Crystal engineering in the gem-alkynol family: the key role of water in the structure of 2,3,5,6-tetrabromo-trans-1,4-diethynyl-cyclohexa-2,5-diene-1,4-diol dihydrate determined by X-ray and neutron diffraction at 150 K.

The structure of the title compound has been determined using low-temperature (150 K) single-crystal X-ray and neutron diffraction data. Crystals adopt the uncommon space group P4(2)/ncm and display a complex set of intermolecular interactions in which the water molecules play the crucial role: the water O-atom [O2(w)] accepts two hydrogen bonds and both water H atoms act as bifurcated donors. A set of O--H...O hydrogen bonds is formed around the 4(2) axis comprising (a) a cyclic tetrameric synthon involving four donor-H from two water molecules and two O(hydroxy) acceptors from two parent molecules, and (b) short discrete O(hydroxy)--H...O2(w) hydrogen bonds which link these tetramers along the c axis. Four Br...Br interactions [3.708 (1) A] form cyclic Br(4) tetramers around the 4 axis and are linked to the O--H...O system via O2(w)--H...Br bonds with H...Br = 2.995 (2) A. Finally, the O--H...O system is further linked to the parent molecules via C identical with C...H...O2(w) bonds of 2.354 (3) A. The supramolecular structure of the title hydrate is compared with that of the non-hydrated parent molecule, which also forms cyclic O--H...O bonded tetrameric synthons, and with its (non-hydrated) tetrachloro analogue, which forms cyclic tetrameric Cl(4) synthons [Madhavi, Desiraju et al. (2000b). Acta Cryst. B56, 1063--1070].

Crystal engineering in the gem-alkynol family: interplay between strong and weak interactions in structures of 2,3,5,6-tetrahalo

Structures of the title compounds are all mediated by strong cooperative arrangements of O-H.O hydrogen bonds, supported by a variety of weaker interactions which affect the type of O-H.O synthon that is formed. The tetrafluoro compound contains hexameric O-H.O synthons in a supramolecular chair conformation, together with C identical withC-H.F interactions. However, the tetrachloro and tetrabromo compounds both form tetrameric O-H.O synthons. This dominant pattern is supported by halogen.halogen interactions having one C-Cl[Br].Cl[Br] angle close to 180 degrees and the other close to 90 degrees, and by C identical withC-H.Cl[Br] interactions.

Crystal engineering in the gem-alkynol family; synthon repetitivity and topological similarity in diphenylethynylmethanols: structures that lack O-H.O hydrogen bonds

The structures of four para-substituted derivatives of diphenylethynylmethanol have been determined [ditolylethynylmethanol, di(4-chlorophenyl)ethynylmethanol, di(4-bromophenyl)ethynylmethanol and bis(4,4'-biphenylyl)ethynylmethanol]. The dimethyl, dichloro, dibromo and diphenyl compounds have been analysed using X-ray diffraction at 150 K, and the dichloro compound has also been studied using neutron diffraction at 150 K. In common with the parent diphenylethynylmethanol [Garcia, Ramos, Rodriguez & Fronczek (1995). Acta Cryst. C51, 2674-2676], all four derivatives fail to form the expected strong O-H.O hydrogen bonds due to steric hindrance. Instead, the supramolecular structural organization in this family of gem-alkynols is mediated by a variety of weaker interactions. The two most acidic protons, O-H and C[triple-bond]C-H, participate in weak hydrogen bonds to pi-acceptors, forming synthons that stabilize all five structures. These primary interactions are reinforced by a variety of other weak hydrogen bonds involving C-H donors and the hydroxy-O as an acceptor, and by halogen.halogen interactions in the dichloro and dibromo compounds.

4-Ethynyl-4-hydroxycyclohexan-1-one and 4-ethynyl-4-hydroxy-2,3,5, 6-tetramethylcyclohexa-2,5-dien-1-one.

The title compounds, C(8)H(10)O(2), (I), and C(12)H(14)O(2), (II), occurred as by-products in the controlled synthesis of a series of bis(gem-alkynols), prepared as part of an extensive study of synthon formation in simple gem-alkynol derivatives. The two 4-(gem-alkynol)-1-ones crystallize in space group P2(1)/c, (I) with Z' = 1 and (II) with Z' = 2. Both structures are dominated by O-H. O=C hydrogen bonds, which form simple chains in the cyclohexane derivative, (I), and centrosymmetric dimers, of both symmetry-independent molecules, in the cyclohexa-2,5-diene, (II). These strong synthons are further stabilized by C[triple-bond]C-H. O=C, C(methylene)-H.O(H) and C(methyl)-H.O(H) interactions. The direct intermolecular interactions between donors and acceptors in the gem-alkynol group, which characterize the bis(gem-alkynol) analogues of (I) and (II), are not present in the ketone derivatives studied here.

Racemic 1,2-diphenylbut-3-yn-2-ol.

Molecules of the title compound, C(16)H(14)O, are chiral and crystallize in space group P-4 with Z' = 2, and with one R and one S molecule in the asymmetric unit. The conformations of the phenyl rings in the two independent molecules differ slightly. Supramolecular organization in the crystal is via tetrameric O-H. H(O) hydrogen-bonded synthons formed separately by each conformer. These tetrameric synthons stack along the c axis via C[triple-bond]C-H.O(H) hydrogen bonds. The only link between the conformer stacks is provided by weaker C(methylene)-H and C(phenyl)-H interactions with pi(arene) density.

Intramolecular hydrogen bonds: common motifs, probabilities of formation and implications for supramolecular organization.

A systematic survey of the Cambridge Structural Database (CSD) has identified all intramolecular hydrogen-bonded ring motifs comprising less than 20 atoms with N and O donors and acceptors. The probabilities of formation P(m) of the 50 most common motifs, which chiefly comprise five- and six-membered rings, have been derived by considering the number of intramolecular motifs which could possibly form. The most probable motifs (P(m) > 85%) are planar conjugated six-membered rings with a propensity for resonance-assisted hydrogen bonding and these form the shortest contacts, whilst saturated six-membered rings typically have P(m) < 10%. The influence of intramolecular-motif formation on intermolecular hydrogen-bond formation has been assessed for a planar conjugated model substructure, showing that a donor-H is considerably less likely to form an intermolecular bond if it forms an intramolecular one. On the other hand, the involvement of a carbonyl acceptor in an intramolecular bond does not significantly affect its ability to act as an intermolecular acceptor and thus carbonyl acceptors display a substantially higher inclination for bifurcation if one hydrogen bond is intramolecular.