To conclude, we have shown that it has been possible to parameterise a FF using a combination of X-ray crystal structure data with high level DFT calculations, and that this FF accurately describes the porous organic cages used in the parameterisation (CC1, CC2, CC3), as well as being transferable to other systems such as CC4 and a co-crystal of two cages, (CC1α, CC3). The main adaption of the FF that was required involved parameters for the imine group conjugated to an aromatic system. It should be expected that the parameters
obtained will improve the description of similar systems when using PCFF. The scaling of the dispersion parameters to obtain reasonable crystal structures appears to be a more general requirement to use PCFF for inter-molecular van der Waals forces. In addition, seven different gases have been inserted into CSFF, so that MD simulations of gases incorporated within cage systems can be simulated. These should aid with the understanding of how and why these porous organic cage systems were important as well as providing insight into their separation capabilities, as discussed in Chapters 4 and 5.
The generation of CSFF32 has made it feasible to simulate how the diffusion of
small gases was altered dependent on their particle size. Using MD simulations, we have unlocked phenomena such as gas selectivity, rare-event hopping, and displacement of gases to regions previously considered inaccessible; all of which help to rationalise existing experimental observations. In addition, a new visualisation technique has been developed to show both the pore topology and dynamic connectivity of the system whilst a MD simulation progresses.33 This
unlocks new ideas and suggests how the pore channels of various systems previously thought too small for gas adsorption are actually ideal candidates for separation.
In the future, it will therefore be possible to use of CSFF to help rationalise the uptake of gases, and also larger guests, such as halogens and solvents, within a whole host of different cage systems. The view here was to use MD analysis for in silico screening of cage materials, in particular molecular separations. If reliable, this could be faster than the associated sorption experiments.
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