Structure and reactivity of the calcite-water interface

The zetapotential of calcite in contact with aqueous solutions of varying composition is determined for pre-equilibrated suspensions by means of electrophoretic measurements and for non-equilibrium solutions by means of streaming potential measurements. Carbonate and calcium are identified as charge determining ions. Studies of the equilibrium solutions show a shift of isoelectric point with changing CO(2) partial pressure. Changes in pH have only a weak effect in non-equilibrium solutions. The surface structure of (104)-faces of single crystal calcite in contact to solutions corresponding to those of the zetapotential investigations is determined from surface diffraction measurements. The results reveal no direct indication of calcium or carbonate inner-sphere surface species. The surface ions are found to relax only slightly from their bulk positions; the most significant relaxation is a ～4° tilt of the surface carbonate ions towards the surface. Two ordered layers of water molecules are identified, the first at 2.35±0.05? above surface calcium ions and the second layer at 3.24±0.06? above the surface associated with surface carbonate ions. A Basic-Stern surface complexation model is developed to model observed zetapotentials, while only considering outer-sphere complexes of ions other than protons and hydroxide. The Basic-Stern SCM successfully reproduces the zetapotential data and gives reasonable values for the inner Helmholtz capacitance, which are in line with the Stern layer thickness estimated from surface diffraction results.     

Investigation of the reactivity and tautomerism of azolidines

The reaction of 2-alkylrhodanines and aromatic amines was studied, and 2-(alkylimino)- and 2-(arylimino)-4-thiazolidones were obtained. It was shown by means of IR spectroscopy that the investigated compounds exist primarily in the imino form; the amino form predominates only for compounds with strong electron-donor substituents.See [I] for communication IX.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 775–779, June, 1972.     

Chemical reactivity of the compressed noble gas atoms and their reactivity dynamics during collisions with protons

Attempts are made to gain insights into the effect of confinement of noble gas atoms on their various reactivity indices. Systems become harder, less polarizable and difficult to excite as the compression increases. Ionization also causes similar effects. A quantum fluid density functional technique is adopted in order to study the dynamics of reactivity parameters during a collision between protons and He atoms in different electronic states for various projectile velocities and impact parameters. Dynamical variants of the principles of maximum hardness, minimum polarizability and maximum entropy are found to be operative.     

Study on the aromaticity and reactivity of chlorophosphinines

Aromaticity of phosphinine derivatives, such as 2-, 3-, and 4-chlorophosphinines and 4-chloro-3-methyl-and 3,5-dimethylphosphinine, has been investigated by means of quantum-chemical calculations and photoelectron spectroscopy. The position of the π-bands in the photoelectron spectra of 4-chloro-3-methyl- and 3,5-dimethylphosphinine indicated that the aromaticity of these species is similar to that of the corresponding benzene derivatives. The MP2/6-31G* and the HF/3-21G(*) geometries of the chlorophosphinines, together with the isodesmic reaction energies and Mulliken charge distributions, showed that the electronic system of phosphinine is not significantly perturbed by the chloro-substitution. 3-methyl-4-chlorophosphinine, similarly to 2-chlorophosphinine, was inert toward nucleophiles even under forced conditions. This unreactivity can more likely be rationalized by kinetic considerations than by thermodynamic arguments (such as increased aromatic stabilization of the chlorophosphinines).     

Synthesis and reactivity of azapalladacyclobutanes

N-Sulfonyl aziridines undergo oxidative addition to palladium(0) complexes generated in situ from mixtures of Pd2(dba)3 and 1,10-phenanthroline. The resulting azapalladacyclobutane complexes undergo intramolecular carbopalladation in the presence of copper(I) iodide to afford azapalladabicyclo[3.2.1]octanes. A deuterium-labeling experiment indicates that the oxidative addition proceeds via SN2-type attack of palladium(0) on the less-hindered carbon of the aziridine ring and that alkene insertion occurs in a syn fashion. The azapalladabicyclo[3.2.1]octane complexes undergo oxidative palladium-carbon bond functionalization in the presence of copper(II) bromide.     

Tautomerism and dual reactivity

Summary Dual reactivity in substances is due to the tautomerism of molecules in accordance with Scheme I or (particularly in cases previously assigned to the field of pseudomerism) to the tautomerism of intermediate ions and radicals in accordance with Scheme III in, in which the tautomeric forms (molecules, ions, radicals) have separate existences. These particles, which exist separately in a state of equilibrium reversible isomerization, can give two series of derivatives under the appropriate conditions, and this is confirmed by physical investigation of the matter [12].This paper is published as a polemic contribution.     

Synthesis and reactivity of trans-Tricyclo

The first synthesis of trans-tricyclo[4.2.0.0(1,3)]oct-4-ene (1), an ethenyl bridged spirohexane, was accomplished in four steps starting from Carpino et al. gem-dichloro ketone 6. An X-ray crystal structure of 1 with one substituent was obtained to provide geometry data on this novel ring system and to confirm the stereochemical assignment of the penultimate synthetic intermediate. Tricyclo[4.2.0. 0(1,3)]oct-4-ene is surprisingly stable. It reacts with glacial acetic acid but only slowly at 145 degrees C; the products were isolated and identified. A unimolecular rearrangement takes place at elevated temperatures (165 degrees C and higher), presumably, via a biradical intermediate to afford tricyclo[4.2.0.0(1,5)]oct-3-ene (23). The structure of this 1,5-bridged bicyclo[2.1.0]pentane derivative was established by NMR and an X-ray crystal structure of its Diels-Alder adduct with isobenzofuran. Tricyclo[4.2.0.0(1, 3)]oct-4-ene equilibrates with 23, so equilibrium constants and reaction rates were measured over a 20 degrees C temperature range from 180 degrees C to 200 degrees C. The difference in the heats of formation (DeltaDeltaH degrees (f) (23 - 1)) is -2.1 kcal/mol, which is in good agreement with ab initio (HF and MP2) calculations using the 6-31G(d) basis set (-1.9 (HF) and -1.4 (MP2) kcal/mol). Computations on trans-tricyclo[4.2.0.0(1,3)]octane and spirohexane also were carried out, and the structures and energies were compared.     

Predicting the reactivity of ambidentate nucleophiles and electrophiles using a single, general-purpose, reactivity indicator

We recently proposed a new reactivity indicator, termed the "general-purpose reactivity indicator", Xi, which describes not only the classical reactivity paradigms, but also describes reactions that are neither frontier-orbital nor electrostatically controlled. This indicator was proposed to be especially useful for reactants with multiple reactive sites, especially if the nature of the reactivity at those sites was different. This suggests that this reactivity indicator is especially appropriate for ambidentate molecules; this paper confirms this hypothesis. The general-purpose reactivity indicator not only identifies the most reactive sites, it also identifies which substrates prefer which reactive sites. In particular, the reactivity indicator allows one to clearly distinguish which sites of an ambidentate molecule are most reactive when electron transfer from the attacking reagent is large (a soft reagent) and which sites are most reactive when the attacking reagent is hard and highly charged (so that electron transfer is relatively insignificant). To illustrate the efficacy of the indicator for nucleophiles we consider SCN(-), SeCN(-), NO(2)(-), SO(3)(2-). For electrophiles we consider dimethyl carbonate, N-methyl-N-nitrosotoluene-p-sulfonamide (MNTS), and 1-chloro-2,4,6-trinitrobenzene (CNB).     

Stereochemistry and reactivity of the HPA-imine Mannich intermediate

Homophthalic anhydride (HPA) typically reacts rapidly with benzalimines to afford the formal [4+2] adduct, a 1,2,3,4-tetrahydroisoquinolin-1-one-4-carboxylic acid. The stereochemical outcome of this reaction is consistent with an open transition state comprising an iminium species and enolized HPA, leading to a short-lived amino-anhydride intermediate. In the case of N-tert-butylbenzalimine, this Mannich-type intermediate, which would normally cyclize at low temperature to a single isomer of the delta-lactam, is intercepted by base treatment to afford beta-lactam products. A pathway featuring ketene formation followed by ring closure is implicated.     

Increasing the reactivity of nitrogen catalysts

This review article presents how nitrogen-centred Lewis bases were modified in order to increase their reactivity in catalytic processes. As examples, we focus on alcohol acylation and Morita-Baylis-Hilman reactions in order to showcase the fundamental parameters at play in transformations initiated by catalysts bearing respectively an active sp(2) or sp(3) nitrogen atoms. These two aspects are epitomised by two leading compounds, the Steglich base 4-dimethylaminopyridine (DMAP), and 1,4-diazabicyclo[2.2.2]octane (DABCO). Throughout this review, we stress the role played and the information brought by physical organic chemistry. Comprehension of these complex transformations relies on the fundamental knowledge of parameters, such as, nucleophilicity, nucleofugality, Lewis basicity, and crucially also the knowledge of their divergent impacts on each elementary step of the catalytic cycle.     

Synthesis and reactivity of the Tl1 salts of

Synthons Tl1[TCNE]*- (1) and Tl12[TCNE]2- (2), for [TCNE]*- and [TCNE]2-, respectively, in metathesis reactions have been quantitatively prepared and characterized. The structure of 1 was solved and refined in a monoclinic unit cell at 27 degrees C [C2/c, a = 12.6966 (12) angstroms, b=7.7599 (7) angstroms, c=15.5041 (15) angstroms, beta = 96.610 (5) degrees , V= 1517.4 (2) angstroms3, Dcalcd = 2.911 gcm-3, Z=8, R1 = 0.0575, omegaR2=0.0701] and exhibits nuCN absorptions at 2,191 (s) and 2,162 (s) cm-1 consistent with metal-bound [TCNE]*-. The structure of 1 consists of a distorted square antiprismatic octacoordinate Tl1 bound to six monodentate [TCNE]*-s with TlN separations ranging from 2.901 to 3.171 angstroms averaging 3.020 angstroms, and one bidentate [TCNE]*- with TlN separations averaging 3.279 angstroms. The TlN bonding is attributed to electrostatic bonding. The [TCNE]*-s form dimerized zigzag chains with intra- and interdimer separations of 2.87 and 3.29 angstroms, respectively. The tight pi-[TCNE](2)2- dimer is diamagnetic and has the shortest intradimer [TCNE]*- distance reported. These synthons for [TCNE]*- and [TCNE]2- in metathesis reactions lead to the precipitation of, for example, TlIX (X = Cl, Br, OAc). Reaction of 1 with MnIII(porphyrin)X (X = Cl, OAc) forms the molecule-based magnets of [MnIII(porphyrin)][TCNE] composition, while the reaction of [CrI(C6H6)2]Br and (Me2N)2CC(NMe2)2Cl2, [TDAE]Cl2, with 1 forms [CrI(C6H6)2] [TCNE] and [TDAE][TCNE]2, respectively. The structure of [TDAE][TCNE]2.MeCN was solved and refined in an orthorhombic unit cell at 21 degrees C [I222, a = 10.2332(15), b = 13.341(6), c = 19.907(8) angstroms, V= 2717.7 angstroms3, Z = 4; Dcalcd = 1.216 gcm-3, R=0.083, Romega = 0.104] and exhibits upsilonCN absorptions at 2,193 (m), 2,174 (s), and 2,163 (s) cm-1 consistent with isolated [TCNE](2)2- , in contrast to the aforementioned TlI bound [TCNE](2)2-. The reaction of 2 with [TDAE]Cl2 forms [TDAE]2+[TCNE]2-.     

Exploring and exploiting the reactivity of glucuronic acid donors

The relative reactivity of glucuronic acid esters was established in a series of competition experiments, in which two thioglucoside and/or thioglucuronic acid ester donors competed for a limited amount of activator (NIS-TfOH). Although glucuronic acid esters are often considered to be of very low reactivity, the series of competition reactions revealed that the reactivity of the glucuronic acid esters studied is sufficient to provide productive glycosylation reactions. The latter is illustrated in the synthesis of two Streptococcus pneumoniae trisaccharides, in which the applicability of the two similarly protected frame-shifted thiodisaccharide donors, Glc-GlcA and GlcA-Glc, were compared. The Glc-GlcA disaccharide, featuring the glucuronic acid donor moiety, proved to be the most productive in the assembly of a protected S. pneumoniae trisaccharide.     

Study on the structure and reactivity of COREX coal

COREX is the primary process in the current smelting reduction method. The process has strict coal quality standards. Combustion processes of coal used in the COREX operating system were analyzed using a synchronous thermogravimetric analyzer combined with a mass spectrometer. The microcosmic structure and macerals were observed by an electronic scanning microscope. The qualitative and quantitative determinations of oxygen functional groups, such as phenolic hydroxyl, carboxyl, carbonyl, and methoxy groups were detected by the Fourier Transform Infrared spectrometer (FT-IR) and through chemical analysis methods. In addition, the evolution of the chemical structure and transformation mechanism of organic oxygen functional groups during COREX coal combustion have been thoroughly investigated. This study proposes a new coal-requirement index system and coal blending method, which will increase the expansion of coal selection and decrease the overall usage of coal during COREX.     

Synthesis and reactivity of aminofuroxans

Chemistry of Heterocyclic Compounds - In this review article, we provide general information on the known methods for directed synthesis of 3- and 4-aminofuroxans, as well as analyze the main...     

Preparation and reactivity of metal-containing monomers

Modified polystyrene and polyacrylonitrile possessing cluster fragments in polymer chains have been obtained by radical copolymerization of styrene or acrylonitrile with triosmium carbonyl cluster complexes having double bonds capable of polymerization in their ligand surroundings. The thermal stability of the copolymers produced has been studied by DTG analysis. The temperature increase characteristic of both polymer and cluster pyrolysis has been observed for clusters containing polystyrene. The cluster complexes have been also shown to influence to a great extent the destruction of polyacrylonitrile, thus preventing the formation of stable fused structures in polymer chains.     

Preparation and reactivity of metal-containing monomers

We have observed the phenomenon of thermally initiated frontal polymerization of acrylamide complexes of nitrates of Mn(II), Co(II), Ni(II), and Zn(II). The velocity of propagation of the polymerization front when using different complexes is (2–9)·10^–2 cm/sec and decreases in the sequence of nitrates: Co(II) > Ni(II) > Mn(II) > Zn(II). Analogous complexes of Cu(II) and Fe(III) under the studied conditions are not polymerized. We have shown that the frontal polyerization products are three-dimensional polymers which are insoluble in organic solvents and slowly swell in water.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 768–772, April, 1990.We thank I. N. Ivlev for making the magnetic susceptibility measurements, and T. I. Ponomarev for studying the thermomechanical properties of the frontal polymerization products; and we acknowledge S. P. Davtyan for discussion of the results obtained.     

Preparation and reactivity of metal-containing monomers

Metal-containing monomers based on vinyl derivatives of heterocycles (4-vinylpyridine, 3,5-dimethyl-1-vinylpyrazole, 2-methyl-5-vinyltetrazole) and NiCl₂, CoCl₂, CrCl₃ have been prepared and characterized by chemical analysis, IR and electron spectroscopy. By means of spectral methods, the complex formation has been shown to involve N atom of the heterocycle, while the exocyclic C = C bond does not participate in the process. The structure of polyhedrons formed has been characterized both in the solid state and in methanol or DMF. The comparison of splitting parameters for different symmetries indicates that in passing from five-membered to six-membered vinylheterocycles, the strength of the ligand field is greatly enhanced.For part 24, seeIzv. Akad. Nauk, Ser. Khim., 1992, 2066 [Bull.Russ.Acad.Sci.Div.Chem.Sci., 1992,41, 1609].Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 76–80, January, 1993.     

Structure and reactivity of GaAs surfaces

First the analytical tools, preparation methods and surface crystallography of clean GaAs surfaces are briefly reviewed. Besides the usual methods of cleaving, ion bombardment and annealing, molecular beam epitaxy is mainly used as a growth method under UHV conditions, and has brought a manifold of differently reconstructed structures on the same crystallographic surface, depending on the exact experimental conditions during growth. Quantitative analysis of the surface composition by AES gives the result that these structures differ only in the amount of As in the topmost layer. From the combination of theoretical LEED analysis, UPS results and arguments considering the different physicochemical nature of Ga and As atoms, rehybridisation of the surface atomic bonds emerges as the driving force for reconstruction: the surface Ga atoms try to assume a trivalent planarsp² and the As atoms a trivalentp³ configuration with three mutually perpendicularp-bonds. The better this rehybridised configuration can be achieved, the better is the chemical stability of the respective structure. The sticking coefficient for oxygen, although generally low, thus varies between ～10^-4 and <10^-9, depending on the crystallographic surface and, on the same surface, on the degree of surface bond saturation given by the respective structure. However, it emerges that, at least on As-depleted polar surfaces, adsorption proceeds via a mechanism of removal of Ga atoms during exposure and adsorption on the defect sites created in this way. The existence of such a complicated mechanism is consistent with the difficulties arising with the preparation of thick stoichiometric oxide layers, the preparation methods and properties of which are reviewed briefly in the last section.