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ABSTRACT: The A-band photodissociation of ammonia is an archetypical and long studied example of non-adiabatic dynamics in a polyatomic system. Due to a well-known conical intersection seam, electronically excited NH(3) can produce either the ground (X̃(2)B(1)) state or the excited (Ã(2)A(1)) state of the NH(2) fragment. In this work, the non-adiabatic dynamics is investigated using a six-dimensional wave packet method and an improved version of a newly developed diabatic Hamiltonian based on high quality ab initio data. The Ã(2)A(1)/X̃(2)B(1) branching ratios are in excellent agreement with experimental estimates, thus validating the non-adiabatically coupled Hamiltonian.
The Journal of chemical physics 12/2012; 137(22):22A541. · 3.09 Impact Factor
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ABSTRACT: The title reaction is thought to be responsible for the production of molecular nitrogen in interstellar clouds. In this work, we report quantum capture calculations on a new two-dimensional potential energy surface determined by interpolating high-level ab initio data. The low-temperature rate constant calculated using a capture model is quite large and has a positive temperature dependence, in agreement with a recent experiment. The origin of the aforementioned behaviors of the rate constant is analyzed.
Physical Chemistry Chemical Physics 08/2012; 14(35):12090-3. · 3.57 Impact Factor
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ABSTRACT: A recently developed method to represent adiabatic electronic states coupled by conical intersections has been used to construct a full six-dimensional quasi-diabatic representation of the 1(1)A and 2(1)A states of NH(3). This representation is expected to be appropriate to simulate the photodissociation of ammonia when it is excited to the 2(1)A electronic state. In this work, the electronic structure aspects of this quasi-diabatic representation are analyzed. This representation is then used as the basis for a simulation of the à ← X absorption spectrum, dominated by a progression in the v(2) mode, using a full six-dimensional quantum mechanical treatment of the nuclear motion. Results are reported for both NH(3) and ND(3). This simulation provides the most accurate computational determination of this absorption spectrum reported to date. These results serve to validate the quasi-diabatic representation and set the stage for subsequent studies of vibrationally mediated photodissociation of NH(3).
The Journal of chemical physics 06/2012; 136(23):234301. · 3.09 Impact Factor
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ABSTRACT: We report extensive quasi-classical trajectory calculations of the HO + CO → H + CO(2) reaction on a newly developed potential energy surface based on a large number of UCCSD(T)-F12/AVTZ calculations. This complex-forming reaction is known for its unusual kinetics and dynamics because of its unique potential energy surface, which is dominated by the HOCO wells flanked by an entrance channel bottleneck and a transition state leading to the H + CO(2) products. It was found that the thermal rate coefficients are in reasonably good agreement with known experimental data in both low and high pressure limits. Excitation of the OH vibration is shown to enhance reactivity, due apparently to its promoting effect over the transition state between the HOCO intermediate and the H + CO(2) product. On the other hand, neither CO vibrational excitation nor rotational excitation in either CO or OH has a significant effect on reactivity, in agreement with experiment. However, significant discrepancies have been found between theory and the available molecular beam experiments. For example, the calculated translational energy distribution of the products substantially underestimates the experiment. In addition, the forward bias in the differential cross section observed in the experiment was not reproduced theoretically. While the origin of the discrepancies is still not clear, it is argued that a quantum mechanical treatment of the dynamics might be needed.
The Journal of Physical Chemistry A 05/2012; 116(21):5057-67. · 2.95 Impact Factor
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ABSTRACT: We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ~35,000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data.
The Journal of chemical physics 01/2012; 136(4):041103. · 3.09 Impact Factor
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ABSTRACT: The normal-to-local transition for the bending modes of acetylene is considered a prelude to its isomerization to vinylidene. Here, such a transition in fully deuterated acetylene is investigated using a full-dimensional quantum model. It is found that the local benders emerge at much lower energies and bending quantum numbers than in the hydrogen isotopomer HCCH. This is accompanied by a transition to a second kind of bending mode called counter-rotator, again at lower energies and quantum numbers than in HCCH. These transitions are also investigated using bifurcation analysis of two empirical spectroscopic fitting Hamiltonians for pure bending modes, which helps to understand the origin of the transitions semiclassically as branchings or bifurcations out of the trans- and cis-normal bend modes when the latter become dynamically unstable. The results of the quantum model and the empirical bifurcation analysis are in very good agreement.
The Journal of chemical physics 01/2012; 136(1):014304. · 3.09 Impact Factor
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Physical Review Letters 01/2012; 109(6). · 7.37 Impact Factor
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Journal of Physical Chemistry Letters 01/2012; 3(17):2482-2486. · 6.21 Impact Factor
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ABSTRACT: Atmospheric ozone is formed by the O + O(2) exchange reaction followed by collisional stabilization of the O(3)(∗) intermediate. The dynamics of the O + O(2) reaction and to a lesser extent the O(3) stabilization depend sensitively on the underlying potential energy surface, particularly in the asymptotic region. Highly accurate Davidson corrected multi-state multi-reference configuration interaction calculations reported here reveal that the minimal energy path for the formation of O(3) from O + O(2) is a monotonically decaying function of the atom-diatom distance and contains no "reef" feature found in previous ab initio calculations. The absence of a submerged barrier leads to an exchange rate constant with the correct temperature dependence and is in better agreement with experiment, as shown by quantum scattering calculations.
The Journal of chemical physics 08/2011; 135(8):081102. · 3.09 Impact Factor
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ABSTRACT: State-to-state differential and integral cross sections for the title reaction were calculated using an exact wave packet method on a recently developed ab initio potential energy surface of the first excited state HO(2)(Ã(2)A'). The calculation results indicate that the reaction is dominated by highly rotationally excited OH products scattered in both the forward and backward directions, consistent with the formation of a long-lived HO(2) reaction intermediate. However, a statistical model was found to overestimate the integral cross sections, due apparently to dynamical bottlenecks. In addition, a unique feature in the OH + O exit channel potential promotes rotational excitation of the departing OH product by exerting a torque force. The role of the title reaction in high temperature combustion is also discussed.
Physical Chemistry Chemical Physics 02/2011; 13(18):8407-13. · 3.57 Impact Factor
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ABSTRACT: The authors report extensive high-level ab initio studies of the first excited (Ã (2)A(')) state of HO(2). A global potential energy surface (PES) was developed by spline-fitting 17 000 ab initio points at the internal contracted multireference configuration interaction (icMRCI) level with the AVQZ basis set. To ascertain the spectroscopic accuracy of the PES, the near-equilibrium region of the molecule was also investigated using three interpolating moving least-squares-based PESs employing dynamically weighted icMRCI methods in the complete basis set limit. Vibrational energy levels on all four surfaces agree well with each other and a new assignment of some vibrational features is proposed. In addition, the dynamics of both the forward and reverse directions of the H+O(2)(ã (1)Δ(g))↔OH+O reaction (J=0) were studied using an exact wave packet method. The reactions are found to be dominated by sharp resonances.
The Journal of chemical physics 10/2010; 133(14):144306. · 3.09 Impact Factor
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ABSTRACT: The authors report a detailed quantum mechanical study of the state-to-state dynamics of the O+OH(v(i)=0, j(i)=0)-->H+O(2)(v(f),j(f)) reaction on an accurate HO(2)(X(2)A") potential energy surface. The scattering dynamics was treated using a reactant coordinate based Chebyshev real wavepacket method with full Coriolis coupling. A total of 84 partial waves were calculated in order to achieve convergence up to the collision energy of 0.17 eV. The differential cross section is near forward-backward symmetric, consistent with the complex-forming mechanism. The O(2) product was found to have a monotonically decaying vibrational distribution and highly excited and inverted rotational distributions, also consistent with the formation of the HO(2) intermediate. These quantum mechanical results were compared with those obtained in earlier quasiclassical trajectory and statistical studies and it is shown that the statistical theory gives a reasonably good description of the product state distributions despite its inability to predict the total reaction cross section.
The Journal of chemical physics 08/2010; 133(5):054302. · 3.09 Impact Factor
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ABSTRACT: The authors report a detailed quantum mechanical study of the
state-to-state dynamics of the O+OH(vi=0,
ji=0)-->H+O2(vf,jf)
reaction on an accurate HO2(X2A'')
potential energy surface. The scattering dynamics was treated using a
reactant coordinate based Chebyshev real wavepacket method with full
Coriolis coupling. A total of 84 partial waves were calculated in order
to achieve convergence up to the collision energy of 0.17 eV. The
differential cross section is near forward-backward symmetric,
consistent with the complex-forming mechanism. The O2 product
was found to have a monotonically decaying vibrational distribution and
highly excited and inverted rotational distributions, also consistent
with the formation of the HO2 intermediate. These quantum
mechanical results were compared with those obtained in earlier
quasiclassical trajectory and statistical studies and it is shown that
the statistical theory gives a reasonably good description of the
product state distributions despite its inability to predict the total
reaction cross section.
Chemical Physics 07/2010; 133(5):4302. · 1.90 Impact Factor
