A silver coordination cage assembled from [Ag₂(bis(isoxazolyl))₃]: DFT approach to the binding forces within the host–guest interactions

Abstract

The storage and detection of different types of molecules using porous materials such as metal–organic frameworks (MOFs) has currently become an area of interest in chemistry. In this sense, non-covalent interactions in host–guest arrays are among the most significant topics to address. In this work, the inclusion of a series of gases in the cavity of a minimal secondary building unit (SBU), namely, [Ag₂(bisox)₃] as the host was studied in terms of their non-covalent interactions and optical properties using different computational approximations. The relevant interaction energies indicated favorable inclusion of incoming guests in the respective host. Consequently, the decomposition of the interaction energy within the Ziegler–Rauk scheme revealed significant differences within the series of studied gases. Hence, the potential selectivity of the [Ag₂(bisox)₃] cage for a particular gas within the series was investigated. Natural orbital for chemical valence (NOCV) analysis was carried out to visualize possible channels of charge transfer between the host–guest pairs. Non-covalent interaction (NCI) analysis supports the fact that different types of weak interaction are involved in the series of studied gases and highlights remarkable differences between polar and non-polar guests. Otherwise, the ability of the studied cage to trap gases was evidenced by changes in the spectral properties of the free cage with respect to those of cages containing guests. This was investigated via computational analysis of the UV-vis absorption spectra. Furthermore, electron density difference maps (EDDMs) were employed to reveal the character of the electron transitions.

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Introduction

In the past, metal–organic frameworks (MOFs) were also frequently referred to as metal coordination polymers, before the term MOFs was widely adopted. They are an emerging class of porous materials that retain their structures and crystallinity after removal of a solvent, which enables them to display an interesting set of properties such as gas storage,^1–5 hydrocarbon separation,^6–10 conductivity,^11–13 CO₂ capture,^14–17 mixed matrix membranes for gas separation,¹⁸ luminescence,^19,20 antenna effects,²¹ catalysis,^22,23 drug delivery,^24–27 and construction of sensors,^28,29 among others. Consequently, these materials are the subject of continuous research in order to discover their basic features and applications. In this sense, both theoretical and experimental studies enable the understanding of properties that these structures might exhibit and contribute to the development of new potential applications.

MOFs may be defined as supramolecular solids that are constructed by the combination of metal ions or clusters and linkers through strong bonds, which provide robustness to the linking units. Furthermore, MOFs can be considered to be materials constructed from secondary building units (SBUs).^30,31

The field of MOFs has become one of the fastest growing areas in chemistry. This is demonstrated by the ever-increasing number of reported structures and the vast number of published articles, as well as by the constant expansion in the scope of research and engagement of researchers.^1–31 The main interest is focused on porous structures in the framework. Recent studies of frameworks of silver formed by the bis(isoxazolyl) ligand 1,4-bis(3,5-dimethylisoxazol-4-ylmethyl)benzene, which is denoted hereafter as bisox, have attracted much attention. This semi-rigid linker contains methylene spacers between the isoxazole rings and the central benzene ring, which allow it to adopt numerous different conformations.

Thus, the reaction between bisox and silver perchlorate in methanol gave crystals of [Ag₂(bisox)₃] cages, which represent an SBU species of an MOF structure, as shown in Fig. 1, which has recently been synthesized by Burrows et al.³² In this paper, we propose to investigate theoretically the energetically favorable molecular arrays formed by [Ag₂(bisox)₃] cages upon the inclusion of a series of gases (such as CH₄, CO₂, and N₂, among others) for subsequent gas storage applications. Specifically, we intend to study the interactions between the [Ag₂(bisox)₃] unit or cage structure and a set of various gases. In this context, we propose to study, by employing computational methods, the interactions involved between the cage and the group of gases via energy decomposition analysis (EDA).^33–35 Furthermore, the non-covalent interaction (NCI)^36,37 properties were determined for the interactions of the subunit (host) with the gas (guest) substrates, which might be captured inside the cage. This analysis provides a graphical index, which enables the characterization of the binding forces within the host–guest pair upon the inclusion of a guest in the MOF structure. The interactions of molecules trapped inside the cage are governed by weak forces. The inclusion of calculations of NCI is of particular importance, because their analysis gives rise to proposed models of interactions for future gas storage applications.

Fig. 1 Representation of the [Ag₂(bisox)₃] free cage.

Finally, to understand whether the optical properties of the cage are affected when gas molecules are trapped in the cage, it is essential to comprehend the effect of gases on the energetic and electronic properties of low-lying excited states. In order to gain insight into these properties, TD-DFT analysis for the calculation of the vertical electronic spectra of the free host and the host–guest pairs was performed. The elucidation of the character of the transitions was supported by electron density difference map (EDDM) plots, which are currently a useful tool for studying charge differences in photochemical processes.³⁸

Computational details

Relativistic density functional theory³⁹ calculations were carried out using the ADF 2014 code,⁴⁰ which incorporated scalar corrections via the two-component ZORA Hamiltonian.⁴¹ The triple-ξ Slater basis set plus two polarization functions (STO-TZ2P) for valence electrons was employed within the generalized gradient approximation (GGA) according to the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional.^42,43 In order to consider attractive van der Waals interactions, the D3-Grimme dispersion correction⁴⁴ was added for both geometry optimizations and energy decomposition analysis. Geometry optimizations were undertaken without any symmetry restraint via the analytical energy gradient method implemented by Versluis and Ziegler,⁴⁵ in which the energy minima for the optimized geometries were confirmed by calculations of vibrational frequencies. Energy decomposition analyses (EDA) were carried out according to the Ziegler–Rauk scheme^33–35 and natural orbital for chemical valence (NOCV) analysis.^46–50 These analyses were carried out with basis set superposition error⁵¹ (BSSE) corrections. Non-covalent interaction (NCI) analysis was carried out using the NCIPLOT program.⁵² on the basis of an analysis of electron density descriptors. Solvation effects were modeled by a conductor-like screening model for real solvents (COSMO).⁵³

The absorption spectra of the [Ag₂(bisox)₃] cage and systems that consisted of the cage structure incorporating a set of various gases (host–guest) were computed by means of time-dependent DFT (TD-DFT)⁵⁴ and 60 excitations were calculated. For this analysis, the CAM-B3LYP-D3 range-separated functional⁵⁵ was employed. This functional provides additional long-range corrections. These computations provided electronic properties such as vertical excitation energies and oscillator strengths. To analyze the nature of electronic transitions via photon absorption, plots of electron density difference maps (EDDMs) are presented. The visualization and plotting of the EDDMs were performed using GaussSum (v. 2.2.6).⁵⁶

Results and discussion

The energy minima structures were determined using the PBE-D3/TZ2P level of theory, which has been shown to be a very reliable method in earlier studies involving weak interaction forces,^57–60 with the incorporation of the COSMO solvation method.⁵³ In Fig. 1 is represented the modeled cage structure of [Ag₂(bisox)₃]. The interatomic distances and bond angles for the calculated cage are in good agreement with previously reported experimental results:³² see Table S1 in the ESI.†

The structure of [Ag₂(bisox)₃] is constructed from three bisox bridge moieties that hold together two silver(I) atoms with a coordination number of 3 and has a semi-rigid structure with a cavity diameter of around 8.0 Å. By this calculation method, the calculated [Ag–N], [N–O], [N–C] and [O–C] bond distances are 2.29 Å, 1.41 Å, 1.32 Å and 1.36 Å, whereas the experimental distances are 2.23 Å, 1.35 Å, 1.30 Å, and 1.35 Å, respectively. Moreover, the theoretical [N₁–Ag–N₂], [C₁–C₂–C₃], and [Ag–N₁–O] bond angles are 119.8°, 113.1° and 118.7° and the corresponding experimental values are 119.6°, 111.9° and 114.3°, which indicates that the results were accurate for the level of theory that was used. This structure might be a suitable receptor for guest gas molecules, which enabled the study of the hypothetical inclusion of a series of guest molecules (N₂, H₂O, CO₂, CH₄ and C₂H₆) in the cavity of the host structure. Furthermore, the obtained structural parameters of the cage-guest system revealed slight deviations from the isolated [Ag₂(bisox)₃] host structure; however, the cavity diameter remained more or less constant (Table S1†).

With the aim of gaining a deeper understanding of the formation of such complexes, we performed energy decomposition analysis within the NOCV scheme,^46–50 which has been employed for the description of the interactions of host–guest systems.⁶¹ Such analysis considers the formation of a molecule AB (with the wavefunction Ψ_AB) from its respective fragments. For the current systems, an energetically favorable situation is observed, which is accounted for by the interaction energy (E_int) between the fragments of the host–guest pair, including the dispersion energy. The interaction energy is calculated according to:

ΔE_int = ΔE_elstat + ΔE_Pauli + ΔE_orb + ΔE_dis

where the ΔE_elstat term accounts for the electrostatic character of the interaction and the ΔE_Pauli term represents destabilizing four-electron two-orbital interactions between occupied orbitals, which denote steric hindrance. The ΔE_orb term is obtained when the densities of the constituent fragments are allowed to relax into the final molecular orbitals (Ψ_AB), which account for the covalent character of the interaction. Finally, the ΔE_dis term considers the dispersion forces involved, which are based on a Grimme pairwise approach (D3).⁴⁴

More insight into the orbital interactions between the fragments can be obtained from a study of NOCV (Ψ_i), which is defined as the eigenvector that diagonalizes the deformation density Δρ(r), that is, decomposes it into deformation densities Δρ_i(r) to provide information about the channels of charge transfer.

To gain an understanding of the interaction energies within the [Ag₂(bisox)₃]–[gas guest] systems, energy decomposition analysis (EDA) was performed, considering the cage and the gas as constituent fragments: see Table 1. When the cage was experimentally prepared it contained diethyl ether (Et₂O) adsorbed inside the cage cavity. This molecule was exchanged with other gases in order to investigate the more stable cage-stored gas interactions. This analysis was performed by considering the hypothetical inclusion of a series of gas guest molecules, namely, N₂, CH₄, CO₂, H₂O and C₂H₆, in the cage cavity of the host.³² Also, the interaction of the cage with Et₂O was studied. Thus, the prominent inclusion of a gas in the cavity of the host structure was investigated via attractive interaction forces. In this sense, the system was studied in terms of the following corresponding fragments, namely, the cage and the gas as fragments A and B, respectively. The results for the overall interaction energy ΔE_int denote energetically favorable inclusion of the incoming guests inside the [Ag₂(bisox)₃] cage structure. Thus, values of non-covalent bonding interactions were observed for the ΔE_int term. This obtained result is in agreement with the weak forces involved in this hypothetical series. The ΔE_int term represents a non-covalent bonding interaction and exhibits a slight increase from the value for N₂ gas, which is the smallest in comparison with those of the more bulky gas molecules, with values of ΔE_int of N₂ (−6.5) < CH₄ (−7.27) < CO₂ (−7.97) < H₂O (−10.02) < C₂H₆ (−11.16) < Et₂O (−16.95 kcal mol⁻¹). This is in accordance with the fact that the bulkier a gas molecule is, it generates greater polarization owing to larger and more dispersed electron clouds, and there arise induced instantaneous polarization multipoles (i.e., dipole and quadrupole moments, among others). Further analysis of the attractive interaction terms, namely, ΔE_elstat, ΔE_orb and ΔE_dis (see Table 1) revealed that the dispersion energy is the major stabilizing term, with values of 52.3% in the case of N₂, 59.2% for CH₄, 58.8% for CO₂, 65.8% for C₂H₆ and 60.6% for Et₂O. Moreover, in the case of H₂O this stabilizing force represented 33.3% of the total attractive terms. This observation is in agreement with the fact that this gas molecule is governed by a permanent dipole moment, which gives rise to greater contributions of the orbital and electrostatic components, which have values of 29.2% and 37.5%, respectively. Thus, when water is the guest the orbital energy makes the largest contribution in the series to the overall stabilizing forces. This fact should reflect major selectivity for H₂O gas, despite the fact that the total interaction energy becomes slightly more stable for larger molecules such as C₂H₆ and Et₂O. In addition, the maximum value of the orbital contribution should result in a tendency to selectivity in the host cage, as will be observed from cases of multiple guest inclusion. The ΔE_orb term accounts for the covalent character of the interaction associated with charge transfer between the fragments. In this sense, the orbital interactions were investigated using NOCV analysis. In this analysis were plotted the main contours of the contributions of deformation density (Δρ_i). The pictures that were obtained show the NOCV orbitals that are involved in charge transfer interactions, which are shown in Fig. 2.

Table 1 Energy decomposition analysis (EDA, kcal mol⁻¹) representing the cage-stored gas interactions for the selected fragments

EDA (kcal mol^−1)	[Ag2(bisox)3][N2]	[Ag2(bisox)3][CH4]	[Ag2(bisox)3][CO2]	[Ag2(bisox)3][H2O]	[Ag2(bisox)3][C2H6]	[Ag2(bisox)3][Et2O]
ΔEPauli	2.66	2.41	3.65	4.60	3.83	5.52
ΔEorb	−2.42 (26.3%)	−1.88 (19.4%)	−2.03 (17.5%)	−4.27 (29.2%)	−2.03 (14.3%)	−3.66 (16.3%)
ΔEelstat	−1.97 (21.4%)	−2.07 (21.4%)	−2.86 (24.6%)	−5.48 (37.5%)	−2.84 (20.0%)	−5.20 (23.2%)
ΔEdis	−4.82 (52.3%)	−5.73 (59.2%)	−6.74 (58.8%)	−4.87 (33.3%)	−9.35 (65.8%)	−13.60 (60.6%)
ΔEint	−6.55	−7.27	−7.97	−10.02	−11.16	−16.95

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Fig. 2 The main contours of the contributions of deformation density (Δρ_i) determined by EDA-NOCV analysis for the [Ag₂(bisox)₃][H₂O] and [Ag₂(bisox)₃][H₂O]₄ complexes.

In order to gain a deeper insight and understanding of the interaction forces involved and owing to the large size of the cavity of the host species (which can include inside its cavity more than one guest molecule), further calculations were performed that considered the inclusion of a greater number of gas guest molecules inside the host cage. Thereby, the inclusion of more species led to favorable structures with attractive interaction energies ΔE_int. This was carried out by taking into account the inclusion of four equivalents of N₂, CH₄, CO₂ and H₂O, which will be distinguished from cases of the inclusion of one molecule by the terms [N₂]₄, [CH₄]₄, [CO₂]₄ and [H₂O]₄. In the case of ethane, only three molecules were included owing to the greater size of this molecule, and this case will be distinguished by the term [C₂H₆]₃. The inclusion of a greater number of molecules results in the collapse of the main host structure.

Consequently, the respective destabilizing term related to the Pauli repulsion energy exhibited a similar tendency as in the case when one gas molecule was included and displayed greater relevance when [CO₂]₄ and polar molecules such as [H₂O]₄ were guests: see Table S2 in the ESI.† As mentioned above in the cases when one gas molecule was included, the interaction energy increased or became more stable as the studied gas became bulkier. However, for the inclusion of multiple guests it was observed that ΔE_int increased from [N₂]₄ (−21.01), [CH₄]₄ (−23.43), [CO₂]₄ (−26.73), and [C₂H₆]₃ (−27.75) to [H₂O]₄ (−32.33 kcal mol⁻¹). Despite the fact that water and carbon dioxide as guest gases have the most unfavorable ΔE_Pauli repulsive terms, these display the most attractive values of both electrostatic and orbital terms. This fact results in more favorable interaction energy. Thus, [CO₂]₄ guests display an almost equivalent value of ΔE_int in comparison with that of [C₂H₆]₃ species, in spite of the fact that fewer atoms interact with the [Ag₂(bisox)₃] host cage. In this context, the electrostatic term stabilizes the interactions in the order of [N₂]₄ (−6.00), [CH₄]₄ (−8.17), [C₂H₆]₃ (−10.88), [CO₂]₄ (−14.42) to [H₂O]₄ (−24.84 kcal mol⁻¹), which indicates greater attractive interaction energy towards the most polar guests. Furthermore, as in the hypothetical counterpart when one gas molecule was included, [H₂O]₄ provided the most stable value of the orbital contribution, as well as the overall interaction energy. This value was analyzed by means of NOCV orbitals, showing the major contours of the contributions of deformation density (Δρ_i) (Fig. 2). In this figure, it can also be observed that charge transfer occurs from the lone pair of the water guest molecule. Consequently, this fact would be reflected in selectivity of the host for water species. In addition, the cases of N₂ gas in both studied systems (the inclusion of single and multiple molecules) display orbital quantities of 26.3% and 23.5%. These favorable calculated percentages are associated with the interaction of the available lone electron pairs of this guest molecule. Otherwise, as an overall tendency, the dispersion energy represents the main contribution within the attractive terms, with percentages of 56.6% for the complex with [N₂]₄, 60.6% for [CH₄]₄, 51.8% for [CO₂]₄, 27.6% for [H₂O]₄ and 59.2% for [C₂H₆]₃ among the inclusion compounds with multiple guests. Therefore, for non-polar guests the dispersion term provides a greater percentage of the total interaction force. Hence, as shown for the guests [N₂]₄, [CH₄]₄ and [C₂H₆]₃, the dispersion term increases for bulky guests. This is attributed to the fact that their polarizability also increases.

In addition, non-covalent interactions were investigated via the use of the non-covalent index (NCI) approach, which relies on topological analysis of the electron density and its derivatives in regions of low density based on the reduced density gradient (s(ρ)), which is defined as:

Regions on the NCI isosurface indicate both stabilizing and destabilizing weak interactions. These can be distinguished according to the sign of the second eigenvalue of the Hessian matrix (λ₂), where the sign of λ₂ can vary accordingly; thus it is suggested to be a useful descriptor for characterizing such situations of weak interactions. Here, negative values of the product represented by ρ* sign(λ₂) denote stabilizing interactions. Values close to zero indicate weak interactions (van der Waals forces), whereas positive values represent cases of weak repulsion.

Moreover, the results of 3D NCI analysis are depicted in Fig. 3 and 4, which reveal stabilizing non-covalent intermolecular interactions between gas molecules and the corresponding cage, which is indicated by a green region. The weak forces involved are related to current dispersive interactions due to induced multipoles, which are denoted as interactions of the induced dipole-induced dipole type. In Fig. 3 are shown the NCI plots for one equivalent gas guest molecule. Significant affinity of each gas molecule for the atoms of the host cage structure is observed. In Fig. 4 can be observed more pronounced dispersive forces for the [CH₄]₄ and [C₂H₆]₃ systems, which are indicated by larger green regions in the corresponding plots. For the [H₂O]₄ complex a decrease in the green surfaces is noted, which is related to a smaller dispersive interaction, which is compensated for by an orbital contribution.

Fig. 3 NCI plots of the studied complexes, where red regions denote strong stabilizing forces and green-yellow regions denote weak van der Waals forces.

Fig. 4 NCI plots for several guest molecules incorporated in the host cage. The red regions denote strong stabilizing forces and the green-yellow regions denote weak van der Waals forces.

Spectral properties

The excitation energies that correspond to λ_max with their corresponding oscillator strengths and active molecular orbitals (MOs) for the host and host–guest pairs (including single and multiple molecules of gases) are listed in Table 2. In addition, the simulated UV-vis absorption spectra of all systems are shown in Fig. 5.

Table 2 Calculated wavelengths of maximum absorption λ_max (nm), energies (eV), and oscillator strengths (f(L)), and the corresponding active molecular orbitals and contributions involved in the electronic transitions

System	λ (nm)	E (eV)	f	Active molecular orbitals			% Contr.
[Ag2(bisox)3]	210	5.91	0.441	HOMO−1		LUMO	20
				HOMO		LUMO+2	14
				HOMO		LUMO+2	14
[Ag2(bisox)3][N2]	213	5.82	0.506	HOMO		LUMO+2	9
				HOMO		LUMO+3	9
				HOMO		LUMO+4	13
[Ag2(bisox)3][N2]4	214	5.79	0.385	HOMO−3		LUMO+2	9
				HOMO		LUMO+2	6
[Ag2(bisox)3][CH4]	215	5.77	0.414	HOMO−2		LUMO+3	8
				HOMO−1		LUMO	12
				HOMO		LUMO+2	23
[Ag2(bisox)3][CH4]4	215	5.77	0.405	HOMO−4		LUMO	12
				HOMO−1		LUMO+4	6
				HOMO		LUMO+1	11
[Ag2(bisox)3][CO2]	208	5.95	0.489	HOMO−6		LUMO+1	9
				HOMO−7		LUMO	9
				HOMO		LUMO+1	13
[Ag2(bisox)3][CO2]4	213	5.8	0.357	HOMO		LUMO+1	10
				HOMO−7		LUMO	12
				HOMO−2		LUMO+2	10
[Ag2(bisox)3][H2O]	215	5.77	0.441	HOMO−5		LUMO+1	14
				HOMO−5		LUMO+4	12
				HOMO		LUMO+4	7
[Ag2(bisox)3][H2O]4	224	5.52	0.111	HOMO−1		LUMO	28
				HOMO−1		LUMO+1	12
				HOMO		LUMO+1	22
[Ag2(bisox)3][C2H6]	214	5.79	0.497	HOMO		LUMO+1	6
				HOMO−6		LUMO+1	9
				HOMO−6		LUMO	9
[Ag2(bisox)3][C2H6]3	215	5.77	0.385	HOMO		LUMO+1	7
				HOMO−3		LUMO+1	8
				HOMO−4		LUMO	10

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Fig. 5 UV-vis absorption spectra simulated for the [Ag₂(bisox)₃] cage and the cages containing the studied molecules. (a) Cages with [N₂] and [N₂]₄, (b) cages with [CH₄] and [CH₄]₄, (c) cages with [CO₂] and [CO₂]₄, (d) cages with [H₂O] and [H₂O]₄ and (e) cages with [C₂H₆] and [C₂H₆]₃.

According to the results, it is observed that the guests induce a red shift in the spectrum of the host and that several H₂O molecules generate a dramatic change, which is mainly represented by new low-energy bands. Moreover, the absorption profiles of the cages with non-polar guest molecules (CH₄ and C₂H₆) are similar and display a red shift effect with respect to the spectrum of the host. The intensity of the lowest-energy band of the spectrum of the cage with CH₄ is the most similar to the intensity for the free host system.

On the other hand, the spectra of the cages with N₂ and CO₂ display a red shift and lower intensities of the bands with respect to those of the free cage. It is worth mentioning that in general the UV-vis absorption profiles of cages containing one molecule and those containing several molecules display similar energies. In the case of CO₂, the largest difference between the cage containing one molecule and the cage containing four molecules was observed, as the profile for one molecule is similar to that of the free host, but a remarkable difference was displayed for four molecules.

The frontier molecular orbitals (MOs), namely, the HOMO and LUMO, are involved in all the described transitions. To perform an appropriate description of the nature of the transitions, EDDM analysis was carried out for the lowest-energy band (λ_max) (Fig. 6). This analysis provides representations of the changes in electron density upon a given electron transition. Generating the relevant maps involves the use of information given in calculations of a singlet excited state and uses the configurations that contribute to the transition of interest. An EDDM can be plotted for the electron density determined for a system before and after excitation.³⁸ These plots display in a more explicit manner the probability of the transfer of an electron from one fragment to another.

Fig. 6 EDDMs for the [Ag₂(bisox)₃] cage and the cages containing the studied molecules upon photoexcitation to the first singlet excited state. The yellow densities represent sources of electrons and the red densities represent target locations after electron transfer.

Therefore, it was possible to determine the localization of the occupied MOs of the systems that would undergo depopulation and the unoccupied MOs that would be populated after excitation. For most of the studied systems, the transitions occur between the p-orbitals of the ligands with little contribution from the metal, except for N₂, which interestingly displays a contribution from the N₂ orbitals. These results were observed for one as well as four guest molecules.

Conclusions

Here, we investigated the weak interactions between the [Ag₂(bisox)₃] cage and a series of gas molecules (N₂, H₂O, CO₂, CH₄, and C₂H₆) included in the cavity of the cage. In order to determine the strengths of the interactions between the species and investigate the potential selectivity for a particular gas guest molecule, various computational techniques were employed. By this method, significant interaction energies ΔE_int ranging from −6.55 to −16.55 kcal mol⁻¹ were found for the inclusion of a single guest molecule. Furthermore, for the inclusion of multiple guest molecules the corresponding attractive interaction energies ranged between −21.01 and −32.33 kcal mol⁻¹, which indicated a favorable association for the host–guest pair systems.

The decomposition of the interaction energy into its Pauli repulsion, electrostatic, orbital and dispersion components showed clear differences between polar and non-polar guest molecules. The dispersion energies provide dominant stabilizing contributions to the overall energy as a general tendency. In the case where H₂O is the guest molecule, the orbital and electrostatic energy represent 66.7% and 72.4% of its stabilizing interaction for the inclusion of single and multiple guest molecules, respectively, which allows it to be inferred that this favorable interaction might result in potential selectivity of the host cage for H₂O molecules. On the other hand, for non-polar guests such as N₂, CH₄ and C₂H₆, the dispersion term displays values that range from 56.6% to 60.6%, which represent a major contribution to the weak interaction force. In addition, the dispersion energy becomes more stable for C₂H₆ than for H₂O, which is in agreement with the fact that more bulky molecules are more polarizable, which leads to a higher dispersive force. However, this fact does not necessarily lead to selectivity within the series, because more than one molecule of N₂, H₂O, and CO₂ can be placed inside the cage, which leads to more favorable interaction energies towards CO₂ and the polar guest molecule H₂O. The trapping of gases in the cage is evidenced by changes in the spectral properties of the free cage with respect to those of cages containing guests. This was determined via computational analysis of the UV-vis absorption spectra, by which features such as new bands, red shifts and changes in the intensity of the peak corresponding to λ_max were observed. EDDMs revealed the participation of the N₂ orbitals, which act as a target of the lowest-energy electronic transition for the cage with one gas molecule as well as the cage with four gas molecules.

Acknowledgements

The authors thank the financial support of Post-Doctoral FONDECYT 3160682, FONDECYT 1161416, and FONDECYT 11140563.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22905k

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