Desvendando Mecanismos Moleculares do Halotano e do Ácido Ascórbico: dinâmica de Surface Hopping na racionalização da Fotodissociação atmosférica, das razões de ramificação e da inversão Quiral do Halotano; e Dinâmica Molecular de Car–Parrinello na Investigação da Solvatação

Abstract

The elucidation of photochemical and solvation processes requires approaches capable of integrating electronic structure, molecular dynamics, and solvent effects. In this thesis, halothane was investigated along two main axes: excited- state photodynamics, with emphasis on the selective dissociation of carbon– halogen bonds and chirality inversion, and solvation in aqueous solution. In parallel, ascorbic acid was used as a water-soluble system to investigate hydration networks stabilized by hydrogen bonds. Ab initio molecular dynamics simulations were performed within the Car–Parrinello formalism (CPMD/PBE), complemented by TDDFT calculations and nonadiabatic dynamics using the Surface Hopping approach. Solvation was characterized through pair radial distribution functions (PRDFs) and mean residence times (MRTs), whereas electronic and topological properties were analyzed using NTO, QTAIM, and RDG methods. For halothane photodynamics, 138 trajectories initiated in S1 and S4 were classified according to their structural outcomes. Molecular stabilization was the most frequent result (51.4%), followed by chirality inversion (25.4%) and permanent dissociation (23.2%). The initial electronic state influenced the selectivity, with a greater contribution of dissociative processes in S4 and an overall predominance of C–Cl cleavage, resulting in a C–Cl:C–Br ratio of 2.20:1. In contrast, 71.4% of the configurational inversion events were associated with the C–Br coordinate, revealing distinct tendencies for fragmentation and stereochemical reorganization. Temporal analysis of 119 trajectories yielded a mean first-arrival time at S0, ts0, of 986.66 fs, with later access to the ground state for trajectories initiated in S4 and for those associated with dissociation or inversion. In aqueous solution, halothane preserved its structural integrity and exhibited weakly structured solvation, characterized by weak and transient interactions consistent with its hydrophobic character. For ascorbic acid, CPMD/PBE simulations revealed selective solvation, particularly at the H(1) and H(2) sites, with persistent hydrogen bonds, characteristic distances close to 2.5 Å, residence times between 10 and 15 ps, and free energies around −10 kcal mol−1. Overall, the results demonstrate that the combination of ab initio molecular dynamics, nonadiabatic dynamics, and electronic and topological analyses enables the connection of electronic structure, nuclear motion, photochemical selectivity, and solvent organization. This thesis advances the molecular understanding of photodissociation, stereochemical reorganization, and solvation processes relevant to atmospheric chemistry and the behavior of biomolecules in solution.