Estudo teórico de ácidos aminobenzóicos usando técnicas de mecânica quântica e simulação computacional

Abstract

In this work we realize a study of the aminobenzoic acid family (Abz) in neutral, anionic and cationic conformations combining a Sequential QM/MM method with mean field approximation (ASEC) with Free Energy Gradient method (FEG), for the solvents: cyclohexane, acetonitrile and water. In all cases, the Abz molecules type present a conformation in aqueous solution has greater geometry distortion compared to the gas phase. According to our results, there is just one cationic conformation in aqueous solution for ortho. For para and meta, anionic molecules the geometry is more stable planar conformation, which is a saddle point in gas phase. For neutral ortho, the rotamer I is the more energetically stable structure. In anionic conformations it is found the largest number of hydrogen bonds in aqueous solution. In acetonitrile and water, the energy stability of conformations can be explained by hydrogen bonds and solute molecule dipole moment. In all cases, the peak of the first absorption band is in good agreement with experimental results. Our results turn out to be more close to experimental result if explicit solvent molecules are included. The theoretical absorption spectra has similar behaviour to those obtained experimentally and the absorption band shifts are well described with ASEC, mainly those involving the neutral, anionic and cationic conformations in water.