Quantum Dynamics of the HO + CO → H + CO₂ Reaction on an Accurate Potential Energy Surface

Full-dimensional quantum dynamics of the HO + CO → H + CO₂ reaction is investigated on a recent global potential energy surface based on a large number of ab initio points. The <i>J</i> = 0 reaction probability is small and essentially a monotonically increasing function with energy, superimposed by overlapping resonances. The reactivity is considerably enhanced by OH vibrational excitation while relatively insensitive to CO vibrational excitation. The rate constant estimated by the <i>J</i>-shifting approximation indicates a much better agreement with experiment than that obtained on a previous potential energy surface

CO Hydrogenation on Pd(111): Competition between Fischer–Tropsch and Oxygenate Synthesis Pathways

The hydrogenation of CO on Pd can lead to methane via the Fischer–Tropsch process and methanol via oxygenate synthesis. Despite the fact that the former is thermodynamically favored, the catalysis is mostly selective to the latter. Given the importance of methanol synthesis in both industry applications and fundamental understanding of heterogeneous catalysis, it is highly desirable to understand the mechanism and selectivity of CO hydrogenation on Pd catalysts. In this work, this process is studied on Pd(111) using periodic plane-wave density functional theory and kinetic Monte Carlo simulations. The barriers and reaction energies for the methanol and methane formation are systematically explored. Our results suggest that methanol is formed via CO* → CHO* → HCOH* → CH₂OH* → CH₃OH*. The HCOH* and CH₂OH* intermediates, which feature a C–O single bond, were found to possess the lowest barriers for C–O bond fission, but they are still higher than those in methanol formation, thus confirming the kinetic origin of the experimentally observed selectivity of the Pd catalysts toward methanol

General Charge Transfer Dipole Model for AMOEBA-Like Force Fields

The development of highly accurate force fields is always an importance aspect in molecular modeling. In this work, we introduce a general damping-based charge transfer dipole (D-CTD) model to describe the charge transfer energy and the corresponding charge flow for H, C, N, O, P, S, F, Cl, and Br elements in common bio-organic systems. Then, two effective schemes to evaluate the charge flow from the corresponding induced dipole moment between the interacting molecules were also proposed and discussed. The potential applicability of the D-CTD model in ion-containing systems was also demonstrated in a series of ion–water complexes including Li+, Na+, K+, Mg2+, Ca2+, Fe2+, Zn2+, Pt2+, F–, Cl–, Br–, and I– ions. In general, the D-CTD model demonstrated good accuracy and good transferability in both charge transfer energy and the corresponding charge flow for a wide range of model systems. By distinguishing the intermolecular charge redistribution (charge transfer) under the influence of an external electric field from the accompanying intramolecular charge redistribution (polarization), the D-CTD model is theoretically consistent with current induced dipole-based polarizable dipole models and hence can be easily implemented and parameterized. Along with our previous work in charge penetration-corrected electrostatics, a bottom-up approach constructed water model was also proposed and demonstrated. The structure-maker and structure-breaker roles of cations and anions were also correctly reproduced using Na+, K+, Cl–, and I– ions in the new water model, respectively. This work demonstrates a cost-effective approach to describe the charge transfer phenomena. The water and ion models also show the feasibility of a modulated development approach for future force fields

General Charge Transfer Dipole Model for AMOEBA-Like Force Fields

The development of highly accurate force fields is always an importance aspect in molecular modeling. In this work, we introduce a general damping-based charge transfer dipole (D-CTD) model to describe the charge transfer energy and the corresponding charge flow for H, C, N, O, P, S, F, Cl, and Br elements in common bio-organic systems. Then, two effective schemes to evaluate the charge flow from the corresponding induced dipole moment between the interacting molecules were also proposed and discussed. The potential applicability of the D-CTD model in ion-containing systems was also demonstrated in a series of ion–water complexes including Li+, Na+, K+, Mg2+, Ca2+, Fe2+, Zn2+, Pt2+, F–, Cl–, Br–, and I– ions. In general, the D-CTD model demonstrated good accuracy and good transferability in both charge transfer energy and the corresponding charge flow for a wide range of model systems. By distinguishing the intermolecular charge redistribution (charge transfer) under the influence of an external electric field from the accompanying intramolecular charge redistribution (polarization), the D-CTD model is theoretically consistent with current induced dipole-based polarizable dipole models and hence can be easily implemented and parameterized. Along with our previous work in charge penetration-corrected electrostatics, a bottom-up approach constructed water model was also proposed and demonstrated. The structure-maker and structure-breaker roles of cations and anions were also correctly reproduced using Na+, K+, Cl–, and I– ions in the new water model, respectively. This work demonstrates a cost-effective approach to describe the charge transfer phenomena. The water and ion models also show the feasibility of a modulated development approach for future force fields

Nonadiabatic Tunneling in Photodissociation of Phenol

Using recently developed full-dimensional coupled quasi-diabatic <i>ab initio</i> potential energy surfaces including four electronic (¹ππ, ¹ππ*, 1¹πσ*, and 2¹πσ*) states, the tunneling dynamics of phenol photodissociation via its first excited singlet state (S₁ ← S₀) is investigated quantum mechanically using a three-dimensional model. The lifetimes of several low-lying vibrational states are examined and compared with experiment. The deuteration of the phenoxyl hydrogen is found to dramatically increase the lifetime, attesting to the tunneling nature of the nonadiabatic dissociation. Importantly, it is shown that owing to the conical intersection topography tunneling in this system cannot be described in the standard adiabatic approximation, which eschews the geometric phase effect since the nonadiabatically computed lifetimes, validated by comparison with experiment, differ significantly from those obtained in that limit

Full-Dimensional Quantum State-to-State Nonadiabatic Dynamics for Photodissociation of Ammonia in its <i>A</i>‑Band

Full-dimensional state-to-state quantum dynamics of the photodissociation of NH₃(<i>Ã</i>¹A₂^″) is investigated on newly developed coupled diabatic potential energy surfaces. For the first time, the rovibrational distributions of the nonadiabatically produced NH₂(<i>X̃</i>²<i>B</i>₁) product have been determined quantum mechanically. In agreement with experimental observations, NH₂(<i>X̃</i>²<i>B</i>₁) produced from the 0⁰ and 2¹ states of NH₃(<i>Ã</i>¹A₂^″) was found to be dominated by its ground vibrational state with an <i>N </i>= <i>K</i>_<i>a</i> propensity, shedding light on the quantum-state-resolved nonadiabatic dynamics facilitated by conical intersections and setting the stage for the elucidation of vibrationally mediated photodissociation

Non-Adiabatic Effects on Excited States of Vinylidene Observed with Slow Photoelectron Velocity-Map Imaging

High-resolution slow photoelectron velocity-map imaging spectra of cryogenically cooled <i>X̃</i>²<i>B</i>₂ H₂CC^– and D₂CC^– in the region of the vinylidene triplet excited states are reported. Three electronic bands are observed and, with the assistance of electronic structure calculations and quantum dynamics on ab initio-based near-equilibrium potential energy surfaces, are assigned as detachment to the ã ³<i>B</i>₂ (T₁), <i>b̃</i> ³<i>A</i>₂ (T₂), and <i>Ã</i> ¹<i>A</i>₂ (S₁) excited states of neutral vinylidene. This work provides the first experimental observation of the <i>Ã</i> singlet excited state of H₂CC. While regular vibrational structure is observed for the <i>ã</i> and <i>Ã</i> electronic bands, a number of irregular features are resolved in the vicinity of the <i>b̃</i> band vibrational origin. High-level ab initio calculations suggest that this anomalous structure arises from a conical intersection between the <i>ã</i> and <i>b̃</i> triplet states near the <i>b̃</i> state minimum, which strongly perturbs the vibrational levels in the two electronic states through nonadiabatic coupling. Using the adiabatic electron affinity of H₂CC previously measured to be 0.490(6) eV by Ervin and co-workers [<i>J. Chem. Phys.</i> <b>1989</b>, <i>91</i>, 5974], term energies for the excited neutral states of H₂CC are found to be <i>T</i>₀(<i>ã</i> ³<i>B</i>₂) = 2.064(6), <i>T</i>₀(<i>b̃</i> ³<i>A</i>₂) = 2.738(6), and <i>T</i>₀(<i>Ã</i> ¹<i>A</i>₂) = 2.991(6) eV