Lily Yeo

Structural diversity, but no polymorphism, in a homologous family of co-crystals of urea and α,ω-dihydroxyalkanes

A family of stoichiometric (2:1 molar ratio) co-crystals formed between urea and α,ω-dihydroxyalkanes of even chain length [HO(CH2)nOH, n = 2m, m = 3–8] is shown to exhibit three well-defined structure types, which are rationalized on the basis of specific hydrogen-bonding motifs. In spite of the structural diversity observed for different α,ω-dihydroxyalkane chain lengths, there is no evidence that any member of this family of co-crystals exhibits polymorphism (i.e. none of the α,ω-dihydroxyalkane/urea systems is observed to exist in more than one of the well-defined co-crystal structure types).

Definitive structural characterization of the conventional low-temperature host structure in urea inclusion compounds

Structural properties of the 1,10-dibromodecane/urea and 1,12-dibromododecane/urea inclusion compounds have been determined by single crystal X-ray diffraction for both the high- and low-temperature phases. In the high-temperature phase both inclusion compounds have the conventional hexagonal urea tunnel structure, with substantial orientational disorder of the guest molecules. In the low-temperature phase the urea tunnel structure distorts to an orthorhombic structure, based on a distorted form of the orthohexaganol cell of the high-temperature structure and with the loss of the C centre. Within this tunnel structure there is evidence that the guest molecules have a narrow distribution of orientations (with respect to rotation about the tunnel axis) and the preferred orientation of the guest molecules correlates well with the observed distortion of the host tunnel. This represents the first accurate and reliable report of the conventional low-temperature structure of urea inclusion compounds. Previous powder X-ray diffraction studies have confirmed that the host structure in the low-temperature phase of 1,10-dibromodecane/urea is the same as that in the low-temperature phase of the alkane/urea inclusion compounds.

Computational investigation of host–guest chiral recognition in incommensurate 2-bromoalkane/urea inclusion compounds

In conventional urea inclusion compounds, the urea molecules form a crystalline solid host structure (space group P6122 or P6522) within which there are linear, parallel tunnels. The walls of these tunnels are formed by a spiral arrangement of urea molecules; appropriate guest molecules may be located within these tunnels. In order to investigate the extent and nature of the chiral recognition between the chiral urea tunnel structure and chiral guest molecules, computational investigations of host–guest interaction in 2-bromoalkane/urea inclusion compounds have been carried out. As the 2-bromoalkane/urea inclusion compounds have an incommensurate relationship between the periodicities of the host and guest structures along the tunnel axis, it is important to consider the way in which the chiral recognition varies as a function of the position of the 2-bromoalkane guest molecule along the tunnel. All 2-bromoalkanes from 2-bromoheptane to 2-bromohexadecane were studied; in each case, four different ‘types’ of 2-bromoalkane guest molecule were considered, representing the R and S enantiomers, both for the conformation with the Br atom trans (CH3 group gauche) and for the conformation with the Br atom gauche (CH3 group trans).For each enantiomer of the 2-bromoalkane guest molecule, it is found that the Br trans/CH3gauche conformation is preferred over the Br gauche/CH3trans conformation at all positions along the tunnel (in contrast, the Br gauche/CH3trans conformation is preferred for isolated 2-bromoalkane molecules). For the preferred Br trans/CH3gauche conformation within the P6122 host structure, the R enantiomer is preferred over the S enantiomer at all positions along the tunnel for all guest molecules studied, with the exception of 2-bromoundecane (for 2-bromoundecane, there are restricted regions within which there is a slight preference for the S enantiomer). On taking into account the incommensurate nature of the 2-bromoalkane/urea inclusion compounds, an overall excess of the R enantiomer within the P6122 host structure is predicted in all cases. Structural reasons underlying the host–guest chiral recognition in the 2-bromoalkane/urea inclusion compounds are discussed.

Direct measurement of the distance between adjacent guest molecules in a disordered solid inclusion compound using solid-state F-19-H-1 NMR spectroscopy

Abstract Three solid-state NMR techniques have been used to determine the effective average intermolecular 19 F , 19 F dipolar interaction, 〈D′〉, between fluorine nuclei in neighbouring guest molecules in the 1,10-difluorodecane/urea inclusion compound using a static sample. An MREV8 pulse sequence was used to isolate the shielding powder pattern. A Carr–Purcell–Meiboom–Gill experiment was employed to obtain the dipolar powder pattern. To confirm the results, the full powder pattern influenced by both interactions was obtained and simulated by computer. In all experiments efficient proton decoupling was used. The value of 〈D′〉 obtained is 1.01 kHz. This is discussed in relation to the conformations and mutual orientation of the end-groups of the guest molecules. The result is consistent with random distribution over all mutual orientations of the gauche–gauche and gauche–trans situations, with little or no population in the trans–trans form.

Chiral recognition in incommensurate one-dimensional inclusion compounds: a computational investigation

Computational investigations of host-guest interaction between the chiral urea tunnel structure and chiral 2-bromoalkane guest molecules have been used as a basis for understanding the nature of chiral recognition in 2-bromoalkane/urea inclusion compounds.

Application of triple-channel 13C{1H,19F} NMR techniques to probe structural properties of disordered solids

Triple-channel 13C MAS NMR experiments are carried out to characterize intermolecular interactions in disordered molecular solids containing fluorinated n-alkanes; an approach involving a combination of 1H→13C and 19F→13C cross-polarization experiments, with both single-channel (1H) and double-channel (1H,19F) decoupling is developed to assign resonances for a given type of 13C nucleus in different intermolecular environments.

Fluorocyclohexane ring inversion in a solid thiourea inclusion compound studied by fluorine-19 magic-angle spinning NMR with high-power proton decoupling

Variable-temperature 19 F magic-angle spinning NMR experiments involving high-power proton decoupling are used to examine the conformational preference and ring-inversion kinetics in the fluorocyclohexane–thiourea inclusion compound.