Search

We present the dynamics of the electronic quenching OH(A2Σ+) + Kr(1S) → OH(X2Π) + Kr(1S), with OH(A2Σ+) in the ground ro-vibrational state. This study relies on a new non-adiabatic quantum theory that uses three diabatic electronic states Σ+, Π′, and Π′′, coupled by one conical-intersection and nine Renner–Teller matrix elements, all of which are explicitly considered in the equation of the motion. The time-dependent mechanism and initial-state-resolved quenching probabilities, integral cross sections, thermal rate constants, and thermally-averaged cross sections are calculated via the real wavepacket method. The results point out a competition among three non-adiabatic pathways: Σ+ ↔ Π′, Σ+ ↔ Π′′, and Π′ ↔ Π′′. In particular, the conical-intersection effects Σ+–Π′ are more important than the Renner–Teller couplings Σ+–Π′, Σ+–Π′′, and Π′–Π′′. Therefore, Π′ is the preferred product channel. The quenching occurs via an indirect insertion mechanism, opening many collision complexes, and the probabilities thus present many oscillations. Some resonances are still observable in the cross sections, which are in good agreement with previous experimental and quasi-classical findings. We also discuss the validity of more approximate quantum methods. [ABSTRACT FROM AUTHOR]

Following previous studies on the O(3P) + H2+(X2Σg+) collisions, we present the nonadiabatic quantum dynamics of the reactions OH(X2Π) + H′+→ OH′(X2Π) + H+, exchange (e), → OH+(X3Σ−) + H′(2S), quenching (q), and → OH′+ (X3Σ−) + H(2S), exchange-quenching (eq). The reactants and products correlate via the ground X˜2A′′ and first excited Ã2A′ electronic states of OH2+, which are the degenerate components of linear 2Π species. Therefore, they are strongly perturbed by nonadiabatic Renner―Teller (RT) effects, opening the (q) and (eq) channels that are closed in the Born–Oppenheimer approximation. Using accurate potential energy surfaces (PESs) and RT matrix elements, initial-state-resolved reaction probabilities, real-time dynamics, cross sections, and rate constants of the product channels are obtained through the time-dependent real wavepacket (WP) method and full coupled-channel calculations. Owing to the nonadiabatic couplings, the WP jumps from the excited Ã2A′ surface to the X˜2A′′ ground PES, avoiding any barrier, opening the quenching channels, and giving many collision complexes into the deep minima of both PESs, as it is clearly shown by the oscillations of the reaction probabilities and by the time-dependent WP dynamics. All the results show that the nonadiabatic-RT channels (q) and (eq) are highly reactive, much more than the adiabatic one (e), pointing out large RT effects. The reactivity of the quenching channels is similar, accounting for 97% of the overall reactivity. In fact, the maximum values of the (q) and (eq) cross sections σq and σeq are equal to 31.6 Å2, whereas the maximum σe value equals 1.34 Å2, and the maximum values of the rate constants kq, keq, and ke are 2.07 × 10−10, 2.45 × 10−10, and 0.23 × 10−10 cm3 s−1. Some calculations show that the centrifugal-sudden and the truncated coupled-channel approximations cannot be employed for the (q) channel. After a sharp increase at the threshold, σq and σeq decrease at larger collision energies while σe and the rate constants depend slightly on the collision energy and temperature, respectively. These findings are consistent with the barrierless nature of both PESs and the exoergicity of the quenching products, with the small role played by the centrifugal and RT barriers in the reactant channel, and with the large RT couplings in the OH2+ intermediates. Finally, we contrast the present results with those for the opposite reactions O + H2+ and for the nonadiabatic-induced quenchings NH + H′ and OH + H′. [ABSTRACT FROM AUTHOR]

The dynamics of the title reaction was studied using mainly the quasiclassical trajectory (QCT) method on the ground 12A′′ (OH+ channel) and first excited 12A′ (OH channel) potential energy surfaces (PESs) employing ab initio analytical representations of the PESs developed by us. Both PESs correspond to exoergic reactions, are barrierless and present a deep minimum along the minimum energy path (MEP). Some extra calculations (cross sections) were also performed with the time dependent quantum real wave packet method at the centrifugal sudden level (RWP-CS method). A broad set of properties as a function of collision energy (Ecol≤ 0.5 eV) was considered using the QCT method: cross sections, average fractions of energy, product rovibrational distributions, two- and three-vector properties, and the microscopic mechanisms analyzing their influence on the dynamics. The proton transfer channel dominates the reactivity of the system and significant differences between the two reaction channels are found for the vibrational distributions and microscopic mechanisms. The results were interpreted according to the properties of the ground and excited PESs. Moreover, the QCT and RWP-CS cross sections are in rather good agreement for both reaction channels. We hope that this study will encourage the experimentalists to investigate the dynamics of this interesting but scarcely studied system, whose two lowest PESs include the ground and first excited electronic states of the H2O+ cation. [ABSTRACT FROM AUTHOR]

The molecular mechanisms of the water gas shift reaction on Cu(321) have been chosen to investigate the effect of dispersion terms on the description of the energy profile and reaction rates. The present study based on periodic DFT calculations shows that including dispersion terms does not change the qualitative picture of the overall reaction, maintaining the rate determining step and the predominant route. However, the effect of dispersion is different for different adsorbates – reactants, intermediates or products – with a clear net effect and with no compensation of errors. Thus, in the OH + OH → H2O + O process the dispersion effects imply up to three orders of magnitude in the calculated reaction rates; the formation of carboxyl is highly disfavoured when dispersion terms are explicitly included and finally, the reaction rate for CO2 production (at 463 K) through cis-COOH dissociation is enhanced by three orders of magnitude by including dispersion terms in the calculation of the energy barrier. Consequently, the inclusion of dispersion terms largely affects the overall potential energy profile and produces tremendous changes in the predicted reaction rates. Therefore, dispersion terms must be included when aiming at obtaining information from macroscopic simulations employing for instance microkinetic or kinetic Monte Carlo approaches, where these effects should be clearly shown. [ABSTRACT FROM AUTHOR]

We present Born–Oppenheimer (BO) and Renner–Teller (RT) time dependent quantum dynamics studies of the reactions O(3P) + H2+(X2Σg+) → OH+(X3Σ−) + H(2S) and OH(X2Π) + H+. We consider the OH2+ X˜2A′′ and Ã2A′ electronic states that correlate with a linear 2Π species. The electronic angular momenta operators L̂ and L̂2 are considered in nonadiabatic coupled-channel calculations, where the associated RT effects are due to diagonal VRT potentials that add up to the PESs and to off-diagonal CRT couplings between the potential energy surfaces (PESs). Initial-state-resolved reaction probabilities PI, integral cross sections σI, and rate constants kI are obtained using recent ab initio PESs and couplings and the real wavepacket formalism. Because the PESs are strongly attractive, PI have no threshold energy and are large, σI decrease with collision energy, and kI depend little on the temperature. The X˜2A′′ PES is up to three times more reactive than the Ã2A′ PES and H2+ rotational effects (j0 = 0, 1) are negligible. The diagonal VRT potentials are strongly repulsive at the collinearity and nearly halve all low-energy observables with respect to the BO ones. The off-diagonal CRT couplings are important at low partial waves, where they mix the X˜2A′′ and Ã2A′ states up to ∼20%. However, VRT effects predominate over the CRT ones that change at most by ∼19% the BO values of σI and kI. The reaction O(3P) + H2+(X2Σg+) → OH+(X3Σ−) + H(2S) is probably one of the most reactive atom + diatom collisions because its RT rate constant at room temperature is equal to 2.26 × 10−10 cm3 s−1. Within the BO approximation, the present results agree rather well with recent quasiclassical and centrifugal-sudden data using the same PESs. [ABSTRACT FROM AUTHOR]

The rate constants (k; T: 200–900 K) and cross-sections (σ; Ecol: 0.010–0.50 eV) of the O + H2+→ OH+ + H (1), OH + H+ (2) reactions, which occur on the ground (12A′′) and first excited (12A′) potential energy surfaces (PESs), respectively, were investigated for the first time, considering also the rate constants for D2+ and HD+. Ab initio multireference configuration interaction calculations were performed on both barrierless PESs (where the minimum energy path involves the insertion of the O atom into the middle of the H2+ bond), and suitable analytical expressions were developed for the first time and used in quasiclassical trajectory (QCT) calculations. k(1) ≈ 3k(2) independent of the isotopic variant, k(H2+) ＞ k(HD+) ＞ k(D2+) for EQNREFreactions (1) and (2)/EQNREF, and the intermolecular and intramolecular isotopic effects are essentially independent of T. Comparison with the Langevin–Gioumousis–Stevenson (LGS) simple capture model shows that these results are similar to the QCT ones, especially for EQNREFreaction (2)/EQNREF; and the isotopic effects are coincident with the QCT ones for both reactions. For O + H2+, σ(1) ≈ 3σ(2) at Ecol≤ 0.10 eV, and σ(1) = 1.5σ(2) at 0.40 and 0.50 eV. The larger value of σ(12A′′) with respect to σ(12A′) arises from the larger value of bmax(12A′′) with respect to bmax(12A′), and this results from the more attractive character of the former PES. Besides, the reaction probabilities are quite large [0.78–0.98 (12A′′) and 0.78–0.93 (12A′)], and the decreasing trend of both cross-sections as Ecol increases arises from the barrierless character of both PESs. We expect that these results (in particular, the competition between proton transfer and hydrogen atom transfer) will encourage experimentalists to carry out investigations on this interesting reaction. [ABSTRACT FROM AUTHOR]

The dependence of the cross section (σ) of the Ne + H2+→ NeH+ + H proton transfer reaction on the vibrational excitation of H2+, v = 0–17 and j = 1, was analyzed in detail at the collision energies (Ecol) of 0.7 and 1.7 eV, using the quasi-classical trajectory (QCT) method and the PHHJ3 and LZHH potential energy surfaces (PESs), taking advantage of the rich experimental data available for this reaction as a function of H2+(v). The efficiency of vibrational excitation to promote the reaction was investigated from the analysis of the σ(QCT) vs. v dependence in terms of the average reaction probability, maximum impact parameter, regions of the (late barrier) PES explored, and taking into account the Ne + H2+→ Ne + H+ + H dissociative channel, which plays a dominant role at high enough total energies. Although the earlier PHHJ3 PES performs rather well, the LZHH PES QCT results show a better agreement with the experiment. On the other hand, some artifacts were found in recently reported QCT calculations (unphysical oscillations in σ(QCT) as a function of v), and the present study shows that special care is needed when carrying out QCT calculations involving highly excited vibrational states. [ABSTRACT FROM AUTHOR]

A microkinetic model is developed to study the reactivity of an O/O2gas mixture over a β-cristobalite (001) surface. The thermal rate constants for the relevant elementary processes are either inferred from quasiclassical trajectory calculations or using some statistical approaches, resting on a recently developed interpolated multidimensional potential energy surface based on density functional theory. The kinetic model predicts a large molecular coverage at temperatures lower than 1000 K, in contrary to a large atomic coverage at higher temperatures. The computed atomic oxygen recombination coefficient, mainly involving atomic adsorption and Eley-Rideal recombination, is small and increases with temperature in the 700–1700 K range (0.01 < γO< 0.02) in good agreement with experiments. In the same temperature range, the estimated chemical energy accommodation coefficient, the main contribution to which is the atomic adsorption process is almost constant and differs from unity (0.75 < βO< 0.80). [ABSTRACT FROM AUTHOR]

A quantum dynamics study for the NH(a1Δ) + D(2S) reactions using coupled channel time dependent real wavepacket formalism is presented. Moreover, the Renner–Teller (RT) interactions between two electronic states of NHD (X2A′′ and Ã2A′) have been taken into account by means of semiempirical RT matrix elements. The introduction of RT effects opens the possibility of studying not only the adiabatic reactions [depletion (d) to N(2D) + HD(X1Σ+) and exchange (e) to ND(a1Δ) + H(2S)] but also nonadiabatic ones [quenching (q) to NH(X3Σ−) + D(2S) and exchange-quenching (eq) to ND(X3Σ−) + H(2S)]. Reaction probabilities, cross sections, isotopic effects, and rate constants are presented for all the before mentioned reactions. RT results are compared with Born–Oppenheimer, quasiclassical, and experimental data. Contrasting with previous NH + H results, we point out interesting RT and isotopic effects, which depend on the D and H masses and on the tunneling of the H atom. In fact, RT effects, near-threshold cross sections, and rate constants are smaller in NH + D than in NH + H, as expected from the masses of the attacking atoms. Our rate constants and quenching branching ratio agree well with previous quasiclassical and experimental data, validating the semiempirical RT coupling we employ. Some small differences between calculated and measured rate constants might be due to the theoretical approximations and to the large experimental error bars. [ABSTRACT FROM AUTHOR]

We present the dynamics of the electronic quenching OH(A2Σ+) + Kr(1S) → OH(X2Π) + Kr(1S), with OH(A2Σ+) in the ground ro-vibrational state. This study relies on a new non-adiabatic quantum theory that uses three diabatic electronic states Σ+, Π', and Π'', coupled by one conical-intersection and nine Renner-Teller matrix elements, all of which are explicitly considered in the equation of the motion. The time-dependent mechanism and initial-state-resolved quenching probabilities, integral cross sections, thermal rate constants, and thermally-averaged cross sections are calculated via the real wavepacket method. The results point out a competition among three non-adiabatic pathways: Σ+ ↔ Π', Σ+ ↔ Π'', and Π' ↔ Π''. In particular, the conical-intersection effects Σ+-Π' are more important than the Renner-Teller couplings Σ+-Π', Σ+-Π'', and Π'-Π''. Therefore, Π' is the preferred product channel. The quenching occurs via an indirect insertion mechanism, opening many collision complexes, and the probabilities thus present many oscillations. Some resonances are still observable in the cross sections, which are in good agreement with previous experimental and quasi-classical findings. We also discuss the validity of more approximate quantum methods.

Following previous studies on the O( 3 P) + H 2 + (X 2 Σ g + ) collisions, we present the nonadiabatic quantum dynamics of the reactions OH(X 2 Π) + H' + → OH'(X 2 Π) + H + , exchange (e), → OH + (X 3 Σ - ) + H'( 2 S), quenching (q), and → OH' + (X 3 Σ - ) + H( 2 S), exchange-quenching (eq). The reactants and products correlate via the ground X[combining tilde] 2 A'' and first excited à 2 A' electronic states of OH 2 + , which are the degenerate components of linear 2 Π species. Therefore, they are strongly perturbed by nonadiabatic Renner-Teller (RT) effects, opening the (q) and (eq) channels that are closed in the Born-Oppenheimer approximation. Using accurate potential energy surfaces (PESs) and RT matrix elements, initial-state-resolved reaction probabilities, real-time dynamics, cross sections, and rate constants of the product channels are obtained through the time-dependent real wavepacket (WP) method and full coupled-channel calculations. Owing to the nonadiabatic couplings, the WP jumps from the excited à 2 A' surface to the X[combining tilde] 2 A'' ground PES, avoiding any barrier, opening the quenching channels, and giving many collision complexes into the deep minima of both PESs, as it is clearly shown by the oscillations of the reaction probabilities and by the time-dependent WP dynamics. All the results show that the nonadiabatic-RT channels (q) and (eq) are highly reactive, much more than the adiabatic one (e), pointing out large RT effects. The reactivity of the quenching channels is similar, accounting for 97% of the overall reactivity. In fact, the maximum values of the (q) and (eq) cross sections σ q and σ eq are equal to 31.6 Å 2 , whereas the maximum σ e value equals 1.34 Å 2 , and the maximum values of the rate constants k q , k eq , and k e are 2.07 × 10 -10 , 2.45 × 10 -10 , and 0.23 × 10 -10 cm 3 s -1 . Some calculations show that the centrifugal-sudden and the truncated coupled-channel approximations cannot be employed for the (q) channel. After a sharp increase at the threshold, σ q and σ eq decrease at larger collision energies while σ e and the rate constants depend slightly on the collision energy and temperature, respectively. These findings are consistent with the barrierless nature of both PESs and the exoergicity of the quenching products, with the small role played by the centrifugal and RT barriers in the reactant channel, and with the large RT couplings in the OH 2 + intermediates. Finally, we contrast the present results with those for the opposite reactions O + H 2 + and for the nonadiabatic-induced quenchings NH + H' and OH + H'.

The dynamics of the title reaction was studied using mainly the quasiclassical trajectory (QCT) method on the ground 1 2 A'' (OH + channel) and first excited 1 2 A' (OH channel) potential energy surfaces (PESs) employing ab initio analytical representations of the PESs developed by us. Both PESs correspond to exoergic reactions, are barrierless and present a deep minimum along the minimum energy path (MEP). Some extra calculations (cross sections) were also performed with the time dependent quantum real wave packet method at the centrifugal sudden level (RWP-CS method). A broad set of properties as a function of collision energy (E col ≤ 0.5 eV) was considered using the QCT method: cross sections, average fractions of energy, product rovibrational distributions, two- and three-vector properties, and the microscopic mechanisms analyzing their influence on the dynamics. The proton transfer channel dominates the reactivity of the system and significant differences between the two reaction channels are found for the vibrational distributions and microscopic mechanisms. The results were interpreted according to the properties of the ground and excited PESs. Moreover, the QCT and RWP-CS cross sections are in rather good agreement for both reaction channels. We hope that this study will encourage the experimentalists to investigate the dynamics of this interesting but scarcely studied system, whose two lowest PESs include the ground and first excited electronic states of the H 2 O + cation.

We present Born-Oppenheimer (BO) and Renner-Teller (RT) time dependent quantum dynamics studies of the reactions O((3)P) + H2(+)(X(2)Σg(+)) → OH(+)(X(3)Σ(-)) + H((2)S) and OH(X(2)Π) + H(+). We consider the OH2(+) X[combining tilde](2)A'' and Ã(2)A' electronic states that correlate with a linear (2)Π species. The electronic angular momenta operators L[combining circumflex] and L[combining circumflex](2) are considered in nonadiabatic coupled-channel calculations, where the associated RT effects are due to diagonal V(RT) potentials that add up to the PESs and to off-diagonal C(RT) couplings between the potential energy surfaces (PESs). Initial-state-resolved reaction probabilities PI, integral cross sections σI, and rate constants kI are obtained using recent ab initio PESs and couplings and the real wavepacket formalism. Because the PESs are strongly attractive, PI have no threshold energy and are large, σI decrease with collision energy, and kI depend little on the temperature. The X[combining tilde](2)A'' PES is up to three times more reactive than the Ã(2)A' PES and H2(+) rotational effects (j0 = 0, 1) are negligible. The diagonal V(RT) potentials are strongly repulsive at the collinearity and nearly halve all low-energy observables with respect to the BO ones. The off-diagonal C(RT) couplings are important at low partial waves, where they mix the X[combining tilde](2)A'' and Ã(2)A' states up to ∼20%. However, V(RT) effects predominate over the C(RT) ones that change at most by ∼19% the BO values of σI and kI. The reaction O((3)P) + H2(+)(X(2)Σg(+)) → OH(+)(X(3)Σ(-)) + H((2)S) is probably one of the most reactive atom + diatom collisions because its RT rate constant at room temperature is equal to 2.26 × 10(-10) cm(3) s(-1). Within the BO approximation, the present results agree rather well with recent quasiclassical and centrifugal-sudden data using the same PESs.

The rate constants (k; T: 200-900 K) and cross-sections (σ; Ecol: 0.010-0.50 eV) of the O + H2(+)→ OH(+) + H (1), OH + H(+) (2) reactions, which occur on the ground (1(2)A'') and first excited (1(2)A') potential energy surfaces (PESs), respectively, were investigated for the first time, considering also the rate constants for D2(+) and HD(+). Ab initio multireference configuration interaction calculations were performed on both barrierless PESs (where the minimum energy path involves the insertion of the O atom into the middle of the H2(+) bond), and suitable analytical expressions were developed for the first time and used in quasiclassical trajectory (QCT) calculations. k(1) ≈ 3k(2) independent of the isotopic variant, k(H2(+)) > k(HD(+)) > k(D2(+)) for , and the intermolecular and intramolecular isotopic effects are essentially independent of T. Comparison with the Langevin-Gioumousis-Stevenson (LGS) simple capture model shows that these results are similar to the QCT ones, especially for ; and the isotopic effects are coincident with the QCT ones for both reactions. For O + H2(+), σ(1) ≈ 3σ(2) at Ecol≤ 0.10 eV, and σ(1) = 1.5σ(2) at 0.40 and 0.50 eV. The larger value of σ(1(2)A'') with respect to σ(1(2)A') arises from the larger value of bmax(1(2)A'') with respect to bmax(1(2)A'), and this results from the more attractive character of the former PES. Besides, the reaction probabilities are quite large [0.78-0.98 (1(2)A'') and 0.78-0.93 (1(2)A')], and the decreasing trend of both cross-sections as Ecol increases arises from the barrierless character of both PESs. We expect that these results (in particular, the competition between proton transfer and hydrogen atom transfer) will encourage experimentalists to carry out investigations on this interesting reaction.

The dependence of the cross section (σ) of the Ne + H2(+)→ NeH(+) + H proton transfer reaction on the vibrational excitation of H2(+), v = 0-17 and j = 1, was analyzed in detail at the collision energies (Ecol) of 0.7 and 1.7 eV, using the quasi-classical trajectory (QCT) method and the PHHJ3 and LZHH potential energy surfaces (PESs), taking advantage of the rich experimental data available for this reaction as a function of H2(+)(v). The efficiency of vibrational excitation to promote the reaction was investigated from the analysis of the σ(QCT) vs. v dependence in terms of the average reaction probability, maximum impact parameter, regions of the (late barrier) PES explored, and taking into account the Ne + H2(+)→ Ne + H(+) + H dissociative channel, which plays a dominant role at high enough total energies. Although the earlier PHHJ3 PES performs rather well, the LZHH PES QCT results show a better agreement with the experiment. On the other hand, some artifacts were found in recently reported QCT calculations (unphysical oscillations in σ(QCT) as a function of v), and the present study shows that special care is needed when carrying out QCT calculations involving highly excited vibrational states.

A microkinetic model is developed to study the reactivity of an O/O(2) gas mixture over a β-cristobalite (001) surface. The thermal rate constants for the relevant elementary processes are either inferred from quasiclassical trajectory calculations or using some statistical approaches, resting on a recently developed interpolated multidimensional potential energy surface based on density functional theory. The kinetic model predicts a large molecular coverage at temperatures lower than 1000 K, in contrary to a large atomic coverage at higher temperatures. The computed atomic oxygen recombination coefficient, mainly involving atomic adsorption and Eley-Rideal recombination, is small and increases with temperature in the 700-1700 K range (0.01 < γ(O) < 0.02) in good agreement with experiments. In the same temperature range, the estimated chemical energy accommodation coefficient, the main contribution to which is the atomic adsorption process is almost constant and differs from unity (0.75 < β(O) < 0.80)., (This journal is © the Owner Societies 2011)