Reactions Of Organic Molecules Research Articles

Organic Reactivity in Confined Spaces under Scanning Tunneling Microscopy Control: Tailoring the Nanoscale World

Submolecularly resolved scanning tunnelling microscopy studies of reactions occurring in confined space (i.e., in two dimensions on a surface, see figure) provide genuine insight into the chemical reaction mechanism and offer new ways to achieve greater control over the reactivity of organic molecules.

Water-medium Ullmann reaction over a highly active and selective Pd/Ph-SBA-15 catalyst

A series of Pd/Ph-SBA-15 catalysts with ordered mesoporous structure are designed by using the phenyl-functionalized SBA-15 (Ph-SBA-15) as supports. These catalysts exhibit higher activity and selectivity toward biphenyl than Pd/SiO2, Pd/MCM-41, Pd/Ph-MCM-41 and Pd/SBA-15 in a water-medium iodobenzene Ullmann coupling reaction. The promoting effects on the activity and selectivity could be mainly attributed to the high dispersion of Pd active sites, the ordered mesoporous channels and the strong surface hydrophobocity which facilitate the diffusion and/or adsorption of organic reactant molecules, especially in aqueous media. Meanwhile, the relatively large pore channels are favorable for the coupling of iodobenzene to form biphenyl, which is much bigger than the byproduct benzene resulting from the dehalogenation of iodobenzene. The Pd/Ph-SBA-15 catalyst could be separated easily from reaction products and used repetitively several times, showing the superiority over the homogeneous catalysts for industrial applications.

Organic Chemistry on Solid Surfaces

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

The interaction of benzene with Cu + sites in zeolites : IR studies and DFT quantum chemical calculations

As Cu + ions in zeolites catalyse several reactions of organic molecules, we undertook IR and DFT studies of the interaction of organic molecules with Cu +. Our previous studies had shown that Cu + was able to activate strongly the double and triple bonds in alkenes and acetylene, and less strongly the C O bond in acetone, which resulted in a distinct multiple bond weakening. The DFT calculations provided evidence that this was realized by π back donation of d electrons of Cu + to π * antibonding orbitals of the molecules. The present study concerns the activation of the benzene molecule, which is different from previously studied ones. It is a good electron acceptor, but the LUMO orbital of benzene has only a small antibonding character, therefore the interaction with Cu + was expected to result in a strong adsorption but only slight activation of the C–C bond. Our IR studies have shown, that the C–C bond weakening was indeed very small (Δ ν C–C = 13 cm −1, a value 10 times less than that for alkenes). DFT calculations showed that the molecule acquired a negative charge (−0.06) indicating that π back-donation prevailed over π donation. Both IR and DFT revealed that benzene adsorption on Cu + ions was relatively strong, which resulted in a withdrawing Cu + from the oxygen ring. DFT provided information on the geometry of the Cu +–benzene adduct and evidenced that the interaction of the benzene molecule with Cu + resulted in non-symmetric adsorption. The distances Cu +–C were in the range: 0.203–0.303 nm. The shorter the Cu +–C distance, the more important the bond activation. The benzene molecule interacting with Cu + was no longer flat, some hydrogens were directed towards Cu + and others in the opposite direction.

Kinetics of organic transformations under mild aqueous conditions: implications for the origin of life and its metabolism.

The rates of thermal transformation of organic molecules containing carbon, hydrogen, and oxygen were systematically examined in order to identify the kinetic constraints that governed origin-of-life organic chemistry under mild aqueous conditions. Arrhenius plots of the kinetic data were used to estimate the reaction of half-lifes at 50 degrees C. This survey showed that hydrocarbons and organic substances containing a single oxygenated group were kinetically the most stable; whereas organic substances containing two oxygenated groups in which one group was an alpha- or beta-positioned carbonyl group were the most reactive. Compounds with an alpha- or beta-positioned carbonyl group (aldehyde or ketone) had rates of reaction that were up to 10(24)-times faster than rates of similar molecules lacking the carbonyl group. This survey of organic reactivity, together with estimates of the molecular containment properties of lipid vesicles and liquid spherules, indicates that an origins process in a small domain that used C,H,O-intermediates had to be catalytic and use the most reactive organic molecules to prevent escape of its reaction intermediates.

Influence of Temperature on Photoinduced Reactions of Organic Molecules in the Gas Phase

The influence of temperature on the rate constants of photoinduced intermolecular electron transfer, representing the first stage of photoinduced reactions, has been investigated based on analysis of the quenching of fluorescence of carbazole vapor by halomethanes (CHCl3, CH2Br2, CCl4, CHBr3) and delayed fluorescence of benzophenone and anthraquinone vapors by aliphatic amines (diethylamine, dibutylamine, cyclohexylamine, triethylamine) and pyridine. It has been established that the rate constants of photoinduced electron transfer in different donor-acceptor pairs in the gas phase can increase or decrease with increase in the temperature from 433 to 623 K. The energies of activation and enthalpy of the fluorescence-quenching process have been determined. The interrelation between the rate constants of fluorescence quenching kq and the free energy of electron transfer ΔGET has been analyzed with account for the mean vibrational energy of the interacting molecules. It is shown that positive and negative temperature dependences kq(T) are characteristic, respectively, of the regions of normal (kET increases with decrease in ΔGET) and inverted (kET decreases with decrease in ΔGET) changes in the rate constants caused by an increase in the exothermicity of the photoinduced electron transfer process.

A Classical Approach to Aromaticity and a New Representation of Benzene

AbstractFor Abstract see ChemInform Abstract in Full Text.

A classical approach to aromaticity and a new representation of benzene

The classical chemical structure theory (CST) provides “localised” qualitative models for the structure and reactivity of organic molecules, while, in addition to quantitative methods, the modern molecular orbitals (MO) theory provides “delocalised” qualitative models. In particular, these two theories provide two different descriptions of the ethylenic double bond, bent bonds and σ/π orbitals, respectively. In the framework of CST, a graphical model of aromaticity and antiaromaticity, related to a classical model of Woodward–Hoffmann rules, is presented. By application of Linnett's double-quartet theory, a new representation for benzene is proposed. It can be easily visualised when a new split representation of Linnett-type formulae is used.

Potassium ferrate [Fe(VI)] does not mediate self-sterilization of a surrogate mars soil

BackgroundMartian soil is thought to be enriched with strong oxidants such as peroxides and/or iron in high oxidation states that might destroy biological materials. There is also a high flux of ultraviolet radiation at the surface of Mars. Thus, Mars may be inhospitable to life as we know it on Earth. We examined the hypothesis that if the soil of Mars contains ferrates [Fe(VI)], the strongest of the proposed oxidizing species, and also is exposed to high fluxes of UV radiation, it will be self-sterilizing.ResultsUnder ambient conditions (25°C, oxygen and water present) K2FeO4 mixed into sand mineralized some reactive organic molecules to CO2, while less reactive compounds were not degraded. Dried endospores of Bacillus subtilis incubated in a Mars surrogate soil comprised of dry silica sand containing 20% by weight K2FeO4 and under conditions similar to those now on Mars (extreme desiccation, cold, and a CO2-dominated atmosphere) were resistant to killing by the ferrate-enriched sand. Similar results were observed with permanganate. Spores in oxidant-enriched sand exposed to high fluxes of UV light were protected from the sporocidal activity of the radiation below about 5 mm depths.ConclusionBased on our data and previously published descriptions of ancient but dormant life forms on Earth, we suggest that if entities resembling bacterial endospores were produced at some point by life forms on Mars, they might still be present and viable, given appropriate germination conditions. Endospores delivered to Mars on spacecraft would possibly survive and potentially compromise life detection experiments.

Chemical ionization of amino and hydroxy group containing arylalkyl compounds with ions in a nitromethane plasma

The gas-phase ion/molecule reactions of organic molecules containing several functional groups, including amino, hydroxy and carboxy groups, have been investigated under nitromethane chemical ionization conditions. Three main reaction channels are observed in the ion source: (a) proton transfer, (b) electron transfer and (c) hydride abstraction. The product ion [M+NO] + is also formed, but in a very low abundance. Initial electrophilic attack of the nitrosonium ion on the aromatic ring is postulated to explain the elimination of HNO from the [M+NO] + adduct ions, observed for all substrates studied. Elimination of water is a characteristic fragmentation pathway for all substrates possessing a benzylic hydroxy group. Fragment ions resulting from cleavage of the molecular ions of the amines, formed by charge transfer, react with the neutral molecules forming two types of adduct ions: [M+immonium] + and [M+C 7H 7] +, which have been characterized through the study of their unimolecular decompositions. The latter provide strong evidence for the existence of two types of structures: a covalent and an ion/molecule complex, that is a non-covalent structure.

Complexation with albumins of chiral aromatic substrates and their chemistry in ground and excited states. Catalytic and chirality recognition properties of the protein in the cases of binaphthol, its photoisomers, and ketoprofen.

The reactivity of organic molecules can be modified upon complexation with proteins: these changes can be different and more significant when the substrate is in an electronically excited state. Here we review UV, CD, and fluorescence spectroscopy studies on the photochemistry and on the chemistry of atropisomeric binaphthols and of ketoprofen, complexed to serum albumins. The chemical and photochemical properties of the organic substrates, complexed to the albumins or free in common solvents, are different. The role of the protein complexation is also evidenced in photoresolution processes of racemate-protein complexes. Catalytic effects due to serum albumins are also reported. In particular, the Arrhenius parameters for the rate of thermal isomerization of a metastable photoproduct of binaphthol in common solvents are compared with those of the bovine serum albumin catalyzed isomerization.

Carbon chains and rings in the laboratory and in space

Seventy-seven reactive organic molecules of astrophysical interest have been identified in a supersonic molecular beam, 73 in the radio band by Fourier-transform microwave spectroscopy, four in the optical by laser cavity ringdown spectroscopy. Most are linear carbon chains, but six consist of carbon chains attached to the compact, highly polar C 3 ring, and two are rhomboidal cyclic configurations of SiC 3. The laboratory astrophysics of the radio molecules is complete for the time being, in the sense that essentially all the rotational transitions of current interest to radio astronomy (including hyperfine structure when present) can now be calculated to a small fraction of 1 km s -1 in equivalent radial velocity; six of the radio molecules have already been detected in space on the basis of the present data. The FTM spectrometer employed in this work is far from fundamental limits of sensitivity, so many more molecules can probably be found by refinements of present techniques. The density of reactive molecules in our supersonic beam is generally high by the standards of laser spectroscopy, and many of the radio molecules probably have detectable optical transitions which we are attempting to find, largely motivated by the long-standing problem of the diffuse interstellar bands. Our most interesting result to date is the detection of a fairly strong molecular band at 443 nm in a benzene discharge, in exact coincidence with the strongest and best known interstellar band. Isotopic shifts measured with partially and totally deuterated benzene suggest that the carrier of the laboratory band is a hydrocarbon molecule with the elemental formula C n H 5, with n most likely in the range 3–6.

Vapor phase transport synthesis of zeolites from sol–gel precursors

A study of zeolite crystallization from sol–gel precursors using the vapor phase transport synthesis method has been performed. Zeolites (ZSM-5, ZSM-48, zeolite P, and sodalite) were crystallized by contacting vapor phase organic or organic–water mixtures with dried sodium silicate and dried sodium alumino-silicate gels. For each precursor gel, a ternary phase system of vapor phase organic reactant molecules was explored. The vapor phase reactant mixtures ranged from pure ethylene diamine, triethylamine, or water, to an equimolar mixture of each. In addition, a series of gels with varied physical and chemical properties were crystallized using the same vapor phase solvent mixture for each gel. The precursor gels and the crystalline products were analyzed via scanning electron microscopy, electron dispersive spectroscopy, X-ray mapping, powder X-ray diffraction, nitrogen surface area, Fourier transform infrared spectroscopy, and thermal analyses. The product phase and purity as a function of the solvent mixture, precursor gel structure, and precursor gel chemistry is discussed.

Academy selects chemists for honors in 1999

Among the 17 individuals the National Academy of Sciences will honor in 1999 outstanding contributions to science are five whose work involves some aspect of chemistry. The recipient of the NAS Award in Chemical Sciences is John D. Roberts, Institute Professor of Chemistry, emeritus, and lecturer at the Gates & Crellin Laboratories of Chemistry at California Institute of Technology. Roberts was chosen for defining modern physical organic chemistry—the integration of physical chemistry and organic synthesis applied to the study of the relations between the structure and reactivity of organic molecules. The award, a medal and prize of $20,000, is presented annually. C. Grant Willson will receive the NAS Award Chemistry in Service to Society. Willson, professor and Rashid Engineering Regents Chair in the department of chemistry and chemical engineering at the University of Texas, Austin, was chosen for his fundamental contributions to the chemistry of materials that produce micropatterns in semico...

Controlling Microscopic Surface Structure, Crystalline Size and Crystallinity of CdS and ZnS Nanocrystallites

Size controlled CdS and ZnS nanocrystallites with well-defined crystallinity were simply prepared by reacting M2+ (M=Cd2+ or Zn2+) and S2- (H2S or Na2S) in N,N-dimethylformamide (DMF) without adding any surface stabilizers. The mean diameter of CdS nanocrystallites can be rigorously controlled over a range from 1.8 to 4.2 nm maintaining the hexagonal crystalline structure by varying the sulfur source and by regulating the preparation temperature. The band-gap dependence estimated from the absorption peak wavelength on the size of the hexagonal CdS nanocrystallites was in good agreement with the pseudopotential calculation using lattice parameters of hexagonal CdS. In the system of ZnS nanocrystallites, crystallinity was controlled by changing the surface modifying reagent. Reactive organic molecules, which can bind onto the surface of ZnS nanocrystallites, induce phase transition of ZnS nanocrystallites from the hexagonal to the cubic phase, maintaining a crystalline size of 3 nm at ambient temperature and pressure.

Molecular-Sieving Gas Sensor Prepared by Chemical Vapor Deposition of Silica on Tin Oxide Using an Organic Template

Abstract A tin(IV) oxide surface was modified by the chemical vapor deposition (CVD) of tetramethoxysilane after the pre-adsorption of benzoate anion as a template to obtain a novel gas sensor with a molecular-sieving function. The sensitivity to large molecules was strongly suppressed by the modification, whereas the sensitivity to small molecules was relatively less modified. It is considered that the silica overlayer with pores whose size was controlled by the template molecule prevented large molecules from reacting with the exposed tin oxide surface, resulting in the molecular-sieving property. The catalytic activity for the oxidation of the alcohol was measured in order to support the idea that the reactivity of organic molecules was suppressed by silica.

Applications of self‐assembled monolayers in electroanalytical chemistry

AbstractThe different approaches in which self‐assembled monolayers (SAMs) have been applied in electroanalytical chemistry are reviewed. Three principal concepts have been used and can be categorized according to the mechanism of attachment of the monolayer to the electrode. These include the formation of covalent bonds between reactive organic molecules, such as chlorosilanes, with activated surfaces, the formation of lipid and bilayer lipid membranes that self‐assemble on conductive surfaces and the adsorption of functionalized alkanes, e.g., alkanethiols, on well ordered surfaces. Systems for detecting inorganic, organic and biological species are described and conclusions are drawn.

Spectroscopic studies on organic molecules intercalated into clays

An overview of clay structures and characterization methods is followed by a discussion of some adsorption reactions of organic molecules which have been studied spectroscopically. These can be divided into two main classes: adsorption with and without reaction.

Metal carbonyl anion loaded molecular reservoir particles — application to the catalytic synthesis of two styrene oxide derivatives

Molecular reservoirs consisting of a porous polydivinylbenzene (PDVB) nutshell surrounding polytriethylvinylbenzyl ammonium chloride (PEVAC) chains were employed to immobilize the homogeneous catalysts: [Co(CO) 4] − or [Rh(CO) 2I 2] −. The molecular reservoirs were prepared by the successive polymerization of vinylbenzyl chloride (VBC) and divinylbenzene (DVB) in a concentrated emulsion, followed by the quaternization of the encapsulated VBC chains. The immobilization was achieved via the formation of ion-pairs between the ammonium cations and the metal carbonyl anions. Two kinds of polymer-bound catalysts dispersed in water were used to catalyze the synthesis of two derivatives of styrene oxide in the presence of CO. It was found that when PDVB (PEVAC)-[Co(CO) 4] − was used, 2,5-dihydro-2-oxo-3-hydroxy-4-phenylfuran was the main product, as in the homogeneous case. When, however, PDVB(PEVAC)-[Rh(CO) 2I 2] − was employed, 2-phenyl-2-(α-styryoxyl)ethanol was obtained as the main product; in the homogeneous case, the selectivity was much lower. Both polymer-bound catalysts were used in five successive cycles at 200 psi CO and 95–98°C, and the conversion of styrene oxide was almost quantitative. The cocatalyst methyl iodide was introduced before the first cycle only. It was found that 72% of the initial Co and 94% of the initial Rh were still present in the molecular reservoirs after the five cycles. In addition to immobilizing the homogeneous catalyst, the molecular reservoir PDVB (PEVAC) also possesses some adsorption capability for the organic reactant molecules, because of its hydrophobic porous PDVB shell. This hydrophobic adsorption capability allows the styrene oxide molecules to approach the encapsulated catalytically active sites.

Calculated infrared spectra of unstable organic molecules and reactive intermediates

The use of calculated IR spectra (ab initio) in the identification of highly reactive organic molecules and intermediates has proven to be a very useful tool to the experimental synthetic organic chemist. The results of recent collaborations between the authors and experimental groups are presented. These include the identification of o-benzyne, cyclopentadienylideneketene, and trimethylenemethane prepared photochemically in low-temperature matrices. In the first two examples, it is shown how the very useful experimental technique of obtaining the dichroic IR spectra of medium-sized molecules greatly aided in the interpretation of the experimental IR spectra. © 1995 John Wiley & Sons, Inc.