Excited-state intramolecular proton transfer in o-hydroxybiaryls: a new route to dihydroaromatic compounds

An excited-state intramolecular proton transfer (ESIPT) from the phenol OH to the 7'-carbon on the naphthyl ring in o-(1-naphthyl)phenol (3) and 1-(1'-naphthyl)-2-naphthol (4) leads to efficient (Phi = 0.1-0.2) formation of the corresponding dihydrobenzoxanthenes (5 and 7) via quinone methide intermediates. This new reaction represents a clean, efficient, and high-yielding route to benzoxanthenes and dihydrobenzoxanthenes. A related ESIPT of similar efficiency has been detected at the 2'-aromatic position in these systems, by deuterium labeling studies.     

Experimental and theoretical charge density study of the neurotransmitter taurine

The experimental electron density distribution in taurine, 2-aminoethane sulfonic acid, 1, has been determined from high-resolution X-ray diffraction data collected at a temperature of 100 K. Taurine crystallizes as a zwitterion in the monoclinic space group P2(1)/c. Topological analysis of the experimental electron density and a comparison with high-level theoretical gas-phase calculations show that the crystal environment has a significant influence on the electronic configuration of the sulfonate moiety in 1, which in the crystal is more delocalized than in the gas phase. This crystal effect is mainly due to hydrogen bonding.     

Anion dependent structures of luminescent silver(i) complexes

The reaction of AgX, where X = trifluoroacetate (CF(3)CO(2)(-), tfa), nitrate (NO(3)(-)), trifluoromethanesulfonate (triflate, CF(3)SO(3)(-), OTf), hexafluorophosphate (PF(6)(-)), or perchlorate (ClO(4)(-)), with 2,2',3' '-tripyridylamine (tpa) yields five novel silver(I) complexes, which have been structurally characterized. The five complexes have the same 1:1 stoichiometry of Ag/tpa but exhibit different modes of coordination, depending upon the counterion present in the compound. Compound 1, [Ag(tpa)(tfa)](n)(), forms a 1D coordination polymer of [Ag(tpa)(tfa)](2) dimer units linked through bridging tfa counterions. Compound 2, [Ag(tpa)(CH(3)CN)(NO(3))](n), forms a zigzag chain 1D coordination polymer exclusively through Ag-N bonds. In compounds 1 and 2, each tpa ligand is bound to two Ag(I) ions via a 2-py and a 3-py group. Compound 3, [Ag(tpa)(OTf)](n), forms a ribbonlike 1D coordination polymer, in which each tpa ligand binds to three different silver centers via all three pyridyl groups, and the counterion remains coordinated to the Ag(I) center. Compounds 4, [Ag(tpa)(CH(3)CN)](n)(PF(6))(n), and 5, [Ag(tpa)(CH(3)CN)](n)() (ClO(4))(n), display ribbonlike structures resembling that of 3, except that the counterions are not coordinated. All complexes are luminescent in acetonitrile solution, with emission maxima in the near-UV region (lambda(max) = 366, 368, 367, 367, and 368 nm for 1-5, respectively). At 77 K, the emission maxima are red-shifted to lambda(max) = 452, 453, 450, 450, and 454 nm for 1-5, respectively.     

Mixed-mode reversed-phase and ion-exchange separations of cationic analytes on polybutadiene-coated zirconia

The retention and selectivity of the chromatographic separation of basic (cationic) analytes on a polybutadiene-coated zirconia (PBD-ZrO2) stationary phase have been studied in greater detail than in previous studies. These separations are strongly influenced by the chemistry of the accessible surface of zirconia. In the presence of buffers which contain hard Lewis bases (e.g., phosphate, fluoride, carboxylic acids) zirconia's surface becomes negatively charged due to adsorption of the buffer anion at the hard Lewis acid sites. Consequently, under most conditions (e.g., neutral pH), cationic analytes undergo both hydrophobic and cation-exchange interactions. This mixed-mode retention process generally leads to greater retention factors for cations relative to those on silica-based reversed phases despite the lower surface areas of the zirconia phase, but, more importantly, adsorption of hard Lewis bases can be used to control the chromatographic selectivity for cationic analytes on these zirconia-based stationary phases. In contrast to our prior work, here we show that when mixed-mode retention takes place, both retention and selectivity are easily adjusted by changing the type of hard Lewis base buffer anion, the type of buffer counter-ion (e.g., sodium, potassium, ammonium), the pH, and the ionic strength of the eluent as well as the type and amount of organic modifier.     

A wide-angle secondary ion probe for organic ion imaging

A secondary ion source has been developed for an organic ion microprobe capable of imaging samples up to 2 em in diameter. The source uses a focused 5 keY Cs⁺ ion beam which is rastered across the sample surface, and secondary ions from each point on the sample are collected and formed into a low energy beam to be analyzed by a quadrupole mass filter. Dynamic emittance matching is employed to deflect ions from off-axis points on the sample back onto the mass analyzer axis. Rastering and dynamic emittance matching are rapidly controlled by assembly language programs using an IBM/AT (80286) type computer. A low energy ion monitor was used to tune and evaluate the secondary ion source by providing a magnified cross-sectional image of the ion beam at the source exit aperture. A well-focused and centered secondary ion beam was obtained from each point on the sample, indicating that large-scale dynamic emittance matching with high collection efficiency is possible. Mass resolved images of grids and glycerol samples are shown to demonstrate the performance of the integrated secondary ion source mass analyzer and control system.     

Free energy perturbation calculations on models of active sites: Applications to adenosine deaminase inhibitors

Free energy perturbation calculations were performed to determine the free energy of binding associated with the presence of perhaps an unusual hydroxyl group in the transition state analog of nebularine, an inhibitor of the enzyme adenosine deaminase. The presence of a single hydroxyl group in this inhibitor has been found to contribute ?9.8 kcal/mol to the free energy of binding, with a 10⁸-fold increase in the binding affinity by the enzyme. In this work, we calculate the difference in solvation free energy for the 1,6-dihydropurine complex versus that of the 6-hydroxyl-1,6-dihydropurine complex to determine if this marked increase in binding affinity is attributed to an unusually hydrophobic hydroxyl group. The calculated ΔG associated for the solvation free energy is ?11.8 kcal/mol. This large change in the solvation free energy suggests that this hydroxyl is instead unusually hydrophilic and that the difference in free energy of interaction for the two inhibitors to the enzyme must be at least ca. 20 kcal/mol. Although the crystal structure for adenosine deaminase is currently not known, we attempt to mimic the nature of the active site by constructing models which simulate the enzyme-inhibitor complex. We present a first attempt at determining the change in free energy of binding for a system in which structural data for the enzyme is incomplete. To do this, we construct what we believe is a minimal model of the binding between adenosine deaminase and an inhibitor. The active site is simulated as a single charged carboxyl group which can form a hydrogen bond with the hydroxyl group of the analog. Two different carboxyl anion models are used. In the first model, the association is modeled between an acetic acid anion and the modified inhibitor. The second model consists of a hydrophobic amino acid pocket with an interior Glu residue in the active site. From these models we calculate the change in free energy of association and the overall change in free energy of binding. We calculate the free energies of interaction both in the absence and presence of water. We conclude from this that the presence of a single suitably placed-CO^?₂ group probably cannot explain the binding effect of the-OH group and that additional interactions will be found in the adenosine deaminase active site.     

Flash photolysis and antioxidant activity

The photo-chemical behaviour of a number of mono- and polyfunctional commercial phenolic antioxidants has been examined using kinetic micro-second flash photolysis. The technique provides useful information on the relationship between antioxidant structure and the efficiency of phenoxy radical production. The kinetics of decay of the phenoxy radicals are also found to be dependent on structure. Mono-functional antioxidants give phenoxy radicals which decay by a second-order process whereas polyfunctional antioxidants give phenoxy radicals that decay by a first-order process. In the former case dimerisation to give bisphenolic coupling products is observed whereas, with the latter, this process is sterically inhibited. The value of flash photolysis as a probe for studying antioxidant activity is discussed.     

Metallcarbonyl-Synthesen,XIV. Neuartige Vanadium-, Niob- und Tantal-Komplexe mit Wasserstoff-Brücken

Syntheses of Metal Carbonyls, XIV. Novel Vanadium, Niobium, and Tantalum Complexes Having Hydrido Bridges The novel homo- and heterodinuclear organometallic μ-hydrido compounds 3a – c of vanadium, niobium, and tantalum have been synthesized by light-induced reactions of the hydridoniobium complex (η⁵-C₅H₅)₂NbH₃ ( 1 ) with the half-sandwich complexes (η⁵-C₅H₅)M(CO)₄ ( 2a : M = V; 2b : M = Nb; 2c : M = Ta). These compounds have the general composition L_xM – H – Nb(CO)(η⁵-C₅H₅)₂. The formation of the M – H – Nb moieties is a dark-reaction preceded by two photoreactions that are independent from each other elimination of CO from 2a – c and elimination of H₂ from 1 , with the extruded carbon monoxide being transfered to the (η⁵-C₅H₅)₂NbH fragment; subsequent fixation of the species (η⁵-C₅H₅)₂Nb(CO)H thus generated to the photogragments (η⁵-C₅H₅)M(CO)₃ results in donor stabilization of these latter groups. The structural architecture of the derivatives 3a und b was established by X-Ray diffraction. The hydrogen bridges are to be considered as three-center two-electron functions that are responsible for a serious lengthening of the otherwise by at least 40 pm shorter metal-to-metal distances amounting to 371.3 pm in 3a and 373.3 pm in 3b (mean values).     

Fe-catalyzed three-component dicarbofunctionalization of unactivated alkenes with alkyl halides and Grignard reagents

A highly chemoselective iron-catalyzed three-component dicarbofunctionalization of unactivated olefins with alkyl halides (iodides and bromides) and sp²-hybridized Grignard reagents is reported. The reaction operates under fast turnover frequency and tolerates a diverse range of sp²-hybridized nucleophiles (electron-rich and electron-deficient (hetero)aryl and alkenyl Grignard reagents), alkyl halides (tertiary alkyl iodides/bromides and perfluorinated bromides), and unactivated olefins bearing diverse functional groups including tethered alkenes, ethers, protected alcohols, aldehydes, and amines to yield the desired 1,2-alkylarylated products with high regiocontrol. Further, we demonstrate that this protocol is amenable for the synthesis of new (hetero)carbocycles including tetrahydrofurans and pyrrolidines via a three-component radical cascade cyclization/arylation that forges three new C–C bonds.

A highly selective iron-catalyzed three-component dicarbofunctionalization of unactivated alkenes with alkyl halides and sp²-hybridized Grignard reagents is reported.     

Facile fabrication of AgCl@polypyrrole-chitosan core-shell nanoparticles and polymeric hollow nanospheres

A one-step sequential method for preparing AgCl@polypyrrole-chitosan core-shell nanoparticles and subsequently the formation of polypyrrole-chitosan hollow nanospheres is reported. The formation of the core and the shell is performed in one reaction medium almost simultaneously. Transmission electron microscopy (TEM) images show the presence of core-shell nanoparticles and hollow nanospheres. Ultraviolet-visible (UV-vis) studies reveal that AgCl was formed first followed by polypyrrole. X-ray diffration (XRD) and UV-vis studies show that AgCl was present in the core-shell nanoparticles and could be removed completely from the core.