Role of vibrationally excited NO in promoting electron emission when colliding with a metal surface: a nonadiabatic dynamic model

A nonadiabatic quantum dynamic model has been developed to study the process of electron emission from a low-work-function metal surface. The process is initiated by scattering a highly vibrationally excited NO molecule from a surface composed of a Cs layer covering a Ru crystal. The model addresses the increasing quantum yield of the electron emission as a function of the molecular vibrational excitation and incident kinetic energy. The reaction mechanism is identified as a long-range harpooning electron transfer to a molecular ion which is then accelerated toward the surface. Upon impact, the molecular ion emits its excess electron.     

High-throughput screening of enzyme libraries: thiolactonases evolved by fluorescence-activated sorting of single cells in emulsion compartments

Single bacterial cells, each expressing a different library variant, were compartmentalized in aqueous droplets of water-in-oil (w/o) emulsions, thus maintaining a linkage between a plasmid-borne gene, the encoded enzyme variant, and the fluorescent product this enzyme may generate. Conversion into a double, water-in-oil-in-water (w/o/w) emulsion enabled the sorting of these compartments by FACS, as well as the isolation of living bacteria cells and their enzyme-coding genes. We demonstrate the directed evolution of new enzyme variants by screening >10(7) serum paraoxonase (PON1) mutants, to yield 100-fold improvements in thiolactonase activity. In vitro compartmentalization (IVC) of single cells, each carrying >10(4) enzyme molecules, in a volume of <10 femtoliter (fl), enabled detection and selection despite the fast, spontaneous hydrolysis of the substrate, the very low initial thiolactonase activity of PON1, and the use of difusable fluorescent products.     

Analysis of the Micropore Structure of Various Microporous Materials from Nitrogen Adsorption at 77 K

The micropore structure of four microporous materials (two zeolites, ZSM-5 and Y-82; an activated carbon and an alumina pillared clay) and their binary physical mixtures (50-50 wt%) have been examined by nitrogen adsorption at 77 K. Various micropore sizes have been considered from the stages on the micropore filling mechanism in the microporous materials. The application of the Dubinin-Astakhov (DA) equation to characterize and obtain the adsorption potential distributions of the microporous materials is presented.     

Deazapurine solid-phase synthesis: combinatorial synthesis of a library of N3,N5,C6-trisubstituted pyrrolo[3,2-d]pyrimidine derivatives on cross-linked polystyrene bearing a cysteamine linker

Solid-phase methodology for the preparation of pyrrolo[3,2-d]pyrimidine-6-carboxylates with diversity at the N3 pyrmidine nitrogen has now been elaborated to allow for the generation of pyrrolopyrimidine libraries with members possessing diversity at the N3, N5, and C6 positions. The diversification of the N5 position was achieved by treating the parent resin-bound pyrrolo[3,2-d]pyrimidines 3 with an alkyl halide in the presence of Cs2CO3 in DMF. Modification of the C6 carboxylate of resin-bound pyrrolopyrimidines 3-5 was first achieved by hydrolysis of the benzyl ester using LiOH in a mixture of THF/H2O/MeOH. Further alteration of the C6 position of resin-bound pyrrolo[3,2-d]pyrimidine-6-carboxylic acids 6-8 was then performed by activation with triphosgene and treatment with an amine to furnish resin-bound pyrrolo[3,2-d]pyrimidine-6-amides. Twenty-two pyrrolo[3,2-d]pyrimidines 1a-v with different substituents at the N3, N5, and C6 positions were obtained in yields of 21-83% and purities of 61-98% after cleavage from the solid support.     

IR study of heterogeneity of OH groups in zeolite HY-splitting of OH and OD bands

We have been studying the problem of heterogeneity of OH groups in zeolites HY for a long time. The heterogeneity was suggested by the shift of the IR band of OH groups restoring upon ammonia desorption and also by the fact that the band of OH groups forming hydrogen bonds was relatively broad (broader than for homogeneous acidic OH). In the present study we present another important argument for heterogeneity: the splitting of the IR band of free OH and OD groups in a zeolite of Si/Al=8.3 dealuminated by (NH₄)₂SiF₆ treatment. Such a splitting is the best seen in low temperature spectra of OD groups. We found less acidic 3640 cm⁻¹ (AlO)(SiO)₂SiO₁HAl(OSi)₃ and more acidic 3625 cm⁻¹ (SiO)₃SiO₁HAl(OSi)₃ groups. The presence of these two kinds of hydroxyls corresponds to the presence of Si(2Al) and Si(1Al), respectively, detected in ²⁹Si MAS NMR spectra. We also found a small amount of strongly acidic 3599 cm⁻¹ hydroxyls interacting with extraframework Al species.     

The solvation of iodine anions in water clusters: PES studies

We have measured the photoelectron-spectra of I^? (H₂O)_n clusters in the size range n=1–60. We have found that the first six water molecules form a solvation layer with an average 0.35 eV electrostatic stabilization of the anion. At larger cluster sizes the electrostatic stabilization of water does not fit a continuous dielectric solvent. The most stable structures of the clusters consist of internally solvated anions. In the size range n=34–40 we have found evidence for existence of cluster structures with surface solvated anions.     

Ferromagnetism in colloidal Mn2+ -doped ZnO nanocrystals

High-temperature hydrolysis of Zn(II) and Mn(II) alkoxides in a high boiling point solvent in the presence of surfactants was used to prepare surfactant-coated Zn(1-x)Mn(x)O nanocrystals with average size of 5.5 nm and x = 0.04 +/- 0.03. The magnetic properties of the nanocrystals were measured both for isolated particles diluted in a hydrocarbon matrix and for a nanocrystal powder. Nanocrystals of manganese oxide and ZnO coated with manganese oxide were prepared for comparison to the Zn(1-x)Mn(x)O nanocrystals. We find that the manganese ions primarily substitute zinc ions in the hexagonal ZnO lattice, and part of them are ferromagnetically coupled up to room temperature even in isolated noninteracting nanocrystals. The rest of the ions are magnetically disordered or uncoupled. Surprisingly, these small Zn(1-x)Mn(x)O nanocrystals poses relatively large low-temperature magnetic coercivity and relatively high blocking temperature in the isolated form, which indicate large magnetic anisotropy. In the nanocrystal powder the coercive field decreased significantly. This study highlights the advantages of working with noninteracting single domain particles of these intriguing materials.     

Alkyl Ammonium Ion Selectivity of Hexahomotrioxacalix[3]arene Triamide Derivative having the Intramolecular Hydrogen-Bonding Group

The lower rim functionalized hexahomotrioxacalix[3]arene triamide 4 with cone-conformation was synthesized from triol 1 by a stepwise reaction. The different extractability for alkali metal ions, transition metal ions, and alkyl ammonium ions from water into dichloromethane is discussed. Due to the strong intramolecular hydrogen bonding between the neighboring NH and CO groups in triamide 4, its affinity to metal cations was weakened. Triamide 4 shows a single selectivity to n-BuNH ₃ ⁺ . The anion complexation of triamide 4 was also studied by ¹H NMR titration experiments. Triamide 4 binds halides through the intermolecular hydrogen bonding among the NH hydrogens of amide in a 1:1 fashion in CDCl₃. The association constants calculated from these changes in chemical shifts of the amide protons are K _a = 223 M^?1 for Cl^? and K _a = 71.7 M^?1 for Br^?. Triamide 4 shows a preference for Cl^? complexation than Br^? complexation.     

Every monotone graph property has a sharp threshold

In their seminal work which initiated random graph theory Erdös and Rényi discovered that many graph properties have sharp thresholds as the number of vertices tends to infinity. We prove a conjecture of Linial that every monotone graph property has a sharp threshold. This follows from the following theorem. Let denote the Hamming space endowed with the probability measure defined by , where . Let be a monotone subset of . We say that is symmetric if there is a transitive permutation group on such that is invariant under . Theorem. For every symmetric monotone , if then for . ( is an absolute constant.)