"Vibrational spectra from atomic fluctuations in dynamics simulations:
II. Solventinduced frequency fluctuations at femtosecond timeresolution" Matthias Schmitz and Paul Tavan
J. Chem. Phys. 121, 1224712258 (2004)
Abstract: The mid infrared (MIR) spectra of molecules in polar solvents exhibit inhomogeneously
broadened bands, whose spectral positions are shifted as compared to the gas phase.
The shifts are caused by interactions with structured solvation shells and the broadenings
by fluctuations of these interactions. The MIR spectra can be calculated from hybrid molecular
dynamics (MD) simulations, which treat the solute molecule by density functional theory (DFT)
and the solvent by molecular mechanics (MM), by the socalled instantaneous normal mode
analysis (INMA) or by Fourier transforming the time correlation function (FTTCF) of the
molecular dipole moment. In
part I of this work
(M. Schmitz and P. Tavan, J. Chem. Phys. 121, 1223312246, 2004)
we explored an alternative method based on generalized virial (GV)
frequencies noting, however,
that GV systematically underestimates frequencies. As shown by us these
artifacts are caused by solventinduced fluctuations of the (i) equilibrium geometry,
(ii) force constants, and (iii) normal mode directions as well as by (iv) diagonal and
(v) offdiagonal anharmonicities.
Here we now show, by analyzing the time scales of fluctuations and
sample MD trajectories of formaldehyde in the gas phase and in water,
that all these sources of computational artifacts can be made visible
by a Fourier analysis of the normal coordinates. Correspondingly, the
error sources (i),(iii)(v) can be removed by bandpass filtering, as long as
the spectral signatures of the respective effects are well separated
from the fundamental band. Furthermore, the artifacts arising from effect (ii) can
be strongly diminished by a timeresolved version of the GV approach (TFGV).
The TFGV method then yields for each mode j a trajectory of the vibrational frequency
ω_j(tτ) at a time resolution τ > τ_j, which is only limited by
the corresponding oscillation time τ_j = 2π / ω_j and, thus, is in the femtosecond
range. A correlation analysis of these trajectories clearly separates
the librational motions from the conformational dynamics of the solvation shells and
yields the inhomogeneously broadened MIR spectra, if the theory of motional narrowing
is properly included.
The MIR spectrum of FA in solution obtained by TFGV agrees very well with the FTTCF result,
if one applies the socalled ``harmonic approximation'' quantum correction factor and
a temperature scaling to the FTTCF intensities. Also for INMA an excellent agreement is
achieved, if one disregards a slight INMA overestimate of linewidths.
BMO authors (in alphabetic order): Matthias Schmitz Paul Tavan
Assoziierte Projekte: QM/MM hybrid descriptions of solutes in complex solvents
