Predicting Thermochemistry

The basic aspects of the BAG-MP4 method used to predict thermochemistry for main-group molecules have been previously described [72] and a detailed description of its extension to compounds containing tin has been published... 

Reaction Eq. 36 is endothermic, suggesting a large kinetic barrier. Giunta et al. estimated a 15 kcal mol barrier height, which is unrealistic based on the predicted thermochemistry. Thus, this step probably represents a minor reaction channel. Peroxide formation from CHsSnCr, however, is very... 

The first two papers in the series by Becke [9, 10] explored the improvement in the predicted thermochemistry of DFT methods due to the inclusion of gradient corrections from exchange (B88) and correlation... 

Our goal in this chapter is to help you learn the laws of thermodynamics, especially the concepts of entropy and free energy. It might be helpful to review Chapter 6 on thermochemistry and the writing of thermochemical equations. The concept of Gibbs free energy (G) will be useful in predicting whether or not a reaction will occur spontaneously. Just like in all the previous chapters, in order to do well you must Practice, Practice, Practice. 

K. K. Irikura, D. J. Frurip, Eds. Computational Thermochemistry Prediction and Estimation of Molecular Thermodynamics. ACS Symposium Series 677 American Chemical Society Washington, DC, 1998. 

In this chapter, we look closely at the performance of several ab initio techniques in the prediction of radical thermochemistry with the aim of demonstrating which procedures are best suited in representative situations. We restrict our attention to several areas in which we have had a recent active interest, namely, the determination of radical heats of formation (AHf), bond dissociation energies (BDEs), radical stabilization energies (RSEs), and selected radical reaction barriers and reaction enthalpies. We focus particularly on the results of our recent studies. 

Miedema s theory and structural information. The Miedema model for energy effects in alloys, presented in 2.2.1.3 has been very useful in an evaluation, albeit approximate, of the formation enthalpies and in the prediction of compound formation capability. For an example of the application and limits of this model, see the comments on the thermochemistry of the Laves phases reported in 3.9.3. However notice that the general usefulness of the Miedema approaches has diminished with time, both for its inherent approximation and for... 

In Fig. 1, various elements involved with the development of detailed chemical kinetic mechanisms are illustrated. Generally, the objective of this effort is to predict macroscopic phenomena, e.g., species concentration profiles and heat release in a chemical reactor, from the knowledge of fundamental chemical and physical parameters, together with a mathematical model of the process. Some of the fundamental chemical parameters of interest are the thermochemistry of species, i.e., standard state heats of formation (A//f(To)), and absolute entropies (S(Tq)), and temperature-dependent specific heats (Cp(7)), and the rate parameter constants A, n, and E, for the associated elementary reactions (see Eq. (1)). As noted above, evaluated compilations exist for the determination of these parameters. Fundamental physical parameters of interest may be the Lennard-Jones parameters (e/ic, c), dipole moments (fi), polarizabilities (a), and rotational relaxation numbers (z ,) that are necessary for the calculation of transport parameters such as the viscosity (fx) and the thermal conductivity (k) of the mixture and species diffusion coefficients (Dij). These data, together with their associated uncertainties, are then used in modeling the macroscopic behavior of the chemically reacting system. The model is then subjected to sensitivity analysis to identify its elements that are most important in influencing predictions. 

As evident from these results, the predictions of the thermochemistry of various C Hg isomers, with the exception of cyclic C4Hg, are in reasonable accord with the available experimental data, i.e., the agreements in AHf are all within + 3 kcal/mol and the results exhibit correct trends. In the case of... 

Quantum Chemistry Methods for the Prediction of Molecular Thermochemistry... 

A theoretical approach that we find predicts main-group thermochemistry quite successfully is the BAG method [67,72-74]. To date, BAG methods have been applied to and thermochemistry reported for compounds of boron [75,76], carbon [73,77], nitrogen [73], aluminum [76,78], sihcon [72,79-86], and galUiun [76]. The BAG methodology recognizes that... 

Our first example that attempts to imravel this complexity is entitled Gas-Phase Thermochemistry and Mechanism of Organometallic Tin Oxide CVD Precursors . The authors, M. Allendorf and A.M.B. van Mol, describe the development of quantum chemistry methods that can predict heats of formation for a broad range of tin compounds in the gas phase, which need to be considered when Sn(CH3)4 or (CH3)2SnCl2 and other tin alkyls are used as precursors together with oxygen and water for tin oxide deposition. 

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