Compatible additive permutations of finite integral bases

Let be an additive permutation of a finite integral base. It is shown that ifB is symmetric, then there is a unique additive permutation ofB which is compatible with in the sense that ^–1 is also an additive permutation; and that, further, ifB is asymmetric, then there is no additive permutation ofB which is compatible with. Thus, in the symmetric case, there are no additively compatible sets (of permutations) forB of size greater than 3. This contrasts with the situation for completely compatible sets (equivalently, additive sequences of permutations) where for certainB compatible sets of size (resp. length) 4 or less are known, but where nothing is known of sets of greater size (resp. length). It is also noted how this result restricts the possibility of a useful multiplication theorem for the additive analogue of perfect systems of difference sets and graceful graphs.     

X-ray photoelectron spectroscopy and differential capacitance study of thiol-functional polysiloxane films on gold supports

Polymeric molecules containing multiple thiol groups (polythiols) provide tenacious attachment to metal surfaces such as gold. Polythiol films are also well suited for subsequent derivatization with biomacromolecules through remnant free thiol groups of the film. In this study, 1-3 nm thick layers of a commercial polythiol, poly((mercaptopropyl)methylsiloxane) (PMPMS), are investigated with X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy. XPS is used to reveal the surface coverage of thiolate-Au bonds between the polythiol and the metal support, which is found to be approximately 30% lower than that in alkanethiol self-assembled monolayers. The surface density of thiolate-Au bonds did not depend on film thickness provided sufficient PMPMS material was present. Differential capacitance measurements show that the effective dielectric barrier presented by PMPMS films under aqueous environments corresponds closely to their physical thickness, with even approximately 1 nm films remaining impermeable to electrolyte species. Modification of the films with an oligoethylene glycol compound was also examined, in anticipation of future applications in label-free, impedance-based biomolecular diagnostics.     

Broadband and band-selective IMPRESS-gHMBC: compensation of refocusing inefficiency with synchronized inversion sweep

Compensation of refocusing inefficiency in a gHMBC experiment by replacing the rectangular pi pulse with a pair of adiabatic pulses with synchronized inversion sweep (CRISIS) significantly improves the performance of the gHMBC experiment. The CRISIS-gHMBC experiment retains the pure absorptive shapes in F1 and hence results in better lineshape and higher resolution than the current versions of magnitude mode gHMBC spectra. When used as a broadband experiment, CRISIS-gHMBC, owing to better refocusing efficiency of the adiabatic pulse pairs, gives improved performance across the 13C spectral width. Moreover, it is shown that CRISIS-gHMBC is a robust and improved alternative and when used along with the IMPRESS (Improved Resolution using Symmetrically Shifted pulses) technique further increases the sensitivity and resolution without additional experimental time. The IMPRESS-CRISIS combination is demonstrated for broadband gHMBC and band-selective gHMBC experiments. The ICbs-gHMBC [IMPRESS-CRISIS-band-selective gHMBC] experiment is an attractive and better alternative to individual band-selective gHMBC.     

Mechanistic studies of nickel(II) alkyl agostic cations and alkyl ethylene complexes: investigations of chain propagation and isomerization in (alpha-diimine)Ni(II)-catalyzed ethylene polymerization

The synthesis of a series of (alpha-diimine)NiR(2) (R = Et, (n)Pr) complexes via Grignard alkylation of the corresponding (alpha-diimine)NiBr(2) precursors is presented. Protonation of these species by the oxonium acid [H(OEt(2))(2)](+)[BAr'(4)](-) at low temperatures yields cationic Ni(II) beta-agostic alkyl complexes which model relevant intermediates present in nickel-catalyzed olefin polymerization reactions. The highly dynamic nature of these agostic alkyl cations is quantitatively addressed using NMR line broadening techniques. Trapping of these complexes with ethylene provides cationic Ni alkyl ethylene species, which are used to determine rates of ethylene insertion into primary and secondary carbon centers. The Ni agostic alkyl cations are also trapped by CH(3)CN and Me(2)S to yield Ni(R)(L)(+) (L = CH(3)CN, Me(2)S) complexes, and the dynamic behavior of these species in the presence of varied [L] is discussed. The kinetic data obtained from these experiments are used to present an overall picture of the ethylene polymerization mechanism for (alpha-diimine)Ni catalysts, including effects of reaction temperature and ethylene pressure on catalyst activity, polyethylene branching, and polymer architecture. Detailed comparisons of these systems to the previously presented analogous palladium catalysts are made.     

Dendritic nanowire ultraviolet laser array

Self-organized dendritic crystal growth is explored to assemble uniform semiconductor nanowires into highly ordered one-dimensional microscale arrays that resemble comb structures. The individual ZnO nanowires have uniform diameters ranging from 10 to 300 nm. They are evenly spaced on a stem with a regular periodicity of 0.1-2 micrometer. Under optical excitation, each individual ZnO nanowire serves as a Fabry-Perot optical cavity, and together they form a highly ordered nanowire ultraviolet laser array.     

Student-Designed Multistep Synthesis Projects in Organic Chemistry

The incorporation of research projects into undergraduate chemistry courses provides a perspective that is fundamentally unavailable in most laboratory experiences. While independent, multistep synthesis projects in organic chemistry have been reported previously, most efforts have been directed at relatively restricted, closely guided research plans with modest student participation in the experimental design. We have implemented a more open-ended synthesis project, limited principally by cost, safety and availability of materials. In the second semester of the sophomore organic sequence, students develop multiple drafts of a plan for a three-to-four-step synthesis. Subsequently, students obtain their own literature protocols for the individual steps. The synthesis is performed over three four-hour laboratory periods. The students conclude this project with a poster presentation of the results at the end of the semester. Evaluation of the students work focuses not only on the successful synthesis of the target but also on planning, troubleshooting, purification, and spectral analysis.     

Some reactions of copper(I) pentafluorothiophenolate as a nucleophile

Several aromatic compounds containing one or two C₆F₅S groups have been prepared by nucleophilic displacement reactions using CuSC₆F₅ in DMF solution. Aromatic iodine or bromine, rather than chlorine of fluorine is replaced by the SC₆F₅ group using CuSC₆F₅. A mechanism is postulated. New compounds prepared include $\text{p}$-(C₆F₆F₅S)₂C₆H₄, $\text{o}$- and $\text{m}$-(C₆F₅S)₂C₆F₄ and $\text{p}$XC₆H₄SC₆F₅(X=C1, NO₂, I, CH₃, CO₂C₂H₅).     

Carbon nanohorns grown from ruthenium nanoparticles

A nanoscale ruthenium/gold bimetallic cluster of clusters has been used as a molecular precursor to produce pure ruthenium nanoparticles (seeds) as catalysts for the growth of carbon nanohorns (CNHs).     

Water adsorption and hydrolysis on molecular transition metal oxides and oxyhydroxides

Addition of water to molecular transition metal oxides (TiO2(g) and CrO3(g)) and oxyhydroxides (ScO(OH)(g), VO2(OH)(g), and MnO3(OH)(g)) was studied by means of quantum chemistry. In the investigated reactions, each reaction step comprised the breaking of one M=O bond and the formation of two OH groups. Exothermicity was observed when the product had tetrahedral or lower oxygen coordination. The reactions were found to involve stable water complexes as intermediates. The stabilities of such complexes were accentuated in the addition reaction Sc(OH)3(g) + H2O(g), in which the formation of a tetrahedral complex was found exothermic. For VO(OH)3(g), CrO2(OH)2(g), and MnO3(OH)(g), water addition to the remaining M=O bonds was found endothermic, whereas the formation of water complexes, using hydrogen bonds and preserving the oxyhydroxide kernel, was preferred. Thus, the sequence of such kernels for water clustering in the investigated reactions was found to be Sc(OH)3.H2O(g), Ti(OH)4(g), VO(OH)3(g), CrO2(OH)2(g), and MnO3(OH)(g). These stability considerations are important, as CrO2(OH)2(g) is believed to be the product of water-induced degradation of the protective chromium oxide scale on stainless steel at elevated temperatures.     

Synthesis and biological activity of novel epothilone aziridines

[reaction: see text]. A series of 12alpha,13alpha-aziridinyl epothilone derivatives were synthesized in an efficient manner from epothilone A. The final semisynthetic route involves a formal double-inversion of stereochemistry at both the C12 and C13 positions. All aziridine analogues were tested for effects on tubulin binding polymerization and cytotoxicity. The results indicate that the aziridine moiety is a viable isosteric replacement for the epoxide in the case of epothilones.