Showing papers on "Potential energy surface published in 2016"
TL;DR: In this review, recent progress in constructing high-dimensional potential energy surfaces for polyatomic molecules interacting with transition metal surfaces based on the plane-wave density functional theory and in quantum dynamical studies of dissociative chemisorption on these potentialEnergy surfaces are summarized.
Abstract: Dissociative chemisorption is the initial and often rate-limiting step in many heterogeneous processes. As a result, an in-depth understanding of the reaction dynamics of such processes is of great importance for the establishment of a predictive model of heterogeneous catalysis. Overwhelming experimental evidence has suggested that these processes have a non-statistical nature and excitations in various reactant modes have a significant impact on reactivity. A comprehensive characterization of the reaction dynamics requires a quantum mechanical treatment on a global potential energy surface. In this review, we summarize recent progress in constructing high-dimensional potential energy surfaces for polyatomic molecules interacting with transition metal surfaces based on the plane-wave density functional theory and in quantum dynamical studies of dissociative chemisorption on these potential energy surfaces. A special focus is placed on the mode specificity and bond selectivity in these gas-surface collisional processes, and their rationalization in terms of the recently proposed Sudden Vector Projection model.
144 citations
TL;DR: A new translation-rotation-internal coordinate (TRIC) system which explicitly includes the collective translations and rotations of molecules, or parts of molecules such as monomers or ligands, as degrees of freedom is introduced.
Abstract: The effective description of molecular geometry is important for theoretical studies of intermolecular interactions. Here we introduce a new translation-rotation-internal coordinate (TRIC) system which explicitly includes the collective translations and rotations of molecules, or parts of molecules such as monomers or ligands, as degrees of freedom. The translations are described as the centroid position and the orientations are represented with the exponential map parameterization of quaternions. When TRIC is incorporated into geometry optimization calculations, the performance is consistently superior to existing coordinate systems for a diverse set of systems including water clusters, organic semiconductor donor-acceptor complexes, and small proteins, all of which are characterized by nontrivial intermolecular interactions. The method also introduces a new way to scan the molecular orientations while allowing orthogonal degrees of freedom to relax. Our findings indicate that an explicit description of molecular translation and rotation is a natural way to traverse the many-dimensional potential energy surface.
132 citations
TL;DR: The modern understanding of the SN2 reaction mechanism is based on work of Hughes and Ingold as discussed by the authors, who proposed that the nucleophile (X−) approaches the carbon atom that bears the leaving group (Y−).
Abstract: The SN2 nucleophilic substitution reaction, X− + RY → XR + Y−, is a paradigm reaction in organic chemistry ( 1 ). The modern understanding of the SN2 reaction mechanism is based on work of Hughes and Ingold ( 2 ), who proposed that the nucleophile (X−) approaches the carbon atom that bears the leaving group (Y−). As a result, the bond between the carbon atom and the leaving group becomes weakened. As this bond breaks and a new bond forms between the nucleophile and the carbon atom, the configuration of the carbon atom is inverted. Analyses of gas-phase reaction rates led to the suggestion of a potential energy surface (PES) with two wells connected by a central barrier transition state ( 3 ). Electronic structure calculations have confirmed this picture for some SN2 reactions ( 4 ), but recent studies have shown that the actual reaction dynamics may be considerably more complex (see the figure) ( 5 – 8 ).
128 citations
TL;DR: In this paper, a fast and accurate method to estimate the partition functions and entropies of adsorbates based on quantum mechanical estimates of the potential energy surface is presented, using the harmonic oscillator (HO) approximation for most of the modes of motion of the adsorbate.
Abstract: With the recent explosion in computational catalysis and related microkinetic modeling, the need for a fast yet accurate way to predict equilibrium and rate constants for surface reactions has become more important. Here we present a fast and accurate new method to estimate the partition functions and entropies of adsorbates based on quantum mechanical estimates of the potential energy surface. As with previous approaches, it uses the harmonic oscillator (HO) approximation for most of the modes of motion of the adsorbate. However, it uses hindered translator and hindered rotor models for the three adsorbate modes associated with motions parallel to the surface and evaluates these using an approach based on a method that has proven accurate in modeling the internal hindered rotations of gas molecules. The adsorbate entropies were calculated with this method for four adsorbates (methanol, propane, ethane, and methane) on Pt(111) using density functional theory (DFT) to evaluate the potential energy surface ...
109 citations
TL;DR: It is found that the new method, which incorporates information from the MD trajectory that connects reactants with products, produces a dramatically distinct set of minimum energy paths compared to existing approaches that start from information for the reaction end points alone.
Abstract: We describe a flexible and broadly applicable energy refinement method, "nebterpolation," for identifying and characterizing the reaction events in a molecular dynamics (MD) simulation. The new method is applicable to ab initio simulations with hundreds of atoms containing complex and multimolecular reaction events. A key aspect of nebterpolation is smoothing of the reactive MD trajectory in internal coordinates to initiate the search for the reaction path on the potential energy surface. We apply nebterpolation to analyze the reaction events in an ab initio nanoreactor simulation that discovers new molecules and mechanisms, including a C-C coupling pathway for glycolaldehyde synthesis. We find that the new method, which incorporates information from the MD trajectory that connects reactants with products, produces a dramatically distinct set of minimum energy paths compared to existing approaches that start from information for the reaction end points alone. The energy refinement method described here represents a key component of an emerging simulation paradigm where molecular dynamics simulations are applied to discover the possible reaction mechanisms.
99 citations
TL;DR: In this article, the authors developed wavelength-tunable femtosecond stimulated Raman spectroscopy to simultaneously achieve pre-resonance enhancement for transient reactant and product species of photoacid pyranine undergoing excited-state proton transfer (ESPT) reaction in solution.
Abstract: Photochemistry powers numerous processes from luminescence and human vision, to light harvesting However, the elucidation of multidimensional photochemical reaction coordinates on molecular timescales remains challenging We developed wavelength-tunable femtosecond stimulated Raman spectroscopy to simultaneously achieve pre-resonance enhancement for transient reactant and product species of the widely used photoacid pyranine undergoing excited-state proton transfer (ESPT) reaction in solution In the low-frequency region, the 280 cm-1 ring deformation mode following 400 nm photoexcitation exhibits pronounced intensity oscillations on the sub-picosecond timescale due to anharmonic vibrational coupling to the 180 cm-1 hydrogen-bond stretching mode only in ESPT-capable solvents, indicating a primary event of functional relevance This leads to the contact ion pair formation on the 3 ps timescale before diffusion-controlled separation The intermolecular 180 cm-1 mode also reveals vibrational cooling time constants, ∼500 fs and 45 ps in both H2O and D2O, which differ from ESPT time constants of ∼3/8 and 90/250 ps in H2O/D2O, respectively Spectral results using H218O further substantiate the functional role of the intermolecular 180 cm-1 mode in modulating the distance between proton donor and acceptor and forming the transient ion pair The direct observation of molecular structural evolution across a wide spectral region during photochemical reactions enriches our fundamental understanding of potential energy surface and holds the key to advancing energy and biological sciences with exceptional atomic and temporal precision
74 citations
TL;DR: A full-dimensional, permutationally invariant potential energy surface (PES) for the cyclic formic acid dimer is reported and 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.
Abstract: 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 15 337 ± 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.
73 citations
TL;DR: It is reported that the excitations in vibrational modes of H2O are more efficacious than increasing the translational energy in promoting the reaction, and the enhancement of the excitation in asymmetric stretch is the largest, but that of symmetric stretch becomes comparable at very low energies.
Abstract: Despite significant progress made in the past decades, it remains extremely challenging to investigate the dissociative chemisorption dynamics of molecular species on surfaces at a full-dimensional quantum mechanical level, in particular for polyatomic-surface reactions. Here we report, to the best of our knowledge, the first full-dimensional quantum dynamics study for the dissociative chemisorption of H2O on rigid Cu(111) with all the nine molecular degrees of freedom fully coupled, based on an accurate full-dimensional potential energy surface. The full-dimensional quantum mechanical reactivity provides the dynamics features with the highest accuracy, revealing that the excitations in vibrational modes of H2O are more efficacious than increasing the translational energy in promoting the reaction. The enhancement of the excitation in asymmetric stretch is the largest, but that of symmetric stretch becomes comparable at very low energies. The full-dimensional characterization also allows the investigation of the validity of previous reduced-dimensional and approximate dynamical models.
72 citations
TL;DR: New vibrational-translational (VT), vibratory-rotational- Translational-Translational(VRT) energy exchange, and dissociation models are developed based on QCT observations and maximum entropy considerations, which makes it feasible to upscale ab initio simulation to full nonequilibrium flow calculations.
Abstract: Quasi-classical trajectory (QCT) calculations are used to study state-specific ro-vibrational energy exchange and dissociation in the O2 + O system. Atom-diatom collisions with energy between 0.1 and 20 eV are calculated with a double many body expansion potential energy surface by Varandas and Pais [Mol. Phys. 65, 843 (1988)]. Inelastic collisions favor mono-quantum vibrational transitions at translational energies above 1.3 eV although multi-quantum transitions are also important. Post-collision vibrational favoring decreases first exponentially and then linearly as Δv increases. Vibrationally elastic collisions (Δv = 0) favor small ΔJ transitions while vibrationally inelastic collisions have equilibrium post-collision rotational distributions. Dissociation exhibits both vibrational and rotational favoring. New vibrational-translational (VT), vibrational-rotational-translational (VRT) energy exchange, and dissociation models are developed based on QCT observations and maximum entropy considerations. Full set of parameters for state-to-state modeling of oxygen is presented. The VT energy exchange model describes 22 000 state-to-state vibrational cross sections using 11 parameters and reproduces vibrational relaxation rates within 30% in the 2500-20 000 K temperature range. The VRT model captures 80 × 10(6) state-to-state ro-vibrational cross sections using 19 parameters and reproduces vibrational relaxation rates within 60% in the 5000-15 000 K temperature range. The developed dissociation model reproduces state-specific and equilibrium dissociation rates within 25% using just 48 parameters. The maximum entropy framework makes it feasible to upscale ab initio simulation to full nonequilibrium flow calculations.
70 citations
TL;DR: A global ground-state triplet potential energy surface for the N2O2 system that is suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation is presented.
Abstract: We present a global ground-state triplet potential energy surface for the N2O2 system that is suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation. The surface is based on multi-state complete-active-space second-order perturbation theory/minimally augmented correlation-consistent polarized valence triple-zeta electronic structure calculations plus dynamically scaled external correlation. In the multireference calculations, the active space has 14 electrons in 12 orbitals. The calculations cover nine arrangements corresponding to dissociative diatom-diatom collisions of N2, O2, and nitric oxide (NO), the interaction of a triatomic molecule (N2O and NO2) with the fourth atom, and the interaction of a diatomic molecule with a single atom (i.e., the triatomic subsystems). The global ground-state potential energy surface was obtained by fitting the many-body interaction to 54 889 electronic structure data points with a fitting function that is a permutationally invariant polynomial in terms of bond-order functions of the six interatomic distances.
62 citations
TL;DR: In this article, the Lagrangian descriptors are used to construct the stable and unstable manifolds of time-varying transition states as dynamic phase space objects governing configurational changes when the ketene molecule is subjected to an oscillating electric field.
Abstract: The time-dependent geometrical separatrices governing state transitions in field-induced ketene isomerization are constructed using the method of Lagrangian descriptors. We obtain the stable and unstable manifolds of time-varying transition states as dynamic phase space objects governing configurational changes when the ketene molecule is subjected to an oscillating electric field. The dynamics of the isomerization reaction are modeled through classical trajectory studies on the Gezelter–Miller potential energy surface and an approximate dipole moment model which is coupled to a time-dependent electric field. We obtain a representation of the reaction geometry, over varying field strengths and oscillation frequencies, by partitioning an initial phase space into basins labeled according to which product state is reached at a given time. The borders between these basins are in agreement with those obtained using Lagrangian descriptors, even in regimes exhibiting chaotic dynamics. Major outcomes of this work are: validation and extension of a transition state theory framework built from Lagrangian descriptors, elaboration of the applicability for this theory to periodically- and aperiodically-driven molecular systems, and prediction of regimes in which isomerization of ketene and its derivatives may be controlled using an external field.
TL;DR: In this article, a selective sampling procedure is proposed to preferentially evaluate a potential energy surface (PES) in a part of the configuration space governing a physical property of interest.
Abstract: In this paper, we propose a selective sampling procedure to preferentially evaluate a potential energy surface (PES) in a part of the configuration space governing a physical property of interest. The proposed sampling procedure is based on a machine-learning method called the Gaussian process, which is used to construct a statistical model of the PES for identifying the region of interest in the configuration space. We demonstrate the efficacy of the proposed procedure for atomic diffusion and ionic conduction, specifically, the proton conduction in a well-studied proton-conducting oxide, barium zirconate $({\mathrm{BaZrO}}_{3})$. The results of the demonstration study indicate that our procedure can efficiently identify the low-energy region characterizing the proton conduction in the host crystal lattice and that the descriptors used for the statistical PES model have a great influence on the performance.
TL;DR: The dynamics of a prototypical SN2 reaction is investigated using a high-dimensional quantum mechanical model on an accurate potential energy surface (PES) and further analyzed by quasi-classical trajectories on the same PES.
Abstract: Despite its importance in chemistry, the microscopic dynamics of bimolecular nucleophilic substitution (SN2) reactions is still not completely elucidated. In this publication, the dynamics of a prototypical SN2 reaction (F– + CH3Cl → CH3F + Cl–) is investigated using a high-dimensional quantum mechanical model on an accurate potential energy surface (PES) and further analyzed by quasi-classical trajectories on the same PES. While the indirect mechanism dominates at low collision energies, the direct mechanism makes a significant contribution. The reactivity is found to depend on the specific reactant vibrational mode excitation. The mode specificity, which is more prevalent in the direct reaction, is rationalized by a transition-state-based model.
TL;DR: In this article, the authors reported the first seven-dimensional quantum dynamics study for the dissociative chemisorption of H2O on Cu(111) using the time-dependent wave-packet approach, based on an accurate nine-dimensional potential energy surface (PES), which is newly developed by neural network fitting to ∼80'000 density functional theory points.
Abstract: We report the first seven-dimensional quantum dynamics study for the dissociative chemisorption of H2O on Cu(111) using the time-dependent wave-packet approach, based on an accurate nine-dimensional potential energy surface (PES), which is newly developed by neural network fitting to ∼80 000 density functional theory points. This seven-dimensional quantum model allows the examination of the influence of azimuthal angles and also the investigation of the quantitative relationship between the seven-dimensional (7D) dissociation probabilities and those results calculated by the six-dimensional (6D) model with the flat surface approximation. The reactivity strongly depends on the azimuthal rotations due to different barrier heights. Very large differences are seen between the 7D dissociation probabilities and the 6D results with fixed azimuthal angles, at different fixed sites of impact, indicating that the 6D model by neglecting the azimuthal rotation can introduce substantial errors in calculating dissociation probabilities and the 7D quantum dynamics is essential to investigate the dissociation process. A new azimuthal angle-averaging approach is proposed that the 7D dissociation probability can be well reproduced by averaging 6D results over 18 azimuthal angles, in particular in low kinetic energy regions.
TL;DR: An integrated computational strategy aimed at providing reliable thermochemical and kinetic information on the formation processes of astrochemical complex organic molecules and state-of-the-art quantum-mechanical computations, second-order vibrational perturbation theory, and kinetic models based on capture and transition state theory together with the master equation approach are described.
Abstract: We describe an integrated computational strategy aimed at providing reliable thermochemical and kinetic information on the formation processes of astrochemical complex organic molecules. The approach involves state-of-the-art quantum-mechanical computations, second-order vibrational perturbation theory, and kinetic models based on capture and transition state theory together with the master equation approach. Notably, tunneling, quantum reflection, and leading anharmonic contributions are accounted for in our model. Formamide has been selected as a case study in view of its interest as a precursor in the abiotic amino acid synthesis. After validation of the level of theory chosen for describing the potential energy surface, we have investigated several pathways of the OH + CH2NH and NH2 + H2CO reaction channels. Our results show that both reaction channels are essentially barrierless (in the sense that all relevant transition states lie below or only marginally above the reactants) and once tunneling is t...
TL;DR: A fully coupled global nine-dimensional potential energy surface for the dissociative chemisorption of CO2 on Ni(100) is constructed and the behavior of the reactivity is rationalized by couplings with the reaction coordinate at each transition state.
Abstract: A fully coupled global nine-dimensional potential energy surface for the dissociative chemisorption of CO2 on Ni(100) is constructed from ∼18,000 density functional points. It reveals a complex reaction pathway dominated by two near iso-energetic transition states. The dissociation probabilities obtained by quasi-classical trajectories on the potential energy surface reproduced experimental trends, and indicate that vibrational excitations of CO2 significantly promote the dissociation. Using the sudden vector projection model, the behavior of the reactivity is rationalized by couplings with the reaction coordinate at each transition state. These results offer plausible rationalization for the observed enhancement of CO2 dissociation in non-thermal plasmas by metal surfaces.
TL;DR: The influence of temperature on NMR chemical shifts and quadrupolar couplings in model molecular organic solids is explored using path integral molecular dynamics (PIMD) and density functional theory (DFT) calculations of shielding and electric field gradient (EFG) tensors.
Abstract: The influence of temperature on NMR chemical shifts and quadrupolar couplings in model molecular organic solids is explored using path integral molecular dynamics (PIMD) and density functional theory (DFT) calculations of shielding and electric field gradient (EFG) tensors. An approach based on convoluting calculated shielding or EFG tensor components with probability distributions of selected bond distances and valence angles obtained from DFT-PIMD simulations at several temperatures is used to calculate the temperature effects. The probability distributions obtained from the quantum PIMD simulations, which includes nuclear quantum effects, are significantly broader and less temperature dependent than those obtained with conventional DFT molecular dynamics or with 1D scans through the potential energy surface. Predicted NMR observables for the model systems were in excellent agreement with experimental data.
TL;DR: This work revisits the chromium dimer system for the third time with multiconfigurational perturbation theory, now in order to increase the active space of the reference wave function and improves the shape of the potential energy surface significantly.
Abstract: The chromium dimer has long been a benchmark molecule to evaluate the performance of different computational methods ranging from density functional theory to wave function methods. Among the latter, multiconfigurational perturbation theory was shown to be able to reproduce the potential energy surface of the chromium dimer accurately. However, for modest active space sizes, it was later shown that different definitions of the zeroth-order Hamiltonian have a large impact on the results. In this work, we revisit the system for the third time with multiconfigurational perturbation theory, now in order to increase the active space of the reference wave function. This reduces the impact of the choice of zeroth-order Hamiltonian and improves the shape of the potential energy surface significantly. We conclude by comparing our results of the dissocation energy and vibrational spectrum to those obtained from several highly accurate multiconfigurational methods and experiment. For a meaningful comparison, we used the extrapolation to the complete basis set for all methods involved.
TL;DR: The results presented here call into question the value of the so-called power stroke model as an explanation of the function of autonomous chemically-driven molecular machines such as are commonly found in biology.
Abstract: Molecular machines use external energy to drive transport, to do mechanical, osmotic, or electrical work on the environment, and to form structure. In this paper the fundamental difference between the design principles necessary for a molecular machine to use light or external modulation of thermodynamic parameters as an energy source vs. the design principle for using an exergonic chemical reaction as a fuel will be explored. The key difference is that for catalytically-driven motors microscopic reversibility must hold arbitrarily far from equilibrium. Applying the constraints of microscopic reversibility assures that a coarse grained model is consistent with an underlying model for motion on a single time-independent potential energy surface. In contrast, light-driven processes, and processes driven by external modulation of the thermodynamic parameters of a system cannot in general be described in terms of motion on a single time-independent potential energy surface, and the rate constants are not constrained by microscopic reversibility. The results presented here call into question the value of the so-called power stroke model as an explanation of the function of autonomous chemically-driven molecular machines such as are commonly found in biology.
TL;DR: This paper presents a comprehensive analysis of the two-dimensional infrared spectrum of O-H stretching vibrations in liquid H2O and their interactions with bending and intermolecular vibrations and argues that the collective nature of water vibrations should be considered in describing aqueous solvation.
Abstract: Water’s extended hydrogen-bond network results in rich and complex dynamics on the sub-picosecond time scale. In this paper, we present a comprehensive analysis of the two-dimensional infrared (2D IR) spectrum of O–H stretching vibrations in liquid H2O and their interactions with bending and intermolecular vibrations. By exploring the dependence of the spectrum on waiting time, temperature, and laser polarization, we refine our molecular picture of water’s complex ultrafast dynamics. The spectral evolution following excitation of the O–H stretching resonance reveals vibrational dynamics on the 50–300 fs time scale that are dominated by intermolecular delocalization. These O–H stretch excitons are a result of the anharmonicity of the nuclear potential energy surface that arises from the hydrogen-bonding interaction. The extent of O–H stretching excitons is characterized through 2D depolarization measurements that show spectrally dependent delocalization in agreement with theoretical predictions. Furthermore, we show that these dynamics are insensitive to temperature, indicating that the exciton dynamics alone set the important time scales in the system. Finally, we study the evolution of the O–H stretching mode, which shows highly non-adiabatic dynamics suggestive of vibrational conical intersections. We argue that the so-called heating, commonly observed within ∼1 ps in nonlinear IR spectroscopy of water, is a nonequilibrium state better described by a kinetic temperature rather than a Boltzmann distribution. Our conclusions imply that the collective nature of water vibrations should be considered in describing aqueous solvation.
TL;DR: This work demonstrates the use of broad-band pump-probe spectroscopy to measure femtosecond solvation dynamics and reveals how protein-buried chromophores are sensitive to the solvent dynamics inside and outside of the protein environment.
Abstract: In this work, we demonstrate the use of broad-band pump–probe spectroscopy to measure femtosecond solvation dynamics. We report studies of a rhodamine dye in methanol and cryptophyte algae light-harvesting proteins in aqueous suspension. Broad-band impulsive excitation generates a vibrational wavepacket that oscillates on the excited-state potential energy surface, destructively interfering with itself at the minimum of the surface. This destructive interference gives rise to a node at a certain probe wavelength that varies with time. This reveals the Gibbs free-energy changes of the excited-state potential energy surface, which equates to the solvation time correlation function. This method captures the inertial solvent response of water (∼40 fs) and the bimodal inertial response of methanol (∼40 and ∼150 fs) and reveals how protein-buried chromophores are sensitive to the solvent dynamics inside and outside of the protein environment.
TL;DR: The results clearly indicate that the electron-hole pair effects are generally small, both on absolute reactivity of each vibrational state and on the mode specificity and bond selectivity in the dissociative chemisorption of CH4/CH3D/CHD3 on Ni(111).
Abstract: The dissociative chemisorption of methane on metal surfaces has attracted much attention in recent years as a prototype of gas-surface reactions in understanding the mode specific and bond selective chemistry. In this work, we systematically investigate the influence of electron-hole pair excitations on the dissociative chemisorption of CH4/CH3D/CHD3 on Ni(111). The energy dissipation induced by surface electron-hole pair excitations is modeled as a friction force introduced in the generalized Langevin equation, in which the independent atomic friction coefficients are determined within the local-density friction approximation. Quasi-classical trajectory calculations for CH4/CH3D/CHD3 have been carried out on a recently developed twelve-dimensional potential energy surface. Comparing the dissociation probabilities obtained with and without friction, our results clearly indicate that the electron-hole pair effects are generally small, both on absolute reactivity of each vibrational state and on the mode specificity and bond selectivity. Given similar observations in both water and methane dissociation processes, we conclude that electron-hole pair excitations would not play an important role as long as the reaction is direct and the interaction time between the molecule and metal electrons is relatively short.
TL;DR: 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, to characterize the roaming phenomenon and insight into the mechanism.
Abstract: 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 co...
TL;DR: In this article, the authors used ab initio molecular dynamics simulations to account for surface atom motion and surface temperature effects, employing the PBE and the RPBE functionals and constructed an accurate potential energy surface incorporating the six adsorbate degrees of freedom of HCl on Au.
Abstract: The reactive scattering of HCl on Au(111) is currently one of the most peculiar reactions in the field of surface chemistry, as it so far eludes an accurate theoretical description. Possible reasons for the observed mismatch between theory and experiment that were not yet all considered in the computations are (i) dissipative effects due to the creation of electron hole pairs and excitations of surface atom motion that might inhibit reaction and (ii) use of an inappropriate density functional theory method or even its failure due to the occurrence of a charge transfer at the transition state. In this work, we address all of these possibilities and perform quasiclassical molecular dynamics simulations employing different methodologies. We use ab initio molecular dynamics simulations to account for surface atom motion and surface temperature effects, employing the PBE and the RPBE functionals. We also construct an accurate potential energy surface incorporating the six adsorbate degrees of freedom of HCl on...
TL;DR: The mode specific reactivity of the F + CHD3 → HF + CD3 reaction is investigated using an eight-dimensional quantum dynamical model on a recently developed ab initio based full-dimensional potential energy surface and results indicate prominent resonance structures at low collision energies and absence of an energy threshold in reaction probabilities.
Abstract: The mode specific reactivity of the F + CHD3 -> HF + CD3 reaction is investigated using an eight-dimensional quantum dynamical model on a recently developed ab initio based full-dimensional potential energy surface. Our results indicate prominent resonance structures at low collision energies and absence of an energy threshold in reaction probabilities. It was also found that excitation of the C-D stretching or CD3 umbrella mode has a relatively small impact on reactivity. On the other hand, the excitation of the C-H vibration (v(1)) in CHD3 is shown to significantly increase the reactivity, which, like several recent quasi-classical trajectory studies, is at odds with the available experimental data. Possible sources of the disagreement are discussed. Published by AIP Publishing.
TL;DR: In this article, a full 9-dimensional ab initio potential energy surface for the methane molecule was constructed using extended electronic structure coupled-cluster calculations with various series of basis sets following increasing X cardinal numbers.
Abstract: Full 9-dimensional ab initio potential energy surfaces for the methane molecule are constructed using extended electronic structure coupled-cluster calculations with various series of basis sets following increasing X cardinal numbers: cc-pVXZ (X = 3, 4, 5, 6), aug-cc-ACVXZ (X = 3, 4, 5), and cc-pCVXZ-F12 (X = 3, 4). High-order dynamic electron correlations including triple and quadrupole excitations as well as relativistic and diagonal Born-Oppenheimer breakdown corrections were accounted for. Analytical potential functions are parametrized as non-polynomial expansions in internal coordinates in irreducible tensor representation. Vibrational energy levels are reported using global variational nuclear motion calculations with exact kinetic energy operator and a full account of the tetrahedral symmetry of CH4. Our best ab initio surface including above-mentioned contributions provides the rms (obs.-calc.) errors of less than 0.11 cm−1 for vibrational band centers below 4700 cm−1, and ∼0.3 cm−1 for all 229 assigned experimentally determined vibrational levels up to the Icosad range <7900 cm−1 without empirically adjusted parameters. These results improve the accuracy of ab initio methane vibrational predictions by more than an order of magnitude with respect to previous works. This is an unprecedented accuracy of first-principles calculations of a five-atomic molecule for such a large data set. New ab initio potential results in significantly better band center predictions even in comparison with best available empirically corrected potential energy surfaces. The issues related to the basis set extrapolation and an additivity of various corrections at this level of accuracy are discussed.
TL;DR: The ab initio equilibrium C-H bond length is in excellent agreement with previous values despite pure rotational energies displaying minor systematic errors as J (rotational excitation) increases, and it is shown that these errors can be significantly reduced by adjusting the equilibrium geometry.
Abstract: A new nine-dimensional potential energy surface (PES) for methane has been generated using state-of-the-art ab initio theory. The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit and incorporates a range of higher-level additive energy corrections. These include core-valence electron correlation, higher-order coupled cluster terms beyond perturbative triples, scalar relativistic effects, and the diagonal Born-Oppenheimer correction. Sub-wavenumber accuracy is achieved for the majority of experimentally known vibrational energy levels with the four fundamentals of (12)CH4 reproduced with a root-mean-square error of 0.70 cm(-1). The computed ab initio equilibrium C-H bond length is in excellent agreement with previous values despite pure rotational energies displaying minor systematic errors as J (rotational excitation) increases. It is shown that these errors can be significantly reduced by adjusting the equilibrium geometry. The PES represents the most accurate ab initio surface to date and will serve as a good starting point for empirical refinement.
TL;DR: The unified reaction valley approach (URVA) for a detailed mechanistic analysis of chemical reactions is improved in three different ways: Direction and curvature of path are analyzed in terms of internal coordinate components that no longer depend on local vibrational modes, and the path analysis is no longer sensitive to path instabilities associated with the occurrences of imaginary frequencies.
Abstract: The unified reaction valley approach (URVA) used for a detailed mechanistic analysis of chemical reactions is improved in three different ways: (i) Direction and curvature of path are analyzed in terms of internal coordinate components that no longer depend on local vibrational modes. In this way, the path analysis is no longer sensitive to path instabilities associated with the occurrences of imaginary frequencies. (ii) The use of third order terms of the energy for a local description of the reaction valley allows an extension of the URVA analysis into the pre- and postchemical regions of the reaction path, which are typically characterized by flat energy regions. (iii) Configurational and conformational processes of the reaction complex are made transparent even in cases where these imply energy changes far less than a kcal/mol by exploiting the topology of the potential energy surface. As examples, the rhodium-catalyzed methanol carbonization, the Diels-Alder reaction between 1,3-butadiene and ethene, and the rearrangement of HCN to CNH are discussed.
TL;DR: Thermal rate coefficients at temperatures between 200 and 1000 K are calculated for the HCl + OH → Cl + H2O reaction on a recently developed permutation invariant potential energy surface, using ring polymer molecular dynamics (RPMD).
Abstract: Thermal rate coefficients at temperatures between 200 and 1000 K are calculated for the HCl + OH → Cl + H2O reaction on a recently developed permutation invariant potential energy surface, using ring polymer molecular dynamics (RPMD). Large deviations from the Arrhenius limit are found at low temperatures, suggesting significant quantum tunneling. Agreement with available experimental rate coefficients is generally satisfactory, although the deviation becomes larger at lower temperatures. The theory–experiment discrepancy is attributed to the remaining errors in the potential energy surface, which is known to slightly overestimate the barrier. In the deep tunneling region, RPMD performs better than traditional transition-state theory with semiclassical tunneling corrections.
TL;DR: In this paper, the authors evaluated the rate constants and several kinetic isotope effects for the OH + CH4 hydrogen abstraction reaction using two kinetics approaches, ring polymer molecular dynamics (RPMD) and variational transition state theory with multidimensional tunneling (VTST/MT), based on a refined full-dimensional analytical potential energy surface, PES-2014.
Abstract: Thermal rate constants and several kinetic isotope effects were evaluated for the OH + CH4 hydrogen abstraction reaction using two kinetics approaches, ring polymer molecular dynamics (RPMD) and variational transition state theory with multidimensional tunneling (VTST/MT), based on a refined full-dimensional analytical potential energy surface, PES-2014, in the temperature range 200–2000 K For the OH + CH4 reaction, at low temperatures, T = 200 K, where the quantum tunneling effect is more important, RPMD overestimates the experimental rate constants due to problems associated with PES-2014 in the deep tunneling regime and to the known overestimation of this method in asymmetric reactions, while VTST/MT presents a better agreement, differences about 10%, due to compensation of several factors, inaccuracy of PES-2014, and ignoring anharmonicity In the opposite extreme, T = 1000 K, recrossing effects play the main role, and the difference between both methods is now smaller, by a factor of 15 Given that