Confinement Effects of Two-dimensional Surfaces on Water Adsorption and Dissociation over Pt(111).

It has been established that the  confined space created by stacking a two dimensional (2D) surface atop a metal catalyst serves as a nano-reactor. According to recent research, when a graphene (Gr) overlayer encloses a Pt catalyst from above, the activation barrier for the water dissociation reaction, a process with major industrial significance, decreases. In order to investigate how the effect of confinement varies among different two-dimensional (2D) materials, we study the adsorption and dissociation barriers of water molecule on Pt(111) under graphene, hexagonal boron nitride (h-BN), and heptazine-based graphitic carbon nitride (g-C3N4) layers using density functional theory calculations. Our findings reveal that the strength of adsorption does not decrease consistently with a reduction in the height of the 2D overlayer. Furthermore, a smaller barrier is not always the consequence of poorer adsorption of the reactant. We also examine the effect of confinement on the shape of the reaction path, on the frequencies of vibrational modes, and on the rate constants derived using the harmonic transition state theory. Overall, all three of the 2D surfaces cause a decrease in barrier height and a weakening of adsorption, though to differing degrees due to a mix of mechanical, geometric and electronic variables.

Efficient Control of Electron Localization and Probability Modulation with Synthesized Two-Color Intense Laser Pulses.

A coupled electron-nuclear dynamical study at attosecond time scale is performed on the HD+ and H2+ molecular ions under the influence of synthesized intense two-color electric fields. We have employed ω - 2ω and also, ω - 3ω two-color fields in the infrared/mid-infrared regime to study the different fragmentation processes originating from the interference of n - (n + i) (i = 1, 2) photon absorption pathways. The branching ratios corresponding to different photofragments are controlled by tuning the relative phase as well as intensity of the two-color pulses, while the effect of the initial nuclear wave function is also studied by taking an individual vibrational eigenstate or a coherent superposition of several eigenstates of HD+ and H2+. By comprehensive analysis, the efficacy of the two different types of synthesized two-color pulses (ω - 2ω and ω - 3ω) are analyzed with respect to one-color intense pulses in terms of controlling the probability modulation and electron localization asymmetry and compared with previous theoretical calculations and experimental findings. Through the detailed investigation, we have addressed which one is the major controlling knob to have better electron localization as well as probability modulation.

Theoretical Chemistry Symposium 2021.

The editorial paints a brief history of the conference - Theoretical Chemistry Symposium, organized by eminent theoretical and computational chemists from India.

Strong Laser Field-Driven Coupled Electron-Nuclear Dynamics: Quantum vs Classical Description.

We have performed a coupled electron-nuclear dynamics study of H2+ molecular ions under the influence of an intense few-cycle 4.5 fs laser pulse with an intensity of 4 × 1014 W/cm2 and a central wavelength of 750 nm. Both quantum and classical dynamical methods are employed in the exact similar initial conditions with the aim of head-to-head comparison of two methodologies. A competition between ionization and dissociation channel is explained under the framework of quantum and classical dynamics. The origin of the electron localization phenomena is elucidated by observing the molecular and electronic wave packet evolution pattern. By probing with different carrier envelope phase (CEP) values of the ultrashort pulse, the possibility of electron localization on either of the two nuclei is investigated. The effects of initial vibrational states on final dissociation and ionization probabilities for several CEP values are studied in detail. Finally, asymmetries in the dissociation probabilities are calculated and mutually compared for both quantum and classical dynamical methodologies, whereas Franck-Condon averaging over the initial vibrational states is carried out in order to mimic the existing experimental conditions.

Oxygen-Induced Dissociation of a Single Water Molecule in Confined 2-D Layers: A Semiempirical Study.

Semiempirical quantum mechanical methods provide a middle ground to molecule-surface interactions between computationally demanding full ab initio quantum chemistry calculations and force-field calculations. In the present study, the PM7 semiempirical method is used to evaluate the adsorption energy values of X@h-BN monolayer [X=O, OH, and H2 O], followed by a mechanistic study of oxygen-induced water dissociation on a free-standing h-BN monolayer. Based on oxygen adsorption configurations, two reaction pathways for water dissociation are studied that yield two distinct configurations of double OH-functionalized h-BN monolayer. The effect of a graphene cover layer on these proposed mechanistic pathways is then investigated by placing the graphene cover layer on the top of the h-BN monolayer and continuously tuning the separation (dGr/h-BN ) between these two layers.

Mode selective chemistry for the dissociation of methane on efficient Ni/Pt-bimetallic alloy catalysts.

The mode selectivity of methane dissociation is studied on three different Ni/Pt-bimetallic alloy surfaces using a fully quantum approach based on reaction path Hamiltonian. Dissociative sticking probability depends on the composition of alloying metals, excited vibrational mode, and symmetry of the reaction path about the plane perpendicular to the catalyst surface containing the carbon atom and two hydrogen atoms. Our calculations show that symmetry of the minimum energy reaction path depends on the surface alloy composition. In the transition state, the dissociating C-H bond elongates significantly for the dissociation of methane on these alloy systems. A significant decrease in the frequency of the symmetric stretching mode and the two bending modes near the transition state is observed on all the alloy surfaces. Under the vibrational adiabatic limit, excitation of these softened modes enhanced the dissociation probability compared to the ground vibrational state. The reaction probability values decrease abruptly at the incident energies less than the zero-point energy corrected barrier height. With the inclusion of non-adiabatic vibrational coupling terms, reaction probability in the low incident energy region increases to a greater extent, and mode selective behavior also becomes different from that observed within the vibrational adiabatic limit. Symmetric stretching mode displayed the highest reactivity on all the alloy surfaces. Overall, Ni8/Pt(111) is found to be the most reactive toward the methane dissociation.

Dissociative ionization of the H2 molecule under a strong elliptically polarized laser field: carrier-envelope phase and orientation effect.

A coupled electron-nuclear dynamical study is performed to investigate the sub-cycle dissociation and ionization of the H2 molecule in a strong 750 nm 4.5 fs elliptically polarized laser pulse. A quasi-classical method is employed in which additional momentum-dependent potentials are added to the molecular Hamiltonian to account for the non-classical effects. The effect of molecular orientation with respect to the laser polarization plane on the probabilities of different dynamical channels and proton energy spectra has been examined. We demonstrate the 2D-control of proton anisotropy by manipulating the carrier-envelope phase of the pulse. We demonstrate that the quasi-classical method can capture the carrier-envelope phase effects in the dissociative ionization of the H2 molecule. Our results indicate that the classical models provide an efficient approach to study the mechanistic insights of strong-field molecular dynamics.

Schematic Design of Metal-Free NHC-Mediated Sequestering and Complete Conversion of SO2 to Thiocarbonyl S-Oxide Derivatives at Room Temperature.

The sequestering and complete conversion of SO2 to valuable chemicals in a metal-free pathway is highly demanded. The recent success of SO2 fixation by N-heterocyclic carbenes instigated further studies in this regard. Previous reports were confined within the carbene-SO2 reaction mechanism and the stability of oxathiirane S-oxide derivatives. The complete conversion of captured SO2 to precious chemicals was not studied. The present inquisition has accomplished the scarcity of the earlier studies. It is observed that in the presence of an excess amount of carbene, the registered SO2 is converted to the ketone derivative and thiocarbonyl S-oxide derivative. An electronic level investigation of these reactions is carried out. From the change of the molecular orbitals along the reaction path, it is concluded that the reaction between the oxathiirane S-oxide derivative and carbene follows a frog's hunting mechanism.

Effect of temperature on CO oxidation over Pt(111) in two-dimensional confinement.

Confined catalysis between a two-dimensional (2D) cover and metal surfaces has provided a unique environment with enhanced activity compared to uncovered metal surfaces. Within this 2D confinement, weakened adsorption and lowered activation energies were observed using surface science experiments and density functional theory (DFT) calculations. Computationally, the role of electronic and mechanical factors responsible for the improved activity was deduced only from static DFT calculations. This demands a detailed investigation on the dynamics of reactions under 2D confinement, including temperature effects. In this work, we study CO oxidation on a 2D graphene covered Pt(111) surface at 90 and 593 K using DFT-based ab initio molecular dynamics simulations starting from the transition state configuration. We show that CO oxidation in the presence of a graphene cover is substantially enhanced (2.3 times) at 90 K. Our findings suggest that 2D confined spaces can be used to enhance the activity of chemical reactions, especially at low temperatures.

Competitive Reactivity of SO2 and NO2 with N-Heterocyclic Carbene: A Mechanistic Study.

Recent DFT based molecular engineering to obtain stable oxathiirane S-oxide derivatives evokes the recommencement of the use of carbenes for the sequestering of SO2, which has been kept separate so far. Carbene is one of the key chemicals for the sequestering of various premier greenhouse gases like CO2, CO, N2O, etc. In this respect, a comparative study of the reactivity of carbenes with variant greenhouse gases is highly demanding. The present investigation is engrossed in the comparative reactivity of SO2 and NO2 with carbenes. All three selected carbenes are highly susceptible to SO2 and NO2. Through an immaculate mechanistic study, we are able to corroborate that the end product of the carbene-SO2 reaction is an adduct which has a preferable structure having a six-membered ring with hydrogen bonding instead of ketone and SO with higher thermodynamic stability than the corresponding oxathiirane S-oxide derivative. Carbene reacts with NO2 to form a stable carbene N, N-dioxide derivative which forms vibrationally excited oxaziridine N-oxide which rapidly dissociates to form a ketone derivative. The formation of carbene S, S-dioxide and carbene N, N-dioxide is a barrierless process. The dissociation of oxaziridene N-oxide is also a barrierless process.

Controlling the Ultrafast Dynamics of HD+ by the Carrier-Envelope Phases of an Ultrashort Laser Pulse: A Quasi-Classical Dynamics Study.

A theoretical study on the coupled electron-nuclear dynamics of HD+ molecular ions under ultrashort, intense laser pulses is performed by employing a well-established quasi-classical model. The influence of the laser carrier-envelope phase on various channel (H + D+, D + H+, and H+ + D+) probabilities is investigated at different laser field intensities. The carrier-envelope phase is found to govern the dissociation (H + D+ and D + H+) and Coulomb explosion (H+ + D+) channel probabilities. The kinetic energy release distributions of the fragments are also found to be sensitive to the carrier-envelope phase of the laser pulse. Our results are in agreement with the previously reported quantum dynamics studies and experiments.

Controlling Electron Dynamics with Carrier-Envelope Phases of a Laser Pulse.

A theoretical study on the ionization dynamics of carbon atom irradiated with a few-cycle, intense laser field is performed within a quasiclassical model to get mechanistic insights into an earlier reported carrier-envelope phase dependency of ionization probabilities of an atom [ Phys. Rev. Lett. 2013, 110, 083602]. The carrier-envelope phase of the laser pulse is found to govern the overall dynamics, reflecting its importance in controlling electronic motion. To understand the origin of this effect, individual trajectories were analyzed at a particular laser intensity. We found that a variation in the carrier-envelope phase affects the angle of ejection of the electrons and subsequently the attainment of the desired final state.

Effects of alloying on mode-selectivity in H2O dissociation on Cu/Ni bimetallic surfaces.

The influence of alloying on mode-selectivity in H2O dissociation on Cu/Ni bimetallic surfaces has been studied using a fully quantum approach based on reaction path Hamiltonian. Both the metal alloy catalyst surface and the normal modes of H2O impact the chemical reactivity of H2O dissociation. A combination of these two different factors will enhance their influence reasonably. Among all the bimetallic surfaces, one monolayer (Ni4_Cu(111)) and 12 monolayer of Ni on Cu surface (Ni2_Cu(111)) show lowest barrier to the dissociation. Excitation of bending mode and symmetric stretching mode enhances the reactivity remarkably due to a significant decrease in their frequencies near the transition state in the vibrational adiabatic approximation. In the presence of non-adiabatic coupling between the modes, asymmetric stretching also shows similar enhancement in reactivity to that of symmetric stretching for all the systems. Inclusion of lattice motion using a sudden model enhances the dissociation probability at surface temperature 300 K and at lower incident energy, compared to that of the static surface approximation. The mode selective behaviour of H2O molecules is almost similar on all the Cu- and Ni-based surfaces. The excitation of symmetric stretching vibration by one quantum is shown to have largest efficacy for promoting reactions for all the systems. Overall, the dissociation probabilities for all the systems are enhanced by vibrational excitation of normal modes and become more significant with the non-adiabatic coupling effect.

Controlling Heterogeneous Catalysis of Water Dissociation Using Cu-Ni Bimetallic Alloy Surfaces: A Quantum Dynamics Study.

Copper-Nickel bimetallic alloys are emerging heterogeneous catalysts for water dissociation which is the rate-determining step of the industrially important water gas shift (WGS) reaction. Yet, the detailed quantum dynamics studies of water-surface scattering in the literature are limited to pure metal surfaces. We present here a three-dimensional wave packet dynamics study of water dissociation on Cu-Ni alloy surfaces, using a pseudodiatomic model of water on a London-Eyring-Polanyi-Sato (LEPS) potential energy surface in order to study the effect of initial vibration, rotation, and orientation of the water molecule on the reactivity. For all the chosen surfaces, reactivity increases significantly with vibrational excitation. In general, for lower vibrational states the reactivity increases with increasing rotational excitation but it decreases in higher vibrational states. Molecular orientation strongly affects reactivity by helping the molecule to align along the reaction path at higher vibrational states. For different alloys, the reaction probability follows the trend of barrier heights and the surfaces having all Ni atoms in the uppermost layer are much more reactive than the ones with Cu atoms. Hence the nature of the alloy surface and initial quantum state of the incoming molecule significantly influence the reactivity in surface catalyzed water dissociation.

HOD on Ni(111): Ab Initio molecular dynamics prediction of molecular beam experiments.

The dissociation of water on a transition-metal catalyst is a fundamental step in relevant industrial processes such as the water-gas shift reaction and steam reforming. Although many theoretical studies have been performed, quantitative agreement between theoretical simulations and molecular beam experiments has not yet been achieved. In this work, we present a predictive ab initio molecular dynamics study on the dissociation of mono-deuterated water (HOD) on Ni(111). The analysis of the trajectories gives useful insight into the full-dimensional dynamics of the process and suggests that rotational steering plays a key role in the dissociation. The computed reaction probability suggests that, in combination with accurate molecular beam experiments, the specific reaction parameter density functional developed for CHD3 (SRP32-vdW) represents a good starting point for developing a semi-empirical functional able to achieve chemical accuracy for HOD on Ni(111).

Water dissociation on Ni(100), Ni(110), and Ni(111) surfaces: Reaction path approach to mode selectivity.

A comparative study of mode-selectivity of water dissociation on Ni(100), Ni(110), and Ni(111) surfaces is performed at the same level of theory using a fully quantum approach based on the reaction path Hamiltonian. Calculations show that the barrier to water dissociation on the Ni(110) surface is significantly lower compared to its close-packed counterparts. Transition states for this reaction on all three surfaces involve the elongation of one of the O-H bonds. A significant decrease in the symmetric stretching and bending mode frequencies near the transition state is observed in all three cases and in the vibrational adiabatic approximation, excitation of these softened modes results in a significant enhancement in reactivity. Inclusion of non-adiabatic couplings between modes results in the asymmetric stretching mode showing a similar enhancement of reactivity as the symmetric stretching mode. Dissociation probabilities calculated at a surface temperature of 300 K showed higher reactivity at lower collision energies compared to that of the static surface case, underlining the importance of lattice motion in enhancing reactivity. Mode selective behavior is similar on all the surfaces. Molecules with one-quantum of vibrational excitation in the symmetric stretch, at lower energies (up to ∼0.45 eV), are more reactive on Ni(110) than the Ni(100) and Ni(111) surfaces. However, the dissociation probabilities approach saturation on all the surfaces at higher incident energy values. Overall, Ni(110) is found to be highly reactive toward water dissociation among the low-index nickel surfaces owing to a low reaction barrier resulting from the openness and corrugation of the surface. These results show that the mode-selective behavior does not vary with different crystal facets of Ni qualitatively, but there is a significant quantitative effect.

Effect of Vibrational Pre-Excitation on the Dissociation Dynamics of HOD(2+).

Preferential breaking of chemical bonds using few-cycle, intense laser pulse to obtain desired products offer a formidable challenge in understanding ultrafast chemical reactivity. In a recent study [J. Chem. Phys. 2015, 143, 244310], it was found that carrier-envelope phase influences the bond-selective fragmentation in HOD with up to 3-fold enhancement. We present a detailed theoretical study to understand the influence of initial vibrational states governing the dissociation dynamics. We have carried out a time-dependent quantum mechanical wave packet study on the ground electronic state (X̃ (3)B1) of HOD(2+). Analytical potential energy surface for the ground electronic states of both the neutral molecule and dication has been developed at multireference configuration interaction level of theory with aug-cc-pVQZ basis set. Branching ratio is computed from the accumulated flux in H(+) + OD(+) and D(+) + OH(+) dissociation channels. Our investigation demonstrate a strong dependency on the initial conditions, and thereby preferential cleavage of bonds can be achieved. We have also compared our results with experimental and other theoretical studies.

Coupled Electron-Nuclear Dynamics on H2+ within Time-Dependent Born-Oppenheimer Approximation.

Quantum dynamical behavior of H2+ in the presence of a linearly polarized, ultrashort, intense, infrared laser pulse has been studied by numerically solving the time-dependent Schrödinger equation with nuclear motion restricted in one-dimension along the direction of laser polarization and electronic motion in three-dimensions. On the basis of the time-dependent Born-Oppenheimer approximation, we have constructed time-dependent potentials for the ground electronic state (1sσg) of H2+. Subsequent nuclear dynamics is then carried out on these field-dressed potential energy surfaces, and the dissociation dynamics is investigated. Our analyses reveal that although the electronic longitudinal degree of freedom plays the major role in governing the dissociation dynamics, contributions from the electronic transverse degree of freedom should also have to be taken into account to obtain accurate results. Also, modeling electron-nuclei Coulomb interactions in a one-dimensional calculation with an artificially chosen constant softening parameter leads to a discrepancy with the exact results. Comparing our results with other quantum and classical dynamical studies showed a good agreement with exact results.

Phase-only laser control in the weak-field limit: Two-pulse control of IBr photofragmentation revisited.

We demonstrate theoretically that laser-induced coherent quantum interference control of asymptotic states of dissociating molecules is possible, starting from a single vibrational eigenstate, after the interaction with two laser pulses-at a fixed time delay-both operating in the weak-field limit. Thus, phase dependence in the interaction with the second fixed-energy phase-modulated pulse persists after the pulse is over. This is illustrated for the nonadiabatic process: I + Br(*)←IBr → I + Br, where the relative yield of excited Br(*) can be changed by pure phase modulation. Furthermore, a strong frequency dependence of the branching ratio is observed and related to the re-crossing dynamics of the avoided crossing in the above-mentioned process.

Dissociative chemisorption of methane on Ni and Pt surfaces: mode-specific chemistry and the effects of lattice motion.

The dissociative chemisorption of methane on metal surfaces is of great practical and fundamental interest. Not only is it the rate-limiting step in the steam re-forming of natural gas, but also the reaction exhibits interesting mode-specific behavior and a strong dependence on the temperature of the metal. Electronic structure methods are used to explore this reaction on various Ni and Pt surfaces, with a focus on how the transition state is modified by motion of the metal lattice atoms. These results are used to construct models that explain the strong variation in reactivity with substrate temperature, shown to result primarily from changes in the dissociation barrier height with lattice motion. The dynamics of the dissociative chemisorption of CH4 on Ni and Pt is explored, using a fully quantum approach based on the reaction path Hamiltonian that includes all 15 molecular degrees of freedom and the effects of lattice motion. Agreement with experiment is good, and vibrational excitation of the molecule is shown to significantly enhance reactivity. The efficacy for this is examined in terms of the vibrationally nonadiabatic couplings, mode softening, mode symmetry, and energy localization in the reactive bond.