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Computational Design of Functionalized Imidazolate Linkers of Zeolitic Imidazolate Frameworks for Enhanced CO2 Adsorption Mohammed Althaf Hussain, Yarasi Soujanya, and G. Narahari Sastry J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08043 • Publication Date (Web): 23 Sep 2015 Downloaded from http://pubs.acs.org on September 27, 2015

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The Journal of Physical Chemistry

Computational Design of Functionalized Imidazolate Linkers of Zeolitic Imidazolate Frameworks for Enhanced CO2 Adsorption M. Althaf Hussain, Yarasi Soujanya*, and G. Narahari Sastry* Centre for Molecular Modeling, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad, Telangana, India, 500007. Email: [email protected]

Abstract Zeolitic imidazolate frameworks (ZIFs) represent the class of metal-organic frameworks (MOFs) that possess high porosity, large surface area, exceptional thermal and chemical stability. Because of these properties, ZIFs are being employed extensively in gas separation and selective CO2 adsorption. We have chosen the structural modification approach to enhance the CO2 binding ability of various imidazolate (Im) linkers of ZIFs by systematically varying the substituents at 2, 4 and 5 positions of Im ring with CH3, Cl, CN, OH, NH2 and NO2 functional groups. Density functional theory (DFT) calculations have been employed to identify and quantify the CO2 binding ability of various adsorption sites present in 137 Im linkers. The study demonstrates that the Im linkers with asymmetrical substitution, viz. NO2/OH, CN/OH and Cl/OH combinations are highly promising linkers of ZIFs for efficient CO2 adsorption. The QTAIM analysis characterizes these interactions as noncovalent interactions which are stabilized by weak hydrogen bond and van der Waals (vdWs) interactions. Localized molecular orbital energy decomposition analysis (LMO-EDA) performed on substituted Im···CO2 complexes reveals that CO2 binding is governed by a combination of H-bonding, electrostatic, and 1 ACS Paragon Plus Environment


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dispersion interactions. The findings of the study will serve as guide-in principle to synthesize new adsorbents with enhanced and selective CO2 adsorption. Keywords: ZIFs, Imidazoalate, linker, adsorption, binding energy, analysis.

1. Introduction Carbon dioxide capture and storage from the flue exhaust is one of the major environmental challenges and plays a significant role in the mitigation of atmospheric CO2.1-3 A variety of solid adsorbents such as Zeolites, ZIFs, MOFs, activated carbon, metal oxides covalent organic frameworks (COFs), clathrate hydrates, carbaneous materials, polymers, graphene and their hybrids have been very widely studied for CO2 physisorption.1-21 Fundamental understanding of the binding mechanism of CO2 with these solid adsorbents is imperative in design of new materials for enhanced CO2 adsorption.16 It has been shown from our earlier studies that amino acids, onium ions and other π systems bind with CO2 by physisorption mechanism.3,16 ZIFs represent a unique class of MOFs in which the network topology and related properties vary greatly while core chemical connectivity is retained.2 ZIFs that comprise tetrahedrally coordinated metal ions and bridging Im linkers exhibit high thermal (>673K) and chemical stability.4,5 One of the advantages of ZIF chemistry is the ability to incorporate various functionalized linkers without much modification in the synthetic scheme of a target structure.7 This simplifies in correlating the gas uptake properties to the nature of the functional group in ZIF structures. Based on this concept, Yaghi et al. synthesized ZIFs with linkers substituted with CH3, Cl, CN, Br and NO2 functionalities to tune the adsorption capacity of CO2 as well as the selectivity of CO2/CH4 and CO2/N2.8,9 Having several feasible avenues for ZIFs to increase its CO2 uptake capacity and sensing metal ions, anions and small molecules, the quest to design new

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ZIFs with anticipated properties assumed outstanding significance in this decade.10-12,22-25 Though many millions of possible ZIFs can be generated by changing the linker, however in practice, synthesis of only a very small fraction of these materials is reported.13,15 In this context, computational screening has been an effective and pragmatic tool in designing a large number of materials and understanding the fundamental structure-function relationship in selection of adsorbents for CO2 capture. Unlike MOFs, experimental evidence based on inelastic neutron-scattering spectroscopy, infrared (IR) and Raman spectroscopic investigations which provide useful information for the CO2 interaction are not available for ZIFs.26,27 Over the past decade there are different theoretical techniques based on Monte Carlo (MC) and molecular dynamics (MD) simulations to explain the adsorption pattern and structure of ZIFs in conjunction with experiments.8,12,15,28 Based on these techniques, Liu et al.29 determined spatial distribution plots for the guest molecules in the ZIF unit cell, Further, Woo et al.30 determined the nature of CO2 binding sites and selectivity in ZIF 68 and ZIF 69. The reported grand canonical Monte Carlo (GCMC) calculations on ZIFs are mainly based on the force fields, which are specific to systems under study. Using universal force fields, few functionalized ZIFs were developed and their adsorption capacity in capturing small gases is studied.31-34 A recent study by Lee et al.35 has shown that the interactions of CO2 with heteroaromatic ring system possessing multi nitrogen atom super bases is essential for designing novel materials to effectively capture the CO2 gas. Although these studies indicate the binding potential wells, regions of high adsorbate density, an obvious route is to substantially improve the adsorption capacity of ZIFs with favorable multiple sites for CO2 binding.36,37 While the ability of only few functional groups of ZIFs to enhance CO2 adsorption has been described in the literature,36-40 so far there is no systematic study aiming to assess the impact of functional 3 ACS Paragon Plus Environment


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groups on CO2 adsorption properties.7,9,34-36,40 Similar strategies were applied for MOFs wherein specific polar groups (OH and NH2 etc.,) were inducted to improve CO2 selectivity from flue gas.41-45 In continuation of our earlier studies in understanding various factors that influence the nature of noncovalent interaction of CO2 with different adsorbents,3,14,16 we aim in the current study to enhance the CO2 adsorption capacity of ZIFs by systematically varying the functional groups. Several studies46-54 have been clearly indicated the fact that the CO2 uptake capacity is influenced primarily by functionality effects rather than the pore metrics of ZIFs. Thus, it is important to choose the functional group which will direct the mode of binding with CO2 in the vicinity of functionalized ZIFs.51-54 The functional groups CH3, Cl, OH, NH2, CN and NO2 are substituted systematically at C2, C4 and C5 positions of Im ring to compute their CO2 binding properties using DFT calculations. The results of the study will serve as a guide to modify/design new adsorbents with anticipated properties.

2. Computational Methods 2.1 Construction and Evaluation of Linker System Earlier studies7,53 have demonstrated the effective method of fragment based approach to explain the binding mechanism of gas molecules (H2 and CO2 in MOFs), wherein the finite simplest model that was large enough to represent infinite periodic ZIF crystal. For ZIFs, fragments of imidazole ring terminated with either hydrogen or Zn (CH3), or Li atom were used in earlier reports to study its CO2 binding.54 In view of several ways of fragmentation used to describe the environment of organic models, we have evaluated four different fragment models based on method of termination and its binding modes of CO2. As shown in Scheme 1 we evaluated four


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fragment models. Model 1, (H-Im-H) is represented by one Im ring terminated with two hydrogen and model 2 (Im), Im ring is connected to two Zn metal atoms instead of hydrogens. Model 3, (Im), is same as model 2 but with constrained positions of zinc metal atoms. In model 4, Zn(Im)3-Im-Zn(Im)3, Im ring is terminated with Zn metal atoms tetrahedrally coordinated to four Im rings. Scheme 1 near here It is clear from the Scheme 1 that the model 1 fails to mimic ZIF environment. As evident from model 2, constraining the dihedral angle to freeze zinc atoms has no impact on its electron densities. Therefore we have chosen a medium sized [Im]+1 fragment i.e., model 3, where two Zn ions were coordinated to one imidazole ring in the current study. Our studies show that the model 4 which provide similar results as model 3 is however expensive to perform calculations on all 137 linker model complexes, considering the huge number of possible binding sites and orientations. Our reasons for choosing the model 3 are a) computationally less expensive and smallest possible model for ZIFs to effectively compute the effect of substituting various functional groups on its interaction with CO2 b) possibility to span the maximum number of conformers with different substituents and c) Im model is repeating unit in ZIFs. Therefore, this model is an ideal and practical choice to represent ZIFs environment for CO2 binding. Scheme 2a and 2b near here In general, to compare the binding energetics and geometries of all possible binding sites, we have classified the Im linker space into five different regions for CO2 to bind, shown in Scheme 2a. The chosen Im model shown in Scheme 2b is substituted with various functional groups, CH3, Cl, CN, OH, NH2 and NO2 at C2, C4 and C5 positions of Im ring in mono, di and tri multipliers. Substitution of these functional groups at different positions of Im ring has generated



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137 model systems. Depending on the number and position of a functional group, we have classified the systems as mono, di and trisubstituted linkers shown in Scheme 2b. It is clear from the scheme that we have explored all the possible binding sites (a, b, c, d and e) of Im linker with CO2. The open metal site as shown in Scheme 1 is does not represent the real structural environment of ZIF. Therefore, we did not include the CO2 binding to zinc atom in the calculations. Besides, the binding of CO2 at sites d and e are also not possible in the periodic structure of ZIFs due to steric reasons. Therefore only interactions of CO2 arising from a, b and c sites of model Im linkers are considered in this study. The a site, where the CO2 is approaching from the C2 position of Im ring, will interact with the hydrogen at C2 or with the functional group if C2 is functionalized. But, in the site b, CO2 binds either at top or bottom face of the linker. Akin to site a, in c site, CO2 interacts with C4 and C5 hydrogens or with functional group. We have computed binding energetics for all possible orientations of CO2 with these model systems at a, b and c sites and the lowest minimum energy structures are reported and analyzed. Figure 1 near here 2.2 Geometry Optimization of Im Linkers All geometry optimizations were carried out at M06-2X/6-31G(d)55,56 level of theory keeping N1–Zn1 and N3- Zn2 bond lengths fixed at 1.96 Å, C2-N1-Zn1 and C2-N3-Zn2 bond angles fixed at 131.0˚ and Zn1-N1-C2-N3 and Zn2-N3-C2-N1 dihedral angles fixed at 180.0˚ as described by Gauss and Stanton.57 M06-2X appears to be an acceptable functional due to its satisfactory treatment of dispersion, hydrogen bonding and mixture of both interactions.58 This method is suitable for quantifying weak vdW interactions, which are sensitive to local environment of Im linker interacting with CO2.56 The binding energies (BEs) thus obtained are corrected for basis set superposition error (BSSE) by counterpoise method of Boys-Bernardi59 at M06-2X/cc-


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pVTZ//M06-2X/6-31G(d) level of theory. All the calculations are done using the Gaussian 09 package.60 The BEs are calculated as the difference between the sum of the total energies of parents ZIF (Im), CO2 and complex (Im···CO2) shown in the equation 1. BE = ( E + E ) − E∙∙∙

(1)

Bader’s61 QTAIM analysis is performed at M06-2X/6-31G(d) level of theory to distinguish the nature of noncovalent interactions involved in the studied complexes using the AIM 2000 program.62,63 This analysis demonstrates the electron density and it’s Laplacian at the bond critical points (BCPs) of interacting monomers and provides a quantitative strength of the noncovalent interaction between functionalized Im···CO2 complexes.16,64,65 For all the structures the topologies of the complexes consistent with the Poincaré-Hopf (PH) relationship.61,66,67 For an isolated molecule or molecular complex the PH relationship is calculated as shown in equation 2. n−b+r−c=1

(2)

Where, n stands for number of nuclear critical points denoted with a rank and signature of (3,−3), b for the number of bond critical points (3,−1), r for the number of ring critical points (3,+1), and c for the number of cage critical points (3,+3). In general, PH relationship is considered as a standard and completeness of the characteristic set.68 QTAIM is a powerful tool to study the nature of interactions for atom-atom interactions such as intermolecular contacts or valence bonds, thus, the characteristics of the corresponding BCPs are very important.61 Electron density (ρ(r)) is usually considered to measure the bond strength at BCP.61,68 The negative and positive values of Laplacian of electron density (∇2ρ(r)) represent the covalent and noncovalent interaction, respectively.69-71 The energetic properties of BCPs are often considered such as the local electron energy density (H(r)) at BCP and its components: the local electron kinetic energy



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density (G(r)), which is always positive and the local electron potential energy density (V(r)), is negative.72 The balance between these two values determines the type of interaction73 and the relationship between them is shown in equation 3. (3)

H(r) = G(r) + V(r) Which is also known from the virial theorem that

(4)

∇ ρ(r) = 2G(r) + V(r)

The -G(r)/V(r) may show the regions belonging to covalent or noncovalent interactions. For noncovalent interaction, -G(r)/V(r) ratio must be greater than 1. In the case of the ratio between 0.5 and 1, the interaction is partly covalent in nature and where –G(r)/V(r) is less than 0.5; thus interaction is a shared interaction provided H(r) and ∇2ρ(r) are negative.73 The energy decomposition analysis has been done using scheme developed by Su and Li popularly known as the LMO-EDA74 to identify the forces attributed to the modulation of these interactions as a function of groups substituted at C2, C4 and C5 position of Im ring. The geometries obtained at M06-2X/6-31G(d) level are further used to perform LMO-EDA at same level using the general atomic and molecular electronic structure system (GAMESS) program code.75 In case of LMO-EDA, the total interaction energy (∆E) of any complex at a given instant is analyzed for contributions from electrostatic (∆E

!

), exchange (∆E " ), repulsion

(∆E# $ ), polarization (∆E$%!) and dispersion (∆E&'$ ) energies. The total interaction energy is calculated as shown in the equation 5. ∆E = ∆E

!

+ ∆E

"

+ ∆E#

$

+ ∆E$%! + ∆E&'$

3. Results and Discussion


(5)

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The variation of BEs as a function of substituent and the underlying mechanism of interaction among all 137 Im···CO2 complexes are discussed in this section. The substituted Im linkers chosen in the current study as shown in the Figure 1 are subjected to interact in all possible orientations of CO2. The calculated BEs and geometries of all substituted Im···CO2 complexes are shown in Tables S1 to S3 and Figures S1 to S5 respectively. The corresponding QTAIM analysis and LMO-EDA of all complexes are shown in Tables S4 to S10. The geometries and their corresponding BEs of selected Im linkers, which are showing optimal BEs towards CO2 are shown in Figure 2 for mono, Figure 4 for di and Figure 5 for trisubstituted Im linkers. Quadruple moments due to the charge separation in the C=O (CO2) bonds allow CO2 to act both as a Lewis acid (LA) and Lewis base (LB). The carbon atom acquires partial positive charge acting as a LA and the two oxygen atoms have partial negative charges acting as a LB, making the carbon atom acting as an electron acceptor in a LA-LB interaction. Besides, oxygen atoms with partial negative charges can invoke weak electrostatic interactions with appropriately positioned electron deficient C-H bonds of Im linker resulting into a cooperative weak hydrogen bond (ImH···O(CO2)).76-78 Considering the possibility of multiple binding sites in Im linkers, we have examined all possible sites of interactions (Scheme 2a) and several possible orientations of the complex, Im···CO2 are generated. The most stable binding mode of a particular site is given in Table S1 for mono, Table S2 for di and Table S3 for trisubstituted linkers. The factors that are responsible for regioselective binding of CO2 in σ and π fashion with Im linkers at all sites are divided into various energy components using LMO-EDA analysis.79 The analysis is based on criteria that the particular group or combination of mixed functional groups should lead to significant enhancement of one or two sites or it should have optimal interaction with CO2 at all three sites and the results are presented in the order mono, di and trisubstituted linker systems.



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3.1 Monosubstituted Imidazolate Linkers Im linker with single group functionalization at C2 or C4 has generated eight and six symmetrical (1-8) and asymmetrical (9-14) linkers respectively as shown in Figure 1. The geometries and their corresponding BEs are depicted in Figure 2. From the Table S1, one can observe that for all the symmetrical linkers, the BEs are increasing in the order b > c > a sites in its interaction with CO2 except in the case of OH (5), wherein a site attain higher BE. Monosubstitution at C2 position of Im ring mostly stabilizes the binding at b site as shown in terms of their high BEs compared to a and c sites for complexes, 2, 3, 4, 6, 7 and 8 (Table S1). On the contrary, monosubstitution at C4 position mostly favors c site binding for 3, 4, 5 and 6. From the optimized geometries of linker complexes given in Figure 2, it is apparent that in view of its acidic proton (O-H), direct binding of CO2 with the OH functional group either at C2 or C4 stabilizes hydrogen bonding type (C-H···O(CO2) interactions in 5a and in 4c giving rise to 4.81 kcal/mol and 5.05 kcal/mol respectively. Figure 2 near here The QTAIM analysis also corroborates the strength of the hydrogen bond (ρ(r), 0.022) with hydrogen bond critical point (HBCP) shown in Table S11. The computed hydrogen bond critical point (ρ(r) and ∇2ρ(r)) values are within the range of the existence of hydrogen bonds proposed by Koch and Popelier (i.e., 0.002-0.034 a.u. for electron densities and 0.024-0.139 a.u. for Laplacian).80 The c site appears to be least interacting with CO2 in all monosubstituted linkers except in cases where CO2 interacts directly with functional group, as seen in Figure 2 for 12 and 14. These observations reveal that the OH substituent has a major effect in enhancing the contribution of electrostatic and polarization terms, as evident in Figure 3. Besides, other functional groups like CN and NO2 are also found to be potential functional groups for the


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enhancement of CO2. This illustrates that the nature of the functional group and its position in the Im ring play a significant role in enhancing CO2 adsorption. Figure 3 near here 3.2 Disubstituted Imidazolate Linkers Disubstitution of imidazole ring has generated 57 linkers which are categorized in to two groups, first group comprises 12 linkers (15-26) containing identical functional groups at C4 and C5 or at C2 and C4/C5, and the second one comprises 45 linkers (27-71) substituted with mixed functional groups at C4 and C5 or at C2 and C4/C5 positions of Im ring. From analyzing the data summarized in Table S2, it can be concluded that 15-20 linkers having same functional groups (CN, NO2, NH2) at C4 and C5 display enhanced binding at all a, b and c sites. From Figure 4 it is clear that substitution with OH group leads to preferential c site binding as a result of direct interaction of CO2, which is reflected as hydrogen bond critical points (HBCPs with ρ(r) of 0.020 a.u. and ∇2ρ(r) of 0.018 a.u.) between hydrogen of OH groups at C4 and C5 and oxygens of CO2. From the LMO-EDA for 18c, it is clear that the presence of two OH groups at C4 and C5 leads to enhancement of both electrostatic and polarization terms. Two NO2 groups at C4 and C5 (20b) induce Im ring as electron deficient, thereby dispersion component (-11.02 kcal/mol) dominates in CO2 binding at b site. These results indeed show a strong Lewis acid–base interaction exists between the gas molecule CO2 and nitro group.81 Another important observation of the study is that the substitution at C2 and C4 (21-26) with two identical functional groups did not improve its performance compared to linkers with same groups at C4 and C5 positions (15-20). Recent experimental studies82 have shown that the incorporation of mixed di functional groups (asymmetric) significantly enhance the binding affinity of CO2 with the Im linkers due to insertion of polar groups.



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Figure 4 near here Di substitution with mixed functional groups, keeping CH3 group constant and varying second functional group has yielded 27-41 linkers. It is interesting to note that a site is not preferred for any of linkers except in 34 for the reasons explained above. The combination of CH3 group with OH and NO2 (33-35 and 39-41) has shown impressive BEs (Table S2) with CO2 than with other groups. Especially 34 substituted with OH at C2 and CH3 at C4 is the ideal combination to experimentally test its binding capacity. In addition to this, 35 substituted with OH and CH3 at C4 and C5 respectively has a strong binding of 4.87 kcal/mol at c site, which is manifested in HBCPs and enhancement of both electrostatic and polarization terms (Figure 3). Disubstitution with mixed functional groups, keeping chlorine moiety constant and varying second functional group has yielded 42-53 linkers. With Cl and OH group combination, the linkers 45-47 are showing enhanced BEs only at selective binding sites. However linker with Cl at C2 and with NO2 group at C4 position (51) is having better BEs at all the sites. While keeping the CN group constant and varying other functional groups has yielded linkers 54-62. The combination of CN and OH groups at C4, C5 respectively has shown best results with the maximum BE of about 6.91 kcal/mol for 56c. Similarly, 54c has shown preferred BE of 5.64 kcal/mol. Among other combinations, NH2/NO2 (63-65), NO2/OH (66-68) and NH2/NO2 (6971), Im linkers with NO2 and OH groups (68) at C4, C5 has shown optimal BE of about 7.50 kcal/mol at c site. The forgoing analysis clearly reveal that the functional group plays a key role in fine tuning the adsorption capacity for CO2 and the best combinations to get optimum BEs are the C2 with CH3 and C4 with groups like OH, NO2, CN, NH2 and Cl. In order to compare the linkers with the combination of OH with other functional groups, we looked into geometries of 33-35, 45-47, 54-56 and 63-68 (Figure 1). It is clear that the


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combination of OH group with NO2 and CN groups seems to be much more efficient than other combinations as reflected by its highest BE of 7.47 kcal/mol (68c). Whereas the other combinations like C2 with NO2 or NH2 and C4 with OH and C4 with OH and C5 with NO2 or NH2 have shown optimal binding with CO2. Besides, substitution of OH group at C2 is found to be better than two OH groups either at C4, C5 or at C2, C4, as manifested in the better BEs at all three sites in 4 than 18 or 24. Nevertheless two NO2 groups at C4 and C5 also make all sites favorable in 20 than 6 or 26. It is interesting to observe that linker 34 with CH3 and OH groups, outperform 33 and 35. LMO-EDA analysis points to high electrostatic and polarization terms for this observation (Figure 3). As noticed earlier, the combination of OH and Cl group did not show multisite enhancement and therefore we did not continue our discussion for these groups in LMO-EDA. Instead, the combination of CN and OH at C4, C5 results multi site enhancement as reflected in their EDA and BEs data. Another important combination is NO2 with OH groups (66-68), which are showing promising binding affinity at multiple sites, especially in 68 with enhanced components of EDA. Similar observation is also seen by Ray et al.83 in a vdW-DFT study on CH4/CO2 adsorption selectivity in ZIF, wherein the organic linkers with two different functional groups are shown to enhance the CO2 binding strength. In a similar study,84 the CO2 binding strength in a set of five ZIFs (ZIF-25, ZIF-71, ZIF-93, ZIF-96 and ZIF-97), possessing Im linkers with different chemical functionalization is found to be governed by a combination of electrostatic, dispersive, and hydrogen-bonding interactions. These experimental results corroborate with our computational studies. 3.3 Trisubstituted Imidazolate Linkers Trisubstitution of imidazole ring has generated 66 linkers (72-137), in which 36 are symmetrical (72-107) and 30 with asymmetrical (108-137). A comparison of 72-77 linkers substituted with all



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three identical functional groups at C2, C4 and C5 with the corresponding disubstituted linkers (for example, compare the BEs of 29 with 108 and 109, given in Figure 4 and Figure 5 respectively) reveals that the substitution with third functional group did not yield any significant effect on BEs except for OH and NO2 groups (75 and 77). Another observation is that linkers with CH3 at C2 are found to exhibit effective binding at multiple sites when it is combined with either OH or NO2 groups at C4 and C5 positions (82, 86 > 83, 87). On contrary, the combination of Cl with OH or NO2 groups has shown enhancement at 90c and 94b. Thus, by keeping CH3 and Cl groups constant and varying the third functional group has no improvement in CO2 adsorption, even with OH and NO2 groups. On the other hand a much enhanced BEs (>6.0 kcal/mol) is noticed in case of OH and CN combination (96, 97), and around 5.0 kcal/mol BEs for OH and NH2 (103) and NO2 (104). Two strong electron withdrawing groups (EWGs), CN or NO2 substituted at C4 and C5 positions when combined with OH group at C2 position make the hydroxyl proton highly acidic facilitating a strong hydrogen bond type interaction at a site, accompanied by enhanced binding at both b, c sites (97, 104 > 96, 104). From LMO-EDA analysis (Figure 3), b site with higher dispersion contribution becomes dominant in most of trisubstituted ones than monosubstituted and disubstituted. Figure 5 near here The asymmetrical trisubstitution of CH3 with other functional groups is not found to be a good combination for the CO2 adsorption. The combinations, where OH with Cl (120 and 121), CN (126 and 127), NH2 (132 and 133) and NO2 (134 and 135) have shown reasonable enhancement in BEs are given in Table S3. The corresponding geometries and topological properties are given in Figure 5 and Table S12 respectively. Among all 30 asymmetrical Im linkers, linkers with the combination of Cl/OH (120-121) and CN/OH (126-127) are found to be


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favorable for multi site (a, b and c) binding to CO2. In contrast, combinations involving OH group leads to enhancement of BE at only c site due to the presence of hydrogen bond type interaction. LMO-EDA reveals the primary role of electrostatic and polarization terms in stabilization of the complexes (Figure 3). Furthermore, the linkers with the combination of OH/NO2 with highest BE of 7.82 kcal/mol appear to be promising candidates for further testing among all mono, di and trisubstitutions. The asymmetrical Im linker systems substituted with the functional groups in di and tri fashion exhibit optimal BEs towards CO2, in agreement with study on a series of isoreticular ZIFs by Yaghi and coworkers.7-9 Thus, BEs are enhanced for Im linkers whenever the OH group is in asymmetrical combination with CN and NO2 groups. For example, 12 with 5.05 kcal/mol, 56 with 6.91 kcal/mol and 68 with 7.47 kcal/mol, 48 with 7.82 kcal/mol. Further, the disubstitution at C4 and C5 positions with OH and NO2 (68 with 7.47 kcal/mol) or OH and CN (56 with 6.91 kcal/mol) display high BEs which can be further improved by the insertion of EWG at C2 with OH and NO2 (135 with 7.82 kcal/mol) or OH and CN (126 with 7.18 kcal/mol). From this study we propose that the combinations shown in Figure 6 is the best combination for CO2 adsorption using OH and NO2 and OH and CN. Besides, the other combinations where all the three a, b and c sites have shown enhancement for CO2 adsorption are 5, 11, 14, 17, 20, 41, 55, 58, 69, 71, 75, 97, 104, 110, 116, 121, 127 and 134. These observations illustrate that the adsorption is highly sensitive to the position of a functional group and the high adsorption of 135, 68 and 134, 126, 127 and 56 may be traced to large contribution arising from electrostatic, polarization terms as well as to hydrogen bond type interactions. Obviously, dipole moment plays an important role in determining the charge directionality of Im linker, because it is related to the polarizability of the linker. The calculated dipole moments of mono, di and trisubstituted Im linkers are compiled in Figure S6, which



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reveals that the polar functional groups like, NO2 and CN displays high dipole moments. Further, the combination of these functional groups with OH group is found to enhance binding energies significantly. Figure 6 near here The topological descriptors obtained for selected Im···CO2 complexes from QTAIM analysis are provided at the BCPs are shown in Table S11 and S12 and their corresponding interactions along with total kinetic energy G(r) and total potential energy V(r) are also provided. From these results, one can see that the condition for noncovalent interactions, i.e. ∇2ρ(r) > 0 and H(r) > 0 is fulfilled in all complexes. Moreover, the balance between local kinetic energy density (G(r)) and local potential energy density (V(r)) also greater than 1, indicating these interactions are noncovalent in nature.16 The ratio, –G(r)/V(r) values are in the range of 1.0 to 1.4 a.u. suggesting that these complexes are stabilized through weak hydrogen bonded and weak vdWs interactions. 3.5 Comparison with Experimental Results The list of ZIFs with functionalized Im likers that were experimentally reported for CO2 uptake properties is given in Table 1. Evident from this table, few linkers of 137 model linkers chosen in the current study are identical to linkers of ZIFs tested experimentally. The adsorption capacity and selectivity of a given ZIF are related to both the affinity of the Im linker for CO2 and the pore size of the ZIF, which in turn is determined by the steric demands of the linker. The combination of CN group with NH2 group in 59 did not show noticeable difference from 11 having only CN group. This is in accordance with experimental observation in ZIF-82 and ZIF96. The small increase in its CO2 uptake for ZIF-82 is reasoned to presence of another linker with NO2 group at C2 position. Interestingly, adding the second chlorine group has shown


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significant improvement in CO2 adsorption, evident from ZIF-69 and ZIF-95 and ZIF-100. Comparison of CO2 uptake values in 15 and 35 substantiate that the effect of combination of a mixed functional groups is much more pronounced than having identical groups. The analysis of the calculations also illustrate that 35 display better performance than 15. Another outcome of the study is that the CO2 adsorption capability of 10 which is tested experimentally can be further improved with the substitution of NO2 group at C5 position (53). Table 1 near here

Conclusions The current investigation aimed at understanding the influence of various factors on CO2 binding efficiency of Im linkers substituted with different functional groups and underlines the possibility to engineer the ZIFs networks by judiciously employing Im linkers. We have employed a novel structural modification approach based on quantum chemical calculations to tune the properties of ZIFs, obtained by systematically varying the strengths of functional groups CH3, Cl, CN, OH, NH2 and NO2 in mono, di and tri fashion at C2, C4 and C5 positions of Im ring. Instead of enhancement at one particular site of a functionalized Im, exhibiting optimal adsorption at all three sites is found to be the favorable approach in design of new linkers. The study demonstrated that the CO2 adsorption capacity of Im linkers of ZIFs mainly depend on nature and the position of the functional group. The QTAIM analysis shows that the criteria for noncovalent interactions in all complexes are fulfilled and the complexes are stabilized by weak hydrogen bond and weak vdWs interactions. Similarly, LMO-EDA reveals that CO2 binding is governed by a combination of H-bonding, electrostatic, and dispersion interactions. In summary, Im linkers with asymmetrical substitution, viz. NO2/OH, CN/OH and Cl/OH combinations are



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highly promising linkers of ZIFs for efficient CO2 adsorption. Linkers with these combinations 56c, 68c, 126c, 127c, 134c and 135c are the most promising ones among the 137 model linkers considered in the study. Experimental efforts in this direction are rewarding.

Author Information Corresponding Author * Tel: (+91)4027193016, Email: [email protected]

Supporting Information (SI) Tables of binding energies at M06-2X/cc-pVTZ//M06-2X/6-31G(d) level, QTAIM analysis and LMO-EDA analysis at M06-2X/6-31G(d) level. Figures of optimized geometries obtained for mono, di and trisubstituted Im···CO2 complexes at M06-2X/6-31G(d) level of theory. Tables of total electron energy densities and its components of selected ZIFs at M06-2X/6-31G(d) level of theory. This material is available free of charge via internet at http://pubs.acs.org.

Acknowledgement Authors thank DST, New Delhi, for the financial assistance in INDO-EU sponsored AMCOS project and 12th five year plan project of CSIR (INTELCOAT and MSM). MAH thanks CSIR, New Delhi for senior research fellowship.


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79. Rao, J. S.; Sastry, G. N. Structural and Energetic Preferences of π, σ, and Bidentate Cation Binding (Li+, Na+, and Mg2+) to Aromatic Amines (Ph-(CH2)n-NH2, n = 2-5): A Theoretical Study. J. Phys. Chem. A 2009, 113, 5446-5454. 80. Koch, U.; Popelier, P. L. A. Characterization of C-H-O Hydrogen Bonds on the Basis of the Charge Density. J. Phys. Chem. 1995, 99, 9747-9754. 81. Hou, X.-J.; Li, H. Unraveling the High Uptake and Selectivity of CO2 in the Zeolitic Imidazolate Frameworks ZIF-68 and ZIF-69. J. Phys. Chem. C 2010, 114, 13501-13508. 82. Thompson, J.; Blad, C. R.; Brunelli, N. A.; Lydon, M. E.; Lively, R. P.; Jones, C. W.; Nair, S. Hybrid Zeolitic Imidazolate Frameworks: Controlling Framework Porosity and Functionality by Mixed-Linker Synthesis. Chem. Mater. 2012, 24, 1930-1936. 83. Ray, K. G.; Olmsted, D.; Houndonougbo, Y.; Laird, B. B.; Asta, M. Origins of CH4/CO2 Adsorption Selectivity in Zeolitic Imidazolate Frameworks: A van der Waals Density Functional Study. J. Phys. Chem. C 2013, 117, 14642-14651. 84. Ray, K. G.; Olmsted, D.; He, N.; Houndonougbo, Y.; Laird, B. B.; Asta, M. van der Waals Density Functional Study of CO2 Binding in Zeolitic Imidazolate Frameworks. Phys. Rev. B 2012, 85, 085410-085418. 85. Thompson, J. A.; Brunelli, N. A.; Lively, R. P.; Johnson, J. R.; Jones, C. W.; Nair, S. Tunable CO2 Adsorbents by Mixed-Linker Synthesis and Postsynthetic Modification of Zeolitic Imidazolate Frameworks. J. Phys. Chem. C 2013, 117, 8198-8207.



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

Model

System

HOMO

Page 30 of 39

LUMO

1

2 H

180.0 131.0 C 131.0 180.0 2

3

Zn

Zn

1.96

N1 C5

H

N3

1.96

C4 H

4

Scheme 1. Pictorial representation of HOMO and LUMO orbitals of different model systems considered in this study.


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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


a.

b.

Scheme 2. a) Im linker with representation of CO2 binding sites b) Schematic representation of various types of substituted Im linkers in symmetrical (S) and asymmetrical (A) fashion considered in the current study.



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

Figure 1. Substituted Im linker systems to study the physisorption of CO2.


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Page 33 of 39

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


5a, 4.81

5b, 3.33

6b, 3.64

7b, 3.91

11a, 3.12

2.352

11c, 3.58

12c, 5.05

14a, 3.13

14c, 4.07

Figure 2. Bond lengths in Å and BSSE corrected binding energies in kcal/mol of monosubstituted Im···CO2 complexes at M06-2X/cc-pVTZ//M06-2X/6-31G(d) level of theory. (Red-oxygen; grey-carbon; white-hydrogen; blue-nitrogen; greenchlorine).


15

15

12

12

9

9

Energies in kcal/mol

6 3 0 -3 -6 -9

3 0 -3 -6 -9

-12

-15

-15

-18

-18

-21 -24

-21

a.

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6

-12

-24

b. 75c 77b 82c 83a 86b 94b 96c 97a 99c 100b 104a 104b 106b 110c 110b 113c 115b 116c 116b 120c 121a 121c 125c 127c 128c 129c 132c 133c 134c 135c

5a 5e 6b 7b 11a 11c 12c 14a 14c 18c 20b 24a 24c 33c 34a 34b 35c 45c 46a 47c 51c 51b 54c 56c 60c 64c 65c 66c 67c 68c 71c

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

Energies in kcal/mol


Complexes

Complexes

Figure 3. Contribution of various factors towards the total BE as calculated at the M06-2X/631G(d) level using the LMO-EDA approach for a. Mono and disubstituted Im···CO2 complexes and b. Trisubstituted Im···CO2 complexes. (Black-electrostatic, red-exchange, green-repulsion, blue-polarization, cyan-dispersion, pink-total interaction energy).


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18c, 5.14

34a, 4.53

47c, 4.86

20b, 5.17

24b, 4.64

24c, 4.90

33c, 4.88

34b, 4.47

35c, 4.87

45c, 5.16

46a, 4.91

51b, 4.10

51c, 4.11

54c, 5.64

56c, 6.91 1.912 2.751

60c, 4.38

64c, 4.89

65c, 4.81

68c, 7.47

66c, 4.08

67c, 5.57

71c, 4.14

Figure 4. Bond lengths in Å and BSSE corrected binding energies in kcal/mol of disubstituted Im···CO2 complexes at M06-2X/cc-pVTZ//M06-2X/6-31G(d) level of theory. (Red-oxygen; grey-carbon; white-hydrogen; blue-nitrogen; greenchlorine).



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

Page 36 of 39

75c, 5.20

77b, 5.64

82c, 4.94

83a, 5.02

86b, 5.34

90c, 5.07

94b, 5.38

96c, 6.04

97a, 6.09

99c, 4.91

100b, 5.35

104a, 5.49

104b, 5.57

106b, 5.55

110b, 4.73

110c, 4.69

113c, 4.79

116b, 4.46

116c, 4.81

120c, 4.97

121a, 5.47

115b, 5.13

121c, 4.97

126c, 7.18

127c, 6.93

1.949 2.881

128c, 4.51

129c, 4.34

133c, 4.70

132c, 4.51

134c, 7.19

135c, 7.82

Figure 5. Bond lengths in Å and BSSE corrected binding energies in kcal/mol of trisubstituted Im···CO2 complexes at M06-2X/cc-pVTZ//M06-2X/6-31G(d) level of theory. (Red-oxygen; grey-carbon; white-hydrogen; blue-nitrogen; green-chlorine).


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Figure 6. Model complexes, Im depicting the best combination of functional group substitution for CO2 adsorption with asymmetrical substitution of Im ring.



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

Page 38 of 39

Table 1. The experimental data on CO2 adsorption with ZIFs, the corresponding model linkers for a given ZIF id reported in parenthesis. ZIF (Im)

Linker 1

Linker 2

Experimental CO2 uptake Density in Conditions at 273 K mmol/g and 1bar

References

ZIF8 (2)

-

8.014

Yes

45, 85

ZIF25 (15)

-

900.0*

Yes

7

ZIF68 (7)

1.679

Yes

5, 71

ZIF69 (7)

1.813

Yes

5, 71

ZIF70 (7,1)

2.455

Yes

0.650

Yes

ZIF78 (7)

2.299

Yes

ZIF79 (7)

1.496

Yes

ZIF81 (7)

1.705

Yes

ZIF82 (10,7)

2.353

Yes

ZIF71 (16)

-

5

7 5 5 5 5

ZIF96 (59)

-

2.180

Yes

7

ZIF97 (35)

-

1840.0*

Yes

7

ZIF100

-

1.70.0 0.957

At 273 K At 293 K

13

*

mmol/m2


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