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Painel 229|PN.229 Painel 229|PN.229 12 a 17/Nov, 2017, Águas de Lindóia/SP, Brasil DFT study of the self-aggregation of asphaltene model compounds Kassem K. Negea(IC), Mateus R. Lageb(PQ), Stanislav R. Stoyanovc,d(PQ), José W. de M. Carneiroa(PQ) aInstituto de Química, UFF, Outeiro de São João Batista s/n, Centro, Niterói, RJ, 24020-141, Brazil bUniversidade Federal do Maranhão, Campus Balsas, Rua José Leão, 484, Centro, Balsas, MA, 65800-000, Brazil c Natural Resources Canada, Canmet ENERGY-Devon, 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada d Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2 V4, Canada Abstract: Asphaltenes are compounds found in the heavier fractions of the oil as a supramolecular aggregates [1]. The chemical complexity of these fractions of oil creates difficulties for their understanding and the prediction of their behavior. However, many authors have performed studies related to their chemical structure and properties, proposing molecules as asphaltene models, mainly based on data obtained by nuclear magnetic resonance (NMR), elemental analysis and molecular weight [2]. Asphaltenes can cause serious problems in oil exploration, triggered mainly to their deposition that is due to the level of molecular aggregation. The problems caused by the deposition of asphaltenes are present from the exploration to the refining of the petroleum. Thus, it is extremely important to know the mechanisms of aggregation of asphaltenes. In this work, we performed a theoretical study of the homodimerization and self- aggregation of asphaltene model compounds A and B (Figure 1). These model compounds contain fused aromatic rings and heterocyclic fragments representative of asphaltenes tethered with butyl linkers to represent archipelago asphaltenes [3]. The ability of the DFT functional ωB97X-D and 6-31+G (d, p) basis set to predict the optimized geometry and the thermochemistry of aggregation was evaluated [4,5]. Figure 1. Chemical structures of the asphaltene model compounds A and B. 470 Livro.indd 470 01/11/2017 16:25:13 12 a 17/Nov, 2017, Águas de Lindóia/SP, Brasil Through the optimized structures A and B, two self-aggregation arrangements, called "Hamburger" and "Self-aggregation" were proposed (Figure 2). The aggregation free energies, given relative to the free energy of the optimized asphaltene structure without self-aggregation (open structure), are listed in Table 1. A - Hamburguer A - Self-Aggregation B - Hamburguer B - Self-Aggregation Figure 2. Self-aggregation structures of the asphaltene model compounds (A and B). Table 1. Values of the free energy difference of the asphaltenes structures. Model Compound Hamburguer Self-aggregation A -9.83 -8.54 B -16.15 -13.86 Therefore, as observed for both cases, the proposed structure of self-aggregation "hamburger" has substantially lower free energy and would be the most probable conformation of these asphaltene model compounds. Keywords: asphaltenes, aggregation, DFT, free energy. Support: KKN and JWMC acknowledges CNPq and MRL acknowledges CAPES for research grants. The research at UFF was supported by FAPERJ. References: [1] M. R. Gray, R. R. Tykwinski, J. M. Stryker and X. Tan, Energy Fuels, 25, 3125 (2011). [2] H. Groezin, O. Mullins. Energy Fuels, 14, 677 (2000). [3] A. Scherer, F. Hampel, M. R. Gray, J. M. Stryker and R. R. Tykwinski, J. Phys. Org. Chem., 25, 597 (2012). [4] L. M. da Costa; S. R.Stoyanov, S.Gusarov, P. R.Seidl, J. W. de M. Carneiro,A. Kovalenko. J. Phys. Chem. A, 118, 896 (2014). [5] L. M. Costa, S. Hayaki, S. R. Stoyanov, S. Gusarov, X. Tan, M. R. Gray, J. M. Stryker, R. Tykwinsky, J. W. M. Carneiro, H. Sato, P. R. Seidl, A. Kovalenko, Phys. Chem. Chem. Phys., 14, 3922 (2012). 471 Livro.indd 471 01/11/2017 16:25:13 Painel 230|PN.230 12 a 17/Nov, 2017, Águas de Lindóia/SP, Brasil Multi-Electronic-State Approach of Tetranitrogen M. X. Silvaa, B. R. L. Galvãob, J. P. Bragaa, J. C. Belchiora aDepartamento de Química – ICEx, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Pampulha, Belo Horizonte, Minas Gerais, Brazil bDepartamento de Química, Centro Federal de Educação Tecnológica de Minas Gerais, Av. Amazonas 5253, Nova Suíça, Belo Horizonte, Minas Gerais, Brazil Abstract: There is a rising interest nowadays in employing nanoscale materials as new possible types of fuels. To be considered an efficient fuel, the candidate material must be sufficiently unstable in order to react with another system and then release a considerable amount of energy. On the other hand, it must be stable enough to be synthesized and stored without spontaneously decomposing itself [1]. To accomplish the goal of being considered “green” these materials must not only generate reasonably less toxic products than the current widely spread fuels do, but also demonstrate higher performances while being technically as well as economically attractive [2]. In this context, polinitrogen systems have been studied due to their potential use as high- energy density materials (HEDM), since their dissociation into inert N2 molecules may release huge amounts of energy [1,3]. Therefore, studies of the photochemistry of polinitrogen systems can be considered important theoretical prototypes. It is relevant either for supporting the detection of their existence through spectroscopic analysis and/or understanding their chemical behavior in Earth’s atmosphere, other astronomical environments or yet as possible fuels [4]. This subject has already been intensively studied for the nitrogen gas [5,6], while quantum chemical calculations are responsible for most of the information on larger polintrogen systems [7,8]. The formation of linear N3 from the reaction between two N2 molecules, for instance, is expected to occur upon photoexcitation and to present a linear N4 species as transition state [4]. While several calculations involving various polinitrogen isomers indicate high potential energies when compared to molecular nitrogen, only some of them exhibit an essential HEDM feature: high dissociation barrier [3]. Within this perspective, the scientific community has been giving great attention to isomers of tetranitrogen (N4), in particular to the tetrahedral form (N4 – Td) [8,9]. In fact, the good understanding of atomic aggregates or clusters in general relies, foremost, on the capacity of generation of plausible structures for those compositions whose available information is scarce. At the same time, approaching systems with typical cluster dimensions requires taking into consideration the effects arising from the quantum behavior of matter, and the applicability of analytical or empirical potentials becomes questionable [10]. In this context, we have employed and upgraded a previously developed genetic algorithm coupled with electronic structure methods [11], namely QGA, tested to survey the potential energy surface of the tetranitrogen system. The QGA approach presented good cost- effectiveness ratio when it comes to the computational effort associated with electronic structure calculations. It12 is a well 17/Nov,-suited 2017, to Águas assess de Lindóia/SP,the potential Brasil energy surface of atomic clusters, mainly in cases where no previous information about the structural arrangement of the system is available.472 The QGA provided structures in good agreement with coupled cluster results, even though it started from structures generated completely randomly. Theoretical investigations of the electronic potential energy Livro.inddcurves 472 associated with the dissociation of the N4 (Td) into two N2 molecules were01/11/2017 also 16:25:13 performed for the electronic ground state and the first excited states using CASSCF(12,12) level of theory implemented in MOLPRO package [12]. The graphs containing the dissociation paths studied involved singlet and triplet states, where it was possible to observe both conical intersections and intersystem crossings. Possible alternative dissociation channels for the N4 (Td) were then inferred from these crossings and photoexcitation of this species. Coupled cluster (CCSD(T)-F12) calculations were also performed on specific structures to generate benchmark results, which were then compared to results provided by density functional theory (DFT) calculations with several different exchange-correlation functionals in order to find the best match. The tetrazete Td and planar D2h isomers were approached by higher level of theory (CCSD(T)-F12/cc-pCVTZ-F12) than already reported until the present date. The N4 -1 -1 (Td) was predicted to be 182 kcal.mol above 2N2 in energy, and 1 kcal.mol above the D2h isomer. The unraveling of a possible reaction mechanism involving the abstraction of a nitrogen atom from a N2 molecule in an excited electronic state, by another excited N2 molecule, to produce linear N3 is also currently in progress. Key-words: polinitrogen, aggregates, cluster, photochemistry Support: This work has been supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). References: [1] K. M. Dunn, K. Morokuma, J. Chem. Phys., 102, 4904 (1995). [2] T. Brinck, M. Rahm, “Theoretical Design of Green Energetic Materials: Predicting Stability, Detection, Synthesis and Performance, in Green Energetic Materials” (2014), John Wiley & Sons, Chichester, United Kingdom. [3] P. C. Samartzis, A. M. Wodtke, Int. Rev. Phys. Chem., 25, 527 (2006). [4] S. L. Chou, J. I. Lo, M. Y. Lin, Y. C. Peng, H. C. Lu, B. M. Cheng, Angew. Chem. Ind. Ed., 53, 738 (2014). [5] A. Lofthus, P. H. Krupenie, J. Phys. Chem. Ref. Data, 6, 113 (1977). [6] H. Okabe, “Photochemistry of Small Molecules” (1978), John Wiley & Sons, New York. [7] M. J. Greshner et al., J. Phys. Chem. A, 120, 2920, (2016). [8] M. Bittererová, H. Östmark, T. Brinck, J. Chem. Phys., 116, 9740, (2002). [9] T. J. Lee, J. E. Rice, J. Chem. Phys., 94, 1215, (1991). [10] M. X. Silva, B. R. L. Galvão, J. C. Belchior, Phys. Chem. Chem. Phys., 16, 8895, (2014). [11] F.
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