Time-dependent Wave Packet Study Research Articles

Ortho-Para Conversion for H+ + H2 Collision in Low Temperature: A Fully Close-Coupled Time-Dependent Wave Packet Study.

The collision-induced rate coefficients of ortho-para conversion for the H+ + H2 reaction provide accurate information to probe the lifetime of cold environments in interstellar media. Rotationally resolved reaction probabilities are calculated at the low collision energy regime (0 < Ecol ≤ 0.3 eV) by employing the coupled three-dimensional (3D) time-dependent wave packet (TDWP) formalism in hyperspherical coordinates on a recently constructed ab initio ground adiabatic potential energy surface of H3+ [J. Chem. Phys. 2014, 141, 204306] for the process H+ + H2 (v = 0, j = 0-5) → H+ + H2 (v' = 0, j'). Cross-sections are then computed from the converged reaction probabilities as a function of total angular momentum (J) over the same energy regime and subsequently employed to obtain the rate constants for the ortho-to-para (O-P) and para-to-ortho (P-O) conversions and their ratio. The ratio of ortho-para conversion shows (a) appropriate convergence with the inclusion of a higher number of initial rotational states as well as a reasonable agreement with the results from a quantum statistical method and (b) a peak at lower temperature that could be due to the available collision energy for transitions involving lower rotational states (j = 0 → j' = 1).

New Diabatic Potential Energy Surfaces for the Li + H2 Reaction and Time-Dependent Quantum Wave Packet Studies.

New global diabatic potential energy surfaces (DPESs) for the ground (12A') and first excited (22A') states for the Li + H2 system were developed, with more than 30,000 energy points at the IC-MRCI+Q level of theory, utilizing the aug-cc-pV5Z basis set for the H atoms and the cc-pCV5Z basis set for the Li atom, fitted by a single neural network (NN) with symmetry. Product state-resolved quantum dynamics calculations of the nonadiabatic reaction Li (2P) + H2 (X 1 ∑g+, v0 = 0, j0 = 0) → LiH (X 1∑+) + H(2S) were carried out using these new DPESs and also the previous HYLC-DPESs. The numerical results suggested that our newly constructed DPESs provided an accurate description of the LiH2 system.

Low energy electron interaction with citric acid: a local complex potential based time-dependent wavepacket study

Low energy electron interaction with citric acid: a local complex potential based time-dependent wavepacket study

N–H photodissociation dynamics of electronically excited aniline: a three dimensional time-dependent quantum wavepacket study

We have simulated the dynamics of 1πσ* state-mediated nonadiabatic N-H bond dissociation in photo-excited aniline (C6H5NH2). A three electronic state diabatic model potential, involving the ground, 1ππ*, and 1πσ* diabatic states, and focussing on the NH2 degrees of freedom alone is constructed using XMS-CASPT2 energies. Using a kinetic energy operator in the polyspherical framework, wavepacket dynamics in three vibrational modes, viz. NH stretch, NH2 out-of-plane wag and torsion, is carried out using the Chebyshev propagation scheme. For optically bright 1ππ* excitation, the wavepacket can access the 1πσ*/1ππ* and 1ππ/1πσ* conical intersections that lie en route to dissociation. For both intersections, NH2 out-of-plane wag and torsional motions are the most dominant coupling coordinates. Carrying out dynamics with initial wavepackets varying in excitation in the three degrees of freedom, we probe their roles in the evolution of the state populations, probability densities, and product branching for the NH dissociation process.

Full-dimensional quantum mechanical calculations of the reaction probability of the H + CH4 reaction based on a mixed Jacobi and Radau description.

A full-dimensional time-dependent wave packet study using mixed polyspherical Jacobi and Radau coordinates for the title reaction has been reported. The non-reactive moiety CH3 has been described using three Radau vectors, whereas two Jacobi vectors have been used for the bond breaking/formation process. A potential-optimized discrete variable representation basis has been employed to describe the vibrational coordinates of the reagent CH4. About one hundred billion basis functions have been necessary to achieve converged results. The reaction probabilities for some initial vibrational states are given. A comparison between the present approach and other methods, including reduced and full-dimensional ones, is also presented.

Time-dependent wave packet studies of the abstraction and exchange channels of the H + NaH+ reaction

The abstraction reaction H + NaH+ (v0 = 0, j0 = 0) → Na+ + H2 and exchange reaction H′ + NaH+ (v0 = 0, j0 = 0) → NaH′+ + H were studied using the time-dependent wave packet method with second order split operator in the collision energy range from 0.01 to 1.0 eV. A new global potential energy surface (PES), which was reported by Yuan and co-workers (2018 Chem. Phys. Lett. 700 122), was used in the present dynamics calculation. The dynamics properties such as reaction probability, integral cross section, differential cross section, and the distribution of the product were presented in the present work. The results show that direct abstraction mechanism is dominated in abstraction reaction whereas the ‘rebound’ mechanism plays a dominant role in exchange reaction.

Non-resonant vibrational excitation of HOD and selective bond breaking.

This paper reports a time-dependent quantum mechanical wave packet study for bond-selective excitation and dissociation of HOD into the H + OD and D + OH channels in the first absorption band. Prior to excitation, the HOD molecule is randomly oriented with respect to a linearly polarized laser field and accurate static dipole moment and polarizability surfaces are included in the interaction potential. Vibrational excitation is obtained with intense, non-resonant 800 nm few-cycle excitation using dynamic Stark effect/impulsive Raman scattering. Dissociation is accomplished by another ultrashort vacuum ultraviolet-laser excitation. A laser control scheme is designed with a train of simple, non-resonant laser pulses in order to enhance the selectivity between the fragmentation channels. The effect of the carrier-envelope-phase of the ultrashort laser pulses is also investigated.

Coupled 3D time-dependent quantum wave-packet study of the O + OH reaction in hyperspherical coordinates on the CHIPR potential energy surface

We report quantum dynamics calculations of the O+OH→H+O2 reaction on the CHIPR (Varandas, 2013) and DMBE IV (Pastrana et al., 1990) potential energy surfaces (PESs) for ground-state HO2 using the 3D time-dependent wavepacket formalism based on hyperspherical coordinates. Reaction probabilities for J=0 are calculated for several initial rovibrational states of the OH radical (v=0; j=0–5). The J-shifting approximation is used to obtain initial state selected rate-coefficients in the range 0–400K. Total and state-to-state rate-coefficients are predicted in good agreement with recent experimental studies and theoretical calculations.

A full-dimensional time-dependent wave packet study of the H + CO2 → OH + CO reaction

A full dimensional quantum dynamics calculation has been carried out to study the prototypical complex forming H+CO2→OH+CO reaction. The total reaction probability is converged with a propagation time much shorter than the reverse reaction, and only exhibits very small oscillatory structures, indicating the reaction proceeds mainly through a direct mechanism. The strong Fermi resonance between the (020) and (100) vibrationally excited states makes it hard to define their efficacy for reactivity, although it is conceivable that the bending excitation is much more effective than the symmetric excitation according to the geometry of the transition state in the entrance channel.

Time-Dependent Quantum Wave Packet Study of the Si + OH → SiO + H Reaction: Cross Sections and Rate Constants.

The dynamics of the Si(3P) + OH(X2Π) → SiO(X1Σ+) + H(2S) reaction is investigated by means of the time-dependent wave packet (TDWP) approach using an ab initio potential energy surface recently developed by Dayou et al. ( J. Chem. Phys. 2013 , 139 , 204305 ) for the ground X2A' electronic state. Total reaction probabilities have been calculated for the first 15 rotational states j = 0-14 of OH(v=0,j) at a total angular momentum J = 0 up to a collision energy of 1 eV. Integral cross sections and state-selected rate constants for the temperature range 10-500 K were obtained within the J-shifting approximation. The reaction probabilities display highly oscillatory structures indicating the contribution of long-lived quasibound states supported by the deep SiOH/HSiO wells. The cross sections behave with collision energies as expected for a barrierless reaction and are slightly sensitive to the initial rotational excitation of OH. The thermal rate constants show a marked temperature dependence below 200 K with a maximum value around 15 K. The TDWP results globally agree with the results of earlier quasi-classical trajectory (QCT) calculations carried out by Rivero-Santamaria et al. ( Chem. Phys. Lett. 2014 , 610-611 , 335 - 340 ) with the same potential energy surface. In particular, the thermal rate constants display a similar temperature dependence, with TDWP values smaller than the QCT ones over the whole temperature range.

Full-dimensional quantum dynamics study of the H2 + C2H → H + C2H2 reaction on an ab initio potential energy surface

This work performs a time-dependent wavepacket study of the H2 + C2H → H + C2H2 reaction on a new ab initio potential energy surface (PES). The PES is constructed using neural network method based on 68 478 geometries with energies calculated at UCCSD(T)-F12a/aug-cc-pVTZ level and covers H2 + C2H↔H + C2H2, H + C2H2 → HCCH2, and HCCH2 radial isomerization reaction regions. The reaction dynamics of H2 + C2H → H + C2H2 are investigated using full-dimensional quantum dynamics method. The initial-state selected reaction probabilities are calculated for reactants in eight vibrational states. The calculated results showed that the H2 vibrational excitation predominantly enhances the reactivity while the excitation of bending mode of C2H slightly inhibits the reaction. The excitations of two stretching modes of C2H molecule have negligible effect on the reactivity. The integral cross section is calculated with J-shift approximation and the mode selectivity in this reaction is discussed. The rate constants over 200-2000 K are calculated and agree well with the experimental measured values.

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.

A new potential energy surface for the H2S system and dynamics study on the S((1)D) + H2(X(1)Σg(+)) reaction.

We constructed a new global potential energy surface (PES) for the electronic ground state (1A′) of H2S based on 21,300 accurate ab initio energy points over a large configuration space. The ab initio energies are obtained from multireference configuration interaction calculations with a Davidson correction using basis sets of quadruple zeta quality. The neural network method is applied to fit the PES, and the root mean square error of fitting is small (1.68 meV). Time-dependent wave packet studies for the S(1D) + H2(X1Σg+) → H(2S) + SH(X2Π) reaction on the new PES are conducted to study the reaction dynamics. The calculated integral cross sections decrease with increasing collision energy and remain fairly constant within the high collision energy range. Both forward and backward scatterings can be observed as expected for a barrierless reaction with a deep well on the PES. The calculated integral cross sections and differential cross sections are in good agreement with the experimental results.

Kinetic isotope effect of the (16)O + (36)O2 and (18)O + (32)O2 isotope exchange reactions: Dominant role of reactive resonances revealed by an accurate time-dependent quantum wavepacket study.

The O + O2 isotope exchange reactions play an important role in determining the oxygen isotopic composition of a number of trace gases in the atmosphere, and their temperature dependence and kinetic isotope effects (KIEs) provide important constraints on our understanding of the origin and mechanism of these and other unusual oxygen KIEs important in the atmosphere. This work reports a quantum dynamics study of the title reactions on the newly constructed Dawes-Lolur-Li-Jiang-Guo (DLLJG) potential energy surface (PES). The thermal reaction rate coefficients of both the (18)O + (32)O2 and (16)O + (36)O2 reactions obtained using the DLLJG PES exhibit a clear negative temperature dependence, in sharp contrast with the positive temperature dependence obtained using the earlier modified Siebert-Schinke-Bittererova (mSSB) PES. In addition, the calculated KIE shows an improved agreement with the experiment. These results strongly support the absence of the "reef" structure in the entrance/exit channels of the DLLJG PES, which is present in the mSSB PES. The quantum dynamics results on both PESs attribute the marked KIE to strong near-threshold reactive resonances, presumably stemming from the mass differences and/or zero point energy difference between the diatomic reactant and product. The accurate characterization of the reactivity for these near-thermoneutral reactions immediately above the reaction threshold is important for correct characterization of the thermal reaction rate coefficients.

Excess electron interaction with radiosensitive 5-bromopyrimidine in aqueous solution: a combined ab initio molecular dynamics and time-dependent wave-packet study.

Radiation-generated secondary electrons can induce resonance processes in a target molecule and fragment it via different pathways. Although the associating electronic resonant states at equilibrium geometry have been well studied for many target molecules in the gas phase, vibrational resonance contributions and the solvent effect are still poorly understood for relevant processes in solution. Taking a radiosensitive drug, 5-bromopyrimidine (5-BrPy), as an example, we here present a combined ab initio molecular dynamics simulation and time-dependent wave packet study with an emphasis on vibrational resonance and solvation effects on excess electron interaction with 5-BrPy in solution. The gaseous results reveal two primary channels for the electron induced C-Br bond cleavage: the highest vibrational resonance on vertical potential energy curve via a tunneling mechanism (e + 5-BrPy → 5-BrPy(*-) →(tunneling) Br(-) + Py(·)), and auto-dissociation along repulsive relaxed potential energy curve (e + 5-BrPy → 5-BrPy(*-) →(relaxation) Br(-) + Py(·)), which account for the two peaks at 0.2 and 0 eV observed in Modelli's experiment. However, a strong solvation effect modifies the mechanism and dynamics of the dissociation of the electron···5-BrPy system. On one hand, the spontaneous dissociation becomes unfavorable due to a barrier on the relaxed free energy surface created by the coupling between the π* and σ* states. Seven vibrational resonances (v = 0-6) are identified for the solution process and only the high-level v = 5, 6 with non-negligible quantum tunneling coefficient can cause the dissociation (e + 5-BrPy →(localization) 5-BrPy(*-) →(tunneling) [Br(δ-)···Py(δ-)] →Br(-) + Py(·)). On the other hand, protonation is also observed at the N sites of the hydrated 5-BrPy anion (e + 5-BrPy →(localization) 5-BrPy(*-) →(relaxation) Prt-5-BrPy), and this inhibits the dissociation along the C-Br bond, suggesting a competing pathway against C-Br bond cleavage. Clearly, this work provides a combination strategy using an ab initio molecular dynamics technique and time-dependent wave packet method to explore the effects of vibrational resonances and solvation on the interaction of radio-generated excess electrons with target biological molecules in complicated solution surroundings.

Effects of reagent rotational excitation on the H + CHD₃ → H₂ + CD₃ reaction: a seven dimensional time-dependent wave packet study.

Seven-dimensional time-dependent wave packet calculations have been carried out for the title reaction to obtain reaction probabilities and cross sections for CHD3 in J0 = 1, 2 rotationally excited initial states with k0 = 0 - J0 (the projection of CHD3 rotational angular momentum on its C3 axis). Under the centrifugal sudden (CS) approximation, the initial states with the projection of the total angular momentum on the body fixed axis (K0) equal to k0 are found to be much more reactive, indicating strong dependence of reactivity on the orientation of the reagent CHD3 with respect to the relative velocity between the reagents H and CHD3. However, at the coupled-channel (CC) level this dependence becomes much weak although in general the K0 specified cross sections for the K0 = k0 initial states remain primary to the overall cross sections, implying the Coriolis coupling is important to the dynamics of the reaction. The calculated CS and CC integral cross sections obtained after K0 averaging for the J0 = 1, 2 initial states with all different k0 are essentially identical to the corresponding CS and CC results for the J0 = 0 initial state, meaning that the initial rotational excitation of CHD3 up to J0 = 2, regardless of its initial k0, does not have any effect on the total cross sections for the title reaction, and the errors introduced by the CS approximation on integral cross sections for the rotationally excited J0 = 1, 2 initial states are the same as those for the J0 = 0 initial state.

Low energy electron induced cytosine base release in 2'-deoxycytidine-3'-monophosphate via glycosidic bond cleavage: a time-dependent wavepacket study.

Low energy electron (LEE) induced cytosine base release in a selected pyrimidine nucleotide, viz., 2'-deoxycytidine-3'-monophosphate is investigated using ab initio electronic structure methods and time dependent quantum mechanical calculations. It has been noted that the cytosine base scission is comparatively difficult process than the 3' C-O bond cleavage from the lowest π* shape resonance in energy region <1 eV. This is mainly due to the high activation energy barrier associated with the electron transfer from the π* orbital of the base to the σ* orbital of the glycosidic N-C bond. In addition, the metastable state formed after impinging LEE (0-1 eV) has very short lifetime (10 fs) which may decay in either of the two competing auto-detachment or dissociation process simultaneously. On the other hand, the selected N-C mode may cleave to form the cytosine base anion at higher energy regions (>2 eV) via tunneling of the glycosidic bond. Resonance states generated within this energy regime will exist for a duration of ~35-55 fs. Comparison of salient features of the two dissociation events, i.e., 3' C-O single strand break and glycosidic N-C bond cleavage in 3'-dCMPH molecule are also provided.

Time-dependent wave packet and quasiclassical trajectory studies of the Au + HD reaction: competition between the reactive channels

Time-dependent wave packet (TDWP) and quasiclassical trajectory (QCT) calculations are carried out for the Au (2S) + HD (X1∑+g) reaction. The reaction probabilities, integral cross sections (ICSs) and differential cross sections (DCSs) of the two reactive channels (AuH + D/AuD + H) are calculated. The ICS results indicate that the dominant reactive channel changes from AuD + H to AuH + D with the collision energy increasing. As is known from the distributions of the complex lifetime, this change mainly results from the difference between the direct reactive mechanisms in the two channels. With collision energy increasing, the growth of direct reactive collisions in AuH + D channel is much larger than that in the AuD + H channel. This is explained by the fact that, in the direct reactive mechanism with a small impact parameter, the D atom with a larger mass can get away easily from the potential well, and the H atom is likely to be captured by the Au atom. To compare vector properties of the two reactive channels, a study of the stereodynamics for the title reaction is also carried out by the QCT method. Due to more direct reactive collisions, product rotational polarization of the AuH + D channel is more obvious than that of the AuD + H channel.

State-Resolved Time-Dependent Wave Packet and Quasiclassical Trajectory Studies of the Adiabatic Reaction S(3P) + HD on the (13A″) State

Time-dependent wave packet (TDWP) and quasiclassical trajectory (QCT) calculations have been carried out for the reaction S(3P) + HD(X1Σg+) at the lowest 13A″ state with both rotational and vibrational excitations of reactant HD. The calculated integral cross sections from QCT agree fairly well with the TDWP calculations. The reaction probability results from TDWP show that the reaction displays a strong tendency to the SD channel. When the reactant HD is vibrationally excited, both channels are promoted apparently. The vibration of the HD bond tends to reduce the difference of reactivity between the two channels. The detailed state-to-state differential cross sections (DCSs) are calculated. These distributions show some significant characters of the barrier-type reactions. At the same time, the scattering width of product SD has a certain relationship with its rotation excitation. For the vector properties, P(θr), P(r), and P(θr,r) distributions are calculated by QCT, and the increased collision energy weakens the rotational polarization of the SD molecule.

The effect of the Coriolis coupling on H+ND reaction: A time dependent wave packet study

We present a quantum scattering calculation of initial-state-resolved reaction probabilities, integral cross section, initial state selected reaction rate constants and thermal rate constant for exchange and depletion channels of the H+ND reaction. The calculations are carried out using Coriolis coupling (CC) method on the modified NH2Ã2A1 potential energy surface(PES). The initial state selected reaction probabilities are calculated for 0.0–0.5eV of collision energy range. The reaction probabilities for depletion channel shows some sharp and large resonances associated with long-lived collision complexes, as compared to exchange channel. Integral cross sections for both reactions depend strongly on initial rotational states. The calculated thermal reaction rate constant for the depletion channel is in a good agreement with the values previously obtained by the centrifugal sudden approximation, experimental and semiclassical results.