Computational Study on the Structure, Stability, and Electronic Feature Analyses of Trapped Halocarbons inside a Novel Bispyrazole Organic Molecular Cage.

Computational experiments on a novel crystal (Bharadwaj et al. Cryst. Growth Des. 2019, 19, 369-375) having a series of seven host-guest complexes (HGCs) where the host species belong to the family of a novel bispyrazole organic cryptand (BPOC) and their structural, stability, and the electronic feature analyses have been reported using the quantum chemical calculation approach. This report systematically unravels an inclusive theory-based experiment on the well-known guest solvents (S) like halocarbon solvents [CCl4, CHCl3/CHCl3' (two orientations), CH2Cl2 , C2H4Cl2 , C2H4Br2 , and C2HCl3 ] and a few model chlorofluorocarbons (CFCs) (CClF3 , CCl2F2 , and CCl3F) trapped inside the host (BPOC) cryptand, which are the crux in forming the structures of biological and supramolecular systems. Using the implicitly dispersion-corrected DFT (M06-2X/6-31G*) approach, the BPOC molecular cage and its host-guest capabilities were evaluated for the encapsulation of the above said halocarbon solvents as well as the CFC models. The encapsulated C2H4Br2 solvent inside the BPOC cage is found to be the most stable among all the HGCs; however, common in the solid phase, similar binary complexes have not been formerly examined in any gas/solvent-phase studies of the BPOC host species. Moreover, very interestingly, the stability pattern of the host-guest complexes enhances for the CFC models when the number of Cl atoms is increased. As the halogenated solvents through halogen and H-bonding are very decisive in understanding and controlling chemical reactions, the NCI-plots support the presence of the halogen bonding (C-Cl/Br···π) and H-bonding (C-H···π) interactions playing an imperative role in stabilizing the guests (solvents) inside the hydrophobic cavity. To get more insights, the HOMO-LUMO and MESP plots as well as natural population analyses have also been highlighted. This theoretical study portrays an inclusive information about the structural, stability, and electronic feature analyses of the host-guest assemblies consisting of the halogen and H-bonding interactions at the atomic level where the influences of such halocarbon solvents play crucial roles in comprehending and managing chemical reactions.

Introduction

It is well known that an apparent attractive contribution of the C–H/π and halogen–aryl π (X−π) interactions is all-important for any rational approach to drug design revealed in solution. Weak intermolecular interaction is a major topic in nonbonding research. The energies of such interactions fall within a few kJ/mol and approach those of the van der Waals forces. We adopted herein molecular adducts constructed by a bispyrazole-based cryptand (balance) and encapsulated solvents which have been used to determine different nonbonding interactions such as C–H/π and X−π interactions via computational exploration. This behavior was shown for the weak intermolecular nonbonding interactions C–H/π,[1] X−π,[2] π–π,[3] and cation−π.[4] With this technique, precise information on the structural, energetic, and dynamical aspects of such interactions is obtained in an environment free of the intermolecular interactions that take place in condensed media.

In most of the cases, the C–H/π interactions engaging aromatic (i.e., donor or acceptor) groups are observed in the interior of the protein; however, it is likely that the C–H/π-interactions contribute significantly to the overall stability of a protein. The C–H/π interaction is an attractive molecular force [hydrogen bond (HB)] that arises between a soft acid (SA) and soft base (SB) [SA: C–H; SB: π–system (double/triple bonds by six and five carbon aromatic rings, heteroaromatics, convex surfaces of nanotubes and fullerenes)]. These weak HBs have been well exercised in the past couple of decades, mainly by crystallography and spectroscopy. The interaction/stabilization energy depends on the nature of the molecular fragments, π-groups, and C–H: the stabilizing effect will be larger if the C–H group has a stronger proton donating ability.[5] The geometric parameters fairly suggest that such kinds of interactions can be classified as weak HBs. In addition to that, higher halogens (X = Cl and Br) are expected to construct overall attractive interactions with the aromatic π-faces of the ring in balances, while the F atom does not hold such kinds of interactions due to its highly electronegative character as well as very low polarizability, in general.

Until now, such kinds of studies for several halocarbon guest solvents, trapped inside a host molecular cage [bispyrazole-based organic cage/cryptand (BPOC)] consisting of a fan like structure shown in Scheme , have been conjectured only from the crystallographic data analyses,[6] and in all states of matter, the structural, energetic, and electronic feature analyses of the endo-complexation of the noble host molecular container (BPOC) possessing a variety of guest molecules along with some modeled guests (S’), have been performed theoretically. In order to probe the structural features, the computationally probed C–H/π and X−π approximate distances were compared with the crystallographic geometrical parameters. Our study supplies a theoretical basis for the experimental finding of binding interactions and further enables us to directly compare the magnitude of such interactions with the experiment-based parameters acquired from the single-crystal X-ray experiments of host–guest complexes (HGCs).

Scheme 1

Pictorial Representation of the Host (Organic Molecular Cage) and Guest (Solvents) Molecules as well as Host–Guest Complex

To the best of our knowledge and understanding, this is the first report based on computational testing that untangles the structural, energetic, and electronic properties of the crystal structures of the encapsulated halocarbons (guest) in a BPOC (host) with a hydrophobic void along with some model CFC guest species. Such a motif of the endo-complexation of solvent molecules has never been probed for any other bispyrazole-based cryptand systems computationally.

Quantum Chemical Methodology

As an aid in characterizing the structural, stability/energetic, and electronic features of the host–guest inclusion complexes (S@BPOC, where S = S1/S2/S’) (vice versa), quantum chemical calculations (QCCs) have been performed in vacuo, using the implicitly dispersion-corrected functional (M06-2X)[7] with the 6-31G* basis set as implemented in the Gaussian 09 electronic structure calculations package[8] with geometry optimizations initiated from the experimentally observed geometries that loosely corresponded to C–H/π and X−π (X = Cl, Br) interacting structures. The geometry optimization of all complexes and their monomer units has been carried out without any symmetry constraint in the potential energy surface. Vibrational frequency calculations were carried out at the same level of study. To authenticate the optimized structures of the HGCs that belong to at least some local minima, the normal modes were scanned and no imaginary frequency was detected. The limited computational resources did allow the computations up to this level of basis set as the entitled systems are quite large, which possess a huge number of atoms.

The natural atomic charges have been reported with the employment of natural bond orbital (NBO) analysis available in the Gaussian 09 software. Also, reaction field calculations were performed in the presence of different solvents using the polarizable continuum model (PCM) of Cramer and Truhlar.[9] Solvation effects were computed using the PCM model with the deployment of very useful implicitly dispersion-corrected functional (M06-2X) approach. Solvents S show slight changes in the encapsulation/binding/interaction energies (EEs/BEs/IEs) of the complexes of interest in comparison to without using the PCM model approach (i.e., gas phase).

Results and Discussion

Out of a total three cases, at first, the BPOC host–guest (S1) inclusion complexes were chosen representing as the S1@BPOC complex [where S1 = CCl, CHCl (existing in two orientations represented as CHCl and CHCl), and CH [i.e., dichloromethane (DCM)] act as guests and the BPOC cryptand acts as the host]. Here, the main emphasis is to investigate the nature and kind of interaction involved between the Cl-substituted methane solvents and their molecular container (BPOC), which is/are responsible for making the complexes stable. In the second case, in order to look into the binding features and charge transfer phenomenon, S crystal assemblies [where S2 = C (1,2-dibromoethane), C (1,2-dichloroethane), and C (trichloroethylene)] have been selected as test cases and the QCCs have been performed.

Moreover, the third case is related to the chlorofluorocarbons (CFCs) (Cl versus F substitution) in which the DFT study was deployed to compare the features indicated above for a few model complexes, S’@BPOC [S’ = CClF, CCl, and CCl]. A fan like structure of the host molecule can be seen in the schematic diagram as shown in Scheme where all seven halocarbon solvents (S = S1 and S2) as well as three CFCs (S = S’) have been highlighted. Here, the comparisons of the results have been shown and discussed into three parts: S1@BPOC host–guest crystallized systems, S2@BPOC host–guest crystallized assemblies, and S1@BPOC crystals versusS’@BPOC models (for example, CHClversusCFCl and CHversusCF).

S1@BPOC Host–Guest Crystallized Systems (S1 = C, CHCl, CHCl, and CCl)

Geometric Feature Analyses

First, let us start the discussion with the S1@BPOC-related complexes. In the host–guest assemblies, the nonbonded distances (NBDs) between the two bridgehead nitrogen atoms of the molecular tank (BPOC) vary from 20.806 to 21.765 Å analyzed by the structural data analyses (see Table , second column) in which the range is in close proximity with the computationally observed data (19.791–21.810 Å). Some useful and selected geometric parameters (bond length, bond angle, and torsional angle) of the experimentally observed (solid state) S1@BPOC crystal assemblies have been tabulated in Tables –3, and the comparison of theoretically observed parameters is shown in detail (wherever required) in the solvent and gaseous phases. The optimized structures of the HGCs are closer with their solid-state forms in which the complexes of endo-type solvents associated with the BPOC trapper illustrate the C–H/π and X−π (X = Cl and Br) interactions with the cryptand (BPOC) (see Tables –3). From the examination of the results obtained by the experiment and the QCCs, numerous remarkable points arise here.

Table 1

Comparison of Bond Distances between the C–H/Halogens(Cl/Br) and π-Framework of the Ring (in Å)

crystal structure				DFT (PCM)
	bridgehead NN distance	Cl/Br···π	C–H···π	bridgehead NN distance	Cl/Br···π	C–H···π
CH2Cl2	21.722	3.780, 3.894	2.920, 3.073	21.694	3.493, 3.820	2.612
CHCl3	21.765	3.172, 3.153	2.789	21.654	3.054, 3.099	2.537
CHCl3′	21.242	3.190, 3.209, 3.269	-	21.344	3.147, 3.149, 3.150	-
CCl4	21.231	3.217, 3.291, 3.324	-	21.447	2.943, 2.994, 3.047	-
C2H4Cl2	21.574	3.607, 3.733	2.557, 2.573, 3.113, 3.349	21.810	3.528, 3.535	2.352, 2.355
C2H4Br2	20.806	3.479, 3.479	2.490, 2.490	19.791	3.334, 3.334	2.632, 2.633
C2HCl3	21.144	3.194, 3.194	-	21.269	3.064, 3.254	2.442

Table 3

Calculated Some Important Geometrical Parameters of the Fully Geometry Optimized Structures for Different Solvents inside the Molecular Cage and Solvents Alone at the M06-2X/6-31G* Level of Theory

crystal	solvent in the cryptand (Exp, PCM)	free solvent
CH2Cl2	C–Cl (1.710, 1.779), C–H (0.970, 1.090), ∠HCH (107.8, 112.3), ∠ClCCl (113.1, 110.5), ∠HCCl (109.0, 108.0)	C–Cl (1.778), C–H (1.086), ∠HCH (111.6), ∠ClCCl (112.3), ∠HCCl (108.2)
CHCl3	C–Cl (1.736, 1.752), C–H (0.979, 1.090), ∠ClCCl (110.8, 110.2), ∠HCCl (108.6, 108.7)	C–Cl (1.772), C–H (1.085), ∠ClCCl (110.9), ∠HCCl (108.0)
CHCl3’	C–Cl (1.741), C–H (1.202), ∠ClCCl(111.8), ∠HCCl (107.0,)	C–Cl (1.772), C–H (1.085), ∠ClCCl (110.9), ∠HCCl (108.0)
CCl4	C–Cl (1.745, 1.769), ∠ClCCl (108.7, 109.5)	C–Cl (1.775), ∠ClCCl (109.5)
C2H4Cl2	C–C (1.462, 1.508), C–H (0.970, 1.092), C–Cl (1.776, 1.745), ∠HCH (108.1, 109.8), ∠ClCC (110.2, 109.2), ∠ClCH (109.7107.4), τ ClCCCl (72.3, 69.9)	C–C (1.090), C–H (1.511), C–Cl (1.796), ∠HCH (109.6), ∠ClCC (112.1), ∠ClCH (107.0), τ ClCCCl (66.4)
C2H4Br2	C–C (1.360, 1.509), C–H (0.970, 1.092), C–Br (1.930, 1.957), ∠HCH (107.1, 110.1), ∠BrCC (119.1, 111.7), ∠BrCH (107.6, 106.5), τ BrCCBr (50.7, 70.6)	C–C (1.510), C–H (1.089), C–Br (1.949), ∠HCH (110.3), ∠BrCC (111.5), ∠BrCH (106.2), τ BrCCBr (65.3)
C2HCl3	C=C (1.420, 1.332), C–H(1.070, 1.086), C–Cl [(1.674, 1.718)H, (1.660, 1.724) Cl-Trans, (1.650, 1.720)Cl-Cis], ∠ClCCl (93.4, 115.6), ∠ClCH (110.1, 116.1), τ ClCCCl (165.5, 177.7)	C=C (1.331), C–H (1.083), −Cl (1.718H, 1.728Cl-Trans, 1.718 Cl-Cis), ∠ClCCl (115.7), ∠ClCH (115.4), τ ClCCCl ( 180.0)

The QCCs belong to the gas phase in vacuo, while the experimental outcomes belong to a molecule in the solid phase. In the solid state, the survival of a crystal field along with the intermolecular interactions links the molecules together via a lattice unit linkage, which results in the differences in geometrical parameters between the calculated and experimental values. This result provides reliability to the level of theory performed in the analysis. The C–H/π and the X−π (X = Cl and Br) NBDs and associated angles of the optimized HGCs are in good agreement with the experimental outcomes (see Tables and 2). In most of the cases, the computed NBDs between several guest molecules (solvents) and the host (cryptand) are shorter than the experimental ones. This outcome is not unexpected since the equilibrium distances of weak complexes are shorter in the isolated form than in the solid phase.

Table 2

Comparison of the Associated Bond Angles between C–H/C-Cl and π-Framework of the Ring (in °)

crystal structure			DFT optimized (PCM)
	C–Cl···π	C–H···π	C–Cl···π	C–H···π
CH2Cl2	127.6, 129.8	131.6, 137.5	138.8–122.4	121.5, 141.4
CHCl3	154.5, 167.7	150.1	166.9–154.1	148.8
CHCl3’	151.6–161.1	-	155.0	-
CCl4	120.8–125.1	-	139.4–162.2	-
C2H4Cl2	135.8, 139.6	138.8, 149.9	135.6, 135.6	142.9, 142.9
C2H4Br2	169.4	168.1	167.8	131.1
C2HCl3	140.4	-	131.5, 156.6	133.9

Now, let us begin the discussion by choosing the optimized CH crystal (see Figure a) containing one cryptand/trapper (BPOC) unit and in which one CH solvent is trapped in the cavity of the BPOC molecular cage. The encapsulated CH solvent shows two halogen bonds (XBs) (C–Cl···π, 3.780 and 3.894 Å) from two Cl atoms and two HBs (C–H···π, 2.920 and 3.073 Å) from two H atoms of the CH solvent molecule. The DFT-optimized bond distances for Cl···π (3.493 and 3.820 Å) and C–H···π (2.612 Å) interactions reveal that such interactions are not very strong (see Figure b). Also, the experimentally probed bond angles, C–Cl/···π (127.6 and 129.8°) and C–H···π (131.6 and 137.5°), are deviating more from the linear angle, 180°, clearly suggesting the loosely bound solvent. The NBD between the bridgehead N atoms is found to be 21.722 Å, which falls in excellent agreement with the computationally observed value (21.694 Å).

Figure 1

Cl···π and C–H···π interactions (bond distance and angles) in (a) the crystal structure and (b) DFT optimized (solvent phase) for CH.

The short distances in the isolated form (i.e., gas phase) compared to those in the solid state is because of the fact that the equilibrium lengths of weak complexes are shorter in the gas (or solvent) phase than in the solid phase. This result gives reliability to the level of theory used in the study. From a crystallographic report,[6] it indicates that methanol and water solvent molecules are not able to penetrate inside the cavity of the cage, which strongly suggests the hydrophobic behavior of the cavity in the cryptand.

The crystal CHCl assembly (see Figure a) having a cryptand unit and one CHCl solvent was optimized. Here, the CHCl is present inside the cavity. The trapped CHCl solvent molecule shows two halogen (C–Cl···π, 3.153 and 3.172 Å) interactions from the two Cl atoms and one HB (C–H···π, 2.789 Å) from the H atom of the CHCl solvent. The DFT-optimized geometries augment the solid-state structure bond distances and angles, with values of 3.054 and 3.099 Å for the XBs (C–Cl···π), 2.537 Å for the HB (C–H···π), and the associated XB (154.1–166.9) and the HB (148.8°) angles. It is interesting to mention that in contrast to the CH crystal, the bond distances are shorter in the case of CHCl assembly and the angles of associated XB (C–Cl···π, 154.5 and 167.7°) strongly favor the well-built directional interaction with the host molecular container (BPOC). The optimized structure of the CHCl complex (excluding H-atoms) can be seen in Figure b. In this case, the NBD between the bridgehead terminal N atoms has been observed to be 21.765 Å, which is in close proximity to the computationally observed value (21.654 Å).

Figure 2

C–Cl···π and C–H···π interactions (bond distance and angles) in (a) the crystal structure and (b) DFT optimized (solvent phase) for CHCl.

It is worth mentioning that in the case of chloroform solvent (CHCl), a rare observation is found where two crystal structures were reported with different orientations of the guest (CHCl) solvent contained by the BPOC molecular tank.[6] The other possible structure (i.e., CHCl complex) of the trapped chloroform solvent is displayed in Figure a. The CHCl complex contains three XBs (C–Cl···π) with no HB, which was crystallized and compared to the CHCl dimer complex consisting of two XBs (C–Cl···π) and one HB (C–H···π). This may be expected because of keeping this crystal at two different temperatures in which competition occurs between the XB and HB interaction(s) where the solvent molecule consists of this kind of nature inside the cage structure (see Figure a). For the CHCl crystal structure, the C–Cl···π, bond distances are 3.190, 3.209, and 3.269 Å along with the associated C–Cl···π angles (see Figure a) ranging from 151.6 to 161.1°. The DFT-based calculated geometrical parameters (C–Cl···π distances, 3.147, 3.149, and 3.150 Å, and their associated three C–Cl···π angles (155.0° for all three) fall in between the above highlighted experimentally observed value as expected. Both experimental and calculated values suggest strong interactions with good directionality of the XB. The optimized structural data (bond distance and bond angle) can be discerned from Figure b.

Figure 3

C–Cl···π interactions (bond distance and angles) in (a) the crystal structure and (b) DFT optimized (solvent phase) for CHCl (CHCl3 is disordered over two positions).

The experiment-based structural analyses of the cryptand showed that the NBD between the two bridgehead nitrogen atoms of the cage is 21.242 Å, which is also favored by the DFT value 21.344 Å in the HGC. The cavity length (in terms of the bridgehead N atoms distance) appears to be decreased when the chloroform solvent goes from one orientation of CHCl (21.765 Å) to the other orientation of the CHCl (21.242 Å) structure by varying the temperature, which appears to be due to the change in positions of the H and Cl atoms. Comparing the C–Cl···π interactions in the above two cases, namely, CHCl and CHCl, the former appears to show stronger interactions while both of them showed nearly equal directionality, which is also supported from the associated bond angles of the XB interactions. Here, the contest between the halogen (C–Cl···π) and hydrogen bonding (C–H···π) is observed, which can be seen in the literature (see ref (9) and therein) in which it is concluded that the former dominates over the latter during the self-assembly process.[10]

The crystal CCl (see Figure a) containing a cryptand (BPOC) unit and one CCl solvent molecule was optimized, and here, the trapped CCl solvent molecule is disordered over two positions with half occupancy of each, and it appears to show three XB (C–Cl···π) interactions with distances of 3.217, 3.291, and 3.324 Å. Compared to the CH (DCM)- and CHCl-related complexes, the aforementioned bond distances and the associated angles (C–Cl···π) (120.8, 123.7, and 125.1°) of the CCl system strongly indicate very weak XB interaction because of the poor directional interaction. Since the CCl guest is disordered, these values cannot be treated as reliable. The DFT-optimized geometries of this species (see Figure b) will give more accurate details, where C–Cl···π distances and angles drop in the ranges of 2.943–3.047 Å (2.943, 2.994, and 3.047 Å) and 139.4–162.2° (139.4, 159.5, and 162.2°), respectively. By looking into the structural features, the theoretical study appears to unveil strong interaction with high directionality of the XB, which is comparable with the CHCl and CHCl complexes (vice versa). Moreover, the experimentally observed NBD between the terminal N atoms of the long cavity is found to be the third lowest value (21.231 Å) among all systems probed in this work, which can be seen in Table , and it closely resembles with the calculated value (21.447 Å).

Figure 4

C–Cl···π and C–H···π interactions (bond distance and angles) in (a) the crystal structure and (b) DFT optimized (solvent phase) for CCl (CCl4 is disordered over two positions).

Encapsulated versus Free Solvents

For S1@BPOC (where S1 = CH, CHCl, and CCl)-related species, it can be seen that there is a decrement in the C–Cl and C–H distances when the S1 solvents have been captured inside the molecular cage (BPOC) (see Table ), and also, the solvent molecule structures are changed along with the associated bond angles due to one of the most important features, the confinement effect.

Energetics and Electronic Feature Analyses

Now second, in order to look into the energetics and electronic features, the analyses of the optimized crystals, S1@BPOC (S = CH/CHCl, and CCl4), have been discussed in both the gas and solvent phases.

The stability (in terms of binding/stabilization/encapsulation energy) of all complexes (crystal and model) can be calculated by considering the dissociation into their corresponding constituting monomers S and BPOC. The encapsulation energy (EE) can be calculated as belowwhere E[..] represents the total electronic energy of the respective species. The calculated EE values of all complexes from the above equation are also listed in Table .

Table 4

Calculated Encapsulation Energies (EEs) and HOMO–LUMO Gaps of the Fully Geometry Optimized Structures for the Interaction Different Solvents and Molecular Cage at the M06-2X/6-31G* Level of Theorya

halocarbon @BPOC	BE (kJ/mol)	HOMO (eV)	LUMO (eV)	Egap (eV)	dipole moment (S@BPOC)	dipole moment (BPOC)	dipole moment (S)
CH2Cl2	–69.9 (−63.6)	–7.00 (−7.18)	–0.46 (−0.66)	6.44 (6.52)	1.31 (2.07)	0.03 (0.01)	1.87 (2.24)
CHCl3	–73.2 (−68.6)	–6.99 (−7.12)	–0.48 (−0.63)	6.51 (6.49)	0.83 (1.20)	0.03 (0.01)	1.30 (1.52)
CHCl3’	–87.7 (−81.1)	–7.16 (−7.28)	–0.54 (0.68)	6.62 (6.60)	0.81 (1.08)	0.03 (0.01)	1.30 (1.52)
CCl4	–60.8 (−59.8)	–6.97 (−7.05)	–0.49 (−0.58)	6.48 (6.47)	0.34 (0.45)	0.03 (0.01)	0.00 (0.00)
C2H4Cl2	–75.8 (−67.4)	–6.96 (−7.15)	–0.44 (−0.65)	6.52 (6.50)	2.49 (3.76)	0.03 (0.06)	2.94 (3.71)
C2H4Br2	–158.6 (−150.6)	–7.06 (−7.18)	–0.42 (−0.55)	6.64 (6.63)	0.19 (0.29)	0.03 (0.23)	2.73 (3.40)
C2HCl3	–60.9 (−57.4)	–7.13 (−7.23)	–0.51 (−0.63)	6.62 (6.60)	0.84 (1.18)	0.03 (0.03)	1.00 (1.18)

Values given without and within parentheses are in the gas and solvent phases, respectively.

It is very interesting to observe that the stability pattern has been determined as CHCl (−87.7) > CHCl (−73.2) > CH (−69.9) > CCl (−60.8) (in kJ/mol) (gas phase) where the CHCl complex is found to be the most stable among all Cl-substituted methane complexes (S1@BPOC). The EE value of the CHCl complex (−87.7 kJ/mol) is about 1.5 times larger than that of the CCl complex (−60.8 kcal/mol), clearly revealing the strong dipole interactions between the CHCl solvent and cryptand BPOC.

The two Cl−π and one C–H/π interactions, of course, reinforce the stability of the CHCl complex, while in the case of the CCl crystal, although three Cl−π exist therein, the CCl solvent having zero dipole moment interacts very weakly with the BPOC cryptand. However, despite the appearance of the reasonably strong interaction acquired from the geometrical parameters (as mentioned earlier), the CCl contains the lowest EE value (−60.8 kJ/mol) compared to the other S-related complexes, which may be due to steric crowding of the fourth Cl atoms (which is not showing any interaction) as the halogen atoms are larger in size and more polarizable than the H atoms and thus are more sensitive to steric hindrance than are the HBs.[11] As a result, the CCl system is observed to be the least stable (i.e., the EE value, −60.9 kJ/mol of the C HGC is very close to the CCl species) among all the systems studied here in the present work. Now, using the PCM approach, a similar stability pattern has been viewed as CHCl (−81.1) > CHCl (−68.6) > CH (−63.6) > CCl (−59.8) (in kJ/mol). Now, to confirm the dominant behavior of the C–Cl···π (XB) and C–H···π (HB) interactions in both CHCl and CHCl complexes, the EEs were calculated to be −68.6 and −81.1 kJ/mol, respectively, which clearly shows that the latter case with three XB interactions dominates over the HB interaction and as a result, the CHCl has higher EE value (more stable) than the CHCl.

It has been recognized that a weak attractive force (i.e., interaction) (a hypothesis was presented) between the dipole of the C–H group and the quadrupole of the π-face of the aromatic moieties is important as subsequent findings based on spectroscopy as well as theoretical calculations led the author to advocate that an attractive force works between these groups.[12−15] Based on theoretical facts, stabilization of C–H/π moiety interaction comes, essentially, from the dispersion force. As mentioned earlier in the Introduction part that the BE/IE/EE is dependent on the character of the molecular segments, the C–H moiety and the π-groups, especially the C–H group (like CHCl protic solvent), have stronger proton donating ability showing the larger stabilizing effect, and as a consequence, it can clearly be seen in both cases of the CHCl- and CHCl-related complexes.

Frontier Molecular Orbitals (FMOs)

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbitals (LUMO) are the frontier molecular orbitals (FMOs), and the magnitude of the HOMO–LUMO gap (Egap) has important chemical implications, even if qualitatively estimated. As these orbitals fall at the outermost boundaries of the electrons of the molecular systems, the HOMO (acts as a Lewis base) is one of the FMOs of the highest energy of any system that is still filled, and it can donate electrons easily and this could be simply donating electron density to form a bond. In contrast, the LUMO (acts as a Lewis acid) is the lowest-lying orbital that is unfilled, and therefore, it is easy to accept electrons. Egap is a key parameter that is equivalent to the determination of the electron mobility from HOMO to the LUMO. A large HOMO–LUMO gap indicates good thermodynamic stability of the species, whereas its small value suggests an easier electronic transition.

The energy eigenvalues EHOMO and ELUMO of HOMOs and LUMOs and the corresponding energy gap (Egap) are listed in Table . The differences of eigenvalues (energy gap between the HOMO and LUMO) (gas phase, PCM), which is one of the important electronic parameters, for the chlorine-substituted methane systems were calculated for CHCl, CHCl, CH, and CCl, which are (6.62, 6.60), (6.51, 6.49), (6.44, 6.52), and (6.48, 6.47) (in eV), respectively, which also depict that the CHCl system is most stable among all the chlorine-substituted methane-related complexes. As energetic contribution is very important for the stability of any complex, cases involving the solvent like chloroform having a strong C–H donor property reveal forming the lowest-energy (most stable) complex. The HOMO–LUMO surface plots (distribution of HOMOs and LUMOs as well as the electronic transition phenomenon involved therein) of all 10 investigated complexes can be seen in the Supporting Information (see Figures S3–S5) along with a brief discussion.

Natural Population Analysis (NPA)

The natural charges (QX) obtained from the natural population analyses (NPA) were calculated for all complexes in the gas and solvent phases as summarized in Table S1. Though the total natural charges calculated for the individual monomers constructing different complexes are lightly significant (range 0.001–0.062 in the gas phase, 0.001–0.03 using PCM), a total charge on the cryptand and total charge on the corresponding encapsulated solvents are in opposite sign having the same magnitude.

In the case of the CH-, CHCl-, and CHCl-related complexes excluding CCl, the QX values on all the three solvents range from 0.013 to 0.015e indicating that the charge transfer (CT) occurs to the cryptand having charge ranging from −0.013 to −0.015e whereas there is a relatively very small CT of 0.001e from the cryptand to the CCl solvent consisting of a charge of −0.001e. The order of CT from solvents to the cryptand is (CHCl = 0.015e) > (CH = 0.013e) > (CCl = 0.001e) in which CHCl also shows the same CT behavior as its other orientation complex in the gas phase.

Despite the above analyses, the natural charges were computed on the C atoms of the free solvents CHCl/CHCl, CH, and CCl in the gas and solvent phases, which are approximately the same as −0.37, −0.49, and −0.3e, respectively. Now, the charges on the same C atoms of the solvents are reduced when they sit inside the cage, appearing due to formation of encapsulated solvent complexes. In the gas and solvent phases, these are (−0.31e, −0.345e)/(−0.334e, −0.334e), (−0.463e, −0.462e), and (−0.251e, −0.251e), respectively, for the aforementioned complexes.

The charges on the Cl atoms of the CHCl/CHCl, CH, and CCl free solvents (gas phase, PCM) are (0.03e, 0.02e), (−0.02e, −0.04e), and (0.08e, 0.07e), respectively, while the decreased charges drop in ranges of (0.015e, 0.023e/0.012e, 0.029e), (−0.03e, −0.039e), and (0.06e, 0.067e) in the case of encapsulated solvents. It is worth noting that the charges on the H atoms of the CHCl/CHCl and CH free solvents (gas phase, PCM) are found to be (0.28e, 0.3e) and (0.26e, 0.28e), respectively, whereas these charges have been altered to a small extent (0.281e, 0.281e)/0.289e, 0.291e), and (0.271e, 0.273e) in the aforesaid trapped solvents, respectively.

The CT from solvent to the cryptand also results in a net electric dipole moment for all complexes (see Table ). The net electric dipole moments for the free cryptand in the gas and solvent phases are 0.03 and 0.01 D, respectively. The order of net electric dipole moments (gas phase, PCM) for the free solvents is found to be CH (1.87 D, 2.24 D) > CHCl/CHCl (1.30 D, 1.52 D) > CCl (0.03 D, 0.00). For example, this leads to CT from CHCl (0.015e) to its molecular container BPOC (−0.015e), which results in a much larger dipole moment of the complex, (gas phase = 0.81 D, PCM = 1.08 D), than the cryptand alone (gas phase = 0.03 D, PCM = 0.01 D).

Molecular Electrostatic Potential (MEP) Surface Map

The molecular electrostatic potential (MEP) surfaces have frequently been applied to obtain the electrostatic potential (ESP) over constant electron density of any chemical species. The importance of the MEP occurs in reality that it concurrently shows the sizes, shapes, and local positive and negative potential regions of any chemical systems supported by the color scheme, which is, of course, quite supportive for analyzing the most possible reactive site in company with the sizes and shapes of the molecular systems. It should be noted that the MEP surface maps always fall approximately in the same region of the molecular container for all the guest–host chemical species exercised here (see Figure ).

Figure 5

General and approximately common MEP surface maps of all guest–host complexes and the molecular container alone.

The void of the molecular container BPOC (alone) itself does not illustrate any significant partial potential. Instead, the collective structural and computational results supply the facts that the alliance of the π-cloud of some of the five/six-membered rings contained by the cryptand BPOC is clearly assisted to interact with the C–H/X moiety of the trapped solvents and the mentioned rings contain electron-rich surfaces although to a small extent, which can be seen in the yellow color scheme, implying that the affinity of the cavity toward anions is not very poor. Now, in contrast to the cavity, the three methylene groups (blue color scheme) sitting over the middle region of the cryptand show an electron-poor (electropositive) area that is more suitable for anion binding.

Noncovalent Interaction (NCI) Plots

Identification and graphical envision of NCI regions along with the nature and strength involved therein have been basically produced by means of the NCI-reduced density gradient (NCI-RDG) approach, which allows an inclusive description of HBs, van der Waals (vdW) interactions, and steric repulsion in all species. Generally, localized blue lentils show the strong attractive interactions (i.e., HBs), thin and delocalized green regions characterize the weak interaction like vdW interactions, and red isosurfaces reveal the steric clashes (for example, see Figure , figures on the right side). Detailed information on the understanding and designing of the NCIs is provided by such above said color schemes.[16,17] To perceive the NCIs engaging in small molecules, molecular complexes, and solids, some interesting highlights on the electron density (ED) and its derivatives-based approach were given by Johnson and co-workers approximately one decade ago.[17] The ED and its derivatives allow synchronized analysis and visualization of a broad range of NCI types as real space surfaces and playing a key role to a chemist’s arsenal, and efforts have been made in giving an outline for the H-bonded species.

Figure 6

3D isosurface (left) and 2D scatter (right) plots of CH, CHCl, CHCl, and CCl (from top to bottom).

The RDG isosurface displayed on the horizontal axis is 0.5 (ranging from −0.05 to +0.05). The Ω(r) values ranging from −0.035 atomic unit (au) to +0.02 au on the vertical axis (right side) show the colored surfaces of the species on a blue-green-red scale. The detailed description of the NCI tool clarifies that the higher-density values (Ω(r) < 0) show stronger attractive interactions while the very low-density values (Ω(r) > 0) indicate repulsive interactions. For example, in the case of the C system, green-colored broad spikes (values lying between −0.002 and −0.013) are seen in the 2D scatter plot. Moreover, in the 3D isosurface representation, two green-colored disc-shaped NCI isosurfaces have been shown (two black-colored rounded rectangles indicating C–Cl···π and one purple-colored rounded rectangle indicating C–H···π interactions) and signify the attractive interaction giving a clear indication of the presence of vDW interaction, which is also supported by the experimental expectations.[6] As lots of attractive interactions are involved in all the probed HGCs, for the sake of clarity, only the C–X···π (where X = Cl or Br) and C–H···π types of interactions have been highlighted in the pictorial representation. The NCI-plot also shows the π–π stacking involved in the HGC (consisting of manifold electron-rich pyrazole aromatic rings where both hydrogen and halogen bonding interactions can be utilized) formation, which is used to evaluate the role of NCIs in the crystal.

The presence of steric effects is evidently shown by the low-gradient spikes (2D scatter plot) appearing at the positive side (+0.006 to +0.022). This effect as shown by the red ellipsoid depicts (3D isosurface) the electron density depletion, which is due to the electrostatic repulsion. If we look into the other S1-related three complexes, two C–Cl···π and C–H···π interactions in CHCl, three C–Cl···π interactions in CHCl, and three C–Cl···π interactions in the CCl complexes have been observed using the NCI-plot tool, which appears to be strongly favored by the experimental outcomes.[6] It should be noted that the rounded black-colored and purple-colored rectangles as shown in the 3D representation are indicative of the C–X···π and C–H···π interactions, respectively. Moreover, it is worth mentioning that the CHCl complex shows two C–Cl···π and one C–H···π interaction whereas the CHCl complex reveals three C–Cl···π interactions, which reveal to be due to the existence of CHCl3 in two different orientations. The zoomed pictures of the NCI 3D isosurface and 2D scatter plots of the S1@BPOC complexes can be seen Figure .[7] Full images of the NCI plots (3D isosurface) of all 10 investigated S@BPOC assemblies can be discerned from Figure S6.

S2@BPOC Host–Guest Crystallized Assemblies (S2 = C, C, and C)

In order to look into the binding and polarization features, another set of systems containing halogen (X = Cl/Br) atoms and C–C/C=C type bonds were chosen as S2@BPOC (S2 = C, C, and C). The crystallized structure of the C complex (see Figure a) having one BPOC unit with one encapsulated solvent molecule of C has been optimized. The trapped C solvent is disordered and is not showing any well-defined interactions, but the other host–guest moiety shows two large C–Cl···π (3.607 and 3.733 Å) and four C–H···π (2.557, 2.573, 3.113, and 3.349 Å) interactions reflecting the weak interactions in the host–guest system. C is present in the gauche conformation having 72.3° of the torsion angle. The C–Cl···π (135.8 and 139.6°) and C–H···π angles (138.8 and 149.9°) reflect much deviation from the directionality of the XBs.

Figure 7

C–Cl···π and C–H···π interactions (bond distance and angles) in (a) the crystal structure and (b) DFT optimized (solvent phase) for C.

The DFT-optimized structure (see Figure b) also resembles the solid-state one with C–Cl···π distances (3.528 and 3.535 Å) with a bond angle of 135.6° for both C–H···π distances (2.352 and 2.355 Å) with a bond angle of 142.9° for both. It is appealing to notice that both values (i.e., for the bond lengths as well as bond angles) of the solid state and DFT-based structures suggest weak interactions of the C solvent molecule with the host cryptand, BPOC. Here, in this, the nonbonded length between the terminal N atoms is measured to be 21.574 Å; however, a marginal change has been observed to be 21.810 Å using the computational approach.

Furthermore, the crystal of C having a BPOC cage unit and one C solvent molecule trapped inside the molecular container (BPOC) was optimized. Here, the crystal form of the C guest solvent (Figure a) exists in the gauche conformation consisting of a torsion angle of 50.7°. In this case, two C–Br···π and two C–H···π interactions were observed here with distances of 3.479 and 2.490 Å, respectively, for its crystalline form. Also, the associated C–Br···π and C–H···π angles are 169.4 and 168.1°, respectively. The optimized structure (3.334 Å for both C–Br···π and 2.632 and 2.633 Å for C–H···π distances) also shows good consistency with the crystallized structure, which is shown in Tables and 2 (see Figure b). The studies on the both experimental and theoretical cases disclose nearly the same bond distances and angles (in terms of symmetrically arranged solvent) suggesting the strong directional nature of the Br atom compared to the Cl atom. As compared to all other cases, the NN distance between the nonbonded nitrogen atoms located over the extreme of the BPOC has been found to be the lowest (20.806 Å), which is also computed by the theoretical investigation (19.791 Å).

Figure 8

C–Br···π and C–H···π interactions (bond distance and angles) in (a) the crystal structure and (b) DFT optimized (solvent phase) for C.

Now, in addition to the above, the C crystal (see Figure a) assembly was optimized in which the C solvent contains the C=C bond consisting of three chlorine (Cl) atoms. The trapped C also persists in a disordered state. The crystal structural analysis appeared to show two C–Cl···π interactions with a distance of 3.194 Å (no C–H···π interaction) and angle of 140.4°. Such values do not reflect the true nature because of the disordered nature of the solvent; therefore, the DFT-optimized structure (see Figure b) was also inspected. Three interactions were found with the deployment of the DFT approach in which there is an existence of two C–Cl···π (3.064 and 3.254 Å) and one C–H···π (2.442 Å) showing short distances. Here, the associated C–Cl···π bond angles are calculated as 131.5 and 156.6° using the theoretical approach. Again, these values suggest strong bonding along with the high directionality of the XBs. Furthermore, the experimental data reveals that the distance between the N atoms lying at the extreme of the long cavity is the second smallest value (21.114 Å) among all the systems, which closely resembles with the calculated data (21.269 Å).

Figure 9

C–Cl···π and C–H···π interactions (bond distance and angles) in (a) the crystal structure and (b) DFT optimized (solvent phase) for C (C is present over 2-fold axes and is therefore disordered).

In the case of the S2@BPOC systems, the C–Cl and C–H distances are found to be reduced when the S2 solvent molecules (S2 = C/C/C) are grabbed by the cryptand (BPOC) and the solvent moiety structures are compressed; however, the associated bond angles are also changed due to the confinement effect (see Table ).

Energetics and Electronic Feature Analyses

The calculated EE values as given in Table clearly reveal that the endo-complexation of the C solvent within the cryptand (BPOC) is favored and has the most salient feature in the sense of stability (EE: −158.6 kJ/mol) over other endo-complexation in all probed species in the gas phase, which demonstrates a good fit of this solvent inside the molecular cage. Apparently, the PCM approach (EE: −150.6 kJ/mol) also provides a similar trend for the C HGC showing the highest stability among all the systems exercised here, and hence, it appears to cause the good performance of M06-2X/6-31G* method based on the QCCs. The EE value of the C complex (−158.6 kJ/mol) in the gas phase is about 2.1 and 2.6 times larger than those of the C (−75.8 kJ/mol) and C (−60.9 kJ/mol) complexes, respectively. A similar pattern has been obtained by performing the PCM approach, and the EE value of the C (−150.6 kJ/mol) system is about 2.6 and 2.2 times greater than those of the C (−57.4 kJ/mol)- and C-related complexes, correspondingly in the solvent phase analysis. The calculated BE (−150.6 kJ/mol) was found to be the highest for the C assembly, suggesting the strong halogen bonding capability of Br compared to the Cl atom.[18]

It is well known that the polarizability increases as the atom/ion gets larger and less electronegative. Hence, the polarizability of the Br atom is higher than that of the Cl atom, and therefore, due to much favorable interactions driven by the polarizability factor involved in the system, the stability (in terms of EE) is expected to be higher in the Br-related complex (vice versa). It is surprising to observe that in all the host–guest assemblies, water and methanol solvent molecules are never found inside the cavity, which suggests the strong hydrophobic nature of the inside of the cryptand and shows that halocarbons are bound more strongly in the hydrophobic void, which seems to be due to the constructive size/shape and multiple interaction sites involving the halo atoms. The EE of the C host–guest dimer system is slightly larger (i.e., about −15 kJ/mol in the gas phase) than that of the C system. As in the CCl4-related complex, here also, the BE for the C-related complex is −57.4 kJ/mol, which is nearly close to the CCl (−59.8 kJ/mol), possibly due to the same reason given for the CCl (one Cl atom is not showing any interaction and creating crowding). Among all the systems, the maximum and minimum EE differences between the gas and solvent phases are 8 and 1 kJ/mol for the C and CCl complexes, respectively.

Frontier Molecular Orbitals

The Egap value of the C system has been compared with those of the S2-related complexes C and C as reported in Table . The order of energy difference between the HOMO and LUMO (Egap) is as follows: (gas phase, PCM); C (6.64, 6.63) > C (6.62, 6.60) > C (6.52, 6.50) (in eV), which demonstrates that the C-related complex is the most stable chemical species, which is also in accordance with the largest EE value (−158.6 kJ/mol) obtained for this system as tabulated in Table . Generally, the occurrence of a dielectric field (PCM approach) corresponding to the solvent molecules exercised here resorts to stabilize the respective complexes more than in the gas phase (see Table ).

Natural Population Analyses

For the C, C, and C complexes, 0.009e is the natural charge on both the solvents C and C whereas 0.062e is for the C solvent, which demonstrates that the CT takes place from all three solvents to the molecular tank. For the former two solvents C- and C-related complexes, the natural charge on the molecular cage (BPOC) is −0.009e whereas a comparatively much large CT of 0.062e from C solvent to its cryptand consisting of a charge of −0.062e has been calculated. The total electric dipole moment for the free cryptand has been computed to be 0.03 D in the gas phase; however, in the solvent phase, the order of a net electric dipole moment for the trapper/cryptand having solvents is C (0.23 D) > C (0.06 D) > C (0.03 D) leading to the largest CT from the encapsulated solvent C (0.062e) to its molecular cage BPOC (−0.062e). This outcome is supported by a much larger EE value (gas phase, PCM) of the C complex (−158.6 kJ/mol, −150.6 kJ/mol) than those with the other two complexes, C (−60.9 kJ/mol, −57.4 kJ/mol) and C (−75.8 kJ/mol, −67.4 kJ/mol). For instance, this fairly escorts to the CT from the C to the trapper BPOC, and such findings give a much larger dipole moment (gas phase, PCM) of the complex (0. 19 D, 0.29 D) than that of the cryptand alone (0.03 D, 0.23 D). However, the order of the total electric dipole moments (gas phase, PCM) for free solvents is found to be C (2.94 D, 3.71 D) > C (2.73 D, 3.40 D) > C (1.00 D, 1.18 D). This finding slightly differs from the abovementioned order, which is quite different from the experimentally observed order: C (1.572 D) > C (1.5 D) > C (1.04 D).

Among the aforementioned three complexes (in the gas phase), the analyzed natural charges on the C (−0.265 and −0.107e), H (0.289e), and Cl (0.04–0.053e) atoms of the trapped C solvent were computed while the natural charges on its free (alone) solvent are C (−0.29 and −0.13e), H (0.28e), and Cl (0.03e–0.07e). The charges on the C, H, and Cl atoms for the C solvent were obtained to be −0.446, 0.264 (i.e., average value taken), and −0.078e, respectively, in the gas phase, whereas for the free solvent, the respective values are −0.43, 0.27, and −0.1e. However, the charges on C, H, and Br atoms of the trapped C solvent were attained to be −0.498, 0.277, and −0.026e, respectively, while these are −0.47, 0.24, and −0.01e, correspondingly, in its isolated form. By looking into the dichloro- and dibromo-related systems, the charges on the C atoms of the both systems have been increased (in magnitude) while the charges on the H atoms show the opposite trend (i.e., in the C case, it has been reduced from 0.27 to 0.264e, whereas it has enhanced from 0.24 to 0.277e in the case of C) (see Table S1). The negative charges (gas phase, PCM) on the Cl (−0.078e, −0.087e) and Br (−0.026e, −0.007e) atoms depict that the Br atoms are more polarizable than chlorine, which may appear that Br has a good capability to form the favorable Br−π interaction, which provides another strong evidence for the Br-related complex being the most stable among all.

NCI-3D Isosurface and 2D Scatter Plots

To visualize the reason for the endo-complexation of the encapsulated trichloro-substituted ethene and dichloro- and dibromo-substituted ethane systems, much clear (i.e., zoomed images) NCI 3D isosurface and 2D scatter plots were mapped, which are shown in Figure . In the case of S2-associated complexes (where S2 = C, C, and C), the 3D isosurface of the C complex shows two C–Cl···π and one C–H···π interactions in which outcomes are consistent with a previous report.[6]

Figure 10

3D isosurface (left) and 2D scatter (right) plots of C, C, and C (from top to bottom).

Furthermore, let us have a closer look into the NCI plots of the former two host–guest species C and C. The four C–Br···π and two C–H···π interactions in the C complex while two C–Cl···π and two C–H···π interactions in the C complex can be seen in Figure (see the 3D isosurface). The higher number of interactions in the former one seems to be due to the higher polarizability of the Br atom having a comparably large σ-hole with regard to the Cl atom, which is responsible for the enhanced and highest stability of the Br-containing complex (i.e., the EE value, −150.6 kJ/mol, of the C complex is 2.3 times higher than the EE value, −67.4 kJ/mol, of the C).

The size of the halogen atom lobes especially, in the case of both halogen-substituted ethane systems, enhances with the radius or polarizability of the halogen. Therefore, it plays an important role suggesting that the Br atoms would form the strongest Br−π type interactions whereas the Cl atoms form moderately strong Cl−π-type interactions. The magnitude of these polarization effects is indeed modulated by the chemical context of the halogen atom. It must be noted that these polarization interactions are supported by a favorable arrangement of the solvent in the cryptand, in agreement with the fact, mentioned above, that the Br-containing complex yields always the Br−π interaction as the most stable one.

S’@BPOC Model Systems versusS1@BPOC Complexes (CFversusCH and CFClversusCHCl)

To get more insights about the nature of the binding and substitution effect of the Cl (or F) atoms, three model encapsulated halocarbon adducts such as S’@BPOC (S’ = CClF, CCl, and CCl) were chosen and optimized followed by the frequency calculations. The optimized structures of the model complexes can be discerned from Figures –13. The geometric features and energetics have also been attempted to show such kinds of determined characteristics observed at the molecular level. Like the S1- and S2-related crystals, all three aforementioned S’ guests sit over the center of the cage (BPOC).

Figure 11

C–Cl···π interactions (bond distance and angles) in DFT optimized (gas phase) for CClF.

Figure 13

C–Cl···π interactions (bond distance and angles) in DFT optimized (gas phase) for CCl.

It would be an interesting exercise if the crystallization for the CFCs (CCl, CCl, and CClF) with the BPOC cryptand can be attempted. Here, in order to get some theoretical understanding for such model complexes, only the DFT (gas phase) optimization studies on three molecules, namely, CCl, CCl, and CClF, have been performed. In order to probe the binding nature, the optimization imitates the experimental trend in the equilibrium Cl···π distances, which are shorter in the complex with monochloro, CClF (3.212 Å), dichloro, CCl (3.181 and 3.182 Å), and trichloro, CCl adducts (3.052, 3.058, and 3.059 Å). The CCl showed three C–Cl···π interactions with distances ranging from 3.052 to 3.059 Å, while the C–Cl···π angles fall in the range of 157.3–157.9°. It should also be noted that the F atom does not show any interaction because it does not have any σ-hole formation to form halogen bonding, which increases in the order of F < Cl < Br < I. Also, CFCl showed very strong halogen bonding as reflected from its bond distance and its angle. Not surprisingly, CCl showed only two HBs, with distances of C–Cl···π, 3.181 and 3.182 Å, since the rest of the two halogen atoms are fluorine. Also, the associated XB angles are 155.4 and 155.4°. These values suggest that CCl is bound loosely compared to the CCl species. Furthermore, the CClF showed only one C–Cl···π interaction with a distance of 3.212 Å and an angle of 157.1°. From this data, we can confirm that as the number of Cl atoms is decreased (or the number of F atoms is increased) in the CFCs, the halogen bonding interaction also decreases gradually. It can also be seen that the NN bridgehead terminal distances decrease as the number of Cl substitution increases (or decrease in F atoms): [1Cl (21.511 Å) > 2Cl (21.501 Å) > 3Cl (21.413 Å)].

In the CFC-related systems of S’@BPOC when the solvents (S’) are trapped by the molecular tank (BPOC), a decrease in the Cl−π bond distances of the solvents has been observed as the number of Cl substitution increases, and the associated bond angles are also influenced because of the confinement effect; however, the corresponding C–F distances are increased.

The estimated EE values of the model complexes (S’@BPOC), CClF, CCl, and CCl, are −77.9, −86.6, and −88.6 (in kJ/mol), respectively, as shown in Table . The DFT calculations using the M06-2X functional predict the EE for the model cryptand–CCl complex to be about 2 and 10 kJ/mol greater (i.e., giving an indication of higher stability) than the other model CCl- and CClF-related complexes, respectively. From the calculated EEs, as the chlorine substitution on methane increases, the stability enhances, which emerges to be caused by the enhancement of the dipole moment ((CCl, 0.27 D) > (CCl, 0.21 D), (CClF, 0.09 D)) (using the PCM approach) and the Cl−π interactions (see Table ). The calculated average Cl−π distances are 3.056, 3.181, and 3.212 Å for the CCl, CCl, and CClF HGCs, respectively. The calculated EE supports the data of the halogen bonding interactions, which also decreases in the same manner.

The dipole moments of the CCl (1.13 D), CCl (0.45 D), and CClF (0.29 D) solvents appear to show that the stability of the complexes (mentioned above) decreases as the dipole interactions reduce between the corresponding solvents and the cryptand (BPOC). As can be seen from Table , the dipole moment of the free CCl (0.27 D) solvent is greater than those of free CCl (0.21 D) and CClF (0.09 D), which is expected from the above compared features of the encapsulated solvent complexes. Note also a very good agreement of this with the calculated EEs for CCl (−88.6 kJ/mol), CCl (−86.6 kJ/mol), and CClF (−77.9 kJ/mol).

Energetics of CHCl/CHClversusCCl and CHversusCCl

It is interesting to note that the substitution of the F atom in place of the H atom in the CHCl solvent does not alter the EEs effectively as the estimated EE value for the complex CCl (−88.6 kJ/mol) is marginally higher than the EE (−87.7 kJ/mol) of the experimentally observed crystal CHCl; however, in the latter case, the H atom of the CHCl3 solvent is able to form a CH/π type of interaction. Moreover, surprisingly, an almost similar trend has also been found in the case of the CCl complex with an EE value of −86.6 kJ/mol (consisting of two Cl−π moiety interactions only), while the EE value (−69.9 kJ/mol) of the CH complex is even much lower than that of the former complex though it possesses two Cl−π and two CH/π moiety interactions. Finally, the EE values fairly indicate that the CCl guest should be better absorbed than the CCl and this one is much better than the CClF. In order to look into the interaction(s) in the CClFversusCHCl-related complexes, it is surprising to see that the EE value of the CClF complex with only one Cl−π interaction (−77.9 k J/mol) is found to be slightly greater than the EE of the CHCl-related complex (−73.2 kJ/mol) associated with one CH/π and two Cl−π interactions. In contrast, amazingly, the EE value of the former complex (CClF) is much smaller than that of the other orientation chloroform complex, CHCl (−87.7 kJ/mol) having three Cl−π interactions.

Among these three models, the natural charges on the [C, F, Cl] atoms contained by the free CClF, CCl, and CCl solvents were obtained to be (1.06, −0.35, −0.01), (0.64, −0.34, 0.02) and (0.19, −0.33, 0.05), respectively (see Table S1). The charges on C and F atoms decrease when Cl substitution increases. Now, when these solvents (S’ = CClF, CCl, and CCl) perch inside the cryptand (BPOC), the charges on the (C, F, Cl) atoms of the respective encapsulated solvents are (1.09, −0.35, −0.02), (0.67, −0.34, 0.02), and (0.22, −0.33, 0.04), respectively. From the above findings, the charges on the C atoms enhance when the solvents are trapped inside the BPOC cryptand while in the case of F and Cl atoms, the changes are the same. Though the total natural charges calculated here for the individual monomers constructing different complexes are lightly significant (range 0.001–0.01), the total charge on the cryptand and the total charge on the corresponding encapsulated solvents are in opposite sign while in the same magnitude. Within the three systems mentioned here in this section, the NPA appears to show that the CT leads from solvent moieties CClF, CCl, and CCl to the BPOC cryptand, which results in much larger dipole moments of the complex, 1.13, 0.45, and 0.29 D, respectively, than that of the free cryptand having a dipole moment of 0.03 D in all three cases.

To visualize especially the C–Cl···π interaction(s) involved in the endo-complexation of the CFC model systems (CClF, CCl, and CCl), clear pictures (in zoomed view) of the NCI 3D isosurface and 2D scatter plots are displayed in Figure . From the given below NCI 3D isosurface diagrams, it is obvious that the C–Cl···π interaction increases as the number of Cl atoms enhance in the CFC-related HGCs, which is because the atomic polarizability of the F atom (3.2 au) is substantially smaller than that of the Cl atom (16 au) and thus the smaller polarizability is the cause of smaller attraction of CClF at the M06-2X/6-31G* level of approach. The replacement of the H atoms of methane by the Cl atoms having large atomic polarizability enhances the electrostatic attraction substantially; in contrast, exchange of the H atoms by the F atoms influences the ESP.

Figure 14

3D isosurface (left) and 2D scatter (right) plots of CCl, CCl, and CClF (from top to bottom).

The observed enhancement of intermolecular interactions followed by the crystallographic facts and using the DFT approach suggests that the substitution by chlorine atoms increases the Cl−π interaction(s) assisted by the polarizability, and the obtained order of stability is favored as CCl > CCl > CClF, which is in accordance with the order of calculated EE values and natural charges (see Figures , 11, 12, and 13).

Figure 12

C–Cl···π interactions (bond distance and angles) in DFT optimized (gas phase) for CCl.

The computational facts not only assist the experimental verdict of the endo-complexation of CCl with the cryptand BPOC, which is more favorable than the other two CCl and CClF complexation with the trapper BPOC, but it also specifies that the guest encapsulation is primarily directed by the weak intermolecular forces between the C–H/X moiety of the trapped solvents and the five-membered π-ring cloud of the molecular cage BPOCvia the C–H/π as well as the X−π interactions, which makes the F/Cl-substituted HGC models having a higher number of Cl atoms act as better guests for the endo-complexation.

Here, the computational study has been well suited to describe the C–H/π and X−π (X = Cl and Br) interactions involved in the crystallized complexes adequately, which is strongly supported by several parameters (obtained from the theory) like geometric/structural parameters, BE/IE/EE, HOMO–LUMO gap (Egap), NPA, NCI and MEP plots, and dipole moments computed at the M06-2X/6-31G* level of theory. This is evidenced from the good performance by the DFT approach. This understanding would be constructively helpful to learn different C–H/π and X−π (X = Cl and Br) interaction-driven structures such as halogen containing systems intercalated between the host and the guest molecules. Despite the fact that similar binary complexes are frequently detected in the solid-state form, this is a distinctive paradigm of such types of neutral guests in a host molecular container in several solutions as well as in the gas phase.

Some interesting extended work on such kinds of attention-grabbing and rarely synthesized complexes will be reported in future communication, which will contain a vast number of useful information like the existence of aromaticity/noncovalent interaction coupling effects. Recently, Pandey et al. have proposed two tools for the quantification of aromaticity [aromaticity index based on interaction coordinates (AIBIC)][19] and noncovalent interactions [hydrogen bond strength based on interaction coordinates (HBSBIC)][20] in which applications will be reported comprehensively in future communication. The rotational motion and dynamics studies using the PES scanning technique, aromaticity (using the HOMA, NICS, AIBIC, and PDI tools) versus noncovalent interaction (using the BE calculations including the BSSE correction, HBSBIC, and QTAIM tools) features, 1H NMR calculations, and some other interesting features of the above reported HGCs in the present work will be divulged in the future.

Conclusions

In summary, it has unambiguously been shown that neutral guest molecules are bound at an endo-binding site in complexes with a bispyrazole-based cryptand through dispersion interactions in the gaseous and solution phases. The structures of the studied crystal complexes and other host–guest model complexes were investigated using the DFT quantum computational approach. The geometry-based parameters of the complexes are also predicted and are in almost good agreement with the solid-state structures with the endo-complexed solvents showing several interactions. Among all the host–guest complexes probed here in this work, the most stable structures of the C complex having encapsulation energies of −158.6 and −150.6 kJ/mol were predicted in the gas and solvent phases, respectively. Although common in the solid phase, similar binary complexes have not been formerly examined in any gas-phase studies of the bispyrazole-based organic cryptand (BPOC). We have investigated the structural, stability, and electronic features along with the binding nature of the BPOC consisting of several solvents in specific host–guest chemistry in more detail.

The results are noteworthy and another interesting template for the progress in materials for complexation of host–guest species in drug-design synthesis. The BPOC facilitates a suitable model of the host intended for binding of several interesting solvent molecules as guests. Such studies may assist researchers to facilitate other suitable interesting motifs in supramolecular self-assemblies or drug design binding processes.