Análise conformacional e correções térmicas em moléculas de cicloalcanos e etanos-substituídos

Abstract

Ab initio quantum mechanical methods in conjunction with standard formalism of statistical thermodynamics can be applied for the determination of macroscopic thermodynamic properties of chemical systems. The inclusion of thermal correction for the theoretical calculation of thermodynamic properties has been done. The evaluation of translational, rotational and vibrational partition function for a molecule can yield a direct comparison and also an agreement with experimental gas phase data. The presence of low frequency vibrational modes constitute in a problem and it is still open, where a general solution has not been found yet. All over the years various methods have been employed aiming at a better description of the thermodynamic properties of chemical species (Gibbs free energy, conformational population and entropy). In this Thesis, high level ab initio methods (MP4(SDTQ), CCSD(T)) combined with standard formalism of statistical thermodynamics were used to investigate cycloalkanes and ethane-substitute molecules, aiming to assess the performance of the level of theory employed and also a detailed analysis of the influence of the low frequency vibrational modes on the vibrational partition function for the calculation of conformational populations. A direct comparison with experiment was also made for all molecules studied in this work. It was shown, in a transparent way, that in the cases where a disagreement with gas phase experimental conformational population data was observed. The reason for the discordance can not be attributed to the ab initio Post-Hartree-Fock level of calculation utilized for the evaluation of thermal correction (MP2) and electronic plus nuclear-repulsion energy (CCSD(T)). It was made clear that the theoretical model employed to treat the low frequency normal modes was not adequate for the description of molecules where the Gauche Effect play an important role.