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Sistemas moleculares fotoativos para armazenamento de energia solar: aplicação da teoria do funcional da densidade para a determinação de parâmetros termodinâmicos da molécula de azobenzeno
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Nesta dissertação, foi efetuado um estudo alargado que incide sobre a determinação de parâmetros termodinâmicos da molécula de azobenzeno através da Teoria do Funcional de Densidade. Desta forma, determinaram-se os valores dos parâmetros geométricos desta molécula, que são de 114,80º e 124,06º quanto ao ângulo de valência NNC para as conformações trans e cis, respetivamente, e de 180,00º e 9,79º quanto ao ângulo diedro CNNC, igualmente para as conformações trans e cis. Através de otimizações de geometrias parciais, foi também possível construir as superfícies de potencial do estado fundamental e do primeiro estado excitado deste sistema, através do funcional B3LYP e da base 6-31G (d, p), e tal como indicado na literatura (Crecca et al). Ainda relativamente à análise destas duas superfícies, foi possível verificar a existência de uma interseção cónica, no mesmo local, de valor mínimo em S1 correspondente a 186,49 kJ.mol-1, e de máximo em S0 correspondente a 172,42 kJ.mol-1. Ao adotar a combinação B3LYP/6-31G (d, p), a diferença de entalpia entre os confórmeros cis e trans (ΔH) foi de 61,85 kJ.mol-1, que revela uma diferença superior a 10 kJ.mol-1 quando comparado com o valor experimental de 49,10 kJ.mol-1 (Dias et al). Contudo, fizeram-se novos cálculos e os melhores resultados para o valor de entalpia deste sistema resultaram do uso de um funcional de correlação e troca que contém uma correção empírica para a energia de dispersão, B3LYP-D, e com a mesma base utilizada anteriormente, 6-31G (d, p). O valor de diferença de entalpia (ΔH) foi calculado como 51,82 kJ.mol-1. Conseguiu-se também calcular a geometria do estado de transição através da identificação de um ponto sela. Testaram-se vários conjuntos de funcionais e bases para as três conformações do azobenzeno (trans, estado de transição, e cis), sendo que os valores calculados de Ea não diferenciam muito entre si. Salienta-se que o conjunto teórico B3LYP-D/6-31G (d, p) apresenta um dos maiores valores de Ea, de 10,34 kJ.mol-1. Quanto à influência de solventes, esta não se fez sentir nos valores de ΔH do sistema de azobenzeno, pelo menos com a utilização de um modelo de cálculos simplista: o modelo de solvente contínuo. Por último, foi possível que se apurassem através de TD-DFT os valores de absorção máximos para cada um dos confórmeros. Para o confórmero trans obteve-se uma absorção a uma energia de 3,77 eV, para o estado transição de 4,74 eV e para o confórmero cis de 6,16 eV.
In this dissertation, a broad study that focuses on the determination of thermodynamic parameters of the azobenzene molecule through the Density Functional Theory was carried out. Therefore, the values of the geometric parameters of this molecule were determined, which are 114,80º and 124,06º for the valence angle NNC for the trans and cis conformations, respectively, and 180,00º and 9,79º for the dihedral angle CNNC, also for the trans and cis conformations. Through partial geometry optimizations, it was also possible to construct the potential surfaces of the stationary state and the first excited state of this system through the B3LYP functional and the 6-31G (d, p) base, as indicated in the literature (Crecca et al). Also, in terms of the analysis of these two surfaces, it was possible to verify the existence of a conic intersection, in the same place, of minimum value in S1 corresponding to 186.46 kJ.mol-1, and maximum in S0 corresponding to 172.42 kJ.mol-1. By adopting the B3LYP/6-31G (d, p) combination, the enthalpy difference between the cis and trans conformers (ΔH) was 61.85 kJ.mol-1 which is 10 kJ.mol-1 above the experimental value of 49.10 kJ.mol-1 (Dias et al). However, new calculations were carried out and the best results for the enthalpy value of this system were from an exchange-correlation functional containing an empirical correction for the dispersion energy, B3LYP-D, and with the same basis, used previously, 6-31G (d, p). The enthalpy difference (ΔH) was calculated as 51.82 kJ kJ.mol-1. It was also possible to calculate the transition state geometry and corresponding energy of azobenzene by identifying a saddle point. Several functionals and basis-sets for the three azobenzene conformers (trans, transition state, and cis) were tested, and the calculated values of Ea do not differ much between each other. It should be noted that the theoretical set B3LYP-D/6-31G (d, p) has one of the highest Ea values, 10.34 kJ.mol-1. As for the influence of solvents, this was not felt in the ΔH values for the azobenzene system, at least with the use of a simplistic calculation model: the continuous solvent model. Finally, it was possible to determine the maximum absorption values for each of the conformers by TD-DFT. For the trans conformer an energy absorption of 3.77 eV was obtained, for the transition state of 4.74 eV and for the cis conformer of 6.16 eV.
In this dissertation, a broad study that focuses on the determination of thermodynamic parameters of the azobenzene molecule through the Density Functional Theory was carried out. Therefore, the values of the geometric parameters of this molecule were determined, which are 114,80º and 124,06º for the valence angle NNC for the trans and cis conformations, respectively, and 180,00º and 9,79º for the dihedral angle CNNC, also for the trans and cis conformations. Through partial geometry optimizations, it was also possible to construct the potential surfaces of the stationary state and the first excited state of this system through the B3LYP functional and the 6-31G (d, p) base, as indicated in the literature (Crecca et al). Also, in terms of the analysis of these two surfaces, it was possible to verify the existence of a conic intersection, in the same place, of minimum value in S1 corresponding to 186.46 kJ.mol-1, and maximum in S0 corresponding to 172.42 kJ.mol-1. By adopting the B3LYP/6-31G (d, p) combination, the enthalpy difference between the cis and trans conformers (ΔH) was 61.85 kJ.mol-1 which is 10 kJ.mol-1 above the experimental value of 49.10 kJ.mol-1 (Dias et al). However, new calculations were carried out and the best results for the enthalpy value of this system were from an exchange-correlation functional containing an empirical correction for the dispersion energy, B3LYP-D, and with the same basis, used previously, 6-31G (d, p). The enthalpy difference (ΔH) was calculated as 51.82 kJ kJ.mol-1. It was also possible to calculate the transition state geometry and corresponding energy of azobenzene by identifying a saddle point. Several functionals and basis-sets for the three azobenzene conformers (trans, transition state, and cis) were tested, and the calculated values of Ea do not differ much between each other. It should be noted that the theoretical set B3LYP-D/6-31G (d, p) has one of the highest Ea values, 10.34 kJ.mol-1. As for the influence of solvents, this was not felt in the ΔH values for the azobenzene system, at least with the use of a simplistic calculation model: the continuous solvent model. Finally, it was possible to determine the maximum absorption values for each of the conformers by TD-DFT. For the trans conformer an energy absorption of 3.77 eV was obtained, for the transition state of 4.74 eV and for the cis conformer of 6.16 eV.
Descrição
Tese de mestrado, Química (Química) Universidade de Lisboa, Faculdade de Ciências, 2018
Palavras-chave
DFT Sistemas moleculares fotoativos Azobenzeno Estado de transição B3LYP-D Teses de mestrado - 2018