Search results for "Thermochemistry"
showing 6 items of 16 documents
Conceptual and epistemological difficulties of future teachers of physics and chemistry in the energetic explanations of physical and chemical phenom…
2016
En este trabajo analizamos las dificultades conceptuales y epistemológicas que tienen sobre termoquímica estudiantes universitarios de los últimos cursos de Física, Química e Ingeniería Química. Para ello se han aplicado a una muestra de 35 de estos futuros profesores dos cuestionarios sobre efectos térmicos en fenómenos físicos y químicos que han de explicar y se han categorizado sus respuestas según los razonamientos usados. Se ha constatado que la mayoría de los estudiantes no saben delimitar en el nivel macroscópico los sistemas que interaccionan, no utilizan el concepto de energía interna ni la primera ley de la termodinámica en los procesos físicos y químicos. Muy pocos de estos profe…
W3 theory: robust computational thermochemistry in the kJ/mol accuracy range
2003
We are proposing a new computational thermochemistry protocol denoted W3 theory, as a successor to W1 and W2 theory proposed earlier [Martin and De Oliveira, J. Chem. Phys. 111, 1843 (1999)]. The new method is both more accurate overall (error statistics for total atomization energies approximately cut in half) and more robust (particularly towards systems exhibiting significant nondynamical correlation) than W2 theory. The cardinal improvement rests in an approximate account for post-CCSD(T) correlation effects. Iterative T_3 (connected triple excitations) effects exhibit a basis set convergence behavior similar to the T_3 contribution overall. They almost universally decrease molecular bi…
High-accuracy extrapolated ab initio thermochemistry. II. Minor improvements to the protocol and a vital simplification
2006
The recently developed high-accuracy extrapolated ab initio thermochemistry method for theoretical thermochemistry, which is intimately related to other high-precision protocols such as the Weizmann-3 and focal-point approaches, is revisited. Some minor improvements in theoretical rigor are introduced which do not lead to any significant additional computational overhead, but are shown to have a negligible overall effect on the accuracy. In addition, the method is extended to completely treat electron correlation effects up to pentuple excitations. The use of an approximate treatment of quadruple and pentuple excitations is suggested; the former as a pragmatic approximation for standard cas…
Kinetics of the formation reactions of trichloro- and tribromomethyl hypohalites and alcohols in the gas-phase: Theoretical study
2007
Abstract The CX3OY molecules (X = Cl, Br and Y = H, F, Cl, Br) can be formed in the atmosphere by the recombination CX3 + OY and CX3O + Y reactions. In the present study the results of a theoretical analysis of the kinetics and thermochemistry of this class of reactions are performed. The molecular properties of the reactants and products were derived from ab initio calculations. The high-pressure limiting rate constants for the recombination reactions were evaluated using a version of the statistical adiabatic channel model. The kinetic equations derived in this study allow a description of the kinetics of the reactions under investigation in the temperature range of 200–400 K.
Molecular “Floppyness” and the Lewis Acidity of Silanes: A Density Functional Theory Study
2001
A comprehensive set of Lewis acid-base adducts of silanes was investigated by means of the density functional theory geometry optimization [B3LYP/6−31G(d)], and thermochemical calculations, [B3LYP/6−311+G(2d,p)//B3LYP/6−31G(d)]. Complex formation was found to weaken Si−Cl and Si−Br bonds more than Si−F or Si−H bonds. Comparable distances between Si and a Lewis base L (L = NH3, OH2, F−) are shorter in hexa- than in pentacoordinated complexes. The molecular structures of the pentacoordinated Si complexes allowed for a mapping of an SN2 reaction pathway by correlating the lengths of the Si−X and Si−L bonds. Complex formation was found to be exothermic for most of the coordination compounds, an…
Benson group additivity values of phosphines and phosphine oxides: Fast and accurate computational thermochemistry of organophosphorus species
2018
Composite quantum chemical methods W1X-1 and CBS-QB3 are used to calculate the gas phase standard enthalpy of formation, entropy, and heat capacity of 38 phosphines and phosphine oxides for which reliable experimental thermochemical information is limited or simply nonexistent. For alkyl phosphines and phosphine oxides, the W1X-1, and CBS-QB3 results are mutually consistent and in excellent agreement with available G3X values and empirical data. In the case of aryl-substituted species, different computational methods show more variation, with G3X enthalpies being furthest from experimental values. The calculated thermochemical data are subsequently used to determine Benson group additivity …