Antonio G. S. de Oliveira-Filho

Quasiclassical Trajectory Calculations of the Rate Constant of the OH + HBr → Br + H2O Reaction Using a Full-Dimensional Ab Initio Potential Energy Surface Over the Temperature Range 5 to 500 K.

We report a permutationally invariant, ab initio potential energy surface (PES) for the OH + HBr → Br + H2O reaction. The PES is a fit to roughly 26 000 spin-free UCCSD(T)/cc-pVDZ-F12a energies and has no classical barrier to reaction. It is used in quasiclassical trajectory calculations with a focus on the thermal rate constant, k(T), over the temperature range 5 to 500 K. Comparisons with available experimental data over the temperature range 23 to 416 K are made using three approaches to treat the OH rotational and associated electronic partition function. All display an inverse temperature dependence of k(T) below roughly 160 K and a nearly constant temperature dependence above 160 K, in agreement with experiment. The calculated rate constant with no treatment of spin-orbit coupling is overall in the best agreement with experiment, being (probably fortuitously) within 20% of it.

New Molecular Species of Potential Interest to Atmospheric Chemistry: Isomers on the [H, S2, Br] Potential Energy Surface

This work reports a state-of-the-art theoretical characterization of four new sulfur-bromine species and five transition states on the [H, S(2), Br] potential energy surface. Our highest level theoretical approach employed the method coupled cluster singles and doubles with perturbative contributions of connected triples, CCSD(T), along with the series of correlation-consistent basis sets and with extrapolation to the complete basis set (CBS) limit in the optimization of the geometrical parameters and to quantify the energetic quantities. The structural and vibrational frequencies here reported are unique and represent the most accurate investigation to date of these species. The global minimum corresponds to a skewed structure HSSBr with a disulfide bond; this is followed by a pyramidal-like structure, SSHBr, 18.85 kcal/mol above the minimum. Much higher in energy, we found another skewed structure, HSBrS (50.29 kcal/mol), with one S-Br dative-type bond, and another pyramidal-like one, HBrSS (109.80 kcal/mol), with two S-Br dative-type bonds. The interconversion of HSSBr into SSHBr can occur via a transfer of either the hydrogen or the bromine atom but involves a very high barrier of about 43 kcal/mol. These molecules are potentially a new route of coupling the sulfur and bromine chemistry in the atmosphere, and conditions of high concentration of H(2)S like in volcanic eruptions might contribute to their formation. We note that HSSBr can act as a reservoir molecule for the reaction between the radicals HSS and Br. Also, an assessment of the methods DFT/B3LYP/CBS and MP2/CBS relative to CCSD(T)/CBS provides insights on the expected performance of these methods on the characterization of polysulfides and also of more complex systems containing disulfide bridges.

Computationally Designed 1,2,4-Triazolylidene-Derived N-Heterocyclic Olefins for CO2 Capture, Activation, and Storage

In this article, triazolylidene-derived N-heterocyclic olefins (trNHOs) are designed using computational quantum tools, and their potential to promote CO2 sequestration is tested and discussed in detail. The low barrier heights related to the trNHO-mediated process indicate that the tailored compounds are very promising for fast CO2 sequestration. The systematic analysis of the presence of distinct substitutes at different N positions of the trNHO ring allows us to rationalize their effect on the carboxylation process and reveal the best N-substituted trNHO systems for CO2 sequestration and improved trNHO carboxylates for faster CO2 capture/release.

Thermal rate constants for the O(3P) + HBr and O(3P) + DBr reactions: transition-state theory and quantum mechanical calculations.

The O((3)P) + HBr → OH + Br and O((3)P) + DBr → OD + Br reactions are studied on a recent high-quality ab initio-based potential energy surface. Thermal rate constants over the 200-1000 K temperature range, calculated using variational transition-state theory (VTST) with the small-curvature tunneling (SCT) correction and quantum mechanical methods with the J-shifting approximation (QM/JS) for zero total angular momentum (J = 0), are reported. These results are compared to the available experimental data, which lie in the ranges of 221-554 and 295-419 K for O + HBr and O + DBr, respectively. The rate constants, in cm(3) molecule(-1) s(-1) and at 298 K, for the O + HBr reaction are 3.66 × 10(-14) for VTST, 3.80 × 10(-14) for QM/JS, and 3.66 × 10(-14) for the average of eight experimental measurements.

The surprising metastability of TeH2

A high-level ab initio investigation of a manifold of electronic states of the diatomic dication TeH(2+) is presented. Potential energy curves for both Λ + S and relativistic (Ω) states are constructed not only making evident the metastability of this system, but also the large energy splitting due to spin-orbit interactions. This effect is also very significant in the region close to the crossing of the (2)Π and (4)Σ(-) states, where avoided crossings between the Ω states have a relatively large impact on the height of the energy barriers. In contrast to TeH, with only two bound states (X1 (2)Π3∕2 and X2 (2)Π1∕2) below about 25,000 cm(-1), in the case of TeH(2+) a much richer energy profile is obtained indicating various possibilities of electronic transitions. Guided by the results of this study, the experimental characterization of these states is now a challenge to spectroscopists. Since close to the equilibrium region the double positive charge is centered on the tellurium atom, the binding in this system can be rationalized as a simple covalent bond between the pz and s orbitals of Te(2+) and H, respectively. As the internuclear distance increases, the electron affinity of Te(2+) overcomes that of H(+) and the system dissociates into two singly charged fragments. A simulation of the double ionization spectra complements the characterization of the electronic states, and results of a mass spectrometric investigation corroborates the predicted transient existence of this metastable species.

How To Arrive at Accurate Benchmark Values for Transition Metal Compounds: Computation or Experiment?

With the objective of analyzing which kind of reference data is appropriate for benchmarking quantum chemical approaches for transition metal compounds, we present the following, (a) a collection of 60 transition metal diatomic molecules for which experimentally derived dissociation energies, equilibrium distances, and harmonic vibrational frequencies are known and (b) a composite computational approach based on coupled-cluster theory with basis set extrapolation, inclusion of core-valence correlation, and corrections for relativistic and multireference effects. The latter correction was obtained from internally contracted multireference coupled-cluster (icMRCC) theory. This composite approach has been used to obtain the dissociation energies and spectroscopic constants for the 60 molecules in our data set. In accordance with previous studies on a subset of molecules, we find that multireference corrections are rather small in many cases and CCSD(T) can provide accurate reference values, if the complete basis set limit is explored. In addition, the multireference correction improves the results in cases where CCSD(T) is not a good approximation. For a few cases, however, strong deviations from experiment persist, which cannot be explained by the remaining error in the computational approach. We suggest that these experimentally derived values require careful revision. This also shows that reliable reference values for benchmarking approximate computational methods are not always easily accessible via experiment and accurate computations may provide an alternative way to access them. In order to assess how the choice of reference data affects benchmark studies, we tested 10 DFT functionals for the molecules in the present data set against experimental and calculated reference values. Despite the differences between these two sets of reference values, we found that the ranking of the relative performance of the DFT functionals is nearly independent of the chosen reference.

The electronic states of TeH+: A theoretical contribution

This work reports the first theoretical characterization of a manifold of electronic states of the as yet experimentally unknown monotellurium monohydride cation, TeH(+). Both Λ + S and Ω representations were described showing the twelve states correlating with the three lowest (Λ + S) dissociation channels, and the twenty five states associated with the five lowest Ω channels. The X (3)Σ(-) state is split into X1 0(+) and X2 1 separated by 1049 cm(-1); they are followed by the states a 2 (a (1)Δ) and b 0(+) (b (1)Σ(+)) higher in energy by 8554 and 17 383 cm(-1), respectively. These states can accommodate several vibrational energy levels. The potential energy curves of the Ω states arising from the bound A (3)Π, the weakly bound (1)Π, and the repulsive (5)Σ(-) states have a complex structure as shown by the very close avoided crossings just above ∼30 000 cm(-1). In particular, a double minima potential results for the state A1 2 that in principle could be probed experimentally through the A1 2-X2 1 system transitions. The states A2 1, b 0(+), and A4 0(+) offer possible routes to experimental investigations involving the ground state X1 0(+). Higher energy states are very dense and mostly repulsive. The high-level of the electronic structure calculations, by providing a global view of the electronic states and reliable spectroscopic parameters, is expected to further guide and motivate experimental studies on this species. Additional discussions on dipole and transition dipole moments, transition probabilities, radiative lifetimes, and a simulation of the single ionization spectrum complement the characterization of this system.

Full-dimensional analytical ab initio potential energy surface of the ground state of HOI

Extensive ab initio calculations using a complete active space second-order perturbation theory wavefunction, including scalar and spin-orbit relativistic effects with a quadruple-zeta quality basis set were used to construct an analytical potential energy surface (PES) of the ground state of the [H, O, I] system. A total of 5344 points were fit to a three-dimensional function of the internuclear distances, with a global root-mean-square error of 1.26 kcal mol(-1). The resulting PES describes accurately the main features of this system: the HOI and HIO isomers, the transition state between them, and all dissociation asymptotes. After a small adjustment, using a scaling factor on the internal coordinates of HOI, the frequencies calculated in this work agree with the experimental data available within 10 cm(-1).

Optical Pumping of TeH+: Implications for the Search for Varying mp/me

Molecular overtone transitions provide optical frequency transitions sensitive to variation in the proton-to-electron mass ratio (

Metastable BrO2+ and NBr2+ molecules in the gas phase

\mu\equiv m_p/m_e