The effect of high pressure on the crystal structures of -, -and -glycine has been investigated. A new polymorph, δ-glycine, is obtained from  glycine. δ-Glycine is monoclinic, P2 1 /a, a = 11.156(4), b = 5.8644(11), c = 5.3417(17) Å,  = 125.83(4)° at 1.9 GPa. The transition, which occurs between 0 and 0.8 GPa, proceeds from a single crystal of -glycine to a single crystal of δ-glycine, resulting in an equal number of NH…O hydrogen bonds, but an increase in the number and strength of CH…O hydrogen bonds, which act to close-up of 'holes' which are formed within the layers of -glycine in the centers of R-type hydrogen bonded motifs. Trigonal -glycine begins to undergo a transition to another high-pressure phase, -glycine, at 1.9 GPa, but the transformation is destructive; it is essentially complete at 4.3 GPa. The structure is monoclinic Pn, a = 4.8887(10), b = 5.7541(11), c = 5.4419(11) Å,  = 116.682(10)° at 4.3 GPa. The structure consists of layers similar those observed in -glycine with inter-layer separations of 2.38 and 3.38 Å and CH…O interactions formed between the layers. Monoclinic -glycine is known to be stable to 23 GPa, and we have obtained a single crystal structure of this polymorph at 6.2 GPa. Super-short NH…O hydrogen bonds are not formed up to 6.2 GPa, and they only shorten significantly if they are formed parallel to CH…O hydrogen bonds which strengthen, or vectors across holes which close-up, under pressure.

The crystal structures of five hemiadducts of paracetamol with 1,4-dioxane, N-methylmorpholine, morpholine, N,N-dimethylpiperazine and piperazine and a related 1:1 adduct of paracetamol with 4,4'-bipyridine are described. All structures are characterized by the formation of chains of paracetamol molecules, which are linked via either OHtriplebondO=C interactions [C(9) chains in graph-set notation] or NHtriplebondO=C interactions [C(4) chains], depending on the presence or absence of substituent groups on the guest molecule. In all cases except for the morpholine and bipyridine adducts these chains are connected by hydrogen-bond interactions with the guest molecules, which reside on crystallographic inversion centres. In the bipyridine adduct this linkage also involves a pi-stacking interaction; in the morpholine adduct it is formed between the OH groups of two opposed paracetamol molecules. Most adducts (that with 4,4'-bipyridine is an exception) decompose on heating to give monoclinic paracetamol. This is the first systematic study of a series of co-crystals containing paracetamol.

The interaction of metal cyclams with carboxylate groups is thought to play an important role in their binding to the CXCR4 chemokine receptor and in their anti-HIV activity. Here we report the synthesis of acetate, phthalate, perchlorate and chloride complexes of Zn(II) cyclam (1,4,8,11-tetraazacyclotetradecane). The X-ray crystal structures of [Zn(cyclam)(phthalate)](n)(CH(3)OH)(2n) and [Zn(cyclam)(H(2)O)(2)](OAc)(2) contain octahedral Zn(II) centres. Phthalate acts as a bridging ligand in the former complex, binding through monodentate carboxylate groups, and giving rise to infinite chains in the lattice together with extensive hydrogen bonding between carboxylate donor oxygen atoms and amine and methanol acceptor atoms. The uncoordinated acetate groups and the aqua ligand in the acetate complex are also involved in a rich network of hydrogen bonds and this may account for the unusually long Zn[bond]O distance (2.27 A). In both crystalline complexes, the macrocycle adopts the trans-III (S,S,R,R) configuration. 1D (1)H NMR spectra of all four complexes have been fully assigned by a combination of 2D [(1)H, (1)H] COSY and TOCSY, and [(1)H, (13)C] and [(1 )H, (15)N] HSQC NMR data. In aqueous solution, the stable trans-III configuration found in the solid-state equilibrates slowly (hours at 298 K) with trans-I (R,S,R,S) and cis-V (R,R,R,R) configurations. The trans-III configuration is predominant in aqueous solution for both the chloride and perchlorate complexes, but for the acetate and phthalate complexes, the cis-V configuration dominates. Carboxylate groups appear to stabilize the cis-V configuration in solution through Zn(II) coordination and hydrogen bonding. Titration of the chloride Zn(II)-cyclam complex with acetate confirmed that carboxylates strongly induce formation of the cis-V configuration. This implies that carboxylates can exert a strong influence over configurational selectivity. Cyclam NH hydrogen bonding is prevalent both in the solid state and in solution, and is relevant to the anti-HIV activity of Zn(II) and other metal cyclam complexes and to their ability to recognize the CXCR4 transmembrane co-receptor.

The high-pressure crystal structure of phenol (C(6)H(5)OH), including the positions of the H atoms, has been determined using a combination of single-crystal X-ray diffraction techniques and ab initio density-functional calculations. It is found that at a pressure of 0.16 GPa, which is just sufficient to cause crystallization of a sample held at a temperature just above its ambient-pressure melting point (313 K), a previously unobserved monoclinic structure with P2(1) symmetry is formed. The structure is characterized by the formation of hydrogen-bonded molecular chains, and the molecules within each chain adopt a coplanar arrangement so that they are ordered in an alternating 1-1-1 sequence. Although the crystal structure of the ambient-pressure P112(1) phase is also characterized by the formation of molecular chains, the molecules adopt an approximate threefold arrangement. A series of ab initio calculations indicates that the rearrangement of the molecules from helical to coplanar results in an energy difference of only 0.162 eV molecule(-1) (15.6 kJ mole(-1)) at 0.16 GPa. The calculations also indicate that there is a slight increase in the dipole moment of the molecules, but, as the high-pressure phase has longer hydrogen-bond distances, it is found that, on average, the hydrogen bonds in the ambient-pressure phase are stronger.

Recrystallisation of paracetamol from a solution in methanol at a pressure of 0.62 GPa gives a new 1:1 solvate that has been characterised by single crystal X-ray diffraction.