Mark D. Hollingsworth

Crystal Engineering: from Structure to Function

Modern crystal engineering has emerged as a rich discipline whose success requires an iterative process of synthesis, crystallography, crystal structure analysis, and computational methods. By focusing on the molecular recognition events during nucleation and growth, chemists have uncovered new ways of controlling the internal structure and symmetry of crystals and of producing materials with useful chemical and physical properties.

An X-Ray Diffraction Study

Abstract X-ray oscillation photographs reveal both diffuse and discrete scattering for alkanone guest molecules within inclusion compounds of 5-undecanonehrea grown from methanol. Guest molecules that give rise to diffuse scattering are ordered only along the channel axis, whereas those guest substructures that give rise to discrete scattering are ordered in three dimensions. Precession photographs and diffractometry show that within domains containing three dimensionally ordered guests, the offset (denoted δg) between the heights of guest molecules in adjacent channels is exactly zero. These guests therefore share overall hexagonal symmetry with the host, as do a large number of n-alkanones of varying chain length and symmetry properties. Although side by side ordering optimizes dipole-dipole interactions between alkanones in adjacent channels, the large inter channel distance (8.22A) and the lack of symmetry in 5-undecanone suggest that other factors could influence guest molecule ordering. A careful di...

Mechanical Stress and Reactivity in Organic Solids

Organic single crystals provide an ideal model for studying the factors that influence chemical processes in structured media. Reaction trajectories are well defined and reveal the influence of spontaneous mechanical stresses equivalent to tens of thousands of atmospheres. Analysis of molecular and crystal structures helps to explain both local mechanical properties, which influence reactions, and bulk properties such as melting point, compressibility, and surface energy.

Structural properties of α,ω-dibromoalkane/urea inclusion compounds: a new type of interchannel guest molecule ordering

Structural properties of urea inclusion compounds containing α,ω-dibromoalkanes [Br(CH2)nBr; n= 7–10] as the guest component are reported. In these inclusion compounds, the host (urea) structure contains linear, parallel, non-intersecting channels (tunnels), within which the guest molecules are located. The structural properties of the guest molecules have been determined at room temperature using single-crystal X-ray diffraction techniques (primarily photographic methods), and represent the main focus of this paper.Rationalization of the complete X-ray diffraction pattern from an α,ω-dibromoalkane/urea inclusion compound requires more than one three-dimensionally periodic reciprocal lattice; the interpretation of the complete diffraction pattern is discussed in detail for 1,10-dibromodecane/urea. For each α,ω-dibromoalkane/urea single crystal, the guest molecules are closely packed along the channels with a periodic repeat distance (|cg|) that is incommensurate with the periodic repeat distance (|ch|) of the host along the channel axis. In some regions of the crystal the guest molecules are ordered only along the channel axis whereas in other regions the guest molecules are ordered in three dimensions. In this three-dimensionally ordered structure, the offset (denoted Δg) between the ‘heights’ of guest molecules in adjacent channels is given by the exact relationship Δg=|cg|/3. Thus, Δg depends on the value of |cg|, and hence depends on the ‘length’ of the guest molecule. This represents a new mode of interchannel ordering of guest molecules in urea inclusion compounds. Within each single crystal, two domains of the guest structure can be distinguished by X-ray diffraction. Each domain has rhombohedral symmetry; the lattices that define the periodicities of the two domains differ in orientation [related by (2n+ 1)π/3 rotation about the channel axis; n= integer], but are otherwise identical. These two domains correspond to the observe and reverse settings of the rhombohedral average guest structure. Finally, the results are discussed in relation to the contrasting structural properties of urea inclusion compounds containing other types of guest molecules.

Superstructure control in the crystal growth and ordering of urea inclusion compounds

A template-directed mechanism of crystal growth is demonstrated for urea inclusion compounds (UICs). For UICs containing n-alkanone or α+1,ω−1-alkanedione guests, x-ray diffraction revealed superstructure relations between host and guest repeats along the channel axis for guests containing 8 to 14 carbons. For a favorable structural match between host and guest, UICs typically grow as flat hexagonal plates, and atomic force microscopy of {001} surfaces revealed molecularly smooth terraces differing in height by multiples of the guest repeat. If the match is poor, protrusion of guests from the {001} surfaces nucleates growth along the channel axis to form hexagonal needles.

Hidden Degrees of Freedom in Aperiodic Materials

Numerous crystalline materials, including those of bioorganic origin, comprise incommensurate sublattices whose mutual arrangement is described in a superspace framework exceeding three dimensions. We report direct observation by neutron diffraction of superspace symmetry breaking in a solid-solid phase transition of an incommensurate host-guest system: the channel inclusion compound of nonadecane/urea. Strikingly, this phase transition generates a unit cell doubling that concerns only the modulation of one substructure by the other—an internal variable available only in superspace. This unanticipated pathway for degrees of freedom to rearrange leads to a second phase transition, which again is controlled by the higher dimensionality of superspace. These results reveal natures capacity to explore the increased number of phases allowed in aperiodic crystals.

Structural Properties of the Guest Species in Diacyl Peroxide/Urea Inclusion Compounds: An X-Ray Diffraction Investigation

In this paper we report single crystal X-ray diffraction studies of urea inclusion compounds containing diacyl peroxides (dioctanoyl peroxide (OP), diundecanoyl peroxide (UP), lauroyl peroxide (LP)) as the guest component. In these inclusion compounds, the host (urea) molecules crystallize in a hexagonal structure that contains linear, parallel, non-intersecting channels (tunnels). The guest (diacyl peroxide) molecules are closely packed inside these channels with a periodic repeat distance that is incommensurate with the period of the host structure along the channel axis. Furthermore, there is pronounced inhomogeneity within the guest structure: within each single crystal, there are regions in which the guest molecules are three-dimensionally ordered, and other regions in which they are only one-dimensionally ordered (along the channel axis). Although it has not proven possible to ‘determine’ the guest structures in the conventional sense, substantial information concerning their average periodicities and their orientational relationships with respect to the host has been deduced from single crystal X-ray diffraction photographs recorded at room temperature. For OP/urea, UP/urea and LP/urea, the guest structure in the three-dimensionally ordered regions is monoclinic, and six types of domain of this monoclinic structure can be identified within each single crystal. The relative packing of diacyl peroxide molecules is the same in each domain, and the different domains are related by 60° rotation about the channel axis. For each of these inclusion compounds, the offset between the ‘heights’ of the guest molecules in adjacent channels is the same (ca. 4.6 Å (4.6 x 10-10 m)) within experimental error, suggesting that the relative interchannel packing of the guest molecules is controlled by a property of the diacyl peroxide group. In addition to revealing these novel structural properties, the work discussed in this paper has more general relevance concerning the measurement and interpretation of single crystal X-ray diffraction patterns that are based on more than one three-dimensionally periodic reciprocal lattice. Seven separate reciprocal lattices are required to rationalize the complete X-ray diffraction pattern from each diacyl peroxide/urea crystal studied here.

Dynamic properties of dioctanoyl peroxide guest molecules constrained within the urea tunnel structure: A combined incoherent quasielastic neutron scattering and solid state 2H nuclear magnetic resonance investigation

The dynamic properties of dioctanoyl peroxide guest molecules within the urea host tunnel structure in the dioctanoyl peroxide/urea inclusion compound have been investigated by incoherent quasielastic neutron scattering (IQNS) and solid state 2H nuclear magnetic resonance (NMR) techniques. The IQNS investigations were carried out on samples of urea inclusion compounds containing perdeuterated urea to ensure that the incoherent scattering is dominated by the dioctanoyl peroxide guest molecules. Using semioriented polycrystalline samples, translational motions of the guest molecules along the tunnel were investigated separately from reorientational motions of the guest molecules about the tunnel axis. The 2H NMR experiments used dioctanoyl peroxide deuterated selectively in both the α CD2 groups and urea with natural isotopic abundance. The dynamic models that have been found to describe the translational and reorientational motions of the guest molecules from the IQNS and 2H NMR data are discussed in detai...

ESR and X-ray diffraction studies of diacyl peroxides in urea and aluminosilicate hosts

Electron spin resonance (ESR) spectroscopy was used to study the photodecomposition of long-chain diacyl peroxides trapped in channels within zeolites (silicalite and ferrierite) and urea clathrates. ESR spectra of radical pairs in single crystals of the urea clathrates of diundecanoyl peroxide (UP), lauroyl peroxide (LP) andbis(6-bromohexanoyl) peroxide (6-BrHP) show that the alkyl radicals respond to the CO2 stress field by recoiling along the channel. In each clathrate, the inter-radical distance for the most relaxed pair is approx. 9.5 Å, suggesting nearly complete relaxation of stress from the CO2s. The rotational mobility and exceptional kinetic stability of the radicals is attributed to relaxation of stress and the lack of a convenient escape route for the CO2s. X-ray diffraction indicates one-dimensional ordering of guests in 6-BrHP/urea and 3-dimensional ordering of guests in UP/urea. Solid state NMR experiments on LP/urea suggest high guest mobility under ambient conditions. When UP and 6-BrHP were intercalated into silicalite, photolysis yielded isolated radicals, but no radical pairs, even as low as 20 K.

Deuterium NMR studies of guest motions in urea inclusion compounds of 1,6-dibromohexane with analytical evaluation of spectra in the fast motion limit

Abstract2H NMR (nuclear magnetic resonance) spectroscopy, in conjunction with X-ray diffraction experiments, was used to characterize the guest motions of 1,6-dibromohexane in its urea inclusion compound. These motions are characterized by alkyl chain jumps between two conformations, each approximately gauche to the terminal bromines, which remain stationary. In this distorted urea channel, one conformer is heavily preferred, but thermally activated population of the unfavorable conformer leads to reversible, temperature-dependent changes in the unit cell parameters. Although rapid motions of the guest chain give rise to ambiguities in the interpretation of the2H NMR spectra, fortuitous temperature-independent spectral features of guests containing deuterium at the α, β and γ positions indicate that the guest motion resembles a two-site jump with unequal probabilities. Analytical lineshape calculations on the three sets of2H NMR spectra indicate a single jump mechanism in which the range of jump angles is narrowly prescribed. This NMR model provided a starting point for successful solution and refinement of the crystal structures at 213 and 298 K, which had been complicated by motional disorder.