Espectro de Absorção da Orto-betaína em Água: um Estudo Teórico usando o Método Seqüencial Monte Carlo/Mecânica Quântica

• Ano de defesa: 2009
• Autor(a) principal: OLIVEIRA, Leonardo Bruno Assis
• Orientador(a): FONSECA, Tertius Lima da
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Goiás
• Programa de Pós-Graduação: Mestrado em Física
• Departamento: Ciências Exatas e da Terra
• País: BR
• Palavras-chave em Português: Método Monte Carlo; Mecânica Quântica; solvatocrômico
• Palavras-chave em Inglês: Monte Carlo simulation; quantum mechanics; solvatochromic
• Área do conhecimento CNPq: CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA::FISICA DA MATERIA CONDENSADA
• Link de acesso: http://repositorio.bc.ufg.br/tede/handle/tde/817

Using sequential Monte Carlo simulation and quantum mechanics calculations(S-QM/MM) based on the TD-DFT methods we have been used to study the solvatochromic shift of the orto-betaine in water. Statistically uncorrelated configurations have been obtained from MC simulations with unpolarized and in-solution polarized solute. To account for the solute electronic polarization, we have used an iterative procedure based on the S-QM/MM methodology to determine the in-solution dipole moment of orto-betaine in presence of water molecules treated as point charges. MP2/cc-pVDZ calculations on the configurations bearing the orto-betaine molecule electrostatically embedded in the 320 water molecules show that the solute polarization increases the dipole moment of orto-betaine, compared to gas phase, by 49%, giving the in-solvent dipole values of 12.2 D. MC simulation results for the Helmholtz free energy as function of the torsion angle indicate that the mosts table conformer of the orto-betaine in water is that with an angle of 60o, after including solute polarization. The average number of solute-solvent hydrogen bonds was analyzed and it was found a large increase of 3,25 in polarized situation to only 1,82 in the unpolarized situation. Modeling the water molecules by point charges (electrostatic embedding), the results show that solvent effects on the absorption spectrum have a significant contribution of the electrostatic interactions. In comparison with the result of gas-phase, TD-BHandHLYP/6311+G(2d,p) calculations give for the π → π∗ electronic transition solvatochromic shifts of 0.42 eV and 0.90 eV for the unpolarized and polarized orto-betaine, respectively. The experimental spectral shift value of 0.90 eV is very well reproduced by the present theoretical results after including the solute polarization. The TD-B3LYP/6311+G(2d,p) model gives the corresponding value for the polarized orto-betaine of 0.52 eV which is an underestimated result in qualitative agreement with experiment. This study indicates that the inclusion of the solute polarization is important for a reliable description of the spectral shifts considered here.