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- An important well studied atmospheric reaction, O (1D) + H2Publication . Rio, Carolina; Brandão, J.; Wang, WenliAmong the chemical reactions in atmosphere, the reaction of an excited oxygen atom, O (View the MathML source), with ground state molecular hydrogen, H2 (View the MathML source), has been one of the most studied both experimentally and theoretically. To describe this reaction, various potential energy surfaces have been calibrated and their dynamics has been studied using quantum mechanical and quasiclassical trajectory methods. The theoretical results have shown to be in good agreement with experiment. The main uncertainties arise in the low temperature rate constants and in the isotopic branching ratio when reacting with HD.
- N-dimensional switch function for energy conservation in multiprocess reaction dynamicsPublication . Mogo, César; Brandão, J.The MReaDy program was designed for studying Multiprocess Reactive Dynamic systems, that is, complex chemical systems involving different and concurrent reactions. It builds a global potential energy surface integrating a variety of potential energy surfaces, each one of them representing an elementary reaction expected to play a role in the chemical process. For each elementary reaction, energy continuity problems may happen in the transition between potential energy surfaces due to differences in the functional form for each of the fragments, especially if built by different authors. A N-dimensional switch function is introduced in MReaDy in order to overcome such a problem. As an example, results of a collision trajectory calculation for H-2 + OH -> H3O are presented, showing smooth transition in the potential energy, leading to conservation in the total energy. Calculations for a hydrogen combustion system from 1000 K up to 4000 K shows a variation of 0.012% when compared to the total energy of the system. VC 2016 Wiley Periodicals, Inc.
- The branching ratio of the O(1D) + HD reaction: a dynamical studyPublication . Rio, Carolina; Brandão, J.The isotopic branching on the reaction of O(1D) + HD has been the subject of several theoretical and experimental studies with results ranging from 1 to 2 for the OD/OH product ratio. We present a study of the dynamics of this reaction using a double-valued potential energy surface for the ground state water molecule. Our results predict a branching ratio close to 2 for this reaction in agreement with statistical studies but in contrast with experiment. This result can be attributed to the formation and dynamics of the intermediate complex.
- A full dimensional potential for H2O2 (X(1)A) covering all dissociation channelsPublication . Coelho, Daniela V.; Brandão, J.This work presents a new full dimensional potential energy surface for the ground singlet state of hydrogen peroxide, H2O2. This potential is based on a 3 x 3 matrix to accurately reproduce all the different dissociation channels in accordance with the Wigner-Witmer rules, namely, O(D-1) + H2O(X(1)A(1)), OH(X-2 Pi) + OH(X-2 Pi), O-2(a(1)Delta(g)) + H-2(X-1 Sigma(+)(g)) e H(S-2) + HO2(X(2)A ''). It has been obtained by fitting more than 38 thousand ab initio energies computed using the aug-cc-pVTZ and aug-cc-pVQZ basis sets and extrapolated to the basis set limit. The functional form used to represent the four-body short-range interactions is based on a sum of polynomial functions of the fourth degree multiplied by a range factor, both built with intrinsic permutation symmetry and centred at specific reference geometries, to which the ab initio points computed are assigned based on a k-means algorithm. It also accounts for the electrostatic dipole-dipole interaction between two OH((2)Pi) fragments.
- A modified potential for HO2 with spectroscopic accuracyPublication . Rio, Carolina; Tennyson, Jonathan; Brandão, J.Seven ground state potential energy surfaces for the hydroperoxyl radical are compared. The potentials were determined from either high-quality ab initio calculations, fits to spectroscopic data, or a combination of the two approaches. Vibration-rotation calculations are performed on each potential and the results compared with experiment. None of the available potentials is entirely satisfactory although the best spectroscopic results are obtained using the Morse oscillator rigid bender internal dynamics potential [Bunker , J. Mol. Spectrosc. 155, 44 (1992)]. We present modifications of the double many-body expansion IV potential of Pastrana [J. Chem. Phys. 94, 8093 (1990)]. These new potentials reproduce the observed vibrational levels and observed vibrational levels and rotational constants, respectively, while preserving the good global properties of the original potential.
- Quasiclassical trajectory calculations of the H + O2 and O + OH reactions on spectroscopically accurate modified DMBE IV PESsPublication . Rio, Carolina; Wang, Wenli; Brandão, J.Here we present dynamical calculations for the O + OH and H + O2 reactions on modified Double Many-Body Expansion DMBE IV potentials. The modifications were carried out to yield spectroscopy accuracy. Our results show that the dynamical behaviour of the original DMBE IV potential has not been changed.
- Dynamical studies and product analysis of the reaction between O(1D) and H2/D2Publication . Rio, Carolina; Brandão, J.In this work we study the dynamics of the O (1D) + H2/D2 reactions at fixed collision energies [4,5] using quasiclassical trajectories calculations on a double-valued potential energy surface for H2O [2].
- Using the reactions O+H2 → OH+H to explore the importance of the atomic quantum states on chemical kinetics and reaction dynamicsPublication . Rio, Carolina; Brandão, J.; Wang, WenliThe O + H2 reaction is a particular example to alert the importance of a clear definition of the quantum state of an atom when referring to a chemical reaction. In its ground state, O (3P), the oxygen atom reacts with H2 through an energy barrier with small rate constant. In contrast, when the oxygen atom is in its first excited state, O (1D), the reaction O + H2 occurs without energy barrier and the rate constant is seven orders of magnitude higher. The dynamic behaviour of this reaction depends also on the quantum state of the oxygen atom.
- Dynamical studies and product analysis of O(1D)1H2/D2 reactionsPublication . Rio, Carolina; Brandão, J.The aim of this study was to analyse the dynamics of O(1D)þH2/D2 reactions using quasiclassical trajectory calculations on a double-valued potential energy surface for H2O. Produced on the photodissociation of stratospheric ozone, the excited oxygen atom is a highly reactive species whose chemistry plays a key role in the ozone depletion cycle. In order to make comparisons with experiment, we studied these reactions at fixed translational collision energies. In particular, we consider the reactive cross sections, the thermal rate constants, the opacity function, and the differential cross sections. In addition, we also study the energy distribution of the products and compare the results with experiment and calculations based on phase space statistical theory. Results for the rotational population of the OH products are also compared with experimental results. The agreement between our results and experiment reinforces the accuracy of the H2O potential energy surface used.
- The ground state water molecule and the reactions O(1D) + H2/D2/HD and H + OD that occur in this systemPublication . Rio, Carolina; Brandão, J.; Wang, WenliDue to their importance in atmospheric and combustion chemistry, the reactions of O(1D) with H2 and its isotopic variants HD and D2 have been subject of several theoret- ical1, 2 and experimental2, 3 studies.