Thermochemistry density functional calculations

Cramer, C. J., Nash, J. J., and Squires, R. R. 1997. A Reinvestigation of Singlet Benzyne Thermochemistry Predicted by CASPT2, Coupled-cluster and Density Functional Calculations , Chem. Phys. Lett., m, 311. 

Density functional models and MP2 models are needed for accurate descriptions of the thermochemistry of reactions which involve net bond making or breaking, and for calculation of absolute activation energies. In practice, MP2 models may only be applied to relatively small molecules, whereas density functional models are roughly comparable in cost to Hartree-Fock models for molecules of moderate size. 

The use of computational chemistry to address issues relative to process design was discussed in an article. The need for efficient software for massively parallel architectures was described. Methods to predict the electronic structure of molecules are described for the molecular orbital and density functional theory approaches. Two examples of electronic stracture calculations are given. The first shows that one can now make extremely accurate predictions of the thermochemistry of small molecules if one carefully considers all of the details such as zero-point energies, core-valence corrections, and relativistic corrections. The second example shows how more approximate computational methods, still based on high level electronic structure calculations, can be used to address a complex waste processing problem at a nuclear production facility (Dixon and Feller, 1999). 

In Section 2.1, we remarked that classical thermodynamics does not offer us a means of determining absolute values of thermodynamic state functions. Fortunately, first-principles (FP), or ab initio, methods based on the density-functional theory (DFT) provide a way of calculating thermodynamic properties at 0 K, where one can normally neglect zero-point vibrations. At finite temperatures, vibrational contributions must be added to the zero-kelvin DFT results. To understand how ab initio thermodynamics (not to be confused with the term computational thermochemistry used in Section 2.1) is possible, we first need to discuss the statistical mechanical interpretation of absolute internal energy, so that we can relate it to concepts from ab initio methods. 

It is important and of value to try to validate the accuracy of the Density Functional method in order to use DFT to estimate thermochemistry in large oxygenated hydrocarbons, where high level ab initio calculations may not be possible. Enthalpies (A are calculated for the... 

Mascal and Nikitin reported the cmiversion of CMF 2 into either HMF 1 or LA 4 with excellent selectivities and yields. HMF was obtained within 30 s in 86% yield by the action of boiling water on CMF. When CMF is hydrolyzed for an extended period (20 min) at 190°C, LA is produced in 91% yield [132]. Similarly, HMF has been obtained in quantitative yield by stirring the more reactive BMF 3 in water at RT [127]. When these same reactions are carried out in alcoholic solution, the corresponding ethers and esters are produced. Thus, both CMF and BMF have been converted into 5-(ethoxymethyl)furfural (EMF) 28 (R=Et), a proposed biofuel, in high yield on treatment with ethanol [50,120,127] (Scheme 11). Levulinate esters 29 are likewise produced in high yields from CMF on heating with alcohols at elevated temperatures [132]. hi a recent theoretical smdy, the thermochemistry of the conversion of CMF into HMF, LA, EMF, and ethyl levulinate was calculated using G4 theory and density functional (DFT) methods [133]. 

The hrst hve chapters (Part 1) present an overview of some methods that have been used in the recent hterature to calculate rate constants and the associated case studies. The main topics covered in this part include thermochemistry and kinetics, computational chemistry and kinetics, quantum instanton, kinetic calculations in liquid solutions, and new applications of density functional theory in kinetic calculations. The remaining hve chapters (Part II) are focused on apphcations even though methodologies are discussed. The topics in the second part include the kinetics of molecules relevant to combustion processes, intermolecular electron transfer reactivity of organic compounds, lignin model compounds, and coal model compounds in addition to free radical polymerization. 

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