Carolina Oliva

New analytical (2A′,4A′) surfaces and theoretical rate constants for the N(4S)+O2 reaction

We report two new analytical fits of the ground potential energy surface (PES) (2A′) and the first excited PES (4A′) involved into the title reaction and its reverse, using ab initio electronic structure calculations from Papers I and II along with new grids of ab initio points by means of the second-order perturbation theory on CASSCF wave function [CASPT2 (17,12) G2/aug-cc-pVTZ] reported here (1250 points for the 2A′ PES and 910 points for the 4A′ PES). Some experimental data were also introduced to better account for the exoergicity and the experimental rate constant at 300 K. The final root-mean-square deviations of the fits were 1.06 and 1.67 kcal/mol for 2A′ and the 4A′ PESs, respectively, for the NOO Cs abstraction and insertion regions of the PESs. Thermal rate constants were calculated (300–5000 K) for both the direct and reverse reactions by means of the variational transition state theory with the inclusion of a microcanonical optimized multidimensional tunneling correction, obtaining a very go...

A theoretical approach to the O(1D)+H2O(X 1A1) reaction: Ab initio potential energy surface and quasiclassical trajectory dynamics study

An ab initio study of the ground potential energy surface (PES) of the O(1D)+H2O system has been performed, employing Moller–Plesset methods. From the stationary and additional points calculated, the ground PES has been modeled as a triatomic system, with an OH group of the H2O molecule treated as a single atom of 17.0 amu. The rate constant of reaction (1), O(1D)+H2O→2OH (main reaction channel), estimated from the quasiclassical trajectory (QCT) calculations is reasonably close to the recommended experimental value. For the relative translational energies explored (ET=0.234, 0.303, and 0.443 eV) and H2O at T=300 K, the QCT OH vibrational populations are in good agreement with the experimental values reported for the new OH fragment, but the QCT OH average rotational energies are in general quite larger than the experimental ones. Regarding the stereodynamics, for ET=0.234 eV there is not a clear tendency to a particular rotational alignment of the OH product with respect to the initial relative velocity ...

Quantum dynamics of the N(4S)+O2 reaction on the X 2A′ and a 4A′ surfaces: Reaction probabilities, cross sections, rate constants, and product distributions

We report real wave packet (WP) calculations of reaction probabilities, cross sections, rate constants, and product distributions of the reaction N(4S)+O2(X 3∑g−)→NO(X 2∏)+O(3P). We propagate initial WPs corresponding to several O2 levels, and employ reactant coordinates and a flux method for calculating initial-state-resolved observables, or product coordinates and an asymptotic analysis for calculating state-to-state quantities. Exact or J-shifting calculations are carried out at total angular momentum J=0 or J>0, respectively. We employ the recent X 2A′ S3 potential energy surface (PES) by Sayos et al. and the earlier a 4A′ PES by Duff et al. In comparing S3 results with the WP ones of a previous X 2A′ S2 PES, we find lower S3 energy thresholds and larger S3 probabilities, despite the higher S3 barrier. This finding is due to the different features of the doublet PESs in the reactant and product channels, at the transition state, and in the NO2 equilibrium region. We analyze the effects of the O2 initi...

Wave packet dynamics of the N(4S)+O2(X 3Σg−)→NO(X 2Π)+O(3P) reaction on the X 2A′ potential energy surface

We report three-dimensional quantum calculations of total angular momentum J=0 reaction probabilities, J-shifting cross sections, and rate constants of the title reaction. Employing the real wave packet approach, we propagate wave packets corresponding to several O2(v,j) initial levels on the X 2A′ potential surface of Sayos et al. As collision energy increases, the average probabilities first increase monotonically and then become nearly constant, while the cross sections rise in the overall energy range. Numerous probability resonances point out the formation of NOO collision complexes and NO final states. Rotational excitation in O2 decreases the collisional energy thresholds and enhances the state-resolved rate constants, mainly at low temperature. O2 vibrational excitation inhibits the reactivity, although the energy thresholds are still reduced. With respect to previous quasiclassical and mixed quantum-classical studies, we obtain lower thresholds and cross sections but similar rate constants, which...

The lowest doublet and quartet potential energy surfaces involved in the N(4S)+O2 reaction. I. Ab initio study of the Cs-symmetry (2A′, 4A′) abstraction and insertion mechanisms

In this work we have carried out ab initio complete active space self-consistent-field (CASSCF) calculations, second-order perturbation calculations based on CASSCF wave functions (CASPT2), uncontracted multireference configuration interaction calculations, and some density functional calculations with standard correlation-consistent Dunning basis sets and atomic natural orbital basis sets on the lowest 2A′ and 4A′ potential energy surfaces involved in the title reaction. The ground 2A′ surface has an average energy barrier of 5.3 kcal/mol in the CASPT2 complete basis set limit. A peroxy NOO minimum is found in agreement with preceding ab initio works, which seems to play an important role in the opening of a double microscopic mechanism: direct Cs abstraction and indirect Cs insertion through the NO2(X 2A1) molecule. The ground 4A′ surface shows an average energy barrier of 13.5 kcal/mol in the CASPT2 complete basis set limit. Despite this excited surface displays another peroxy minimum, in this case onl...

The lowest doublet and quartet potential energy surfaces involved in the N(4S)+O2 reaction. II. Ab initio study of the C2v-symmetry insertion mechanism

In the present work we have carried out ab initio complete active space self-consistent field (CASSCF) and second-order perturbation theory on CASSCF wave function (CASPT2) calculations and also some density functional theory calculations with the aug-cc-pVTZ Dunning’s basis set on the lowest A1, B1, A2, and B2 doublet and quartet potential energy surfaces (PES) that could be involved in the title reaction. Thus, several minima, transition states, and surface crossings have been found for the C2v-insertion reaction mechanism. The results agree very well with available experimental data [i.e., for NO2 (2A1), MIN2 (2B2), NO2 (2Πu)] and with other previous ab initio calculations. Six A′/A′- and four A′/A″-type surface crossings were located and classified for these PES’, whose only one (i.e., 2B2/2A1) has been previously reported in theoretical and experimental studies. High-energy barriers were found for the direct C2v-insertion mechanism (3.11 and 2.54 eV for the lowest doublet and quartet PES’ at the CASPT2/aug-cc-pVTZ level, respectively), clearly showing that this competitive mechanism is much less favorable than the direct Cs-abstraction or the indirect Cs-insertion reaction mechanisms reported in Paper I.In the present work we have carried out ab initio complete active space self-consistent field (CASSCF) and second-order perturbation theory on CASSCF wave function (CASPT2) calculations and also some density functional theory calculations with the aug-cc-pVTZ Dunning’s basis set on the lowest A1, B1, A2, and B2 doublet and quartet potential energy surfaces (PES) that could be involved in the title reaction. Thus, several minima, transition states, and surface crossings have been found for the C2v-insertion reaction mechanism. The results agree very well with available experimental data [i.e., for NO2 (2A1), MIN2 (2B2), NO2 (2Πu)] and with other previous ab initio calculations. Six A′/A′- and four A′/A″-type surface crossings were located and classified for these PES’, whose only one (i.e., 2B2/2A1) has been previously reported in theoretical and experimental studies. High-energy barriers were found for the direct C2v-insertion mechanism (3.11 and 2.54 eV for the lowest doublet and quartet PES’ at the CASP...

Ab initio CASPT2//CASSCF study of the O(1D)+H2O(X 1A1) reaction

The ground potential energy surface (PES) of the O(1D)+H2O system was studied with the CASPT2//CASSCF ab initio method. We analyzed the degree of validity of an earlier ab initio study by us that used the Moller–Plesset (MP) method. Both the present CASPT2//CASSCF calculations and the highest level MP calculations (PUMP4//UMP2) showed that the main reaction channel (OH+OH) has no energy barrier along the minimum energy path. This result is consistent with the absence of experimental activation energy. The CASPT2//CASSCF and PUMP4//UMP2 results, however, show important differences, mainly concerning the energy, due to the dominant open-shell singlet character of the ground PES. To make an accurate general description of this system, ab initio calculations using multireference methods like the one discussed here are required. Nevertheless, the earlier PUMP4//UMP2 calculations can be taken as a reasonable starting point for characterizing the ground PES of this system. Moreover, the pseudotriatomic (O(1D)+H–...

A quantum mechanics/molecular mechanics study of the reaction mechanism of the hepatitis C virus NS3 protease with the NS5A/5B substrate

Combined quantum mechanics and molecular mechanics (QM/MM) calculations were carried out to characterize the reaction mechanism of the NS3 protease with its preferred substrate (NS5A/5B). The main purpose of this study was to locate the barrier states and intermediates along the distinguished coordinate path (DCP) involved in this process. These structures, and in particular the one corresponding to the first barrier state and intermediate (B1 and I1), could be a starting point for the synthesis of inhibitors of this protease, which could be used to treat hepatitis C. The two first steps of the reaction mechanism were studied, i.e., the acylation step and the breaking of the peptide bond. The first step takes place through a tetracoordinated intermediate, as suggested from previous works on other Serine proteases. The importance of the different amino acid residues was also considered (perturbation study where the MM charges of each residue were set to zero independently). The residues of the oxyanion hole were confirmed as the most important for the electrostatic stabilization of the tetracoordinate intermediate. Moreover, the role of other residues, e.g., Arg‐155 and Asp‐79, was also explained. Proteins 2007.

Quasiclassical Trajectory Study of the O(1D) + H2O → 2OH, H + HO2 Reactions

The dynamics of the O(1D) + H2O → OH,H + HO2 reactions has been studied using the quasiclassical trajectory (QCT) method on a pseudotriatomic (O - H - (OH)) analytical representation of the ground potential energy surface (PES), where an OH bond of the H2O molecule has been treated as an atom of 17 a.m.u.. The OH + OH and H + HO2 reaction channels show a very different behaviour, although the H2O2 (hydrogen peroxide) deep minimum plays a very important role in the dynamics (insertion mechanism) of both channels.