Effect Of Halogen Research Articles

Aluminum Foils: The Contrasting Characters of Hyperconjugation and Steric Repulsion in Aluminum Dimetallocenes

The novel sandwich complex Cp2*Al2I2, which was recently synthesized by Minasian and Arnold, has been characterized using ab initio and density functional methods. A large family of related compounds was also investigated. Although a few Al(II)–Al(II) bonds are known, this is the first such bond to be supported by Cp-type ligands. In addition, in the remarkable Cp4*Al4 synthesis by Roesky, Cp2*Al2I2 is the Al(II) intermediate; Cp4*Al4 is important as a precursor to novel organoaluminum species. Halogen and ligand effects on the Al–Al bond in Cp2*Al2I2 were systematically explored by studying a series of 20 Cp2*Al2I2 derivatives using density functional theory with relativistic basis sets for the halogens. Comparison was made with the focal point treatment, which uses extrapolation to estimate the full configuration interaction and complete basis set limit energy. Torsional potential energy curves, natural population analyses, and enthalpies of hydrogenation were computed. Using the focal point approach, torsional barriers were computed with 0.05 kcal mol(–1) uncertainty. The interplay of steric and electronic effects on the torsional potential energy curves, enthalpies of dehydrogenation reactions, and geometries is discussed. In species with small ligands (R = H, Me), hyperconjugative effects determine the torsional landscape, whereas steric repulsions dominate in species with Cp* alkyl ligands. Species with Cp ligands represent an intermediate case, thus providing insight into how ligands modulate the structures and properties of small metal clusters.

Rationalizing the Effect of Halogenation on the Molecular Structure of Simple Cyclobutene Derivatives by Topological Real-Space Analysis of Their Electron Density

The accurate gas-phase equilibrium structures on the ground-state potential energy surface of the complete series of fluorinated and chlorinated cyclobutene derivatives with C(2v) symmetry have been evaluated at DFT PBE0/6-311++G(d,p) theory level. The optimized geometries have been compared with all the available experimental data reported in the literature, as obtained by microwave spectroscopy (MW) and gas-phase electron diffraction (GED) techniques. For hexafluorocyclobutene and 1,2-dichloro-3,3',4,4'-tetrafluorocyclobut-1-ene, the results of accurate low-temperature single-crystal X-ray diffraction experiments have also been considered. Structural changes within the cyclobutene ring, as induced by fluorination and chlorination at allylic and vinylic positions, have been correlated with changes in the corresponding theoretical charge densities. To this aim, several local and nonlocal topological descriptors provided by the quantum theory of atoms in molecules, QTAIM, have been employed, with particular emphasis on the delocalization indices and integrated source function decomposition schemes. Key factors for the resulting molecular structures are the chemical nature and the steric hindrance of the substituents, as well as quantum-mechanical effects, such as delocalization and partial conjugation. When fluorine atoms replace hydrogens at allylic or vinylic positions, the corresponding Csp(3)-Csp(3) or Csp(2)═Csp(2) bonds between the substituted carbons undergo a significant strengthening, while chlorination has just the opposite effect. In the latter case the steric hindrance between bulky chlorine atoms occupying vicinal positions is crucial in determining the single Csp(3)-Csp(3) bond length. These findings are discussed in the context of the reactivity of chemically related chlorofluorocarbon compounds.

Formation of a Protective Alumina Scale on Ni-Base Superalloys by Using the Halogen Effect

A new method is proposed to achieve a dense protective alumina scale for Ni-base superalloys with an Al-content lower than 10 wt.% at temperatures above 1000°C. The method is based on the halogen effect. Thermodynamical calculations show the existence of a region for a positive fluorine effect at temperatures between 900-1200°C for the alloys IN738 and IN939. By using fluorine ion implantation in combination with Monte Carlo simulation of the fluorine profiles these results were transformed into a region of F-concentrations at the metal surface. A dense protective alumina scale was formed for IN738 after oxidation at 1050°C. Due to the very low Al-content no alumina scale was found for IN939.

Comprehensive Core-Level Study of the Effects of Isomerism, Halogenation, and Methylation on the Tautomeric Equilibrium of Cytosine

Core-level X-ray photoemission and near-edge X-ray absorption fine structure spectra of 5-methylcytosine, 5-fluorocytosine, and isocytosine are presented and discussed with the aid of high-level ab initio calculations. The effects of the methylation, halogenation, and isomerization on the relative stabilities of cytosine tautomers are clearly identified spectroscopically. The hydroxy-oxo tautomeric forms of these molecules have been identified, and their quantitative populations at the experimental temperature are calculated and compared with the experimental results and with previous calculations. The calculated values of Gibbs free energy and Boltzmann population ratios are in good agreement with the experimental results characterizing tautomer equilibrium.

Assessing the halogen effect in Diels–Alder reactions involving chiral α-halo enones. A combined experimental and DFT computational approach

An experimental and computational study was conducted to assess the effect of chlorine and bromine substitution in Diels–Alder reactions involving chiral α-halo enones as dienophiles. An important rate enhancement was observed in the case of acyclic dienes, while the use of cyclic dienes resulted in prolonged reaction times and lower yields. DFT calculations suggest that these reactions are governed by finely balanced geometric and electronic features at the TSs.

Structural, vibrational and quantum chemical investigations on 5-chloro-2-hydroxybenzamide and 5-chloro-2-hydroxybenzoic acid

The Fourier transform infrared (FTIR) and FT-Raman spectra of 5-chloro-2-hydroxybenzamide (5CBA) and 5-chloro-2-hydroxybenzoic acid (5C2HBA) have been recorded in the range 4000–400 and 4000–100 cm −1, respectively. The complete vibrational fundamental modes of the compounds were assigned and analysed using the observed FTIR and FT-Raman data. The vibrational frequencies determined experimentally were compared with the theoretical wavenumbers calculated from ab initio HF and DFT–B3LYP gradient methods employing 6-31G** and 6-311++G** basis sets. The effect of halogen, hydroxyl groups and hydrogen bonding on the characteristic frequencies of the –COOH and –CONH 2 group frequencies have been investigated. In 5CBA and 5C2HBA intramolecular hydrogen bond between a hydroxyl group and C O group makes a six membered ring, which causes the O⋯H interaction onto the resonance of the benzene ring. Comparison of the positions of the ν(OH) bands shows the ν(OH) band of 5CBA is located at considerably higher frequency which confirms a weaker hydrogen bond than in 5C2HBA.

Oxidation protection of Ni-base superalloys by halogen treatment

A new concept of oxidation protection is proposed for Ni-base alloys with Al-amounts of 2–5 wt%. Following Wagner's theory of oxidation the formation of a dense protective alumina scale is possible if the Al-activity is sufficiently high. This condition can be fulfilled by using the halogen effect. Thermodynamical calculations reveal the existence of regions for a positive effect of F and Cl for the alloys IN738 and IN939. In the case of fluorine these results are transformed into F concentrations in a screening by using ion implantation. By the F-effect the oxidation mechanism at 1050 °C/air was changed from internal oxidation of Al to an external formation of an Al2O3 – scale. Optimal implantation parameters were defined. The formation and stability of the protective alumina scale was proved within 1000 h/1050 °C.

The effects of halogen and light-emitting diode light curing on the depth of cure and surface microhardness of composite resins

Aim:Light-emitting diode light curing units (LED LCUs) have become more popular than halogen LCUs in routine dental restorative treatment. The aim of the study was to compare the effects of two conventional halogen (Hilux Plus and VIP) and two LED (Elipar FreeLight 2 and Smart Lite) light curing units on the depth of cure and the microhardness of various esthetic restorative materials.Materials and Methods:The curing depth and microhardness of a compomer (Dyract Extra), a resin-modified glass ionomer (Vitremer), a packable composite (Sculpt It), an ormocer (Admira), a hybrid composite (Tetric Ceram), two microhybrid composites (Miris and Clearfil Photo Posterior) and, a nanofil composite (Filtek Supreme) were determined using a scraping method and a hardness tester. A total of 320 samples were prepared using the eight different materials (n = 10 samples for each subgroup). The scraping test was based on ISO 4049:2000. Vicker's microhardness testing was carried out using hardness tester (Zwick 3212). Data were analyzed using one-way analysis of variance (ANOVA), Bonferroni and the Kolmogorov–Smirnov tests.Results:Best microhardness results were obtained with the LED light curing units and Tetric EvoCeram and Filtek Supreme achieved the highest hardness values. The nanofil composite, Filtek Supreme, showed the best curing depth results in all the tested light curing systems.Conclusions:The LEDs were found to be more successful than the halogen units with respect to both curing depth and microhardness properties.

Comparative evaluation of marginal adaptation between nanocomposites and microhybrid composites exposed to two light cure units

Recent advances in resin adhesives and restorative materials, as well as an increased demand for esthetics, have lead to the introduction of newer resin-based composites like nanocomposites and light-curing units like light emitting diodes (LEDs). The present study was conducted to evaluate the effect of conventional quartz tungsten halogen (QTH) curing unit and blue LED on marginal adaptation of microhybrid composite (Filtek Z250) and nanocomposite (Filtek Z350) resins. Eighty Class V cavities were prepared on, extracted human premolars and were divided into four groups of 20 each. The four groups were designed according to the restorative resin and light cure unit used microhybrid/QTH, nanocomposites/QTH, microhybrid/LED and nanocomposites/LED. After thermocycling and immersion in 2% Basic Fuschin dye solution, the teeth were sectioned and dye penetration was observed under a stereomicroscope at 20X magnification. All the samples were scored and results were analyzed using the "Kruskal Wallis and Mann Whitney tests" with "Wilcoxone correction." The results revealed least microleakage in microhybrid composites exposed to QTH and maximum in nanocomposites exposed to LED. Microhybrid composites exhibited lesser microleakage than nanocomposite resins.

Halogenated Materials as Organic Semiconductors

Organic semiconductors have great potential as the active material in low-cost, large area plastic electronics, whether as light-emitting diodes (LEDs), field-effect transistors (FETs) or solar cells. Organic semiconducting materials retain the processability associated with polymers while maintaining good optoelectronic properties, for example, high absorption coefficients for photons in the visible, and field-effect mobilities comparable with that of amorphous silicon. The elucidation of important structure−property relationships is vital for the design of functional, high-performance organic semiconductors. In this short review, we summarize such relationships stemming from the halogenation of organic semiconductors. While it has been known in the past decade that fluorination lowers the energy levels in carbon based systems, induces stability and electron transport, less is known about the effect of the other halogens. Chlorination has recently been shown to be a viable route to n-type materials. The bandgap of conjugated compounds can also be decreased slightly by the addition of Cl, Br, and I to the aromatic core. The effect of the halogenated moieties on the packing of molecules is discussed.

Receptor activity and conformational analysis of 5′-halogenated resiniferatoxin analogs as TRPV1 ligands

A series of 5′-halogenated resiniferatoxin analogs have been investigated in order to examine the effect of halogenation in the A-region on their binding and the functional pattern of agonism/antagonism for rat TRPV1 heterologously expressed in Chinese hamster ovary cells. Halogenation at the 5-position in the A-region of RTX and of 4-amino RTX shifted the agonism of parent compounds toward antagonism. The extent of antagonism was greater as the size of the halogen increased (I>Br>Cl>F) while the binding affinities were similar, as previously observed for our potent agonists. In this series, 5-bromo-4-amino RTX (39) showed very potent antagonism with Ki (ant)=2.81nM, which was thus 4.5-fold more potent than 5′-iodo RTX, previously reported as a potent TRPV1 antagonist. Molecular modeling analyses with selected agonists and the corresponding halogenated antagonists revealed a striking conformational difference. The 3-methoxy of the A-region in the agonists remained free to interact with the receptor whereas in the case of the antagonists, the compounds assumed a bent conformation, permitting the 3-methoxy to instead form an internal hydrogen bond with the C4-hydroxyl of the diterpene.

Synthesis of Functional Polymer Brushes Containing Carbohydrate Residues in the Pyranose Form and Their Specific and Nonspecific Interactions with Proteins

Three novel N-substituted acrylamide monomers containing different carbohydrate residues, 2'-acrylamidoethyl-α-d-mannopyranoside, 2'-acrylamidoethyl-β-d-glucopyranoside, and 2'-acrylamidoethyl-β-d-galactopyranoside, in the pyranose form were synthesized. The corresponding glycopolymer brushes were prepared on silicon substrates by surface-initiated atom transfer radical polymerization (SI-ATRP) using unprotected glycomonomers. The formation of glycopolymer brushes was well-characterized using ellipsometry, ATR-FTIR, water contact angle analysis, atomic force microscopy analysis, and X-ray photoelectron spectroscopy. The effects of halogen, ligand, and solvent on the polymerization were thoroughly investigated. It was shown that CuCl/CuCl2/tris(2-dimethylaminoethyl)amine (Me6TREN) catalytic system with an optimized ratio of Cu(I)/Cu(II) produced glycopolymer with high molecular weight (Mn = 44-140 kDa) and relatively narrow molecular weight distribution (PDI = 1.4). The dry thickness of resulting glycopolymer brushes (10-36 nm) showed a proportional relationship with the molecular weight of free polymer generated in the solution. The grafting densities of obtained glycopolymer brushes were between 0.12 and 0.17 chains/nm(2). The grafting of glycopolymer resulted in highly hydrophilic surface layer with very low water contact angles (<10°). The glycopolymer brushes showed ultralow protein adsorption from bovine serum albumin (BSA) and fibrinogen (Fb) solutions. Glycopolymer brushes containing glucose units showed relatively better protection against BSA and Fb adsorption than those brushes containing mannose and galactose units. Synthesized glycopolymer brushes retained specific protein interactions, as evident from the interaction with Concanavalin A (Con A). The interaction of surface-grafted glycopolymer brushes with Con A depended on both the stereochemistry of carbohydrate units and the chemical structures present. In addition, the newly synthesized glycopolymer brushes performed significantly better in comparison with currently available structures in terms of specific protein interactions.

Halogenation of 4-hydroxy/amino-3-methoxyphenyl acetamide TRPV1 agonists showed enhanced antagonism to capsaicin

As an extension of our analysis of the effect of halogenation on thiourea TRPV1 agonists, we have now modified selected 4-hydroxy(or 4-amino)-3-methoxyphenyl acetamide TRPV1 agonists by 5- or 6-halogenation on the aromatic A-region and evaluated them for potency for TRPV1 binding and regulation and for their pattern of agonism/antagonism (efficacy). Halogenation shifted the functional activity at TRPV1 toward antagonism with a greater extent of antagonism as the size of the halogen increased (I > Br > Cl), as previously observed for the thiourea series. The extent of antagonism was greater for halogenation at the 5-position than at the 6-position, in contrast to SAR for the thiourea series. In this series, compounds 55 and 75 showed the most potent antagonism, with K i (ant) = 2.77 and 2.19 nM, respectively, on rTRPV1 expressed in Chinese hamster ovary cells. The compounds were thus ca. 40–60-fold more potent than 6′-iodononivamide.

Enhanced Catalytic Activities and Characterization of Ruthenium-Grafted Halogenous Hydroxyapatite Nanorod Crystallites

The nanorod crystallites of ruthenium-grafted halogenous hydroxyapatite (RuXAp, X = F, Cl or Br) were newly developed through a facile method and identified as highly efficient catalysts for the aerobic oxidation of alcohols. Compared with RuHAp for selective oxidation of benzyl alcohol, the existence of F, Cl, and Br elements in hydroxyapatite dramatically enhanced the catalytic activity with a prominent selectivity of far more than 99%. In particular, the RuClAp and RuFAp catalysts, respectively, showed the excellent catalytic activity of TOF = ∼233 and 210 h−1, which was nearly 3 times higher than that of RuHAp. The RuFAp catalyst was furthermore demonstrated to be recyclable and available to be applied for various alcohols. On the basis of the DRIFT and XAFS results, the enhanced activities could be preliminarily ascribed to the electron-withdrawing effect of halogens and the greater amounts of active species existing in the surface of RuXAp compared with that of RuHAp.

ChemInform Abstract: Comparison of the Photochemistry of Diarylchloronium, Diarylbromonium, and Diaryliodonium Salts.

The direct and triplet-sensitized photochemistry of diphenylchloronium, diphenylbromonium, bis(4-methylphenyl) chloronium, and bis(4-methylphenyl) bromonium salts was studied. Direct photolysis of the diarylhalonium salts gave haloarene 2-, 3-, and 4-halobiaryls, and acetanilide, predominantly from a heterolytic cleavage mechanism via the aryl cation-haloarene pair, via in-cage recombination and cage-escape reactions. However, triplet-sensitized photolysis gave haloarene, 2-, 3-, and 4-halobiaryl, and reduced arene mainly from homolytic cleavage to the arene radical-haloarene radical cation pair, which also gave in-cage recombination and cage-escape products. There is evidence for singlet-triplet interconversion of the excited states by spin-orbit coupling and interconversion of the singlet-triplet radical pairs by spin-spin coupling. The effect of halogen and arene substituent on these interconversions is discussed

ChemInform Abstract: Mechanistic Aspects on the Formation of Chiral Allenes from Propargylic Ethers and Organocopper Reagents.

Propargylic ethers react with organocopper reagents to afford allenes by a syn addition to the triple bond followed by a β-elimination of the resulting alkenyl copper species. With use of chiral propargylic ethers and stoichiometric organocopper reagent, it was shown that the β-elimination step is purely anti, resulting in the formation of a chiral allene with 96% optical yield. The same reaction, run with a Grignard reagent RMgX and a catalytic amount of a Cu I salt, affords allenes through an anti or syn overall process. The crucial step is the β-elimination of the intermediate alkenyl organometallic species, which is of anti type with RMgI and of syn type whith RMgCl. Propargylic acetates, wich also afford allenes in this reaction, but through a Cu III intermediate, are not sensitive to this «halogen effect»

ChemInform Abstract: Stereochemical Aspects on the Formation of Chiral Allenes from Propargylic Ethers and Epoxides

Chiral propargylic ethers react with organocopper reagents to afford optically active allenes by a u addition to the triple bond followed by an anti p- elimination of the resulting alkenyl copper species. However, the same reaction, run with a Grignard reagent and a catalytic amount of copper (I) salt affords allenes through an anti or svn process. The crucial step is the p-elimination of the intermediate alkenyl metal species, which is of & type with FWa and of type with RMgCJ. Propargylic acetates, which also afford allenes in this reaction, but through a Cu(II1) intermediate, are not sensitive to this halogen effect. By close analogy to ethers, propargylic epoxides react with Grignard reagents and catalytic amount of Cu(1) salt, leading to a-allenic alcohols. The reaction is highly diastereoselective and its stereochemical outcome can be fully controlled.The diastereomer, probably arising through an addition-elimination mechanism, is better obtained with RM&l and copper (I) bromide, whereas the anti diastereomer is best obtained with RM& and a complexed copper (I) salt.

ChemInform Abstract: Reaction of 1-Substituted Pyrroles with N-Fluorodibenzenesulfonamide: Effect of Halogen on Electrophilic Substitution by Addition- Elimination.

ChemInform Abstract: Reaction of 1-Substituted Pyrroles with N-Fluorodibenzenesulfonamide: Effect of Halogen on Electrophilic Substitution by Addition- Elimination.

Impact of halogen based disinfectants in seawater on polyamide RO membranes

The effect of halogen based disinfectants including monochloramines (NH 2Cl), free chlorine (HOCl/OCl −), and free oxidants (mixture of HOCl/OCl − and HOBr/OBr) on polyamide membrane was studied in synthetic Ocean seawater. Formation and stability of these oxidants were also examined. Permeability and salt rejection of flat sheet polyamide RO membranes following exposure to the halogen based oxidants were compared to the baseline performance of unexposed membranes. The ratio between free chlorine and free bromine was found to depend on the ratio between the bromides, naturally found in seawater, and the added chlorine. Bromide enhanced the degradation of monochloramines but did not affect the stability of free chlorine. All the oxidants damaged the polyamide membranes studied while the free oxidants appeared to be the most aggressive.

Kinetics of the reaction of sodium arsenite with bromo- and iodoacetic acids

By an example of the reaction of sodium arsenite with bromoacetic and iodoacetic acids the effect of halogen on the rate of Meyer reaction was evaluated. Rate constants and the activation energy and entropy values were estimated. Effects of the nature of halogen and temperature on the ratio of rate constants of the Meyer reaction and the competing alkaline hydrolysis of haloacetic acids were evaluated.