Proposta de demarcação da estrutura supramolecular cristalina de sais de di-, tri- e tetrametilamônio

Abstract

Using the supramolecular cluster demarcation, whose criteria is the contact area between molecules, a series bromide salts of di-, tri- and tetramethylammonium was used as models to propose a demarcation of the supramolecular crystalline structure, thus allowing to understand the formation of the crystalline network. The comprehension was that the ions interact while in solution and as the solvent slowly evaporates these ions start forming specific cation anion pairs. The most stable pairs tend to predominate and interact creating the crystalline network. Thus, the anion (AN) which shows the highest stabilization energy with C1 was identified. This pair was named D1 (dimer one). The dimeric pairs of cation and anion interact originating the crystalline network. Variations of the calculation methods were made. The theory levels Hartree-Fock (HF), Moller-Plesset (MP2) and Density Functional Theory (DFT) with the functional ωb97xd were tested to choose the most appropriate method to obtain the interaction energies. Different basis-set were tested as well, -21G, cc-pvDz, aug-cc-pvDz, aug-cc-pvTz e aug-cc-pvDz-pp. The theory level DFT with the functional ωb97xd and the cc-pvDz basis set was chosen. This choice was based in the correlation coefficient between the functional results versus MP2 results. The stabilization energies and contact areas between the cation-anion dimers were evaluated. To better comprehend, and compare the results of each compound, the energetic and contact area data was normalized. Several destabilizing interaction energies between dimers were identified in all compounds. A parameter was proposed to assess the stabilization of the supramolecular cluster. The Stabilization Quotient (SQ), calculated by the quotient of destabilizing and stabilizing energies of dimers in the supramolecular cluster. From the intermolecular energy data, both raw and normalized, and crystalline network observations an explanation for the destabilizing energies was identified. The dimers which showed destabilizing energies presented poles of same charge near one another were found, thus justifying the electrostatic repulsion. Furthermore, it was possible to verify that the increase in the organic character of the ammonium ion decreased the repulsive energies between dimers due to inductive effect stabilizing the partial positive charge and shielding the cation against destabilizing interactions. The more CH 3 substituents were added to ammonium, more similar the energetic profile was to neutral organic compounds. In addition, the QE parameter proposed reflected this increase in stabilization becoming closer to zero. Finally, with the energetic and contact area analysis and observing the crystalline lattice it was possible to propose crystallization mechanisms for the compounds. After the formation of the dimer other stages were proposed. Compound 1 dimers interact to form 2D nuclei. These nuclei interact creating the 3D network. Compound 2 shows a similar crystallization mechanism. The dimers interact creating 2D nuclei which aggregate creating the 3D network. However, these energies are not as destabilizing as in 1. Compound 3 dimers form 1D nuclei which aggregate generating the 3D crystalline network almost with no destabilizing interactions.