In Scheme 1, the general mechanism for the reduction of this type of aryl-radical precursor is shown. There are two important mechanisms to be considered, namely, concerted (direct reduction without intermediates) and stepwise (with the generation of an intermediate) mechanisms.8 In our case, the NBD reduction followed a concerted mechanism, whereas the corresponding INB reduction was stepwise.22 In addition, there was another fact of the electrochemical response that was important to analyze: the change in the consecutive cycles for both precursors. Figure 3 shows the electrochemical responses after one, two, and five cycles.

gloves, safety glasses or goggles, etc.) is needed when handling chemicals.

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EXPERIMENTS AND RESULTS This laboratory experiment combines a computational and an electrochemical exercise. Three upper-division students performed the electrochemical experiments as part of their finalgrade project. All the experimental results were repeated at least three times. In addition, 12 graduate students from the course Computational Chemistry and Molecular Modelling as a part of the Master in Chemistry program performed the computational exercises between 2015 and 2018. Both exercises were accomplished by students in two sessions of 3−4 h each. The characterization of the electrode was done first. For this purpose, cyclic voltammetry of the bare graphite surface in a solution without the electroactive species but with the supporting electrolyte was accomplished (see the Supporting Information for details). Once the bare surface was characterized, cyclic voltammograms were registered for the INB and NBD solutions (Figure 2).

Figure 2. Cyclic voltammetry of a graphite electrode in a 10 mM solution of NBD (pink) and a 10 mM solution of INB (cyan) in 0.1 M TBABF4/ACN. Starting potentials of E = 0.75 V (NBD) and 0 V (INB); v = 100 mV s−1.

The product of these electrochemical reactions was the generation of a radical species (aryl radical) that could suffer a grafting process on the surface. This meant the formation of a C−C bond, implying the anchoring of the molecule on the carbon electrode.21 The electrochemical reduction of both precursors can be observed in Figure 2. When the potential was scanned in the cathodic direction (toward more negative potentials), an irreversible wave could be observed. These signals were related to the C−N2+ and C−I bond cleavage for NBD and INB, respectively.22,23 In general terms, the value of the reductionpeak potential is related to the facileness of the reaction.23 This means that the more negative the potential value, the more difficult it is for the precursor reduction to occur. The reduction peak appeared at ≈−1.23 ± 0.02 V for INB and at ≈+0.41 ± 0.03 V for NBD. Thus, the difference between the two signals in the absolute value is around 1.64 ± 0.03 V. When organic layers grow under these experimental conditions, an aryl-based multilayered system is obtained. This is a consequence of the molecule chemisorption on the surface

Figure 3. Cyclic voltammograms obtained for NBD (pink) and INB (cyan). Solid line: first cycle, dashed line: second cycle, dotted line: fifth cycle. The same conditions as those in Figure 2 were used.

The charge of the reduction peak in each run decreased as a result of the partial coverage of the graphite surface after each cycle. This fact is directly related to the production of aryl radicals that could graft on the surface. For NBD, in the first cycle almost all the pencil-tip surface became covered because of the high amount of aryl radicals produced. Nevertheless, after five cycles, the peak corresponding to INB was still visible. The reason for this behavior is the high negative potential for INB reduction (≈−1.2 V). At this potential, certain reduction of the aryl radicals to nongrafting species occurs.22 Thus, only a C




small amount of radicals were formed in each cycle, and available areas could be grafted in consecutive cycles. The second laboratory period is focused on the understanding of the electrochemical differences found for the NBD and INB precursors. For this purpose, two different type of calculations, namely, geometry optimization and frequency calculations, were used. Theoretical support for the basis of the calculations as well as a detailed procedure for carrying them out can be found in the Supporting Information. As commented on in the Experimental Section, from the calculated enthalpy and Gibbs free energy of each species of reaction 1, the enthalpy and the Gibbs free energy of reaction 1 were determined through eqs 2 and 3 The computed electronic energy (at 0 K), enthalpy, and Gibbs free energy (at 298.15 K) for all the species involved in reaction 1 with ACN as the solvent are shown in Table 1 (the results in phase gas are shown in the Supporting Information). Table 1. Comparison by Species of Total Electronic and Zero-Point-Energy Values, Individual-Enthalpy Values, and Gibbs-Energy Values with ACN as the Solvent E0,a uab

ZPE,c ua

H(X),d ua

G(X),e ua

−545.41491 −447.52433 −436.06821 −109.52454 −11.57119

0.10172 0.09279 0.09027 0.00560 0.00000

−545.30357 −447.42217 −435.97023 −109.51563 −11.56883

−545.34798 −447.46756 −436.01046 −109.53738 −11.58804

Species f

NBD INBg Ar*h N2i I −j

Figure 4. Electrostatic potential maps (ESP) for NBD (top) and INB (bottom). Left: precursors. Right: intermediates.

stepwise mechanism, from the stable intermediate, in the case of INB). Next, the electron-transfer reaction from the graphite to the precursor was explained from the point of view of frontierorbital theory. The greater energy gain takes place because the energy difference between the HOMO of the nucleophile and the LUMO of the electrophile is lower. Taking this fact into account, the LUMO energies for NBD and INB in ACN as the solvent have been calculated: −4.54 and −2.72 eV, respectively. The HOMO energy of the nucleophile (the electron) corresponds to the Fermi level of the working electrode, that is, graphite (EF = −4.6 eV). Consequently, for the NBD reaction, the resulting HOMO and LUMO were closer than those for the INB case. In conclusion, NBD behaved as a better electrophile making reaction 1 a more favored process for the NBD precursor (see Figure 5).

a

E0: total electronic energy. bua: hartree energy unit. cZPE: zero-point energy. dH(X): individual enthalpy. eG(X): individual Gibbs energy. f NBD: 4-nitrobenzene diazonium. gINB: 1-iodo-4-nitrobenzene. h Ar*: Aryl radical. iN2: nitrogen molecule. jI−: iodide anion.

From the individual enthalpy values collected in Table 1, the reaction enthalpy could be calculated from eq 2, and the change in Gibbs free energy was obtained from eq 3. The results are shown in Table 2. Table 2. Comparison by Precursor of Reaction-Enthalpy Values and Gibbs-Energy Values with ACN as the Solvent ACN Solvent Precursor a

NBD INBb

−1

ΔHr, kJ mol −478.59 −306.89

ΔGr, kJ mol−1 −524.75 −343.80

a

NBD: 4-nitrobenzene diazonium. bINB: 1-iodo-4-nitrobenzene.

As it was commented before, it is known that for the NBD case, the mechanism is concerted, and for INB, the mechanism is stepwise. This could be explained in terms of the stability of an intermediate after the incorporation of an electron. To this purpose, the representation of the electrostatic maps for both possible intermediates was very enlightening. These maps are shown in Figure 4. As it can be observed, after the incorporation of an electron, the NBD intermediate showed a negative charge (red) located in the nitro group, whereas a positive charge (blue) was found in the diazonium-group position. This charge distribution favors the leaving of the diazonium group rather than the iodide in the INB intermediate, where the resulting charge was resulted uniformly distributed. This explained the different mechanisms followed by both precursors (i.e., a concerted mechanism, from the nonstable intermediate, for NBD and a

Figure 5. Energy-level diagram of graphite and the precursors.

Finally, the correlation of the computational thermodynamic data, reaction enthalpy, and Gibbs free energy with electrochemical data was analyzed. As can be seen in Table 2, the reaction enthalpy for NBD was significantly more negative than that for INB. Thus, the reaction was more favored and exothermic in the case of NBD compared with that of INB, and consequently, the facileness of forming the grafting species was higher. D




Notes

It was interesting to correlate the difference of electrochemical potential between the two precursors, δΔEp = (Ep)INB − (Ep)NBD = −1.23 − (+0.41) = −1.64 V, with the difference of the reaction Gibbs free energy in the presence of the ACN solvent, δΔGp = (ΔGr)INB − (ΔGr)NBD = −343.80 − (−524.75) = 180.95 kJ mol−1. We estimated from the ΔGr difference a theoretical electrochemical potential, ΔEp, making use of the relation to the Gibbs free energy and the maximum electrical work: δ ΔGr = −nF(δ ΔEp)

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.C.R.G. thanks the Spanish MECD for an FPU grant (FPU2014/00886). The authors acknowledge MINECO (ENE2016-74889-C4-2-R, AEI-FEDER-UE).

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(1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press, Inc.: San Diego, CA, 1991. (2) Cea, P.; Martín, S.; Gonzalez-Orive, A.; Osorio, H. M.; Quintín, P.; Herrer, L. Nanofabrication and Electrochemical Characterization of Self-Assembled Monolayers Sandwiched between Metal Nanoparticles and Electrode Surfaces. J. Chem. Educ. 2016, 93, 1441−1445. (3) McCreery, R. L.; Bergren, A. J. Surface functionalization in the nanoscale domain. In Nanofabrication: Techniques and Principles; Stepanova, M., Dew, S., Eds.; Springer: New York, 2012; pp 163−190. (4) Van Benschoten, J. J.; Lewis, J. Y.; Heineman, W. R.; Roston, D. A.; Kissinger, P. T. Cyclic voltammetry experiment. J. Chem. Educ. 1983, 60, 772−776. (5) Mahouche-Chergui, S.; Gam-Derouich, S.; Mangeney, C.; Chehimi, M. M. Aryl diazonium salts: a new class of coupling agents for bonding polymers, biomacromolecules and nanoparticles to surfaces. Chem. Soc. Rev. 2011, 40, 4143−4166. (6) Bard, A. J. Chemical modification of electrodes. J. Chem. Educ. 1983, 60 (4), 302. (7) Ito, T.; Neluni, D. M.; Perera, T.; Nagasaka, S. Gold Electrodes Modified with Self-Assembled Monolayers for Measuring L-Ascorbic Acid: An Undergraduate Analytical Chemistry Laboratory Experiment. J. Chem. Educ. 2008, 85 (8), 1112−1115. (8) Elgrishi, N.; Rountree, K. J.; McCarthy, B. D.; Rountree, E. S.; Eisenhart, T. T.; Dempsey, J. L. A Practical Beginner’s Guide to Cyclic Voltammetry. J. Chem. Educ. 2018, 95 (2), 197−206. (9) Martel, D.; Sojic, N.; Kuhn, A. A Simple Student Experiment for Teaching Surface Electrochemistry: Adsorption of Polyoxometalate on Graphite Electrodes. J. Chem. Educ. 2002, 79, 349. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (11) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864−B871. (12) Kohn, W.; Sham, L. J. Self-consistent equations including exchange and correlation effects. Phys. Rev. 1965, 140, 1133−1138. (13) Parr, R.G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (14) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic-behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (15) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (16) Dunning, T. H., Jr.; Hay, P. J. Gaussian Basis Sets for Molecular Calculations. In Modern Theoretical Chemistry; Schaefer, H. F., III, Ed.; Plenum: New York, 1977; Vol. 3, pp 1−28. (17) Ochterski, J. W. Gaussian, Inc. Personal communication via [email protected], 2000. (18) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical continuum solvation models. Chem. Rev. 2005, 105, 2999−3093. (19) Barone, V.; Cossi, M. Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model. J. Phys. Chem. A 1998, 102, 1995−2001. (20) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies, structures, and electronic properties of molecules in solution with the C-PCM solvation model. J. Comput. Chem. 2003, 24, 669−681. (21) Bélanger, D.; Pinson, J. Electrografting: a powerful method for surface modification. Chem. Soc. Rev. 2011, 40, 3995−4048.

where n is the number of electrons, which in this case is equal to 1, and F is the Faraday constant equal to 96,485 C mol−1. The value obtained for δΔEp is −1.87 V, which is in reasonably good agreement with the experimental δΔEp estimation. Thus, an error of around 12% is found. The origin of this error could be explained in terms of the complexity of the electrochemical process, which has been simplified in this exercise. This electrochemical process includes both thermodynamic and kinetic parameters as well as the important influence of the double-layer structure. Nevertheless, here it has been proved that the thermodynamic parameters for bond cleavage of the precursors ab initio is a good approach for understanding the differences in reduction-peak potentials.

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CONCLUSIONS In this exercise, experimental and theoretical aspects of a very useful method for electrode-surface modification have been combined. The laboratory experiment is based on a simple way to learn fundamental points and concepts of Electrochemistry and Surface Modification. The students observed the differences in the electrochemical behaviors of commonly used molecules, and the computational chemistry allowed them to understand the origins of those differences. The acquisition of the thermochemical parameters of a chemical reaction showed very good results with upper-division students. Increases in the student’s understanding of the performance and consequences of theoretical results have been tested. The electrochemical experiments about electrografting on carbon surfaces have been a good approach to introduce upper-level students to surface modification and electrochemical processes on surfaces. We expect that this activity, which explores the basis of multiple applications in the Nanoscience field, serves as a motivation and inspiration to introduce students to research and teaching in this discipline.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00147. Additional experiments, an instructor guide, and a student handout (PDF, DOCX)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.C.). *E-mail: [email protected] (M.C.R.G.). ORCID

Miriam C. Rodríguez González: 0000-0002-7084-1599 Alberto Hernández Creus: 0000-0002-8793-6236 Pilar Carro: 0000-0001-8073-9857 E




(22) González, M. C. R.; Carro, P.; Creus, A. H. Morphological Changes in Electrografted Aryl-Based Thin Films Induced by using Diazonium Salts or Aryl Iodides. ChemElectroChem 2018, 5, 464−470. (23) Koefoed, L.; Vase, K.; Stenlid, J. H.; Brinck, T.; Yoshimura, Y.; Lund, H.; Pedersen, S. U.; Daasbjerg, K. On the Kinetic and Thermodynamic Properties of Aryl Radicals Using Electrochemical and Theoretical Approaches. ChemElectroChem 2017, 4, 3212−3221.

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