Análise de síntons intermoleculares de bases N-heterocíclicas e suas correlações com propriedades óticas
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2021-12-17
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Universidade Federal de Goiás
Resumo
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study fifteen unpublished structures were obtained, among which only one was solved in a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.
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Citação
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.