Quantifying Host–Guest Interaction Energies in Clathrates of Dianin's

Oct 12, 2016 - The subtle intermolecular host–guest interaction energies have been quantified for 17 different clathrates of the Dianin's compound. ...
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Quantifying Host−Guest Interaction Energies in Clathrates of Dianin’s Compound Espen Eikeland,† Mark A. Spackman,‡ and Bo B. Iversen*,† †

Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus C, Denmark School of Chemistry and Biochemistry, M310, University of Western Australia, 35 Stirling Hwy, Crawley WA, 6009, Australia

‡

S Supporting Information *

ABSTRACT: The subtle intermolecular host−guest interaction energies have been quantified for 17 different clathrates of the Dianin’s compound. Energy framework analysis of the host structure reveals that, in addition to strong electrostatic forces due to H-bonding, the framework is stabilized by very strong dispersion interactions, resulting in a three-dimensional energy framework. Compared to the host framework, the host−guest interactions are rather weak, and the enclathration only perturbs the host energy framework. Larger guest molecules result in more attractive host−guest interactions, although the shape of the guest molecule is also found to be important. Easy rotation about the c-axis was found for the hexane guest molecule, while the rotation is hindered in the cases of CCl4, CCl3CN, and C(CH3)3CN. For the Dianin clathrates containing the C(CH3)3CN or the acetone guest species, attractive interaction energies between guest molecules in adjacent cavities suggest short-range ordering of the guest molecules. For the clathrates containing multiple guest molecules per cavity, intermolecular interaction energies were used to determine probable guest molecule configurations. In the same way, simple energy calculations like the ones presented here can help crystallographers solve disordered molecular structures by discarding unstable configurations.

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INTRODUCTION The rapidly growing research areas of crystal engineering and supramolecular chemistry studies focus on intermolecular interactions which have broad implications in natural science.1 Noncovalent interactions are the key driver in complex phenomena such as self-assembly and nucleation, ligand receptor binding, and enzymatic catalysis.2−6 Simple organic clathrates are key supramolecular entities, with a structural diversity controlled largely by subtle interactions between the host and the enclosed guest molecules. Quantifying these intermolecular host−guest interactions provides insight vital for understanding the interplay between molecules in more complex structures, such as the ligand binding in proteins. The simplicity of the clathrates also allows for theoretical studies on guest transport and molecular dynamics.7−9 Racemic Dianin’s compound (4-(2,2,4-trimethylchroman-4-yl)phenol, abbreviated DC) is known to form clathrates with a wide variation of guest molecules, and it also forms a stable guestfree apohost compound isostructural to the clathrates, which allows for studying the distortions in the host crystal structures caused by the guest species.10−14 The stability of the host structure is emphasized by the fact that synthetic modifications to the DC molecule such as thiolation of the phenolic group, removal of one of the 2-methyl groups, or replacement of the oxygen heteroatom by S or Se do not change the coordination of the host molecules. Even deprotonating the OH ring by introducing morpholine as guest molecules result in a similar packing motif.15 Since the discovery of the DC clathrates in © XXXX American Chemical Society

1914, more than 50 crystalline DC adducts have been extensively studied, in particular their preparation, melting points, and the host/guest molar ratio.16−18 The symmetry and structure of the apohost (space group R3)̅ have been known since the 1950s, whereas the guest molecule positions have only recently been determined for the main part of the clathrates. The reason why the guest positions have eluded researchers for such a significant period is due to the symmetry related disorder of the guest molecules. The cavities formed by the host structure are highly symmetric (3̅ symmetry element), which result in disorder of nonsymmetric guest molecules. Recently, Lee et al.11 reported a crystallographic study, solving the guest positions in 17 different DC clathrate structures, with data measured at 100 K (the guest positions of the DC/CCl4 and DC/CCl3CN structures were known from the literature).19−22 The study yielded considerable insight into the host response to enclathration. The CrystalExplorer software was used to further understand short host−guest distances via Hirshfeld surface analysis, and cavity volumes were estimated using void space analysis.23,24 A new addition to CrystalExplorer is an efficient computational approach to calculating accurate intermolecular energies.25,26 The interaction energy between any nearest-neighbor molecular pairs is expressed in terms of four energy components: electrostatic, polarization, dispersion, Received: July 1, 2016 Revised: September 24, 2016

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and exchange-repulsion. The energies may be graphically represented as energy frameworks, providing an easily accessible method to quantitatively compare and study host−guest interaction energies in the DC clathrates. By calculating the electrostatic, polarization, dispersion, and exchange-repulsion energies between all nearest neighbors, one ensures not to overlook any important energy contributions to the cohesive energy of the structure.

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THEORETICAL CALCULATIONS

Model energies have been calculated between all unique molecular pairs in 17 DC clathrate structures together with the empty DC apohost structure, using CrystalExplorer and Gaussian 09.27,28 The model (termed CE-B3LYP) uses B3LYP-D2/6-31G(d,p) molecular wave functions calculated at the crystal geometry, taken from the work by Lee et al.10,11 and Jacobs et al.,22 with X−H bond lengths fixed to standard neutron diffraction values.29 The approach uses electron densities of unperturbed monomers to obtain four separate energy terms: electrostatic, polarization, dispersion, and exchange-repulsion. Each term is scaled to fit a large training set of B3LYP-D2/6-31G(d,p) counterpoise-corrected energies from both organic and inorganic crystals. The CE-B3LYP energies reproduce the training set energies with a mean absolute deviation of ∼1 kJ/mol.26 Pairwise energies in the clathrates are depicted via energy f rameworks, whereby cylinders with a thickness proportional to the magnitude of the interaction energy link the center of mass of the molecules. For the DC/acetic acid structure, the guest molecule geometries were optimized using CRYSTAL1430 and the B3LYP/pob_TZVP31 level of theory including dispersion corrections proposed by Grimme.29 Unit cell parameters and host molecule coordinates were fixed to the experimental values, and the symmetry was reduced to P1̅. More details are found in the Supporting Information.

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RESULTS AND DISCUSSION The Host Structure. All the analyzed DC clathrate structures have trigonal symmetry with space group R3.̅ The apohost structure has the following unit cell parameters, a = 26.778(1) Å and c = 10.9031(4) Å, using the hexagonal cell setting. All the structures were determined at 100 K.11 The host structure motif, illustrated in Figure 1b, consists of six host

Figure 2. Energy frameworks for the empty apohost structure. The electrostatic, dispersion, and total energies are colored red, green, and blue, respectively, with cylinder thickness proportional to the magnitude of the interaction energy. Only energies of magnitude > 20 kJ/mol are included in the figure.

Figure 1. (a) Dianin’s compound. (b) View of the DC/apohost structure including a 0.0003 au procrystal electron density void surface, illustrating the hourglass shape of the host cavities oriented along the c-axis.

strongest interactions consist mainly of dispersion forces while a single interaction is dominated by a large electrostatic energy term. From the total energy framework, we can see that the host structure forms a very stable structure, with strong intermolecular interactions in all three dimensions. This is in sharp contrast to the literature on Dianin’s compound which assigns the stability of the structure only to the H-bonded network, neglecting the strength of the dispersion interactions. Host Response to Guest Enclathration. From a void space analysis, Lee et al. found that enclathration of large guest molecules expands the cavities, whereas small guest molecules shrink the cavity volumes.11 The authors suggested that this is due to significant attractive interactions between the host molecule and the guest. For the apohost structure total host− host interaction energy (i.e., interaction energy per molecule),

molecules related by the threefold rotoinversion symmetry. Each cavity is enclosed by three host molecules pointing upward and three pointing downward. The result is an hourglass-shaped cavity with the top and bottom of each cavity consisting of a hexagonal cyclic motif of O−H···O hydrogen bonds as displayed in Figure 1b. The relatively large hourglassshaped cavities allow for the inclusion of multiple small guest molecules or a single larger guest molecule. The four strongest intermolecular interaction energies are depicted in Figure 2, with total interaction energies varying from −31 to −40 kJ/ mol. All other pairwise intermolecular interactions have total interaction energies with magnitudes less than 15 kJ/mol. The B

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Figure 3. (a) Changes in total host−host interaction energies as a function of cavity volume (using a 0.0005 au isosurface), with respect to the apohost structure. Negative values correspond to guest molecules lowering the total host−host interaction energies with respect to the apohost structure. For the apohost structure, the total host−host interaction energy is −258 kJ/mol and the void space volume = 420 Å3. Void space values are taken from the work by Lee et al. (b) Total host−guest interaction energies for the different clathrates plotted as a function of the guest volume per cavity. Benzene related guest molecules are highlighted in red.

Table 1. Host−Guest Interaction Energies Per Cavity (kJ/mol)a

a

DC with:

VUC/Å3

Coulomb

polarization

dispersion

repulsion

total

fluorobenzene trichloroacetonitrile carbon tetrachloride dichloroethane chlorobenzene benzene toluene bromobenzene bromoform iodobenzene acetic acid trimethylacetonitrile ethanol n-hexane nitromethane tetrahydrofuran acetone

6790.6(2) 6772.5(8) 6795.3(2) 6758.8(2) 6807.0(2) 6823.3(2) 6819.4(2) 6811.7(2) 6815.8(5) 6794.4(5) 6787.1(2) 6867.5(2) 6750.1(4) 6770.9(2) 6719.3(5) 6840.9(2) 6816.5(3)

−40.9 −35.7 −37.0 −16.5 −41.4 −33.8 −43.6 −38.9 −46.6 −60.1 −16.4 −35.7 −31.0 −19.3 −21.2 −76.6 −56.7

−6.0 −2.9 −2.0 −3.9 −6.4 −5.1 −4.8 −6.2 −4.1 −7.7 −11.6 −10.0 −7.6 −4.7 −25.6 −11.8 −14.7

−107.9 −95.1 −93.1 −73.7 −116.1 −105.0 −117.4 −122.7 −102.9 −136.2 −113.0 −120.1 −126.7 −121.6 −104.8 −192.7 −153.9

101.2 77.5 75.7 32.7 102.2 82.3 101.6 97.7 83.4 123.7 56.0 79.3 74.9 47.5 40.8 166.9 99.5

−53.6 −56.3 −56.4 −61.4 −61.6 −61.6 −64.1 −70.1 −70.2 −80.3 −84.5* −86.6 −90.4* −98.1 −100.8* −114.2* −125.8*

Asterisks indicate structures with two guest molecules per cavity.

guest interactions have been summed together. The interaction energies between the guest molecules and the 12 nearest host molecules have been summed together, which include all the host molecules belonging to the top and bottom OH ring of each cavity. For most of the clathrates, only the 6 enclosing molecules contribute significantly to the total host−guest energies, although the contributions from the remaining neighbors are added for consistency. The summed host− guest energies per cavity are listed in Table 1, and the total host−guest interaction energies per cavity are plotted against the guest molecule size in Figure 3b. The plot shows a general trend of larger guest molecule volumes per cavity resulting in more attractive host−guest interactions, although there is quite a spread in the plotted values. This spread is due to guest molecule shape influencing the energies. By inspecting Table 1, it can be seen that the smallest guest species have the most

summing the 14 nearest-neighbor interaction energies amounts to −258 kJ/mol. The host−host interaction energies are lowered slightly where the guest enclathration results in a cavity contraction, whereas the cavity expansions results in slightly higher host−host interactions energies. A plot of these changes in relation to the changes in cavity volume is seen in Figure 3a. From the smallest cavities in the DC/methanol structure to the largest cavity expansion in the DC/trimethylacetonitrile, the total host−host interactions only differ by 8 kJ/mol. In other words, the host can adjust to incorporate the guest molecules without causing large changes in the host−host interaction energies. The small energetically favorable changes in the host framework energies as we shrink the cavities are most likely compensated by changes in the H-bonding. To quantify and compare the host−guest interaction energies for different clathrates, the individual nearest-neighbor host− C

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Figure 4. Guest molecule positions in the DC clathrates containing substituted benzene derived guest molecules. From left to right: DC/benzene, DC/toluene, DC/bromobenzene.

Figure 5. Summed host−guest interaction energies as a function of rotating the hexane guest molecule (clockwise) around the threefold rotation axis.

attractive host−guest interaction energies per cavity, since they have fewer geometry restrictions compared to the larger bulkier molecules. In the DC/hexane clathrate, the hexane molecule has a shape which results in an especially snug fit inside the host cavity, leading to particularly attractive host−guest interaction energies, despite its size. Focusing only on the four halobenzenes together with the toluene clathrate, all having the same molecular shape, a nice linear tendency can be found where larger guest molecules result in more attractive total host−guest interaction energies. The trend is highlighted in Figure 3b. In the following, the intermolecular interactions in selected clathrates are discussed. For the clathrate structures with guest molecules located on the threefold rotation axis, the changes in host−guest interaction energies with respect to rotation are explored. Furthermore, implications of guest ordering are found in several of the clathrates. In the six DC clathrates involving benzene derived guest molecules, certain trends in host−guest interaction energies are evident. Structurally, the guest geometries are all similar to only a small tilt difference; see Figure 4. For all six structures, four of the enclosing host molecules result in host−guest interactions with relatively large exchange-repulsion energy terms, whereas the remaining two enclosing host molecules have interactions composed largely of dispersion forces. For the halobenzene

clathrates, all the 12 individual host−guest interactions get more attractive when going from DC/fluorobenzene to DC/ iodobenzene, resulting in the described trend in total host− guest interaction energy. Rotation of Guest Molecules. DC/n-Hexane. The DC/nhexane clathrate crystallizes with the hexane guest molecule in the all-anti conformer. Using MM2 calculations and 13C NMR, Imashiro showed that the all-anti configuration is by far the most dominant configuration in the DC/n-hexane clathrate, and also that the molecule fits well inside the cavity, leading to a large stabilization of steric energy. Furthermore, the 13C NMR results indicate the hexane rotates about the c-axis. The large stabilization energy is also seen here with the total host−guest interaction energy of −98 kJ/mol. The magnitude of the interaction is the largest energy per guest molecule of the studied clathrates. The good guest fit is also seen when comparing the near identical cavities in the DC/hexane and the DC/apohost structure; i.e., the host structure can enclathrate hexane molecules with negligible changes to the host geometry. The host−guest interaction energy is made up almost exclusively of dispersion forces between the guest molecule and the 6 enclosing host molecules. To explore the guest rotation, the change in host−guest interaction energies as a function of a rotation of the hexane molecule around the c-axis D

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Figure 6. Summed host−guest energies as a function of rotating the guest molecule around the threefold rotation axis, in the DC/CCl4 and DC/ CCl3CN clathrate structure.

CCl3CN molecules are rotated. The large repulsion term entails a maximum in the total host−guest interaction energy at a rotation of 60°. The difference between the energy maximum and minimum corresponds to 29 and 31 kJ/mol for the DC/ trichloroacetonitrile and the DC/carbon tetrachloride compounds, respectively. No changes are observed in the dispersion energy term, with respect to rotation, and we, therefore, conclude that the guest ordering is not due to particularly strong Cl···π interactions, but to minimize repulsion between the host structure and the guest molecules. It should further be noted that, in these analyses, the host framework is kept fixed during the rotation of the guest molecules. The difference between the minimum and maximum total host−guest interaction energies should, therefore, not be interpreted as a an upper bound for the energy barrier to rotation. The same arguments are valid in the following. DC/Trimethylacetonitrile. The trimethylacetonitrile guest molecules are found at two different positions in the cavity, though with only a single guest molecule in each cavity. Both guest geometries coincide with the threefold rotation axis. One of the guest geometries is similar to the DC/trichloroacetonitrile structure, while the other guest site is oriented with the cyano group centered in the middle of the OH ring (Figure 7). The summed total host−guest energies for the former guest geometry amount to −82 kJ/mol and the latter −91 kJ/mol. To explain why it is energetically favorable to have two different guest positions, one has to look at the possible guest−guest

has been calculated and is displayed in Figure 5. The hexane molecule has been rotated a total of 120° in steps of 15°. Only a rotation of 120° is necessary since the hexane molecule lies along the threefold axis. The changes in host−guest interaction energies as the molecule is rotated are all very small. The magnitude of the repulsion energy term increases slightly, but is compensated by changes in the electrostatic energy term. The result is a difference of only 2 kJ/mol for rotating the molecule. This supports the 13C NMR results and indicates that the guest molecule can rotate without the need for relaxation of the host framework. DC/Carbon Tetrachloride and DC/Trichloroacetonitrile. In the DC/trichloroacetonitrile (CCl3CN) clathrate structure, the guest molecule is located on the threefold rotation axis with the cyano group pointing through the waist of the cavity and the chlorine atoms oriented toward the benzene rings of the surrounding host molecules. The structure was first solved by Jacobs et al., who attributed the guest ordering to “strong Cl···π interactions”. In the DC/carbon tetrachloride structure, the CCl4 guest molecule is located at almost exactly the same position as the CCl3CN molecule structure only with an additional chloride atom instead of the cyano group. Changes to the host−guest energies as the two different guest molecules are rotated around the c-axis are plotted in Figure 6. As one would assume, the interaction energies are almost identical for the two structures. Compared with the DC/hexane structure, we see a large increase in the repulsion term when the CCl4 or E

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Figure 7. Summed host−guest energies as a function of rotating the two guest molecules in DC/trimethylacetonitrile around the threefold rotation axis. The upper diagram corresponds to energies for the depicted upper guest molecule and the lower diagram to the lower guest molecule.

Figure 8. Structure of DC/acetone, DC/nitromethane, and DC/acetic acid. Hirshfeld surfaces for the different guest molecules mapped with HF/ STO-3G molecular electrostatic potentials are depicted. The potential range from −0.05 au (red) to +0.05 au (blue).

structure to only have a single guest position. By artificially positioning a second trichloroacetonitrile guest molecule to adopt the DC/trimethylacetonitrile guest geometry, it is observed that the second trichloroacetonitrile geometry has less attractive host−guest interaction energies compared to the original guest position. The difference in total host−guest interaction energies for the two guest geometries amount to 15 kJ/mol, which is also larger than the possible total interaction energy between adjacent guest molecules of only −2 kJ/mol.

interactions. If the trimethylacetonitrile guest molecules are arranged in adjacent cavities like displayed in Figure 7, the total interaction energy between the guest molecules amounts to −15 kJ/mol, where any other guest configurations result in repulsive or negligible interaction energies. The guest−guest interaction is mainly stabilized by attractive electrostatic forces and indicates at least short-range ordering of the guest molecules. In the same way, it is possible to argue why it is energetically favorable for the related DC/trichloroacetonitrile F

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Figure 9. DC/ethanol and DC/tetrahydrofuran clathrate structures.

arrangement, depicted in Figure 8, should result in attractive dipole−dipole interactions. This configuration of nitromethane molecules is similar to the arrangement found in the nitromethane crystal, with only a slightly longer distance between the molecules inside the cavity. The guest−guest interaction energy between antiparallel nitromethane molecules amounts to −13 kJ/mol, stabilized by attractive electrostatic forces as expected. In comparison, the alternate guest configuration, rotating one of the molecules 120° about the c-axis, results in a total guest interaction energy of 2 kJ/mol, with the difference of 15 kJ/mol indicating that the guest molecules are only found in the antiparallel configuration. By plotting the molecular electrostatic potential for the nitromethane molecules, we see that the antiparallel configuration displays complementarity between the electronegative and the electropositive regions of the guest molecules. The total host− guest interaction energy amounts to −101 kJ/mol per cavity, mainly stabilized by dispersion forces. The DC/acetic acid structure is an interesting structure, in its own right, as the guest forms a centrosymmetric H-bonded dimer inside the H-bonded apohost network. The guest forms two slightly different dimers, although the resulting host−guest interaction energies are very similar. As already stated, the cavity volumes are hourglassshaped, with the least accessible space at the waist. Acetic acid forms a strong H-bond across this cavity waist, which results in an elongation of the cavity with the structure having by far the longest c-axis and shortest a-axis of the studied clathrates. The interaction energy between the molecules in the H-bonded dimer is highly dependent on the position of the hydroxyl H atom, which generally cannot be accurately determined from Xray diffraction. Therefore, the acetic acid guest geometry has been optimized with DFT using CRYSTAL14. In the calculation, unit cell parameters and host molecule coordinates have been fixed to experimental values, and the space group symmetry was reduced to P1̅. The new guest geometry, displayed in Figure 8, results in a total guest−guest interaction energy of −78 kJ/mol, an energy comparable to the total host− guest interaction energy of −85 kJ/mol per cavity. The host− guest interactions mainly consist of dispersion forces. The distinction between the experimental and the calculated guest geometry is only evident in a slight difference in the H positions. Implication of Guest Dynamics in DC/Ethanol and DC/ Tetrahydrofuran. The DC/ethanol clathrate structure contains two guest molecules per cavity. A recent molecular dynamics study supports the guest molecule positions while

These estimates of intermolecular interaction energies can readily explain why the DC/trimethylacetonitrile and not the DC/trichloroacetonitrile contains two distinct guest configurations. Returning to the DC/trimethylacetonitrile structure, changes to the host−guest energies as the two different guest molecules are rotated around the c-axis are plotted in Figure 7. For both guest geometries, the difference between the total host−guest energy maximum and minimum corresponds to 13 kJ/mol. Comparing the energies for the two geometries, we see that, for the trimethylacetonitrile molecule pointing the cyano group into the OH ring, the magnitude of the different energy terms is much larger. Since the larger attractive host−guest interactions are compensated by the repulsion term, the resulting difference in total host−guest interaction energies is only 8 kJ/mol at the equilibrium positions. Indications of guest molecule ordering are also found in DC/acetone, DC/nitromethane, and the DC/ acetic acid clathrates. Guest Molecule Ordering in DC/Acetone, DC/Nitromethane, and DC/Acetic Acid. The DC/acetone clathrate crystallizes with two acetone molecules in each cavity. From dielectric measurements by Davies and Williams, we know that the guest molecules dipole moments are aligned (Figure 8).32 While aligned, the two guest molecules can sit in three distinct configurations with respect to each other, and the three configurations can be visualized by imagining fixing one of the acetone molecules and rotating the other molecule about the threefold rotation axis. The total interaction energy between the acetone molecules inside the cavity is relatively independent of their mutual orientation (between −10 and −11 kJ/mol), while the interaction energy with the molecule from the adjacent cavity is −6 kJ/mol. These interactions are stabilized largely by electrostatic forces. The energies suggest that there is no preferred orientation about the threefold rotation with respect to guest−guest interactions, although they indicate the presence of minimum short-range order of the acetone molecules across cavities. Together, the acetone molecules have the largest host−guest interactions energy per cavity. The acetone molecule pointing the carbonyl group toward the OH ring has the strongest host−guest interaction energies with a total energy of −72 kJ/mol, while the remaining acetone molecule has a total energy of −54 kJ/mol. In the DC/ nitromethane clathrate, two crystallographically equivalent nitromethane molecules occupy each cavity. From the 3̅ symmetry element, two different guest molecule arrangements are possible. Lee et al. reasonably suggest that the antiparallel G

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also suggesting a reorientation energy of the guest molecules of only 5 kJ/mol. The energy barrier for guest transport through the waist of the cavity was found to be ∼20 kJ/mol, while transport across the OH ring was found to be as much as ∼70 kJ/mol.7 A single crystal 2H NMR study also indicated rapid guest dynamics.33 Owing to the rapid guest movement, and assumed “free rotation” of the hydroxyl group, host−guest interactions will not be elaborated further. From a static point of view, the two different guest molecules are positioned too close to one another to be present at the same time and, therefore, thought to be the result of guest dynamics.34 Similar indications of guest dynamics are found in the DC/tetrahydrofuran structure. Each cavity can accommodate two different guest molecule geometries, as is depicted in Figure 9. Regardless of how the two guest molecules are oriented with respect to each other, exploiting the threefold rotoinversion symmetry element in the middle of the cavity, the intermolecular interaction energy between the guest molecules is always large and repulsive. This could indicate, similarly to the case with the DC/ethanol structure, rapid guest movement inside the cavities with the guest dynamics relieving the static “repulsive” guest interactions. The crystallographic data suggest that the guest molecules have approximately C2 symmetry (twist form), but we know from theory that the interconversion from C2 to Cs symmetry (envelope) requires very little energy (∼0.3 kJ/mol in the gas phase). This very recently discovered DC clathrate, therefore, could be interesting to study using molecular dynamics.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Danish National Research Foundation (Center for Materials Crystallography, DNRF93), the Danish Research Council for Nature and Universe (Danscatt), and the Australian Research Council (DP130103304). Maja K. Thomsen is acknowledged for performing the DFT calculations.

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CONCLUSION Subtle intermolecular interaction energies have been quantified for 17 different clathrates of Dianin’s compound. Energy framework analysis shows that the host structure consists of strong intermolecular interactions in all three dimensions and not only in the planes containing hydrogen bonds. This is in contrast to a lot of the literature assigning the H-bonded network to be the single important structure determining entity. Owing to the strength of the host structure interactions, changing the guest molecule was found to have very little impact on the host energy framework. In general, a larger molecular guest volume results in increased host−guest interactions, although the molecular shape was also found to have considerable impact on the interactions. Indications of short-range ordering of guest molecules across cavities were found in the DC/trimethylacetonitrile and DC/acetone clathrate structures. Rotation of the hexane guest molecule was found to have very little net effect on the host−guest interaction energies, indicating that the guest molecule can rotate without relaxation of the host framework. For the structures containing multiple disordered guest molecules, the calculated interaction energies helped in finding the most plausible guest configurations. In that respect, implementing interaction energies in crystallographic software could greatly help assist in structure solving, enabling users to filter away molecular configurations leading to repulsive interactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00986. The presented intermolecular interaction energies and information on the theoretical calculations (PDF) H

DOI: 10.1021/acs.cgd.6b00986 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.6b00986 Cryst. Growth Des. XXXX, XXX, XXX−XXX