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Letter

Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity. David C. Cantu, Juntaek Lee, Mal-Soon Lee, David J. Heldebrant, Phillip K Koech, Charles J. Freeman, Roger Rousseau, and Vassiliki-Alexandra Glezakou J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00395 • Publication Date (Web): 28 Mar 2016 Downloaded from http://pubs.acs.org on March 28, 2016

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The Journal of Physical Chemistry Letters

Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity David C. Cantu 1 , Juntaek Lee 2 , Mal-Soon Lee 1 , David J. Heldebrant 2 , Phillip K. Koech 2 , Charles J. Freeman 2 , Roger Rousseau 1 , Vassiliki-Alexandra Glezakou 1*

1Physical

Sciences Division, 2Energy Processes and Materials Division

Pacific Northwest National Laboratory, Richland WA 99352

Abstract The deployment of transformational non-aqueous CO2-capture solvent systems is encumbered by high viscosity even at intermediate uptakes. Using single-molecule CO2 binding organic liquids as a prototypical example, we present key molecular features controlling bulk viscosity. Fast CO2-uptake kinetics arise from close proximity of the alcohol and amine sites involved in CO2 binding in a concerted fashion, resulting in a Zwitterion containing both an alkyl-carbonate and a protonated amine. The population of inter-molecular hydrogen bonding between the two functional groups determines the solution viscosity. Unlike the ion pair interactions in the ionic liquids, these observations are novel and specific to a hydrogen-bonding network that can be controlled by chemically tuning single molecule CO2 capture solvents. We present a molecular design strategy to reduce viscosity by shifting the proton transfer equilibrium towards a neutral acid/amine species, as opposed to the ubiquitously accepted zwitterionic state. The molecular design concepts proposed here are readily extensible to other CO2 capture technologies.

1 ACS Paragon Plus Environment


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Keywords: CO2 capture solvent systems, ab initio molecular dynamics, classical molecular dynamics, switchable ionic liquids, viscosity.

TOC


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Aqueous amine solvents, such as monoethanolamine, are the industrial benchmark liquid systems being considered for CO2 capture from flue gas streams.1-2 However, their regeneration energy requirements are cost prohibitive for most flue gas applications under consideration,2-4 largely as a result of the aqueous water content carried in these solvents. A number of advanced solvents are currently being developed to adsorb and release CO2 without the need for an aqueous carrier. Carbon dioxide binding organic liquids (CO2BOLs) represents a promising class of water-free or water-lean solvents due to their ability to switch between the ionic and non-ionic forms with the binding and release of CO2.5-6 Their functionality stems from their ability to bind CO2 onto an aliphatic alcohol group and form a zwitterionic species upon CO2-uptake (Figure 1). Despite the considerably lower regeneration energy penalties of CO2BOLs compared to aqueous amines, high viscosities after binding CO2 renders them impractical for post combustion flue gas applications.1-2 Other non-aqueous alternative solvent classes under development are also limited by high viscosity behavior upon loading with CO2.

Task-specific ionic liquids7 are two-component

systems that turn into carbamate-ammonium or carbamate-imidazolium salts after carbon binding8. Similarly, single component siloxylated amines are switchable and turn into a two-component carbamate-ammonium ionic liquid after reaction with CO2.9 Ionic liquids with aprotic heterocyclic anions10 have the advantage that their CO2 binding energies can be tuned with chemical modifications, and that their viscosities do not increase much upon reaction with CO211, however they exhibit intrinsic high viscosities before reacting with CO212.



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These challenges inspired the work reported here, in which the conceptual framework that relates the molecular-level properties to the bulk physical properties of CO2BOL capture solvent systems was developed. Our approach lies on two main accomplishments: (i) a computational protocol that can provide reliable estimates of the viscosity for a variable composition system, since the liquid’s composition changes with CO2 loading, and (ii) the mechanism and the quantification of the freeenergetics of CO2-binding in water-free solvents, as well as the acid-base properties and equilibrium behavior in its CO2-bound state. The single component alkanolguanidine CO2BOL material family was used as the initial solvent class of study, with solvent viscosity reduction being a primary focus. However, many of our findings can be transferred to other solvent classes. To the best of the authors’ knowledge, this work contains the first ab initio simulations of CO2 capture by a single component switchable ionic liquid, as well as identifying and quantifying the molecular level interactions, such as internal hydrogen bonding and acid-base equilibria, that ultimately control bulk viscosity. This study is also the first to propose that single-component CO2BOLs in their CO2-bound state do not necessarily exist as purely zwitterionic species but rather exhibit a dynamic acid-Zwitterion equilibrium that could be exploited to further reduce viscosity. O

O

C O

O

C

O O

C O

HO

O H N

H N

N N

N N

N

N

N

Neutral

Zwitterion

Figure 1: The structure of neutral IPADM-2-BOL not binding CO2, and zwitterionic binding CO2.


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The CO2 binding free energy is one of the deciding criteria in the design of gas separation solvents. Herein, a concerted mechanism for CO2 binding is proposed based on Blue Moon ensemble13 simulations, where a CO2 molecule was placed in a simulation box with 34 IPADM-2-BOL molecules, and a series of ab initio molecular dynamics (AIMD) simulations are performed at fixed values of the distance between the CO2 carbon atom and the alcohol oxygen atom of a IPADM-2-BOL molecule, rCO.

Details on the computational approach, free energy profile calculations, and error estimates appear in

the Methods Section and Supplementary Information (SI). We note that CO2 capture involves both solvation and binding; herein we focus on the binding event. Further discussion about overall capture appears in the SI. Figure 2 shows the free energy profile of CO2 addition reaction as a function of rC-O, where 2A corresponds to the CO2-bound system, 2B is the transition state, and 2C corresponds to the solvated CO2 in the vicinity of the alcohol. For rC-O distances less than 2.00 Å (Figure 2A), CO2 is bound in the form of an alkylcarbonate, while the H atom that originally belonged to the OH group remains on the guanidine N. For distances greater than 2.20 Å (Figure 2C), IPADM-2-BOL remains in its alcohol form, and CO2 is mostly linear with the ∠OCO angle averaging ~175º. The angle decreases to ~165° for rC-O distances between 2.0 and 2.2 Å. CO2 binding happens in an effectively concerted mechanism: at rC-O ~ 2.00 Å, the ∠OCO angle becomes ~150° with the simultaneous H transfer to the nitrogen of the guanidine base. The CO2 structure is consistent with a partial charge transfer to form a CO2δ-

14-15

and

subsequent formation of a CO3- moiety in IPADM-2-BOL (Figure 1, Figure 2B).



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Figure 2: Free energy (red) and energy (blue) profile of CO2 binding by IPADM-2-BOL at 40 °C obtained with Blue Moon ensemble simulations as a function of the CO2 carbon to IPADM-2-BOL alcohol oxygen distance, see SI. In the images, dark gray is C, white is H, red is O, and blue is N. The H atom that moves between alcohol to guanidium base is highlighted in turquoise. The energy diagram on the right summarizes the whole capture and binding process.

From the free energy profile, we determine that CO2 binding by IPADM-2-BOL proceeds with a barrier of 16.5±1.2 kJ/mol and a binding free energy of -5.8±1.6 kJ/mol. The binding free energy is consistent with the experimentally obtained values of diazabicyclo[5.4.0]-undec-7-ene (DBU) containing dual-component CO2BOLs that range between -5.7 to -9.7 kJ/mol.6 This implies that at 40 °C, there is an equilibrium between solvated and bound CO2, in accord with NMR measurements reported by Heldebrant et al. 6 The free energy barrier of 16.5 kJ/mol and the activation energy of 9.8 kJ/mol (Figure 2) are compatible with the experimental observation that this process readily occurs at 40 °C.


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For monoethanolamine (MEA) the energy barrier is more than twice that of CO2BOLs. In dry MEA, density functional methods give a barrier of 35.5 kJ/mol.16 For wet MEA, the energy barrier was estimated based on density functional calculations to be 50.0 kJ/mol17, in good agreement with the activation energy of 46.7 kJ/mol estimated with the Arrhenius relation from experimental data.18 However, these large barriers are likely associated with high-energy intermediate states involving protonation of primary alcohols and carbamate formation. On the other hand, Ozturk et al. measured lower activation energies for the dual-component CO2BOL systems 1,1,3,3 tetramethylguanidine (TMG)/1-hexanol (9.7 kJ/mol)19 and DBU/1-hexanol (13.7 kJ/mol),20 while proposing similar intermediates as for MEA. Based on our mechanistic results, we believe that a direct carbonate formation is possible, and the only requirement for the low activation barrier is acid/base proximity. Our estimate of activation free energy is only ~7 kJ/mol higher than the activation energy, which is indicative of a small entropic contribution at the transition state, owing to the proximity of the alcohol/amine moieties in the single component systems: unlike dual component systems, solvent reorganization at the transition state is not required. The relatively low barrier then suggests that capture in CO2BOLs is likely to be diffusion limited. Because the solvent viscosity increases exponentially with CO2 loading21, the capture rate will decrease as more CO2 is added. These phenomena were observed when the CO2 absorption rates of single and dual-component CO2BOL solvents where measured with wetted-wall experiments.22 Motivated by the conventional picture of acid/base equilibrium, where an organic acid is in dynamic equilibrium with its conjugate base, we made the connection with the CO2-bound zwitterionic alkylcarbonate species, which acts as both an acid and a base, and asked the question whether non-ionic CO2-binding species are feasible. Given that the rheological properties (e.g. high viscosity) of CO2BOLs are intimately coupled to the population of the charged species, tuning the equilibrium between charged zwitterionic and non-charged acid species could alter their fluid properties.



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To test this hypothesis, we conducted a series of AIMD simulations, with the metadynamics23 protocol to accelerate the proton transfer between the carboxylate and guanidium groups, on three target molecules that cover a range of relative acid/base strengths. All simulations started with a carbon loaded CO2BOL molecule in its zwitterionic form placed in a simulation box of the capture solvent. The free energy profile of the proton transfer process was calculated with respect to an appropriately tailored collective variable that describes both zwitterionic (charged) and acid (non-charged) CO2bound states; see Methods and SI for details. The IPADM-2-BOL frame was modified in two different ways: (i) by introducing an oxime group to reduce the acidity of alkanol moiety (EODM-2-BOL), and (ii) by fluorination of the guanidium core to decrease the basicity at the N site (IPATFMM-2-BOL). Figure 3 shows the resulting free energy landscape for proton shuttling between the charged (Zwitterion) and non-charged (acid) forms for IPADM-2-BOL (blue), EODM-2-BOL (green) and IPATFMM-2-BOL (red). Table S1 in SI summarizes the results and an error analysis of the metadynamics simulations is also presented in SI, Fig. S3. Based on the computed free energy landscapes, equilibrium constants and relative populations of the acid and zwitterion were calculated. The relative acid:Zwitterion populations thus determined, 1:4000 for IPADM-2-BOL, 3:1 for EODM-2-BOL and 8:1 for IPATFMM-2-BOL, clearly demonstrate how simple molecular modifications can influence the ratio of charged and non-charged species that ultimately provide an appropriate description of the liquid phase. In silylamines24, increased CO2 capture capacity was attributed to stabilization of carbamic acid by the carbamate and ammonium species of the ionic liquid. Similarly for monoethanolamine, a carbamic acid - carbamate equilibrium has also been proposed, but not verified.25 Although, the concept of such acid/zwitterionic equilibrium has been discussed in the context of enhancing the CO2 adsorption capacity, the present study is the first to consider it as a molecular design strategy for the reduction of viscosity.


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O

O O

C

C

O

=CH2 in IPADM-2-BOL =O in EODM-2-BOL =CH2 in IPATFMM-2-BOL

O

O

H

H N N

N

N

N

Zwitterion (Charged)

=CH3 in IPADM-2-BOL =CH3 in EODM-2-BOL =CF3 in IPATFMM-2-BOL

N

Acid (Uncharged)

0 -1.5

-1

-0.5

-5

0

0.5

1

1.5

-10

Free Energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-15 -20

IPADM-2-BOL EODM-2-BOL IPATFMM-2-BOL

-25 -30 -35 -40 -45

Figure 3: Free energy profiles, obtained from ab initio molecular dynamics simulations with the metadynamics technique to accelerate proton transfer, of the acid-zwitterion equilibrium for IPADM-2BOL, the oxime derivative (EODM-2-BOL), and the fluoro-substituted derivative (IPATFMM-2-BOL). See abbreviations for compound definition.

For all tested CO2BOL compounds, the free energy barriers to switch between the CO2-bound states are low (~23 kJ/mol for IPADM-2-BOL zwitterion to acid, the rest below 16 kJ/mol, Figure 3, SI Table S1), indicating that at carbon capture conditions, an equilibrium between the charged and noncharged species will be facile. Proton transfer kinetics will not determine equilibrium populations, only their relative free energies. To determine if a correlation between solvent dielectric and the acidZwitterion equilibrium exists, we computed ∆G values between the different CO2-bound states for the three species (Figure 3) with electronic structure calculations (see Methods) using a continuum dielectric model for a range of values (SI Table S2). For all compounds, as the dielectric constant of the solvent decreases, the equilibrium shifts towards the acid state. The ∆G values obtained with AIMD 9 ACS Paragon Plus Environment


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agree within ~6 kJ/mol with implicit solvent electronic structure calculations for dielectric constants ranging from 12 to 6.8. We note that the relative CO2 capture (CO2 solvation and binding) energies (see SI) of EODM2-BOL and IPATFMM-2-BOL are within ~15 kJ/mol of that for IPADM-2-BOL using gas phase calculations indicating possible similar CO2 capture capacities. Given that CO2 solvation energies using a continuum dielectric model for single molecule calculations are not reliable for IPADM-2-BOL (see SI), we cannot properly assess if the modified compounds will have enhanced or lessened CO2 capture capacity. The single molecule calculation performed with a continuum dielectric model (mentioned in the previous paragraph to determine the relative energies of acid/Zwitterion states) all have CO2 bound and do not need to consider CO2 solvation. EODM-2-BOL and IPATFMM-2-BOL were chosen for AIMD simulations to demonstrate that the equilibrium could be shifted toward the acid state significantly. We are currently studying full CO2 capture (solvation and binding) with AIMD in CO2BOLs and will appear in an upcoming publication. Since CO2BOLs are switchable ionic liquids5, the extended solvent dielectric will vary as the polarity of individual molecules change with CO2 capture. We estimated solvent dielectric constants with from the dipole moments based on the Debye-Onsager model (SI Tables S3, S4) and from classical molecular dynamics (MD) simulations (Methods) for IPADM-2-BOL at different CO2 loadings (SI Table S5). These range from 4 to ~12, showing an inverse dependency with CO2 loading, suggesting that at higher loadings (lower dielectrics), the acid state of CO2-bound molecules will be more prevalent. An increase in dielectric constant with CO2 loading would be expected given that more polar molecules (Zwitterions) are present at higher loading. However, the dielectric constant of the solvent decreases because they tend to form clusters that could result in smaller net dipoles. The extended solvent structure obtained from classical MD simulations (Figure 4B) is highly heterogeneous, with different polar (CO2-bound charged zwitterion) and non-polar (unbound neutral alcohol) regions. This finding is supported by the fact that heterogeneous domains in the solvent structure were detected, 10 ACS Paragon Plus Environment

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with ultraviolet visible and infrared spectroscopic experiments, of dual component CO2BOLs.26-27 We note that simple models such as the Debye-Onsager equation do not predict the correct trends in changes of static dielectric constants, since their estimation using single molecule dipole moments does not take into account the spatial arrangement between neighboring molecular species, see SI for more details. The impact of acid/base equilibrium on the viscosity is illustrated by two independent sets of classical MD simulations on the IPADM-2-BOL system at varying CO2 loading: (i) one with all CO2bound molecules in zwitterionic form, and (ii) another with 1:1 acid:Zwitterion populations. The first system represents the liquid state consistent with an equilibrium shifted mainly towards the zwitterionic form, as discussed above. The second hypothetical system allows us to probe the impact of charged state keeping all other factors the same, see SI for details and force field parametrization. Figure 4 shows very good agreement between the computed (orange) and experimental22 (yellow circles) viscosities for IPADM-2-BOL. However, the hypothetical 1:1 mixture (green) shows a pronounced drop in viscosity, on the order of 30-50% for the higher loadings. This appreciable viscosity reduction points to potential for viable, non-ionic CO2 capture solvent systems that can be brought upon with simple molecular modifications once the appropriate equilibria drivers are taken into account.



500.0

A log Viscosity (cP)

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50.0 Experiment All Zwitterion 1:1 Acid:Zwitterion 5.0 0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

% mol CO2 loading

B

0%

10%

20%

30%

Figure 4: (A) IPADM-2-BOL viscosities. Experimental data points (yellow), calculated with all CO2loaded molecules zwitterionic (orange) and CO2-loaded molecules in 1:1 acid:Zwitterion (green). (B) Snapshots from classical MD simulations show that as CO2 loading increases, the extended solvent structure becomes a highly heterogeneous mixture of CO2-bound molecules (red) and solvent molecules (blue).

To further probe this hypothesis, we computed the viscosities of EODM-2-BOL and IPATFMM-2-BOL with solvent molecules in different CO2-bound states at 25% mol loading. The 25% mol CO2 loading was chosen for this analysis because for IPADM-2-BOL it has been projected to be the thermodynamically optimal lean-solvent loading for post-combustion CO2 capture.28 For IPADM-2BOL, at 25% mol loading, the 1:1 acid:Zwitterion system undergoes ~ 48% viscosity reduction compared to the all-Zwitterion system. At the same loading, viscosity reductions of 34% and 61% were 12 ACS Paragon Plus Environment

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observed in the 1:1 state for IPATFMM-2-BOL and EODM-2-BOL solvents, with respect to the allZwitterion state, see Table 1, and SI Table S7 for additional details. As a final point, we note that the intrinsic viscosities of the three solvents varied greatly (Table 1), regardless of the charged state of the CO2-bound molecules (i.e. Zwitterion vs. acid). After careful analysis of the extended solvent structures of zwitterionic systems, we identified that strong H-bonds between the carboxylate and protonated amine sites of neighboring CO2-bound molecules in a charged (Zwitterion) state are the decisive structural factor for controlling viscosity, see SI Figures S4 and S5. Lower viscosities are observed when significant populations of charged CO2-bound species keep intermolecular H-bonds. High internal H-bonding is an indicator of localized and directional charge separation that is however contained within each Zwitterion and therefore, less likely to be involved in agglomeration of charged species in the liquid. As a result, significant COO----H+N population between neighboring Zwitterions results in high viscosities, while internal H-bonding leads to lower viscosities. Since in two component ionic liquids internal hydrogen bonding is not feasible, it has been suggested that the strong hydrogen bonding between COO- and NH3+ groups of neighboring molecules in twocomponent CO2-bound alkanolamines is the leading factor for their increased viscosity.29

Table 1: Viscosities of the solvents at 25% mol loading, and the percentage of CO2-bound molecules (all zwitterionic) with an internal hydrogen bond. See SI for details, and a visual representation of the internal hydrogen bond. 25% mol Loading

100% Zwitterion

1:1 Zwitterion:Acid

System

Viscosity (cP)

% Internal H-bond

Viscosity (cP)

EODM-2-BOL

45.5

92

17.9

IPADM-2-BOL

149.5

34

77.7

IPATFMM-2-BOL

328.5

13

214.2



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Therefore, we propose two strategies to reduce CO2BOL viscosity in the CO2-bound state: (i) have a significant concentration of acid (non-charged) molecules, and/or (ii) a high population of Zwitterions (charged) that sustain a high degree of intermolecular H-bonding. We also note that even the case of the acid (non-charged species) internal hydrogen bond will also help localize the acid-base properties and stabilize the bound CO2. This paper demonstrates several critical aspects of CO2 capture by water-lean solvent systems that can be controlled by deliberate molecular modifications. We have identified two key structural motifs that play a pivotal role in determining CO2 adsorption kinetics and bulk liquid viscosity. The first one is ascribed to the close proximity of the amine and alcohol moieties. This is reflected in the concerted mechanism of CO2 binding by the nucleophilic alcohol and concurrent proton transfer to the amine. The overall effect is fast CO2 binding kinetics associated with low entropic contribution to the free energy barrier. Proximity also enables a higher likelihood of CO2-bound zwitterionic species to have internal H-bonding that reduces viscosity. The second emerging factor is a tunable acid/base equilibrium. Enthalpically, a high acidity at the alcohol site allows for a more efficient CO2 activation at the transition state and an efficient proton transfer to the amine. Here, we introduced the concept of non-charged CO2 capture solvent systems by adjusting the acid/base properties of the solvent molecules, so that a significant fraction of the CO2-loaded molecules can exist in a non-charged (acid) form. This can be achieved by either increasing the acidity of the alcohol or by decreasing the basicity of the amine. Non-charged CO2 capture systems exhibit appreciably lower viscosities than the analogous zwitterionic form. A quantitative structure-viscosity relationship of CO2BOLs will be the subject of a forthcoming publication. We should also point out that the concepts outlined here are distinct from similar discussions in the literature due to fact the CO2BOLs are single molecule switchable ionic liquids, with CO2-bound molecules exhibiting an acid-Zwitterion equilibrium, as opposed to the typical two component ionic liquids. In two component ionic liquids, hydrogen bonding and salt bridges

29

as well as molecular stacking due to side chain length30 have been identified as

viscosity contributors. Analogous to having a larger population of acid (non-charged) species in single 14 ACS Paragon Plus Environment

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molecule systems, molecular liquids have been proposed to decrease viscosity in typical two component ionic liquid mixtures31, however this results in reduced CO2 loading capacity. This work specifically shows that one can decrease the extended H-bond network by enhancing intermolecular H-bonding and decrease the overall ionic strength by tuning the inter-molecular acid/base equilibrium. Both factors strongly contribute to substantial viscosity reduction without impacting CO2 loading. Finally, we believe that the chemical guidelines outlined here for controlling CO2 uptake kinetics and viscosity reduction can be ubiquitously applied to both carbonate and carbamate solvent systems.

Computational Methods All electronic structure calculations were performed using Gaussian0932 with the M06L33 functional and the 6-31++G** basis set. Structure optimizations were done for all individual molecules to get force field equilibrium bond lengths and angles, electrostatic potential (ESP) charges, and CO2 binding energies. A polarizable continuum model34 was used for implicit solvent calculations for a range of dielectric constant values, see SI. The CP2K35-36 package was used to perform spin-polarized density functional theory based AIMD simulations using the gradient-corrected PBE functional37 for exchange correlation. Dispersion corrections were included through the third generation of the empirical Grimme DFT-D3 approximation.38 For all atoms, core electrons were modeled with GTH pseudopotentials39, and a double-zeta quality basis set40 for valence electrons. A 340 Ry cutoff was used for the plane wave basis for the electrostatic energy. AIMD simulations were done in the NVT ensemble with a 0.5 fs time step using a Nose-Hoover41-42 thermostat to keep the solvent box at 40 °C. To simulate IPADM-2-BOL CO2 capture and uncover its mechanism, the Blue Moon13 procedure was used. The distance between the CO2 carbon and the IPADM-2-BOL oxygen (Figure 5A, rC-O) was used as a collective variable for thermodynamic integration. A series of 11 points was used



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and the forces on the constraint were compiled during ~10 ps of well-equilibrated trajectory to construct the potential of the mean force. The metadynamics23 technique was employed to induce the internal proton transfer in acidZwitterion equilibrium simulations. The direct metadynamics version43 as implemented in the CP2K package was used. Gaussian hills were added every 10 fs with a hill height of 0.00016 a.u. (0.1 kcal/mol, 0.42 kJ/mol) and a width of 0.08 in units of CV. Simulations were carried out until multiple CV crossing events were observed (see SI Figure S2), typically ~6 ps. The CV used (Figure 5B, equation 1) is the difference between the coordination number (CN) of the carboxylate oxygen and H atom, and the CN of the guanidium nitrogen and H atom. CV values range from -1 (acid where CNNH=0 and CNOH=1) to 1 (zwitterion where CNNH=1 and CNOH=0). The collective variable (CV) is defined as the difference between the N-H and O-H coordination numbers (CNab) and have the following functional forms: CV=CNNH - CNOH

(1)

r  1−  ab   r0  CN ab = ND  rab  1−    r0 

(2)

NN

where CNab is the coordination number (CN) between atoms a and b, rab the distance between atoms a and b, ro the threshold distance for bonding, and the NN and ND exponents that determine the curve. The values for ro, NN, and ND were 2.46 (a.u.), 10, and 22, for both oxygen-hydrogen and nitrogenhydrogen CNs.


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Figure 5: Rare event AIMD simulation variables: A) CO2 carbon to IPADM-2-BOL alcohol oxygen distance is the reaction coordinate for the Blue Moon simulations, and B) difference of coordination numbers is the collective variable for metadynamics.

Classical MD simulations of CO2BOL solvents at various CO2 loadings were done at 40 °C to calculate viscosities, self-diffusion coefficients, and dielectric constants. CO2-bound molecules were either all zwitterionic, 1:1 zwitterionic:acid, or all acid to gauge how non-ionic acid-state CO2-bound CO2BOLs change solvent viscosity. The GROMACS44 package version 4.6.6 was used for all classical molecular dynamics (MD) simulations. Optimized geometries and charges obtained from electronic structure calculations were used with the OPLS-AA45 force field. Certain dihedral angle parameters were obtained from electronic structure calculations (see SI). Mixtures of CO2-bound and unbound CO2BOLs were constructed at varying CO2 loadings (mol%: 0, 10, 15, 20, 25, 30), with a total of 1,728 molecules. Energy minimizations and high-temperature runs for proper mixing were done, followed by NPT ensemble equilibrations at 1 bar and 40 °C until the volume and total energy display steady-state behavior, typically occurring between 5 and 10 ns of simulation. Equilibrated box dimensions appear in the SI. Viscosities were calculated with three methods: (1) the Green-Kubo approach by calculating the integral of the pressure tensor autocorrelation function46, (2) the non-equilibrium method47 as 17 ACS Paragon Plus Environment


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implemented in GROMACS, which is based on the fluctuation-dissipation theorem, where added energy is consumed through viscous friction and viscosities can be calculated from the relaxation times following an applied acceleration, and (3) by comparing the calculated self diffusion coefficient with the self diffusion coefficient and viscosity of water48. The recently proposed approach49 to calculate viscosity in two-component ionic liquids, based on ion pair lifetimes, is not compatible with single molecule solvent systems. Additional details on the calculations and their errors appear in Tables S6 and S7 of the SI.

Acknowledgments The authors acknowledge the U.S. Department of Energy’s Office of Fossil Energy for funding award number FWP-65872. Computational resources were provided through a NERSC User Proposal. PNNL is proudly operated by Battelle for the U.S. Department of Energy. The authors wish to thank Dr. T. Brouns for his constant support of this work, and the editor Prof. B. Mennucci and the reviewers for their insightful comments and criticisms.

Author contributions D.C.C. planned and executed the simulations, J.L. and M.-S.L. constructed classical potentials and helped with data analysis. D.J.H., P.K.K. and C.J.F. provided and discussed experimental data and insights on CO2 capture systems. R.R. and V.-A.G. planned and supervised the research. All authors contributed to the writing of the paper.

Author Information Corresponding Author *[email protected]

Abbreviations AIMD: ab initio molecular dynamics


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MD: molecular dynamics DBU: diazabicyclo[5.4.0]-undec-7-ene TMG: 1,1,3,3 tetramethylguanidine IPADM-2-BOL: 1-((1,3-dimethylimidazolidin-2-ylidene)-amino)-propan-2-ol IPATFMM-2-BOL: (Z)-1-((1-methyl-3-(trifluoromethyl)imidazolidin-2-ylidene)amino)-propan-2-ol EODM-2-BOL: 1,3-dimethylimidazolidin-2-one oxime

Supporting Information Supplementary data as noted in the text. This material is available free of charge via the Internet http://pubs.acs.org.

Competing financial interests. The authors declare no competing financial interests.

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