The protonated water tetramer H+(H2O)4, often written as the Eigen cluster, H3O+(H2O)3, 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 H3O+(H2O)3 and D3O+(D2O)3 isomers compare very well with experiment. The calculated spectra for the cis and trans-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.

The phenomenon of roaming in chemical reactions has now become both commonly observed in experiment and extensively supported by theory and simulations. Roaming occurs in highly-excited molecules when the trajectories of atomic motion often bypass the minimum energy pathway and produce reaction in unexpected ways from unlikely geometries. The prototypical example is the unimolecular dissociation of formaldehyde (H2CO), in which the “normal” reaction proceeds through a tight transition state to yield H2 + CO but for which a high fraction of dissociations take place via a “roaming” mechanism in which one H atom moves far from the HCO, almost to dissociation, and then returns to abstract the second H atom. We review below the theories and simulations that have recently been developed to address and understand this new reaction phenomenon.

We combine path-integral Monte Carlo methods with a new intramolecular potential energy surface to quantify the equilibrium enrichment of doubly substituted ethane isotopologues due to clumped-isotope effects. Ethane represents the simplest molecule to simultaneously exhibit 13C–13C, 13C–D, and D–D clumped-isotope effects, and the analysis of corresponding signatures may provide useful geochemical and biogeochemical proxies of formation temperatures or reaction pathways. Utilizing path-integral statistical mechanics, we predict equilibrium fractionation factors that fully incorporate nuclear quantum effects, such as anharmonicity and rotational-vibrational coupling which are typically neglected by the widely used Urey model. The magnitude of the calculated fractionation factors for the doubly substituted ethane isotopologues indicates that isotopic clumping can be observed if rare-isotope substitutions are separated by up to three chemical bonds, but the diminishing strength of these effects suggests that enrichment at further separations will be negligible. The Urey model systematically underestimates enrichment due to 13C–D and D–D clumped-isotope effects in ethane, leading to small relative errors in the apparent equilibrium temperature, ranging from 5 K at 273.15 K to 30 K at 873.15 K. We additionally note that the rotameric dependence of isotopologue enrichment must be carefully considered when using the Urey model, whereas the path-integral calculations automatically account for such effects due to configurational sampling. These findings are of direct relevance to future clumped-isotope studies of ethane, as well as studies of 13C–13C, 13C–D, and D–D clumped-isotope effects in other hydrocarbons.

This Review summarizes recent research on vibrational predissociation (VP) of hydrogen-bonded clusters. Specifically, the focus is on breaking of hydrogen bonds following excitation of an intramolecular vibration of the cluster. VP of the water dimer and trimer, HCl clusters, and mixed HCl–water clusters are the major topics, but related work on hydrogen halide dimers and trimers, ammonia clusters, and mixed dimers with polyatomic units are reviewed for completion and comparison. The theoretical focus is on generating accurate potential energy surfaces (PESs) that can be used in detailed dynamical calculations, mainly using the quasiclassical trajectory approach. These PESs have to extend from the region describing large amplitude motion around the minimum to regions where fragments are formed. The experimental methodology exploits velocity map imaging to generate pair-correlated product translational energy distributions from which accurate bond dissociation energies of dimers and trimers and energy disposal in fragments are obtained. The excellent agreement between theory and experiment on bond dissociation energies, energy disposal in fragments, and the contributions of cooperativity demonstrates that it is now possible, with state-of-the-art experimental and theoretical methods, to make accurate predictions about dynamical and energetic properties of dissociating clusters.

We report a new potential energy surface (PES) for hydronium that dissociates to H+ + H2O. The PES is a permutationally invariant fit to a data set of nearly 100,000 electronic energies, of which most are CCSD(T)-F12/aug-cc-pVQZ, plus a small set of MRCI-aug-cc-pVTZ diabatic energies in the region where the CCSD(T) method fails. The long-range part of the PES is described accurately by a classical Coulomb interaction between the proton and H2O using partial charges obtained from an accurate, full-dimensional dipole moment surface. A switching function connects the fitted PES to this long-range interaction.The fidelity of this global PES is determined by a combination of standard geometry and harmonic analyses at the minimum and inversion saddle point. In addition, VSCF/VCI calculations of the fundamentals and tunneling splittings are reported; all of these are within 3 cm–1 or less of experimental values. A diffusion Monte Carlo calculation is also reported for the zero-point state. The PES is used in a two-body representation of the interaction of the proton with two water molecules, including a 2-body H2O–H2O interaction, and is shown to give a realistic description of the Zundel cation H5O2+. This demonstrates that the PES may be usable as a component in a many-body potential describing the hydrated proton, especially for vibrational calculations of protonated water clusters.

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 (syn-CH3CHOO) 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 syn-CH3CHOO (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.

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, H5O2+, 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 H7O3+ and H9O4+ 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+(H2O)21.

Semiclassical quantization of vibrational energies, using adiabatic switching (AS), is applied to CH4 using a recent ab initio potential energy surface, for which exact quantum calculations of vibrational energies are available. Details of the present calculations, which employ a harmonic normal-mode zeroth-order Hamiltonian, emphasize the importance of transforming to the Eckart frame during the propagation of the adiabatically switched Hamiltonian. The AS energies for the zero-point, and fundamental excitations of two modes are in good agreement with the quantum ones. The use of AS in the context of quasi-classical trajectory calculations is revisited, following previous work reported in 1995, which did not recommend the procedure. We come to a different conclusion here.

The local monomer model is applied, with ab initio potential energy and dipole moment surfaces, to a calculation of the IR spectrum of liquid D2O at 300 K, over the spectral range 0 to 4000 cm–1. The spectrum is an incoherent superposition of spectra of many monomers over snapshots of a molecular dynamics trajectory, where both intramolecular and intermolecular coupling in each monomer is treated. The comparison to experiment shows an unprecedented level of agreement for the stretch, bend, and libration bands and also the bend+libration and stretch+bend combination bands. This indicates that the incoherent approach captures much of the dynamics underlying the spectrum, provided monomer couplings are considered. The calculated spectrum is compared to the recently calculated IR spectrum of H2O, using the same method and potential energy and dipole moment surfaces, and shifts relative to that spectrum are presented and discussed.

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–1 (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.

We recently reported a second-generation, ab initio dipole moment surface (DMS) for water and applied it successfully to the IR spectrum of liquid water at 300 K. Here the transferability of this DMS is demonstrated in three applications. One is the distribution of monomer dipole moments, considering two definitions, and effective atomic charges of liquid water at 300 K and also for a model of ice Ih at 0 K. The second one is a calculation of the dielectric constant of liquid water at 280, 300, 320, 340, 360 K, and the third one is correcting the intensities of the IR spectrum of liquid water at 300 K, obtained using the q-TIP4P/F potential, bringing them into much improved agreement with experiment. For the purpose of obtaining statistical ensembles we use molecular dynamics simulations with the TIP4P+E3B water model developed by Skinner and co-workers. The average monomer dipole moments for 300 K water and 0 K ice Ih are 2.94 and 3.54 D, respectively, in good agreement with literature values. Effective monomer charge distributions are derived from the monomer dipoles and give average values of −1.02 e for O and 0.51 e for H in liquid water, which are also in agreement with values reported from experiment. The calculated dielectric constant of liquid water at the above five temperatures is compared to experiment and is roughly 10–15% lower than experiment.

A modified, computationally efficient method to provide permutationally invariant polynomial bases for molecular energy surface fitting via monomial symmetrization (Xie Z.; Bowman J. M. J. Chem. Theory Comput. 2010, 6, 26–34) is reported for applications to complex systems, characterized by many-body, non-covalent interactions. Two approaches, each able to ensure the asymptotic zero-interaction limit of intrinsic potentials, are presented. They are both based on the tailored selection of a subset of the polynomials of the original basis. A computationally efficient approach exploits reduced permutational invariance and provides a compact fitting basis dependent only on intermolecular distances. We apply the original and new techniques to obtain a number of full-dimensional potentials for the intrinsic three-body methane–water–water interaction by fitting a database made of 22,592 ab initio energies calculated at the MP2-F12 level of theory with haTZ (aug-cc-pVTZ for C and O, cc-pVTZ for H) basis set. An investigation of the effects of permutational symmetry on fitting accuracy and computational costs is reported. Several of the fitted potentials are then employed to evaluate with high accuracy the three-body contribution to the CH4–H2O–H2O binding energy and the three-body energy of three conformers of the CH4@(H2O)20 cluster.

The potential energy surface of the methane–water dimer is represented as the sum of a new intrinsic two-body potential energy surface and pre-existing intramolecular potentials for the monomers. Different fits of the CH4–H2O intrinsic two-body energy are reported. All these fits are based on 30467 ab initio interaction energies computed at CCSD(T)-F12b/haTZ (aug-cc-pVTZ for C and O, cc-pVTZ for H) level of theory. The benchmark fit is a full-dimensional, permutationally-invariant analytical representation with root-mean-square (rms) fitting error of 3.5 cm−1. Two other computationally more efficient two-body potentials are also reported, albeit with larger rms fitting errors. Of these a compact permutationally invariant fit is shown to be the best one in combining precision and speed of evaluation. An intrinsic two-body dipole moment surface is also obtained, based on MP2/haTZ expectation values, with an rms fitting error of 0.002 au. As with the potential, this dipole moment surface is combined with existing monomer ones to obtain the full surface. The vibrational ground state of the dimer and dissociation energy, D0, are determined by diffusion Monte Carlo calculations, and MULTIMODE calculations are performed for the IR spectrum of the intramolecular modes. The relative accuracy of the different intrinsic two-body potentials is analyzed by comparing the energetics and the harmonic frequencies of the global minimum well, and the maximum impact parameter employed in a sample methane–water scattering calculation.

This article describes theoretical/computational techniques that are aimed at addressing both vibrational spectroscopy and large-amplitude nuclear dynamics. Two new examples from our group are discussed in detail. These are the syn and anti-conformers of the Crigee intermediate CH3CHOO and isotope effects in the nitrate radical vibrational energies. For both, potential energy surfaces that are mathematical fits to thousands of electronic energies are described. Dipole moment surfaces for syn and anti-CH3CHOO are also described. These surfaces are used in new, full-dimensional vibrational calculations for syn and anti-CH3CHOO and survey infrared spectra are reported up to 6200 cm−1. For NO3 the focus is on further studies of isotope effects, using an adiabatic PES that was recently reported in a study of the vibrational energies of 14N16O3 and 15N16O3 (Homayoon and Bowman, 2014).

This Perspective highlights progress in ab initio quantum approaches to IR spectroscopy of water and hydrates. Here, “ab initio” refers to many-body potentials and dipole moment surfaces for flexible water and hydrates. Specifically, these are mathematical representations of two-body and three-body interactions based on permutationally invariant fitting of tens of thousands of ab initio electronic energies, a spectroscopically accurate one-body monomer potential, and four- and higher-body interactions described by the long-range interactions incorporated into, for example, the TTM3-F family of potentials. There are currently two such potentials of this type, denoted WHBB and MB-pol, which are being used in expanding applications. Here, the focus is on infrared spectroscopy, using the WHBB potential and dipole moment surface, with an embedded, local monomer quantum method to obtain vibrational energies and dipole transition moments. Comparisons are also made with the popular q-TIP4P/F potential. Brief mention is made of an application to small HCl–H2O clusters.

A model for energy transfer in the collision between an atom and a highly excited target molecule has been developed on the basis of classical mechanics and turning point analysis. The predictions of the model have been tested against the results of trajectory calculations for collisions of five different target molecules with argon or helium under a variety of temperatures, collision energies, and initial rotational levels. The model predicts selected moments of the joint probability distribution, P(Jf,ΔE) with an R2 ≈ 0.90. The calculation is efficient, in most cases taking less than one CPU-hour. The model provides several insights into the energy transfer process. The joint probability distribution is strongly dependent on rotational energy transfer and conservation laws and less dependent on vibrational energy transfer. There are two mechanisms for rotational excitation, one due to motion normal to the intermolecular potential and one due to motion tangential to it and perpendicular to the line of centers. Energy transfer is found to depend strongly on the intermolecular potential and only weakly on the intramolecular potential. Highly efficient collisions are a natural consequence of the energy transfer and arise due to collisions at “sweet spots” in the space of impact parameter and molecular orientation.

A semiglobal potential energy surface (PES) and quartic force field (QFF) based on fitting high-level electronic structure energies are presented to describe the structures and spectroscopic properties of NNHNN+. The equilibrium structure of NNHNN+ is linear with the proton equidistant between the two nitrogen groups and thus of D∞h symmetry. Vibrational second-order perturbation theory (VPT2) calculations based on the QFF fails to describe the proton “rattle” motion, i.e., the antisymmetric proton stretch, due to the very flat nature of PES around the global minimum but performs properly for other modes with sharper potential wells. Vibrational self-consistent field/virtual state configuration interaction (VSCF/VCI) calculations using a version of MULTIMODE without angular momentum terms successfully describe this motion and predict the fundamental to be at 759 cm–1. This is in good agreement with the value of 746 cm–1 from a fixed-node diffusion Monte Carlo calculation and the experimental Ar-tagged result of 743 cm–1. Other VSCF/VCI energies are in good agreement with other experimentally reported ones. Both double-harmonic intensity and rigorous MULTIMODE intensity calculations show the proton-transfer fundamental has strong intensity.

Using a new potential energy surface, based on fitting around 33 000 CCSD(T)-F12/aug-cc-pVTZ energies, a robust set of predictions is made for mode-specific isomerization of trans-hydroxymethylene to formaldehyde in the deep tunneling region. The calculations make use of a recent projection model for mode-specific tunneling based on the rectilinear Qim path [Wang, Y.; Bowman, J. M. J. Chem. Phys. 2013, 139, 154303]. The most interesting prediction is a large decrease in the half-life from roughly 5 h for the ground vibrational state to roughly 1.5 min and 1 s by excitation of the fundamental and first overtone of the asymmetric bending normal mode, respectively. The properties of the new PES are described along with variational calculations of low-lying vibrational states of trans- and cis-hydroxymethylene.

We report vibrational self-consistent field/virtual state configuration interaction energies of nitromethane using the code MULTIMODE and a new full-dimensional potential energy surface (PES). The PES is a precise, permutationally invariant linear least-squares fit to 17 049 electronic energies, using the CCSD(T)-F12b method with HaDZ basis (cc-pVDZ basis for H atoms, and aug-cc-pVDZ basis for C, O, N atoms). Nitromethane has 15 vibrational degrees of freedom, including one that is a nearly free internal methyl torsion, which is accurately described by the PES. This torsional mode makes vibrational calculations very challenging and here we present the results of calculations without it. Nevertheless, 14-mode calculations are still challenging and can lead to very large Hamiltonian matrices. To address this issue, we apply a pruning scheme, suggested previously by Handy and Carter, that reduces the size of matrix without sacrificing accuracy in the eigenvalues. The method is briefly described here in the context of partitioning theory. A new and more efficient implementation of it, coded in the latest version of MULTIMODE program, is described. The accuracy and efficiency are demonstrated for 12-mode C2H4 and then applied to CH3NO2. Agreement with available experimental values of the CH3NO2 14 fundamentals is very good. Diffusion Monte Carlo calculations in full dimensionality are done for the zero-point energy and wavefuction. These indicate that the torisonal motion is nearly a free-rotor in this state.

Quasi-classical trajectory studies have been performed for the collision of internally excited methane with water using an accurate methane–water potential based on a full-dimensional, permutationally invariant analytical representation of energies calculated at a high level of theory. The results suggest that most energy transfer takes place at impact parameters smaller than about 8 Bohr; collisions at higher impact parameters are mostly elastic. Overall, energy transfer is fairly facile, with values for ⟨ΔEdown⟩ and ⟨ΔEup⟩ approaching almost 2% of the total excitation energy. A classical model previously developed for the collision of internally excited molecules with atoms (Houston, P. L.; Conte, R.; Bowman, J. M. J. Phys. Chem. A 2015, 119, 4695–4710) has been extended to cover collisions of internally excited molecules with other molecules. For high initial rotational levels, the agreement with the trajectory results is quite good (R2 ≈ 0.9), whereas for low initial rotational levels it is only fair (R2 ≈ 0.7). Both the model and the trajectories can be characterized by a four-dimensional joint probability distribution, P(J1,f,ΔE1,J2,f,ΔE2), where J1,f and J2,f are the final rotational levels of molecules 1 and 2 and ΔE1 and ΔE2 are the respective changes in internal energy. A strong anticorrelation between ΔE1 and ΔE2 is observed in both the model and trajectory results and can be explained by the model. There is evidence in the trajectory results for a small amount of V ↔ V energy transfer from the water, which has low internal energy, to the methane, which has substantial internal energy. This observation suggests that V ↔ V energy transfer in the other direction also occurs.

Can a molecule be efficiently activated with a large amount of energy in a single collision with a fast atom? If so, this type of collision will greatly affect molecular reactivity and equilibrium in systems where abundant hot atoms exist. Conventional expectation of molecular energy transfer (ET) is that the probability decreases exponentially with the amount of energy transferred, hence the probability of what we label “super energy transfer” is negligible. We show, however, that in collisions between an atom and a molecule for which chemical reactions may occur, such as those between a translationally hot H atom and an ambient acetylene (HCCH) or sulfur dioxide, ET of chemically significant amounts of energy commences with surprisingly high efficiency through chemical complex formation. Time-resolved infrared emission observations are supported by quasi-classical trajectory calculations on a global ab initio potential energy surface. Results show that ∼10% of collisions between H atoms moving with ∼60 kcal/mol energy and HCCH result in transfer of up to 70% of this energy to activate internal degrees of freedom.

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.

We report a global potential energy surface (PES) for the N(2D) + H2O reaction based on fitting roughly 312 000 UCCSD(T)-F12/aug-cc-pVTZ electronic energies. The surface is a linear least-squares fit using a permutationally invariant basis with Morse-type variables. Quasiclassical trajectory calculations of the N(2D) + H2O(D2O) reaction with focus on the NH(D) + OH(D) exit channel are performed. An analysis of the internal-state distributions shows that the NH(D) fragment has more internal energy, both rotational and vibrational than the OH(D) fragment, in good agreement with experiment. This difference is traced to nonstatistical dynamics.

We report mode-specific tunneling in the unimolecular dissociation of cis-HOCO to H + CO2 using a recent projection theory that makes use of a tunneling path along the imaginary-frequency normal mode, Qim, of a relevant saddle point. The tunneling probabilities and lifetimes are calculated for the ground vibrational state of cis-HOCO and highly excited overtones and combination bands of the modes that have large projections onto the Qim path. To go beyond the harmonic approximation, which is important for the OH stretch, energies and classical turning points are calculated using the anharmonic one-dimensional (1D) potential in that normal mode and used in the projection theory. The tunneling lifetimes calculated for a number of combination states of the OCO bend and CO stretch are in good accord with those estimated in a previous five degrees-of-freedom quantum wavepacket simulation of the dissociative photodetachment of HOCO–. The present results are also consistent with the interpretation of the tunneling of cis-HOCO to H + CO2 seen in recent experiments.

The bimolecular hydrogen abstraction reactions of methane with atoms have become benchmark systems to test and extend our knowledge of polyatomic chemical reactivity. We review the state-of-the-art methodologies for reaction dynamics computations of X + methane [X = F, O(3P), Cl, Br] reactions, which consist of two key steps: (1) potential energy surface (PES) developments and (2) reaction dynamics computations on the PES using either classical or quantum methods. We briefly describe the permutationally invariant polynomial approach for step 1 and the quasiclassical trajectory method, focusing on the mode-specific polyatomic product analysis and the Gaussian binning (1GB) techniques, and reduced-dimensional quantum models for step 2. High-quality full-dimensional ab initio PESs and dynamical studies of the X + CH4 and CHD3 reactions are reviewed. The computed integral cross-sections, angular, vibrational, and rotational product distributions are compared with available experiments. Both experimental and theoretical findings shed light on the rules of mode-selective polyatomic reactivity.

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)n 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–1 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–1 of experiment.

The influence of rotational excitation on energy transfer in single collisions of allyl with argon and on allyl dissociation is investigated. About 90 000 classical scattering simulations are performed in order to determine collision-induced changes in internal energy and in allyl rotational angular momentum. Dissociation is studied by means of about 50 000 additional trajectories evolved for the isolated allyl under three different conditions: allyl with no angular momentum (J = 0); allyl with the same microcanonically sampled initial conditions used for the collisions (J*); allyl evolving from the corresponding exit conditions after the collision. The potential energy surface is the sum of an intramolecular potential and an interaction one, and it has already been used in a previous work on allyl–argon scattering (Conte, R.; Houston, P. L.; Bowman, J. M. J. Phys. Chem. A 2013, 117, 14028–14041). Energy transfer data show that increased initial rotation favors, on average, increased relaxation of the excited molecule. The availability of a high-level intramolecular potential energy surface permits us to study the dependence of energy transfer on the type of starting allyl isomer. A turning point analysis is presented, and highly efficient collisions are detected. Collision-induced variations in the allyl rotational angular momentum may be quite large and are found to be distributed according to three regimes. The roles of rotational angular momentum, collision, and type of isomer on allyl unimolecular dissociation are considered by looking at dissociations times, kinetic energies of the fragments, and branching ratios. Generally, rotational angular momentum has a strong influence on the dissociation dynamics, while the single collision and the type of starting isomer are less influential.

We report a permutationally invariant, ab initio potential energy surface (PES) for the OH + HBr → Br + H2O reaction. The PES is a fit to roughly 26 000 spin-free UCCSD(T)/cc-pVDZ-F12a energies and has no classical barrier to reaction. It is used in quasiclassical trajectory calculations with a focus on the thermal rate constant, k(T), over the temperature range 5 to 500 K. Comparisons with available experimental data over the temperature range 23 to 416 K are made using three approaches to treat the OH rotational and associated electronic partition function. All display an inverse temperature dependence of k(T) below roughly 160 K and a nearly constant temperature dependence above 160 K, in agreement with experiment. The calculated rate constant with no treatment of spin–orbit coupling is overall in the best agreement with experiment, being (probably fortuitously) within 20% of it.Keywords: ab initio calculations; atmospheric chemistry; chemical kinetics; non-Arrhenius behavior; potential energy surfaces; quasiclassical trajectory calculation; reaction rate constant;

Diffusion Monte Carlo calculations for all the deuterated H7+ isotopologues and isotopomers were performed to determine their zero-point energies, and thus the stability of them. Based on these calculations, we conclude that the deuterium atom prefers the unbonded position in the central H3+, and then the bonded position in H3+. When two deuterium atoms are in the outer H2 units, forming a D2 is more stable than one deuterium in each H2 unit. We also discovered that some unstable isotopomers can rearrange to a more stable isotopomer through two types of isomerization: one is that a new H3+ core is formed with more deuterium atoms in it; the other is that the deuterium in the central H3+ goes from the interior (the bonded position) to the exterior (the unbonded position) while the number of deuterium atoms in the H3+ does not change. Three transition states related to the isomerization were identified, two of which have not been reported previously. The corresponding reaction paths were also determined.

Dilute mixtures of HOD in pure H2O and D2O ices and liquid have been used by experimentalists to focus on the spectrum and vibrational dynamics of the local OH and OD stretches and bend of HOD in these complex and highly heterogeneous environments. The hexamer version of the mixture is HOD(D2O)5. The cage isomer of this cluster was recently studied and analyzed theoretically using local-mode calculations of the IR spectrum by Skinner and co-workers. This and the further possibility of experimental investigation of this cluster have stimulated us to study HOD(D2O)5 using the three-mode, local-monomer model, with the ab initio WHBB dipole moment and potential energy surfaces. Both the cage and prism isomers of this cluster are considered. In addition to providing additional insight into the HOD portion of the spectrum, the spectral signatures of each D2O are also presented in the range of 1000–4000 cm–1. The OH stretch bands of both the prism and cage isotopomers exhibit rich structures in the range of 3100–3700 cm–1 that are indicative of the position of the HOD in these hexamers. A preliminary investigation of the site preference of the HOD is also reported for both cage and prism HOD(D2O)5 using an harmonic zero-point energy analysis of the entire cluster. This indicates that the energies of free-OH sites are lower than the ones of H-bonded OH sites. Finally, following our earlier work on the IR spectra of H2O ice models, we present IR spectra of pure D2O and HOD.

The N(2D) + H2O is a reaction with competitive product channels, passing through several intermediates. Dynamics of this reaction had been investigated by two of the present authors at two collision energies, Ec, using the crossed molecular beams mass spectrometric method ( Faraday Discuss. 2001, 119, 27−49). The complicated mechanism of this reaction and puzzling results encouraged us to investigate the reaction in a joint experimental/theoretical study. Quasiclassical trajectory (QCT) calculations on an ab initio potential energy surface describing all channels of the title reaction are done with a focus on the N/H exchange channels. Interesting results of QCT calculations, in very good agreement with experimental data, reveal subtle details of the reaction dynamics of the title reaction to HNO/HON + H exit channels by disentangling the different routes to formation of the two possible HNO/HON isomers and therefore assisting in a critical manner the derivation of the reaction mechanism. Results of the present study show that the nonstatistical HNOH intermediate governs exit channels; therefore, the HON channel is as important as that of HNO. The study also confirms that the H2 + NO molecular channel is negligible even though the barrier to its formation is calculated to be well below the reactant asymptote.Keywords: crossed molecular beams; isomers; N(2D) reactions; nonstatistical dynamics; quasiclassical trajectories; reaction dynamics;

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)n–(H2O)m 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.Keywords: ab initio potentials; anharmonic vibrations; diffusion Monte Carlo; many-body; tetramers; trimer;

We report a theoretical study of mode-specific tunneling splittings in double-hydrogen transfer in trans-porphycene. We use a novel, mode-specific “Qim path method”, in which the reaction coordinate is the imaginary-frequency normal mode of the saddle point separating the equivalent minima. The model considers all 108 normal modes and uses no adjustable parameters. The method gives the ground vibrational-state tunneling splitting, as well the increase in the splitting upon excitation of certain modes, in good agreement with experiment. Interpretation of these results is also transparent with this method. In addition, predictions are made for mode excitations not investigated experimentally. Results for d1 and d2 isotopolgues are also in agreement with experiment.Keywords: d1, d2 isotopolgues; double-hydrogen transfer; mode-specific tunneling; porphycene; projection model;

We calculate the probabilities for the association reactions H+HCN→H2CN* and cis/trans-HCNH*, using quasiclassical trajectory (QCT) and classical trajectory (CT) calculations, on a new global ab initio potential energy surface (PES) for H2CN including the reaction channels. The surface is a linear least-squares fit of roughly 60 000 CCSD(T)-F12b/aug-cc-pVDZ electronic energies, using a permutationally invariant basis with Morse-type variables. The reaction probabilities are obtained at a variety of collision energies and impact parameters. Large differences in the threshold energies in the two types of dynamics calculations are traced to the absence of zero-point energy in the CT calculations. We argue that the QCT threshold energy is the realistic one. In addition, trajectories find a direct pathway to trans-HCNH, even though there is no obvious transition state (TS) for this pathway. Instead the saddle point (SP) for the addition to cis-HCNH is evidently also the TS for direct formation of trans-HCNH.

We report classical (CT) and quasiclassical (QCT) trajectory calculations of the association reaction H + C2D2 → C2D2H∗, using an ab initio global potential energy surface. The QCT threshold is roughly 4 kcal/mol, in very good agreement with the vibrationally adiabatic ground-state barrier. In contrast, the 0 and 300 K CT threshold energies are roughly 10 and 8 kcal/mol, respectively. These contrasting results are analyzed in detail as functions of the impact parameter and collision energy, and are of potential significance for general molecular dynamics simulations, which typically do not include zero-point energy of the reactants.Graphical abstractHighlights► We report trajectory calculations of the association probability for H + C2D2 → C2D2H∗. ► The classical threshold energy is higher than the barrier height to form C2D2H. ► The quasiclassical threshold energy is close to the barrier height. ► We analyze the source of the surprisingly high threshold in the classical results. ► A general deficiency of classical trajectory method was indicated by our analysis.

We report a joint experimental-theoretical study of the predissociation dynamics of the water trimer following excitation of the hydrogen bonded OH-stretch fundamental. The bond dissociation energy (D0) for the (H2O)3 → H2O + (H2O)2 dissociation channel is determined from fitting the speed distributions of selected rovibrational states of the water monomer fragment using velocity map imaging. The experimental value, D0 = 2650 ± 150 cm–1, is in good agreement with the previously determined theoretical value, 2726 ± 30 cm–1, obtained using an ab initio full-dimensional potential energy surface (PES) together with Diffusion Monte Carlo calculations [Wang; Bowman. J. Chem. Phys. 2011, 135, 131101]. Comparing this value to D0 of the dimer places the contribution of nonpairwise additivity to the hydrogen bonding at 450–500 cm–1. Quasiclassical trajectory (QCT) calculations using this PES help elucidate the reaction mechanism. The trajectories show that most often one hydrogen bond breaks first, followed by breaking and re-forming of hydrogen bonds (often with different hydrogen bonds breaking) until, after many picoseconds, a water monomer is finally released. The translational energy distributions calculated by QCT for selected rotational levels of the monomer fragment agree with the experimental observations. The product translational and rotational energy distributions calculated by QCT also agree with statistical predictions. The availability of low-lying intermolecular vibrational levels in the dimer fragment is likely to facilitate energy transfer before dissociation occurs, leading to statistical-like product state distributions.

We report ab initio potential energy and dipole moment surfaces that span the regions describing the minima of trans- and cis-HOCO and the barrier separating them. We use the new potential in three types of variational calculations of the vibrational eigenstates, for zero total angular momentum. Two use the code MULTIMODE (MM) in the so-called single-reference and reaction path versions. The third uses the exact Hamiltonian in diatom–diatom Jacobi coordinates. The single-reference version of MM is limited to a description of states that are localized at each minimum separately, whereas the reaction-path version and the Jacobi approach describe localized and delocalized states. The vibrational IR spectrum for zero total angular momentum is also reported for the trans and cis fundamentals and selected overtone and combination states with significant oscillator strength.

We report a global potential energy surface (PES) for CH3NO2 based on fitting roughly 114 000 density functional theory (UB3LYP/6-311+g(d,p)) electronic energies. The PES is a precise, permutationally invariant fit to these energies. Properties of the PES are described, as are some preliminary quasiclassical trajectory calculations. An isomerization-roaming pathway to the CH3ONO isomer and then to the CH3O + NO products is found. Although the pathway occurs at larger distances than a related loose saddle-point on the PES, the pathway supports the supposition of such a pathway based on locating a loose first-order saddle point and associated IRC, reported previously by Zhu and Lin [Zhu, R. S. and Lin, M. C. Chem. Phys. Lett. 2009, 478, 11].

Predicting the results of collisions of polyatomic molecules with a bath of atoms is a research area that has attracted substantial interest in both experimental and theoretical chemistry. Energy transfer, which is the consequence of such collisions, plays an important role in gas-phase kinetics and relaxation of excited molecules. We present a study of energy transfer in single collisions of highly vibrationally excited allyl radical in argon. We evolve a total of 52 000 classical trajectories on a potential energy surface, which is the sum of an ab initio intramolecular potential for the allyl and a pairwise interaction potential describing the argon’s effect on the allyl. The former is described by means of a permutationally invariant full-dimensional potential, whereas the interaction potential between allyl and argon is obtained by means of a sum of pairwise potentials dependent on nonlinear parameters that have been fit to a set of MP2/avtz counterpoise corrected ab initio energies. Results are reported for energy transfers and related probability densities at different collisional energies. The sensitivity of results to the interaction potential is considered and the potential is shown to be suitable for future applications involving different isomers of the allyl. The impact of highly efficient collisions in the energy transfer process is examined.

We present an analysis of the vibrational modes of a model of hexagonal ice, ice Ih, comprised of 192 monomers with a core region of 105 monomers, using the ab initio WHBB potential energy surface [Wang, Y.; Shepler, B.; Braams, B.; Bowman, J. M. J. Chem. Phys. 2011, 134, 094509]. A standard normal-mode analysis and a local-monomer normal-mode analysis of 105 core monomers are performed to obtain harmonic frequencies and state densities of the “pseudo-translation” (0–400 cm–1), “libration” (500–1100 cm–1), monomer bend fundamental (∼1600 cm), and O–H stretch (∼3000–3700 cm–1) bands. In addition, the coupled local-monomer model is used to obtain the vibrational density of states in the bend fundamental and O–H stretch regions. The harmonic and local-monomer vibrational density of states obtained from core monomers are in good agreement with those of inelastic neutron scattering spectra, especially the latter, which accounts for anharmonic coupling of monomer modes. Full deuteration is also considered, and the vibrational density of states is again compared to experiment, where good agreement is found.

Signature IR spectra of isomers of the water hexamer in the spectral range 3000–3800 cm–1 have been reported by experimentalists, but crucial theoretical interpretation has still not been definitive. Using ab initio potential and dipole moment surfaces and a fully coupled quantum treatment of the intramolecular modes, the ring and book are assigned to spectra obtained in the He nanodroplet and Ar tagging experiments, respectively. The overtone of the intramolecular bend at ca. 3200 cm–1 is a new calculated feature that completes an important missing piece in previous experimental and theoretical comparisons and leads to a consistent assignment of these two experimental spectra. Calculated IR spectra for the lowest energy forms of the water heptamer and octomer are also presented and compared to experiment. In all the calculated spectra, the bend overtone is demonstrated to be a noticeable feature, and this is one important conclusion from the work. Also, the danger in using scaled double-harmonic spectra to assign spectra is demonstrated.Keywords: bend-overtone; heptamer; local-monomer; octomer; ring hexamer; water book hexamer;

We present ab initio calculations of the infrared (IR) spectra of the strongly anharmonic clusters F–(H2O) and F–(H2O)2 using full-dimensional potential energy and dipole moment surfaces. The full-dimensional F–(H2O)2 potential energy surface is composed of previously published one- and two-body water potentials, a two-body F–H2O potential, and a new three-body F–(H2O)2 potential. The three-body water–fluoride potential is a fit to 16 111 second-order Møller–Plesset perturbation theory (MP2/aug-cc-pVTZ) energies using permutationally invariant polynomials. The IR spectrum of F–(H2O)2 is computed using 6-, 8-, 10-, and 13-dimensional VSCF/VCI in order to illustrate the effects of mode coupling and intensity borrowing between the high-frequency intramolecular modes and the low-frequency intermolecular modes. Comparisons with Ar-tagged action spectra are also made.Keywords: intensity sharing; mode coupling; vibrational relaxation; water−fluoride clusters; water−ion clusters;

The hydrogen bonding in water is dominated by pairwise dimer interactions, and the predissociation of the water dimer following vibrational excitation is reported here. Velocity map imaging was used for an experimental determination of the dissociation energy (D0) of (D2O)2. The value obtained, 1244 ± 10 cm–1 (14.88 ± 0.12 kJ/mol), is in excellent agreement with the calculated value of 1244 ± 5 cm–1 (14.88 ± 0.06 kJ/mol). This agreement between theory and experiment is as good as the one obtained recently for (H2O)2. In addition, pair-correlated water fragment rovibrational state distributions following vibrational predissociation of (H2O)2 and (D2O)2 were obtained upon excitation of the hydrogen-bonded OH and OD stretch fundamentals, respectively. Quasi-classical trajectory calculations, using an accurate full-dimensional potential energy surface, are in accord with and help to elucidate experiment. Experiment and theory find predominant excitation of the fragment bending mode upon hydrogen bond breaking. A minor channel is also observed in which both fragments are in the ground vibrational state and are highly rotationally excited. The theoretical calculations reveal equal probability of bending excitation in the donor and acceptor subunits, which is a result of interchange of donor and acceptor roles. The rotational distributions associated with the major channel, in which one water fragment has one quantum of bend, and the minor channel with both water fragments in the ground vibrational state are calculated and are in agreement with experiment.

We report the IR spectra of two forms of ice in the monomer bend and OH-stretching regions, using recently developed ab initio potential and dipole moment surfaces for arbitrarily many water monomers. Coupling and anharmonicity of the intramolecular vibrational modes are taken into account using coupled three-mode variational calculations, within the local-monomer model. Spectra for the surface and core regions of these ice models are presented. The calculated spectra for the core region, with no adjustments, are in good agreement with experiment for the intramolecular OH-stretch and bend regions. Our analysis also shows a significant contribution from the overtone of the monomer bend to the OH-stretch region of the spectra.Keywords: ab initio water potential; ice models; intramolecular vibrational modes; IR spectra; local-monomer model; overtone of bend;

The O(3P) + C2H4 reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H + CH2CHO, H + CH3CO, H2 + CH2CO, CH3 + HCO, CH2 + CH2O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4 kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross-sections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH3 + HCO and H + CH2CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.

Full-dimensional, three-state, surface hopping calculations of the photodissociation dynamics of formaldehyde are reported on ab initio potential energy surfaces (PESs) for electronic states S1, T1, and S0. This is the first such study initiated on S1 with ab initio-calculated spin–orbit couplings among the three states. We employ previous PESs for S0 and T1, and a new PES for S1, which we describe here, as well as new spin–orbit couplings. The time-dependent electronic state populations and the branching ratio of radical products produced from S0 and T1 states and that of total radical products and molecular products at three total energies are calculated. Details of the surface hopping dynamics are described, and a novel pathway for isomerization on T1 via S0 is reported. Final translational energy distributions of H + HCO products from S0 and T1 are also reported as well as the translational energy distribution and final rovibrational distributions of H2 products from the molecular channel. The present results are compared to previous trajectory calculations initiated from the global minimum of S0. The roaming pathway leading to low rotational distribution of CO and high vibrational population of H2 is observed in the present calculations.

There has been great progress in the development of potential energy surfaces (PESs) for reaction dynamics that are fits to ab initio energies. The fitting techniques described here explicitly represent the invariance of the PES with respect to all permutations of like atoms. A review of a subset of dynamics calculations using such PESs (currently 16 such PESs exist) is then given. Bimolecular reactions of current interest to the community, namely, H + CH4 and F + CH4, are focused on. Unimolecular reactions are then reviewed, with a focus on the photodissociation dynamics of H2CO and CH3CHO, where so-called “roaming” pathways have been discovered. The challenges for electronically non-adiabatic reactions, and associated PESs, are presented with a focus on the OH* + H2 reaction. Finally, some thoughts on future directions and challenges are given.

We present a quasiclassical trajectory study of the photodissociation of CH3CHO using a global ab initio-based potential energy surface. Calculations are performed at a total energy of 160 kcal/mol, corresponding to a photolysis wavelength of 230 nm, and with all trajectories initiated from the acetaldehyde global minimum. Many product channels are energetically accessible and observed at this energy, and the branching ratios to these are presented. We identify a minor channel giving H2O plus vinylidene or acetylene. Mechanisms are identified for these products, both of which originate from dissociation of vinyl alcohol an isomer of acetaldehyde. The channel CH2═CHOH → HCCH + H2O produces acetylene with relatively low internal energy, while the acetylene which arises from the subsequent isomerization of vinylidene via the channel CH2═CHOH → [CH2═C + H2O] →HCCH + H2O, is highly excited. The dissociation of CD3CHO is also studied, and both HOD+DCCD (D2CC) and D2O+DCCH products are formed from the same mechanisms as in the CH3CHO dissociation.Keywords: acetaldehyde; acetylene; photodissociation; vinylidene; water;

High-level ab initio calculations are performed to examine previously unexplored regions of full-dimensional potential energy surfaces that connect the conventional and recently reported “roaming” saddle points for the H2CO and CH3CHO unimolecular dissociations to form molecular products, H2 + CO and CH4 + CO, respectively. The aim of this investigation is to determine whether or not there are large barriers separating these saddle points and their associated intrinsic reaction pathways. The results are of fundamental significance in formulating statistical and reduced dimensionality dynamical approaches to model these reactions, including both pathways.Keywords: acetaldehyde; formaldehyde; photodissociation; roaming; saddle points;

The Born−Oppenheimer potential energy surface(s) underlies theoretical and computational chemistry (whether one considers a single or multiply coupled surfaces). The recent progress in representing these surfaces, rigorously obtained from electronic structure calculations, is the focus of this Perspective. Examples of potentials of complex molecules, namely, CH3CHO, CH5+, and H5+, and molecular complexes, namely, water clusters, are given.

Quasiclassical trajectories on an ab initio potential energy surface show that a stereodynamical effect steers the F atom away from the stretching-excited CH bond, thereby promoting the DF + CHD2 product channel of the F + CHD3(ν1 = 1) reaction at low collision energies. This explains the unexpected results of a recent crossed molecular beam experiment.

We report quasiclassical trajectory calculations of the HO2 + NO reaction using a new full dimensional, singlet potential energy surface (PES) which is a fit to more than 67000 energies obtained with density functional theory-B3LYP/6-311G(d,p)-calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO2, saddle points connecting them and the OH + NO2, HO2 + NO channels. Quasiclassical trajectory calculations of cross-sections for the HO2 + NO to form HOONO, HONO2 and OH + NO2 are done using this PES, for reactants in the ground vibrational state and rotational states sampled from a 300 K Boltzmann distribution. Trajectory calculations illustrate the pathway that HO2 + NO takes to the energized HOONO complex, which dissociates to products OH + NO2, reactants HO2 + NO, or isomerizes to HONO2. The association cross sections are used to obtain rate constants for formation of HOONO and HONO2 in the high-pressure limit, and formation of products OH + NO2 in the low-pressure limit.

We describe multidimensional extensions to a one-dimensional approach to tunneling splittings using a relaxed potential in the imaginary-frequency normal mode of the relevant saddle point ( J. Chem. Phys. 2008, 129, 121103). Tests of these extensions are given for H3O+ and NH3 where full dimensional tunneling splittings are available and for the vinyl radical using a new full-dimensional potential energy surface.

We present a quasiclassical trajectory study of the photodissociation of CH3CHO to molecular and radical products, CH4 + CO and CH3 + HCO, respectively, using global ab initio-based potentials energy surfaces. The molecular products have a well-defined potential barrier transition state (TS) but the dynamics exhibit strong deviations from the TS pathway to these products. The radical products are formed via a variational TS. Calculations are reported at total energies corresponding to photolysis wavelengths of 308, 282, 264, 248 and 233 nm. The results at 308 nm focus on a comparison with experiment [Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079] and the elucidation of the nature and extent of non-TS reaction dynamics to form the molecular products, CH4 + CO. At the other wavelengths the focus is the branching ratio of these products and the radical products, CH3 + HCO.

The photodissociation dynamics of H2CO is known to involve electronic states S1, T1 and S0. Recent quasiclassical trajectory (QCT) calculations, in conjunction with experiment, have identified a “roaming” H-atom pathway to the molecular products, H2+CO [Townsend; et al. Science 2004, 306, 1158.]. These calculations were initiated at the global minimum (GM) of S0, which is where the initial wave function is located. The “roaming” mechanism is not seen if trajectories are initiated from the molecular transition state saddle point (SP). In this Letter we identify the minimum energy-crossing configurations and energy of the T1/S0 potentials as a step toward studying the multisurface nature of the photodissociation. QCT calculations are initiated at these configurations on a revised potential energy surface and the results are compared to those initiated, as previously, from the S0 GM as well as the S0 SP. The product state distributions of H2 + CO from trajectories initiated at the T1/S0 crossing are in excellent agreement with those initiated at the S0 GM.

Abstract

We present two quantum calculations of the infrared spectrum of protonated methane (CH₅⁺) using full-dimensional, ab initio–based potential energy and dipole moment surfaces. The calculated spectra compare well with a low-resolution experimental spectrum except below 1000 cm^–1, where the experimental spectrum shows no absorption. The present calculations find substantial absorption features below 1000 cm^–1, in qualitative agreement with earlier classical calculations of the spectrum. The major spectral bands are analyzed in terms of the molecular motions. Of particular interest is an intense feature at 200 cm^–1, which is due to an isomerization mode that connects two equivalent minima. Very recent high-resolution jet-cooled spectra in the CH stretch region (2825 to 3050 cm^–1) are also reported, and assignments of the band origins are made, based on the present quantum calculations.

We review the photodissociation dynamics of formaldehyde with an emphasis on recent calculations that make use of a global ab initio-based potential energy surface for the S0 state. These calculations together with recent experiments reveal striking departures from conventional transition state theory for the formation of the molecular products H2 + CO. The evidence for this departure is reviewed in detail by examining properties of the new potential surface and results of quasiclassical trajectory dynamics calculations using this surface. We also review very recent work on the dynamics governing the formation of radical products, H + HCO. These products can be formed on the T1 surface as well as the S0 one, and we present some results contrasting the dynamics on these two surfaces. This work makes use of a new semi-global ab initio-based T1 potential energy surface.

Abstract

We present a combined experimental and theoretical investigation of formaldehyde (H₂CO) dissociation to H₂ and CO at energies just above the threshold for competing H elimination. High-resolution state-resolved imaging measurements of the CO velocity distributions reveal two dissociation pathways. The first proceeds through a well-established transition state to produce rotationally excited CO and vibrationally cold H₂. The second dissociation pathway yields rotationally cold CO in conjunction with highly vibrationally excited H₂. Quasi-classical trajectory calculations performed on a global potential energy surface for H₂CO suggest that this second channel represents an intramolecular hydrogen abstraction mechanism: One hydrogen atom explores large regions of the potential energy surface before bonding with the second H atom, bypassing the saddle point entirely.

We report a full-dimensional, permutationally invariant potential energy surface (PES) for the cyclic formic acid dimer. This PES is a least-squares fit to 13475 CCSD(T)-F12a/haTZ (VTZ for H and aVTZ for C and O) energies. The energy-weighted, root-mean-square fitting error is 11 cm−1 and the barrier for the double-proton transfer on the PES is 2848 cm−1, in good agreement with the directly-calculated ab initio value of 2853 cm−1. The zero-point vibrational energy of 15337 ± 7 cm−1 is obtained from diffusion Monte Carlo calculations. Energies of fundamentals of fifteen modes are calculated using the vibrational self-consistent field and virtual-state configuration interaction method. The ground-state tunneling splitting is computed using a reduced-dimensional Hamiltonian with relaxed potentials. The highest-level, four-mode coupled calculation gives a tunneling splitting of 0.037 cm−1, which is roughly twice the experimental value. The tunneling splittings of (DCOOH)2 and (DCOOD)2 from one to three mode calculations are, as expected, smaller than that for (HCOOH)2 and consistent with experiment.

The potential energy surface of the methane–water dimer is represented as the sum of a new intrinsic two-body potential energy surface and pre-existing intramolecular potentials for the monomers. Different fits of the CH4–H2O intrinsic two-body energy are reported. All these fits are based on 30467 ab initio interaction energies computed at CCSD(T)-F12b/haTZ (aug-cc-pVTZ for C and O, cc-pVTZ for H) level of theory. The benchmark fit is a full-dimensional, permutationally-invariant analytical representation with root-mean-square (rms) fitting error of 3.5 cm−1. Two other computationally more efficient two-body potentials are also reported, albeit with larger rms fitting errors. Of these a compact permutationally invariant fit is shown to be the best one in combining precision and speed of evaluation. An intrinsic two-body dipole moment surface is also obtained, based on MP2/haTZ expectation values, with an rms fitting error of 0.002 au. As with the potential, this dipole moment surface is combined with existing monomer ones to obtain the full surface. The vibrational ground state of the dimer and dissociation energy, D0, are determined by diffusion Monte Carlo calculations, and MULTIMODE calculations are performed for the IR spectrum of the intramolecular modes. The relative accuracy of the different intrinsic two-body potentials is analyzed by comparing the energetics and the harmonic frequencies of the global minimum well, and the maximum impact parameter employed in a sample methane–water scattering calculation.

We report quasiclassical trajectory calculations of the HO2 + NO reaction using a new full dimensional, singlet potential energy surface (PES) which is a fit to more than 67000 energies obtained with density functional theory-B3LYP/6-311G(d,p)-calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO2, saddle points connecting them and the OH + NO2, HO2 + NO channels. Quasiclassical trajectory calculations of cross-sections for the HO2 + NO to form HOONO, HONO2 and OH + NO2 are done using this PES, for reactants in the ground vibrational state and rotational states sampled from a 300 K Boltzmann distribution. Trajectory calculations illustrate the pathway that HO2 + NO takes to the energized HOONO complex, which dissociates to products OH + NO2, reactants HO2 + NO, or isomerizes to HONO2. The association cross sections are used to obtain rate constants for formation of HOONO and HONO2 in the high-pressure limit, and formation of products OH + NO2 in the low-pressure limit.

There has been great progress in the development of potential energy surfaces (PESs) for reaction dynamics that are fits to ab initio energies. The fitting techniques described here explicitly represent the invariance of the PES with respect to all permutations of like atoms. A review of a subset of dynamics calculations using such PESs (currently 16 such PESs exist) is then given. Bimolecular reactions of current interest to the community, namely, H + CH4 and F + CH4, are focused on. Unimolecular reactions are then reviewed, with a focus on the photodissociation dynamics of H2CO and CH3CHO, where so-called “roaming” pathways have been discovered. The challenges for electronically non-adiabatic reactions, and associated PESs, are presented with a focus on the OH* + H2 reaction. Finally, some thoughts on future directions and challenges are given.