Photochemistry of HOSO radical in the gas phase

Spectroscopy and photochemistry of ClSSO

Sulfur-chlorine cycles play a role in the atmosphere of Venus. It is thought that many sulfur-chlorine bearing molecules could be present in Venus's atmosphere and play an important role in its chemical processes. The goal of this work is to provide new insight into the electronic structure and spectroscopy of the [Cl, S, S, O] molecular system. Eight isomers could be formed, but only three were found to be thermodynamically stable relative to the first dissociation limit. We spectroscopically characterized the two lowest energy stable isomers, C1-ClSSO and trans-ClSSO, using the accurate CCSD(T)-F12/aug-cc-pVTZ method. The dipole moments of the two lowest energy stable isomers are predicted to be 1.90 and 1.60 debye, respectively. The C1-ClSSO isomer is suitable for laser induced fluorescence detection since the lowest excited electronic states absorb in the visible, ∼610 nm, and near UV region, 330 nm. We mapped the evolution of the low-lying excited electronic states along the ClS, SS, and SO bond lengths to find that the production of ClS, SO, or S₂O is plausible, whereas the production of ClS₂ is not allowed.

Photochemical and thermochemical pathways to S2 and polysulfur formation in the atmosphere of Venus

Polysulfur species have been proposed to be the unknown near-UV absorber in the atmosphere of Venus. Recent work argues that photolysis of one of the (SO)₂ isomers, cis-OSSO, directly yields S₂ with a branching ratio of about 10%. If correct, this pathway dominates polysulfur formation by several orders of magnitude, and by addition reactions yields significant quantities of S₃, S₄, and S₈. We report here the results of high-level ab-initio quantum-chemistry computations that demonstrate that S₂ is not a product in cis-OSSO photolysis. Instead, we establish a novel mechanism in which S₂ is formed in a two-step process. Firstly, the intermediate S₂O is produced by the coupling between the S and Cl atmospheric chemistries (in particular, SO reaction with ClS) and in a lesser extension by O-abstraction reactions from cis-OSSO. Secondly, S₂O reacts with SO. This modified chemistry yields S₂ and subsequent polysulfur abundances comparable to the photolytic cis-OSSO mechanism through a more plausible pathway. Ab initio quantification of the photodissociations at play fills a critical data void in current atmospheric models of Venus.

Polysulfur compounds have been ascribed as the unknown near-UV absorbers in Venusian atmosphere and play a key role in the sulfur chemical cycle of this planet. Here, authors establish their production from (SO)₂ on the grounds of quantifications of photochemical and thermal pathways involved in the sulfur chemical cycle of the planet.

Spectroscopic Characterization of HSO2• and HOSO• Intermediates Involved in SO2 Geoengineering

Sulfur-containing radicals HSO₂^• and HOSO^• are key intermediates involved in stratospheric sulfur geoengineering by SO₂ injection. The spectroscopic characterization and photochemistry of both radicals are crucial to understanding the chemical impact of SO₂ chemistry in the stratosphere. On the basis of the efficient generation of HOSO^• by flash pyrolysis of gaseous sulfinic acid, CHF₂S(O)OH, a strong absorption is observed at 270 nm along with a shoulder up to 350 nm for HOSO^• isolated in low-temperature noble gas matrixes (Ar and Ne). These mainly arise from the excitations from the ground state (X²A) to the C²A/D²A and A²A/B²A states, respectively. Upon a 266 nm laser irradiation, the broad absorption band in the range 320-500 nm for HSO₂^• appears, and it corresponds to the combination of three excitations from the X²A state to the first (A²A), second (B²A), and third (C²A) excited states. Assignment of the UV-vis spectra is consistent with the photochemistry of HOSO^• and HSO₂^• as observed by matrix-isolation IR spectroscopy and also by the agreement with high-level ab initio calculations.

Photochemistry of HOSO2 and SO3 and Implications for the Production of Sulfuric Acid

Sulfur trioxide (SO₃) and the hydroxysulfonyl radical (HOSO₂) are two key intermediates in the production of sulfuric acid (H₂SO₄) on Earth's atmosphere, one of the major components of acid rain. Here, the photochemical properties of these species are determined by means of high-level quantum chemical methodologies, and the potential impact of their light-induced reactivity is assessed within the context of the conventional acid rain generation mechanism. Results reveal that the photodissociation of HOSO₂ occurs primarily in the stratosphere through the ejection of hydroxyl radicals (^•OH) and sulfur dioxide (SO₂). This may decrease the production rate of H₂SO₄ in atmospheric regions with low O₂ concentration. In contrast, the photostability of SO₃ under stratospheric conditions suggests that its removal efficiency, still poorly understood, is key to assess the H₂SO₄ formation in the upper atmosphere.

Spectroscopic Characterization of the First and Second Excited States of the HOSO Radical

The spectroscopic properties of the ground and first two excited states of the HOSO radical are investigated using the internally contracted multireference configuration interaction method, including the Davidson correction (MRCI+Q) and explicit treatment of the electron correlation (MRCI-F12). The vertical and adiabatic excitation energies are also determined. The results reveal that both the 1 ²A and 2 ²A electronic states contain minima in their potential energy surfaces. The first excited state 1 ²A possesses a nonplanar structure and has an adiabatic excitation energy of 1.45 eV (855 nm), lying in the near-infrared region. The second excited state 2 ²A has a planar geometry and an adiabatic excitation energy of 2.91 eV (426 nm) existing in the visible region. The calculated oscillator strengths for the vertical electronic excitations to the 1 ²A (327 nm) and 2 ²A (270 nm) states are 0.003 and 0.022, respectively, indicating experimental intensity should be observed. The small but non-negligible Franck-Condon factors for excitations ∼300 nm, and the broad and intense absorption feature in the 225-275 nm region suggest that detection of the HOSO radical with electronic spectroscopy may be feasible.

Photochemistry and Non-adiabatic Photodynamics of the HOSO Radical

Hydroxysulfinyl radical (HOSO) is important due to its involvement in climate geoengineering upon SO₂ injection and generation of the highly hygroscopic H₂SO₄. Its photochemical behavior in the upper atmosphere is, however, uncertain. Here we present the ultraviolet-visible photochemistry and photodynamics of this species by simulating the atmospheric conditions with high-level quantum chemistry methods. Photocleavage to HO + SO arises as the major solar-induced channel, with a minor contribution of H + SO₂ photoproducts. The efficient generation of SO is relevant due to its reactivity with O₃ and the consequent depletion of ozone in the stratosphere.

Photochemistry from low-lying states of HOSO. J Chem Phys 2020;152:134302. [PMID: 32268736 DOI: 10.1063/5.0001867]

Using configuration interaction ab initio methods, the evolution of the lowest electronic states of singlet and triplet spin multiplicities of HOSO⁺ along the stretching and bending coordinates of is investigated. Equilibrium geometries, rotational constants, and harmonic vibrational frequencies of the lowest electronic states are calculated, i.e., X¹A', 1¹A″, 1³A', and 1³A″. The global minimum of the 1¹A″ state is located below the first dissociation limit and its calculated lifetime is predicted to be 0.40 µs, making it suitable for detection by laser-induced fluorescence. According to the potential energy surfaces, HOSO⁺ should produce SO₂ ⁺ and H after ultraviolet photon absorption to the 2¹A' state. This work opens the door to investigate the branching ratio and the production rates of SO₂ ⁺, SO⁺, and OH from HOSO⁺. These insights can help understand the SO₂ cycle in the earth's atmosphere and its effect on cooling our planet.