Edward J. Valente

Electron diffraction studies of supersonic jets. V. Low temperature crystalline forms of SF6, SeF6, and TeF6

Condensation of SF6, SeF6, and TeF6 in nozzle flows with inert carrier gases produces microcrystals of these materials. All form the higher temperature body‐centered cubic structure at higher partial pressures of hexafluoride. At lower partial pressures and colder nucleation conditions a lower symmetry form of each has been produced. Electron diffraction powder patterns are consistent with the space group Pnma to which metal hexafluorides of UF6 type belong. Low temperature phases of the present materials differ from those of the metal compounds, however, in being less dense than the cubic forms. Aspects of the gas dynamics affording a control over nucleated species are briefly discussed.

Electron diffraction studies of supersonic jets. VI: Microdrops of benzene

Large clusters of benzene were generated in supersonic flow through a Laval nozzle with carrier gases He, Ne, or Ar. Benzene concentration and carrier pressures were varied over wide ranges. Diffraction patterns recorded from s=0.8 to 12 A−1 revealed strong intermolecular interference features indicative of supercooled bulk liquid at 100–200 K. Computer simulations of nucleation and cluster growth were carried out and agreed with the following observations. An initial benzene partial pressure of ∼0.1 bar expanded through a nozzle of our design requires a carrier pressure of 2–3 bar to give reasonably complete condensation. Cluster diameter and fraction condensed decrease if (1) benzene mole fraction is reduced at constant carrier pressure, (2) carrier pressure is reduced at constant benzene partial pressure, and (3) carrier molecular weight is decreased. The simulations identify the factors accounting for the trends. Cluster intensities are compared with those calculated for small clusters proposed by Wil...

Electron diffraction studies of supersonic jets. II. Formation of benzene clusters

Expansions of benzene at 1-10 mol % in neon or helium with various nozzle types have produced clusters of benzene molecules. Preliminary comparisons with intensities from the bulk liquid and with model calculations based on solid state crystal structures suggest that on the average the clusters are probably considerably larger than a 13 molecule unit, and have a vibrational temperature on the order of 100--150 K. Unlike clusters of spherical or quasispherical molecules previously found to pack in nearly crystalline arrays, benzene molecules appear to be unable to organize into regular arrays in the time of our experiments. Local order in clusters is similar but not identical to that in the solid, resembling that expected for a supercooled liquid. The present approach, exploiting several advantages of electron diffraction over alternative methods, shows promise as a new means of studying liquids or glasses.

Electron diffraction studies of supersonic jets. VII. Liquid and plastic crystalline carbon tetrachloride

Carbon tetrachloride at 3–11 mol % in neon carrier produces clusters of CCl4 molecules in flow through a Laval nozzle. Electron diffraction patterns of the clusters formed at a total pressure below 2.5 bar are intermediate between x‐ray diffraction patterns of bulk liquid CCl4 and neutron patterns of the amorphous material formed by condensation at 10 K. Strong, sharp interference features at low angle imply appreciable long range order, perhaps beyond 50 A, in the clusters, whose temperatures are about 210 K. When carrier partial pressure is increased beyond 2.6 bar, clusters begin to exhibit crystallinity. Crystallites belong to the rhombohedral plastic‐crystalline phase Ib, a=14.27 A and α=90.0°.

Hexacoordinate Ru-based olefin metathesis catalysts with pH-responsive N-heterocyclic carbene (NHC) and N-donor ligands for ROMP reactions in non-aqueous, aqueous and emulsion conditions

Summary Three new ruthenium alkylidene complexes (PCy3)Cl2(H2ITap)Ru=CHSPh (9), (DMAP)2Cl2(H2ITap)Ru=CHPh (11) and (DMAP)2Cl2(H2ITap)Ru=CHSPh (12) have been synthesized bearing the pH-responsive H2ITap ligand (H2ITap = 1,3-bis(2’,6’-dimethyl-4’-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidene). Catalysts 11 and 12 are additionally ligated by two pH-responsive DMAP ligands. The crystal structure was solved for complex 12 by X-ray diffraction. In organic, neutral solution, the catalysts are capable of performing standard ring-opening metathesis polymerization (ROMP) and ring closing metathesis (RCM) reactions with standard substrates. The ROMP with complex 11 is accelerated in the presence of two equiv of H3PO4, but is reduced as soon as the acid amount increased. The metathesis of phenylthiomethylidene catalysts 9 and 12 is sluggish at room temperature, but their ROMP can be dramatically accelerated at 60 °C. Complexes 11 and 12 are soluble in aqueous acid. They display the ability to perform RCM of diallylmalonic acid (DAMA), however, their conversions are very low amounting only to few turnovers before decomposition. However, both catalysts exhibit outstanding performance in the ROMP of dicyclopentadiene (DCPD) and mixtures of DCPD with cyclooctene (COE) in acidic aqueous microemulsion. With loadings as low as 180 ppm, the catalysts afforded mostly quantitative conversions of these monomers while maintaining the size and shape of the droplets throughout the polymerization process. Furthermore, the coagulate content for all experiments stayed <2%. This represents an unprecedented efficiency in emulsion ROMP based on hydrophilic ruthenium alkylidene complexes.

CONTROL OVER THE STRUCTURE OF CLUSTERS GENERATED IN SUPERSONIC FLOW

Clusters of a variety of volatile substances have been generated in flow with carrier gases through a miniature Laval nozzle. Cluster structures were investigated by electron diffraction and found to be liquid-like in some cases, crystalline in others. Solid clusters tended to occur in experiments with substances exhibiting plastic crystalline phases. In a. number of examples the cluster structure could be controlled by the flow conditions. Factors governing the type of cluster formed are discussed.