A Theoretical Study

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Physical Chemistry Chemical Physics

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Published on 15 August 2017. Downloaded by University of Windsor on 15/08/2017 17:58:28.

Endohedral Gas Adsorption by Cucurbit[7]uril: A Theoretical Study Sudip Pan,1,2,* Gourhari Jana,2 Ashutosh Gupta,3 Gabriel Merino,1,* and Pratim K. Chattaraj2,* 1

Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados Unidad Mérida. km 6 Antigua carretera a Progreso. Apdo. Postal 73, Cordemex, 97310, Mérida, Yuc., México

2

Department of Chemistry and Centre for Theoretical Studies, Indian Institute of Technology, Kharagpur, 721302, India 32

Department of Chemistry, Udai Pratap Autonomous College, Varanasi, Uttar Pradesh, 221002, India

* Corresponding authors: [email protected] (SP), [email protected] (GM), [email protected] (PKC)

1

Physical Chemistry Chemical Physics Accepted Manuscript

DOI: 10.1039/C7CP03984K


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Abstract The selectivity of cucurbit[7]uril (CB[7]) towards adsorbing a series of 14 molecules


encompassing four hydrocarbons (C 2 H2 , C2 H4 , C2 H6 , and CH4 ), diatomic molecules of halogens (F2 and Cl2 ), nitrogen oxides (NO 2 and NO), carbon oxides (CO 2 and CO), SO2 , H2 S, N 2 , and H2 is explored via density functional theory based study. CB[7] is noted to have high selectivity towards adsorbing SO 2 over the other considered molecules, highlighting its probable utility to separate SO 2 from the flue gas or other gas mixtures containing these molecules. The nature of bonding is deciphered via the computations of non-covalent interaction indices and energy decomposition analysis. Although in all cases the dispersion interaction is turned out to be the most dominating contributor in stabilizing these complexes, the electrostatic contribution is also considerable. In fact, the combined effect of these two energy terms in SO 2 @CB[7] is responsible for the obtained selectivity.

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DOI: 10.1039/C7CP03984K

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DOI: 10.1039/C7CP03984K

glycoluril [=C4 H2 N4 O2 =] units, are pumpkin shaped methylene linked macrocyclic host molecules. Over the years they have been proved to act as better hosts than other traditional


host molecules such as resorcinarenes and calixarenes.1 CB[n]s are symmetrical with a hydrophobic inner cavity and two identical hydrophilic carbonyl edge portals and have high ability in encapsulating various types of guest molecules including drugs, organic dyes, metal cations, halide ions, different guest molecules etc., indicating their possible application in industry in different ways.2 Particularly, great attention has been provided on the gas adsorbing ability of CB[6].2k-2m,2p The selective adsorption of CO 2 by CB[6] over CO and CH4 in the solid state was showed by Kim et al.2m Recently, CB[6] was noted to encapsulate Cl2 followed by C 2 H2 selectively over a series of 19 gas molecules.3 CB[7] possesses 14 carbonyl and 28 methylene groups that surround a large cavity having 7.3 Å diameter, which is close to that of β-cyclodextrin, and accordingly it exhibits better molecular recognition properties with guest molecules than CB[6].4 CB[7] was reported to bind aromatic compounds including stilbenes, viologen, naphthalene and even metal complexes, showing its application in cancer treatment.5 Quite recently, in experiment Tian et al.6 noted CB[7] to adsorb CO2 selectively over N2 and CH4 molecules, which was further theoretically verified by Shau and Ganguly.7 The former group6 also found exceptionally high encapsulating capacity of CB[7] towards SO 2 and H2 S. A detailed scrutiny regarding the adsorption of H2 S by CB[7] was also studied by Ganji and Danesh8 via an ab initio van der Waals density functional approach. Given that the size of the cavitand in CB[n]s would have an impact on the adsorption ability, obviously the selectivity of CB[7] to adsorb a particular gas molecule over a wide 3


Cucurbit[n]urils, commonly abbreviated as CB[n]s, where n is the number of


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Article Online range of gas molecules would differ from that of CB[6]. For example, SO 2 cannot be View fitted DOI: 10.1039/C7CP03984K

spectrum of gas molecules would be worthy in order to get a proper knowledge of utilizing CB[7] to separate a particular gas from a given mixture. Herein, we have considered 14 such molecules encompassing four hydrocarbons (C 2 H2 , C2 H4 , C2 H6 , and CH4 ), diatomic Published on 15 August 2017. Downloaded by University of Windsor on 15/08/2017 17:58:28.

molecules of halogens (F2 and Cl2 ), nitrogen oxides (NO 2 and NO), carbon oxides (CO 2 and CO), SO 2 , H2 S, N 2 , and H2 . Their energetics and the bonding analysis are performed by employing density functional theory (DFT) based computations to understand the relative stability of different gas molecules encapsulated by CB[7] and the factors responsible for the interaction, respectively. CB[7] is found to bind SO 2 the most strongly followed by Cl2 and C2 H2 among these considered molecules. The full optimization without any geometry constraints followed by the harmonic vibrational frequency characterization of CB[7] and its different molecule encapsulated analogues were carried out using the long-range corrected ωB97X-D functional,9 in conjunction with 6-31G(d,p) basis set. Note that ωB97X-D is parameterized in such a way that it includes the empirical atom-atom dispersion corrections and is appropriate to describe non-covalent interactions. Taking the most stable minimum energy structures obtained at the ωB97X-D/6-31G(d,p) level, further geometry optimization was performed at the ωB97XD/6-311+G(d,p) level. Due to the larger sizes of the present systems, the frequency computation at the latter level is not affordable, rather we used the thermal corrections from the former levels to compute and thermochemical parameters like enthalpies and free energies at 298 K. All these computations were done by using Gaussian 09 program package.10

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within the cavitand of CB[6]. Therefore, a scrutiny regarding the selectivity over a large

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Energy

decomposition

analysis

(EDA)11

was

carried

out

at

the

View Article Online BP86-

DOI: 10.1039/C7CP03984K

total interaction energy (ΔEint ) between two fragments is decomposed into four energy terms, viz., the electrostatic interaction energy (ΔVelstat ), Pauli repulsion energy (ΔEPauli), orbital interaction energy (ΔEorb) and dispersion interaction energy (ΔEdisp ). Therefore, ΔEint can be Published on 15 August 2017. Downloaded by University of Windsor on 15/08/2017 17:58:28.

written as, ΔEint = ΔEPauli + ΔVelstat + ΔEorb + ΔEdisp

(1).

For more details about EDA, the reader is referred to the related reviews.13 The real space characteristics of the interaction were further examined via the evaluation of non-covalent interaction (NCI) indices,14 which describe the nature of the interactions based on electron density (ρ) and its derivatives. The gradient isosurface in real space was visualized by NCIPLOT program15 where the color of the isosurfaces is decided by the value of sign(λ 2 ). For more information about NCI, the reader is referred to the excellent papers by Contreras-Garcia and co-workers.16 In the present cases, blue, green, and red color codes were used to represent stabilizing H-bonding, van der Waals, and destabilizing steric interactions, respectively. The NCI plots were generated at the ωB97X-D/6-311+G(d,p) level. Different guest molecules in this series are found to occupy different positions and orientations in order to maximize the interaction with the host moiety (see Fig. 1). In the cases of hydrocarbons, owing to the acidic behaviour of hydrogen in C 2 H2 , it orients itself in a vertical way with respect to the cage in front of the –C2 H4 N2 – moiety in order to be connected through H-bonding with the four edged oxygen centers simultaneously. In moving along C2 H2 -C2 H4 -C2 H6 , because of the gradually lowered acidic character of hydrogen, the guest inclines itself in larger degree towards the horizontal orientation to maximize the van 5


D3/TZP//ωB97X-D/6-311+G(d,p) level using ADF 2013.01 program package. 12 In EDA, the


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View Article Online der Waals interaction. On the other hand, while carbon oxides (CO and CO 2 ), nitrogen oxides DOI: 10.1039/C7CP03984K

that unlike the schematic diagram proposed by Tian et al., 6 CO2 does not reside exactly at the center position of the cage, rather it moves towards the wall of CB[7], which was also noted earlier by Sahu and Ganguly.7 In case of SO 2 , two oxygen centers direct towards the wall and Published on 15 August 2017. Downloaded by University of Windsor on 15/08/2017 17:58:28.

sulphur atom is tilted inward. All these guest molecules remain well inside the cavitand of CB[7]. However, on the contrary H2 S is located near the open face of the cage, where two H centers of H2 S interact with two O centers of CB[7]. Similar to the most of the van der Waals complexes, due to the encapsulation neither the CB[7] cage nor the guest molecules get affected by each other as reflected in their very small deformation energies (0.0-0.9 kcal/mol) and very negligible charge alteration (less than0.02 e-). Among all the studied systems, SO 2 is found to be encapsulated with the highest binding enthalpy (Hb: 14.3 kcal/mol) followed by Cl2 (11.6 kcal/mol) and C2 H2 (10.4 kcal/mol) (see Table 1). Further, in terms of Hb, CB[7] also shows higher efficacy by 2.0 and 2.3 kcal/mol towards adsorbing C 2 H4 and C2 H6 , respectively, than CO 2 (7.7 kcal/mol). On the other hand, NO 2 and H2 S show almost a similar tendency for encapsulation within CB[7] as that of CO 2 . The corresponding Hb values for NO, F2 , N 2 , CO and CH4 encapsulated systems lie within the range of 4.7-5.8 kcal/mol with the highest value for the case of CH4 .

6


(NO and NO 2 ) and N 2 remain in a horizontal position, F2 , Cl2 , and H2 orient vertically. Note

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Fig. 1 ωB97X-D/6-311+G(d,p) geometries of guest molecule encapsulated CB[7] systems. As expected from the smallest size, H2 has the least affinity to be adsorbed inside the cavitand. Therefore, the present enthalpy results reflect the high selectivity towards uptaking SO2 over the considered series of molecules. This highlights the possible role of CB[7] as an agent for the removal of SO 2 from the flue gas and other related gas mixtures. So, it would have huge industrial importance. The possibility of multiple SO 2 molecules to be adsorbed within the cavitand is explored further; however, the space permits only one SO 2 molecule to remain inside the cavitand at a time.

7



DOI: 10.1039/C7CP03984K


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View Article Online Table 1. The binding enthalpy (Hb, kcal/mol) and the free energy change at 298 DOI: K (G 298K, 10.1039/C7CP03984K

X + CB[7]  X@CB[7], respectively, where X is the guest molecule at the ωB97X-D/6-


311+G(d,p) level. G298K

Systems

Hb

SO 2 @CB[7]

14.3

-3.4

CO2 @CB[7]

7.7

0.7

CO@CB[7]

4.9

4.0

C2 H2 @CB[7]

10.4

-1.4

C2 H4 @CB[7]

9.7

0.6

C2 H6 @CB[7]

10.0

1.4

CH4 @CB[7]

5.8

3.6

N2 @CB[7]

4.8

2.4

NO2 @CB[7]

7.4

1.3

NO@CB[7]

5.1

2.8

F2 @CB[7]

4.7

2.5

Cl2 @CB[7]

11.6

-2.0

H2 S@CB[7]

8.1

2.4

H2 @CB[7]

1.1

6.1

Note that Tian et al.6 found an uptake capacity of 5.5 mol of SO 2 per mole of CB[7] in the solid state at 1 bar and 297 K, indicating other possible adsorption sites at interstitial 8


kcal/mol) computed for the desorption and adsorption process, X@CB[7]  X + CB[7] and

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View Article Online voids generated during molecular aggregation in solid state. Therefore, the present study DOI: 10.1039/C7CP03984K

where the host-guest interaction is dominated only in the molecular cavitand. In fact, since the last decade the utility of CB[n] and cage structures in solution and gas phases draws significant attention as gas storage and gas separation complementing the solid-state Published on 15 August 2017. Downloaded by University of Windsor on 15/08/2017 17:58:28.

chemistry.17 More realistic prediction regarding the selectivity can be made through the inclusion of entropic effects at standard conditions. Based on the computed Gibbs free energy change (G 298K) values, the first three preferable molecules (SO 2 > Cl2 > C2 H2 ) for the adsorption remain exactly the same as that based on the Hb values. Thereafter, a little bit change in the adsorption order is noted in comparison to the order obtained based on the corresponding Hb values. For example, in contrast to the Hb results, after the consideration of entropic effects C2 H6 is found to be less prone to be encapsulated than CO 2 . Note that CO 2 is a hazardous byproduct, produced in many processes. Hence, if the intent is to separate CO 2 selectively from a given gas mixture by employing CB[7], hydrocarbons, C2 H2 and C2 H4 , including SO 2 and Cl2 must be absent from there. Otherwise, the substitution reaction like CO 2 @CB[7] + X  X@CB[7] + CO 2 would take place exergonically with the G298K values of -4.1 (SO 2 ), -2.7 (Cl2 ), -2.1 (C2 H2 ) and -0.1 (C2 H4 ) kcal/mol. It is well-known that the adsorption process is entropically unfavourable and hence, the process would become exergonic only if enthalpy is strong enough to overbalance the entropy term. In the present cases, only SO 2 , Cl2 , and C2 H2 are found to be adsorbed exergonically at standard condition with G298K values of -3.4, -2.0, and -1.4 kcal/mol, respectively. The adsorption of C 2 H4 and CO 2 is slightly endergonic in nature (by only 0.6-0.7 kcal/mol). Of course, it would become feasible at slightly lower

9


reflects the selectivity of CB[7] better at the molecular level as in solution and gas phases


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temperature. The corresponding G 298K values for the C 2 H6 , N 2 , NO 2 , NO, DOI: F2 10.1039/C7CP03984K and View H2Article S Online

The results of EDA, performed by taking guest molecule as one interacting fragment and CB[7] as another, are provided in Table 2. In all cases, the ΔEdisp term is the major


contributing factor (ca. 48-67% of the total attraction) to stabilize these systems. Nevertheless, the results show that in all these cases electrostatic interaction is also appreciable (ca. 17-39% of the total attraction). As expected from the possibility of H-bond formation between the acidic hydrogens of C 2 H2 and H2 S, and oxygen centers of CB[7], the contribution from ΔVelstat (ca. 35-39% of the total attraction) is larger than that in the others. The interaction between CB[7] and SO 2 also results in a quite high electrostatic contribution, presumably because of the strong dipole-dipole interactions between polar SO 2 molecule and charged portal of CB[7]. It is also apparent from Table 2 that in moving from C 2 H2 to C2 H4 to C2 H6 , the ΔVelstat contribution decreases gradually due to the reduced acidic character of hydrogens. Further, although the contribution from the ΔEorb term is the least in all cases, it is not negligible; rather it is responsible for the 9-21% of the total attraction. While the higher affinity of CB[7] towards SO 2 molecule than towards C 2 H2 is a consequence of the larger dispersion interaction (ΔΔEdisp = 3.7 kcal/mol) in the former case than that in the latter, a higher ΔVelstat and a lower ΔEPauli values in former make the interaction between CB[7] and SO2 stronger than those in C 2 H4 and C2 H6 analogues. On the other hand, with respect to Cl2 and H2 S enhanced electrostatic and dispersion interactions in SO 2 @CB[7] mainly dictate the selectivity, respectively. Therefore, a combined effect of ΔEdisp and ΔVelstat terms is responsible for the obtained selectivity towards adsorbing SO 2 over others.

10


adsorption lie within the range of 1.3-2.8 kcal/mol.

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View Article Online Table 2. The results of EDA taking the guest molecules as one fragment and CB[7] as DOI: 10.1039/C7CP03984K


kcal/mol. Systems

ΔEint

ΔEPauli

ΔVelstat

ΔEorb

ΔEdisp

SO 2 @CB[7]

-13.4

11.3

-8.4 (33.9)

-2.7 (10.7)

-13.7 (55.3)

CO 2 @CB[7]

-8.2

8.2

-4.4 (26.5)

-1.6 (9.4)

-10.6 (64.1)

CO@CB[7]

-6.1

5.6

-2.3 (19.2)

-1.6 (13.8)

-7.9 (67.0)

C2 H2 @CB[7]

-12.2

9.6

-8.4 (38.4)

-2.4 (11.2)

-11.0 (50.4)

C2 H4 @CB[7]

-11.7

11.8

-6.3 (26.6)

-2.8 (11.8)

-14.5 (61.5)

C2 H6 @CB[7]

-11.4

13.2

-5.6 (22.9)

-3.1 (12.5)

-15.9 (64.6)

CH4 @CB[7]

-7.2

8.2

-3.4 (21.9)

-2.0 (12.8)

-10.0 (65.3)

N2 @CB[7]

-6.0

5.6

-2.0 (17.1)

-1.9 (16.6)

-7.7 (66.3)

NO 2 @CB[7]

-8.6

8.7

-3.7 (21.3)

-3.5 (20.4)

-10.1 (58.3)

NO@CB[7]

-3.1

7.8

-2.4 (21.8)

-1.3 (11.8)

-7.2 (66.4)

F2 @CB[7]

-2.8

4.4

-1.7 (23.9)

-0.9 (12.5)

-4.6 (63.6)

Cl2 @CB[7]

-10.6

8.6

-5.3 (27.5)

-1.8 (9.2)

-12.2 (63.3)

H2 S@CB[7]

-10.3

10.0

-7.1 (35.1)

-3.5 (17.0)

-9.7 (47.8)

H2 @CB[7]

-2.6

4.2

-1.8 (26.5)

-0.9 (13.2)

-4.1 (60.3)

The presence of the non-covalent interactions between the guest molecules and CB[7] can be further deciphered by the NCI analysis (see Figs. 2 and 1-SI in supporting information). In every case, the green surfaces are developed in the middle region of the guest molecule and the closest CB[7] surface, highlighting the van der Waals contact therein, according to the presently used color-coding scheme. The amplification in the size of the 11


another at the BP86-D3/TZP//ωB97X-D/6-311+G(d,p) level. All energy terms are in


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View Article Online green surface implies an increased van der Waals interaction in these systems. For example, DOI: 10.1039/C7CP03984K

consequently, the stabilization originated from van der Waals interaction gradually increases along the same order. In case of the best system, the isosurface mainly lies in between O


centers of SO 2 and glycouril units of CB[7].

Fig. 2 NCI isosurface plots of gas@CB[7] at the ωB97X-D/6-311+G(d,p) level.

In conclusion, CB[7] exhibits the affinity towards encapsulating SO 2 as the largest over the other considered molecules because of the combined effects of electrostatic and dispersion interactions. Therefore, the present results highlight the prospect of utilizing CB[7] in order to separate SO 2 from the flue gas or other gas mixtures comprising these studied molecules.

12


in moving from C 2 H2 to C2 H4 to C2 H6 the green surface area gradually enhances and

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Acknowledgements

View Article Online

DOI: 10.1039/C7CP03984K

thanks IIT, Kharagpur for his fellowship. This work is supported by Conacyt (Grant CB2015-252356). The CGSTIC (Xiuhcoalt) and ABACUS at Cinvestav (Conacyt grant


EDOMEX-2011-COI-165873) are acknowledged for allocation of computational resources.

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PKC would like to thank DST, New Delhi for the J. C. Bose National Fellowship. GJ


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View Article Online Fundamental Aspects of Chemical Bonding, p. 121-158, G. Frenking, and S. Shaik DOI: 10.1039/C7CP03984K

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Graphical abstract

Cucurbit[7]uril adsorbs SO 2 selectively over the other gas molecules like C 2 H2 , C2 H4 , C2 H6 , CH4 , F2 , Cl2 , NO 2 , NO, CO 2 , CO, H2 S, N2 , and H2 , because of the combined effects of electrostatic and dispersion interactions.

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DOI: 10.1039/C7CP03984K