2 research outputs found
Solvent Effect on the Stokes Shift and on the Nonfluorescent Decay of the Daidzein Molecular System
The flavonoids have been the target
of several experimental works
due to its influence in the human health as antioxidant elements.
The fluorescence properties of these compounds have been widely studied
due to the large Stokes shifts experimentally observed and the variety
of processes that lead to the fluorescence. In the present work the
role of the solvent in the large Stokes shift experimentally observed
in the daidzein molecular system in water is theoretically studied.
Also studied is the nonfluorescent decay mechanism in a polar aprotic
solvent like acetonitrile. The solvent effect in the ground and in
the low-lying excited electronic states is taken into account by using
the sequential-QM/MM methodology. Excited state properties like equilibrium
geometries and transition energies were studied by using multiconfigurational
calculations, CASSCF and CASPT2. The excited electronic state responsible
for the fluorescence spectrum in water was identified, and the large
Stokes shift seems to be the result of the large interaction of the
system in this electronic state with the solvent. On the other hand,
spināorbit coupling calculations, between the singlet and triplet
electronic states, indicate favorable conditions for intersystem crossing,
in agreement with the experimental result of nonfluorescence observation
Cost-Effective Method for Free-Energy Minimization in Complex Systems with Elaborated Ab Initio Potentials
We
describe a method to locate stationary points in the free-energy
hypersurface of complex molecular systems using high-level correlated <i>ab initio</i> potentials. In this work, we assume a combined
QM/MM description of the system although generalization to full <i>ab initio</i> potentials or other theoretical schemes is straightforward.
The free-energy gradient (FEG) is obtained as the mean force acting
on relevant nuclei using a dual level strategy. First, a statistical
simulation is carried out using an appropriate, low-level quantum
mechanical force-field. Free-energy perturbation (FEP) theory is then
used to obtain the free-energy derivatives for the target, high-level
quantum mechanical force-field. We show that this composite FEG-FEP
approach is able to reproduce the results of a standard free-energy
minimization procedure with high accuracy, while simultaneously allowing
for a drastic reduction of both computational and wall-clock time.
The method has been applied to study the structure of the water molecule
in liquid water at the QCISD/aug-cc-pVTZ level of theory, using the
sampling from QM/MM molecular dynamics simulations at the B3LYP/6-311+GĀ(d,p)
level. The obtained values for the geometrical parameters and for
the dipole moment of the water molecule are within the experimental
error, and they also display an excellent agreement when compared
to other theoretical estimations. The developed methodology represents
therefore an important step toward the accurate determination of the
mechanism, kinetics, and thermodynamic properties of processes in
solution, in enzymes, and in other disordered chemical systems using
state-of-the-art <i>ab initio</i> potentials