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Activation Entropy (activation + entropy)
Selected AbstractsThermal Behavior and Non-isothermal Decomposition Reaction Kinetics of NEPE Propellant with Ammonium DinitramideCHINESE JOURNAL OF CHEMISTRY, Issue 5 2010Weiqiang Pang Abstract Thermal decomposition behavior and non-isothermal decomposition reaction kinetics of nitrate ester plasticized polyether NEPE propellant containing ammonium dinitramide (ADN), which is one of the most important high energetic materials, were investigated by DSC, TG and DTG at 0.1 MPa. The results show that there are four exothermic peaks on DTG curves and four mass loss stages on TG curves at a heating rate of 2.5 K·min,1 under 0.1 MPa, and nitric ester evaporates and decomposes in the first stage, ADN decomposes in the second stage, nitrocellulose and cyclotrimethylenetrinitramine (RDX) decompose in the third stage, and ammonium perchlorate decomposes in the fourth stage. It was also found that the thermal decomposition processes of the NEPE propellant with ADN mainly have two mass loss stages with an increase in the heating rate, that is the result of the decomposition heats of the first two processes overlap each other and the mass content of ammonium perchlorate is very little which is not displayed in the fourth stage at the heating rate of 5, 10, and 20 K·min,1 probably. It was to be found that the exothermal peak temperatures increased with an increase in the heating rate. The reaction mechanism was random nucleation and then growth, and the process can be classified as chemical reaction. The kinetic equations of the main exothermal decomposition reaction can be expressed as: d,/dt=1012.77(3/2)(1,,)[,ln(1,,)]1/3 e,1.723×104/T. The critical temperatures of the thermal explosion (Tbe and Tbp) obtained from the onset temperature (Te) and the peak temperature (Tp) on the condition of ,,0 are 461.41 and 458.02 K, respectively. Activation entropy (,S,), activation enthalpy (,H,), and Gibbs free energy (,G,) of the decomposition reaction are ,7.02 J·mol,1·K,1, 126.19 kJ·mol,1, and 129.31 kJ·mol,1, respectively. [source] Investigation of the stereodynamics of tris-(, -diimine),transition metal complexes by enantioselective dynamic MEKCELECTROPHORESIS, Issue 2 2009Sabrina Bremer Abstract Enantiomerization of octahedral tris(, -diimine),transition metal complexes was investigated by enantioselective dynamic MEKC. Varying both the transition metal ion (Fe2+, Fe3+, and Ni2+) and the bidentate diimine ligand (1,10-phenanthroline and 2,2,-bipyridyl), the enantiomer separations were performed either in a 100,mM sodium tetraborate buffer (pH 9.3) or in a 100,mM sodium tetraborate/sodium dihydrogenphosphate buffer (pH 8.0) both containing sodium cholate as chiral surfactant. The unified equation of dynamic chromatography was employed to determine apparent reaction rate constants from the electropherograms showing distinct plateau formation. Apparent activation parameters ,H, and ,S, were calculated from temperature-dependent measurements between 10.0 and 35.0°C in 2.5,K steps. It was found that the nature of the central metal ion and the ligand strongly influence the enantiomerization barrier. Surprisingly, complexes containing the 2,2,-bipyridyl ligand show highly negative activation entropies between ,103 and ,116,J (K,mol),1 while the activation entropy of tris(1,10-phenanthroline) complexes is positive indicating a different mechanism of interconversion. Furthermore, it was found that the Ni2+ complexes are stereostable under the conditions investigated here making them a lucent target as enantioselective catalysts. [source] Role of electrostatics in the interaction between plastocyanin and photosystem I of the cyanobacterium Phormidium laminosumFEBS JOURNAL, Issue 23 2002Beatrix G. Schlarb-Ridley The interactions between photosystem I and five charge mutants of plastocyanin from the cyanobacterium Phormidium laminosum were investigated in vitro. The dependence of the overall rate constant of reaction, k2, on ionic strength was investigated using laser flash photolysis. The rate constant of the wild-type reaction increased with ionic strength, indicating repulsion between the reaction partners. Removing a negative charge on plastocyanin (D44A) accelerated the reaction and made it independent of ionic strength; removing a positive charge adjacent to D44 (K53A) had little effect. Neutralizing and inverting the charge on R93 slowed the reaction down and increased the repulsion. Specific effects of MgCl2 were observed for mutants K53A, R93Q and R93E. Thermodynamic analysis of the transition state revealed positive activation entropies, suggesting partial desolvation of the interface in the transition state. In comparison with plants, plastocyanin and photosystem I of Phormidium laminosum react slowly at low ionic strength, whereas the two systems have similar rates in the range of physiological salt concentrations. We conclude that in P. laminosum, in contrast with plants in vitro, hydrophobic interactions are more important than electrostatics for the reactions of plastocyanin, both with photosystem I (this paper) and with cytochrome f[Schlarb-Ridley, B.G., Bendall, D.S. & Howe, C.J. (2002) Biochemistry41, 3279,3285]. We discuss the implications of this conclusion for the divergent evolution of cyanobacterial and plant plastocyanins. [source] Selectivity of nitro versus fluoro substitution in arenes in their reactions with charged O - and S -nucleophilesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6 2007Irina A. Khalfina Abstract The relative mobility of nitro and fluoro substituents in 1,3-dinitro- and 1-fluoro-3-nitrobenzenes, 3,5-dinitro- and 3-fluoro-5-nitrobenzotrifluorides under the action of the nucleophiles (2ArYH)·K2CO3 and ArY,K+ in solution and the nucleophiles ArYH·K2CO3 (Y,=,O, S) under heterogeneous conditions was studied by a competitive method in DMF at 40,140,°C. The unique dependences of ,,H, on ,,S, and ,,H, on ,,G, were determined for all the substrates and nucleophiles. The dependence of the mechanistic pathway on the nucleophile is discussed. Two results are relevant to the reactions studied: (i) substituent effects in the nucleophiles (2ArYH)·K2CO3 and ArYH·K2CO3 on the activation entropies suggest that the entropy favours the displacement of nitro group; (ii) the negative signs of ,,H, and ,,S, for the reactions of the nucleophiles ArY,K+ indicate that the enthalpy determines the displacement of nitro group. It is concluded that the selectivity of the reactions with aryloxide and arylthioxide ions cannot be explained by the hard,soft acid,base principle only. Copyright © 2007 John Wiley & Sons, Ltd. [source] H/D exchange reactions;CHEMPHYSCHEM, Issue 6 2003sigma-bond metathesis; The mechanism of the H/D exchange reaction in alkane/hydrogen mixtures on silica-supported zirconium hydride was investigated by a modelling study using density functional theory (DFT) calculations. The electronic activation enthalpy (,H) for the CH bond activation step (TS3) was calculated to be around 92 kJ,mol,1, whereas it would be 258 kJ,mol,1for a direct exchange process (TS1, also called the kite TS). These data clearly speak in favour of the former as a mechanism for CH bond scrambling. Moreover, the calculated enthalpy of activation (,H) for H/D exchange in H2/D2mixtures (TS2) is 33.5 kJ,mol,1, which shows that this reaction is much faster than the H/D scrambling in alkane/H2mixtures, as shown experimentally. Additionally, the calculated activation entropies (For TS1,4,,S ranges between ,129 and ,174 J,mol,1,K,1) are very negative. Although the calculated activation entropies are also in full agreement with experimental data (,S=,113 J,mol,1,K,1), overall, the calculated activation enthalpies are much higher than the experimental ones. This suggests that the actual catalyst is probably more electrophilic than the model chosen for the calculations. [source] Investigation of the stereodynamics of tris-(, -diimine),transition metal complexes by enantioselective dynamic MEKCELECTROPHORESIS, Issue 2 2009Sabrina Bremer Abstract Enantiomerization of octahedral tris(, -diimine),transition metal complexes was investigated by enantioselective dynamic MEKC. Varying both the transition metal ion (Fe2+, Fe3+, and Ni2+) and the bidentate diimine ligand (1,10-phenanthroline and 2,2,-bipyridyl), the enantiomer separations were performed either in a 100,mM sodium tetraborate buffer (pH 9.3) or in a 100,mM sodium tetraborate/sodium dihydrogenphosphate buffer (pH 8.0) both containing sodium cholate as chiral surfactant. The unified equation of dynamic chromatography was employed to determine apparent reaction rate constants from the electropherograms showing distinct plateau formation. Apparent activation parameters ,H, and ,S, were calculated from temperature-dependent measurements between 10.0 and 35.0°C in 2.5,K steps. It was found that the nature of the central metal ion and the ligand strongly influence the enantiomerization barrier. Surprisingly, complexes containing the 2,2,-bipyridyl ligand show highly negative activation entropies between ,103 and ,116,J (K,mol),1 while the activation entropy of tris(1,10-phenanthroline) complexes is positive indicating a different mechanism of interconversion. Furthermore, it was found that the Ni2+ complexes are stereostable under the conditions investigated here making them a lucent target as enantioselective catalysts. [source] The nonisothermal decomposition kinetics of copper(II) complexes with phthalanilic acids and amino acidsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2003Li-Ping Ren Fifteen new kinds of mixed ligand complexes Cu(x-P)A [P = 2-(COO)C6H4CONH-C6H4; x = H, 2-Cl, 2-CH3, 2-OCH3, 4-OCH2CH3; A = dehydronium Met, Leu, Phen] were synthesized. The thermal decomposition behavior of each complex is studied by TG. Coats--Redfern, MacCallum--Tammer, and Zsako methods are adopted to estimate the values of the apparent activation energy Ea, the activation entropy ,S,, the reaction order n, and the frequency factor A. The results showed that the reaction order is 2/3 for each of the complex. Studies on the mechanism of the thermal decomposition reactions suggested that these reactions all fit the kinetic equation 1 , (1 , ,)1/3 = © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 623,628, 2003 [source] Effects of entropy on the gas-phase pyrolysis of ethyl N,N -dimethylcarbamateJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2007Chang K. Kim Abstract In this study, we examined the gas-phase pyrolysis of ethyl N,N -dimethylcarbamate theoretically at various theoretical levels. The reaction consists of a two-step mechanism, with N,N -dimethylcarbamic acid and ethylene as reaction intermediates. In the first step, the reaction proceeds via a six-membered cyclic transition state (TS), which is more favorable than that via a four-membered cyclic TS. Here, the contribution of entropy to the overall potential energy surface was found to play an important role in determining the rate-limiting step, which was found to be the second step when viewed in terms of the enthalpy of activation (,H,), but the first step when entropy changes (,T,S,) were considered. These results are consistent with experimental findings. Moreover, the experimental activation entropy can be reproduced by using the hindered rotor approximation, which converts some low vibration frequencies that correspond to internal rotational modes into hindered rotors. © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 625,631, 2007 [source] On the mechanism of the N,N -dimethyl amination of Grignard reagents: a kinetic studyJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 9 2007Ender Erdik Abstract A direct kinetic study is reported for the electrophilic amination of substituted phenylmagnesium bromides with N,N -dimethyl O -(mesitylenesulfonyl)hydroxylamine in THF. Rate data, Hammett relationship, and activation entropy are consistent with a SN2 displacement involving the attack of carbanions to sp3N in the amination reagent (AR). Copyright © 2007 John Wiley & Sons, Ltd. [source] Catalytic and inhibitory effects of ,-cyclodextrin on the hydrolysis of benzoic anhydrideJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2004Tiago A. S. Brandão Abstract The hydrolysis of benzoic anhydride (Bz2O) in the presence of ,-cyclodextrin (,-CD) was studied in aqueous solution as a function of pH, temperature and ionic strength, at 25°C. The experimental rate constant versus pH profiles show that, in the region of spontaneous water reaction (pH 3.0,6.5), ,-CD inhibits the reaction and the isotope effect indicates that the rate determining step of the reaction corresponds to the water-catalyzed nucleophilic attack of water on the carbonyl group of Bz2O. Conversely, whereas inhibition is observed at pH 6.0, catalysis of the hydroxide ion reaction is observed at pH 8.0 and it is found that the activation entropy is responsible for the catalytic phenomena in the basic hydrolysis of benzoic anhydride. Copyright © 2004 John Wiley & Sons, Ltd. [source] Secondary Ion Mass Spectroscopy Study of Oxygen-18 Tracer Diffusion in 2/1-Mullite Single CrystalsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2001Peter Fielitz Oxygen 18O tracer diffusion in Czochralski-grown mullite single crystals is investigated along [010] and [001]. Oxygen diffusion coefficients range between ,5 × 10,20 m2/s (1250°C) and ,9 × 10,18 m2/s (1525°C). The data does not show any significant anisotropy. The values of the activation enthalpy (4.5 eV) and of the activation entropy ((3.4 ± 1.6)kB, where kB is the Boltzmann constant) suggest that the atomic transport occurs via thermally activated vacancies. [source] In-vitro antimicrobial, thermal and spectral studies of mixed ligand Cu(II) heterochelates of clioquinol and coumarin derivativesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 4 2010G. J. Kharadi Abstract The mixed-ligand heterochelates of Cu(II) with 5-chloro-7-iodo-8-hydroxyquinoline (clioquinol) and various uninegative bidentate ligands were prepared. The structure of mixed-ligand heterochelates was investigated using spectral, physicochemical, elemental analysis and thermal studies. The FAB-mass spectrum of [Cu(A2)(CQ)(H2O)2].2H2O has been carried out. Magnetic moment and reflectance spectral studies reveal that an octahedral geometry has been assigned to all the prepared heterochelates. The kinetic parameters such as order of reaction (n), the energy of activation (Ea), the pre-exponential factor (A), the activation entropy (,S#), the activation enthalpy (,H#) and the free energy of activation (,G#) have been reported. The ligands, metal salts, heterochelates, control and standard drug were tested for their in-vitro antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, Serratia marcescens and Bacillus substilis. The metal complexes exhibit good activity against bacterial strains compared with parental compounds, and moderate compared with the standard drug (clioquinol). Copyright © 2010 John Wiley & Sons, Ltd. [source] Novel drug-based Fe(III) heterochelates: synthetic, spectroscopic, thermal and in-vitro antibacterial significanceAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 2 2010D.H. Jani Abstract A series of novel heterochelates of the type [Fe(An)(L)(H2O)2],mH2O [where H2An = 4,4,-(arylmethylene)bis(3-methyl-1-phenyl-4,5-dihydro-1H-pyrazol-5-ol); aryl = 4-nitrophenyl, m = 1 (H2A1); 4-chlorophenyl, m = 2 (H2A2); phenyl, m = 2 (H2A3); 4-hydroxyphenyl, m = 2 (H2A4); 4-methoxyphenyl, m = 2 (H2A5); 4-hydroxy-3-methoxyphenyl, m = 1.5 (H2A6); 2-nitrophenyl, m = 1.5 (H2A7); 3-nitrophenyl, m = 0.5 (H2A8); p -tolyl, m = 1 (H2A9) and HL = 1-cyclopropyl-6-fluoro-4-oxo-7-(piperazin-1-yl)-1,4-dihydroquinoline-3-carboxylic acid] were investigated. They were characterized by elemental analysis (FT-IR, 1H- & 13C-NMR, and electronic) spectra, magnetic measurements and thermal studies. The FAB-mass spectrum of [Fe(A3)(L)(H2O)2],2H2O was determined. Magnetic moment and reflectance spectral studies revealed that an octahedral geometry could be assigned to all the prepared heterochelates. Ligands (H2An) and their heterochelates were screened for their in-vitro antibacterial activity against Bacillus subtilis, Staphylococcus aureus, Escherichia coli and Serratia marcescens bacterial strains. The kinetic parameters such as order of reaction (n), the energy of activation (Ea), the pre-exponential factor (A), the activation entropy (,S#), the activation enthalpy (,H#) and the free energy of activation (,G#) are reported. Copyright © 2009 John Wiley & Sons, Ltd. [source] Antibacterial, spectral and thermal aspects of drug based-Cu(II) mixed ligand complexesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 10 2009G. J. Kharadi Abstract The antibiotic agent clioquinol is well known for its drug design and coordinating ability towards metal ions. Copper(II) mixed-ligand complexes of clioquinol with various uninegative bidentate ligands were prepared. The structure of the synthesized complexes was characterized using elemental analyses, infrared spectra, 1H-NMR spectra, electronic spectra, magnetic measurements, FAB mass spectrum and thermo gravimetric analyses. The kinetic parameters such as order of reaction (n) and the energy of activation (Ea) are reported using the Freeman,Carroll method. The pre-exponential factor (A), the activation entropy (,S#), the activation enthalpy (,H#) and the free energy of activation (,G#) were calculated. Complexes were also screened for their in vitro antibacterial activity against a range of Gram-positive and Gram-negative bacteria in order to set the precursors for anti-tumourigenic agent. Copyright © 2009 John Wiley & Sons, Ltd. [source] | ||||||||||