Structure property relationships in the ATi2O4 (A=Na, Ca) family of reduced titanates

Reduced titanates in the ATi₂O₄ (A=Li, Mg) spinel family exhibit a variety of interesting electronic and magnetic properties, most notably superconductivity in the mixed-valence spinel, Li_1+xTi_2−xO₄. The sodium and calcium analogs, NaTi₂O₄ and CaTi₂O₄, each differ in structure, the main features of which are double rutile-type chains composed of edge-sharing TiO₆ octahedra. We report for the first time, the properties and band structures of these two materials. XANES spectroscopy at the Ti K-edge was used to probe the titanium valence. The absorption edge position and the pre-edge spectral features observed in the XANES data confirm the assignment of Ti³⁺ in CaTi₂O₄ and mixed-valence Ti³⁺/Ti⁴⁺ in NaTi₂O₄. Temperature-dependent resistivity and magnetic susceptibility studies are consistent with the classification of both NaTi₂O₄ and CaTi₂O₄ as small band-gap semiconductors, although changes in the high-temperature magnetic susceptibility of CaTi₂O₄ suggest a possible insulator-metal transition near 700 K. Band structure calculations agree with the observed electronic properties of these materials and indicate that while Ti-Ti bonding is of minimal importance in NaTi₂O₄, the titanium atoms in CaTi₂O₄ are weakly dimerized at room temperature.     

Refinements of the crystal structures of 1,4-diphenylbutadiyne and its complex with two moles of Co2(CO)6

1,4-diphenylbutadiyne, C₁₆H₁₀, crystallizes in monoclinic space group P2₁/n witha=6.6152(6),b=6.0768(7),c=14.634(1) Å, =100.981(8)⁰,V=577.5(2) ų,Z=2. The structure was refined toR=0.077 andR _w=0.069 for 1007 observed reflections with CuK radiation. The molecule lies on an inversion center. Our determination represents and increase in precision of a factor of approximately three over the original determination. The 21 cobalt complex, C₁₆H₁₀[Co₂(CO)₆]₂, also crystallizes in monoclinic space group P2₁/n witha=8.9568(12),b=17.7057(10),c=18.5671(12) Å, =90.606(12)^o,V=2944.3(5) ų,Z=4. The structure was refined toR=0.032 andR _w=0.031 for 4312 observed reflections. The diphenylbutadiyne triple bonds elongate to 1.346(3) Å and the angles at the diyne C atoms decrease to 134.1(3)–144.9(3)^o upon complexation. The Co-Co distance is 2.4580(4)Å. Our data represent an increase in precision by a factor of 2–3 over the original determination.     

193-nm Photodissociation of ions from saturated and unsaturated aliphatic molecules

To determine the analytical utility of photodissociation as a general fragmentation technique for tandem mass spectrometry of organic ions, the ability to fragment those ions considered least likely to absorb photons efficiently was investigated. To this end, the ability to photodissociate ions of aliphatic compounds by using 193-nm photons has been studied. Three fragment ions, the C₄H ₉ ⁺ ion from n-hexane, the C₄H ₇ ⁺ ion from 2-hexene, and C₄H ₅ ⁺ from 2-hexyne, have been photodissociated. The fragmentation efficiencies for all three ions studied were between 25 and 45%. The photofragment ion spectrum for each precursor ion studied is made up of characteristic fragments. These spectra demonstrate the ability to photodissociate aliphatic ions that originate from both saturated and unsaturated molecules. This provides substantial hope that virtually all organic ions will be able to be photodissociated by using 193-nm photons.     

Sorption and partitioning parameters of benzotriazole compounds

Benzotriazole compounds have major commercial applications as anticorrosive agents in automotive antifreeze and airplane deicer fluids. This study assesses the sorption of benzotriazole (BT), 5-methylbenzotriazole (MBT), and 5-chlorobenzotriazole (CBT) from aqueous solutions to four top soils. The concentration range of 10-500 mg l⁻¹ was used with soils differing in total organic carbon content from 0.27 to 1.72%. Batch systems facilitated the equilibrium sorption with analysis by HPLC. The sorption of these compounds was as much as 60% by mass to a soil with 0.33% Org. C. The log octanol-water partition coefficients (log K_ow) were determined to be 1.23 for BT, 1.89 for MBT, and 2.17 for CBT. The relationship between the log of the sorption partition coefficient, log K_oc and log K_ow differed from previous correlations because hydrophobicity was not the only factor affecting sorption. These compounds have substantial permanent dipole moments as well as being hydrophobic. At high pH where CBT molecules approach their pK_a, sorption was approximately 50% less (by mass) than that of relatively non-ionized CBT molecules.     

Bioenergetics and bioremediation of contaminated soil

The microbial biodegradation of xenobiotic compounds in soil and ground water is constrained by the laws of thermodynamics. Bioremediation is being investigated in a rhizosphere environment in which higher plants provide carbon and energy to sustain the microbial population. Toluene, phenol, trichloroethylene and trichloroethane have been fed in separate experiments to a pilot scale system with alfalfa growing in sandy soil containing less than 10% of silt. It is well known that microbial populations are numerous in the root zone of healthy vegetation. Root exudates can stimulate aerobic microbial biodegradation of compounds which by themselves support growth poorly or not at all. Polynuclear aromatic compounds such as phenanthrene, anthracene, and pyrene, which are not very soluble in water, and chlorinated aliphatic hydrocarbons such as trichloroethylene are examples of compounds that can be biodegraded in the rhizosphere when root exudates are present to enhance and sustain microbial activity. Solar driven transport processes such as water and solute movements due to evapotranspiration increase the likelihood that the contaminants will come into contact with the microorganisms and be degraded. The thermodynamic and bioenergetic aspects of transport and biodegradation in the rhizosphere are examined through a review of the literature and the analysis of experimental data collected in the pilot scale system.     

Synthesis of functionalized cannabinoids

An effective synthesis of tricyclic, nonclassical cannabinoids has been developed on the basis of a cation-olefin cyclization that forms the two nonaromatic rings with the desired stereochemistry in a single step.     

Paths of random evolutions

We study the paths of general random evolution processes obtained by piecing together deterministic evolution functions according to the dictates of a regular step process. If the state space is metrizable we show that such processes are strong Markov; and they are even standard under a certain continuity condition on paths. We apply this result to solutions of stochastic delayed differential equations, and we make a connection between our processes and random evolutions associated with classes of semigroups.     

Preprocessing chains for fast dihedral rotations is hard or even impossible

We examine a computational geometric problem concerning the structure of polymers. We model a polymer as a polygonal chain in three dimensions. Each edge splits the polymer into two subchains, and a dihedral rotation rotates one of these subchains rigidly about the edge. The problem is to determine, given a chain, an edge, and an angle of rotation, if the motion can be performed without causing the chain to self-intersect. An Ω(nlogn) lower bound on the time complexity of this problem is known.We prove that preprocessing a chain of n edges and answering n dihedral rotation queries is 3 -hard, giving strong evidence that Ω(n²) preprocessing is required to achieve sublinear query time in the worst case. For dynamic queries, which also modify the chain if the requested dihedral rotation is feasible, we show that answering n queries is by itself 3 -hard, suggesting that sublinear query time is impossible after any amount of preprocessing.     

Valence band X-ray photoemission and soft X-ray emission studies of Pt

The soft X-ray N_6,7 (5d to 4f transition) emission spectrum and the X-ray photoemission spectrum of Pt (the latter under two conditions of surface cleanliness) have been obtained, and are compared with the results of photoemission and band theoretical studies.