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The quantum dynamics of three CH(X(2)Π) + D((2)S) reactions is studied by means of the coupled-channel time-dependent real-wavepacket (WP) and flux methods at collision energy Ecol ≤ 0.6 eV and on three potential energy surfaces (PESs): the Born-Oppenheimer (BO) ground PES X̃(3)A″ and the excited ones ã(1)A' and b̃(1)A″, coupled by nonadiabatic (NA) Renner-Teller (RT) effects. This three-state model is suitable for obtaining initial-state-resolved observables, is based on a complete analysis of the correlation diagram of the lowest electronic states of the CHD intermediate and of their NA interactions, and neglects the smaller coupling effects due to the asymptotic electronic angular momenta that become important in state-to-state dynamics. WPs are propagated on each PES at total angular momentum values J ≤ 70, with CH in the two lowest vibrational states v0 and in the ground rotational state j0 = 1. Reaction probabilities are obtained for three possible final products (f): (dP) CH decay and C((3)P) + HD(X(1)Σ(+)) formation that occurs on the uncoupled ground PES, (dD) CH decay and C((1)D) + HD(X(1)Σ(+)) formation that depends on the RT-coupled singlet species, and (ex) exchange to CD(X(2)Π) + H((2)S) available adiabatically from the X̃(3)A″ PES and nonadiabatically from ã(1)A' and b̃(1)A″. Observable cross sections σf,v0j0 and rate constants kf,v0j0 in the temperature range T = 100-2000 K are obtained for (dP), (dD), and (ex) channels. Comparing BO with RT probabilities, we show that NA effects are important at high J values for the (ex) channel at v0 = 1. Real time mechanisms on the three PESs show that RT couplings are opened after some time and clearly point out the formation of the product channels. Both cross sections and rate constants present the same sequence, for example σex,11 > σdP,01 ∼ σex,01 > σdP,11 ≫ σdD,11 ≫ σdD,01, and the CH vibrational excitation enhances the total removal CH+D reactivity by a factor of ∼1.7, mainly due to the increase of the (ex) channel contribution from ∼47% at v0 = 0 to ∼76% at v0 = 1. This fact implies a considerable vibrational enhancement of combustion processes at high temperature. In agreement with the probability results, the ã(1)A'/b̃(1)A″ RT coupling increases both σex,11 and kex,11 up to ∼30%. Moreover, including the three PESs in the dynamics simulation of CH+D increase by far the (ex)/(dP) branching ratio with respect to the CH + H' reaction. Thus, at room temperature, kdP,01 changes from 10.8 × 10(-11) to 3.4 × 10(-11) cm(3) s(-1) substituting H atom by D.

We present the adiabatic quantum dynamics of the proton-transfer reaction H((2)S) + HeH(+)(X(1)Σ(+)) → H2(+)(X(2)Σg(+)) + He((1)S) on the HeH2(+) X̃(2)Σ(+) RMRCI6 (M = 6) PES of C. N. Ramachandran et al. ( Chem. Phys. Lett. 2009, 469, 26). We consider the HeH(+) molecule in the ground vibrational–rotational state and obtain initial-state-resolved reaction probabilities and the ground-state cross section σ0 and rate constant k0 by propagating time-dependent, coupled-channel, real wavepackets (RWPs) and performing a flux analysis. Three different wavepackets are propagated to describe the wide range of energies explored, from cold (0.0001 meV) to hyperthermal (1000 meV) collision energies, and in a temperature range from 0.01 to 2000 K. We compare our time-dependent results with the time-independent ones by D. De Fazio and S. Bovino et al., where De Fazio carried out benchmark coupled-channel calculations whereas Bovino et al. employed the negative imaginary potential and the centrifugal-sudden approximations. The RWP cross section is in good agreement with that by De Fazio, except at the lowest collision energies below ∼0.01 meV, where the former is larger than the latter. However, neither the RWP and De Fazio results possess the huge resonance in probability and cross section at 0.01 meV, found by Bovino et al., who also obtained a too low σ0 at high energies. Therefore, the RWP and De Fazio rate constants compare quite well, whereas that by Bovino et al. is in general lower.

We present the conical-intersection quantum dynamics of the nonreactive quenching (NQ) OH(A(2)Σ(+)) + H'((2)S) → OH(X(2)Π) + H'((2)S), exchange (X) OH(A(2)Σ(+)) + H'((2)S) → OH'(A(2)Σ(+)) + H((2)S), exchange-quenching (XQ) OH(A(2)Σ(+)) + H'((2)S) → OH'(X(2)Π) + H((2)S), and reaction (R) OH(A(2)Σ(+)) + H'((2)S) → O((1)D) + H2(X(1)Σg (+)) collisions. We obtain initial-state-resolved reaction probabilities, cross sections, and rate constants by considering OH in the ground vibrational state and in the rotational levels j0 = 0, 1, 2, and 5. Coupled-channel real wavepackets (WPs) on the X̃(1)A(') and B̃(1)A(') coupled electronic states are propagated by using the Dobbyn and Knowles diabatic potential surfaces and coupling [A. J. Dobbyn and P. J. Knowles, Mol. Phys. 91, 1107 (1997) and A. J. Dobbyn and P. J. Knowles, Faraday Discuss. 110, 207 (1998)], and performing asymptotic or flux analysis. NQ is the preferred product channel, followed by XQ, R, and X. Moreover, the nonadiabatic quenching processes account for more than 80% of the total rate constants. WP snapshots show a reaction mechanism in good agreement with reaction probabilities. NQ, XQ, and R cross sections, and NQ rate constants decrease with the collision energy and j0, whereas the X reactivity increases, and XQ and R rates are nearly constant with j0. In general, quantum rate constants are smaller than experimental or quasiclassical data.

The dynamics and kinetics of the O((3)P) + CS(X(1)Σ(+)) → CO(X(1)Σ(+)) + S((3)P) chemical laser reaction was studied theoretically in detail for the first time, as a function of collision energy (0.0388-2.0 eV) and rovibrational excitation of CS. This was made using the quasi-classical trajectory (QCT) method and employing the best ab initio analytical ground potential energy surface (1(3)A' PES) available. A broad set of properties was determined, including scalar and vector properties, and the reaction mode. The behaviors observed and the considerable formation of OCS collision complexes were interpreted from some characteristics of the PES (early barrier, shallow minimum in the exit channel, and high exoergicity (mainly channeled into CO vibration; up to ∼81% of the available energy)) and the kinematics. The QCT vibrational and rotational CO populations and the vector properties show a quite good agreement with experiments, but the QCT rate constants disagree. To better account for the kinetics, we performed CASPT2/aug-cc-pVTZ ab initio calculations on the stationary points along the minimum energy path of the ground and first excited (1(3)A'') PESs. The transition state theory, which can be satisfactorily applied here, leads to rate constants (100-2000 K) that are quite close to the measured ones, where comparison is possible (150-300 K). We expect that these results will encourage further theoretical and experimental developments.

The kinetics and dynamics of the title reactions were studied using the quasiclassical trajectory (QCT) method and two ab initio analytical potential energy surfaces (PESs) developed by our group. In addition to the rate constant (T: 10-5000 K), we also considered a broad set of dynamic properties as a function of collision energy (up to 1.0 eV) and the rovibrational state of NO (v=0-2,j=1,8,12). The production of N(2)+O, reaction (1), dominates the reactivity of the N+NO system over the conditions studied, as expected from the large energy barriers associated to the NO+N exchange reaction, reaction (2). Moreover, the ground PES, which is barrierless for reaction (1), plays a dominant role. Most of the results were interpreted according to the properties of the PESs involved and the kinematics of the system. The QCT rate constants of reaction (1) are in agreement with the experimental data (T: 47-3500 K), including very recent low temperature measurements, and also with variational transition state kinetics and most of quantum dynamics calculations. In addition, the QCT average vibrational energy content of the N(2) product also agrees with the experimental and quantum data. The PESs used here could also be useful to determine equilibrium and nonequilibrium reaction rates at very high temperatures (e.g., 5000-15 000 K).

We present the spin-orbit (SO) and Renner-Teller (RT) quantum dynamics of the spin-forbidden quenching O(1D) + N2(X1Σg+)→O(3P) + N2(X1Σg+) on the N2O X1A′, ã3A″, and b3A′ coupled PESs. We use the permutation-inversion symmetry, propagate coupled-channel (CC) real wavepackets, and compute initial-state-resolved probabilities and cross sections σj0 for the ground vibrational and the first two rotational states of N2, j0 = 0 and 1. Labeling symmetry angular states by j and K, we report selection rules for j and for the minimum K value associated with any electronic state, showing that ã3A″ is uncoupled in the centrifugal-sudden (CS) approximation at j0 = 0. The dynamics is resonance-dominated, the probabilities are larger at low K, σj0 decrease with the collision energy and increase with j0, and the CS σ0 is lower than the CC one. The nonadiabatic interactions play different roles on the quenching dynamics, because the X1A′-b3A′ SO effects are those most important while the ã3A″-b3A′ RT ones are negligible. [ABSTRACT FROM AUTHOR]

The kinetics and dynamics of the title reactions were studied using the quasiclassical trajectory (QCT) method and two ab initio analytical potential energy surfaces (PESs) developed by our group. In addition to the rate constant (T: 10–5000 K), we also considered a broad set of dynamic properties as a function of collision energy (up to 1.0 eV) and the rovibrational state of NO (v=0–2,j=1,8,12). The production of N2+O, reaction (1), dominates the reactivity of the N+NO system over the conditions studied, as expected from the large energy barriers associated to the NO+N exchange reaction, reaction (2). Moreover, the ground PES, which is barrierless for reaction (1), plays a dominant role. Most of the results were interpreted according to the properties of the PESs involved and the kinematics of the system. The QCT rate constants of reaction (1) are in agreement with the experimental data (T: 47–3500 K), including very recent low temperature measurements, and also with variational transition state kinetics and most of quantum dynamics calculations. In addition, the QCT average vibrational energy content of the N2 product also agrees with the experimental and quantum data. The PESs used here could also be useful to determine equilibrium and nonequilibrium reaction rates at very high temperatures (e.g., 5000–15 000 K). [ABSTRACT FROM AUTHOR]

We present the Born–Oppenheimer (BO) and Renner–Teller (RT) coupled-channel dynamics of the N(2D)+HD→NH+D and N(2D)+HD→ND+H reactions, considering the X 2A″ and à 2A′ states and RT and Coriolis couplings. We use the best available potential energy surfaces and obtain initial-state-resolved probabilities, cross sections, and rate constants via the real wave packet and flux methods for both electronic states. In contrast with the RT results of BO-X 2A″ ones, we point out the role of RT and Coriolis interactions for both reactions and discuss the importance of the excited state on the initial-state-resolved dynamics and on the thermal kinetic rate. Moreover the competition for the formation of two products is discussed, showing some snapshots of the RT wave-packet density on the ground state. However, the BO approximation gives thermal rates that are smaller than those obtained via full RT calculation, especially at 300 K. Our calculated RT rate constants at room temperature are in good agreement with the experimental ones. The branching ratio is also calculated at T=150–300 K at BO and RT levels. At 300 K the calculated value overestimates slightly the experimental data. [ABSTRACT FROM AUTHOR]

We present the Born–Oppenheimer dynamics of the depletion reaction NH(a1Δ)+H′→N(2D)+H2 and of the exchange one NH(a1Δ)+H′→NH′(a1Δ)+H, using the real wavepacket and flux methods and an accurate NH2(Ã 2A1) surface. We report coupled-channel reaction probabilities, cross sections, and rate constants, taking into account Coriolis couplings. Because the surface is barrierless and strongly bound, probabilities have small centrifugal thresholds and present sharp and large resonances, associated with long-lived collision complexes. Large J’s enhance the high-energy reactivity and favor the exchange reaction, owing to the negative Coriolis couplings, and to K that inhibits depletion but not exchange. Coriolis couplings are important in the collision complexes, and the centrifugal sudden approximation thus gives large errors. Cross sections are large at the threshold and minimal at intermediate collision energies and increase moderately at larger energies. Exchange is preferred, and both reactions are inhibited by the NH rotational excitation. Finally, the present rate constants are in good agreement with previous experimental and semiclassical results. [ABSTRACT FROM AUTHOR]

The reaction N+NO→N2+O was studied by means of the time-dependent real wave-packet (WP) method and the J-shifting approximation. We consider the ground 1 3A″ and first excited 1 3A′ triplet states, which correlate with both reactants and products, using analytical potential energy surfaces (PESs) recently developed in our group. This work extends our previous quantum dynamics study, and probabilities, cross sections, and rate constants were calculated and interpreted on the basis of the different shapes of the PESs (barrierless 1 3A″ and with barrier 1 3A′ surfaces, respectively). The WP rate constant (k1) shows a weak dependence on T(200–2500 K), as the dominant contribution to reactivity is provided by the barrierless ground PES. There is a good agreement of WP k1 with the measurements and variational transition state theory (VTST) data, and also between the WP and VTST k1(1 3A″) results. Nevertheless, there is a large discrepancy between the WP and VTST k1(1 3A′) results. Product state distributions were also calculated for the much more reactive 1 3A″ PES. There is an excellent agreement with the experimental average fraction of vibrational energy in N2(25±3%), the only measured dynamics property of this reaction. [ABSTRACT FROM AUTHOR]

We present the quantum dynamics of the title reaction using the Gray–Balint–Kurti wave-packet (WP) method, several NO vibro-rotational levels, product coordinates, and an asymptotic analysis. We calculate accurate reaction probabilities at J=0, estimate those at J>0 via a capture model, and discuss the reaction mechanism analyzing the WP time evolution. We also obtain cross sections and rate constants. The potential is barrier-less and thus both probabilities and cross sections do not have a collision-energy (E[sub col]) threshold. The probabilities present many sharp resonances, due to the E[sub col] redistribution on the NNO-internal and N[sub 2]-product degrees of freedom. The reaction is stereo-specific and occurs via a bent abstraction mechanism. The cross sections decrease with E[sub col], in agreement with the expected behavior for threshold-less reactions. The present values of the rate constant support previous and less accurate calculations, and are in excellent agreement with laboratory experimental data. This confirms the accuracy of the present 1 [sup 3]A[sup ″] PES. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigated the oscillationsfound in the integral cross sectionof the title reaction, which are particularly evident for Ne + H2+(v0= 2,j0= 1) [essentially isoenergetic with NeH+(v′= 0,j′= 0) + H] at lowcollision energy (Ecol< 0.30 eV).We employed mainly an exact time-independent (TI) quantum dynamicsmethod and used the best potential energy surface available. Fromanalysis of TI initial state selected to all integral cross sections,state-to-state integral cross sections, and the corresponding differentialcross sections (DCSs), we showed that the oscillations correspondto resonances. They arise from the influence of the global [Ne–H–H]+(collinear) minimum on dynamics and probably correspond toFeshbach resonances. Besides, the forward–backward peakingDCS (which oscillates with Ecol) behaviorobserved could be a signature for this type of resonances. Finally,as most data on resonances in bimolecular reactions correspond toneutral systems, we hope that the present results will encourage experimentaliststo re-examine this benchmark system. [ABSTRACT FROM AUTHOR]

Abstract: The computer simulation of reaction dynamics has nowadays reached a remarkable degree of accuracy. Triatomic elementary reactions are rigorously studied with great detail on a straightforward basis using a considerable variety of Quantum Dynamics computational tools available to the scientific community. In our contribution we compare the performance of two quantum scattering codes in the computation of reaction cross sections of a triatomic benchmark reaction such as the gas phase reaction Ne + H2 + → NeH++ H. The computational codes are selected as representative of time-dependent (Real Wave Packet [1]) and time-independent (ABC [2]) methodologies. The main conclusion to be drawn from our study is that both strategies are, to a great extent, not competing but rather complementary. While time-dependent calculations advantages with respect to the energy range that can be covered in a single simulation, time-independent approaches offer much more detailed information from each single energy calculation. Further details such as the calculation of reactivity at very low collision energies or the computational effort related to account for the Coriolis couplings are analyzed in this paper. [Copyright &y& Elsevier]

Abstract: We present the quantum mechanical (QM) and quasiclassical trajectory (QCT) dynamics of the title reaction on two uncoupled surfaces, using a wavepacket (WP) method and considering some N2(v, j) vibrational and rotational states. The reaction is investigated by calculating initial-state-resolved reaction probabilities, cross sections, and rate constants, which are explained in terms of energy profiles and collision mechanisms. These properties reflect the large endo-thermicity of the reaction and the features of the surfaces. Indeed, at low v values we obtain large thresholds and the lower surface is more reactive than the higher one, whereas at high v the thresholds decrease or disappear and the upper surface becomes more reactive. QM and QCT results are in satisfactory agreement, except some differences at low or high collision energy and temperature. QCT rate coefficients agree also with some published results. WP snapshots and movies of QCT time evolution show clearly abstraction and insertion mechanisms depending on the initial conditions. Insertion proceeds via a reaction complex and we observe a QM Feshbach resonance for a specific initial condition. On the overall, the dynamical observables are consistent with the collision mechanisms. [Copyright &y& Elsevier]