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Molecular Level Understanding of the Factors Affecting the Stability of Dimethoxy Benzene Catholyte Candidates From First Principles Investigations Rajeev S. Assary, Lu Zhang, Jinhua Huang, and Larry A Curtiss J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04263 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

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The Journal of Physical Chemistry

Molecular level Understanding of the Factors Affecting the Stability of Dimethoxy Benzene Catholyte Candidates from First principles Investigations Rajeev S. Assarya,b*, Lu Zhanga,c, Jinhua Huanga,c, Larry A. Curtissa,b a) Joint Center for Energy Storage Research, Argonne National Laboratories, Argonne, IL, USA, 60439 b) Materials Science Division, Argonne National Laboratory, Argonne, IL, USA, 60439 c) Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL, USA, 60439 *: corresponding author RSA

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Email: [email protected], Tel: 630-252-3536, Fax: 630-252-9555

Abstract

First principles simulations are performed to gain molecular level insights into the factors affecting the stability of seven 1,4-dimethoxy benzene (DMB) derivatives. These molecules are potential catholyte candidates for non-aqueous redox flow battery systems. Computations are performed to predict oxidation potentials in various dielectric mediums, intrinsic-reorganization energies, and structural changes of these representative catholyte molecules during the redox process. In order to understand the stability of the DMB-based radical cations, the thermodynamic feasibility of the following reactions is computed using density functional theory: (a) deprotonation, (b) dimerization, (c) hydrolysis, and (d) demethylation. The computations indicate that radical cations of the 2, 3-dimethyl and 2, 5-dimethyl derivatives are the most stable among the DMB derivatives considered in this study. In the presence of solvents with high-proton solvating ability (water, dmso, acetonitrile), degradation of cation radical occurring via deprotonation is the most likely mechanism. In the presence of solvents such as propylene carbonate (PC), demethylation was found to be the most likely reaction that causes degradation of radical cations. From the computed enthalpy of activation (∆H‡) for a demethylation reaction in PC, the 2, 5-dimethyl DMB cation radical would exhibit better kinetic stability in comparison to the other candidates. This investigation suggests that computational studies of structural properties such as redox potentials, reorganization energies) and the computed reaction energetics (deprotonation and demethylation) of charged species can be used to predict the relative stability of a large set of molecules required for the discovery of novel redox active materials for flow battery applications.

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Introduction

Effective energy storage in organic molecules has a role to play in the area of the grid electricity storage due to the increased production of intermittent renewable electricity 1-3. Redox couples for such applications need to be chemically and electrochemically (oxidation and reduction) stable to perform thousands of cycles with no significant degradation. In a redox couple, molecules that undergo oxidation and reduction are classified into ‘catholyte’ and ‘anolyte’, respectively. Prerequisites of ideal candidates for flow battery applications are their adequate electrochemical window, solubility, fast redox kinetics, stability, and cost. Detailed molecular level understandings of all the above properties are crucial to the development of new and improved candidates for grid storage applications. Recently, the use of redox active organic molecules for energy storage has made exceptional progress in aqueous systems4-7, while, finding desirable redox flow molecules (catholytes and anolytes) in nonaqueous media was found to be exceptionally challenging8-11. Among, many possible catholyte candidates as redox active materials in non-aqueous media, 2,5-ditert-butyl-1,4-dimethoxy benzene was found to be promising material in the battery operating conditions by Dahn et al. 12-13.These materials have a reasonable oxidation window (> 3.5 V Li/Li+) and stability as radical cations compared to many materials investigated. 14 Similarly, further studies suggest that derivatives of 1,4,-dimethoxy benzene (DMB) can be used as stable material as redox shuttle in lithium ion batteries14-16. In general, due to the electrochemical window, any DMB derivatives with solubility and stability can be used as catholyte materials in flow batteries. In terms of assessing the feasibility of redox materials, there have been many studies of their redox properties.17-23 However, the reasons for the differences in stability of organic molecules for flow batteries are poorly understood due to the complexity of the reactivity patterns24-25 of the radical cations (RC) in solution. However, information regarding the reactivity of radical cations is critical towards understanding the factors that control stability. Therefore, in this investigation we utilize the predictive ability of density functional theory (DFT) to gain insights into factors controlling the stability of seven catholyte candidates based on DMB derivatives. The structures of the seven molecules are shown in Figure 1.

Figure 1. Schematic of dimethoxy benzene (DMB) derivatives considered in this study.

A critical bottleneck for understanding the stability is the complexity of the radical cations due to their many intrinsic and extrinsic properties. Intrinsic properties of radical cations include: (i) delocalization of spin and charge, (ii) release of strain, (iii) stereo electronic factors, and (iv) steric hindrance. Extrinsic


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properties include: (v) solvent medium, (vi) anode materials and (vii) electrolytes. A detailed understanding of these intrinsic and extrinsic effects requires comprehensive experimental and theoretical studies. In this paper we present a theoretical investigation focused on understanding key features that affect the stability of radical cations (RC) formed during the oxidation of the catholytes. These features and associated quantum chemical descriptors that can be computed are schematically shown in Figure 2. We present results for the electrochemical windows, structural distortion, and reactivity of radical cations of the seven DMB derivatives. The oxidation potential and reorganization energy are hypothesized as the descriptors for electrochemical window and structural distortion, respectively. Additionally, computed free energies of reactions such as deprotonation, dimerization, hydrolysis, and demethylation of cation radicals are hypothesized as the descriptors for their reactivity.

Figure 2. Key properties controlling the stability of redox active species and the quantum chemical descriptors that can assess the properties are schematically shown. Note: G, E, and λ represent Gibbs free energy, redox potential, and reorganization energy, respectively

Details of the computational approach employed in this investigation are presented in the next section. In the results and discussion section, computational are studies presented that rank the stability of selected catholyte molecules using structural and energetic parameters.

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Computational Details

All computations were performed using the Gaussian 09 software26. The B3LYP/6-31G(2df,p) level of theory was used to compute the structure, electronic energy, vibrational frequencies, and free energy corrections of all species. Solvation free energies were computed with the SMD solvation model by performing single point energy evaluations with a specific dielectric medium: diethyl ether (ε =4.24),


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acetone (ε =20.49), dimethyl sulfoxide (ε = 46.83), or water (ε =78.36). Reorganization energies were computed (section 4.12) from the structures of the molecules optimized in the diethyl ether solvent medium. We note that explicit interactions of solvent molecules may be essential to represent charged states of the molecules/ions are not included in this investigation due to the computational complexity; which is beyond the scope of this manuscript. The oxidation potentials (EOx w.r.t. Li/Li+) were computed from the computation of Gibbs free energy change (ΔGOx, eV ) at 298 K in the solution (dielectric) for the removal of an electron from the species of interest, using the following equation; =

∆

− 1.24 , where F is the Faraday constant (in eV) and n is the number of electrons involved

in the oxidation process. The addition of the constant ‘-1.24 V’ is required to convert the free energy changes to oxidation potential (Li/Li+ reference electrode), a commonly used convention to compute the redox potentials in solution27-28. The change in energy of electrons when going from vacuum to nonaqueous solution is treated as zero, similar to what has been used by others29. Further details regarding the computation of redox potential can be found elsewhere29-34. Solvation energy contributions of a species are added to the gas phase enthalpies and free energies to approximate enthalpies (Hsoln = Hgas phase + ΔGsolv) and free energies (Gsoln = Ggas phase + ΔGsolv) in solution. The demethylation (C-O bond breaking) reaction barriers presented in Section 4.2.2 are apparent enthalpy barriers (see SI for activation free energy barriers), computed as the difference between the enthalpy of the transition state structure (H†) and the sum of enthalpies of reactants in solution at 298 K. Here we used the solution phase enthalpy approximation because the main free energy contributions (GCDS: cavitation, dispersion, and solvent structure terms) cancel out (or are negligible) when computing the apparent barriers (H(TS)-H(reactants)). Therefore, the dominant solvation contributions are electronic, nuclear, and polarization terms (GENP) which are included in the computation of solution enthalpies. A similar approach was successfully employed elsewhere35. The B3LYP/6-31G(2df,p) level of theory is used to compute the transition state structures in the gas phase and subsequently, a single point solvation energy calculation is performed using the SMD solvation model (diethyl ether and water) at the same level of theory. Thus, the enthalpy barriers (∆H†) presented in section 4.2.2 are the sum of the gas phase enthalpy barrier and the solvation energy (ESMD): ∆ = − − !" + !" .

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Results and Discussion

In this section, computation of various structural and reactivity related properties (Figure 2) of the seven DMB-based molecules are described. The structure related properties, electrochemical windows and structural deviation upon oxidation, are discussed in section 4.1. The reactivity related properties such as thermochemical data for deprotonation, dimerization, hydrolysis, and demethylation reactions of radical cations and their relevance are discussed in section 4.2.


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4.1 Structure Three properties related to structural stability of the dimethoxy benzene (DMB) derivatives were computed at the B3LYP/6-31G(2df,p) level of theory. These properties are: (i): oxidation potential, (ii) reorganization energy (λ) and (iii): the Kabsch root mean square deviation (RMSD) between the optimized coordinates of the oxidized and neutral state of the molecules.

Figure 3.Schematic of the computation of reorganization energy during oxidation (λOx) and reduction (λRed) process from the computed energies of a molecule in its neutral and singly positive charge state. Total reorganization energy ((λTotal) is the sum of reorganization energy required during the oxidation (λOx) and reduction (λRed) process. Table 1. Computed oxidation potentials (V), reorganization energies (λTotal, λOx ), and root mean squared deviation (RMSD) between the optimized Cartesian coordinates of neutral and oxidized species of DMB derivatives (see figure 1). Note: The total reorganization energy (λTotal) is defined as the sum of the reorganization energy during the oxidation (λOx) and reduction 12 (λred). Abbreviations, DEE: diethyl ether:, DMSO: dimethyl sulfoxide, H2O: water Notes: (a) : experimentally measured oxidation potentials in propylene carbonate are 3.9 and 4.1V for DMB (a) and 2356-DMB (b) molecules respectively.

Species

DMBa 2-DMB 23-DMB 25-DMB 26-DMB 235-DMB 2356-DMBb

Computed oxidation potential (V, Li/Li+) EDEE 4.2 4.1 3.9 3.9 4.1 4.5 4.3

EAcetone 3.8 3.7 3.6 3.6 3.8 4.1 3.9

EDMSO 3.7 3.7 3.5 3.5 3.7 4.1 3.9

EH2O 3.8 3.7 3.6 3.6 3.7 4.1 3.9

Reorganization energy (kcal/mol) λOx λTotal 5.6 11.3 5.5 11.2 15.6 19.7 5.4 11.1 10.9 21.3 9.1 20.4 14.6 31.9


Kabsch RMSD 0.04 0.04 0.63 0.04 0.59 0.44 0.52

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4.1.1 Simulated oxidation potentials (EOx) The electrochemical windows of the seven DMB derivatives in various dielectric media including diethyl ether (DEE) (ε=4.24), acetone (ε =20.49), dimethyl sulfoxide (DMSO) (ε =46.83), and water (ε =78.36) were computed and are presented in Table 1. In a low dielectric medium such as in diethyl ether, the oxidation potentials are in the range of 3.9-4.5 V for the DMB candidate molecules. Among the molecules in diethyl ether medium, the 25 DMB and 23 DMB exhibit lower oxidation potentials (3.9 V each), while 235-DMB exhibits a high oxidation potential (4.5 V). At high dielectrics such as DMSO (ε =46.83) and water (ε =78.36), the computed oxidation potentials are on the order of 3.5-4.1 V. This slight reduction in oxidation potential in a high dielectric medium compared to low dielectric conditions is expected due to the increased stabilization of the charged state by the high dielectric medium compared to the neutral state. 4.1.2 Reorganization energy (λ) The reorganization energy (λ) of a molecule during a redox process is computed using the energies and geometries of the neutral and oxidized states. The connection between the nuclear configuration and ground state energies of a given molecule and its cation radical is schematically shown in Figure 3. The total reorganization energy (λTotal) is defined as the sum of the reorganization energy during the oxidation (λOx) and reduction (λred). We note that zero-point energy correction is not considered in the reorganization energy computations. The λOx is the energy difference between the vertical detachment energy (VDE) and adiabatic detachment energy (ADE). The (λred) is the energy difference between ADE and vertical electron affinity (VEA). The absolute values of reorganization energy are used here and are tabulated in Table 1. Lower reorganization energy suggests that the molecular system achieves energy closer to the minimum energy upon electron addition or removal. For oxidation processes, based on the computed λOx presented in Table 1, the 25-DMB has the lowest and the 23-DMB has the highest requirement of reorganization energy. We note that, the 2356-DMB molecule requires highest total reorganization energy as expected due to six substituent groups in the benzene ring. In general, from Table 1, the unsubstituted DMB, 2-DMB, and the 25-DMB have lower values for λOx, and λTotal. 4.1.3 Root Mean Squared Deviation (RMSD) between the geometries Another measure of the structural deviation during the redox processes is the RMSD between the optimized Cartesian coordinates of neutral and oxidized species of the DMB derivatives. From the neutral and unipositive DFT optimized (xyz) structures of molecules (All optimized structures are given in the supporting information Figure S1 and Table S1); the root mean square deviation (RMSD) was computed using the Kabsch algorthm36. The computed RMSD is also shown in Table 1. Based on the computations, the 25-DMB exhibits a lowest RMSD for geometry change (0.04) and also the lowest reorganization energy, suggesting stabilization over the rest of the candidate molecules. Two other derivatives (DMB and 2-DMB) have similarly low RMSDs during oxidation consistent with the prediction of reorganization energy. To compare the oxidation potentials (EOx), reorganization energies (λOx) and RMSD during oxidation of the candidate molecules, these three properties are plotted in Figure 4. From the Figure, it is evident that that the 25-DMB exhibits low values for all three properties. Having lower oxidation potentials (EOx) in a given series of molecules is an indicator of relative stability of cation radicals. This is


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based on the assumption that these cation radicals are less reactive towards typical deprotonation reactions. Methylated DMB derivatives have similar C-H bond dissociation energies, therefore the acidity of the corresponding cation radicals increase with the oxidation potential of neutral species37. In this aspect, the 25-DMB and 23-DMB are less acidic (lower computed oxidation potential, see Table 1) compared to the rest of the candidates. In addition by incorporating other structural features such as reorganization energy (λ) or RMSD during the oxidation, it is evident that 25-DMB can be considered to be the most stable catholyte material from the seven DMB derivatives considered.. In general, plots similar to that of Figure 4 can be obtained for a series of molecule to assess the relative stability potential redox active materials. Further understanding of the fate of the radical cation is also essential to understand the relative stability of radical cations and the overall stability of materials. We consider this in the next section (4.2).

xo

+

Figure 4. Comparison of computed oxidation potentials (E n V unit w.r.t. Li/Li ref electrode), reorganization energy during Ox oxidation (λ ) and root mean square deviation (RMSD) between the optimized XYZ coordinates of neutral and oxidized states of various DMB derivatives (X-axis). Note: The data associated with the plot is given in Table 1.

4.2 Reactivity To assess the reactivity of cation radicals, it is essential to understand the thermodynamic driving forces and kinetic feasibilities of likely reactions. In subsection 4.2.1, the computed thermochemistry of selected probe reactions of radical cations (deprotonation, dimerization, hydrolysis, and demethylation) is discussed. In subsection 4.2.2, the kinetic barriers of demethylation reactions are evaluated and discussed.


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4.2.1 Thermodynamic feasibility of chemical reactions The thermodynamic feasibility (∆G) of deprotonation (rxn1), demethylation (rxn2), hydrolysis (rxn3), and, dimerization (rxn4) reactions were computed for the radical cations of (RCs) the candidates. These four reactions are schematically shown for DMB in Figure 5. The deprotonation (rxn 1) of cation radical in water is chosen as the probe reaction, where deprotonation results into the formation of neutral radical and a solvated proton. This reaction is considered due to the available experimental data of free energy of solvation of the proton in aqueous medium ∆Gprotonsolvation=-265.9 kcal/mol38, which is needed to understand the thermochemistry of protonation and deprotonation 39. Deprotonation of DMB derivatives could occur from the methoxy (-OCH3) or from the methyl (-CH3) group. Note that the deprotonation from the benzene ring is unlikely due to the largely endothermic nature (~+ 40 kcal/mol) of the reaction. Similar to deprotonation, demethylation (rxn 2) is another way of eliminating the positive charge of the radical cations. To address rxn 2, we have computed the thermochemistry for methyl transfer between the radical cation (RC) and propylene carbonate (PC), a probe molecule, as shown in Figure 5. In addition to the deprotonation and demethylation reactions, a hydrolysis reaction (rxn 3), a probe reaction for solvolysis is considered. In this reaction the thermochemistry of etheric cleavage by a water molecule to form CH3OH and hydroxyl radical cations (see figure 5, rxn3) was studied. Finally, dimerization (rxn4) of the cation radical was also considered as a probe reaction. The computed Gibbs free energies of rxns1-4 are presented in Table 2. Using the Gibbs free energy of deprotonation (∆G(H+)aq) and Gibbs free energy of hydrolysis (∆GHOH), the pKa and KHOH equilibrium constants, respectively, were also computed and presented in Table 2.

Figure 5. Schematic of selected reactions (rxn 1 to 4) of 1,4-dimethoxy benzene (DMB) radical cation. Abbreviation: PC: propylene carbonate molecule. All reactions are modeled in aqueous dielectric medium. In rxn1, the cation radical reacts with water to form solvated proton and neutral radical. In rxn 4, two cation radicals interact with water to form dimer and two solvated protons.


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Table 2. Computed thermochemical data (in kcal/mol) of DMB cation radicals at the B3LYP/6-31G(2df,p) level of theory. + dimer Computed data includes deprotonation free energies (∆G(H )aq), dimerization energies (∆G aq), and demethylation + energies (∆G(CH3 )) of DMB cation radicals. Deprotonation from ‘-OCH3’ group

Radical Cation (RC) DMB 2-DMB 23-DMB 25-DMB 26-DMB 235-DMB 2356-DMB

a

+

∆G(H )aq

28.3 29.9 31.3 32.4 27.2 19.3 22.7

b

pKa

7.3 7.7 8.1 8.3 7.0 5.0 5.9

c

Deprotonation from ‘-CH3’ group

dimer

∆G

-12.0 -8.3 -5.0 -3.3 -9.3 -27.0 -20.4

d aq

+

∆G(H )aq

NA 21.1 22.7 23.4 20.8 e 10.8 14.9

b

pKa

NA 5.4 5.8 6.0 5.4 2.8 3.8

c

dimer

∆G

NA -1.1 4.2 3.6 -3.6 -19.9 -10.7

f aq

+

∆G(CH3 )PC

14.0 13.7 14.9 15.4 10.0 9.4 5.9

g

HOH

∆G

0.6 0.1 -0.2 0.2 -4.5 -6.9 -7.1

h

KHOH

1.0E-01 6.5E-01 2.3E+00 5.0E-01 3.6E+07 4.2E+11 8.6E+11

a) Free energy change during the deprotonation reaction (rxn1) from the methoxy group of the DMB derivative using the experimental Gibbs free energy of proton in aqueous solution (-265.9 kcal/mol) b) pKa = ∆G/2.303RT, where R is the gas constant and T is the absolute temperature. c) ∆Gdimer is the free energy change during the dimerization (rxn4) of two cation radicals to form neutral dimer, where two protons are solvated by an aqueous medium. d) The computed free energy change for deprotonation (rxn1) from the methyl group to the aqueous medium using the Gibbs free energy of a proton in aqueous solution :-265.9 kcal/mol38. e) For 235-DMB, the deprotonation from the 3-methyl position (∆G(H+)aq = 10.8 kcal/mol) is thermodynamically more preferred than from the 2-methyl (∆G(H+)aq = 12.3 kcal/mol ) and from the 5-methyl (∆G(H+)aq = 11.7 kcal/mol) position f) ∆G = G(RC) + G(PC) G(PC—CH3+) + G(RC-demethylated), see rxn2 in Figure 5. g) ∆G = G(RC) + G(H2O) G(CH3OH) + G(RC-H)radical, barrier is not computed h) The equilibrium constant for hydrolysis, KHOH = exp(-∆GHOH/2.303RT) Based on the computed data presented in Table 2, the following important points can be drawn: 1. Deprotonation of DMB radical cations are endothermic processes in aqueous solution (1032 kcal/mol). Deprotonation from a methyl group (10-23 Kcal/mol) is thermodynamically more likely than from the methoxy groups (19-32 kcal/mol) for all DMB derivatives. 2. Dimerization of cation radicals are generally thermodynamically downhill and it is likely that the rate-determining deprotonation is the first step. Dimerization of neutral radical formed at the methoxy group (-OCH2-) is thermodynamically more favorable than that formed at the methyl (-CH2-group). See Figure S3 of the supporting information for details. 3. Demethylation reactions of RCs by a PC molecule in aqueous medium are endoergic reactions (5-16 Kcal/mol). This is the most likely reaction based on the free energy of the reaction.


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4. Similar to demethylation, hydrolysis can also cleave the etheric C-O bond. Based on the computations in Table 2, hydrolysis of DMB analogues with steric constraints (due to methyl groups) are thermodynamically favorable. Additionally, deprotonation energetics are likely to be more thermodynamically uphill in solvents such as methanol (∆Gprotonsolvation=-263.5 kcal/mol38) or acetonitrile (∆Gprotonsolvation=-260.2 kcal/mol38) than in water medium, due to the reduced proton solvation energy of methanol and acetonitrile compared to the aqueous medium (∆Gprotonsolvation=-265.9 kcal/mol). In terms of the reactivity of cation radicals, deprotonation and demethylation are the most likely reactions since both of them are ideal for the radical cations to lose its charge to the solvent medium. The dimerization reaction is a consequence of the deprotonation and hydrolysis reaction ( or solvolysis) is one of the routes for the demethylation reaction. In general, based on the computed reaction free energies of demethylation reaction, most likely based on the free energies, the 25-DMB radical cations would exhibit the least reactivity in the solution indicating better relative stability as catholyte materials.

4.2.2

Demethylation: Computed Activation barriers

Figure 6. Computed solution phase enthalpy profile (∆Hsoln) of demethylation of the 25-DMB cation radical by a propylene ‡ carbonate molecule. Computed apparent enthalpy of activation (∆H ) of all DMB candidates are given in Table 3.


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Table 3. Computed enthalpy of activation (kcal/mol) for demethylation of DMB radical cations by a propylene carbonate molecule at the B3LYP/6-31G(2df,p) level of theory in water and diethyl ether solvent dielectric. Using the enthalpy of activation in solution, the rate constant (K) and half-life assuming first order kinetics are shown.

‡

(CH3+)PC

Water dielectric a b K(s-1) T1/2 (hours)

a

‡

Diethyl ether dielectric b c K(s-1) T1/2 (hours)

(CH3+)PC

Radical Cation ∆H ∆H (RC) DMB 26.7 1.6 x 10-7 1.2 x 103 23.6 3.0 x 10-5 6.4 -7 2 -5 2-DMB 26.5 2.5 x 10 7.9 x 10 23.8 2.6 x 10 8.5 23-DMB 27.2 6.8 x 10-8 2.8 x 103 24.7 4.9 x 10-6 3.9 x 101 25-DMB 27.9 2.1 x 10-8 9.1 x 103 25.6 9.9 x 10-7 1.9 x 102 26-DMB 23.8 2.3 x 10-5 8.5 21.0 2.5 x 10-3 7.7 x 10-2 -5 2 256-DMB 16.5 4.8 4.0 x 10 13.9 4.4 x 10 4.4 x 10-7 -5 -3 2356-DMB 23.7 2.0 x 10 9.6 21.5 1.0 x 10 1.9 x 10-1 a. ∆G‡(CH3+)PC is gven in the Table S2 of the supporting information; bK = (KbT/hc)exp(-∆G‡/RT), cT1/2 = ln (2)/K(h-1)

+

‡

+

Figure 7. Comparison of the computed free energies changes (∆G(CH3 )PC) and activation enthalpies (∆H (CH3 )PC ) required + for the demethylation of DMB cation radicals by propylene carbonate. The data associated with (∆G(CH3 )PC) and ‡ + (∆H (CH3 )PC ) are given in Table 2 and 3, respectively.

In addition to the relative stability predictions based on the computed free energy changes associated with the deprotonation/demethylation reactions, the activation barriers for demethylation of DMB analogues were also computed. In general, calculation of activation barriers using the demethylation model can provide qualitative trends for describing the initial decomposition reactions of cation radicals. Demethylation of DMB cation analogues by a propylene carbonate molecule in water dielectric medium is used as a model to compute the activation barriers. In Figure 6, a computed enthalpy profile for the demethylation reaction of 25-DMB is shown, where the computed apparent activation enthalpy (∆H‡) is


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~28 kcal/mol. In the energy profile (Figure 6), first a weakly bound complex between 25-DMB cation radical and a PC molecule is formed. Subsequently, the complex undergoes of C-Omethyl bond cleavage via the transition state shown in the Figure 6. Complete transfer of methyl cation to the PC results in the formation of a 25-DMB demethylated radical. Note that the demethylation reactions are endothermic in nature, as explained in the previous section (see Table 2). In Table 3, the computed apparent activation barriers (∆H‡) for all radical cations in aqueous and diethyl ether solvent model are presented. A comparison of computed activation enthalpies and the free energy changes associated with the demethylation of DMB cation radicals by propylene carbonate is schematically shown in Figure 7. Using this activation enthalpy in solution, we have approximated the rate constant (K s-1) using the Eyring Equation (see Table 3). Additionally, using first order kinetics approximation, the half-life of the reactions (T1/2 h) is also computed, also shown in Table 3. Based on the computations, the activation barriers for the demethylation are smaller in diethyl ether compared to water. Most importantly, the computed activation barriers are in the order: 25-DMB > 23-DMB > 2-DMB > DMB > 2356-DMB> 26DMB> 256-DMB in ether dielectric medium. The computed half-life of the demethylation reaction suggest that both 25-DMB, and 23-DMB would require approximately hundreds of hours to decompose via demethylation reaction, while rest of the cation radicals require tens of hours or less. This is consistent with the conclusion above based on the computed thermochemistry that these two would be the most stable.

Summary Stable, electrochemically active and soluble organic materials are required for redox flow energy storage applications. Understanding the stability of organic materials in the battery operating conditions is a complex problem, but essential to provide guidelines for materials discovery. In this study first principles simulations are performed to gain molecular level insights into the factors affecting the stability of seven 1,4-dimethoxy benzene (DMB) derivatives. These molecules are potential catholyte candidates for nonaqueous redox flow battery systems. Computations are performed to predict oxidation potentials in various dielectric mediums, intrinsic-reorganization energies, and structural changes of these representative catholytes during the redox process. In order to understand the stability of the DMBbased radical cations, the thermodynamic feasibility of the following reactions is computed using density functional theory: (a) deprotonation, (b) dimerization, (c) hydrolysis, and (d) demethylation. The computations indicate that radical cations of the 2, 3-dimethyl and 2, 5-dimethyl derivatives are the most stable among the DMB derivatives considered in this study. In the presence of solvents with highproton solvating ability (water, dmso, acetonitrile), degradation of the cation radical likely occurs via deprotonation. In the presence of solvents such as propylene carbonate (PC), demethylation was found to be the most likely reaction that causes degradation of radical cations. From the computed enthalpy of activation (∆H‡) for a demethylation reaction in PC, the 2, 5-dimethyl DMB cation radical would exhibit better kinetic stability in comparison with the other candidates. This investigation suggests that computational studies of structural properties such as redox potentials, reorganization energies) and the computed reaction energetics (deprotonation and demethylation) of


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charged species could be used to predict the relative stabilities of large sets of molecules required for the discovery of novel redox active materials for flow battery applications. First, computationally less demanding, reorganization energies or RMSD between structures can be used as a descriptor for relative stability. Second, a reactivity indicator from computed free energy change of the most likely reaction can be used to screen a large set of molecules to rank the stability trends. Finally as part of the deep dive study, for a small subset of radical cations kinetic feasibility of the most likely reaction can be computed to assess the reactivity and life-time.. We also anticipate that these first principles based computations can be used in the future to derive quantitative relationships to predict the stability of materials for redox flow applications. Supporting information available Optimized structures of DMB derivatives are shown in Figure S1. Figure 2 suggests most inmportant structural parameters and these parameters are tabulated in Table S1. Figure S3 presents the computed deprotonation energies (rxn 1 of Figure 5) and dimerization energies (rxn 4 of the figure 5) of radical cations at the B3LYP/6-31G(2df,p) level of theory and the Table S 2 shows computed enthalpies and free energies of activation required for the demethylation reactions in water and diethyl ether dielectric media.

Acknowledgements This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U. S. Department of Energy, Office of Science, Basic Energy Sciences. We gratefully acknowledge the computing resources provided on "Blues," a 320-node computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory.


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Schematic of dimethoxy benzene (DMB) derivatives considered in this study. 6x1mm (600 x 600 DPI)


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Molecular Level Understanding of the Factors ... - ACS Publications

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