Estudo do efeito de diferentes substituintes no equilíbrio conformacional de acilatos de cis-3-hidroxi-cicloexila

Abstract

It’s been a long time since computational chemistry has achieved a competitive position with experimental methods, generating several data such as geometric arrangement and relative energies, allowing conformational analysis and hydrogen bond studies. The present work aimed the effect study of different substituents (R = CF3, Cl, F, H, N(CH3)2, NH2, OCH3 e OH) in the conformational equilibrium of cis-3-hidroxy-cyclohexyl R-methanoates, using, through the softwares Gaussian03, Gaussian09 and Gamess, the theoretical methods: density functional theory (DFT), using the functionals B3LYP and M06-2X; second order Møller-Plesset’s perturbation theory (MP2); quantum theory of atoms in molecules (QTAIM); natural bond orbital (NBO); potential energy surfaces (PES). Among the used methods, the DFT/M06-2X turned out to be the most appropriate method, through a calibration study, followed by DFT/B3LYP, SCS-MP2 and MP2. Preliminary studies showed that the energy correction for internal rotations and the approximation by ( ) were not adequate to the present study, then not being considered. Cis-3-hidroxy-cyclohexyl methanoates studies revealed that the rotamers aa13 and ee14 have presented as the most stable for diaxial and diequatorial conformers, respectively, with a = -0,555 kcal/mol in diequatorial-diaxial conversion, and with the H22 substitution for electronic density donors groups (N(CH3)2, NH2, OCH3 and OH) strengthened the O20-H21...O17 intramolecular hydrogen bond, favoring the diaxial conformer, lowering the conversion to values such as -1,09 kcal/mol to OCH3. On the other hand, the exchange of H22 to electronic density withdrawing groups (CF3, Cl e F), weakening the O20-H21...O17 intramolecular hydrogen bond, increasing the conversion to values such as -0,450 kcal/mol with CF3. QTAIM along with NBO has proved to be a very simple and useful method to identification and characterization of intramolecular hydrogen bonds.