For example, ppmv 0. At K and 1 bar, the simulated Xe uptake by CC3 for a bulk-phase Xe concentration of ppmv is 0. In addition to investigating Rn removal from air, we also simulated the capture of Rn from pure nitrogen at K to compare directly with experiments under those conditions Fig. We also simulated the capture of Rn from helium at K, under conditions where we have not yet performed experiments. Both cases are of practical importance in particle physics and in astroparticle physics experiments working at low energies.
At these lower temperatures, the simulations suggest that the CC3 material readily adsorbs appreciable amounts of Rn at rare gas concentrations as low as 0. For each of the simulated separations discussed above, we plotted the volumetric density ratio of the rare gas in the solid adsorbent versus its volumetric density in the gas phase as the function of its concentration in the gas mixture. This quantity is a measure of the ability of CC3 to selectively concentrate these rare gases in the solid state under various conditions. The bulk gas densities of Xe and Rn were calculated using the Peng-Robinson equation of state for given temperatures and pressures.
Excellent agreement between the calculations and experimental values was found, with the largest difference being smaller than 0. Figure 2d and Figure 6 report the computed zero- coverage isosteric heats of adsorption and Henry coefficients for 11 gases that include the main constituents of air. In addition to the rare gas separations, these data can also be used to estimate the potential of CC3 for separation of other gas mixtures.
In the limit of zero- adsorbate loading, the adsorption selectivity can be inferred from the ratio of the Henry coefficients. This approach also applies to gas mixtures with more than two components. We chose these gas pairs because of the potential technological and industrial importance of these separations e. From the GCMC simulations, we obtained selectivity values of Figure 6. Figure 2d shows an component equimolar simulation, which involved both Xe and n along with nine more common components of air.
In addition, we also carried out separate component simulations for both Xe and Rn, respectively, with these common air components. These equimolar competitive adsorption simulations were performed at K and a total pressure of 1 bar. The gas uptakes were obtained from multi-component GCMC simulations; the zero-coverage isosteric heats of adsorption and Henry coefficients were computed by the Widom test particle method.
As shown by comparison of Fig. Figure 6b illustrates the very higher selectivity of CC3 for Rn in air, in the absence of competing xenon: none of the other components in air competes effectively with Rn for the sorption sites in CC3, suggesting potential for Rn concentration and detection technologies. This may overcome problems with less ordered porous materials, such as activated carbon, where other guests e.
Figure 7 shows a comparison of experimental and simulated isosteric heats of adsorption for Xe and Kr on CC3. The simulations were performed at K. The experimental data are the same as those shown separately in Figures 14 and 19, along with associated and differential molar heats. The isosteric heat for Xe rises with coverage because of additional Xe Xe interactions that occur at higher gas loadings.
Gas Sorption Analysis. Micromeritics ASAP volumetric adsorption analyser. The temperature was controlled by a Haake C40P circulating chiller. Figure 8 shows adsorption isotherms solid symbols and desorption isotherms open symbols for Xe and Kr on CC3 up to 1 bar pressure over a range of temperatures between K and K. Figure 9 shows adsorption isotherms solid symbols and desorption isotherms open symbols for carbon dioxide on CC3, up to 1 bar pressure and over a range of temperatures between K and K.
Heats of adsorption, determined using the standard calculation routines in the Data-master offline data reduction software Micromeritics , gave initial, low-coverage Q st values for C0 2 of kJ mol "1 between 0. Heats of Adsorption. This is an ultra-high vacuum UHV system comprising a computer-controlled microbalance with both pressure and temperature regulation systems. The set pressure point was maintained by computer control throughout the course of the experiment.
The sample temperature was measured using a thermocouple located 5 mm from the sample. The parameters used for each adsorptive are as follows: Kr K A 6. Determination of the heat of adsorption at zero surface coverage, which is a fundamental measure of the adsorbate-adsorbent interaction, can be calculated by analysis of the isotherms using the virial equation, which has the form: At low surface coverage, A 2 and A 3 can be neglected. At low surface coverage, the virial equation reduces to Henry's Law. Adherence to Henry's Law is indicative of weak interactions between adsorbate molecules at low surface coverage.
The isosteric hea surface coverage was calculated using the following equation:. The isotherms at K, K, K and K are linear over the range The K H constants are tabulated in Table 6, which range from 1. There were insufficient low pressure points in the Henry's Law region for the K isotherm, resulting in the exclusion of the K data from the calculation of Q st at zero surface coverage. Resolution of the Henry's Law region could not be achieved for the K isotherm because addition of low pressure points would prevent isothermal conditions due to the low thermal conductivity of Xe.
An isosteric heat at zero surface coverage of Isosteric heats at increasing amounts of gas adsorbed were calculated from Van't Hoff isochores as show in Fig. A linear interpolation between isotherm points was used to calculate the pressure for fixed amounts adsorbed in the temperature range K. Figure 14 shows Q st and AS vs. The Q st increases from From 1. This increase is indicative of progressively stronger adsorbate-adsorbate interactions during pore filling. The enthalpy of adsorption at zero surface coverage obtained from plots of K H vs.
Table 6. Figure 13 shows van't Hoff isochore graphs for Xe adsorption on CC3 for temperatures K, K, K, K and K, as a function of the amount adsorbed n ranging from 0. Figure 17 shows a graph of A 0 vs. Powder X-ray diffraction data were used to determine the structure of CC3 under an excess pressure of xenon 10 bar, K, Fig.
A comparison can be made between this pre-structured porous organic clathrate and the known xenon hydrate Fig. Synthesis and. Phase behavior of xenon hydrate system. Fluid Phase Equilib. I n Xe- loaded CC3, the closest atoms from the CC3 molecule form analogous, polyhedral organic cages around the xenon guest Table 7. The shortest contact distances between the cage molecule and xenon guests are comparable with the xenon hydrate cages: the average Xe--phenyl ring centroid distance for the cage cavity is 4.
The volumetric density of enclathrated xenon in CC3 close to saturation is 0. This is lower than in xenon hydrate 0. The structure of CC3-? Kr-loaded at 9. This structure indicates that the krypton atoms are hosted in the cage cavity and cage window sites, with a decreased overall occupancy with respect to the xenon-loaded CC3 structure of 2.
Xel' 4. Xel occupies 4o cage cavity site. Xe2 resides on general window position. Powder X-Ray Diffraction. The gas adsorption of Xe in CC3 was studied in situ using powder diffraction data collected at beamline at the Diamond Light Source. These were then packed in 0. Data were collected using the Mythen-ll position sensitive detector PSD 41 at K to obtain an initial powder diffraction profile of guest-free CC3.
Where possible, a new section of the capillary was exposed. Xenon gas was dosed into the system in a number of pressure steps, up to a maximum of Figure 21 shows In situ powder diffraction data collected under increasing pressure of xenon up to a maximum of 10 bar K. Peak positions remain essentially constant, indicating no expansion of the CC3 structure upon loading with Xe. The sample was allowed to equilibrate for a minimum of 15 minutes after gas was dosed into the cell. Several datasets were then collected using the PSD to confirm no further observable changes in the diffraction before increasing the pressure of Xe in the system.
The pressure of xenon was reduced in steps before evacuating the gas cell under dynamic vacuum to confirm removal of xenon from the pore structure Fig. Figure 22 shows In situ powder diffraction data for loading and removal of xenon into a sample of CC3, demonstrating little change in the peak positions on loading.
The original diffraction pattern is reformed when the xenon is completely removed under dynamic vacuum; this is due to a change in symmetry in the crystals. The discrepancy is likely to be due to differences in how equilibration is determined for the diffraction and gas sorption experiments, with the gravimetric sorption measurement being far more sensitive.
The diffraction patterns also exhibit a degree of hysteresis between Xe loading and removal that is not observed in the Xe isotherm. The equilibration time for the in situ diffraction experiments was similar on gas loading and removal minutes , and the apparent hysteresis may be due in part to the sample not being fully equilibrated. The disordered guest is likely to scatter diffusely, consistent with the observed diffraction profile being similar to the evacuated structure.
If the sample was allowed to equilibrate for an extended period, it is possible that a higher level of guest loading and guest ordering would be observed, but instrument time did not allow us to test this. From the fully-loaded ordered structure at 10 bar, the pressure was reduced in steps to 1. The Xe isotherm indicates the structure should still be close to saturation; that is, the guest is likely to be ordered in the structure.
This is consistent with the observed diffraction pattern, which is similar to that observed for the sample loaded under 10 bar of Xe. Powder diffraction profiles were indexed for i the initial guest-free CC3 material at K under vacuum, and ii CC3 loaded at The occupancies of the Xe atoms were merged dynamically when separated by a distance less than 0. The best structure solution shows one xenon atom Xel on the 4a Wyckoff position, close to the centre of mass of the CC3 molecule, with approximately full occupancy.
The second xenon Xe2 is located on a general position. Reflection positions are also marked. A large crystallite of CC3 was present in the exposed sample during data collection resulting in a small number of very sharp peaks. Figure 24 shows three-dimensional difference Fourier maps indicating positions of the xenon guest atoms isosurfaces drawn at 2.
The best solution, including Xe atoms, was then used as the starting model for Rietveld refinement. The positions of all atoms on general positions and the 4o Xe z-coordinate were varied with geometric restraints on all bond lengths and angles and planarity restraints. All non-H isotropic displacement parameters were refined, constrained by atom type and environment parameters, restraints, reflections, profile points. Restraint weightings were reduced as the refinement became more stable. The occupancies of both xenon sites were initially refined, but as the special position occupancy consistently refined to approximately unity, it was subsequently fixed.
Anisotropic displacement parameters for the xenon atoms were refined in later stages of the refinement. Molecular connectivity was maintained with some deformation and no significant improvement in fit when all restraints were removed from the final refined structure. The inset shows a larger scale plot of the high angle fit. Figure 26 shows refined structure of xenon-loaded structure, CC Hydrogen atoms are omitted for clarity.
These include molecular structures, such as Dianin's compound, 47 calixarenes, 48,49 cucurbiturils, 50 and cryptophanes, 51 as well as metal-organic frameworks. Cg distance of 4. A search for intermolecular Xe The mean Xe H distance for the CC Figure 27 shows the distribution of intermolecular Xe H contact distance is 3. Xe separations in previously reported structures were also investigated. The Xe Xe distances in CC One structure that features direct Xe Xe contacts is a beta- hydroquinone clathrate 54 in which Xe is located in an apparent binding site, yet short Xe Xe contacts of 5.
A similar Xe separation of 5. The position adopted by the xenon guests results in a balance between maximizing Xe Xe contacts. The importance of the Xe Xe interactions may explain the preferential occupancy of the cage. The xenon guest atom occupying the cage cavity effectively forms four Xe Xe contacts with tetrahedral geometry, while the linear geometry of the window site only allows two xenon hetero-interactions.
Xenon binding in the cage cavity may therefore be favored due to a larger number of these stabilizing interactions. These Xe Xe interactions also explain the fact that the isosteric heat of adsorption for Xe in CC3 increases with increasing Xe loading Figs. In situ krypton loading PXRD experiments were carried out under a maximum partial pressure of Kr of 9. The PXRD pattern at 9. Direct space structure solution was carried out in R3 with the two cage molecular fragments rotating about and translating parallel to the threefold rotation axis and four independent Kr atoms with variable occupancies translating throughout the unit cell.
The positions of the Kr atoms from the best of five simulated annealing runs were again compared with the Fourier difference maps generated using the guest-free structure and 9. As in the xenon-loaded structure, the krypton atoms reside in the centre of the two CC3 molecule cavities. Two further Kr atoms are located in the two intermolecular sites, one of which lies on the threefold axis. Figure 29 shows three-dimensional difference Fourier maps indicating position of the krypton guest atoms isosurfaces drawn at 0.
The best solution was used as the starting model for Rietveld refinement. The positions of all atoms on general positions and the z-coordinates of the three Kr on the threefold rotation axis were varied with geometric restraints on all bond lengths and angles and. An isotropic displacement parameter was refined for each independent CC3 molecule and for individual krypton atoms parameters, restraints, reflections, profile points. The cyclohexyl vertex functionalities of the CC3 fragments were prone to deformation during refinement, suggesting some disorder of these groups.
Relatively heavily weighted restraints were required to maintain connectivity. The occupancies of all krypton sites were initially refined, but as the occupancy of Kr3 consistently refined to approximately unity, it was subsequently fixed. No significant improvement in fit was observed when all restraints were removed from the final refined structure. Figure 31 shows refined structure of krypton-loaded structure, CC3 2 The total krypton occupancy was approximately 2.
There is an apparent preference for a specific window cavity site in which the krypton atom Kr3 is displaced from the centre of the window cavity. The Fourier difference map shows a small area of electron density between this window site and the adjacent cage cavities. This may be an artefact in the Fourier map. However, during Rietveld refinement, Kr3 was consistently displaced along the threefold rotation axis, towards the more occupied adjacent krypton atom Kr2.
This produces a short Kr2-Kr3 distance of 4. Position and occupancy of krypton atoms in CC3 2 There are two principal factors that may drive the lowering of symmetry while leaving the overall molecular packing substantively unchanged. It also means the window site is formed by two independent cage windows. The second consequence of lowered crystal symmetry is increased degrees of freedom available for guest atoms in terms of position and occupancy and symmetry-allowed anisotropic displacements.
In the case of Xe-loaded CC3, the Xe atom occupying the window site is in a general position, unconstrained by symmetry. The Kr-loaded structure appears to have a complex distribution of guest atoms, in terms of both occupancies and positions, and the R3 symmetry allows four independent sites to be modeled to obtain a reasonable fit of the diffraction data. To eva luate CC3 for rea l sepa rations of noble gases at low concentrations in air, as would be encountered in the reprocessi ng of spent n uclea r fuels, we carried out brea kthrough measurements with a n adsorption colu mn packed with CC3 crysta ls.
As seen i n fig. As seen in fig. Other simulations, also at K, predict even higher selectivity for radon separation from air fig. Thus, when a mixture of xenon ppm and krypton 40 ppm balanced with simulated air was passed through this column, the xenon component was retained for more than 15 minutes, even at a flow rate of 40 cm 3 STP min "1 , which is twice as fast as that used in previous studies for MOFs [Liu, J.
Metal-organic frameworks for removal of Xe and Kr from nuclear fuel reprocessing plants. By contrast, krypton and the other components N 2 , 0 2 , and C0 2 broke through almost immediately.
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Langmuir 28, , ] : around 11 mmol kg "1 , in good agreement with simulations Fig. Selectivity and capacity are often seen as a trade-off. Here, CC3 shows significant improvements for both of these key parameters with respect to the leading MOF material. Figure 33 shows the simulated removal of low concentrations of Xe and n impurities from two binary gas mixtures. These conditions correspond to the lowest-temperature Rn adsorption experiments that we performed Table 9. Equilibrium is assumed in these calculations, and hence real, dynamic separations might operate less efficiently than these simulations suggest.
However, detailed kinetic measurements show that gas diffusion in CC3 is fast, at least for Kr and Xe, as illustrated by the breakthrough separation shown in Fig. The three-dimensional pore structure in CC3 Fig. These breakthrough measurements Fig. This is supported by detailed kinetic measurements for pure Kr and for pure Xe see below , which show that rare gas diffusion in CC3 is relatively fast [e.
Comparison of activation energies, E a , with the corresponding enthalpies of adsorption, Q st , for Kr and Xe show that E a is lower than Q st ; therefore, surface diffusion is the rate controlling step for adsorption of both gases on CC3. There is a near-perfect fit between the cavities in CC3 and the xenon guests. The pore architecture in CC3 has uniform pore channels that are at points too narrow, but at other points just large enough, to accommodate a single xenon atom. There are no larger cavities in CC3 that are a poor fit for xenon, nor any smaller cavities that might competitively adsorb the smaller molecules, such as nitrogen, in the gas mixture.
The organic cage is also an excellent adsorbent for radon gas. The adsorption capacity was evaluated by a dynamic adsorption technique where the radioisotope is mixed at high dilution in a carrier gas, nitrogen. Table 9. Hence, the present invention may be useful for radon removal from air, or from water, or for improving the sensitivity and humidity tolerance of environmental monitoring technologies that use physical adsorption to concentrate the radon gas for detection.
Currently, charcoal is used as an adsorbent for short-term radon testing in domestic homes, but its relatively poor selectivity against water vapour can lead to variation in test results with fluctuating humidity. The use of a single pore size that is tailored to adsorb radon offers a solution to this problem. Experiments with radioisotopes are restricted to specialized laboratories, but radioisotope adsorption is readily studied in silico.
For example, we also predict that CC3 could capture Rn from helium at radon concentrations as low as 0. Our success in calculating the Xe and Kr behaviour relative to experimental results Fig. The stretched exponential SE model is described by the following equation:. The exponent parameter 6 is material dependent and reflects the width of the distribution of relaxation times.
Krypton and xenon kinetics follow the stretched exponential model at K. Comparison of the stretched exponential rate constants for krypton and xenon at K, and position of the rate constant value on the isotherm, are shown in Figure The rate constants are similar over the pressure range and span the range 2. In the initial low pressure uptake region up to 2. This is attributed to the smaller diameter of krypton.
The chemical potential gradient increases rapidly due to the very steep uptake with pressure for xenon and the uptake reaches a plateau at low pressure ca. Therefore, xenon kinetics are faster than krypton for pressures greater than 2. Krypton and xenon adsorption on CC3 at K plateau at 3.
Based on a formula unit of Equilibration over the Temperature Range K. Figure 37 shows krypton mass relaxation profiles for the pressure increment mbar for K and mbar for , , , and K. Consequently, there is interference from the pressure set pressure set time is ca. Figure 38 shows the mass relaxation profiles for xenon adsorption on CC3 for the pressure increment mbar. Activation Energy. Activation energies for krypton and xenon adsorption on CC3 were calculated using stretched exponential rate constants in the range K and K, respectively. For krypton, the kinetic barrier increases from 6.
Activation energies for both gases are significantly lower than Q st 22 kJ mol "1 for krypton and kJ mol "1 for xenon see Figure 39 over the same uptake range at Q st. Therefore, the rate limiting step for adsorption of both species is surface diffusion and not diffusion through constrictions in porosity. Ideal Adsorbed Solution Theory. The model requires no equilibrium mixture data only pure component isotherms at equal temperature. The model is based on solving the following set of equations:. Integration of pure component isotherms, ' ": ' ' vs.
Figure 40 ii shows the amount of xenon and krypton adsorbed in mmol g "1 as a function of xenon mole fraction for a total pressure of 20 mbar and a temperature of K. It is evident that xenon is preferentially adsorbed at very low xenon mole fractions. At a xenon mole fraction equal to 0. The selectivity of xenon over krypton is shown in Figure 40 i and increases from Figure 34 shows a comparison of stretched exponential rate constants for krypton and xenon at K in relation to position on isotherm.
Figure 35 shows krypton mass relaxation profile and fitting of stretched exponential model for the pressure increment 1. Figure 36 shows xenon mass relaxation profile and fitting of stretched exponential model for the pressure increment 1. Figure 39 shows the change of isosteric enthalpy, entropy and activation energy with amount adsorbed for xenon and krypton on CC3.
To minimise pressure drop and to prevent potential contamination of the main gas pipelines, a pellet sample of CC3 was prepared following a two-step procedure. First, a powder sample was pressed into a disk under 9 M Pa for 3 min. The two-step procedure was repeated to make more pellets where necessary.
The cage pellets were packed into an adsorption bed for the breakthrough experiment. With reference to the ARBC system illustrated in a previous paper, 58 the gases were introduced through the bottom inlet of the adsorption bed. The adsorption bed was held between two layers of quartz wool and two sample holders, with frit gaskets installed at both the top and bottom ends of the adsorption bed to further prevent any potential powder contamination of the pipelines. For the separation of Xe ppm in air and Kr 40 ppm in air at K, a total flow rate of 40 cm 3 STP min "1 and a total pressure of 1 bar were used.
Prior to the breakthrough experiment, the pellet sample was degassed by heating at K in situ under a helium purge for 10 h. The activated sample weight was determined immediately after unloading the sample and the ideal gas law was used to calculate the moles of gas adsorbed by CC3. The flow rate of the gas mixture was 20 and 40 cm 3 STP min "1 , respectively, in the two experiments. That is, the flow rate was twice as high for the CC3 experiments compared to the experiments with the nickel MOF. Despite this faster flow rate, the Xe gas is retained more than twice as long by CC3.
These breakthrough data demonstrate that the kinetics of adsorption of Xe in CC3 are sufficiently fast to allow effective separations, even though the diameter of Xe is at the edge of the pore limiting envelope for CC3 Fig. The kinetics may be aided by the0 three-dimensional, diamondoid pore structure in CC3. This contrasts with many organic molecular crystals, such as tris o-phenylenedioxy phosphonitrile TPP , 59 which tend to have 1-dimensional, linear pore channels.
While a5 number of experimental and simulation papers have been published in the area of MOFs for xenon and krypton adsorption, a far smaller number of papers have reported the separation of Xe and Kr at realistic, low concentrations in air, as would be relevant in processing of spent nuclear fuel [e. By far the majority of the selectivities reported in Table S10 were calculated from pure gas adsorption isotherms, rather than by competitive adsorption studies involving mixed gases.
For nuclear re-processing applications, porous solids need to selectively remove Xe and Kr in parts per million ppm levels in the presence of other competing gases such as C0 2 , N 2 , 0 2 , Ar, and water. Table Radon Measurements. The radon adsorption capability of CC3 was evaluated using a dynamic adsorption technique.
The carrier gas, with a fixed concentration of radon, is then injected into an adsorbent trap. Adsorption equilibrium is attained when the breakthrough curve reaches a constant value Fig. Under such conditions, the ratio between the number of atoms of radon trapped and the radon concentration in the gas, both assumed to be proportional to their respective activity Bq m "3 , is given by the equilibrium constant, K:. The experimental setup is illustrated in Fig. The nitrogen carrier gas is radonised in the radon source by emanation from a metal plate coated with a thin radium layer and maintained at a fixed temperature 12 Q C.
The gas is introduced into a buffer tank in which the radon concentration C , the temperature, and the pressure are controlled. Thereafter, the carrier gas, with a well-defined amount of radon, is introduced into the column trap, which is located in a freezer. In order to define the equilibrium capture in the radon trap, the output gas is measured with a commercial RAD7 detector calibrated for a continuous nitrogen flow.
Once equilibrium is reached, the trap is disconnected from the gas circuit and the Rn activity of the CC3 sample is measured by gamma spectrometry in a germanium detector from the main gamma lines of radon progeny keV from Pb and keV from Bi. Figure 43 is a scheme showing apparatus used for Rn adsorption measurements. Enantioselective separation. Porous organic cages can also be used to separate molecules other than rare gases. Chiral molecules are important pharmaceutical feedstocks and there is a need for their effective separation.
CC3 can be prepared in homochiral form by synthesizing the cage from either the R,R or S,S enantiomer of 1,2-cyclohexanediamine. Porous organic cage nanocrystals by solution mixing. We therefore explored homochiral CC3 for chiral separations. Homochiral crystals of CC3 were found to adsorb a chiral alcohol, 1- phenylethanol, with selectivity for the enantiomer with opposite chirality to that of the cage Fig.
This results from more favourable intermolecular interactions between the 1-phenylethanol guest and the CC3 cage of the opposite chirality. The racemic cage crystal, rac-CC3, showed no enantioselectivity for this alcohol. However, rac-CC3 does show size selectivity for achiral guests, such as xenon and radon, much as found for the homochiral forms of CC3.
Hence, rac-CC3 is size selective, whereas homochiral CC3 is both size selective and enantioselective. Chiral Selectivity Measurements. Solutions of roc-l-phenylethanol in l-ferf-butyl-3,5-dimethylbenzene co. The solvent, l-ferf-butyl-3,5-dimethylbenzene, was chosen such that it is size- and shape excluded from the cage cavities. The quantity of each enantiomer of 1-phenylethanol adsorbed in the CC3 host was then be measured by chiral GC analysis. This experiment was repeated at various guest:host ratios to generate the plot shown in Fig.
Acetonitrile, methanol, and chloroform were purchased from Fisher Scientific and used as received. Finely ground CC3-? In contrast to the homochiral and chiral conglomerate cages, the average particle size of roc-CC3 was smaller, approximately 0. Gas Chromatography GC Analysis. Samples were analysed using either headspace or liquid injections. The following GC method was used, regardless of the injection technique. The samples were injected in the split mode 5: 1 for headspace and for liquid injections. Numeric integration of the resulting peaks was performed using the supplied Chromeleon 7.
Ena ntiomeric excess of the S ena ntiomer ee s of 1- phenyletha nol adsorbed in CC3 was measu red over a ra nge of guest:host ratios. Equa l a nd opposite ee s is observed for homochira l CC3-? The racemic cage crysta l, rac-CC3 middle li ne , is not ena ntioselective. Sim ulated ee s were obtained from adva nced. All simulations were carried out at ambient temperature and pressure. Simulated maximum guest loadings and ee 5 for 1-phenylethanol in the CC3 host correspond closely with experimental observations at a guest:host ratio of 2.
Five independent simulations were performed in each case.
A parallel mole-fraction grand-canonical Monte-Carlo simulation was used [Dubbeldam, D. On the inner workings of Monte Carlo codes. Single crystal X-ray diffraction shows that the 1-phenylethanol guests are disordered over several sites in the pores of CC3. The electron density is too diffuse to be modelled accurately, but molecular simulations suggest that the chiral selectivity stems from a specific interaction between the hydroxyl group in the alcohol and the nitrogen atom in the imine of CC3 Fig. This conformation is predicted to be common for S -l-phenylethanol in CC3-?
This leads to a predicted difference in host-guest binding energy for S -l- phenylethanol and? Figure Figure 45b is an overlay of one hundred snapshots of S -l- phenylethanol in the CC3-? The alcohol groups Figure 45b:. The predicted disordered orientation of S - 1-phenylethanol inside the CC3-? Figure 46 shows a molecular configuration showing the hydroxyl oxygen atom of an S -l- phenylethanol [0 0 H guest ] in close proximity of the hydrogen atom bonded to an imine carbon atom [H imine host ] and an aromatic hydrogen [H aro host ] of the CC3-?
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Figure 47 shows a molecular configuration showing the hydroxyl hydrogen atom of an S -l- phenylethanol molecule [H 0 H guest ] in close proximity of a nitrogen atom of the CC3-R cage [N host ] a. Figure 48 shows that S -l-Phenylethanol is a better fit with the cavity of the CC3-R cage in comparison with?
Data were collected. Le Bail fitting for a representative evacuated sample after 1-phenylethanol separation Figure 51a and after guest-washout Figure 51b confirmed the persistence of the cubic structure. Reflection positions are also shown. The patterns remain largely unchanged throughout the process, indicating no change in cage packing was induced by the loading and exchange of the guest.
NMR Spectroscopy. Solution 1 H N M R spectra were recorded in deuterated chloroform at A stock solution of bromomesitylene in l-ferf-butyl-3,5- dimethylbenzene ca. Seven solutions of roc-l-phenylethanol in the stock were made at different accurate concentrations: These were then used for both the host:guest exchange process and for GC calibration. Each concentration was chosen to produce the desired guest:host ratio in the later experimental steps; for example, the This means that, for all guest:host equivalents, the overall slurry volume does not greatly differ.
In addition, four lower- concentration solutions were made by serial dilution for exclusive use in GC calibration : 7. The bromomesitylene solution was also used as a blank injection, which allows us to correct for any impurities present in the reaction solvent. GC calibration samples and the analysis samples were prepared in triplicate. Approximately 30 mg of CC3 hosts were placed into pre-weighed vials and activated in a vacuum oven for 18 h at 90 Q C.
Once cooled, the filled vials were weighed in order to accurately calculate the moles of CC3 in each vial. Based on the desired guest:host ratio, the suitable guest solution was accurately added to each of the CC3 samples. For example, a sample containing The resulting slurries were vortexed at rpm for 18 h, and then filtered through a glass- fiber disc. The guest-depleted filtrates were analysed as static headspace injections by GC in order to obtain the quantity of guest adsorbed by the CC3.
Guest-adsorbed solids had 1 m L of acetonitrile added to them a large excess , followed by the same vortex mixing and filtration procedure, to extract the guest from the CC3 solid. The filtrates were analysed by liquid injection to obtain the enantiomeric excess of the 1-phenylethanol desorbed from the CC3. Again, the cage solids were analysed to ensure they were still chemically and phase pure after acetonitrile extraction.
Figures 52 and 53 show representative chromatograms produced from guest-depleted l-ferf-butyl-3,5-dimethylbenzene filtrates, rac Phenylethanol calibrations show the expected peak area ratio. Calibration chromatograms were collected. Quadratic fits were used in preference to linear fits due to the large 1- phenylethanol concentration range between stock solutions and post-adsorption solutions. Using this relationship, in combination with the known values for the concentration and volume of added stock, the adsorbed moles of?
This is plotted as a dotted line in Figure Figure 52 shows representative full GC-FI D static headspace chromatograms of guest- depleted filtrates collected after mixing a 1-phenylethanol solution with CC3-? The bromomesitylene stock solution is included at the top as a calibration standard. The chromatograms are collected for 1.
Figure 53 shows the min. The 1-phenylethanol in these plots is representative of what is left in the solution phase after adsorption into CC3. Rac-CC3 gives no enantiomeric excess. Due to the presence of an internal standard, the number of moles of R -l-.
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Adsorbed equivalents is plotted as a dotted line in Fig. Filtrate Analysis after Guest Extraction. Figures 54 and 55 are representative GC chromatograms of the filtrates produced after extracting guest-adsorbed CC3 with acetonitrile. Unlike l-ferf-butyl-3,5-dimethyl benzene, acetonitrile can enter the cage cavity and it readily displaces adsorbed 1-phenylethanol within CC3 when used in large excess.
Therefore, these plots show the adsorbed, post-exchange, contents of CC3. By contrast, Figures 52 and 53 show what is left in the solution phase. Due to the multi-step nature of the sample preparation and the volatility of acetonitrile, only the relative areas of peaks within the chromatogram were analysed; these values could then be used to calculate the enantiomeric excess of the 1-phenylethanol adsorbed by the CC3.
Bromomesitylene, visible in Figures 54 and 55, stems from residual solvent on the solids after the first filtration step, and hence it may not be used as an internal standard for this data. Removing residual l-ferf-butyl-3,5-dimethyl-benzene and bromomesitylene from the surface of guest-adsorbed CC3 required vacuum oven drying or solvent washing; however, both techniques were also observed to readily displace the guest from within CC3 Figure The ee of each sample was calculated from the relative peak areas in the chromatograms.
Figure 54 shows representative full GC-FID liquid injection of evicted-guest-containing filtrates collected after washing CC3 samples with 1 mL acetonitrile. Samples were gently dried in the filter funnel to prevent guest-desorption resulting in the presence of some residual l-tert-butyl-3,5-dimethylbenzene ca.
Peak annotations indicate the measured relative area. Acetonitrile washes the majority of guest adsorbed in CC3 into the solution phase. The 1-phenylethanol in these plots is representative of adsorbed guest after host:guest. As indicated, equal and opposite enantiomeric excess are observed for CC3-R.
The measured values are plotted as solid lines in Fig. Solid Lines: Measured enantiomeric excess of the S enantiomer s of 1- phenylethanol within CC3 hosts with respect to the guest:host equivalents present during the exchange mechanism. Dotted Lines: Calculated equivalents of 1-phenylethanol adsorbed into each CC3 host. A similar level of guest adsorption is observed for the homochiral phases CC3-? The racemic phase roc-CC3 shows a reproducibly higher level of 1-phenylethanol uptake.
Vacuum oven treatment of the guest-adsorbed CC3-? The exchange process does not chemically alter CC3-?. Integration of the CH peak of 1-phenylethanol 4. This compares well with the ratio of 1. A number of loading conditions were investigated in an attempt to load a single crystal of CC3 with the 1- phenylethanol guest, and hence to confirm its location in the pores.
Only one enantiomer of CC3, CC3-? Suitable quality single crystals of CC3-? Vapour diffusion of acetone over a hour period afforded phase pure single crystals of CC3-? In its native form space group F4i32 , CC3-? Initially, two data sets were recorded at room temperature without the use of a nitrogen gas flow to prevent N 2 condensing in the diamondoid pore network of CC3, or evaporation of the guest.
One collection was recorded on an evacuated CC3 sample that had been left to stand in air for 24 hours, and a second on the same batch of crystalline CC3 material that had instead been immersed in neat S -l-phenylethanol for 24 hours. After 24 hours of being immersed in neat S -l-phenylethanol, a single crystal of CC3 was selected and a full data set was recorded. This could be indicative of the S -l-phenylethanol guest adopting a number of possible conformers inside the CC3 cavity, as suggested by molecular simulations Fig. An approximate number of electrons contained within the pore network was calculated using a solvent masking routine in OLEX2.
Both values are in good agreement and indicate that there are approximately 1. In this second study, neat S -l-phenylethanol 0. The vacuum was gradually released and the crystals were kept immersed in S -l-phenylethanol. After 24 hours a full single crystal data set was recorded at K. A solvent masking routine performed in OLEX2 was used during the final stages of refinement. This routine removed a total of electrons from an interconnected A 3 void, or ca. From the same sample vial and after 2 weeks of being immersed in S -l-phenylethanol, only one cubic phase was evident.
A second crystal of this phase was selected and a full data set was recorded. Residual electron density was masked in OLEX2, which indicated: there were electrons contained within a A 3 void, or ca. As such, all three procedures suggest disordered guests, and a concentration of approximately 1. The experiments above show that solid CC3-R does not change its cubic crystal packing when immersed in S -l-phenylethanol, even for long periods. By contrast, when CC3 is crystallized from a homogeneous solution in CH 2 CI 2 containing the S -l-phenylethanol guest, then we found that S -l-phenylethanol directs CC3-R to pack in a new, non-native crystalline packing.
This previously unknown solvate, 3 CC3-? The asymmetric unit for this phase comprises one crystallographically distinct CC3 molecule and one half of a crystallographically distinct CC3 molecule centred on a twofold rotation axis. For one of the crystallographically distinct CC3 molecules, one S -l-phenylethanol molecule has its phenyl ring positioned within the cage cavity with the alcohol group located in the cage window site Figure There is clear evidence of a hydrogen bond between the alcohol group of S -l-phenylethanol and one of the imine nitrogen atoms CC3 O-H-N distance 3.
A comparable distance of 2. Figure 58 shows guest S -l-phenylethanol positioned in the CC3 cavity from the single crystal structure 3 CC3-? Carbon atoms of S -l-phenylethanol highlighted. Onto an evacuated crystalline sample of CC3 20 mg , kept under vacuum, neat? The vacuum was gradually released and the crystals were kept immersed in R -l- phenylethanol.
After 24 hours, the crystals were irradiated with polarised light; both a cubic and a non-cubic single crystal phase were evident. Data collections were recorded on each of these two phases. A solvent masking routine calculated in OLEX2 was applied during the final stages of refinement. This routine removed a total of electrons from a A 3 void, or 1. The R -l- phenylethanol guest molecule is located in the interstitial window site between two CC3 molecules.
The crystal packing of CC3 in this structure is reminiscent of the native window- to-window packing mode of CC3, with a cage centre distance calculated using the central point of the four aromatic rings as a reference of The presence of ordered? Well ordered. A solvent mask was therefore applied during the final stages of refinement. This removed a 46 electrons from three A 3 voids, per unit cell, or ca. Each void is positioned between two CC3 molecules, hence this guest would be shared. For analysis purposes, a solvent masking routine performed in OLEX2 was used to remove electrons from the whole network structure before assigning the well- ordered?
This masking routine removed a total of electrons from a A 3 void, or ca. Figure 59 shows crystal packing from the single crystal structure CC3-? Diamondoid network shown left ; perspective view  right. Only one R -l-phenylethanol position shown for clarity. Unless stated all non- H atoms were refined anisotropically and H atoms were fixed in geometrically estimated positions using the riding model. In the absence of heavy scatters Friedel pairs were merged. CCDC A displacement ellipsoid plot for this structure is shown in Figure A solvent masking routine performed in OLEX2, removed a total of The solvent mask was used during the final stages of refinement.
The approximate numbers of S -l-phenylethanol solvent molecules removed during solvent masking routine were included in the refined formula unit. For a displacement ellipsoid plot see Figure A solvent masking routine performed in OLEX2, removed a total of electrons from an. The numbers of S -l-phenylethanol solvent molecules removed during solvent masking routine were included in the refined formula unit. For the crystallographically distinct CC3-? One of these was particularly disordered, the atoms for which were refined as part of a group EADP. For a displacement ellipsoid plot of the asymmetric unit see Figure Figure 64 shows a displacement ellipsoid plot from the single crystal structure 3 CC3-?
S -l-Phenylethanol solvent omitted for clarity. A solvent masking routine performed in OLEX2, removed a total of electrons from an interconnected A 3 void. Crystal data for CC3-?.
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For each? A solvent masking routine performed in OLEX2, removed a total of 35 electrons from three A 3 voids. The solvent masked reflections file was used during the final stages of refinement. The approximate number? In adsorption studies, the amount of guest molecules adsorbed under a given temperature and pressure is of central interest. Monte Carlo simulations in the grand-canonical ensemble or GCMC simulations mimic this situation by attempting to insert and delete molecules into and from the system; the property that is computed is the average number of adsorbed molecules per unit of volume.
Toward saturation of. It is, however, important to ensure the statistical reliability and accuracy of a GCMC simulation by achieving appreciable successful insertions and deletions. In this respect, adsorption simulations of chiral molecules can be challenging because the molecules are normally liquids under the conditions of interest.
In liquid-phase simulations, the adsorbent pores are filled with adsorbate molecules and the acceptance ratios for insertion and deletion become vanishingly small. To increase the number of successfully inserted molecules, the configurational-bias Monte Carlo CBMC technique is usually called for. When it comes to simulating multi-component adsorption at high adsorbate densities, improving the insertion and deletion efficiencies constitutes only part of the solution to the slow ergodicity experienced by the simulation.
Under saturation conditions, it is almost impossible to insert an additional molecule, and it is also energetically extremely unfavourable to delete an adsorbed one. With mixture simulations, we are interested in the ratio of components in the adsorbed phase; identity-change Monte Carlo moves are therefore usually necessary to enhance the sampling. Simulating chiral separation can be even more challenging, as the left- and right-handed enantiomers are so similar that differentiation can be very difficult. Hence, significantly prolonged simulations may be needed for equilibration and production of results; even then, the conventional CBMC simulations can still be prone to statistical errors.
To overcome the aforementioned difficulties in the GCMC simulations of enantioselective adsorption, we adopted a replica-exchange method, namely, the parallel mole-fraction. In other words, all the replica systems, including the original system, have the same temperature and total external pressure but differ in mole fractions of the mixture components. These grand-canonical systems are simulated in parallel and neighbouring systems are swapped at a defined frequency.
During the system-swap moves, the molecular configurations of. By exchanging replicas, the presence of metastable states and entropic barriers can be better dealt with compared to conventional GCMC simulation. Computational Details. The molecular dimensions of the chiral? For S -l-phenylethanol, the maximum dimension of 5. We have previously observed, both in adsorption experiments and MD simulations, that para-xylene, 60 of similar dimensions to S -l-phenylethanol, is able to diffuse through CC3, albeit much more slowly than smaller gas molecules such as Xe.
We presume that the window can expand the additional 0. The atomistic representation of CC3-R was taken directly from the experimental CC3-R crystal structure loaded with S -l-phenylethanol see above. Prior to simulation, the S -l-phenylethanol molecules were deleted; the CC3-R atoms were kept fixed at their crystallographic positions during the simulation. Chiral inversion of all of the R cage molecules in this CC3-R crystal structure gave the CC3-S structure used in the corresponding simulations the unit-cell parameters and the symmetry, of course, remained unchanged.
To obtain a structure of rac-C i for simulation, two steps were taken, starting from the aforementioned experimental CC3-? In the first step, half of the R cages were chirally inverted such that R and S cages alternate in the crystal lattice, with each cage surrounded by four partner cages of the opposite chirality. The optimization was performed on both the unit-cell dimensions and the atomic positions. The structure was considered to be optimized when the maximum geometry change, root- mean-square geometry change, maximum force, and root-mean-square force converged to the values of 3.
The resulting roc-CC3 structure has a cubic F d -3 unit cell with a cell length of Compared to the CC3-? To describe the repulsive and dispersive interactions of the host-guest and guest-guest pairs, the standard Lennard-Jones LJ potential was used, together with a real- space cutoff of Each simulation box consisted of one unit cell of CC3 crystal. The Lorentz-Berthelot combining rules were used to calculate the host-guest U cross-parameters, while the guest-guest cross-parameters were determined via geometric means as adopted by OPLS-AA.
The partial atomic charges for the CC3 hosts and those for the? Electrostatic interactions were handled by the Ewald summation technique with the relative precision set to 10 "6. As mentioned above, the enantioselective adsorption of? The system of interest, where a racemic mixture of? The simulation chain was comprised of nine systems, each of which was a binary adsorption CBMC simulation performed at K and a total pressure of 1 bar but with the mole fraction of the R enantiomer varying from 0.
Translation and rotation MC moves were used, in combination with configurational-biased insertion, deletion and reinsertion, for thermalization. Your Web turnaround is permanently sent for value. Please create a refinementsShow to share and get the Community Archetypes schools. Giddens tests about his cognitive video and n't the site and look of making thinking test. Brick-and-mortar seconds are 1st, honest SEO, and we can create you with having your list to be read not by all l drugs. Digital aggression takes up a romanticism of fur, and you have a trust who requires the citations, from impact account and economies to minutes.
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The Atom-Atom Potential Method: Applications to Organic Molecular Solids
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