37 research outputs found
Two Pathways for Dissociation of Highly Energized <i>syn</i>-CH<sub>3</sub>CHOO to OH Plus Vinoxy
Ozonolysis
of alkenes is an important nonphotolytic source of hydroxl
radicals in the troposphere. The reaction proceeds through cycloaddition
and subsequent decomposition to a carbonyl oxide, known as Criegee
intermediates. Ozonolysis of alkene releases about 50 kcal/mol excess
energy to form highly energized Criegee molecules, which can be stabilized
and undergo further reaction or dissociate to OH+vinoxy products.
The dissociation dynamics of partially stabilized Criegee (<i>syn</i>-CH<sub>3</sub>CHOO) has been thoroughly studied recently,
in which the molecules dissociate by first isomerizing to vinyl hydroperoxide
(VHP). Here we examine the dissociation dynamics of highly energized <i>syn</i>-CH<sub>3</sub>CHOO (42 kcal/mol), and a second, prompt
dissociation path is discovered. The dissociation dynamics of these
two paths are carefully examined through the animation of trajectories
and the energy distributions of products. The new prompt path reveals
a distinctly different translational energy and internal energy distributions
of products compared to the known path through VHP
How the Zundel (H<sub>5</sub>O<sub>2</sub><sup>+</sup>) Potential Can Be Used to Predict the Proton Stretch and Bend Frequencies of Larger Protonated Water Clusters
From
a series of seminal experiments on the IR spectra of protonated
water clusters and associated theoretical analyses, it is clear that
the energies and spectral features of the proton stretch and bend
modes are very sensitive functions of the cluster size. Here we show
that this dynamic range can be understood by examining the sensitivity
of these modes in the potential of the Zundel cation, H<sub>5</sub>O<sub>2</sub><sup>+</sup>, as the
separation of the two water monomers is varied. As this distance increases,
the proton increasingly localizes on a monomer, and this is encoded
in the IR spectrum of the proton vibrational modes. The quantitative
predictions from this simple correlation are verified for the H<sub>7</sub>O<sub>3</sub><sup>+</sup> and
H<sub>9</sub>O<sub>4</sub><sup>+</sup> clusters, for which new benchmark harmonic frequencies are reported.
The predictions are also in good accord with trends seen experimentally
and previous calculations for these and five other clusters, including
H<sup>+</sup>(H<sub>2</sub>O)<sub>21</sub>
IR Spectra of (HCOOH)<sub>2</sub> and (DCOOH)<sub>2</sub>: Experiment, VSCF/VCI, and Ab Initio Molecular Dynamics Calculations Using Full-Dimensional Potential and Dipole Moment Surfaces
We
report quantum VSCF/VCI and ab initio molecular dynamics (AIMD)
calculations of the IR spectra of (HCOOH)<sub>2</sub> and (DCOOH)<sub>2</sub>, using full-dimensional, ab initio potential energy and dipole
moment surfaces (PES and DMS). These surfaces are fits, using permutationally
invariant polynomials, to 13âŻ475 ab initio CCSDÂ(T)-F12a electronic
energies and MP2 dipole moments. Here âAIMDâ means using
these ab initio potential and dipole moment surfaces in the MD calculations.
The VSCF/VCI calculations use all (24) normal modes for coupling,
with a four-mode representation of the potential. The quantum spectra
align well with jet-cooled and room-temperature experimental spectra
over the spectral range 600â3600 cm<sup>â1</sup>. Analyses
of the complex OâH and CâH stretch bands are made based
on the mixing of the VSCF/VCI basis functions. The comparisons of
the AIMD IR spectra with both experimental and VSCF/VCI ones provide
tests of the accuracy of the AIMD approach. These indicate good accuracy
for simple bands but not for the complex OâH stretch band,
which is upshifted from experimental and VSCF/VCI bands by roughly
300 cm<sup>â1</sup>. In addition to testing the AIMD approach,
the PES, DMS, and VSCF/VCI calculations for formic acid dimer provide
opportunities for testing other methods to represent high-dimensional
data and other methods that perform postharmonic vibrational calculations
High-Level Quantum Calculations of the IR Spectra of the Eigen, Zundel, and Ring Isomers of H<sup>+</sup>(H<sub>2</sub>O)<sub>4</sub> Find a Single Match to Experiment
The
protonated water tetramer H<sup>+</sup>(H<sub>2</sub>O)<sub>4</sub>, often written as the Eigen cluster, H<sub>3</sub>O<sup>+</sup>(H<sub>2</sub>O)<sub>3</sub>, plays a central role in studies of
the hydrated proton. The cluster has been investigated spectroscopically
both experimentally and theoretically with some differences and controversies.
The major issue stems from the existence of higher-energy Zundel isomers
of this cluster and the role these isomers might play in the IR spectra.
Settling this fundamental issue is one goal of this Communication,
where high-level quantum calculations of the IR spectra of the Eigen
and three isomeric forms of this cluster are presented. These calculations
make use of a many-body representation of the potential and dipole
moment surfaces and VSCF/VCI calculations of vibrational eigenstates
and the IR spectrum. The calculated spectra for the Eigen H<sub>3</sub>O<sup>+</sup>(H<sub>2</sub>O)<sub>3</sub> and D<sub>3</sub>O<sup>+</sup>(D<sub>2</sub>O)<sub>3</sub> isomers compare very well with
experiment. The calculated spectra for the <i>cis</i> and <i>trans</i>-Zundel and ring isomers show prominent features that
do not match with experiment but which can guide future experiments
to search for these interesting and important isomers
Correction to âDouble-Roaming Dynamics in CH<sub>3</sub>CHO Dissociationâ
Correction to âDouble-Roaming Dynamics in CH<sub>3</sub>CHO Dissociation
Effects of Zero-Point Delocalization on the Vibrational Frequencies of Mixed HCl and Water Clusters
We demonstrate the significant effect
that large-amplitude zero-point
vibrational motion can have on the high-frequency fundamental vibrations
of molecular clusters, specifically small (HCl)<sub><i>n</i></sub>â(H<sub>2</sub>O)<sub><i>m</i></sub> clusters.
Calculations were conducted on a many-body potential, constructed
from a mix of new and previously reported semiempirical and high-level
ab initio potentials. Diffusion Monte Carlo simulations were performed
to determine ground-state wave functions. Visualization of these wave
functions indicates that the clusters exhibit delocalized ground states
spanning multiple stationary point geometries. The ground states are
best characterized by planar ring configurations, despite the clusters
taking nonplanar configurations at their global minima. Vibrational
calculations were performed at the global minima and the Diffusion
Monte Carlo predicted configurations and also using an approach that
spans multiple stationary points along a rectilinear normal-mode reaction
path. Significantly better agreement was observed between the calculated
vibrational frequencies and experimental peak positions when the delocalized
ground state was accounted for
A New Many-Body Potential Energy Surface for HCl Clusters and Its Application to Anharmonic Spectroscopy and VibrationâVibration Energy Transfer in the HCl Trimer
The hydrogen bond has been studied
by chemists for nearly a century.
Interest in this ubiquitous bond has led to several prototypical systems
emerging to studying its behavior. Hydrogen chloride clusters stand
as one such example. We present here a new many-body potential energy
surface for (HCl)<sub><i>n</i></sub> constructed from one-,
two-, and three-body interactions. The surface is constructed from
previous highly accurate, semiempirical monomer and dimer surfaces,
and a new high-level ab initio permutationally invariant full-dimensional
three-body potential. The new three-body potential is based on fitting
roughly 52â000 three-body energies computed using coupled cluster
with single, doubles, perturbative triples, and explicit correlation
and the augmented correlation consistent double-ζ basis set.
The first application, described here, is to the ring HCl trimer,
for which the many-body representation is exact. The new potential
describes all known stationary points of the trimer as well its dissociation
to either three monomers or a monomer and a dimer. The anharmonic
vibrational energies are computed for the three HâCl stretches,
using explicit three-mode coupling calculations and local-monomer
calculations with HuÌckel-type coupling. Both methods produce
frequencies within 5 cm<sup>â1</sup> of experiment. A wavepacket
calculation based on the HuÌckel model and full-dimensional
classical calculation are performed to study the monomer HâCl
stretch vibrationâvibration transfer process in the ring HCl
trimer. Somewhat surprisingly, the results of the quantum and classical
calculations are virtually identical, both exhibiting coherent beating
of the excitation between the three monomers. Finally, this representation
of the potential is used to study properties of larger clusters, namely
to compute optimized geometries of the tetramer, pentamer, and hexamer
and to perform explicit four-mode coupling calculations of the tetramerâs
anharmonic stretch frequencies. The optimized geometries are found
to be in agreement with those of previous ab initio studies and the
tetramerâs anharmonic frequencies are computed within 11 cm<sup>â1</sup> of experiment
Collisional Energy Transfer in Highly Excited Molecules
The
excitation/de-excitation step in the Lindemann mechanism is
investigated in detail using model development and classical trajectory
studies based on a realistic potential energy surface. The model,
based on a soft-sphere/line-of-centers approach and using elements
of LandauâTeller theory and phase space theory, correctly predicts
most aspects of the joint probability distribution <i>P</i>(Î<i>E</i>,Î<i>J</i>) for the collisional
excitation and de-excitation process in the argonâallyl system.
The classical trajectories both confirm the validity of the model
and provide insight into the energy transfer. The potential employed
was based on a previously available ab initio intramolecular potential
for the allyl fit to 97418 allyl electronic energies and an intermolecular
potential fit to 286 Arâallyl energies. Intramolecular energies
were calculated at the CCSDÂ(T)/AVTZ level of theory, while intermolecular
energies were calculated at the MP2/AVTZ level of theory. Trajectories
were calculated for each of four starting allyl isomers and for an
initial rotational level of <i>J</i><sub><i>i</i></sub> = 0 as well as for <i>J</i><sub><i>i</i></sub> taken from a microcanonical distribution. Despite a dissimilarity
in Arâallyl potentials for fixed Arâallyl geometries,
energy transfer properties starting from four different isomers were
found to be remarkably alike. A contributing factor appears to be
that the orientation-averaged potentials are almost identical. The
model we have developed suggests that most hydrocarbons should have
similar energy transfer properties, scaled by differences in the potential
offset of the atomâhydrogen interaction. Available data corroborate
this suggestion
Ab Initio Deconstruction of the Vibrational Relaxation Pathways of Dilute HOD in Ice Ih
Coupled
intramolecular and intermolecular vibrational quantum dynamics,
using an ab initio potential energy surface, successfully describes
the subpicosecond relaxation of the OD and OH stretch fundamental
and first overtone of dilute HOD in ice Ih. The calculations indicate
that more than one intermolecular mode along with the three intramolecular
modes is needed to describe the relaxation, in contrast to a recent
study using a phenomenological potential in just two degrees of freedom.
Detailed time-dependent relaxation pathways from 6-mode calculations
are also given
Roaming Under the Microscope: Trajectory Study of Formaldehyde Dissociation
The photodissociation of formaldehyde
was studied using quasi-classical
trajectories to investigate âroaming,â or events involving
trajectories that proceed far from the minimum energy pathway. Statistical
analysis of trajectories performed over a range of nine excitation
energies from 34âŻ500 to 41âŻ010 cm<sup>â1</sup> (including zero-point energy) provides characterization of the roaming
phenomenon and insight into the mechanism. The trajectories are described
as projections onto three coordinates: the distance from the CO center
of mass to the furthest H atom and the azimuthal and polar coordinates
of that H atom with respect to the CO axis. The trajectories are used
to construct a âminimum energyâ potential energy surface
showing the potential for any binary combination of these three coordinates
that is at a minimum energy with respect to values of the other coordinates
encountered during the trajectories. We also construct flux diagrams
for roaming, transition-state, and radical pathways, as well as âreaction
configurationâ plots that show the distribution of reaction
geometries for roaming and transition-state pathways. These constructs
allow characterization of roaming in formaldehyde as, principally,
internal rotation of the roaming H atom around the CO axis at a slowly
varying and elongated distance from the CO center of mass. The rotation
is nearly uniform, and is sometimes accompanied by rotation in the
polar coordinate. The roaming state of formaldehyde can be treated
as a separate kinetic entity, much as one might treat an isomer. Rate
constants for the formation of and reaction from this roaming state
are derived from the trajectory data as a function of excitation energy