Home About us Contact
|
Home About us Contact | ||
|
|
||
J/mol K (mol + k)
Selected AbstractsModified calculations of hydrocarbon thermodynamic propertiesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2006Min Hsien Liu Abstract A test set of 65 hydrocarbons was examined to elucidate theoretically their thermodynamic properties by performing the density-functional theory (DFT) and ab initio calculations. All the calculated data were modified using a three-parameter calibration equation and the least-squares approach, to determine accurately enthalpies of formation (,Hf), entropies (S), and heat capacities (Cp). Calculation results demonstrated that the atomization energies of all compounds exhibited an average absolute relative error ranging between 0.11, 0.13%, and an ,Hf of formation with a mean absolute absolute error (M.|A.E.|) ranging from only 5.7,6.8 kJ/mol (1.3,1.6 kcal/mol) (i.e., those results correlated with those of Dr. Herndon's 1.1 kcal/mol). Additionally, the entropy ranged from 3.5,4.2 J/mol K (0.8,1.0 cal/mol K) M.|A.E.|; a heat capacity between 2.3,2.9 J/mol K (0.5,0.7 cal/mol K) M.|A.E.| was obtained as well. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 537,544, 2006 [source] Dynamic Charge Equilibration-Morse stretch force field: Application to energetics of pure silica zeolitesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2002Jan Sefcik Abstract We present the Dynamic Charge Equilibration (DQEq) method for a self-consistent treatment of charge transfer in force field modeling, where atomic charges are designed to reproduce electrostatic potentials calculated quantum mechanically. Force fields coupled with DQEq allow charges to readjust as geometry changes in classical simulations, using appropriate algorithms for periodic boundary conditions. The full electrostatic energy functional is used to derive the corresponding forces and the second derivatives (hessian) for vibrational calculations. Using DQEq electrostatics, we develop a simple nonbond force field for simulation of silica molecular sieves, where nonelectrostatic interactions are described by two-body Morse stretch terms. Energy minimization calculations with the new force field yield accurate unit cell geometries for siliceous zeolites. Relative enthalpies with respect to quartz and third-law entropies calculated from harmonic vibrational analysis agree very well with available calorimetric data: calculated SiO2 enthalpies relative to ,-quartz are within 2 kJ/mol and entropies at 298 K are within 3 J/mol K of the respective experimental values. Contributions from the zero point energy and vibrational degrees of freedom were found to be only about 1 kJ/mol for the free energy of mutual transformations between microporous silica polymorphs. The approach presented here can be applied to interfaces and other oxides as well and it is suitable for development of force fields for accurate modeling of geometry and energetics of microporous and mesoporous materials, while providing a realistic description of electrostatic fields near surfaces and inside pores of adsorbents and catalysts. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1507,1514, 2002 [source] Influence of La doping on the properties of molybdenum perovskite Sr1,xLaxMoO3 (0 , x , 0.2)PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2006S. B. Zhang Abstract The effects of La-doping on the structural, magnetic, electrical transport and specific-heat properties in 4d perovskites Sr1,xLaxMoO3 (0 , x , 0.2) have been investigated. The substitution of Sr ions by La ions does not change the space group of the samples, but increases the lattice parameter a (Å). The resistivity , and magnetic susceptibility , decrease monotonously with the increase of x , while the electronic specific heat coefficient ,e increases. The resistivity of all samples shows a T2 dependence in the low-temperature region of 2 K < T < 125 K and a T dependence in the high-temperature range of 130 K < T < 350 K, related to the electron,electron (e,e) and electron,phonon (e,ph) scattering, respectively. The specific-heat data agrees with the classical Dulong,Petit phonon specific heat, Ccl = 3kBrNA = 124.7 J/mol K at high temperatures and Cp(T )/T = ,e + ,pT2 at low temperatures. These behaviors can be explained according to the decrease of the density of states (DOS) at the Fermi energy level (EF), N (EF). (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Modified calculations of hydrocarbon thermodynamic propertiesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2006Min Hsien Liu Abstract A test set of 65 hydrocarbons was examined to elucidate theoretically their thermodynamic properties by performing the density-functional theory (DFT) and ab initio calculations. All the calculated data were modified using a three-parameter calibration equation and the least-squares approach, to determine accurately enthalpies of formation (,Hf), entropies (S), and heat capacities (Cp). Calculation results demonstrated that the atomization energies of all compounds exhibited an average absolute relative error ranging between 0.11, 0.13%, and an ,Hf of formation with a mean absolute absolute error (M.|A.E.|) ranging from only 5.7,6.8 kJ/mol (1.3,1.6 kcal/mol) (i.e., those results correlated with those of Dr. Herndon's 1.1 kcal/mol). Additionally, the entropy ranged from 3.5,4.2 J/mol K (0.8,1.0 cal/mol K) M.|A.E.|; a heat capacity between 2.3,2.9 J/mol K (0.5,0.7 cal/mol K) M.|A.E.| was obtained as well. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 537,544, 2006 [source] Kinetic study of the reaction of dimethyl carbonate with trialkylaminesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2010Duane E. Weisshaar Quaternary ammonium compounds are produced worldwide in hundreds of millions of pound volume annually for a plethora of end-uses from fabric-care formulations to asphalt emulsifiers, typically from nongreen alkylating reagents. The kinetics of a reaction employing dimethyl carbonate (DMC) as a green alkylating agent was investigated using three trialkylamines (tributylamine, trihexylamine, and trioctylamine) at several temperatures. Arrhenius and Eyring analysis of the data showed that values of Ea (79 kJ/mol), ,H, (75 kJ/mol), and ,S, (220 J/(mol K)) were the same for all three amine reactants, consistent with a report that Ea is independent of alkyl chain length when the chain length is greater than three carbons. Although rates are significantly slower with DMC than with other alkylating reagents, the resulting methyl carbonate anion has advantages for clean anion metathesis, which is important for some applications, especially those involving ionic liquids. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 221,225, 2010 [source] | ||||||||||