Polimorfos do ácido bórico e metabórico: espectroscopia óptica e vibracional (IR/Raman), e cálculos DFT considerando ligações de hidrogênio e interações Van der Waals inter e intraplanares

Abstract

Boric acid (B(OH)3) is a compound widely used by society, such as pesticide and fertilizer; in the glass, porcelain and ceramics industries; as flame retardant and wood preservative; in tannery and textile industries; etc. Recently, it has drawn attention to the use of small concentrations (~1%) of boric acid in sweet solutions, which are sprayed on plants or used in traps, and causes a marked decrease in the Aedes aegypit (Linnaeus) and Aedes albopictus (Skuse) mosquitoes populations, responsible vectors for the propagation of dengue, Zika, chikungunya and malaria. From the boric acid it is obtained the metaboric acid, (BOH)3O3, by thermal decomposition (thermolysis) at low temperature (< 150 oC). In this work, we study the boric acid polymorphs H3BO3-2A triclinic and H3BO3-3T trigonal of stacked monolayers ABAB... and ABCABC...,respectively, as well as the metaboric acid polymorphs α-(BOH)3O3 triclinic of stacked monolayers, β-(BOH)3O3 monoclinic polymer-like, and γ-(BOH)3O3 cubic covalent. We have performed optical UV-Vis and Infrared/Raman spectroscopic measurements for H3BO3-2A, while for all three metaboric acid polymorphs we have taken advantage from the already published experimental data of infrared and Raman vibrational modes. Pursuing a deeper understanding of the experimental results, we have calculated the structural, electronic, optical and vibrational (including phonons and thermodynamic properties) of both the boric and metaboric acid polymorphs, their monolayers and isolated molecules within the context of the Density Functional Theory (DFT). As the most important results, we have obtained that: (i) energy gap of the polymorph H3BO3-2A is in very good agreement with the indirect gap obtained from our band structure and optical absorption DFT calculations; (ii) the H3BO3-2A DFT calculated vibrational wavelengths are in excellent agreement with the experimental values when it is considered its bulk, isolated monolayer and molecule description, respectively. Assignments of the atomic displacements were performed for each vibrational mode, and identified vibrational signatures related to them; (iii) the Kohn-Sham band structures for the three metaboric acid polymorphs suggest they are isolator materials; (iv) a reasonably small heating of the boric and metaboric acid polymorphs can change their light emission efficiency by transforming their indirect gaps in direct ones, pointing to a change from an inefficient to an efficient emission of photons; (v) the simulated IR and Raman spectra of the metaboric acid polymorphs showed a very good agreement with the already published experimental data; (vi) the calculated vibrational modes of an isolated monolayer of boric acid molecules are in good agreement with the α-polymorph spectra , which suggest that the noncovalent interplanar interactions produce only subtle effects on the lattice vibrations of α-(BOH)3O3, the stacked monolayers metaboric acid polymorph; (vii) there is an excellent agreement between the theoretical and experimental results for all the bulk metaboric acid polymorphs; (viii) attributions of atomic motions were performed for each of the vibrational modes, and were identified molecular vibrational signatures related to intra and interplanar motions for all the three metaboric acid polymorphs.