2 research outputs found
Polarizability Anisotropy Relaxation in Nanoconfinement: Molecular Simulation Study of Acetonitrile in Silica Pores
We
present the results of a molecular simulation study of polarizability
anisotropy relaxation of liquid acetonitrile confined in approximately
cylindrical silica pores of diameters in the range of 20–40
Ă…. Grand Canonical Monte Carlo simulation is used to determine
the density of acetonitrile in pores in equilibrium with the bulk
liquid, and canonical-ensemble molecular dynamics is then used to
calculate the trajectories of the filled pores prepared in this way.
We find that the pores are wetting, partially due to hydrogen bonding
between acetonitrile nitrogen and pore silanol groups and that acetonitrile
molecules have preferential orientations relative to the interface.
The mobility of molecules in interfacial regions is considerably reduced
and dependent mainly on their proximity to the interface. We include
the contributions of molecular and interaction-induced polarizabilities
to the collective polarizability anisotropy relaxation. We find that
this relaxation includes a slowly relaxing component absent from the
corresponding process in bulk acetonitrile and that the amplitude
of this component increases as the pore diameter decreases. These
results are in agreement with optical Kerr effect experiments on acetonitrile
in silica pores in a similar diameter range. Further analysis of our
data indicates that collective reorientation and predominantly translational
“collision-induced” polarizability dynamics both contribute
to the slowly relaxing portion of polarizability anisotropy decay.
We further find that pore anisotropy plays a role, giving rise to
different relaxation rates of polarizability anisotropy components
with a different mix of axial and radial character and that collective
reorientation contributing to polarizability anisotropy relaxation
is somewhat faster at long times than single-molecule orientational
relaxation
Effects of Electronic-State-Dependent Solute Polarizability: Application to Solute-Pump/Solvent-Probe Spectra
Experimental studies of solvation
dynamics in liquids invariably
ask how changing a solute from its electronic ground state to an electronically
excited state affects a solution’s dynamics. With traditional
time-dependent-fluorescence experiments, that means looking for the
dynamical consequences of the concomitant change in solute–solvent
potential energy. But if one follows the shift in the dynamics through
its effects on the macroscopic polarizability, as recent solute-pump/solvent-probe
spectra do, there is another effect of the electronic excitation that
should be considered: the jump in the solute’s own polarizability.
We examine the spectroscopic consequences of this solute polarizability
change in the classic example of the solvation dye coumarin 153 dissolved
in acetonitrile. After demonstrating that standard quantum chemical
methods can be used to construct accurate multisite models for the
polarizabilities of ground- and excited-state solvation dyes, we show
via simulation that this polarizability change acts as a contrast
agent, significantly enhancing the observable differences in optical-Kerr
spectra between ground- and excited-state solutions. A comparison
of our results with experimental solute-pump/solvent-probe spectra
supports our interpretation and modeling of this spectroscopy. We
predict, in particular, that solute-pump/solvent-probe spectra should
be sensitive to changes in both the solvent dynamics near the solute
and the electronic-state-dependence of the solute’s own rotational
dynamics