The prediction of thermochemical properties is central in chemistry and is essential in industry to predict the stability of materials and to gain understanding about properties of reactions of interest such as enthalpies of formation, activation energies and reaction enthalpies. Advances in state-of-the-art computing and algorithms as well as high-level ab initio methods have accounted for the generation of a considerable amount of thermochemical data. Today, the field of thermochemistry is largely dominated by computational methods, particularly with their low cost relative to the cost of experiment.There are many computational approaches used for the prediction of thermochemical properties. In selecting an approach, considerations about desired level of accuracy and computational efficiency need to be made. Strategies that have shown utility in the prediction of thermochemical properties with high accuracy and lower computational cost than high-level ab initio methods are ab initio composite approaches, or model chemistries, such as the correlation consistent Composite Approach (ccCA). ccCA has been shown to predict enthalpies of formation within "chemical accuracy", which is considered to be 1 kcal mol-1, on average, for main group elements with respect to well-established experiments. In this dissertation, ccCA and the commonly used Gn composite methods have been utilized to establish effective routes for the determination of structural and thermochemical properties of oxygen fluorides species and for organoselenium compounds. To assess the reliability of these approaches, enthalpies of formation were calculated and compared to experimental data. Density functionals have also been employed in these projects to examine their performance in comparison to experiments as well as to composite methods. The impact of several thermochemical approaches on the accuracy of the predicted enthalpies of formation via various computational methods has been also considered such as the traditional atomization approach and molecular reaction approaches. Additionally, in this dissertation, the reaction of a direct amination of benzene to produce aniline on the Ni(111) surface was investigated to identify possible reaction intermediates and to determine the thermodynamically preferred reaction pathways. The adsorption behaviors and energetics of all species involved in this reaction are presented. Periodic density functionals were used to consider this heterogeneous catalytic process. Because DFT is based on the uniform electron gas model, which in principle resembles the band theory of metallic systems, DFT is particularly good at modeling metallic systems and thus well suited for the study of heterogeneous catalysts at the molecular level.
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