Following the interest generated by two previous blind tests of crystal structure prediction (CSP1999 and CSP2001), a third such collaborative project (CSP2004) was hosted by the Cambridge Crystallographic Data Centre. A range of methodologies used in searching for and ranking the likelihood of predicted crystal structures is represented amongst the 18 participating research groups, although most are based on the global minimization of the lattice energy. Initially the participants were given molecular diagrams of three molecules and asked to submit three predictions for the most likely crystal structure of each. Unlike earlier blind tests, no restriction was placed on the possible space group of the target crystal structures. Furthermore, Z' = 2 structures were allowed. Part-way through the test, a partial structure report was discovered for one of the molecules, which could no longer be considered a blind test. Hence, a second molecule from the same category (small, rigid with common atom types) was offered to the participants as a replacement. Success rates within the three submitted predictions were lower than in the previous tests - there was only one successful prediction for any of the three ;blind' molecules. For the ;simplest' rigid molecule, this lack of success is partly due to the observed structure crystallizing with two molecules in the asymmetric unit. As in the 2001 blind test, there was no success in predicting the structure of the flexible molecule. The results highlight the necessity for better energy models, capable of simultaneously describing conformational and packing energies with high accuracy. There is also a need for improvements in search procedures for crystals with more than one independent molecule, as well as for molecules with conformational flexibility. These are necessary requirements for the prediction of possible thermodynamically favoured polymorphs. Which of these are actually realised is also influenced by as yet insufficiently understood processes of nucleation and crystal growth.

Based on computational studies, we propose new metal-organic framework materials, in which the bridging ligands have been functionalized by different substituents, with the aim of improving the CO 2 adsorption capacity of the material. The materials are based on the large-pore form of MIL-53(Al 3þ ), with the following functional groups: OH-, COOH-, NH 2 -, and CH 3 -. For each form, adsorption heats and isotherms were simulated using the Grand Canonical Monte Carlo method which were found to be consistent with DFT calculations. The study illustrates the enormous impact of the functional groups in enhancing CO 2 capture in the pressure range 0.01-0.5 bar and at room temperature. It also provides important insights into the structural factors which play a key role in the CO 2 adsorption process in the functionalized MOFs. We propose the material (OH) 2 -MIL-53(Al 3þ ) as an optimal candidate for improved CO 2 capture at low pressures.

ZIF-8, a prototypical zeolitic porous coordination polymer, prepared via the self-assembly of tetrahedral atoms (e.g. Zn and Co) and organic imidazolate linkers, presents large cavities which are interconnected by narrow windows that allow, in principle, molecular sieving. However, ZIF-8 shows flexibility due to the swing of the imidazolate linkers, which results in the adsorption of molecules which are too large to fit through the narrow window. In this work, we assess the impact of this flexibility, previously only observed for nitrogen, and the level of agreement between the experimental and simulated isotherms of different energy-related gases on ZIF-8 (CO(2), CH(4) and alkanes). We combine experimental gas adsorption with GCMC simulations, using generic and adjusted force fields and DFT calculations with the Grimme dispersion correction. By solely adapting the UFF force field to reduce the Lennard-Jones parameter ε, we achieve excellent agreement between the simulated and experimental results not only for ZIF-8 but also for ZIF-20, where the transferability of the adapted force field is successfully tested. Regarding ZIF-8, we show that two different structural configurations are needed to properly describe the adsorption performance of this material, demonstrating that ZIF-8 is undergoing a structural change during gas adsorption. DFT calculations with the Grimme dispersion correction are consistent with the GCMC and experimental observations, illustrating the thermodynamics of the CH(4) adsorption sites and confirming the existence of a new adsorption site with a high binding energy within the 4-ring window of ZIF-8.

Intermolecular interactions between the CO(2) molecule and a range of functionalized aromatic molecules have been investigated using density functional theory. The work is directed toward the design of linker molecules which could form part of new metal-organic framework materials with enhanced affinity for CO(2) adsorption at low pressure. Here, the focus was on the effect of introducing polar side groups, and therefore functionalized benzenes containing -NO(2), -NH(2), -OH, -SO(3)H, and -COOH substituents were considered. The strongest types of intermolecular interactions were found to be: (i) between lone pair donating atoms (N,O) of the side groups and the C of CO(2) (enhancement in binding energy of up to 8 kJ mol(-1) compared to benzene); and (ii) hydrogen bond interactions between acidic protons (of COOH and SO(3)H groups) and CO(2) oxygen (enhancement of 3-4 kJ mol(-1)). Both of these types of interaction have the effect of polarizing the CO(2) molecule. Weaker types of binding include hydrogen-bond-like interactions with aromatic H and pi-quadrupole interactions. The strongest binding is found when more than one interaction occurs simultaneously, as in C(6)H(5)SO(3)H and C(6)H(5)COOH, where simultaneous lone pair donation and H-bonding result in binding energy enhancements of 10 and 11 kJ mol(-1), respectively.

Intermolecular interactions between the CO(2) molecule and a range of functionalized aromatic molecules have been investigated using density functional theory. The work is directed toward the design of linker molecules which could form part of new metal-organic framework materials with enhanced affinity for CO(2) adsorption at low pressure. Two classes of substituted benzene molecules were considered: (i) with halogen substituents (tetrafluoro-, chloro-, bromo-, and dibromobenzene) and (ii) with methyl substituents (mono-, di-, and tetramethylbenzene). In the benzene-CO(2) complex, the main interaction is between the delocalized pi aromatic system and the molecular quadrupole of CO(2). Halogen substituents have an electron-withdrawing effect on the ring which destabilizes the pi-quadrupole interaction. Weak "halogen-bond" and hydrogen bondlike interactions partially compensate for this, but not to the extent that any significant enhancement of the intermolecular binding energy is observed. Methyl groups, on the other hand, have a positive inductive effect which strengthens the CO(2)-aromatic interaction by up to 3 kJ mol(-1) in the case of tetramethylbenzene. Weak hydrogen bondlike interactions with methyl H also contribute to the stability of the complexes.