One-Electron Reduction of 2-Nitrotoluene ... - ACS Publications

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One-Electron Reduction of 2‑Nitrotoluene, Nitrocyclopentane, and 1‑Nitrobutane in Room Temperature Ionic Liquids: A Comparative Study of Butler−Volmer and Symmetric Marcus−Hush Theories Using Microdisk Electrodes Eden E. L. Tanner,† Edward O. Barnes,† Peter Goodrich,‡ Christopher Hardacre,‡ and Richard G. Compton*,† †

Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom ‡ School of Chemistry and Chemical Engineering, QUILL Centre, Queens University Belfast, Belfast BT9 5AG, United Kingdom ABSTRACT: The voltammetry for the reduction of 2-nitrotoluene at a gold microdisk electrode is reported in two ionic liquids: trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) and 1ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][NTf2]). The reduction of nitrocyclopentane (NCP) and 1-nitrobutane (BuN) was investigated using voltammetry at a gold microdisk electrode in the ionic liquid [P14,6,6,6][FAP]. Simulated voltammograms, generated through the use of Butler−Volmer theory and symmetric Marcus−Hush theory, were compared to experimental data, with both theories parametrizing the data similarly well. An experimental value for the Marcusian parameter, λ, was also determined in all cases. For the reduction of 2-nitrotoluene, this was 0.5 ± 0.1 eV in both solvents, while for NCP and BuN in [P14,6,6,6][FAP], it was 2 ± 0.1 and 5 ± 0.1 eV, respectively. This is attributed to the localization of charge on the nitro group and the primary nitro alkyl’s increased interaction with the environment, resulting in a larger reorganization energy.

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INTRODUCTION Two theories dominate our current understanding of election transfer, Butler−Volmer and symmetric Marcus−Hush, which both describe a one-electron transfer: (1) A + e− ⇌ B 1,2 Of these two theories, Butler−Volmer theory (B−V; eqs 2 and 3) is used more widely as it reliably parametrizes experimental data.3 It describes the rate constant, kred or kox, in terms of the heterogeneous rate constant, k°/cm s −1, the formal potential, E⊖ f /V, and the transfer coefficient, α, which for a reduction process describes how product- or reactant-like the transition state is, where 0.5 reflects a product-like transition state.4,5 However, B−V theory is phenomenological and so provides little additional physical insight into electron transfer processes. kred and kox are given by ⎡ −αF(E − E ⊖) ⎤ f ⎥ k red = k° exp⎢ RT ⎦ ⎣

(2)

⎡ βF(E − E ⊖) ⎤ f ⎥ kox = k° exp⎢ RT ⎦ ⎣

(3)

ΔG⧧ =

2

⎡ −ΔG⧧ ⎤ ⎥ k° = A exp⎢ ⎣ RT ⎦

(4)

(5)

The reorganization energy, λ, is composed of two components, inner and outer sphere reorganization (λi and λo, respectively, as shown in eq 6). λ = λ i + λo

(6)

Each component is established separately, with the inner sphere, or molecular, reorganization being estimated computationally through changing bond angles and lengths, using methods such as density functional theory,8 and the outer sphere, or solvent, reorganization, through the Born−Marcus solvation equation.9 A comparative method that both critically evaluates B−V and SMH theory and establishes a value for λ and that includes both inner and outer sphere components has been published for a range of compounds in aqueous10,11 and molecular nonaqueous12−14 solvents. Henstridge et al.15 conducted a review of solution-phase redox couples and found that, in cases where

The second theory, symmetric Marcus−Hush theory6,7 (SMH; eqs 4 and 5) relates the heterogeneous rate constant, k°, to the reaction Gibbs energy, ΔG⊖, and reorganization energy, λ: © XXXX American Chemical Society

λ⎛ ΔG⊖ ⎞ ⎟ ⎜1 + 4⎝ λ ⎠

Received: December 13, 2014 Revised: January 22, 2015

A

DOI: 10.1021/jp512419d J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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the transfer coefficient, α, deviated significantly from 0.5, B−V theory was more successful in parametrization of the experimental data than SMH theory. One important insight resulting from these studies16,17 is that the B−V transfer coefficient insofar that it deviates from 0.5 can be understood in terms of asymmetric MH theory. This insight suggests that the transfer coefficient will be 0 (or larger than 0.5 if γ > 0). This parameter reflects the change in force constants between reactants and products. Room temperature ionic liquids (RTILs) consist of asymmetric, bulky cations and inorganic anions18 and are molten at or below room temperature.19 RTILs are useful solvents for electrochemistry,20−23 due to their wide electrochemical windows24 and ability to tailor their charged components to alter properties such as hydrophobicity and viscosity. The structures of two RTILs used throughout this paper, trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) and 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][NTf2]), are shown in Figure 1.

In the present work, a comparative study of B−V theory and SMH theory will be conducted on a range of nitro compounds, 2-nitrotoluene (2-NT), nitrocyclopentane (NCP), and 1nitrobutane (BuN) (structures in Figure 2), and an

Figure 2. Nitro compounds used in this study: 2-nitrotoluene, nitrocyclopentane, and 1-nitrobutane.

experimental λ will be established in each case. 2-NT will be analyzed in the two ILs in Figure 1. 2-NT is aromatic, while both NCP and BuN are aliphatic, which allows us to study the effects of introduction of charge onto the different classes of nitro compounds in an ionic solvent and how this affects the respective abilities of B−V theory and SMH theory to parametrize the data.

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THEORY In this study we investigate the electrochemistry of the three nitro compounds 2-nitrotoluene, nitrocyclopentane, and 1nitrobutane at a microdisk electrode in the ionic liquids [P14,6,6,6][FAP] and [Emim][NTf2], with the aim of first comparing and contrasting two models of electron transfer, Butler−Volmer and symmetric Marcus−Hush theories, in RTIL media and second obtaining and interpreting Marcusian parameters. The electrochemistry is modeled as a single-electron reduction: (7) A + e− ⇌ B Double-potential-step chronoamperometry and cyclic voltammetry measurements are made and simulated, in the latter case using two models for electron transfer kinetics: Butler−Volmer kinetics and Marcus−Hush kinetics. Only species A is assumed to be initially present in solution. A schematic diagram of a microdisk electrode, defining the spatial coordinates used in simulations, is shown in Figure 3. In all cases, the mass transport equation is given by Fick’s second law in cylindrical coordinates:

Figure 1. RTILs used in this study: 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][NTf2]) and trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]).

Computational work has been undertaken to better understand reorganizational processes in ionic solvents. In particular, Lynden-Bell25 has suggested via simulation studies that, in the case of outer sphere electron transfer redox processes, Marcus theory is applicable, and ionic reorganization, although occurring through translation, produces an analogous reorganization energy as is seen with a nonionic polar compound, whereby charged molecules reorient themselves to redistribute the charge. Xiao and Song 26 estimated a value for reorganization in molten salts using molecular Debye−Hückel theory, of ca. 3 eV, which uses the Poisson equation as a starting point and models the solvent as a continuum and the solute as a hard object. A comparative study of B−V theory and SMH theory, as well as the extraction of an experimental λ, was undertaken with the one-electron reduction of oxygen in two ionic liquids.27 It concluded that, similarly to molecular solvents, B−V theory was superior to SMH theory in terms of parametrization of the experimental voltammetry. This finding is consistent with the work in molecular solvents.12 The reorganization energy of the O2 system was determined to be 0.5 eV, which was attributed to inner sphere reorganization.

⎛ ∂ 2c ∂ci ∂ 2c ⎞ 1 ∂ci = Di⎜ 2i + + 2i ⎟ ∂t r ∂r ∂z ⎠ ⎝ ∂r

(8)

−3

where ci (mol m ) is the concentration of species i, t (s) is time, Di (m2 s−1) is the diffusion coefficient of species i, and r and z (m) are the spatial coordinates defined in Figure 3. All symbols are defined in Table 1. Electrical migration may be neglected due to the high ionic strength of an ionic liquid.28,29 This equation is normalized by the introduction of dimensionless parameters to remove the scaling effects of the electrode radius and concentration. These parameters are listed in Table 2, and upon their introduction the mass transport equation becomes ⎛ ∂ 2C ∂Ci ∂ 2Ci ⎞ 1 ∂Ci ⎟ = Di′⎜ 2i + + ∂τ R ∂R ∂Z2 ⎠ ⎝ ∂R B

(9)



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described to within 0.6% error by the Shoup and Szabo equation,30−32 where I = −4nFDA cAre f (τ )

(10)

f (τ ) = 0.7854 + 0.4432τ −0.5 + 0.2146 exp(− 0.3912τ −0.5) (11)

and τ=

DA re 2

t (12)

Simulation of Chronoamperometry. No such equation as the Shoup and Szabo equation exists for double-potential-step chronoamperometry, and hence, numerical simulation is necessary.33 Initial conditions, at time t < 0, are set as cA = cA*

cB = 0

(13)

over the whole simulation space, where cA* is the bulk concentration of species A. At the r = 0 symmetry boundary, and at the insulating surface surrounding the electrode (r ≥ re, z = 0), a zero flux condition is applied to both species at all times:

∂ci =0 ∂n

Figure 3. Schematic diagram of a microdisk electrode, defining the coordinate system used.

where n is the appropriate spatial coordinate r or z. The bulk solution boundaries, rmax and zmax, are set at a distance of 6(Dmaxtmax)1/2 in both directions, where Dmax is the largest diffusion coefficient in the system and tmax is the total time of the experiment. This distance has been shown to be sufficiently far from the edge of the diffusion layer to ensure it represents the bulk solution.34−38 At these boundaries

Table 1. List of Symbols parameter

description

units

α ci c*i c°i Di E E⊖ f F I k° kred/ox λ ν R r re T t z

transfer coefficient concentration of species i bulk solution concentration of species i electrode surface concentration of species i diffusion coefficient of species i applied potential formal potential Faraday constant = 96485 current electrochemical rate constant reduction/oxidation rate constant reorganization energy scan rate gas constant = 8.314 radial coordinate radius of the electrode temperature time z coordinate

unitless mol m−3 mol m−3 mol m−3 m2 s−1 V V C mol−1 A m s−1 m s−1 eV V s−1 J K−1 mol−1 m m K s m

rmax = re + 6 Dmax tmax

definition

Ci D′i K° Kred/ox Λ

ci/cA* Di/DA (re/DA)k° (re/DA)kred/ox (F/RT)λ

dimensionless parameter

definition

R θ τ Z

r/re (F/RT)(E − E⊖ f ) (DA/re2)t z/re

zmax = 6 Dmax tmax

(15)

concentrations are fixed at their bulk values, again at all times. When the first potential step is applied at time t = 0, the boundary conditions at the electrode surface (r ≤ re, z = 0) become cA = 0

⎛ ∂c ⎞ ⎛ ∂c ⎞ DB⎜ B ⎟ = −DA ⎜ A ⎟ ⎝ ∂z ⎠ ⎝ ∂z ⎠

(16)

At some time t = ts, the electrode is subjected to the second potential step, after which the electrode surface boundary conditions become ⎛ ∂c ⎞ ⎛ ∂c ⎞ DA ⎜ A ⎟ = −DB⎜ B ⎟ ⎝ ∂z ⎠ ⎝ ∂z ⎠

Table 2. Dimensionless Parametersa dimensionless parameter

(14)

cB = 0

(17)

These boundary conditions are summarized, in their normalized form, in Table 3. The first step of double-potential-step simulations may be validated by comparison to the Shoup and Szabo equation (eqs 10−12). Cyclic Voltammetry. To simulate cyclic voltammetry, the boundary conditions at the electrode surface must be changed accordingly. The flux of species A through the electrode surface as a result of its net loss via reduction or gain via oxidation is given by

a

Species A refers to the species initially present in solution before the experiment/simulation begins.

⎛ ∂c ⎞ DA ⎜ A ⎟ = k redcA° − koxc B° ⎝ ∂Z ⎠

Chronoamperometry. The Shoup and Szabo Equation. Single-step chronoamperometry at a microdisk electrode is C

(18) DOI: 10.1021/jp512419d J. Phys. Chem. C XXXX, XXX, XXX−XXX


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Table 3. Dimensionless Boundary Conditions for DoublePotential-Step Chronoamperometry Simulations at a Microdisk Electrode

Table 4. Normalized Boundary Conditions To Simulate Cyclic Voltammetry at a Microdisk Electrodea

a

The forms of Kred and Kox depend on the model of electron transfer being used; see the text for details.

state (if α < 0.5, the transition state behaves electrically more like species A; if α > 0.5, it behaves more like species B). As noted above, asymmetric Marcus−Hush theory makes use of an asymmetry factor, γ, which is closely linked to the Butler− Volmer α value.17,39 Marcus−Hush Kinetics. Marcus−Hush theory is often thought to offer a more fundamental physical insight into the electron transfer process, as discussed in the Introduction. Defining the dimensionless parameters

where c°A and c°B are the concentrations of A and B at the electrode surface. The form of the rate constants kred and kox is dependent on which model of electron transfer is being used, Butler−Volmer kinetics or Marcus−Hush kinetics. Applied Potential. In both models of electron transfer, the potential applied to the electrode requires definition. This depends on the start and vertex potentials of the scan, Es (V) and Ev (V), and the scan rate ν (V s−1): E = |Es − Ev − νt | + Ev

(19)

Λ=

which upon introduction of dimensionless parameters becomes θ = |θs − θv − στ | + θv

(20)

⎡ βF(E − E ⊖) ⎤ f ⎥ kox = k° exp⎢ RT ⎦ ⎣

F (ε − E ) (26) RT where ε is the energy of one of the levels in the continuum of levels in the electrode, the rate constants for reduction and oxidation in eq 18 are given by40

kox = k° (22)

where k° (m s−1) is the standard electrochemical rate constant, α and β are the transfer coefficients (α + β = 1), E (V) is the potential applied to the electrode, and E⊖ f (V) is the formal potential of the A/B couple. Using the dimensionless parameters defined in Table 2, these rate constants become (23)

Kox = K ° exp[βθ ]

(24)

Sred(θ , Λ) Sred(0, Λ)

(27)

Sox(θ , Λ) Sox(0, Λ)

(28)

k red = k°

(21)

K red = K ° exp[−αθ ]

(25)

x=

Butler−Volmer Kinetics. In the Butler−Volmer model of electron transfer, the rate constants are considered to be exponentially dependent on the potential applied to the electrode:1,2 ⎡ −αF(E − E ⊖) ⎤ f ⎥ k red = k° exp⎢ RT ⎦ ⎣

F λ RT

where ∞

Sred/ox =

∫−∞

⎡ Λ exp⎢ − 4 1 ± ⎣

(

θ + x 2⎤ Λ ⎦⎥

1 + exp[∓x]

)

dx

(29)

In dimensionless parameters Sred(θ , Λ) Sred(0, Λ)

(30)

Sox(θ , Λ) Sred(0, Λ)

(31)

K red = K °

These boundary conditions are summarized, in their dimensionless form, in Table 4. Butler−Volmer theory has been extensively used in electrochemical simulation, but offers relatively less physical insight beyond an empirical knowledge of the position of the transition

Kox = K °

Table 4 again summarizes these boundary conditions. D



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Reversibility of Cyclic Voltammograms. The heterogeneous rate constant k° is used in both Butler−Volmer and Marcus−Hush theories described above. The magnitude of this parameter has an effect on the observed voltammetry and in some instances may place restrictions on the parameters which may be extracted. Three cases are discussed below. Reversible Voltammetry. If k° is large compared to mass transport, then fast, reversible electrode kinetics will be observed. In this limit, the resultant voltammogram is insensitive to the value of k°, and the waveshape and the peak separation are fixed. This means an actual value of k° cannot be extracted, via simulation or any other means. All that can be said is that k° is above a certain value. Simulated examples of this kind of voltammetry are shown in Figure 4 (circles and black line).

Simulated voltammograms exemplifying this are shown in Figure 4 (blue and green lines). Calculation of the Current. In each experiment, the total flux of species A through the electrode, j, is found by integrating the dimensionless flux at each point over the whole electrode area j = 2π

∫0

1

⎛ ∂C ⎞ R⎜ A ⎟ dR ⎝ ∂Z ⎠ z = 0

(32)

and this dimensionless flux is converted to a current using I = −nFDA cArej

(33)

Numerical Methods. The mass transport equation and boundary conditions must be discretized in space and time to be solved numerically. For double-step chronoamperometry, the same grid as used in previous simulation studies is used.41 In this grid, an initial time point τ0 is defined as 0. The discrete time points then increase uniformly up to some defined time τc, after which the grid expands: τ ≤ τc τc < τ < τs

τk + 1 = τk + Δτ

(34)

τk + 1 = τk + γτ(τk − τk − 1)

(35)

At τs, the point of the potential step, the time grid reverts to its original form and repeats itself, offset by an amount τs. For cyclic voltammetry, a regular time grid is employed, where each unit of θ swept out is divided into Nθ time points. The same form of spatial grid is employed for both chronoamperometry and cyclic voltammetry, which has been successfully employed in the past.27 In the Z direction, the initial spatial point is defined as Z0 = 0. The next spatial point is set at a distance Δs away from Z0, after which the spatial grid expands:

Figure 4. Simulated voltammograms showing the effect of changing the value of k°. Simulated using the Butler−Volmer model with re = 1 mm, DA = 1 × 10−5 cm2 s−1, cA* = 1 mM, α = 0.5, E⊖ f = 0 V, ν = 0.2 V s−1, and k° = 10 cm s−1 (circles), 1 cm s−1 (black), 0.01 cm s−1 (red), 1 × 10−5 cm s−1 (blue), and 1 × 10−6 cm s−1 (green).

Quasi-Reversible Voltammetry. If k° is intermediate in value, then both the waveshape and peak separation are sensitive to it. The forward and (if present) reverse peaks become more separated for smaller values of k°, and the waves become less steep with a smaller peak height. If both peaks are present, then both the peak separation and waveshape can be used while fitting experimental data to simulations to deduce a value of k° (and E⊖ f ). If the back peak is absent, then the waveshape alone can be used. An example of quasi-reversible voltammetry is shown in Figure 4 (red line). Irreversible Voltammetry. In the limit of small k°, the waveshape once again becomes insensitive to the value of k°, but peak separation continues to increase as k° decreases. If then a forward peak and a reverse peak are both observed, k° can be determined from the peak separations. If, however, a back peak is not observed (either because it falls outside the potential window or a homogeneous reaction occurs, trapping the species which would be involved in the formation of this peak), then all we have is a waveshape, which is insufficient to establish a value for k°. Instead, all which may be reported is a composite parameter, k°exp(αE⊖ f F/RT). Changing k° in a simulation will simply move the voltammetric wave to more negative potentials for smaller k° and vice versa. The original response can then be recovered by changing E⊖ f appropriately.

Z0 = 0

(36)

Z1 = Δs

(37)

Zj = Zj + γs(Zj − Zj − 1)

(38)

In the R direction, the spatial grid has the same form and expands away from R = 1 in both directions, until either the center of the electrode (R = 0) or the edge of the simulation space (R = Rmax) is reached. Convergence studies found the following grid parameters appropriate to return a result within 0.5% of a fully converged result. For chronoamperometry, Δτ = 1 × 10−7, τc = 1 × 10−4, γτ = 1.0001, Δs = 1 × 10−5, and γs = 1.1. For cyclic voltammetry, Nθ = 1000, Δs = 8 × 10−5, and γs = 1.25. The mass transport equation and boundary conditions were discretized using the central finite difference method42 and solved in matrix form using the alternating direction implicit (ADI) method43 in conjunction with the Thomas algorithm.44 Programs were coded in C++, and simulation was carried out on a 3.0 GHz Intel Core2 Duo CPU with 3.84 GB RAM. Typical run times are approximately 10 min for doublepotential-step chronoamperometry, 5 min for Butler−Volmer cyclic voltammetry, and 10 min for Marcus−Hush cyclic voltammetry. Simulations are validated by comparison to analytical equations such as the Shoup and Szabo equation30 and the Randles−Ševčı ́k equation for cyclic voltammetric peak heights.45 E



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Error. To quantify the percentage error between simulated and experimental data, the mean scaled absolute deviation (MSAD) is used. The MSAD value is defined as MSAD =

1 N

∑ N

Iexptl − Isim Iexptl

nominal diameter, was polished prior to use using a water− alumina slurry (1, 0.3, and 0.05 μm, 5 min on each grade) on soft lapping pads (Buehler, Illinois).48 Precise radii were determined through calibration of the electrode with a 2.0 mM solution of ferrocene in acetonitrile containing 0.1 M TBAP (silver wire as both the counter and quasi-reference electrodes); chronoamperometry was recorded at 298 K, and assuming a diffusion coefficient of 2.3 × 10−9 m2 s−1,49 the data were analyzed with respect to the Shoup and Szabo equation30 (described in the Theory section). This gave electrode radii of 5.04 ± 0.05 μm (for 2-NT in [P14,6,6,6][FAP] and BuN in [P14,6,6,6][FAP]) and 4.90 ± 0.05 μm (for 2-NT in [Emim][NTf2] and NCP in [P14,6,6,6][FAP]). A 0.5 mm silver wire was used both as the counter and quasi-reference electrodes. Stock solutions were prepared by mixing 0.1 μL of reagent with 20 μL of RTIL. A solution containing 2 μL of the aforementioned stock and 10 μL of RTIL was prepared in a plastic collar fixed on top of the working electrode, and a T-cell was used for structural stability, as described previously.49 A minimum wait time of 15 min was imposed between each experiment to ensure a stable concentration of analyte at the electrode surface. Each experimental cyclic voltammogram was baseline corrected to zero current prior to further analysis.

× 100 (39)

where Iexptl and Isim are experimental and simulated data points and N is the total number of data points. This value is used to optimize the fit between simulated and experimental data for both chronoamperometry and cyclic voltammetry, as described below. Chronoamperometry. The Shoup and Szabo equation (eqs 10−12) can be used to generate theoretical data for the first step of a double-potential-step transient. This is done for a range of values of cA and DA (with a known and fixed re), and each combination is compared to experimental data. Equation 39 is used to generate an MSAD value for each pair of values. This is then repeated for a smaller range of cA and DA around the minimum MSAD until optimal values are reached. These optimal values are then used in the double-potential-step chronoamperometry program described above, and the value of DB is optimized to give a best fit for the second step. Cyclic Voltammetry. To generate an MSAD value for a fit between simulated and experimental cyclic voltammetry, each experimental data point is compared to a linearly extrapolated theoretical value from the two simulated data points surrounding it (since it is unlikely that an experimental data point will be at the same potential as a simulated one). Equation 39 is again used to generate an MSAD for the fit. Parameters are then optimized by minimizing the MSAD value. To avoid the MSAD value becoming misleadingly large by overemphasizing background currents, all data points in a cyclic voltammogram less than 5% of the maximum current are neglected.

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RESULTS AND DISCUSSION The subsequent subsections outline the outcome of experimental voltammetry and associated simulations based on B−V theory and SMH theory for the one-election reduction of 2-NT in two ionic liquids, [P14,6,6,6][FAP] and [Emim][NTf2], first to explore the relative success of the two models and second to compare the Marcusian parameter, λ, for the same analyte in the two solvents. Next, two additional nitro-based compounds, NCP and BuN, are considered in [P14,6,6,6][FAP], again to compare the theories and also to examine the effect on the measured Marcusian solvent reorganization energy and the comparative ability of each theory to parametrize the experimental data. Reduction of 2-NT in [P14,6,6,6][FAP] and [Emim][NTf2]. In this subsection we report the experimental cyclic voltammetry of the one-electron reduction of 2-NT in the ionic liquids [P14,6,6,6][FAP] and [Emim][NTf2] and the comparative outcome of the use of B−V theory and SMH theory to model the experimental voltammograms:

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EXPERIMENTAL SECTION Chemical Reagents. Ferrocene (Fe(C5H5)2; Aldrich, 98%), tetra-n-butylammonium perchlorate (TBAP; Fluka, Puriss electrochemical grade, 99%), 2-nitrotoluene (2-NT; Aldrich, >99%), nitrocyclopentane (NCP; Aldrich, 99%), and 1-nitrobutane (BuN; Aldrich, 98%) were used as received. Ionic liquids trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) (Merck) and 1-ethyl-3methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][NTf2]) were dried under a Schlenk line for ca. 1 week before use, at 70 °C with stirring. [Emim][NTf2] was prepared according to previously reported methods46 and characterized using NMR and CHN analysis. Argon (99.5%), for use with the Schlenk line and glovebox, was purchased from BOC, Surrey, U.K. Instrumentation. All experiments were carried out in an acrylic MBRAUN glovebox (GB-2202-P-VAC) under an inert argon atmosphere. The glovebox was purged in totality a dozen times before initial use. Items entering the glovebox after this initial period were transferred through the antechamber, which went through three purge and refill cycles prior to their introduction to the main chamber. Electrochemical experiments (cyclic voltammetry (CV) and chronoamperometry) were conducted using a μ-Autolab potentiostat (Eco-Chemie, The Netherlands). All experiments were conducted inside a temperature-controlled Faraday cage.47 The working gold microdisk electrode (IJ Cambria Scientific Ltd., U.K.), 10 μm

2‐MeC6H4NO2 + e− ⇌ [2‐MeC6H4NO2 ]•−

An experimental value for solvent reorganization energy, λ, is determined, and physical implications are inferred for both RTILs. Reduction of 2-NT in [P14,6,6,6][FAP]. The first system being examined is the one-electron reduction of 2-NT in [P14,6,6,6][FAP]. This RTIL was selected due to the disparity of sizes between the cation and anion and its relative hydrophobicity compared with other ionic liquids.24 CV was performed on ca. 10 mM solutions of 2-NT in [P14,6,6,6][FAP] over a range of scan rates (100−1000 mV/s). The potential was swept from −1.10 V vs Ag, a potential at which ca. 0 Faradaic current flows, to a potential cathodic of the reduction of 2-NT, −1.60 V (vs Ag), and then back to −1.10 V vs Ag. At a scan rate of 200 mV/s, the peak corresponding to the reduction of nitrotoluene appeared on the forward scan at ca. −1.40 V vs Ag with a peak current of −1.0 nA, while the oxidative peak appeared at −1.27 V vs Ag with a peak current of 0.50 nA. The limiting current was −0.90 nA, and the half-peak F



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Figure 5. Experimental cyclic voltammetry of the reduction of 2-nitrotoluene in [P14,6,6,6][FAP] on a μAu electrode (solid line) at 298 ± 1 K, compared to Butler−Volmer theory (dashed line) and symmetric Marcus−Hush theory (circles), for scan rates of (a) 100 mV/s, (b) 400 mV/s, and (c) 1000 mV/s.

potential was −1.35 V vs Ag wire. All experimental cyclic voltammograms were baseline corrected prior to simulations being conducted. Representative voltammograms are shown in Figure 5. Having established the potential at which 2-NT is fully reduced, double-step chronoamperometry was conducted to determine the precise concentration (cA*) and diffusion coefficients (DA and DB) and was undertaken by moving the potential from −1.15 V vs Ag to −1.60 V vs Ag for 2 s, a point at which 2-NT is fully reduced, and then back to −1.15 V vs Ag for a further 2 s. Three overlaying chronoamperograms were recorded, and the analysis was carried out on the second experiment. The initial step, detailing the reduction, was analyzed with respect to the Shoup and Szabo equation30 (detailed in the Theory section, Figure 6) to yield the bulk concentration (c*A ) and the diffusion coefficient of 2-NT (DA). A fitting program (as described in the Theory section) was used to determine the values of cA* and DA of 2-NT with the lowest possible error, in this case a concentration of 16.3 ± 0.3 mM and a DA of 2.2 ± 0.1 × 10−11 m2 s−1, with an error of 0.68%, calculated as described in the Theory section. The program described in the Theory section was used to simulate the full chronoamperogram, using the values described above for c*A and DA, to obtain a starting point for DB of 8 × 10−12 m2 s−1. The diffusion coefficient of 2-NT, DA, is an order of magnitude larger than that of the reduced species, DB. The disparity in diffusion coefficients is likely to stem from the additional charge on the molecule, which results in an increased interaction with the ionic solvent, as has been reported previously for O2 and O2•−.50 2-NT has not, to our knowledge, been studied electrochemically in any ionic solvents; however, 2-methyl-2nitropropane (2-MeNP) has been studied in acetonitrile, with

Figure 6. Representative experimental chronoamperometry of the reduction of 2-nitrotoluene in [P14,6,6,6][FAP] on a μAu electrode (solid line) compared with fitting obtained using the Shoup−Szabo analysis (circles).

equal diffusion coefficients of 2.7 × 10−9 m2 s−1 measured for both the parent compound and reduced anion. In moving to an ionic solvent, the slower diffusion is expected due to the increased viscosity of the solvent when compared with a molecular solvent. Cyclic voltammograms were first modeled with a program utilizing B−V kinetics (as described in the Theory section). Taking the values of cA*, DA, and DB from the chronoamperograms, optimized values for the remaining parameters (that is, G



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the formal potential, E⊖ f , the transfer coefficient, α, and the heterogeneous rate constant, k°) were obtained, with the fit quantified by calculation of the MSAD (as described in the Theory section). The parameters were kept constant across all scan rates, except the formal potential, which was allowed to deviate slightly (±0.003 V) to account for the use of a pseudoreference electrode. The optimized values and mean MSADs, averaged across all scan rates, are listed in Table 5.

order of magnitude as the analogous nitro compound in acetonitrile discussed above. The cyclic voltammograms were then modeled with a program utilizing symmetric Marcus−Hush kinetics (described in the Theory section). Once again, the values for DA, DB, and cA* were taken from the chronoamperograms, and the other parameters (E⊖ f and k°) were established independently by finding the combination of values with the lowest possible error, as quantified by calculation of the MSAD (described in the Theory section). In the case of SMH theory, a value for reorganization, λ, was determined, as described in the Introduction. Figure 5 shows the experimental cyclic voltammograms with generated voltammograms from both B−V and SMH theories. The values for all of the common parameters (DA, DB, c*A , E⊖ f , and k°) remained the same using both B−V and SMH theory. A reorganization energy of 0.5 ± 0.1 eV was determined, which suggests that the charge on the anion is strongly delocalized on the aromatic portion of the molecule so that relatively little inner sphere contribution is made for the reorganization energy. The MSADs for both theories across the scan rates were comparable, suggesting that both theories are equally capable of parametrization of the data. This contrasts with a previous study of the one-electron reduction of oxygen in an ionic liquid.27 This difference is likely due to the differing transfer coefficients; in the reduction of oxygen the transfer coefficient is ca. 0.3, while for the reduction of 2-NT in [P14,6,6,6][FAP] the transfer coefficient is 0.5. This is consistent with studies conducted in molecular solvents, where deviation in the transfer coefficient from 0.5 results in a

Table 5. Simulation Results for the Reduction of 2Nitrotoluene in [P14,6,6,6][FAP] on a μAu Electrode parameter

Butler−Volmer

symmetric Marcus−Hush

DA/m2 s−1 DB/m2 s−1 concentration/mM k°/cm s−1 E⊖ f /V α λ/eV MSAD(average)/%

(2.2 ± 0.1) × 10−11 (4.3 ± 0.1) × 10−12 16.3 ± 0.3 0.0014 ± 0.0005 −1.305 ± 0.003 0.5 ± 0.05

(2.2 ± 0.1) × 10−11 (4.3 ± 0.1) × 10−12 16.3 ± 0.3 0.0014 ± 0.0005 −1.305 ± 0.003

8.89

0.5 ± 0.1 7.92

Given the quasi-reversible nature of the voltammetry, k° is sensitive to the peak separation and waveshape and is therefore able to be determined by fitting the simulations to this experimental feature, as described in the Theory section. The value for the transfer coefficient, α, of 0.50 ± 0.05 suggests a transition state symmetrically located between reactant and product. The value for k° for the reduction of 2NT in [P14,6,6,6][FAP], 1.4 ± 0.5 × 10−3 cm s−1, is of the same

Figure 7. Experimental cyclic voltammetry of the reduction of 2-nitrotoluene in [Emim][NTf2] on a μAu electrode (solid line) at 298 ± 1 K, compared to Butler−Volmer theory (dashed line) and symmetric Marcus−Hush theory (circles), for scan rates of (a) 100 mV/s, (b) 800 mV/s, and (c) 1500 mV/s. H



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disparity in the relative abilities of B−V and SMH theories to parametrize the experimental data.12 Reduction of 2-NT in [Emim][NTf2]. The next system to be examined is the reduction of 2-NT in the imidazolium-based ionic liquid [Emim][NTf2]. This ionic liquid was chosen for its relatively low viscosity of 34 cP46 and ease of purification. Its component ions are more equally sized than the previously examined RTIL, which allows us to examine any possible sizedisparity effects of the solvents. CV was performed on ca. 10 mM solutions of 2-NT in [Emim][NTf2] over a range of scan rates (10−4000 mV/s) at 298 K. The potential was swept from −0.90 V vs Ag, a value at which no Faradaic current flows, to the potential at which 2-NT is fully reduced, −1.16 V vs Ag, and back to −0.90 V vs Ag. At a scan rate of 200 mV/s, the reductive peak appeared on the forward scan at −1.08 V vs Ag with a peak current of −2.2 nA, while the peak corresponding to the reoxidation of the reduced species appeared on the reverse scan at −0.99 V vs Ag with a peak current of 0.54 nA. The limiting current was −2.2 nA, and the half-peak potential was 1.035 V vs Ag wire. All cyclic voltammograms were baseline corrected prior to the simulations being carried out. Representative voltammograms are shown in Figure 7. Having established the potential at which 2-NT is fully reduced in this solvent, double-step chronoamperometry was carried out at 298 K to establish the precise concentration, c*A , and diffusion coefficients, DA and DB, and was conducted by stepping the potential from −0.91 V vs Ag to −1.15 V vs Ag for 2 s, a potential cathodic of the reduction of 2-nitrotoluene, and then to −0.91 V vs Ag for a final 2 s. Three chronoamperograms were recorded, and as they overlaid, the second experiment was selected arbitrarily for further analysis. The first step, corresponding to the reduction of 2-NT, was analyzed with respect to the Shoup and Szabo equation (described in the Theory section) to yield c*A and DA. A fitting program (described in the Theory section) was run to establish the values for these parameters with the lowest possible error, in this case a concentration of 12.4 ± 0.2 mM and a DA of 8.2 ± 0.1 × 10−11 m2 s−1, with an error of 0.27%, calculated as described in the Theory section. A chronoamperometry simulation program (described in the Theory section) was then run, using the parameters derived above, to ascertain a starting point for the below B−V simulations for DB of 8.2 × 10 −11 m2 s −1. A simulation program using B−V kinetics (described in the Theory section) was used to simulate the voltammograms, and they were then compared with the experimental data collected above (Figure 7). Values for c*A , DA, and DB were informed by chronoamperometry and simulations detailed immediately above, and the other parameters (α, E⊖ f , and k°) were optimized to give the lowest possible calculated MSAD (see the Theory section) across the scan rates. The optimized values and mean MSADs, averaged across all scan rates, are listed in Table 6. The diffusion coefficients measured are of the same order of magnitude, but differ in value, with DA [(8.2 ± 0.1) × 10−11 m2 s−1] being over 3 times greater in magnitude than DB [(2.5 ± 0.1) × 10−11 m2 s−1]. Once again, this is attributable to the charged molecule experiencing a greater interaction with the charged solvent components, but to a lesser degree than seen in the phosphonium ionic liquid. This is likely due to the smaller sized imidazolium cation in Emim NTf2, particularly the lack of

Table 6. Simulation Results for the Reduction of 2Nitrotoluene in [Emim][NTf2] on a μAu Electrode parameter 2

−1

DA/m s DB/m2 s−1 concentration/mM k°/cm s−1 E⊖ f /V α λ/eV MSAD(average)/%

Butler−Volmer


−11

(8.2 ± 0.1) × 10 (2.5 ± 0.1) × 10−11 12.2 ± 0.2 0.022 ± 0.001 −1.006 ± 0.004 0.45 ± 0.05 7.12

(8.2 ± 0.1) × 10−11 (2.5 ± 0.1) × 10−11 12.2 ± 0.2 0.022 ± 0.001 −1.010 ± 0.006 0.5 ± 0.1 6.24

multiple, long alkyl chains that are present in the phosphonium cation. The transfer coefficient, α, is 0.45, which suggests a slightly early transition state. The heterogeneous rate constant, k°, is (2.2 ± 0.1) × 10−2 cm s−1, which is an order of magnitude faster than in the previously examined ionic liquid. As this system is also quasi-reversible, peak separation and waveshape are sensitive to k°, so fitting to these features enabled the rate constant to be determined. A simulation program constructed with SMH kinetics (described in the Theory section) was then run. As occurred previously, values for c*A , DA, and DB were informed by the chronoamperometry and associated chronoamperometric simulations, while the remaining parameters (λ, E⊖ f , and k°) were established through optimization of a fit with experimental data, as reflected by calculated MSAD values. As in the previous section, the values for c*A , DA, DB, and k° were found to be the same as those established using B−V theory. The reorganization energy, λ, was determined to be 0.5 ± 0.1, which again suggests delocalization of the charge on the aromatic molecule. In [Emim][NTf2], both B−V and SMH theories are able to parametrize the data well, with SMH performing slightly but not significantly better. A representative selection of experimental cyclic voltammograms with both B−V and SMH theories overlaid appear in Figure 7. Comparison of Reduction of 2-NT in [P14,6,6,6][FAP] and [Emim][NTf2]. The reduction of 2-NT was studied experimentally and by conducting simulations according to both B− V and SMH theories in a phosphonium-based and an imidazolium-based ionic liquid, as described in the previous section. In [Emim][NTf2], both diffusion coefficients increase relative to those in the phosphonium-based IL; however, DB increases by an order of magnitude. This increased speed of diffusion is likely due to the difference in viscosities of the solvents, whereby [Emim][NTf2] has a viscosity of 34 cP,46 while [P14,6,6,6][FAP] has a viscosity of 464 cP.51 The value of k° also increases by an order of magnitude in moving from the phosphonium-based ionic liquid to the imidazolium-based ionic liquid. The transfer coefficient, α, in [Emim][NTf2] is the same within uncertainty as the reduction in [P14,6,6,6][FAP]. In both cases λ is 0.5 eV, suggesting that the primary contribution to reorganizational energy is inner sphere, or molecular, reorganization. Reduction of Nitrocyclopentane and 1-Nitrobutane in [P14,6,6,6][FAP]. The following subsections detail the experimental CV of the reduction of NCP and BuN in [P14,6,6,6][FAP] and the consequent comparison of the experimental data with B−V and SMH theories: I



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Figure 8. Experimental cyclic voltammetry of the reduction of nitrocyclopentane in [P14,6,6,6][FAP] on a μAu electrode (solid line) at 298 ± 1 K, compared to Butler−Volmer theory (dashed line) and symmetric Marcus−Hush theory (circles), for scan rates of (a) 10 mV/s, (b) 400 mV/s, and (c) 1000 mV/s.

generated values for cA* and DA. A fitting program (described in the Theory section) was used to find the values of cA* and DA that have the lowest calculated MSAD, in this case, 15.0 mM and 1.04 × 10−11 m2 s−1, with an error of 1.22%. A simulation program (as described in the Theory section) was used to obtain a starting point for the below B−V simulations for DB using values for c*A and DA that were obtained above. The diffusion coefficient of NCP, DA, is on the same order of magnitude as that of 2-NT in this solvent, suggesting a similar level of interaction of the molecule with the charged components of the solvent, particularly the cation. However, the larger DB (relative to DA) suggests that fast diffusion of the reduced species is occurring, as the anion is less strongly connected with the IL cation. This is probably partly a size effect. CV simulations were then conducted with a program utilizing B−V kinetics (as described in the Theory section), and the generated voltammograms were compared with the experimental cyclic voltammograms collected as described above (Figure 8). Values for c*A , DA, and DB were taken from the chronoamperometric fitting program discussed above, and the remaining parameters (α, E⊖ f , and k°) were optimized to the lowest possible calculated average MSAD values. The optimized values and mean MSADs, averaged across all scan rates, are listed in Table 7. As this system did not have a discernible back peak, values for k° and E⊖ f were able to be obtained through fitting of the waveshape. This is possible due to the quasi-reversible nature of the reduction, since the waveshape is sensitive to k°, as outlined in the Theory section. The transfer coefficient, at 0.65 ± 0.05, suggests a relatively “late” transition state and contrasts the previously more symmetric transition states seen for 2-NT. The value for the

RNO2 + e− ⇌ RNO2•− R = n‐butyl, cyclopentyl

An experimental value for the reorganization energy will also be determined and described. Reduction of Nitrocyclopentane in [P14,6,6,6][FAP]. NCP was selected as the next compound of interest, as its nonaromatic ring structure provides a good comparison with 2-NT (above) and BuN (below). Cyclic voltammetry was performed on ca. 10 mM solutions of NCP in [P14,6,6,6][FAP] over a range of scan rates (10−2000 mV/s). The potential was swept from a potential at which no Faradaic current flows, −1.60 V vs Ag, to a potential at which NCP is fully reduced, −1.95 V vs Ag, and then back to −1.60 V vs Ag. At a scan rate of 200 mV/s, the peak corresponding to the reduction of NCP appeared on the forward scan at −1.88 V vs Ag, with a peak current of −0.58 nA, while a very small steady-state-like feature appeared on the reverse scan at −1.75 V vs Ag. The limiting current was −0.51 nA, while the half-peak potential was −1.815 V vs Ag. Representative voltammograms are shown in Figure 8. Having once again determined the point at which the analyte is fully reduced, double-step chronoamperometry was conducted to determine precise values for the concentration, cA*, and diffusion coefficient, DA. To undertake this, the potential was jumped from −1.65 V vs Ag and held at −1.95 V vs Ag for 2 s and then returned to −1.65 V vs Ag for a further 2 s. The chronoamperometry was conducted in triplicate, and as the data overlaid, one experiment was subjected to further analysis. The data were then analyzed in accordance with the Shoup and Szabo equation, as described in the Theory section, which J



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suggests a much more significant reorganization than that which occurs with 2-NT, in terms of both inner and outer sphere reorganization. With an average MSAD of ca. 7%, both B−V and SMH theories were equally able to model the experimental cyclic voltammograms and parametrize the data. Reduction of 1-Nitrobutane in [P14,6,6,6][FAP]. The final nitro compound that was considered was BuN. This was chosen as it lacks the ring structure of the previous two compounds; it possesses an alkyl chain instead. Experimental cyclic voltammetry was conducted on ca. 10 mM solutions of BuN at 297 K over a range of scan rates (100−2000 mV/s). The potential was swept from −1.50 V vs Ag, a point at which no Faradaic current flows, to −1.85 V vs Ag, a potential cathodic of the reduction of BuN, and then back to −1.50 V vs Ag. At a scan rate of 200 mV/s, a peak corresponding to the reduction of BuN appeared on the forward scan at −1.73 V vs Ag, with a peak current of −1.61 nA. On the reverse scan, a steady-state-like feature appeared at −1.58 V vs Ag, and even at the highest scan rates, there was no discernible back peak. The limiting current was −1.46 nA, and the half-peak potential was −1.655 V vs Ag. Representative voltammograms are shown in Figure 9. This voltammetry allowed the potential at which BuN is fully reduced to be established, and following this, double-step chronoamperometry was undertaken to establish the precise concentration and to elucidate the diffusion coefficients through further analysis. This was achieved by holding the potential at −1.55 V vs Ag, followed by −1.85 V vs Ag for 2 s, and then returning the potential to −1.55 V vs Ag for a final 2 s. The experimental chronoamperograms were analyzed according to the Shoup and Szabo equation (as described in the

Table 7. Simulation Results for the Reduction of Nitrocyclopentane in [P14,6,6,6][FAP] on a μAu Electrode parameter 2

−1

DA/m s DB/m2 s−1 concentration/mM k°/cm s−1 E⊖ f α λ/eV MSAD(average)/%

Butler−Volmer

symmetric Marcus−Hush −11

(1.04 ± 0.1) × 10 (8.0 ± 0.1) × 10−12 15.0 ± 0.1 0.004 ± 0.0005 −1.855 ± 0.006 0.65 ± 0.05 7.60

(1.04 ± 0.1) × 10−11 (8.0 ± 0.1) × 10−11 15.0 ± 0.1 0.0032 ± 0.0005 −1.855 ± 0.006 2 ± 0.1 7.11

rate constant, k°, was established to be 0.004 ± 0.0005 cm s−1, and is in the same order of magnitude as 2-NT. Simulations were then carried out with a program utilizing SMH kinetics (as described in the Theory section). Similarly to those above, values for cA*, DA, and DB were taken from chronoamperometric simulations, with other parameters (λ, E⊖ f , and k°) optimized through establishment of the best fit to the experimental cyclic voltammograms, as quantified by calculated MSAD values. Once again, values for k° and E⊖ f were able to be obtained despite the lack of a back peak through fitting of the waveshape. Experimental cyclic voltammograms from various scan rates are shown with simulated cyclic voltammograms from both B−V and SMH theories in Figure 8. Values for diffusion coefficients and concentration were the same as established through the B−V program; however, in this case, the rate constant differed slightly, with SMH theory optimizing a slightly smaller value for k°, 0.0032 ± 0.0005 cm s−1. The reorganization energy was optimized to 2.0 ± 0.1 eV, which

Figure 9. Experimental cyclic voltammetry of the reduction of 1-nitrobutane in [P14,6,6,6][FAP] on a μAu electrode (solid line) at 298 ± 1 K, compared to Butler−Volmer theory (dashed line) and symmetric Marcus−Hush theory (circles), for scan rates of (a) 100 mV/s, (b) 400 mV/s, and (c) 2000 mV/s. K



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Theory section) to obtain values for c*A and DA. A fitting program (as described in the Theory section) was used to find values for cA* and DA with the lowest possible error, in this case 18.21 mM and 2.23 × 10−11 m2 s−1, with an error of 0.62%, as calculated in the Theory section. A simulation program, as outlined in the Theory section, was then run, using the values for cA* and DA determined above to establish a starting value for DB of 1 × 10−9 m2 s −1. The value for DA was on the same order of magnitude as those of the other nitro compounds. The concentration of BuN was much higher than those of the other nitro compounds in this IL, indicating a higher solubility of BuN in this ionic liquid. BV simulations were then undertaken, with the values for c*A , DA ,and DB from the above optimization used as a starting point, and the other parameters (α, E⊖ f , and k°) being determined by the generation of a simulation with the lowest calculated MSAD values. The optimized values and mean MSADs, averaged across all scan rates, are listed in Table 8.

elucidated from the BV simulations, while the concentration varied slightly (0.5 mM). The value for k° was on the order of 0.0095 ± 0.0005 cm s−1 and lies between those of 2-NT and NCP. The value established for λ, 5 ± 0.1 eV, is the largest value seen for any of the compounds discussed thus far, indicating that a significant reorganization is occurring. SMH and B−V theories were equally able to parametrize the data, with the average MSAD across scan rates being ca. 5%. Figure 9 shows representative experimental and simulated cyclic voltammograms across the scan rate range. Discussion of the Reduction of NCP and BuN in [P 14,6,6,6 ][FAP]. The reduction of NCP and BuN in [P14,6,6,6][FAP] was studied both experimentally and by conducting simulations informed by B−V theory and SMH theory. The concentration of BuN was much higher than that of NCP, by 5 mM. Both compounds had similar values of DA, but DB was 2 orders of magnitude larger for the reduced form of BuN than for the reduced form of NCP. k° was also an order of magnitude larger in the reduction of BuN than it was in the reduction of NCP. The transition state, α, gleaned through optimization of parameters in B−V theory, was more productlike, at 0.65, in the case of NCP, and closer to symmetric in the reduction of BuN, at 0.55. The value for the reorganization energy established through the use of SMH, λ, was more than 2 times as large in the case of BuN, at 5 eV, when compared to the reduction of NCP, at 2 eV, and much larger than the value extracted for 2-NT, 0.5 eV. These differences are likely attributable to the difference in structure of the molecules. In the case of 2-NT, the lower reorganization is attributable to the delocalization of the charge in the aromatic portion of the molecule. In moving to aliphatic systems, with NCP and BuN, the charge is localized on the nitro group itself, which interacts more stongly with the environment. The primary nitroalkyl BuN will experience a much greater interaction with the other nitro molecules than the secondary nitroalkyl NCP. In both cases, B−V theory and SMH theory were equally adept at parametrization of the data.

Table 8. Simulation Results for the Reduction of 1Nitrobutane in [P14,6,6,6][FAP] on a μAu Electrode parameter

Butler−Volmer


DA/m2 s−1 DB/m2 s−1 concentration/mM k°/cm s−1 E⊖ f α λ/eV MSAD(average)/%

(2.6 ± 0.1) × 10−11 (1.00 ± 0.1) × 10−9 21.5 0.011 ± 0.001 −1.735 ± 0.005 0.55 ± 0.05

(2.6 ± 0.1) × 10−11 (1.00 ± 0.1) × 10−9 21 0.0095 ± 0.0005 −1.735 ± 0.005

5.19

5 ± 0.1 4.46

The lack of a back peak, combined with the extremely large value for DB, which is greater than DA by 2 orders of magnitude, suggests that extremely fast diffusion of the reduced species is occurring. [However, the possibility of secondary reactions cannot be excluded, as it is not easily possible under conditions of convergent or nearly convergent diffusion to distinguish between a simple E mechanism and an EC mechanism with the determined diffusion coefficients, where DB is much larger than DA.] Despite the lack of a back peak, both k° and E⊖ f were able to be extracted through fitting to the experimental waveshape, since the waveshape is sensitive to k° in this quasi-reversible regime, as discussed above in the Theory section. All parameters were kept the same across scan rates, except E⊖ f , which was allowed to vary by ±0.005 V due to the use of a pseudoreference electrode. The value of the transfer coefficient, at 0.55 ± 0.05, is symmetric. k° is significantly larger than the rate constant calculated for the reduction of NCP. A simulation program written with SMH kinetics (as described in the Theory section) was used to generate voltammograms that were compared with the experimental data collected above. Data from the chronoamperometric simulations above were used as a starting point for values of cA*, DA, and DB. In the case of SMH theory, the other parameters (λ, Ef⊖, and k°) were optimized through fitting to the experimental data, as quantified by the lowest calculated MSAD values. As in B−V theory, values for k° and E⊖ f were able to be extracted through fitting of the waveshape. All parameters were kept constant across the scan rates, except E⊖ f , which was allowed to vary by ±0.005 V due to the use of a pseudoreference electrode. The values for D A and D B established by SMH theory were consistent with the values

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CONCLUSIONS The reduction of 2-nitrotoluene at a gold microdisk electrode in two ionic liquids ([P14,6,6,6][FAP] and [Emim][NTf2]) was examined with cyclic voltammetry at 298 K. The reduction of nitrocyclopentane and 1-nitrobutane at a gold microdisk electrode in the ionic liquid [P14,6,6,6][FAP] was also studied with cyclic voltammetry at 298 K. B−V and SMH theories were used to create a program to simulate voltammograms, which were then compared with the experimental voltammetry obtained above. In all cases, B−V and SMH theories were equally capable of parametrization of the data. The Marcusian parameter, λ, from SMH theory, was determined to be 0.5 ± 0.1 eV for the reduction of 2-NT in [P14,6,6,6][FAP] and [Emim][NTf2], 2 ± 0.1 eV for the reduction of NCP in [P14,6,6,6][FAP], and 5 ± 0.1 eV for the reduction of BuN in [P14,6,6,6][FAP]. This increase in reorganization energy likely corresponds to the charge being delocalized on the aromatic portion of the molecule in the case of 2-NT and to the localization on the nitro group itself in the case of NCP and BuN. The increased interaction with the environment of the BuN molecule as compared with NCP likely explains the increased reorganization energy. L



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +44(0) 1865 275957. Fax: +44 (0) 1865 275410. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS For funding, E.E.L.T. thanks the Clarendon Fund and St. John’s College, Oxford, and E.O.B. thanks the Engineering and Physical Sciences Research Council (EPSRC). Merck is thanked for supplying the [P14,6,6,6][FAP].

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