Estudo termodinâmico da rotação interna em etanos substituídos

Abstract

Statistical Thermodynamics is a widely used formalism to determinate thermodynamic quantities of chemical systems from ab initio results. However, some movements are commonly neglected in calculation of these quantities. The Internal rotation is a good example of those kinds of motions, once this motion, as well as other, is also associatedthermal quantities, such as entropy and internal energy. In this dissertation we have developed a methodology based on numerical solution of the Schrödinger equation for calculating the thermal corrections related to this movement. This methodology depends on the calculation of the potential energy surface (PES) for the internal rotationand the reduced moment of inertia of the top. First, tests were performed on simple systems, such as hydrogen peroxide, H2O2, and ethane, C2H6. The results for these two systems showed a good agreement with the experimental data available and then allowed that the main target of the work was studied: 1,2-diuoroethane (1,2-DFE) and 1,2-dichloroethane (1 ,2-DCE) molecules. The interest of these studies is the anti!gauche process in order to quantify the populations of the two species. SEPs were built through a CCSD(T)/6-311++G(3df; 3pd)//MP2/6-311++G(3df; 3pd) scan calculation and Eckart's reduced inertias were used to obtain the eigenvalues related to internalrotation, and consequently, thermodynamic corrections related to this movement. Once these corrections were obtained, free Gibbs energy of conformational process and anti population of 1,2-DFE and 1,2-DCE conformers were then calculated. The results were well consistent with the results available in the literature. For the 1,2-DFE, theestimated population of anti conformer, with T = 298:15 K, was 38%, while the experimental value corresponds to 37 +- 5%. For 1,2-DCE, the anti population at T = 298:15 K was estimated in 75% while the experimental value is 78 +- 5%. Furthermore, the valuescalculated for other temperatures were also consistent with available experimentalvalues.