Análise de síntons intermoleculares de bases N-heterocíclicas e suas correlações com propriedades óticas

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Universidade Federal de Goiás


One of the aims of crystal engineering is the analysis of synthons. The understanding of synthons formation allows modifying them to obtain chemical and physical properties of interest. Among those properties, structure dependents, are optical and electronic properties. The second harmonic generation (SHG) is one example of nonlinear optics phenomena and has a great potential aaplication. In this area, several organic compounds, such as nitrogenous aromatic ones, show SHG. Although robust synthons exist, their modification is delicate, since various parameters induce their formation. Moreover, the SHG has a requirement of inversion center absence in the structure. In this context, guiding structure to specific symmetries are still a crystallographic hard task. In this work the synthons of monoprotonated nitrogenous aromatic compounds salts were analyzed. The salts were synthesized throughout the reaction of nitrogenous compounds (dissolved in isopropanol) with organic and inorganic acids. The used compounds belong to the classes of pyrimidines and pyrazines. After the synthesis, the acidified solutions were submitted to crystallization using the slow evaporation technique. The single crystals had their X-Ray Diffraction data collected. Yet, reflectance diffuse spectroscopy data were collected for some samples. Further, calculations of electronic structure to obtain geometric optimization and spectra absorption data were performed. These calculations were done with density functional theory (and time-dependent density functional theory), with the functional M06-2X or B3LYP and the base 6-31G** or Def2-TZV. In this study fifteen unpublished structures were obtained, among which only one was solved in a noncentrosymmetic space group. The analyses of synthons in all structures revealed two tendencies. The first was observed for the structures obtained from 2-aminopyrazine. In those structures, the protonation of 2-aminopyrazine nitrogen (from aromatic ring) not depended on exclusively of 𝑝𝑘𝑎 from compounds, but also has relation with the final synthons. Accordingly, it was possible to noted that not only a chemical propriety dictates the protonation in the formation of a crystalline medium. Beyond that, the energy of finals intra and intermolecular interactions were essential to understand this result. The other tendency was verified for the compound derived of 2-aminopyrazine substituted by halogen atoms in the aromatic ring, in which, the synthons for the aromatic compounds with or without halogen atoms were the same. Thus, it was observed that a structural modification as an insertion of a halogen atom did not alter the interactions pattern in aromatic compounds. Regarding the physical properties, the direct band gap value for some nitrogenous compound was determined experimentally. Finally, we related the approximationof theoretical and experimental data on electronic transition with the energy of synthons or interactions for a set of atoms present in the structures. For some data, we observed that the greater the stability of the interactions involved, the closer were the theoretical data to the experimental one for the electronic transitions. However, this did not extend to all datasets analyzed in this work.



VALDO, A. K. S. M. Análise de síntons intermoleculares de bases N-heterocíclicas e suas correlações com propriedades óticas. 2021. 163 f. Tese (Doutorado em Química) - Universidade Federal de Goiás, Goiânia, 2021.