The nature of intermolecular interactions between carbon-bonded halogens (C-X, X ) F, Cl, Br, or I) and electronegative atoms (El ) N, O and S) has been analysed, focusing on the role of specific attractive forces and the anisotropic repulsive wall around halogen atoms. Searches of the Cambridge Structural Database show that electronegative atoms in various hybridization states clearly prefer to form contacts to Cl, Br, and I (but not F) in the direction of the extended C-X bond axis, at interatomic distances less than the sum of the van der Waals radii. Ab initio intermolecular perturbation theory calculations show that the attractive nature of the X‚‚‚El interaction is mainly due to electrostatic effects, but polarization, charge-transfer, and dispersion contributions all play an important role. The magnitude of the interaction for the chloro-cyanoacetylene dimer is about 10 kJ/mol, demonstrating the potential importance of these kinds of nonbonded interactions. The directionality of the interaction is explained by the anisotropic electron distribution around the halogen atom, causing a decreased repulsive wall and increased electrostatic attraction for electronegative atoms in the observed preferred position. In contrast, carbon-bonded hydrogens show no directionality in their contacts to the halogen atoms, because the angular dependence of the electrostatic energy is reversed and acts to counter rather then to reinforce the effect of the anisotropic repulsive wall.

The geometries and attractive energies of carbonylcarbonyl interactions have been investigated using crystallographic data and ab initio molecular-orbital calculations. Analysis of crystallographic data for 9049 carbon-substituted >C~O groups shows that 1328 (15%) form contacts with other >C~O groups, in which d(C...O) < 3.6,~. Three common interaction motifs are observed in crystal structures: (a) a slightly sheared antiparallel motif (650 instances) involving a pair of short C...O interactions, together with (b) a perpendicular motif (116 instances) and (c) a highly sheared parallel motif (130 instances), which both involve a single short C...O interaction. Together, these motifs account for 945 (71%) of the observed interactions. Ab-initio-based molecular-orbital calculations (6-31G** basis sets), using intermolecular perturbation theory (IMPT) applied to a bis-propanone dimer model, yield an attractive interaction energy of -22.

A collaborative workshop was held in May 1999 at the Cambridge Crystallographic Data Centre to test how well currently available methods of crystal structure prediction perform when given only the atomic connectivity for an organic compound. A blind test was conducted on a selection of four compounds and a wide range of methodologies representing, the principal computer programs currently available were used. There were 11 participants who were allowed to propose at most three structures for each compound. No program gave consistently reliable results. However, seven proposed structures were close to an experimental one and were classified as "correct". One compound occurred in two polymorphs, but only one form was predicted correctly among the calculated structures. The basic problem with lattice energy based methods of crystal structure prediction is that many structures are found within a few kJ mol(-1) of the global minimum. The fine detail of the force-field methodology and parametrization influences the energy ranking within each method. Nevertheless, present methods may be useful in providing a set of structures as possible polymorphs for a given molecular structure.

The first collaborative workshop on crystal structure prediction (CSP1999) has been followed by a second workshop (CSP2001) held at the Cambridge Crystallographic Data Centre. The 17 participants were given only the chemical diagram for three organic molecules and were invited to test their prediction programs within a range of named common space groups. Several different computer programs were used, using the methodology wherein a molecular model is used to construct theoretical crystal structures in given space groups, and prediction is usually based on the minimum calculated lattice energy. A maximum of three predictions were allowed per molecule. The results showed two correct predictions for the first molecule, four for the second molecule and none for the third molecule (which had torsional flexibility). The correct structure was often present in the sorted low-energy lists from the participants but at a ranking position greater than three. The use of non-indexed powder diffraction data was investigated in a secondary test, after completion of the ab initio submissions. Although no one method can be said to be completely reliable, this workshop gives an objective measure of the success and failure of current methodologies.

Crystallographic and theoretical (ab initio) data on intermolecular nonbonded interactions have been gathered together in a computerised library ('IsoStar'). The library contains information about the nonbonded contacts formed by some 250 chemical groupings. The data can be displayed visually and used to aid protein-ligand docking or the identification of bioisosteric replacements. Data from the library show that there is great variability in the geometrical preferences of different types of hydrogen bonds, although in general there is a tendency for H-bonds to form along lone-pair directions. The H-bond acceptor abilities of oxygen and sulphur atoms are highly dependent on intramolecular environments. The nonbonded contacts formed by many hydrophobic groups show surprisingly strong directional preferences. Many unusual nonbonded interactions are to be found in the library and are of potential value for designing novel biologically active molecules.