Home About us Contact
|
Home About us Contact | ||
|
|
||
Standard Enthalpy (standard + enthalpy)
Selected AbstractsChemInform Abstract: Standard Enthalpies of Formation of Some Lanthanide Gallides by High Temperature Direct Synthesis CalorimetryCHEMINFORM, Issue 31 2001S. V. Meschel Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Standard Enthalpy of Formation of Al0.28Fe0.72 at 298 KCHEMINFORM, Issue 38 2001Y. Feutelais Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Energetic polymer salts from 1-vinyl-1,2,4-triazole derivativesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2008Hong Xue Abstract Energetic polymers salts from 1-vinyl-1,2,4-triazole derivatives have been synthesized via free radical polymerization of 1-vinyl-1,2,4-triazolium monomer salts or by protonation of poly(1-vinyl-1,2,4-triazole) with inorganic or organic acids. Standard enthalpies of formation of the new monomer salts were calculated using the computationally feasible DFT(B3LYP) and MP2 methods in conjunction with an empirical approach based on densities of salts. Compared with the monomer salts, the polymer salts have good thermal properties with high densities > 1.5 g cm,3. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2414,2421, 2008 [source] Ways to Create New CHNO-Oxidizers for Stoichiometric Gas-Generating Compositions with Low Combustion TemperaturePROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 5 2007David Abstract The investigation is aimed to study the possibility of creation of new CHNO-oxidizers for smokeless gas-generating compositions for airbag inflators. For ensuring low amount of CO and nitrogen oxides in combustion products it is necessary to create stoichiometric compositions with a relatively low combustion temperature. Ways to create new oxidizers acceptable to this requirement are examined, mainly by introducing low-enthalpy oxygen-containing groups into the oxidizer molecule. Standard enthalpy of formation (,fH°) has been calculated for substances with unknown ,fH°, thermal stability has been qualitatively estimated, and combustion temperatures of stoichiometric compositions have been calculated. [source] Direct ab initio dynamics calculations of the rate constants for the reaction of CHF2CF2OCH3 with ClINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2007Lei Yang A dual-level direct dynamics method is employed to reveal the dynamical properties of the reaction of CHF2CF2OCH3 (HFE-254pc) with Cl atoms. The optimized geometries and frequencies of the stationary points and the minimum energy path (MEP) are calculated at the B3LYP/6-311G(d,p) level by using GAUSSIAN 98 program package, and energetic information is further refined by the G3(MP2) method. Two H-abstraction channels have been identified. For the reactant CHF2CF2OCH3 and the two products, CHF2CF2OCH2 and CF2CF2OCH3, the standard enthalpies of formation are evaluated with the values of ,256.71 ± 0.88, ,207.79 ± 0.12, and ,233.43 ± 0.88 kcal/mol, respectively, via group-balanced isodesmic reactions. The rate constants of the two reaction channels are evaluated by means of canonical variational transition-state theory (CVT) including the small-curvature tunneling (SCT) correction over a wide range of temperature from 200 to 2000 K. The calculated rate constants agree well with the experimental data, and the Arrhenius expressions for the title reaction are fitted and can be expressed as k1 = 9.22 × 10,19 T2.06 exp(219/T), k2 = 4.45 × 10,14T0.90 exp(,2220/T), and k = 4.71 × 10,22 T3.20) exp(543/T) cm3 molecule,1 s,1. Our results indicate that H-abstraction from CH3 group is the main reaction pathway in the lower temperature range, while H-abstraction from CHF2 group becomes more competitive in the higher temperature range. © 2007 Wiley Periodicals, Inc. 39: 221,230, 2007 [source] Accurate prediction of thermodynamic properties of alkyl peroxides by combining density functional theory calculation with least-square calibrationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2009Cun-Xi Liu Abstract Owing to the significance in kinetic modeling of the oxidation and combustion mechanisms of hydrocarbons, a fast and relatively accurate method was developed for the prediction of ,fH of alkyl peroxides. By this method, a raw ,fH value was calculated from the optimized geometry and vibration frequencies at B3LYP/6-31G(d,p) level and then an accurate ,fH value was obtained by a least-square procedure. The least-square procedure is a six-parameter linear equation and is validated by a leave-one out technique, giving a cross-validation squared correlation coefficient q2 of 0.97 and a squared correlation coefficient of 0.98 for the final model. Calculated results demonstrated that the least-square calibration leads to a remarkable reduction of error and to the accurate ,fH values within the chemical accuracy of 8 kJ mol,1 except (CH3)2CHCH2CH2CH2OOH which has an error of 8.69 kJ mol,1. Comparison of the results by CBS-Q, CBS-QB3, G2, and G3 revealed that B3LYP/6-31G(d,p) in combination with a least-square calibration is reliable in the accurate prediction of the standard enthalpies of formation for alkyl peroxides. Standard entropies at 298 K and heat capacities in the temperature range of 300,1500 K for alkyl peroxides were also calculated using the rigid rotor-harmonic oscillator approximation. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Interpretation of the temperature dependence of equilibrium and rate constantsJOURNAL OF MOLECULAR RECOGNITION, Issue 5 2006Donald J. Winzor Abstract The objective of this review is to draw attention to potential pitfalls in attempts to glean mechanistic information from the magnitudes of standard enthalpies and entropies derived from the temperature dependence of equilibrium and rate constants for protein interactions. Problems arise because the minimalist model that suffices to describe the energy differences between initial and final states usually comprises a set of linked equilibria, each of which is characterized by its own energetics. For example, because the overall standard enthalpy is a composite of those individual values, a positive magnitude for ,Ho can still arise despite all reactions within the subset being characterized by negative enthalpy changes: designation of the reaction as being entropy driven is thus equivocal. An experimenter must always bear in mind the fact that any mechanistic interpretation of the magnitudes of thermodynamic parameters refers to the reaction model rather than the experimental system For the same reason there is little point in subjecting the temperature dependence of rate constants for protein interactions to transition-state analysis. If comparisons with reported values of standard enthalpy and entropy of activation are needed, they are readily calculated from the empirical Arrhenius parameters. Copyright © 2006 John Wiley & Sons, Ltd. [source] 298,K enthalpies of formation of monofluorinated alkanes: theoretical predictions for methyl, ethyl, isopropyl and tert -butyl fluorideJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2004Bethany L. Kormos Abstract Experimentally measured 298,K enthalpies of formation are not well established for monofluoroalkanes. To supplement available experimental data, the multi-coefficient G3 (MCG3) quantum mechanical model has been applied to estimate this thermochemical quantity for methyl fluoride, ethyl fluoride, 2-fluoropropane (isopropyl fluoride) and 2-fluoro-2-methylpropane (tert -butyl fluoride). The following 298,K standard enthalpies of formation are suggested for these monofluoroalkanes: ,H (MeF),=,,57.1,±,0.2,kcal,mol,1, ,H (EtF),=,,66.5,±, 0.4,kcal,mol,1, ,H (i -PrF),=,,75.4±,0.5,kcal,mol,1, and ,H (t -BuF),=,,86.0,±,2.0,kcal,mol,1 (1,kcal,=, 4.184,kJ). Copyright © 2004 John Wiley & Sons, Ltd. [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] Mechanism of interaction of DNA bases with dichloro-[1-alkyl-2-(naphthylazo)imidazole]palladium(II) complexes: A cytosine caseINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2009Pradip Kumar Ghosh Interaction of cytosine (C) with dichloro-[1-alkyl-2-(,-naphthylazo)imidazole]-palladium(II) [Pd(,-NaiR)Cl2, 1] and dichloro-[1-alkyl-2-(,-naphthylazo)imidazole]-palladium(II) [Pd(,-NaiR)Cl2, 2] complexes {where alkyl R = Me (a), Et (b) or Bz (c)} in acetonitrile (MeCN)-water (50% v/v) medium to yield [{1-alkyl-2-(,-naphthylazo)-imidazole}bis(cytosine)]palladium(II)dichloride (3a, 3b, 3c) and [{1-alkyl-2-(,-naphthylazo)-imidazole}bis(cytosine)]palladium(II)dichloride (4a, 4b, 4c) was studied. The products were characterized by microanalytical data and spectroscopic techniques (FT-IR, UV,vis, and NMR). The reaction kinetics show first-order dependence of the rate on each of the concentration of Pd(II) complex and C. External addition of Cl, ion (LiCl) did not influence this nucleophilic substitution rate process and has proved the cleavage of first PdCl bond is the rate-determining step. Thermodynamic parameters standard enthalpy of activation (,,Ho) and standard entropy of activation (,,So) were determined from variable temperature kinetic studies. The negative values of ,,So indicate that the reaction proceeds through an associative inner sphere mechanism. The magnitude of the second-order rate constant k2 increases in the following order: Pd(NaiEt)Cl2 (b) < Pd(NaiMe)Cl2 (a) < Pd(NaiBz)Cl2 (c) as well as Pd(,-NaiR)Cl2 (1) < Pd(,-NaiR)Cl2 (2), which corroborates with the experimental ,,Ho values. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 463,472, 2009 [source] Optimization and analysis of nickel adsorption on microwave irradiated rice husk using response surface methodology (RSM)JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2 2009Magesh Ganesa Pillai Abstract BACKGROUND: The removal of heavy metals using adsorption techniques with low cost biosorbents is being extensively investigated. The improved adsorption is essentially due to the pores present in the adsorbent. One way of improving the porosity of the material is by irradiation of the precursor using microwaves. In the present study, the adsorption characteristics of nickel onto microwave-irradiated rice husks were studied and the process variables were optimized through response surface methodology (RSM). RESULT: The adsorption of nickel onto microwave-irradiated rice husk (MIRH) was found to be better than that of the raw rice husk (RRH). The kinetics of the adsorption of Ni(II) from aqueous solution onto MIRH was found to follow a pseudo-second-order model. Thermodynamic parameters such as standard Gibbs free energy (,G°), standard enthalpy (,H°), and standard entropy (,S°) were also evaluated. The thermodynamics of Ni(II) adsorption onto MIRH indicates that it is spontaneous and endothermic in nature. The response surface methodology (RSM) was employed to optimize the design parameters for the present process. CONCLUSION: Microwave-irradiated rice husk was found to be a suitable adsorbent for the removal of nickel(II) ions from aqueous solutions. The adsorption capacity of the rice husk was found to be 1.17 mg g,1. The optimized parameters for the current process were found as follows: adsorbent loading 2.8 g (100 mL),1; Initial adsorbate concentration 6 mg L,1; adsorption time 210 min.; and adsorption temperature 35 °C. Copyright © 2008 Society of Chemical Industry [source] Interpretation of the temperature dependence of equilibrium and rate constantsJOURNAL OF MOLECULAR RECOGNITION, Issue 5 2006Donald J. Winzor Abstract The objective of this review is to draw attention to potential pitfalls in attempts to glean mechanistic information from the magnitudes of standard enthalpies and entropies derived from the temperature dependence of equilibrium and rate constants for protein interactions. Problems arise because the minimalist model that suffices to describe the energy differences between initial and final states usually comprises a set of linked equilibria, each of which is characterized by its own energetics. For example, because the overall standard enthalpy is a composite of those individual values, a positive magnitude for ,Ho can still arise despite all reactions within the subset being characterized by negative enthalpy changes: designation of the reaction as being entropy driven is thus equivocal. An experimenter must always bear in mind the fact that any mechanistic interpretation of the magnitudes of thermodynamic parameters refers to the reaction model rather than the experimental system For the same reason there is little point in subjecting the temperature dependence of rate constants for protein interactions to transition-state analysis. If comparisons with reported values of standard enthalpy and entropy of activation are needed, they are readily calculated from the empirical Arrhenius parameters. Copyright © 2006 John Wiley & Sons, Ltd. [source] Structural and Preliminary Explosive Property Characterizations of New 3,4,5-Triamino-1,2,4-triazolium (Guanazinium) SaltsPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 5 2008Chaza Darwich Abstract Two new highly stable energetic salts were synthesized in reasonable yield by using the high nitrogen-content heterocycle 3,4,5-triamino-1,2,4-triazole and resulting in its picrate and azotetrazolate salts. 3,4,5-Triamino-1,2,4-triazolium picrate (1) and bis(3,4,5-triamino-1,2,4-triazolium) 5,5,-azotetrazolate (2) were characterized analytically and spectroscopically. X-ray diffraction studies revealed that protonation takes place on the nitrogen N1 (crystallographically labelled as N2). The sensitivity of the compounds to shock and friction was also determined by standard BAM tests revealing a low sensitivity for both. B3LYP/6,31G(d,,p) density functional (DFT) calculations were carried out to determine the enthalpy of combustion (,cH(1)=,3737.8,kJ mol,1, ,cH(2)=,4577.8,kJ mol,1) and the standard enthalpy of formation (,fH°(1)=,498.3,kJ mol,1, (,fH°(2)=+524.2,kJ mol,1). The detonation pressures (P(1)=189×108,Pa, P(2)=199×108,Pa) and detonation velocities (D(1)=7015,m s,1, D(2)=7683,m s,1) were calculated using the program EXPLO5. [source] Thermochemical Study on the Chiral Sodium Zincophosphate NanocrystallineCHINESE JOURNAL OF CHEMISTRY, Issue 4 2006Jian Wu Abstract Chiral sodium zincophosphate nanocrystalline has been prepared and characterized. The standard molar enthalpy of the following reaction 12Na3PO4·12H2O(s)+12ZnSO4·7H2O(s)=Na12(Zn12P12O48)·12H2O(s)+12Na2SO4(s)+216H2O(l) was determined by solution reaction calorimetric at 298.15 K, and calculated to be 33.666±0.195 kJ/mol. From the results and other auxiliary quantities, the standard enthalpy of formation for sodium zincophosphate nanocrystalline was derived to be , fH,m [Na12(Zn12P12O48)·12H2O(s), 298.15 K]=,24268.494±0.815 kJ/mol. [source] standard enthalpy of formationCHINESE JOURNAL OF CHEMISTRY, Issue 10 2002The solid complexes of Cr(AA)2 Cl3 · nH2O of CrCl3, with L-, -amino acids (AA= Val, Leu, Thr, Met, Arg, Phe, Try and His) have been prepared in 95% EtOH medium, and characterized structurally by elemental analysis, chemical analysis, IR spectra and TG-DTG. The constant-volume combustion energies of the complexes have been determined by RBC-II type rotating-bomb calorimeter. The standard enthalpies of formation of the complexes have been calculated as well, which are (-2543.16 ± 3.71) (Val), ( ,2561.32 ± 4.06) (Leu), (-2284.02 ± 2.95) (Thr), (-1418.77 ± 4.60) (Met), (-3218.91 ± 4.67) (Arg), (-2643.90 ± 5.02) (Phe), (-1707.18 ± 3.23) (Try) and (-2838.05 ± 3.45) (His) kJ/mol, respectively. [source] | ||||||||||||||||||||||