Ab initio potential energy surface and quantum dynamics for the H + CH4 → H2 + CH3 reaction

A new full-dimensional potential energy surface for the title reaction has been constructed using the modified Shepard interpolation scheme. Energies and derivatives were calculated using the UCCSD(T) method with aug-cc-pVTZ and 6-311++G(3df,2pd) basis sets, respectively. A total number of 30,000 data points were selected from a huge number of molecular configurations sampled by trajectory method. Quantum dynamical calculations showed that the potential energy surface is well converged for the number of data points for collision energy up to 2.5 eV. Total reaction probabilities and integral cross sections were calculated on the present surface, as well as on the ZBB3 and EG-2008 surfaces for the title reaction. Satisfactory agreements were achieved between the present and the ZBB3 potential energy surfaces, indicating we are approaching the final stage to obtain a global potential energy surface of quantitative accuracy for this benchmark polyatomic system. Our calculations also showed that the EG-2008 surface is less accurate than the present and ZBB3 surfaces, particularly in high energy region.     

Semiclassical calculation of the reaction probability for the process C + O → CO on a Pt(111) surface

The formation of CO on a Pt(111) surface is studied using a semiclassical approach. The reaction probability is calculated as a function of kinetic energy of the carbon atom and initial oxygen position on the surface.     

An ab initio molecular dynamics study on the dissociative recombination reaction of HD2O+ + e−

An ab initio molecular dynamics simulations have been carried out for the dissociative recombination reaction of the deuterium-substituted hydronium cation, HD₂O⁺ + e ⁻, at the state-averaged multiconfigurational self-consistent field level. In the present simulations, five electronic states of HD₂O were included explicitly, and nonadiabatic transitions among adiabatic electronic states were taken into account by the Tully’s fewest switches algorithm. It is shown that the dominant products, OD + D + H, were generated in 63% of trajectories, while the products, OH + 2D, were generated in only 11% of trajectories, indicating that the release of a light fragment H is favored over the release of a heavy fragment D. This result is in conformity with the observation that there is a larger amount of deuterium substituted species than the non-substituted species in the interstellar space. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Interaction and reaction of the hydroxyl ion with β-d-galactose and its hydrated complex: an ab initio molecular dynamics study

The interaction of OH(-) with the sugar β-d-galactose is studied computationally, with Ab Initio Molecular Dynamics (AIMD) as the prime tool. The main findings are: (1) the OH(-) abstracts a proton from the sugar in a barrier-less process, yielding H(2)O and a Deprotonated beta-d-Galactose anion, (Dep-beta-d-G)(-). (2) This reaction can be reversed when two additional H(2)O molecules are present in the sugar. (3) At 500 K, a ring-opening reaction occurs in (Dep-beta-d-G)(-) within a timescale of 10 ps. The (neutral) sugar itself is stable over this timescale, and well beyond. This indicates that OH(-) can catalyze the degradation of β-d-galactose. Implications of this process are briefly discussed.     

H(D) → D(H) + Cu(111) collision system: molecular dynamics study of surface temperature effects

All the channels of the reaction dynamics of gas-phase H (or D) atoms with D (or H) atoms adsorbed onto a Cu(111) surface have been studied by quasiclassical constant energy molecular dynamics simulations. The surface is flexible and is prepared at different temperature values, such as 30 K, 94 K, and 160 K. The adsorbates were distributed randomly on the surface to create 0.18 ML, 0.28 ML, and 0.50 ML of coverages. The multi-layer slab is mimicked by a many-body embedded-atom potential energy function. The slab atoms can move according to the exerted external forces. Treating the slab atoms non-rigid has an important effect on the dynamics of the projectile atom and adsorbates. Significant energy transfer from the projectile atom to the surface lattice atoms takes place especially during the first impact that modifies significantly the details of the dynamics of the collisions. Effects of the different temperatures of the slab are investigated in this study. Interaction between the surface atoms and the adsorbates is modeled by a modified London-Eyring-Polanyi-Sato (LEPS) function. The LEPS parameters are determined by using the total energy values which were calculated by a density functional theory and a generalized gradient approximation for an exchange-correlation energy for many different orientations, and locations of one- and two-hydrogen atoms on the Cu(111) surface. The rms value of the fitting procedure is about 0.16 eV. Many different channels of the processes on the surface have been examined, such as inelastic reflection of the incident hydrogen, subsurface penetration of the incident projectile and adsorbates, sticking of the incident atom on the surface. In addition, hot-atom and Eley-Rideal direct processes are investigated. The hot-atom process is found to be more significant than the Eley-Rideal process. Furthermore, the rate of subsurface penetration is larger than the sticking rate on the surface. In addition, these results are compared and analyzed as a function of the surface temperatures.     

Ab initio study on the dynamical properties of the hydrogen abstraction reaction NH2 + OH → NH + H2O

 The geometry of the transition state of the title reaction was optimized at the unrestricted Hartree–Fock, the spin-unrestricted second-order M?ller–Plesset, and the spin-unrestricted quadratic configuration interaction with all single and double substitutions levels of theory. The changes in the geometry, the bound vibrational modes, and the potential energy along the minimum energy path are discussed. Variational transition-state theory rate constants calculated with the tunneling and curvature effect correction agree very well with the experimental values. Received: 23 April 1999 / Accepted: 9 June 1999 / Published online: 15 December 1999     

Is the structure of hydroxide dihydrate OH−(H2O)2? An ab initio path integral molecular dynamics study

We carried out ab initio path integral molecular dynamics simulations at room temperature for OH^?(H₂O) _n (n = 1, 2) clusters to elucidate the ionic hydrogen bond structure with full thermal and nuclear quantum effects. We found that the hydrogen-bonded proton is located near the water molecule in the case of n = 2, while the proton is located at the center between hydroxide ion and the water molecule in the case of n = 1. Thus, the solvated hydroxide structure \({\text{HO}}{-}{\text{H}} \cdots{\text{OH}}\) is found in n = 2, while the proton sharing hydroxide structure \({\text{HO}} \cdots {\text{H}} \cdots {\text{OH}}\) is in n = 1. We found that the nature of hydrogen bonds significantly changes with the number of water molecules around the hydroxide. We also compared these results with those of F^?(H₂O) _n (n = 1, 2) clusters.     

Ab initio electron correlated studies on the intracluster reaction of NO+ (H2O)(n) → H3O+ (H2O)(n-2) (HONO) (n = 4 and 5)

Hydrated nitrosonium ion clusters NO(+)(H(2)O)(n) (n = 4 and 5) were investigated by using MP2/aug-cc-pVTZ level of theory to clarify isomeric reaction pathways for formation of HONO and fully hydrated hydride ions. We found some new isomers and transition state structures in each hydration number, whose lowest activation energies of the intracluster reactions were found to be 4.1 and 3.4 kcal mol(-1) for n = 4 and n = 5, respectively. These thermodynamic properties and full quantum mechanical molecular dynamics simulation suggest that product isomers with HONO and fully hydrated hydride ions can be obtained at n = 4 and n = 5 in terms of excess hydration binding energies which can overcome these activation barriers.     

New ab initio potential energy surface and quantum dynamics of the reaction H(2S) + NH(X3Σ-) → N(4S) + H2

A new global potential energy surface is reported for the ground state ((4)A(")) of the reaction H((2)S) + NH(X(3)Σ(-)) → N((4)S) + H(2) from a set of accurate ab initio data, which were computed using the multi-reference configuration interaction with a basis set of aug-cc-pV5Z. The many-body expansion and neural network methods have been used to construct the new potential energy surface. The topographical features of the new global potential energy surface are presented. The predicted barrier height is lower than previous theoretical estimates and the heat of reaction with zero-point energy is closer to experimental results. The quantum reactive scattering dynamics calculation was carried out over a range of collision energies (0-1.0 eV) on the new potential energy surface. The reaction probabilities, integral cross-section, and rate constants for the title reaction were calculated. The calculated rate constants are in excellent agreement with the available experimental results.     

Studies of surface processes of electrocatalytic reduction of CO_2 on Pt(210), Pt(310) and Pt(510)

Surface processes of CO2 reduction on Pt(210), Pt(310), and Pt(510) electrodes were studied by cyclic voltammetry. Different surface structures of these platinum single crystal electrodes were obtained by various treatment conditions. The experimental results illustrated that the electrocatalytic activity of Pt single crystal electrodes towards CO2 reduction is decreased in an order of Pt(210)>Pt(310)>Pt(510), i.e., with the decrease of (110) step density on well-defined surfaces. When the surfaces were reconstructed due to oxygen adsorption, the catalytic activity of all the three electrodes has been enhanced to a cer- tain extent. Although the activity order remains unchanged, the electrocatalytic activity has been en- hanced more significantly as the density of (110) step sites is more intensive on the Pt single crystal surface. It has revealed that the more open the surface structure is, the more active the Pt single crystal electrode will be, and the easier for the electrode to be transformed into a surface structure that exhib- its higher activity under external inductions. However, the relatively ordered surfaces of Pt single crystal electrode are comparatively stable under the same external inductions. The present study has gained knowledge on the interaction between CO2 and Pt single crystal electrode surfaces at a micro- scopic level, and thrown new insight into understanding the surface processes of electrocatalytic re- duction of CO2.     

An ab initio based full-dimensional global potential energy surface for FH(2)O(X(2)A(')) and dynamics for the F + H(2)O → HF + HO reaction

A global potential energy surface (PES) for the ground electronic state of FH(2)O is constructed based on more than 30 000 ab initio points at the multi-reference configuration interaction level. The PES features a pre-reaction van der Waals well and two post-reaction hydrogen-bonded complexes, as well as a "reactant-like" transition state with a classical barrier of 3.8 kcal∕mol. The adiabatic F + H(2)O → HF + OH reaction dynamics on this PES was investigated using a standard quasi-classical trajectory method. In agreement with experiment, the HF product contains significant vibrational excitation with limited rotational excitation, while the OH product is internally cold, reflecting its spectator role in the reaction. The products are primarily scattered in the backward direction, consistent with a direct abstraction mechanism.     

Adsorption and decomposition of cyclohexanone (C6H10O) on Pt(111) and the (2 × 2) and (√3 × √3)r30°-Sn/Pt(111) surface alloys

Adsorption and decomposition of cyclohexanone (C(6)H(10)O) on Pt(111) and on two ordered Pt-Sn surface alloys, (2 × 2)-Sn/Pt(111) and (√3 × √3)R30°-Sn/Pt(111), formed by vapor deposition of Sn on the Pt(111) single crystal surface were studied with TPD, HREELS, AES, LEED, and DFT calculations with vibrational analyses. Saturation coverage of C(6)H(10)O was found to be 0.25 ML, independent of the Sn surface concentration. The Pt(111) surface was reactive toward cyclohexanone, with the adsorption in the monolayer being about 70% irreversible. C(6)H(10)O decomposed to yield CO, H(2)O, H(2), and CH(4). Some C-O bond breaking occurred, yielding H(2)O and leaving some carbon on the surface after TPD. HREELS data showed that cyclohexanone decomposition in the monolayer began by 200 K. Intermediates from cyclohexanone decomposition were also relatively unstable on Pt(111), since coadsorbed CO and H were formed below 250 K. Surface Sn allowed for some cyclohexanone to adsorb reversibly. C(6)H(10)O dissociated on the (2 × 2) surface to form CO and H(2)O at low coverages, and methane and H(2) in smaller amounts than on Pt(111). Adsorption of cyclohexanone on (√3 × √3)R30°-Sn/Pt(111) at 90 K was mostly reversible. DFT calculations suggest that C(6)H(10)O adsorbs on Pt(111) in two configurations: by bonding weakly through oxygen to an atop Pt site and more strongly through simultaneously oxygen and carbon of the carbonyl to a bridged Pt-Pt site. In contrast, on alloy surfaces, C(6)H(10)O bonds preferentially to Sn. The presence of Sn, furthermore, is predicted to make the formation of the strongly bound C(6)H(10)O species bonding through O and C, which is a likely decomposition precursor, thermodynamically unfavorable. Alloying with Sn, thus, is shown to moderate adsorptive and reactive activity of Pt(111).     

Ab initio potential energy surface and excited vibrational states for the electronic ground state of Li_2H

A 285-pomt multi-reference configuration-interaction involving single and double excitations ( MRS DCI) potential energy surface for the electronic ground state of L12H is determined by using 6-311G (2df,2pd)basis set.A Simons-Parr-Finlan polynomial expansion is used to fit the discrete surface with a x2 of 4.64×106 The equn librium geometry occurs at Rc=0.172 nm and,LiHL1=94.10°.The dissociation energy for reaction I2H(2A)→L12(1∑g)+H(2S) is 243.910 kJ/mol,and that for reaction L12H(2A')→HL1(1∑) + L1(2S) is 106.445 kl/mol The inversion barrier height is 50.388 kj/mol.The vibrational energy levels are calculated using the discrete variable representation (DVR) method.     

H·(H2O)n clusters: microsolvation of the hydrogen atom via molecular ab initio gradient embedded genetic algorithm (GEGA)

A new version of the ab initio gradient embedded genetic algorithm (GEGA) program for finding the global minima on the potential energy surface (PES) of mixed clusters formed by molecules and atoms is reported. The performance of the algorithm is demonstrated on the neutral H·(H(2)O)(n) (n = 1-4) clusters, that is, a radical H atom solvated in 1-4 water molecules. These clusters are of a fundamental interest. The solvated hydrogen atom forms during photochemical events in water, or during scavenging of solvated electrons by acids, and transiently exists in biological systems and possibly in inclusion complexes in the deep ocean and in the ice shield of earth. The processes associated with its existence are intriguingly complex, however, and have been the subject of decades-long debates. Using GEGA, we explicate the apparently extreme structural diversity in the H·(H(2)O)(n) (n = 1-4) clusters. All considered clusters have four basic structural types: type I, where the H radical is weakly coordinated to the oxygen atom of one of the water molecules; type II, where H is weakly coordinated to a H atom of one of the water molecules; type III, consisting of H(2), the OH radical, and n - 1 H(2)O molecules; and type IV, consisting of H(3)O and n - 1 H(2)O. There are myriads of isomers of all four types. The lowest energy species of types I and II are the isoenergetic global minima. H·(H(2)O)(n) clusters appear to be a challenging case for GEGA because they have many shallow minima close in energy some of which are significantly less stable than the global minimum. Additionally, the global minima themselves have high structural degeneracy, they are only weakly bound, and they are prone to dissociation. GEGA performed exceptionally well in finding both the global and the low-energy local minima that were subsequently confirmed at higher levels of theory.     

Ab initio study of the HCO 3 − /H2O exchange in the (NH3)3 ZnII(HCO 3 − ) complex

Summary The displacement of bicarbonate anion in the (NH₃)₃Zn^II(HCO ₃ ^– ) complex with water has been studied throughab initio calculations. It has been found that H₂O binds to the (NH₃)₃Zn^II(HCO ₃ ^– ) species yielding a stable pentacoordinate (NH₃)₃Zn^II(HCO ₃ ^– )(H₂O) complex. The results also indicate that deprotonation of water in the pentacoordinate species facilitates the release of HCO ₃ ^– , although, the presence of HCO ₃ ^– in the coordination sphere of Zn^II makes such deprotonation more difficult. Environmental effects have been considered in the study of HCO ₃ ^– /H₂O exchange.A contribution from the Grup de Química Quàntica de l'Institut d'Estudis Catalans     

Ab initio surface hopping excited-state molecular dynamics approach on the basis of spin–orbit coupled states: An application to the A-band photodissociation of CH3I

Ab initio molecular dynamics approach has been extended to multi-state dynamics on the basis of the spin–orbit coupled electronic states that are obtained through diagonalization of the spin–orbit coupling matrix with the multi-state second-order multireference perturbation theory energies in diagonal elements and the spin–orbit coupling terms at the state-averaged complete active space self-consistent field level in off-diagonal elements. Nonadiabatic transitions over the spin–orbit coupled states were taken into account explicitly by a surface hopping scheme with utilizing the nonadiabatic coupling terms calculated by numerical differentiation of the spin–orbit coupled wavefunctions and analytical nonadiabatic coupling terms. The present method was applied to the A-band photodissociation of methyl iodide, CH₃I + hv → CH₃ + I (²P_3/2)/I* (²P_1/2), for which a pioneering theoretical work was reported by Amatatsu, Yabushita, and Morokuma. The present results reproduced well the experimental branching ratio and energy distributions in the dissociative products. © 2018 Wiley Periodicals, Inc.     

A theoretical study of the H2SO4+H2O → HSO4 −+H3O+ reaction at the surface of aqueous aerosols

The surface region of sulfate aerosols (supercooled aqueous concentrated sulfuric acid solutions) is the likely site of a number of important heterogeneous reactions in various locations in the atmosphere, but the surface region ionic composition is not known. As a first step in exploring this issue, the first acid ionization reaction for sulfuric acid, H₂SO₄ + H₂O HSO₄^– + H₃O⁺, is studied via electronic structure calculations at the Hartree–Fock level on an H₂SO₄ molecule embedded in the surface region of a cluster containing 33 water molecules. An initial H₂SO₄ configuration is selected which could produce H₃O⁺ readily available for heterogeneous reactions, but which involves reduced solvation and is consistent with no dangling OH bonds for H₂SO₄. It is found that at 0 K and with zero-point energy included, the proton transfer is endothermic by 3.4 kcal/mol. This result is discussed in the context of reactions on sulfate aerosol surfaces and, further, more complex calculations.Contribution to the Jacopo Tomasi Honorary Issue     

Ab initio study of the intermolecular potential energy surface in the ion-induced-dipole hydrogen-bonded O2(-)(X2Πg)-H2(X1Σg(+)) complex

This work presents the first investigation on the intermolecular potential energy surface of the ground electronic state of the O2(-)(2Πg)-H2(1Σg(+)) complex. High level correlated ab initio calculations were carried out using the Hartree-Fock spin-unrestricted coupled cluster singles and doubles including perturbative triples correction [RHF-UCCSD(T)]/aug-cc-pVXZ levels of calculations, where XZ = DZ, TZ, QZ, and 5Z. Results of full geometry optimization and the intermolecular potential energy surface (IPES) calculations show four equivalent minimum energy structures of L-shaped geometry with Cs symmetry at equilibrium along the 2A″ surface of the complex. For these equilibrium minimum energy structures, the most accurate value for the dissociation energy (De) was calculated as 1407.7 cm(-1), which was obtained by extrapolating the counterpoise (CP) corrected De values to the complete basis set (CBS) limit. This global minimum energy structure is stabilized by an ion-induced-dipole hydrogen bond. Detailed investigations of the IPES show that the collinear structure is unstable, while the C2v geometries present saddle points along the 2A″ surface. The barrier height between the two equivalent structures that differs in whether the hydrogen-bonded hydrogen atom is above or below the axis that connects centers of masses of the H2 and O2(-) moieties within the complex was calculated as 70 cm(-1). This suggests that the complex exhibits large amplitude motion. The barrier height to rotation of the H2 moiety by 180° within the complex is 1020 cm(-1). Anharmonic oscillator calculations predicted a strong H-H stretch fundamental transition at 3807 cm(-1). Results of the current work are expected to stimulate further theoretical and experimental investigations on the nature of intermolecular interactions in complexes that contain the superoxide radical and various closed-shell molecules that model atmospheric and biological molecules. These studies are fundamental to understanding the role of the O2(-) anion in chemistry in the atmosphere and in biological systems.     

Ab initio Study of H2O–HCl (1:1, 1:2, 2:1) Complexes

This paper reports on an ab initio (6-31G**) study of 1:1, 1:2, and 2:1 (H₂O) _n (HCl) _m complexes. Stable configurations of the 1:2 and 2:1 (H₂O) _n (HCl) _m complexes and their geometrical and energy characteristics were determined. The vibrational analysis of the complexes was carried out. The effect of hydrogen bonding due to S₁S₀ and T₀T₁ electronic excitations is considered.     

Metal (Co,Fe) tribenzotetraazachlorin–fullerene conjugates: Impact of direct π-bonding on the redox behaviour and oxygen reduction reaction

Novel hexabutylsulphonyltribenzotetraazachlorin–fullerene (C₆₀) complexes of iron (FeHBSTBTAC–C₆₀) and cobalt (CoHBSTBTAC–C₆₀) have been synthesized and their electrochemistry and oxygen reduction reaction (ORR) compared with their octabutylsulphonylphthalocyanine analogues (FeOBSPc and CoOBSPc). It is proved that electron-withdrawing substituents (–SO₂Bu and C₆₀) on phthalocyanine macrocycle exhibit distinct impact on the solution electrochemistry of these metallophthalocyanine (MPc) complexes. The more electron-withdrawing C₆₀ substituent suppressed ORR compared to the –SO₂Bu in alkaline medium. FeOBSPc showed the best ORR activity involving a direct 4-electron mechanism, a rate constant of ～1 × 10⁸ cm³ mol^?1 s^?1 and a Tafel slope of ?171 mV dec^?1.