In silico studies on the origin of selective uptake of carbon dioxide with cucurbit[7]uril amorphous material

Abstract

The efficient capture and storage of flue gases is of current interest due to environmental problems. We report the adsorption of flue gases (CO₂, N₂ and CH₄) on amorphous solid Cucurbit[7]uril (CB[7]) computationally. The DFT calculations revealed that CO₂ can be adsorbed more strongly inside the cavity of CB[7] compared to N₂ and CH₄ molecules. The glycoluril units of CB[7] are the preferential sites for the adsorption of CO₂ gas molecules. The cooperative binding of CO₂ molecules inside the cavity of CB[7] has been observed. The geometrical analysis reveals that the carbon atom of CO₂ is in close proximity to the nitrogen atom of the glycoluril of CB[7] and the CO₂ oxygen atom is in close contact to the carbonyl carbon of the glycoluril unit. The calculated results show that four CO₂ and four CH₄ molecules can reside inside the CB[7] cavity. However, five N₂ gas molecules can be accommodated inside the CB[7] cavity. The energy decomposition analysis (EDA) performed with the adsorbed CO₂ on the wall of CB[7] shows that the dispersive force is playing an important role for the uptake of CO₂ inside the cavity. The process of desorption was also examined with the desorption enthalpies (ΔH_DE) calculated per gas molecule, which suggests that both adsorption and desorption processes are kinetically feasible. The origin of the interactions between the amorphous solid CB[7] and the flue gases can help to design materials to maximize the capture and separation of such gases.

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Introduction

Molecular gas recognition is an emerging area of research that has various applications in gas sensing, gas storage, gas purification, gas conversion, and as biosensors.^1,2 Gas encapsulation within the cavities of synthetic organic molecules has been explored with different systems in solution and/or in the solid state.¹ The threat of global warming is one of the most pressing environmental concerns in recent times.³ Power plants and steel mills are the main stationary source for the emission of CO₂ in nature.⁴ The 40–50% of total CO₂ emission is coming out of such stationary sources.⁴ In the steel making process, CO gas, in particular, is used as a reducing agent, which is converted into CO₂ gas and emitted from the plants.⁴ This is a challenge in developing new materials for the storage, separation and recycling of the important gases such as CO₂ and H₂ in industrial plants.⁵ In recent years, metal–organic frameworks (MOFs)^6–10 or covalent organic frameworks (COFs)^11,12 have been reported in the literature, which showed remarkable CO₂ sorption capacity and selectivity with an exceptionally high surface area. Such MOFs and COFs can be modified easily to enhance the storage capacity. However, these materials are sensitive to moisture (a major component in flue gas) that dampens the efficient capture of CO₂.^4,13 A few organic molecules like calixarenes,¹⁴ cucurbit[n]uril,^1,4,13,15,16 carbon nanotubes and activated carbon⁵ have been explored as porous materials for gas storage and separation. These organic porous materials can be used as appropriate candidate for such adsorbents that can be easily synthesized from the readily available organic molecules. Recently, organic host cucurbit[n]uril (CB[n], n = 6, 7) is used as good alternatives for the CO₂ adsorbents^4,13,15 and can also be used effectively for the separation of CO₂ from different gas mixtures.¹⁷ This pumpkin shape cucurbiturils (CBs) have become an extremely attractive class of supramolecular synthetic receptors^18,19 with different molecular applications.^20,21 Cucurbiturils (CBs) are cyclic oligomers of n glycoluril units linked by 2n methylene groups.²² The CBs possess two hydrophilic carbonylated rims and a hydrophobic nano-cavity.²³ The synthetic macro-polycyclic receptors CB[n] have excellent ability to bind various organic, inorganic, biological molecules and ions in the aqueous phase as well as in the solid state.^24,25 The amorphous solids CB[6] and CB[7] have been explored as sorbents for the flue gases such as CO₂, N₂ and CH₄.¹³ CB[7] has shown remarkable selectivity of CO₂ over N₂ and CH₄.¹³ At 297 K and 1 bar pressure, the CO₂ storage capacity of amorphous CB[7] is 2.3 mmol g⁻¹ (50 cm³ g⁻¹) and 1.2 mmol g⁻¹ (25.2 cm³ g⁻¹) for CB[6], respectively.¹³ In this article, we have examined the origin and magnitude of the possible interactions of the flue gases with CB[7] employing DFT (M06-2X) functional level of theory.^26,27 The interactions have been segregated by EDA and non-covalent interactions (NCI) surface analyses.

Computational details

Full geometrical optimizations have been carried out in the gas phase employing the M06-2X level^28–30 with standard 6-31G(d) basis set. The hybrid meta-functional M06-2X has been considered as an excellent DFT functional considering the non-covalent interactions³¹ and it predicts the accurate valence and Rydberg electronic excitation energies for main group chemistry.²⁸ Frequency calculations were performed at the same level of theory, to confirm that each stationary point is a local minimum (with zero imaginary frequency). The single-point calculations with M06-2X/6-311+G(d,p) have also been performed on M06-2X/6-31G(d) optimized geometries. The calculations were performed with the placement of gas molecules either at the center of the host molecule or else near to the surface walls. The calculated results provide the optimum positions of gas molecules with respect to the host cavity site. Similar study was also performed with the gas molecules outside the surface of CB[7]. Generally, Minnesota functional M06-2X shows good results for short- and medium-range dispersions however, this functional is not suitable for long-range dispersion in larger systems.^32–35 The calculated results obtained with M06-2X functional was further compared with MO6-2X-D3 results for representative systems, where the long-range dispersion is considered using DFT-D3 treatment.³² Furthermore, the importance of dispersion corrections was revealed through the calculations performed with the standard other functional like PBEPBE³⁶ using 6-31G(d) basis sets. NMR chemical shifts were calculated by the GIAO method³⁷ at the M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory to correlate the observed experimental NMR shifts. All the calculations have been performed with GAUSSIAN 09 suite of program.³⁸

The adsorption enthalpy (ΔH) is calculated as:

ΔH = [H_S(gas)n − (H_S + nH_gas)] (1)

The average binding enthalpy per gas molecule (H_b) is calculated as:

H_b = (1/n)[H_S(gas)n − (H_S + nH_gas)] (2)

Similarly, the adsorption energy (ΔE)^39,40 is calculated as:

ΔE = [E_S(gas)n − (E_S + nE_gas)] (3)

The desorption enthalpy (ΔH_DE)^39–41 is calculated as:

ΔH_DE = H_gas + (1/m)[H_S(gas)n−m − H_S(gas)n] (4)

where, ‘S’ denotes the host molecule (CB[7]) and ‘gas’ denotes flue gas (CO₂/N₂/CH₄). [n = total no. of gas molecule and m = no. of the desorbed gas molecules.]

The wave function analysis in this study is performed in the multifunctional MULTIWFN-3.2.1 suite of program.⁴² To study the electron transfer of the trapped gas molecules inside the CB[7] cavity, we have plotted the difference map of electron density by using the MULTIWFN-3.2.1 program.⁴² The Energy Decomposition Analysis (EDA)⁴³ has been performed at the same level of theory using GAMESS software.⁴⁴ In the EDA analysis, the total interaction energy is divided into the five parts, viz. electrostatic, exchange, repulsion, polarization and DFT dispersion energies.⁴⁵

Results and discussion

We have examined the origin and nature of the binding affinity of gas molecules (CO₂, N₂ and CH₄) with CB[7] using DFT calculations. The physisorption of the gas molecules was performed with the single unit of CB[7] for computational simplicity. The gas molecules (CO₂, N₂ and CH₄) were adsorbed sequentially inside the cavity of the CB[7] and the binding enthalpies (ΔH) were calculated with M06-2X/6-31G(d) level of theory. The adsorption process was initiated with only one gas molecule inside the cavity of CB[7] (Fig. 1). The M06-2X/6-31G(d) calculated results showed that the CO₂ and CH₄ molecules reside near the glycoluril-wall of the CB[7] cavity, however, the N₂ molecule is present at the center of the CB[7] cavity due to the repulsive interactions with the peripheral wall of the CB[7] (Fig. 1). The calculated structure shows that the carbon atom of CO₂ is in close proximity to the nitrogen atom of the glycoluril of CB[7] (Fig. 1). Further, the CO₂ oxygen atom is also in close contact to the carbonyl carbon of glycoluril unit. Such interactions were observed in the ¹³C MAS NMR spectrum study.¹³ The ¹³C MAS NMR spectrum showed that there is an upfield shift of Δδ_c = −1.8 ppm for the stored CO₂ and is presumably due to the interaction of gas molecule with the negatively charged carbonyl oxygens and or electron donating nitrogen atoms of the host molecule.¹³ The NMR (GIAO) calculations performed with CB[7] and CO₂ molecule, we have also observed the similar upfield shift (Δδ_c = −1.7 ppm) for the stored CO₂ inside the cavity of the CB[7] at M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory. The M06-2X/6-31G(d) calculated binding enthalpy (ΔH) of single CO₂ is −8.4 kcal mol⁻¹, which is in good agreement with the experimental enthalpy at zero coverage.¹³ The M06-2X/6-31G(d) binding enthalpies (ΔH) calculated for single N₂ and CH₄ are 0.0 kcal mol⁻¹ and −4.6 kcal mol⁻¹, respectively (Fig. 1). The calculated free energy results also support the higher adsorption of CO₂ inside the CB[7] cavity compared to the other two gas molecules (see ESI, Table S1†). The role of temperature on the adsorption of gases with CB[7] has also been considered. The increase in temperature can make the adsorption of CO₂ be more selective than N₂ and CH₄ gas molecules with CB[7] host molecule assuming the pressure remains constant in such cases (see ESI, Table S1†). The volume of host molecule to adsorb the gas molecules may also be important. The binding ability of CB[6] host molecule with single CO₂ is better than CB[7], however, the volume of the host systems can dictate the gas adsorption capacity. The experimental results reveal the higher CO₂ adsorption capacity of CB[7] host molecule.¹³ The computed binding energies (ΔE) with higher basis set with M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory also showed similar trend of interaction of these gas molecules with CB[7] (Fig. 1).

Fig. 1 M06-2X/6-31G(d) calculated binding enthalpies (ΔH) of one CO₂, N₂ and CH₄ inside the cavity of CB[7]. The computed binding energies (ΔE) at the M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory are given in the parentheses. All the energies are in kcal mol⁻¹ and distances are in Å. (Colour of atoms: grey – C, white – H, blue – N and red – O.)

We have extended our computational experiment with two gas molecules placed inside the CB[7] cavity. The calculated M06-2X/6-31G(d) results show that the gas molecules are close to the glycoluril units of the host molecule in parallel fashion in all cases (Fig. 2). Here, the binding enthalpy of two CO₂ molecules inside the cavity of CB[7] is −17.6 kcal mol⁻¹, which is higher than the corresponding N₂ (−9.9 kcal mol⁻¹) and methane gas molecules (−10.1 kcal mol⁻¹). Interestingly, the N₂ molecules prefer to reside near the glycoluril unit of the host molecule and such a situation arises due to the electrostatic repulsions between the two N₂ molecules inside the cavity of CB[7] (Fig. 2). The interaction of glycoluril units of CB[7] with the CH₄ molecules is similar to that obtained with the single CH₄ molecule (Fig. 1 and 2). The average binding enthalpies (H_b) of CO₂, N₂ and CH₄ with CB[7] are −8.8, −4.9 and −5.0 kcal mol⁻¹, respectively, which are comparable to the binding affinities of single gas molecule with CB[7] (Fig. 1 and 2). Since the long-range dispersion is not considered in the M06-2X functional, we have examined the long range dispersion by using D3-zero damping parameters for M06-2X functional.^32–35 The dispersion corrected M06-2X-D3 binding enthalpies are slightly higher than the uncorrected M06-2X binding enthalpies however, the trends is similar in both cases (see ESI, Table S2†). M06-2X-D3 calculated results predict the binding enthalpy even closer to the reported enthalpy of adsorption of CO₂ molecules at zero coverage.¹³ The calculations performed with the standard PBEPBE/6-31G(d) level showed much lower binding enthalpies of the gas molecules with CB[7] compared to the M06-2X results (see ESI, Table S2†).

Fig. 2 M06-2X/6-31G(d) calculated binding enthalpies (ΔH) of two CO₂, N₂ and CH₄ inside the cavity of CB[7] and their corresponding average binding enthalpies (H_b). The computed binding energies (ΔE) at the M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory are given in the parentheses. All the energies are in kcal mol⁻¹ and distances are in Å. (Colour of atoms: grey – C, white – H, blue – N and red – O.)

We have extended our computational study with three gas molecules placed inside the CB[7] cavity. The binding enthalpy (ΔH) of three CO₂ molecules with the host molecule is −26.5 kcal mol⁻¹, which is higher than N₂ (−13.7 kcal mol⁻¹) and CH₄ (−15.7 kcal mol⁻¹), respectively (Fig. 3). The average binding enthalpies (H_b) of CO₂, N₂ and CH₄ remain to be similar as observed with the previous cases (Fig. 1–3). The gas molecules entrapped inside the cavity of CB[7] showed cooperative interactions among them. The gas molecules were taken out from the cavity of CB[7] and calculations were performed with orientations as obtained in the host cavity. The cooperative binding energy of three CO₂ molecules is −4.3 kcal mol⁻¹, whereas, such interactions with N₂ and CH₄ are −0.9, −1.0 kcal mol⁻¹ respectively (Fig. 4). Similar cooperative interactions for CO₂ molecules were also reported for the MOF [Zn₂(Atz)₂(ox), Atz: 3-amino-1,2,4-triazole; ox: oxalate] in the literature.⁴⁶ These results further suggest the cooperative effect among CO₂ molecules is also responsible for the enhanced binding energy compared to N₂ and CH₄ inside the cavity of CB[7].

Fig. 3 M06-2X/6-31G(d) calculated binding enthalpies (ΔH) of three CO₂, N₂ and CH₄ inside the cavity of CB[7] and the corresponding average binding enthalpies (H_b). The computed binding energies (ΔE) at the M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory are given in the parentheses. All the energies are in kcal mol⁻¹ and distances are in Å. (Colour of atoms: grey – C, white – H, blue – N and red – O.)

Fig. 4 Cooperative intermolecular interactions of the three CO₂, N₂ and CH₄ inside the cavity of CB[7]. M06-2X/6-31G(d) calculated energies are in kcal mol⁻¹ and distances are in Å.

The similar trend of binding enthalpies are also observed for the four gas molecules inside the CB[7] cavity. The calculated binding enthalpy (ΔH) of the CO₂ is −34.2 kcal mol⁻¹ compared to N₂ (−19.6 kcal mol⁻¹) and CH₄ (−21.5 kcal mol⁻¹) (Fig. 5) and, the average binding enthalpies (H_b) of CO₂, N₂ and CH₄ are −8.6, −4.9 and −5.4 kcal mol⁻¹ respectively (Fig. 5). M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) calculated binding energy (ΔE) results show the similar trend as obtained with M06-2X/6-31G(d) calculated binding enthalpies (ΔH) (Fig. 5). The CO₂ molecules are close to the glycoluril wall of the CB[7] like the previous cases (Fig. 1–3) and interacting strongly with each other (Fig. 6). The cooperative enhancement effect of CO₂ molecules is also responsible for the strong CO₂ adsorption (Fig. 6).⁴⁶ The N₂ molecules oriented inside the cavity of CB[7] also showed some attractive interactions, presumably due to the induced dipoles created in presence of these gas molecules. Desorption of gas molecules from the host molecule is also important for practical purposes. We have performed the calculations to examine the desorption (inverse of the adsorption process)^39,40 of the gas molecules from the CB[7]. The M06-2X/6-31G(d) calculated desorption enthalpies (ΔH_DE) for CO₂, N₂ and CH₄ are 9.5, 3.9 and 5.5 kcal mol⁻¹ respectively. These results suggest that both adsorption and desorption processes are kinetically feasible.

Fig. 5 M06-2X/6-31G(d) calculated binding enthalpies (ΔH) of four CO₂, N₂ and CH₄ inside the cavity of CB[7] and the corresponding average binding enthalpies (H_b). The computed binding energies (ΔE) at the M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory are given in the parentheses. All the energies are in kcal mol⁻¹ and distances are in Å. (Colour of atoms: grey – C, white – H, blue – N and red – O.)

Fig. 6 Cooperative intermolecular interactions of the four CO₂, N₂ and CH₄ inside the cavity of CB[7]. M06-2X/6-31G(d) calculated energies are given in kcal mol⁻¹ and distances are in Å.

The cooperative effect among the gas molecules and their interactions with the CB[7] was further exemplified with non-covalent interactions (NCI) isosurface plots (Fig. 7). NCI isosurfaces are generally used to understand the nature of interaction in different types of systems.⁴⁷ The NCI results reveal that the flue gas molecules have attractive van der Waals interaction^14,47–49 among themselves and also with the host walls as represented in colour-coding scheme.

Fig. 7 Noncovalent interactions (NCI) isosurface plots for the trapped gas molecules (CO₂, N₂ and CH₄) inside the CB[7] cavity.

The nicely packed gas molecules inside the cavity of CB[7] can exhibit electron transfer between the host and the trapped gas molecules (CO₂, N₂ and CH₄). We have calculated the difference map of electron density of the CB[7] and the trapped gas molecules to understand the electron transfer between the host and guest systems in MULTIWFN-3.2.1 program⁴² (Fig. 8). The electron transfer is significantly higher in the case of CO₂ compared to N₂ and CH₄ trapped gas molecules inside the CB[7].

Fig. 8 The difference map of electron density of the CB[7] and the trapped gas molecules. The red and yellow isosurfaces represent the region in which electron density is increased and decreased after gas molecules trapped to CB[7], respectively.

To examine further, we have incorporated gas molecules inside the cavity of CB[7], and it appears that the fifth gas molecule for CO₂ and CH₄ moves away from the cavity of the host molecule (Fig. 9). However, N₂ being smaller in size, five such molecules can reside inside the cavity of a single CB[7].

Fig. 9 M06-2X/6-31G(d) optimized structures of the five gas molecules (CO₂, N₂ and CH₄) with CB[7] (side view) with their corresponding binding enthalpies (ΔH) and average binding enthalpies (H_b). The computed binding energies at the M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory are given in the parentheses. All the energies are in kcal mol⁻¹ and distances are in Å. (Colour of atoms: grey – C, white – H, blue – N and red – O.)

The experimental results support that four CO₂ gas molecules can be accommodated inside the cavity of CB[7].¹³ Further to note that the maximum sorption capacity of amorphous CB[7] for CO₂ is 118 cm³ g⁻¹ (5.3 mmol g⁻¹) at 1 bar and 196 K, which corresponds to 6.1 CO₂ molecules per CB[7]. These results suggest that the two CO₂ molecules are present in the interstitials voids of the host molecules.¹³ We have placed two CO₂ molecules outside the cavity of CB[7], while keeping the four CO₂ molecules inside the host molecule (Fig. 10). The M06-2X/6-31G(d) calculated results showed that all six CO₂ molecules are bound strongly with the host system compared to the other two gas molecules (N₂ and CH₄) (Fig. 10).

Fig. 10 M06-2X/6-31G(d) optimized structures of the four gas molecules (CO₂, N₂ and CH₄) inside the cavity of CB[7] while two gas molecules at the outside the cavity of CB[7] with their corresponding binding enthalpies (ΔH) and average binding enthalpies (H_b). The computed binding energies at the M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory are given in the parentheses. All the energies are in kcal mol⁻¹ and distances are in Å. (Colour of atoms: grey – C, white – H, blue – N and red – O.)

The study was performed with the adsorption of mixed gas molecules inside the cavity of CB[7]. The adsorption of N₂ and CH₄ gas molecules in the presence of two CO₂ molecules is weaker compared to the adsorption of another CO₂ molecule at the same level of theory. Therefore, it appears that the selectivity of CO₂ adsorption with CB[7] is higher even in mixed environment of flue gas molecules (see ESI, Fig. S2†).

The origin and magnitude of CO₂ interaction with CB[7] was examined with Energy Decomposition Analysis (EDA).⁴³ The EDA analysis was modelled with three glycoluril units of CB[7] and the CO₂ molecule. This geometry has been taken from the adsorbed CO₂ molecule inside the cavity of CB[7] (Fig. 1). The M06-2X/6-31G(d) calculated results with the model geometry reveal that the total interaction energy is −5.9 kcal mol⁻¹, which is further segregated into five energy components i.e., electrostatic (−7.0 kcal mol⁻¹), exchange (−6.4 kcal mol⁻¹), repulsion (29.6 kcal mol⁻¹), polarization (−1.4 kcal mol⁻¹) and DFT dispersion energies (−20.5 kcal mol⁻¹). EDA calculations are performed with the model systems, (Fig. 11) which show that the role of dispersive interaction is predominant in nature towards the adsorption of CO₂ on the cavity of CB[7]. This result is in agreement with the reports on CO₂ adsorption with MOF systems [Zn₂(Atz)₂(ox), Atz: 3-amino-1,2,4-triazole; ox: oxalate] reported in the literature.⁴⁶ The interaction energy of CO₂ with the CB[7] moiety was further calculated with basis set superposition error (BSSE) correction via counterpoise procedure (CP) method with M06-2X/6-31G(d) level of theory. The calculated corrected complexation energy of CO₂ with CB[7] moiety is −5.9 kcal mol⁻¹, which is in reasonable agreement with the uncorrected BSSE interaction energy (−5.9 kcal mol⁻¹).

Fig. 11 CO₂ molecule interacting towards the inner side of the CB[7] moiety.

Therefore, the higher binding enthalpy of CO₂ with the walls of CB[7] arises due to the dispersion interactions. The strong quadruple moment of the CO₂ molecule may help to associate with other CO₂ gas molecules inside the cavity of CB[7], which in turn can augment the adsorption process.^46,50 The preferred binding of CH₄ than N₂ in all the cases can be explained based on the polarizability data.⁴⁶ The higher polarizability of CH₄ (12.5 Bohr³) compared to N₂ (8.4 Bohr³) calculated with M06-2X/6-31G(d) level of theory rationalizes the observed results. It can be mentioned here that the binding enthalpies of the CO₂, N₂ and CH₄ outside the cavity of CB[7] are generally lower compared to the inside the cavity of CB[7] cases (see ESI, Fig. S1†).

Conclusions

This work demonstrates the origin of the physisorption of the gas species (CO₂, N₂ and CH₄) by CB[7] in a systematic study. The DFT (M06-2X) calculated results show that four CO₂ and CH₄ molecules can reside inside the CB[7] cavity, while five N₂ gas molecules can be accommodated in the cavity of host molecule. The M06-2X/6-31G(d) calculations suggest that the binding enthalpy of CO₂ with CB[7] is higher compared to the other flue gases (N₂ and CH₄). The binding enthalpy of four CO₂ molecules is −34.2 kcal mol⁻¹ inside the CB[7] cavity, which is much higher than the N₂ (−19.6 kcal mol⁻¹) and CH₄ (−21.5 kcal mol⁻¹), respectively. The DFT-D3 corrected M06-2X functional results indicate that the binding enthalpy of the adsorbed CO₂ gas molecule inside the CB[7] cavity much closer to the reported enthalpy of adsorption of CO₂ molecules at zero coverage.¹³ The ¹³C NMR (GIAO) calculations at M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) level of theory level of theory shows that the up-field shift of the carbon atom of CO₂ while interacting with the glycoluril wall of CB[7], which corroborates the experimentally observed results. The Energy Decomposition Analysis (EDA) reveals that the dispersive force is predominant for the adsorption of CO₂ with the glycoluril units of CB[7]. The preferential adsorption of CH₄ over N₂ attributes to the higher polarizability of the CH₄ over N₂. These results may help to explore the potentials of cucurbiturils for separation and storage of gas molecules.

Acknowledgements

CSIR-CSMCRI Communication No. 149/2014. The authors thank DBT, DST, CSIR (MSM, SIP) New Delhi, India and DAE-BRNS Mumbai, India for financial support. D. S. is thankful to UGC, New Delhi, India, for awarding a senior research fellowship. D. S. is also thankful to AcSIR for enrolment in the Ph.D. program. The authors thankfully acknowledge the computer resources provided by NCL, Pune, India, and CMMACS, Bangalore, India. The authors are also thankful to Dr Rabindranath Lo for his valuable suggestions in this work. We thank the reviewers' for their valuable comments/suggestions that have helped us to improve the paper.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 and S2 and Tables S1 and S2. See DOI: 0.1039/c5ra13394g

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