Reactivity properties of the HOSO and HSO2 isomers in liquid medium: a sequential Monte Carlo/quantum mechanics study.

CONTEXT: The rationalization of acid rain formation steps is fundamental for mitigating its effects. It is believed the hydroxysulfinyl radical is an intermediate species for the production of atmospheric sulfuric acid. Two stable configurations HOSO and HSO2 have been reported for such a radical in the gas phase. This work aims at studying these isomers in the aqueous medium. The electrical and reactivity quantities - electronic chemical potential ([Formula: see text]), chemical hardness ([Formula: see text]), and electrophilicity ([Formula: see text]) - are here calculated and compared. Considering first solvation shells (15 H2O for HSO2 and 23 H2O for HOSO), an increase of 25% in the dipole moment of HSO2 was obtained, while the dipole moment of HOSO decreases in 11%. Both solvated isomers grow softer ([Formula: see text] decreases) contrasted to the gas phase. METHODS: HOSO and HSO2 are studied through a sequential Monte Carlo/quantum mechanics approach. Lennard-Jones plus the Coulomb potentials were used to represent intermolecular potential interaction in the frame of the DICE package. Molecular structure calculations were performed at CASPT2/aug - cc - pV(T + d)Z level of theory using the MOLPRO suite of programs.

Relaxation of Vibrationally Excited OH Radical by SO.

Molecular energy transfer in gas-phase collisions is of large interest in kinetics models. Vibrational excitation and deactivation processes can be concurrent with chemical reaction and/or sources of nascent excited species. In the present work, a full-dimension quasi-classical trajectory study of the vibrational deactivation of OH(v' = 1,5) in collisions with SO is presented. A global potential energy surface for the ground electronic state of HSO2, previously reported, is used to represent the interatomic interactions. The specific initial-state OH deactivation cross section and the corresponding SO activation cross sections are reported. Models for the maximum impact parameter and excitation function are discussed, while some details on the energy transfer mechanisms are also given. Specific initial-state deactivation thermal rate coefficients and vibrational average deactivation thermal rate coefficients are also presented and compared to previous results in the literature.

Quasi-Classical Trajectory Study of NH(3∑-) + NH(3∑-) Reactive Collisions.

A full-dimension quasi-classical trajectory study of collisions between two NH radicals is presented. Interatomic interactions are represented by a previously reported global six-dimensional potential energy surface for singlet electronic state of the N2H2 system. This study suggests that the formation of N2 from the collision of two NH radicals may occur via a one-step (NH + NH → N2 + H + H) or two-step (NH + NH → N2H + H → N2 + H + H) microscopic reaction mechanism. A fast vibrational energy redistribution is observed in the four-body complex in the latter mechanism. Excitation functions are presented and discussed. A variant of the vibrational energy quantum mechanical threshold method was used to correct the zero-point energy leakage in the classical calculations. The influence of reactant's rotational and vibrational energy on reactivity was investigated by state-specific calculations with one or both reactants excited. Reaction rate constants for the ground and some rotationally excited states are presented using an Arrhenius-Kooij-like functional form.

A comparative study of analytic representations of potential energy curves for O2, N2, and SO in their ground electronic states.

In this work, a review of six functional forms used to represent potential energy curves (PECs) is presented. The starting point is the Rydberg potential, followed by functions by Hulburt-Hirschfelder, Murrell-Sorbie, Thakkar, Hua and finalizing with the potential for diatomic systems by Aguado and Paniagua. The mathematical behavior of these functions for the short- and long-range regions is discussed. A comparison highlighting the positive and negative aspects of each representation is also presented. As study cases, three diatomic systems O2, N2 and SO in their respective ground electronic states were selected. To obtain spectroscopic parameters, ab initio energies were first calculated at multi-reference configuration interaction (MRCI) with the Davidson modification (MRCI+Q) level of theory, using aug-cc-pVXZ (X = T,Q,5,6) Dunning basis sets. Such energies were then fitted to respective functional forms. The so-obtained spectroscopic constants are compared also with available literature data.

Gas phase Elemental abundances in Molecular cloudS (GEMS): I. The prototypical dark cloud TMC 1.

GEMS is an IRAM 30m Large Program whose aim is determining the elemental depletions and the ionization fraction in a set of prototypical star-forming regions. This paper presents the first results from the prototypical dark cloud TMC 1. Extensive millimeter observations have been carried out with the IRAM 30m telescope (3 mm and 2 mm) and the 40m Yebes telescope (1.3 cm and 7 mm) to determine the fractional abundances of CO, HCO+, HCN, CS, SO, HCS+, and N2H+ in three cuts which intersect the dense filament at the well-known positions TMC 1-CP, TMC 1-NH3, and TMC 1-C, covering a visual extinction range from A V ~ 3 to ~20 mag. Two phases with differentiated chemistry can be distinguished: i) the translucent envelope with molecular hydrogen densities of 1-5×103 cm-3; and ii) the dense phase, located at A V > 10 mag, with molecular hydrogen densities >104 cm-3. Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C+/C/CO transition zone (A V ~ 3 mag) where C/H ~ 8×10-5 and C/O~0.8-1, until the beginning of the dense phase at A V ~ 10 mag. This is consistent with the grain temperatures being below the CO evaporation temperature in this region. In the case of sulfur, a strong depletion should occur before the translucent phase where we estimate a S/H ~ (0.4 - 2.2) ×10-6, an abundance ~7-40 times lower than the solar value. A second strong depletion must be present during the formation of the thick icy mantles to achieve the values of S/H measured in the dense cold cores (S/H ~8×10-8). Based on our chemical modeling, we constrain the value of ζ H2 to ~ (0.5 - 1.8) ×10-16 s-1 in the translucent cloud.

A theoretical study of energy transfer in Ar(1S) + SO2( X ̃ 1 A ') collisions: Cross sections and rate coefficients for vibrational transitions.

Vibrational transitions, induced by collisions between rare-gas atoms and molecules, play a key role in many problems of interest in physics and chemistry. A theoretical investigation of the translation-to-vibration (T-V) energy transfer process in argon atom and sulfur dioxide molecule collisions is presented here. For such a purpose, the framework of the quasi-classical trajectory (QCT) methodology was followed over the range of translational energies 2 ≤ Etr/kcal mol-1 ≤ 100. A new realistic potential energy surface (PES) for the ArSO2 system was developed using pairwise addition for the four-body energy term within the double many-body expansion. The topological features of the obtained function are compared with a previous one reported by Hippler et al. [J. Phys. Chem. 90, 6158 (1986)]. To test the accuracy of the PES, additional coupled cluster singles and doubles method with a perturbative contribution of connected triples calculations were carried out for the global minimum configuration. From dynamical calculations, the cross sections for the T-V excitation process indicate a barrier-type mechanism due to strong repulsive interactions between SO2 molecules and the Ar atom. Corrections to zero-point energy leakage in QCT were carried out using vibrational energy quantum mechanical threshold of the complex and variations. Rate coefficients and cross sections are calculated for some vibrational transitions using pseudo-quantization approaches of the vibrational energy of products. Main attributes of the title molecular collision are discussed and compared with available information in the literature.

A quasi-classical study of energy transfer in collisions of hyperthermal H atoms with SO2 molecules.

A deep understanding of energy transfer processes in molecular collisions is at central attention in physical chemistry. Particularly vibrational excitation of small molecules colliding with hot light atoms, via a metastable complex formation, has shown to be an efficient manner of enhancing reactivity. A quasi-classical trajectory study of translation-to-vibration energy transfer (T-V ET) in collisions of hyperthermal H(2S) atoms with SO2(X̃1A') molecules is presented here. For such a study, a double many-body expansion potential energy surface previously reported for HSO2(2A) is used. This work was motivated by recent experiments by Ma et al. studying collisions of H + SO2 at the translational energy of 59 kcal/mol [J. Ma et al., Phys. Rev. A 93, 040702 (2016)]. Calculations reproduce the experimental evidence that during majority of inelastic non-reactive collision processes, there is a metastable intermediate formation (HOSO or HSO2). Nevertheless, the analysis of the trajectories shows that there are two distinct mechanisms in the T-V ET process: direct and indirect. Direct T-V processes are responsible for the high population of SO2 with relatively low vibrational excitation energy, while indirect ones dominate the conversion from translational energy to high values of the vibrational counterpart.

Coupled-Cluster Study of the Lower Energy Region of the Ground Electronic State of the HSO2 Potential Energy Surface.

This work reports CCSD(T)/aug-cc-pV(T+d)Z ab initio calculations for the lower energy region of the ground electronic state of the HSO2 system. Optimized geometries, total energies, zero-point vibrational energies, frequencies, complete basis set extrapolations, and reaction paths are reported at the same level of calculation. The connection of the two minima (synperiplanar HOSO and HSO2) with the dissociation limit H + SO2 through the van der Waals minimum H···SO2 was established. An important quantitative discrepancy with previous works is the fact that at the present level of calculation the energy difference between transition states connecting the global minimum synperiplanar HOSO to the HSO2 minimum (TS5) and to the van der Waals minimum H···SO2 (TS6) is negligible, implying that the forward barriers after the synperiplanar HOSO global minimum have practically the same height. This result suggests that these two transition states may be involved in the path of the global minimum toward the exit channel H + SO2. As a consequence, trajectories for the OH + SO collisions could evolve through the well formed by the HSO2 minimum, therefore opening two competitive channels for the OH + SO → H + SO2 reaction, a fact never reported in trajectory calculations.

Connection between the upper and lower energy regions of the potential energy surface of the ground electronic state of the HSO2 system.

The importance of the HSO(2) system in atmospheric and combustion chemistry has motivated several works dedicated to the study of associated structures and chemical reactions. Nevertheless, controversy still exists about a possible connection between the upper and lower energy regions of the potential energy surface (PES) for the ground electronic state of the system. Very recently, a path to connect these regions was proposed based on studies at the CASPT2/aug-cc-pV(T+d)Z level of calculation but the small energy difference between some of the transitions states along that path suggested the necessity of calculations at a higher level of theory. In the present work, we report a CCSD(T)/aug-cc-pV(T+d)Z study of the stationary states associated to the proposed connection path, including assessment of the most reliable complete basis set (CBS) extrapolation scheme for the system. Among the new features, the present calculations show that there are no structures corresponding to the HSO(2)(b) minimum and the TS3 saddle point obtained at the CASPT2 level and that the connection path between the upper and lower energy regions of the PES for the ground electronic state involves only one transition state and most probably more than one electronic state.

CASPT2 study of the potential energy surface of the HSO2 system.

The importance of the HSO(2) system in atmospheric and combustion chemistry has motivated several works dedicated to the study of associated structures and chemical reactions. Nevertheless controversy still exists in connection with the reaction SH + O(2)→ H + SO(2) and also related to the role of the HSOO isomers in the potential energy surface (PES). Here we report high-level ab initio calculation for the electronic ground state of the HSO(2) system. Energetic, geometric, and frequency properties for the major stationary states of the PES are reported at the same level of calculations: CASPT2/aug-cc-pV(T+d)Z. This study introduces three new stationary points (two saddle points and one minimum). These structures allow the connection of the skewed HSOO(s) and the HSO(2) minima defining new reaction paths for SH + O(2) → H + SO(2) and SH + O(2) → OH + SO. In addition, the location of the HSOO isomers in the reaction pathways have been clarified.