John A. Ripmeester

13C CP MAS NMR of halogenated (Cl, Br, I) pharmaceuticals at ultrahigh magnetic fields

This work reports significantly improved spectral resolution of 13C CP MAS NMR spectra of chlorinated, brominated and iodinated solid organic compounds when such spectra are recorded at ultrahigh magnetic field strengths. The cause of this is the residual dipolar coupling between carbon atoms and quadrupolar halogen nuclides (chlorine‐35/37, bromine‐79/81 or iodine‐127), an effect inversely proportional to the magnetic field strength which declines in importance markedly at 21.1 T as compared to lower fields. In favorable cases, the fine structure observed can be used for spectral assignment, e.g. for Cl‐substituted aromatics where the substituted carbon as well as the ortho‐carbons show distinct doublets. The experimental results presented are supported by theoretical modeling and calculations. The improved spectral resolution in the studied systems and similar halogenated materials will be of particular interest and importance for polymorph identification, drug discovery and quality control in the pharmaceutical industry. Copyright

Cationic guests in extended anionic C-methylcalix[4]resorcinarene-inorganic frameworks: exercising conformational Control over C-methylcalix[4]resorcinarene

Abstract Under acidic conditions and in the presence of appropriate inorganic ions, bipyridyl-type molecules will self-assemble with C-methylcalix[4]resorcinarene to yield a host–guest supramolecular structure in which the resorcinarene adopts the open boat conformation, with the resorcinarene chains linked by the inorganic ions, and bipyridinium-type molecules included as guests that do not interact specifically with the host.

Guest-induced asymmetry in the structure of p-tert-butylcalix[4]arene-nitrobenzene

Abstract Nitrobenzene and p-tert-butylcalix[4]arene form a 1:1 inclusion compound in which the calixarene is significantly distorted from its usual 4-fold symmetry as evident from 13C NMR spectroscopy and X-ray diffraction.

The ability of lower peptides to form co-crystals: inclusion compounds of Leu-Leu-Leu tripeptide with pyridine and picolines

Co-crystals of L-leucyl-L-leucyl-L-leucine tripeptide (LLL) with pyridine (Py), α-picoline (2-MePy), β-picoline (3-MePy) and γ-picoline (4-MePy) have been prepared and studied. Single-crystal XRD analyses reveal the following formulas: LLL*(Py) (1), LLL*(2-MePy)*0.12(H2O) (2), LLL*(3-MePy) (3), and LLL*(4-MePy)*0.14(H2O) (4). The compounds may be described as inclusion compounds of LLL (host), pyridine or picolines (guest) and water (secondary guest). The host LLL molecules are arranged in anti-parallel β-sheet layers and have conformations consistent with this type of peptide organization. The layers are packed by van der Waals forces with an interlayer distance of 11.5–11.8 A. The isobutyl side groups of leucyl residues protrude into the interlayer space and form large channels on one side of the layer. The guest molecules reside in the channels and form H-bonds to the host NH3+ groups. The crystal structures have two host molecules in the asymmetric unit, which in most cases, exhibit rarely observed whole-molecule disorder. Three of the compounds invariably form as twinned crystals. TGA measurements of bulk products reveal the following compositions: LLL*1.04(Py) (1), LLL*0.99(2-MePy) (2), LLL*0.96(3-MePy) (3), and LLL*0.92(4-MePy) (4), in good agreement with the XRD results. The character of thermal dissociation implies the preservation of the host layered structure as guest molecules diffuse out of the channels. Compound 2 dissociates with the formation of an intermediate with the stoichiometry of LLL*0.5(2-MePy).

Resorcinarene configuration and inclusion behaviour: Isolation of a guest free form of pyrogall[4]arene

Both single crystals and bulk powder of the apo form of the rctt isomer of c-methylpyrogall[4]arene can be obtained, the former by molecular recognition in solution, the latter by desolvation of a clathrate.

Guest capture, storage and removal in the TATM host framework: a single-crystal study

The organic host molecule, tris(5-acetyl-3-thienyl)methane (TATM) is known to form a variety of van der Waals inclusion compounds. Surprisingly, this host framework has been shown to be stable despite near-complete guest loss. In the present study we examine the degree to which guests can be removed from the host cavities without framework collapse, as well as the ability of the depleted host framework to re-adsorb guest molecules, including adsorption of a two-component mixture of guests, all in a host system that has no channels for transport. The response of the host framework to changes in guest content is reported as well.

KINETICS OF HYDRATE FORMATION AND DECOMPOSITION OF METHANE IN SILICA SAND.

Kinetics of hydrate formation and decomposition of methane hydrate formed in silica sand particles were studied in detail at three temperatures of 7.0, 4.0 and 1.0°C, respectively. A new apparatus was setup to study the decomposition behavior of the methane hydrate formed in the bed of silica sand particles. Six thermocouples are placed in different locations to study the temperature profiles during hydrate formation and decomposition experiments. Gas uptake measurement curves for the formation experiments and the gas release measurement curves for the decomposition experiment were determined from the experimental data. Percent conversion of water to hydrates was significantly higher for the experiments conducted at 4.0 and 1.0°C compared to 7.0°C. Recovery of methane occurred in two stages during the decomposition experiments carried out with a thermal stimulation approach at constant pressure. Methane recovery in the range of 95 to 98% was achieved.

AB INITIO STRUCTURE DETERMINATION OF GAS HYDRATES AND REFINEMENT OF GUEST MOLECULE POSITIONS BY POWDER X-RAY DIFFRACTION

Structure determination of powdered crystals is still not a trivial task. For gas hydrates, the difficulty lies in how to determine the rotational disorder and cage occupancies of the guest molecules without other supporting information or constraints because the complexity of the problem for the powder diffraction technique generally depends on the number of atoms to be located in the asymmetric unit. Here, the crystal structures of gas hydrates of CO2, C2H6, C3H8, and Methylcyclohexane/CH4, as determined by the direct-space and Rietveld techniques are reported. The resultant structures and cage occupancies were consistent with results found from conventional experimental methods using single crystal x-ray diffraction or solid-state 13 C-NMR. It was shown that the procedures reported in this study make it possible to determine guest disorder and absolute cage occupancy of gas hydrates even from powder crystal.

A LOW SYMMETRY FORM OF STRUCTURE H CLATHRATE HYDRATE

In this paper we report a low symmetry version of structure H hydrate that results from the hexagonal form on cooling below 167 K. Phase changes with temperature in the common clathrate hydrates structural families I, II and H have not been observed before, except in doped systems where ordering transitions take place or in the structure I hydrate of trimethylene oxide where the guest molecule dipoles are known to order. Since there is an inverse relationship between the effect of temperature and pressure on ices, it may well be that the low symmetry form reported at low temperature can also be reached by applying high pressure, and that in fact some of the observed high pressure phases are lower symmetry versions of