Synthesis and characterization of a new structure of gas hydrate.

Atoms and molecules <0.9 nm in diameter can be incorporated in the cages formed by hydrogen-bonded water molecules making up the crystalline solid clathrate hydrates. For these materials crystallographic structures generally fall into 3 categories, which are 2 cubic forms and a hexagonal form. A unique clathrate hydrate structure, previously known only hypothetically, has been synthesized at high pressure and recovered at 77 K and ambient pressure in these experiments. These samples contain Xe as a guest atom and the details of this previously unobserved structure are described here, most notably the host-guest ratio is similar to the cubic Xe clathrate starting material. After pressure quench recovery to 1 atmosphere the structure shows considerable metastability with increasing temperature (T <160 K) before reverting back to the cubic form. This evidence of structural complexity in compositionally similar clathrate compounds indicates that the reaction path may be an important determinant of the structure, and impacts upon the structures that might be encountered in nature.

What we have learned from the study of solid p-tert-butylcalix[4]arene compounds.

The study of solid p-tert-butylcalix[4]arene and its compounds with a variety of techniques has provided a good understanding of the versatility of this host molecule, how to induce a number of distinct host-guest motifs, its molecular recognition properties, the complex phase relationships and unique properties such as gas adsorption without having obvious channels.

Probing the geometry and interconnectivity of pores in organic aerogels using hyperpolarized 129XE NMR spectroscopy.

Hyperpolarized (HP) 129Xe NMR was used for the first time to probe the geometry and interconnectivity of pores in RF aerogels and to correlate them with the [resorcinol]/[catalyst] (R/C) ratio. We have demonstrated that HP 129Xe NMR is an ideal method for accurately measuring the volume-to-surface-area (Vg/S) ratios for soft mesoporous materials without using any geometric models. The Vg/S parameter, which is related both to the geometry and the interconnectivity of the porous space, has been found to correlate strongly with the R/C ratio and exhibits an unusually large span: an increase in the R/C ratio from 50 to 500 results in about 5-fold rise in Vg/S. Unlike conventional techniques, HP 129Xe NMR spectroscopy probes the geometry and interconnectivity of the nano- or mesopores in soft materials, providing new insights into the pore structure.

(6,17-Dimethyl-8,15-diphenyldibenzo[b,i][1,4,8,11]-tetra-aza[14]annulenato)nickel(II) in solids: two guest-free polymorphs and inclusion compounds with methylene chloride and fullerene (C(60))-carbon disulfide.

Two guest-free polymorphs and two inclusion compounds of the macrocyclic title complex [NiL] have been isolated and characterized with single-crystal and/or powder XRD, solid-state (13)C NMR, and other methods. The inclusion compound with methylene chloride, [NiL](CH(2)Cl(2)), is stable in air and thermally stable up to approximately 128 degrees C. Its crystal structure is consistent with van der Waals packing of the host [NiL] and guest CH(2)Cl(2) molecules. The host complex has square-planar coordination of the nickel(II) center with four nitrogen atoms of the macrocycle with an average Ni-N distance of 1.86 A. The molecule has a saddle-shaped conformation with the guest molecule located between one phenylene and two phenyl rings of the host molecule. Isostructural compounds with chloroform and 2-chloropropane form only as mixtures along with a guest-free host polymorph. The inclusion compound with C(60) has a composition 3[NiL]*(C(60))*2(CS(2)) and here also the crystal structure is consistent with a van der Waals type of packing. Three crystallographically inequivalent [NiL] molecules have geometries similar to that in the inclusion compound with methylene chloride. The concave surfaces of the complex molecules form a spherical cavity for the C(60) molecule. At -100 degrees C the C(60) molecule is disordered over two orientations centered at the same site. (13)C NMR studies at room temperature show that the C(60) molecule is undergoing rapid pseudo-isotropic rotation. The stability and other properties of the title and related complexes are discussed.

Nickel(II) and zinc(II) dibenzoylmethanates: molecular and crystal structure, polymorphism, and guest- or temperature-induced oligomerization.

Four forms of nickel(II) and two of zinc(II) dibenzoylmethanates have been isolated and characterized with powder and single-crystal X-ray diffraction analyses, differential scanning calorimetry, magnetic susceptibility measurements, and solid-state 13C cross-polarization/magic angle spinning NMR. Nickel dibenzoylmethanate, Ni(DBM)2 (DBM = PhCOCHCOPh-), forms three polymorphic forms (light-green, brown, and green) and a fourth clathrate form with guest benzene included. The light-green polymorph is metastable. Substituted benzenes induce recrystallization of the polymorph into a stable brown form (C30H22NiO4; a = 26.502(3) A, b = 5.774(1) A, c = 16.456(2) A, beta = 116.03(1) degrees; monoclinic, C2/c; Z = 4). Unlike the other forms, the brown form is diamagnetic and is comprised of monomers of the low-spin [Ni(DBM)2] complex. The Ni(II) is chelated by two DBM ligands in a square planar environment by four donor oxygen atoms. When heated, the brown form transforms to a green form which is stable above 202 degrees C (C90H66Ni3O12; a = 13.819(2) A, b = 16.252(2) A, c = 17.358(2) A, beta = 108.28(1) degrees; monoclinic, P2(1)/n; Z = 2). This polymorph is formed by van der Waals packing of trimers [Ni3(DBM)6] containing linear Ni3 clusters with an Ni-Ni distance of 2.81 A. The cluster is surrounded by six DBM ligands, providing a distorted octahedral environment about each Ni by six oxygen atoms. Benzene stabilizes the trimeric structure at room temperature, forming a [Ni3(DBM)6].2(benzene) inclusion compound (Ni-Ni distance of 2.83 A) with guest benzene molecules located in channels (C90H66Ni3O12 + 2(C6H6); a = 17.670(2) A, b = 20.945(3) A, c=11.209(2) A, beta = 102.57(1) degrees; monoclinic, P2(1)/c; Z = 2). Zinc dibenzoylmethanate has been prepared in two polymorphic forms. The monomeric form contains [Zn(DBM)2] molecules with the zinc center in a distorted tetrahedral environment of four oxygens from the two chelated DBMs (C30H22O4Zn; a = 10.288(2) A, b = 10.716(2) A, c = 12.243(2) A, alpha = 89.19(1) degrees, beta = 75.39(1) degrees, gamma = 64.18(1) degrees; triclinic, P1; Z = 2). Another, dimeric form contains [Zn2(DBM)4] species, with two zinc atoms separated by a distance of 3.14 A and each zinc coordinated by five oxygen atoms (C60H44O8Zn2; a = 25.792(3) A, b = 7.274(1) A, c = 24.307(2) A, beta = 90.58(1) degrees; monoclinic, C2/c; Z = 4). The polymorphic variety of the title complexes and the peculiarities of the Ni(II) and Zn(II) coordination environments are discussed in the context of using the complexes as precursors for new metal complex hosts.

Chemical shift imaging with continuously flowing hyperpolarized xenon for the characterization of materials.

In this contribution we report new approaches to the MRI of materials using continuously produced laser-polarized (129)Xe gas. This leads to vastly improved sensitivity and makes new kinds of information available. The hyperpolarized xenon is produced in a continuous flow system that conveniently delivers the xenon at low partial pressure to probes for NMR and MRI experiments. We illustrate applications to the study of micropore and other kinds of void space and show for the first time that with flowing hyperpolarized xenon it is possible to obtain chemical-shift-resolved images in a relatively short time.