Espectro de Absorção da Orto-betaína em Água: um Estudo Teórico usando o Método Seqüencial Monte Carlo/Mecânica Quântica
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Data
2009-03-26
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Universidade Federal de Goiás
Resumo
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.
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OLIVEIRA, Leonardo Bruno Assis. Absorption spectrum of the ortho-betaine in Water: a Study
Theoretical Method using Sequential Monte Carlo/Mechanics Quantum. 2009. 71 f. Dissertação (Mestrado em Ciências Exatas e da Terra) - Universidade Federal de Goiás, Goiânia, 2009.