AbstractFor Abstract see ChemInform Abstract in Full Text.

Depending on the details of the technique employed, desolvation of tBC inclusion compounds leads to either a dense, guest-free polymorph, or a guest-free low density polymorph, the latter also having distinct high and low temperature forms.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Cl2[C16H12Cl2]2 Te(IV)(1) and Br2[C16H12Br2]2 Te(IV) (2).In both compounds the primary geometry about the Te(IV) atom is disphenoidal, with the halides in the apical positions and the lone pair of electrons and C atoms in the equatorial plane.In (1), the Te(IV) atom participates in a π-C6H5 interaction with one of the peripheric phenyl rings of a neighbouring molecule (Te ... centroid = 3.757 Å), resulting in the self-assembly of a chain like supramolecular arrangement.The chains are packed by van der Waals interactions between the dangling phenyl rings of the second phenylchlorovinyl substituent.In (2) a Te ... Br [3.253(1) Å] secondary bond links the molecules in a chain fashion.The adjacent chains are extended into a three dimensional supramolecular array by a Br .. π-C6H5 inter-chain interaction (3.680 Å).Due to the secondary interactions, which includes intramolecular Te ... X secondary bonds, the polyhedron around the tellurium can be best described as a distorted pseudo-pentagonal bipyramid, with two halides in axial positions and the centriod of a phenyl ring in (1) and a bromine in (2) occupying the fifth equatorial position.Acknowledgements

The low-temperature crystal structure of the p-tert-butylcalix[4]arene-toluene inclusion compound, C(44)H(56)O(4) x C(7)H(8), exhibits significant guest-induced distortion of the host calixarene molecule. This distortion persists long enough to establish a correlation in the orientation of guests in adjacent host molecules. Remarkably, the space group and unit cell that best describe the structure appear to depend on the wavelength of the incident radiation. This behaviour appears to arise from spatial averaging over the different scattering volumes required to establish the diffraction peaks.

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As-synthesized AlPO4-5 aluminophosphate molecular sieve prepared in a fluoride medium, using the quaternary tetrapropylammonium (TPA(+)) cation template, is characterized by a variety of solid-state NMR techniques. F-19 MAS spectra show two distinct populations, with the same isotropic chemical shift. While P-31 MAS experiments distinguish four different crystalline environments, both F-19-P-31 CP-MAS and HETCOR experiments reveal that only two signals belong to the AlPO4-5 framework, whereas the remaining two are attributed to crystalline byproducts. Al-27 MAS and 3Q-MAS spectra indicate, in addition to the tetrahedral environment, the presence of pentacoordinated Al species in close proximity to fluorine atoms, as is additionally shown by the F-19-Al-27 HETCOR experiment. An Al-27-{F-19} REDOR experiment strongly supports the existence of an Al-F bond (r(Al-F) < 2 Angstrom which forms the fifth coordination position of one of the twelve aluminum sites of the unit cell.

As illustrated elsewhere in this volume, calixarenes have proven to be a remarkably versatile class of materials. Numerous schemes have been devised that employ the simple calixarenes, modified molecules, surface-bound species or larger assemblies for a variety of functions. With the large amount of information now available, it is quite appropriate to assess our state of knowledge, as ultimately the rational design of the next generation of materials depends on our level of understanding of structural and dynamic details of the currently known species. In this work we summarise studies that have contributed to our understanding of calixarenes. In previous reviews we have noted the difficulty that structural studies of even the simplest calixarenes has caused [1,2]: in general, the compounds are disordered, often with involvement of both guest and host sub-lattices and there is evidence for dynamic coupling between them. We have also advanced the use of synoptic approaches, where we use complementary techniques to develop improved structural models. Without a doubt, the combination of crystallography and solid-state nuclear magnetic resonance (NMR) spectroscopy has proven to be remarkably effective in solving a number of complex problems, and continuing progress in technique development will advance these capabilities further. During the course of our work on calixarenes we have stayed with the simple ptert-butylcalix[4]arenes (tBC), starting with the very first tBC structure reported, that with toluene as guest. This compound has been under study in our laboratory for nearly ten years, and it has shown some very unusual effects, such as superlattice diffraction peaks that fade with time and a wavelength-dependent structure. Also, a good structural model was found that refined very well but turned out to have the wrong symmetry as shown by NMR spectroscopy. At the start of this work, only a few guest species were known for the simple tBCs, presumably because of the low solubility in many solvents. Many commonly held views on calixarenes derive from a very limited database and are easily shown to be vast oversimplifications, e.g., the suitability of specific guest types for the tBCs, general structural features of the host-guest complexes and the contributing factors to compound stability. With some less conventional approaches we were able to prepare compounds Chapter 16

Two newly identified structures arise upon the crystallisation of the p-tert-butylcalix[4]arene host molecule from tetradecane: the guest-free, self-included host structure, and a 1:1 host-guest structure in which the paraffin guest is anchored in the bowl-shaped host cavity and intercalates layers of host molecules in a lamellar structure reminiscent of clays.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

This crystal structure study examines systematically the effect of guest size and guest substituents on the guest–host interactions in a number of simple p-tert-butylcalix[4]arene host–guest compounds. With increasing chain length, the guest conformation becomes increasingly complex, going from a simple all-trans (C6) to s-shaped (C8) and helical (C12) conformations. For Cl-substituted alkanes, the Cl atom is excluded from the deep cavity, again by the chain taking on complex conformations. However, the Cl atom can be forced into the deep cavity with rigid guests such as 1,4-dichlorobenzene. The results suggest that it is space filling rather than specific interactions that give these compounds stability, and that they should be properly described as clathrates rather than as stoichiometric host–guest materials.

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The supramolecular 1:1 host–guest inclusion compound, p-tert-butylcalix[4]arene·α,α,α-trifluorotoluene, 1, is characterized by 19F and 13C solid-state NMR spectroscopy. Whereas the 13C NMR spectra are easily interpreted in the context of earlier work on similar host–guest compounds, the 19F NMR spectra of solid 1 are, initially, more difficult to understand. The 19F{1H} NMR spectrum obtained under cross-polarization and magic-angle spinning conditions shows a single isotropic resonance with a significant spinning sideband manifold. The static 19F{1H} CP NMR spectrum consists of a powder pattern dominated by the contributions of the anisotropic chemical shift and the homonuclear dipolar interactions. The 19F MREV-8 experiment, which minimizes the 19F–19F dipolar contribution, helps to identify the chemical shift contribution as an axial lineshape. The full static 19F{1H} CP NMR spectrum is analysed using subspectral analysis and subsequently simulated as a function of the 19F–19F internuclear distance (DFF=2.25±0.01 Å) of the rapidly rotating CF3 group without including contributions from additional libration motions and the anisotropy in the scalar tensor. The shielding span is found to be 56 ppm. The width of the centerband in the 19F{1H} sample-spinning CP NMR spectrum is very sensitive to the angle between the rotor and the magnetic field. Compound 1 is thus an attractive standard for setting the magic angle for NMR probes containing a fluorine channel with a proton-decoupling facility.

Leonardite humic acid (LHA) and soil humic acid (SHA), purchased from the IHSS, as well as purified humic acid from Aldrich (HA) are characterised by cross-polarisation magic angle spinning (CP-MAS) and Bloch decay 13C NMR spectroscopy and X-ray photoelectron spectroscopy (XPS). For the 13C CP-MAS NMR experiments, the effects of both the sample spinning sidebands and the influence of the cross-polarisation contact time are investigated. The functional groups distribution obtained by CP-MAS NMR is compared to the one resulting from chemical analyses. It is found that CP-MAS NMR overestimates the concentration of CO2H groups for certain humic compounds, whereas the concentration of phenolic groups is in reasonable agreement with the values determined by chemical methods. XPS was used as a complementary technique to understand the overestimation of carboxylic groups by NMR. The results from XPS, a surface characterisation technique, are in good agreement with those measured by elemental analyses of the bulk structure. High resolution C 1s and N 1s spectra show that the overestimation of the CO2H content by NMR is due to the presence of amide groups which give overlapping signals in the CO2H region of the NMR spectra.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

A novel structure involving a 2(p-tert-butylcalix[4]arene)·3.5(1,4-butanediamine) inclusion compound shows amine sites both exo and endo to the cavity, with the amine hydrogen-bonding to itself as well as to the host hydroxyl groups; for bulky amines with large pKa values, steric factors are more important than basicity, and the formation of a 1∶1, high-symmetry host–guest structure is preferred over a low-symmetry hydrogen-bonded structure.

Magnetic resonance microimaging (MRM) is used to characterize petroleum coke formed in the upgrading of Athabasca bitumen. Formation of large coke materials poses processing difficulties, and so the study of the structure and formation mechanism is important. The larger, “peas and beans” coke (approximately spherical, 4.1 mm diameter) possesses significant porosity, and is ideal for MRM. Coke is soaked in cyclohexane, and then 7V-, diffusion-, and density-weighted 1H images are collected with an unprocessed image resolution of 24 μm pixel-1. Scanning electron microscopy indicates that the coke is surrounded by a dense, 100 μm-thick outer shell. MRM shows that this shell is permeable to cyclohexane, and that a significant pore volume is present. The cyclohexane diffusion coefficient (Dmax = 1.4 x 10-9 m2 s-1) is roughly one-half of the Dmax in pure solution indicating that the cyclohexane inside the coke is liquid-like. The MRM images indicate that the coke is formed by an agglomeration mechanism from smaller fluid coke particles (nominal diameter 150 μm), which in turn posess microporosity. MRM shows great promise as a non-destructive, structural and dynamic characterization method for petroleum coke. Strategies to enhance resolution will be presented and discussed.

The host cavity of 1:1 host-guest compounds of p-tert-butylcalix [4] arene is well suited for the study of weak interactions in the solid state, as the motional freedom of the guests tests the weak intermolecular interactions in a very direct way. Benzene and pyridine are guests with a controlled number of similar (size, shape) as well contrasting (dipole moment, lone electron pair) properties that allow a meaningful comparative investigation of the structural and dynamic features. The 150 K single-crystal X-ray diffraction studies for the two host-guest compounds show that in both cases the guests occupy essentially the same orientation in the host cavity, with pyridine situated 0.11 Angstrom deeper into the cavity than benzene. The complementarity of the diffraction and solid-state NMR techniques is illustrated, in particular, by incorporating the pyridine structural information obtained from H-2 NMR studies into the diffraction data, thus resolving the ambiguity of the nitrogen atom position. Despite similar structural environments, the guests exhibit quite different dynamic behavior. Variable temperature 2H NMR spectra of the perdeuterated pyridine and benzene guests are interpreted in terms of specific motional models; benzene undergoes in-plane rotation followed by reorientation about the compound's 4-fold axis of symmetry. in contrast, pyridine reorients about the pyridine C-2 molecular symmetry axis (rather than in-plane rotation), followed by guest reorientation about the compound's C-4 axis of symmetry. A significant point is-that the pyridine nitrogen has definite orientations in the cavity that cannot be explained by any specific directional electrostatic interactions between the host and guest. Both the dynamically averaged N-15 NMR chemical shift tensor components and the absence of short contacts rule out a C-H ... N hydrogen bonding interaction of the host to the guest. Despite the fact that the pyridine molecule is tightly docked in the host cavity, intermolecular interactions must be ascribed strictly to steric interactions acting in concert rather than specific directional interactions. Both guests are oriented in the host cavity such that the aromatic plane minimizes rather than maximizes contact with the host CH3 groups. This result questions the ability to ascribe structural features in the solid state to isolated directional interactions, such as the importance and role of CHhost...pi(guest) interactions which have been suggested many times as having a stabilizing influence on this system.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationMagnetic Resonance Microimaging of Petroleum CokeEric B. Brouwer, Igor Moudrakovski, Keng H. Chung, Gerald Pleizier, John A. Ripmeester, and Yves DeslandesView Author Information Institute for Chemical Process and Environmental Technology, National Research Council, Ottawa, Ontario, Canada K1A 0R6, Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada K1A 0R6, and Syncrude Research Centre, 9421-17 Avenue, Edmonton, Alberta, Canada T6G 2G6 Cite this: Energy Fuels 1999, 13, 5, 1109–1110Publication Date (Web):August 28, 1999Publication History Received24 May 1999Revised28 July 1999Published online28 August 1999Published inissue 1 September 1999https://pubs.acs.org/doi/10.1021/ef990096mhttps://doi.org/10.1021/ef990096mrapid-communicationACS PublicationsCopyright © 1999 American Chemical SocietyRequest reuse permissionsArticle Views122Altmetric-Citations2LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Chemical structure,Coke,Diffusion,Magnetic resonance microscopy,Probes Get e-Alerts