37 research outputs found

    Two Pathways for Dissociation of Highly Energized <i>syn</i>-CH<sub>3</sub>CHOO to OH Plus Vinoxy

    No full text
    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

    No full text
    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

    No full text
    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 13 475 ab initio CCSD­(T)-F12a electronic energies and MP2 dipole moments. Here “AIMD” 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 600–3600 cm<sup>–1</sup>. Analyses of the complex O–H and C–H 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 O–H stretch band, which is upshifted from experimental and VSCF/VCI bands by roughly 300 cm<sup>–1</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

    No full text
    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 “Double-Roaming Dynamics in CH<sub>3</sub>CHO Dissociation”

    No full text
    Correction to “Double-Roaming Dynamics in CH<sub>3</sub>CHO Dissociation

    Effects of Zero-Point Delocalization on the Vibrational Frequencies of Mixed HCl and Water Clusters

    No full text
    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>–(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 Vibration–Vibration Energy Transfer in the HCl Trimer

    No full text
    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 52 000 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 H–Cl stretches, using explicit three-mode coupling calculations and local-monomer calculations with Hückel-type coupling. Both methods produce frequencies within 5 cm<sup>–1</sup> of experiment. A wavepacket calculation based on the Hückel model and full-dimensional classical calculation are performed to study the monomer H–Cl stretch vibration–vibration 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 tetramer’s anharmonic stretch frequencies. The optimized geometries are found to be in agreement with those of previous ab initio studies and the tetramer’s anharmonic frequencies are computed within 11 cm<sup>–1</sup> of experiment

    Collisional Energy Transfer in Highly Excited Molecules

    No full text
    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 Landau–Teller theory and phase space theory, correctly predicts most aspects of the joint probability distribution <i>P</i>(Δ<i>E</i>,Δ<i>J</i>) for the collisional excitation and de-excitation process in the argon–allyl 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 Ar–allyl energies. Intramolecular energies were calculated at the CCSD­(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 Ar–allyl potentials for fixed Ar–allyl 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 atom–hydrogen interaction. Available data corroborate this suggestion

    Ab Initio Deconstruction of the Vibrational Relaxation Pathways of Dilute HOD in Ice Ih

    No full text
    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

    No full text
    The photodissociation of formaldehyde was studied using quasi-classical trajectories to investigate “roaming,” 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 34 500 to 41 010 cm<sup>–1</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 “minimum energy” 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 “reaction configuration” 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
    corecore