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Isomerization Reaction (isomerization + reaction)
Selected AbstractsPhotochemical syn,anti Isomerization Reaction in N4 -Hydroxycytosine.PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2001An Experimental Matrix Isolation, Theoretical Study ABSTRACT Infrared spectra of N4 -hydroxycytosine isolated in argon and nitrogen low-temperature matrixes are reported. The compound was found to adopt the syn structure of the imino-oxo tautomeric form exclusively. A photoreaction (, > 250 nm) converting this form into the anti isomer was observed. The reaction is reversible and leads to a photostationary point. The initial infrared spectrum and the spectrum of the photoproduct were assigned to the syn and anti isomers, respectively. This assignment is based on a close agreement between the experimental spectra and the spectra theoretically simulated at the DFT(B3LYP)/6-31++G** level of theory. [source] Kinetic Study of Thermal Z to E Isomerization Reactions of Azobenzene and 4-Dimethylamino-4,-nitroazobenzene in Ionic Liquids [1-R-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide with R=Butyl, Pentyl, and Hexyl]CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2006Keita Baba Abstract Thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4,-nitroazobenzene were examined in three ionic liquids of general formula 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (R=butyl, pentyl, and hexyl). The first-order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4-dimethylamino-4,-nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers,Grote,Hynes model or by the Agmon,Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 104,107 s,1. [source] Tin-Free Radical Alkoxyamine Addition and Isomerization Reactions by Using the Persistent Radical Effect: Variation of the Alkoxyamine StructureCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2005Kian Molawi Dipl.-Chem. Abstract Various C-centered radicals can efficiently be generated through thermal CO-bond homolysis of alkoxyamines. This method is used to perform environmentally benign radical cyclization and intermolecular addition reactions. These alkoxyamine isomerizations and intermolecular carboaminoxylations are mediated by the persistent radical effect (PRE). In the paper, the effect of the variation of the alkoxyamine structure,in particular steric effects in the nitroxide moiety,on the outcome of the PRE mediated radical reactions will be discussed. Fourteen different nitroxides were used in the studies. It will be shown that reaction times can be shortened about 100 times upon careful tuning of the alkoxyamine structure. Activation energies for the CO-bond homolysis of the various alkoxyamines are provided. The kinetic data are used to explain the reaction outcome of the PRE-mediated processes. [source] Isomeric hexyl-ketohydroperoxides formed by reactions of hexoxy and hexylperoxy radicals in oxygenINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2003François Jorand Isomerization reactions of peroxy radicals during oxidation of long-chain hydrocarbons yield hydroperoxides, and therefore play an important role in combustion and atmospheric chemistry, because of their action as branching agents in these chain reaction processes. Different formation mechanisms and structures are involved. Three isomeric hexyl-ketohydroperoxides are formed via isomerization reactions in oxygen of either hexoxy RO or hexylperoxy RO2 radicals. In the temperature range 373,473 K, 2-hexoxy (C6H13O) radical in O2/N2 mixtures gives 2-hexanone-5-hydroperoxide via two consecutive isomerizations. The second one is a H transfer from a HC(OH) group occurring via a seven-membered ring intermediate: Its rate constant has been determined at 453 and 483 K, and the general expression can be written as Hexylperoxy C6H13O2 radical, present in n -hexane oxidation by oxygen/nitrogen mixtures in the temperature range 543,573 K, gives 2-hexanone-4-hydroperoxide, 3-hexanone-5-hydroperoxide, and 2-hexanone-5-hydroperoxide. The first two are formed through an isomerization reaction via a six-membered ring intermediate, and the last through an isomerization reaction via a seven-membered ring intermediate. The ratio of the rate constant of the isomerization reactions of RO2 radicals via a seven-membered ring intermediate to that via a six-membered ring is found to be 0.795, and the rate constant expression via a seven-membered ring intermediate is proposed: The role of these reactions in the formation of radicals in the troposphere is discussed. Other products arising in the reactional path, such as ketones, furans, and diketones, are identified. Identification of these ketohydroperoxides was made using gas chromatography/mass spectrometry with electron impact, and with NH3 (or ND3) chemical ionization. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 354,366, 2003 [source] A theoretical investigation on the structures, densities, detonation properties, and pyrolysis mechanism of the nitro derivatives of phenolsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2010Guixiang Wang Abstract The nitro derivatives of phenols are optimized to obtain their molecular geometries and electronic structures at the DFT-B3LYP/6-31G* level. Detonation properties are evaluated using the modified Kamlet,Jacobs equations based on the calculated densities and heats of formation. It is found that there are good linear relationships between density, detonation velocity, detonation pressure, and the number of nitro and hydroxy groups. Thermal stability and pyrolysis mechanism of the title compounds are investigated by calculating the bond dissociation energies (BDEs) at the unrestricted B3LYP/6-31G* level. The activation energies of H-transfer reaction is smaller than the BDEs of all bonds and this illustrates that the pyrolysis of the title compounds may be started from breaking OH bond followed by the isomerization reaction of H transfer. Moreover, the CNO2 bond with the smaller bond overlap population and the smaller BDE will also overlap may be before homolysis. According to the quantitative standard of energetics and stability as a high-energy density compound, pentanitrophenol essentially satisfies this requirement. In addition, we have discussed the effect of the nitro and hydroxy groups on the static electronic structural parameters and the kinetic parameter. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] New Indenylidene-Schiff Base-Ruthenium Complexes for Cross-Metathesis and Ring-Closing MetathesisADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 16 2009Abstract We here report on the stability and catalytic activity of new indenylidene-Schiff base-ruthenium complexes 3a,f through representative cross-metathesis (CM) and ring-closing metathesis (RCM) reactions. Excellent activity of the new complexes was found for the two selected RCM reactions; prominent conversion was obtained compared to the commercial Hoveyda,Grubbs catalyst 2. Moreover, excellent results were obtained for a standard CM reaction. Higher conversions were achieved with one of the indenylidene catalysts compared with Hoveyda,Grubbs catalyst. Unexpectedly, an isomerization reaction was observed during the CM reaction of allylbenzene. To the best of our knowledge, isomerization reactions in this model CM reaction in closed systems have never been described using first generation catalysts, including the Hoveyda,Grubbs catalyst. The first model CM reactions as well as the RCM reactions have been monitored using 1H,NMR. The course of the CM reaction of 3-phenylprop-1-ene (8) and cis -1,4-diacetoxybut-2-ene (9) was monitored by GC. The isomerization reaction was studied by means of GC-mass spectrometry and in situ IR spectroscopy. All catalysts were structurally characterized by means of 1H, 13C, and 31P,NMR spectroscopy. [source] Gibbs energy of activation for thermal isomerization of (1Z)-acetaldehyde hydrazone and (1Z)-acetaldehyde N,N-dimethylhydrazone by Gaussian-4 theory and CCSD(T)/CBS computationsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 14 2009Shih-I Lu Abstract In this article, we examined the Gibbs energy of activation for the Z/E thermal isomerization reaction of (1Z)-acetaldehyde hydrazone and (1Z)-acetaldehyde N,N-dimethylhydrazone, at 298.15 K in the solvent of cyclohexane. We carried out computations employing both the Gaussian-4 (G4) theory and the coupled cluster method using both single and double substitutions and triple excitations noniteratively, CCSD(T). The CCSD(T) energy is extrapolated to the complete basis set (CBS). We compared the calculated results to the available experimental observation. It appeared that both G4 and CCSD(T)/CBS computations overestimated the experimental value by as much as about 6 and 12 kcal/mol in the present two cases. We discussed possible sources of error and proposed the experimental kinetic data could be questionable. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Protein Diffusion Probed by the Transient Grating Method with a New Type of Photochromic Molecule,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2008Takeshi Eitoku A new type of photochromic molecule that can be used for diffusion coefficient (D) measurements of various proteins in solution is described. The absorption spectrum of this molecule is changed upon photoexcitation by the trans,cis isomerization reaction. Target proteins were labeled by this photochromic molecule in the dark and the translational motion of the proteins was detected by the transient grating (TG) method. The TG signal was simple enough to determine D accurately and was stable even for long-time irradiation by the laser light. The TG method using this probe molecule improves many drawbacks of the other techniques. [source] Direct evidence by H/D exchange and ESI-MS for transient unproductive domain interaction in the refolding of an antibody scFv fragmentPROTEIN SCIENCE, Issue 3 2000Marcus Jäger Abstract The refolding kinetics of a single-chain Fv (scFv) fragment, derived from a stabilized mutant of the phosphorylcholine binding antibody McPC603, was investigated by H/D exchange and ESI-MS and compared with the folding kinetics of its constituting domains VH and VL. Both VH and VL adopt essentially native-like exchange protection within the dead time of the manual-mixing H/D exchange experiment (10 s) and in the case of VL, which contains two cis -prolines in the native conformation, this fast protection is independent of proline cis/trans isomerization. At the earliest time point resolvable by manual mixing, fewer deuterons are protected in the scFv fragment than in the two isolated domains together, despite the fact that the scFv fragment is significantly more stable than VL and VH. Full H/D exchange protection in the scFv fragment is gained on a time scale of minutes. This means that the domains in the scFv fragment do not refold independently. Rather, they associate prematurely and in nonnative form, a kinetic trap. Unproductive domain association is observed both after equilibrium- and short-term denaturation. For the equilibrium-denatured scFv fragment, whose native structure formation is dependent on a cis conformation of an interface proline in VL, this cis/trans isomerization reaction proceeds about one order in magnitude more slowly than the escape from the trap to a conformation where full H/D exchange protection is already achieved. We interpret these data in terms of a general kinetic scheme involving intermediates with and without domain association. [source] A Feasible Enzymatic Process for d -Tagatose Production by an Immobilized Thermostable l -Arabinose Isomerase in a Packed-Bed BioreactorBIOTECHNOLOGY PROGRESS, Issue 2 2003Hye-Jung Kim To develop a feasible enzymatic process for d -tagatose production, a thermostable l -arabinose isomerase, Gali152, was immobilized in alginate, and the galactose isomerization reaction conditions were optimized. The pH and temperature for the maximal galactose isomerization reaction were pH 8.0 and 65 °C in the immobilized enzyme system and pH 7.5 and 60 °C in the free enzyme system. The presence of manganese ion enhanced galactose isomerization to tagatose in both the free and immobilized enzyme systems. The immobilized enzyme was more stable than the free enzyme at the same pH and temperature. Under stable conditions of pH 8.0 and 60 °C, the immobilized enzyme produced 58 g/L of tagatose from 100 g/L galactose in 90 h by batch reaction, whereas the free enzyme produced 37 g/L tagatose due to its lower stability. A packed-bed bioreactor with immobilized Gali152 in alginate beads produced 50 g/L tagatose from 100 g/L galactose in 168 h, with a productivity of 13.3 (g of tagatose)/(L-reactor·h) in continuous mode. The bioreactor produced 230 g/L tagatose from 500 g/L galactose in continuous recycling mode, with a productivity of 9.6 g/(L·h) and a conversion yield of 46%. [source] A Stereodivergent Synthesis of All Stereoisomers of Centrolobine: Control of Selectivity by a Protecting-Group ManipulationCHEMISTRY - A EUROPEAN JOURNAL, Issue 44 2009Bernd Schmidt Prof. Abstract All stereoisomers of the natural product centrolobine are selectively synthesized, by starting from a common precursor. Key steps are an enantioselective allylation with enantiomerically pure allylsilanes, a tandem ring-closing metathesis,isomerization reaction, and a Heck reaction by using an arene diazonium salt. By choosing appropriate conditions for the final deprotection step, either the cis -configured centrolobines or their epimers are selectively obtained. [source] Supramolecular Solids as a Medium for Single-Crystal-to-Single-Crystal E/Z Photoisomerization: Kinetic Study of the Photoreactions of Two Zn-Coordinated Tiglic Acid MoleculesCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2008Shao-Liang Zheng Dr. Abstract [Zn(TA)2(H2O)2] (H-TA=tiglic acid) has been embedded in a framework composed of CECR (CECR=C -ethylcalix[4]resorcinarene) molecules to examine its E,Z photoisomerization in a periodic framework. The photoisomerization of tiglic acid in CECR-[Zn(TA)2(H2O)2],4,H2O proceeds without the [2+2]-dimerization reaction that often occurs in crystals of uncomplexed analogues, and without breakdown of the crystal lattice that frequently occurs in neat crystals. The two Zn-coordinated TA molecules are located in different size cavities. The rate constants of the isomerization reaction are strongly affected by the size of the reaction cavity. Analysis of the temperature dependence of the reaction rates and the occupancies in the final photostationary state shows that the activation energies and the standard enthalpies of activation are dependent on the difference between the reaction cavities. This is the first quantitative diffraction study of solid-state E/Z isomerization of a metal-coordinated ligand in a periodic host environment. [source] Kinetics and mechanisms of transalkylation and disproportionation of meta -diethylbenzene by triflic acid catalystINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 11 2003S. M. Al-Zahrani The kinetics of transalkylation and isomerization of meta -diethylbenzene in the presence of benzene using triflic acid as a catalyst has been investigated. High catalytic activity of the triflic acid catalyst was observed in homogeneous liquid-phase reactions. On the basis of the product distribution obtained, transalkylation, disproportionation, and isomerization reactions have been considered and the main product of the reaction was ethylbenzene. These reactions are conducted in a closed liquid batch reactor with continuous stirring under dry nitrogen and atmospheric pressure over the temperature range of 288,308 K. The main transalkylation, disproportionation, and isomerization reactions occurred simultaneously and were considered as elementary reactions. The apparent activation energy of the transalkylation reaction was found to be 35.5 kJ/mol, while that of disproportionation reaction was 42.3 kJ/mol. The reproducibility of the experimental product distribution occurred with an average relative error of ±2%. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 555,563, 2003 [source] Isomeric hexyl-ketohydroperoxides formed by reactions of hexoxy and hexylperoxy radicals in oxygenINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2003François Jorand Isomerization reactions of peroxy radicals during oxidation of long-chain hydrocarbons yield hydroperoxides, and therefore play an important role in combustion and atmospheric chemistry, because of their action as branching agents in these chain reaction processes. Different formation mechanisms and structures are involved. Three isomeric hexyl-ketohydroperoxides are formed via isomerization reactions in oxygen of either hexoxy RO or hexylperoxy RO2 radicals. In the temperature range 373,473 K, 2-hexoxy (C6H13O) radical in O2/N2 mixtures gives 2-hexanone-5-hydroperoxide via two consecutive isomerizations. The second one is a H transfer from a HC(OH) group occurring via a seven-membered ring intermediate: Its rate constant has been determined at 453 and 483 K, and the general expression can be written as Hexylperoxy C6H13O2 radical, present in n -hexane oxidation by oxygen/nitrogen mixtures in the temperature range 543,573 K, gives 2-hexanone-4-hydroperoxide, 3-hexanone-5-hydroperoxide, and 2-hexanone-5-hydroperoxide. The first two are formed through an isomerization reaction via a six-membered ring intermediate, and the last through an isomerization reaction via a seven-membered ring intermediate. The ratio of the rate constant of the isomerization reactions of RO2 radicals via a seven-membered ring intermediate to that via a six-membered ring is found to be 0.795, and the rate constant expression via a seven-membered ring intermediate is proposed: The role of these reactions in the formation of radicals in the troposphere is discussed. Other products arising in the reactional path, such as ketones, furans, and diketones, are identified. Identification of these ketohydroperoxides was made using gas chromatography/mass spectrometry with electron impact, and with NH3 (or ND3) chemical ionization. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 354,366, 2003 [source] New Indenylidene-Schiff Base-Ruthenium Complexes for Cross-Metathesis and Ring-Closing MetathesisADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 16 2009Abstract We here report on the stability and catalytic activity of new indenylidene-Schiff base-ruthenium complexes 3a,f through representative cross-metathesis (CM) and ring-closing metathesis (RCM) reactions. Excellent activity of the new complexes was found for the two selected RCM reactions; prominent conversion was obtained compared to the commercial Hoveyda,Grubbs catalyst 2. Moreover, excellent results were obtained for a standard CM reaction. Higher conversions were achieved with one of the indenylidene catalysts compared with Hoveyda,Grubbs catalyst. Unexpectedly, an isomerization reaction was observed during the CM reaction of allylbenzene. To the best of our knowledge, isomerization reactions in this model CM reaction in closed systems have never been described using first generation catalysts, including the Hoveyda,Grubbs catalyst. The first model CM reactions as well as the RCM reactions have been monitored using 1H,NMR. The course of the CM reaction of 3-phenylprop-1-ene (8) and cis -1,4-diacetoxybut-2-ene (9) was monitored by GC. The isomerization reaction was studied by means of GC-mass spectrometry and in situ IR spectroscopy. All catalysts were structurally characterized by means of 1H, 13C, and 31P,NMR spectroscopy. [source] Formulation considerations for proteins susceptible to asparagine deamidation and aspartate isomerizationJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 11 2006Aditya A. Wakankar Abstract The asparagine (Asn) deamidation and aspartate (Asp) isomerization reactions are nonenzymatic intra-molecular reactions occurring in peptides and proteins that are a source of major stability concern in the formulation of these biomolecules. The mechanisms for the deamidation and isomerization reactions are similar since they both proceed through an intra-molecular cyclic imide (Asu) intermediate. The formation of the Asu intermediate, which involves the attack by nitrogen of the peptide backbone on the carbonyl carbon of the Asn or the Asp side chain, is the rate-limiting step in both the deamidation and the isomerization reactions at physiological pH. In this article, the influence of factors such as formulation conditions, protein primary sequence, and protein structure on the reactivity of Asn and Asp residues in proteins are reviewed. The importance of formulation conditions such as pH and solvent dielectric in influencing deamidation and isomerization reaction rates is addressed. Formulation strategies that could improve the stability of proteins to deamidation and isomerization reactions are described. The review is intended to provide information to formulation scientists, based on protein sequence and structure, to predict potential degradative sites on a protein molecule and to enable formulation scientists to set appropriate formulation conditions to minimize reactivity of Asn and Asp residues in protein therapeutics. © 2006 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 95:2321,2336, 2006 [source] Tunable haptotropic metal migration in fused arenes: towards organometallic switches,THE CHEMICAL RECORD, Issue 2 2004Karl Heinz Dötz Abstract Tricarbonyl chromium complexes of naphthalene derivatives are synthesized by chromium-templated [3,+,2,+,1]-benzannulation and subjected to thermally induced haptotropic rearrangement experiments. Thermodynamic and kinetic parameters for the metal shift demonstrate the influence of the arene substitution pattern. In turn, the chromium template may be tuned as well by phosphorus coligands which allow to accelerate or slow down the isomerization process; this effect quantitatively reflects the steric and electronic properties of the coligand sphere. Proper adjustment of the template allows for a photo-induced reverse migration of the chromium moiety which results in a switchable organometallic device. Experiments with enantiopure arene chromium complexes indicate a stereospecific metal migration. The rearrangement proceeds by an intramolecular mechanism in both directions. Haptotropic isomerization reactions are not limited to bicyclic arenes and can be extended from naphthalenes to phenanthrene or tetra- and pentacyclic heteroarene systems. © 2004 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 4: 61,71; 2004: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20007 [source] Kinetic Study of Thermal Z to E Isomerization Reactions of Azobenzene and 4-Dimethylamino-4,-nitroazobenzene in Ionic Liquids [1-R-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide with R=Butyl, Pentyl, and Hexyl]CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2006Keita Baba Abstract Thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4,-nitroazobenzene were examined in three ionic liquids of general formula 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (R=butyl, pentyl, and hexyl). The first-order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4-dimethylamino-4,-nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers,Grote,Hynes model or by the Agmon,Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 104,107 s,1. [source] | |||||||||||||