Photochemistry and Non-adiabatic Photodynamics of the HOSO Radical

Abstract

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.

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

“…In summary, a joint theoretical and experimental characterization is reported for the electronic properties of two sulfur-containing radicals, HSO 2 • and HOSO • , which are considered as key intermediates in SO 2 geoengineering. In line with the MRCI+Q/aug-cc-pV­(T+d)­Z calculated vertical transitions above 300 nm for HSO 2 • , a very broad absorption band in the range 320–500 nm has been observed in both solid Ar and Ne matrixes for the first time, which reasonably accounts for the previously reported photochemistry of HSO 2 • in solid noble gas (Ar, Kr, and Xe) matrixes. , Additionally, the stronger absorptions for HOSO • in the range 250–350 nm have been identified, coinciding with the photodissociation of HOSO • to • OH and SO under ultraviolet light irradiation conditions in the stratosphere as disclosed by multiconfigurational quantum chemistry . Therefore, the electronic spectra of HSO 2 • and HOSO • computed with high-level multireference and multiconfiguration quantum chemical methods are experimentally validated.…”

“…28,29 Additionally, the stronger absorptions for HOSO • in the range 250−350 nm have been identified, coinciding with the photodissociation of HOSO • to • OH and SO under ultraviolet light irradiation conditions in the stratosphere as disclosed by multiconfigurational quantum chemistry. 30 Therefore, the electronic spectra of HSO 2…”

“…In this work, multireference configuration interaction (MRCI) methods are used to map the excited state potential energy surface of the lowest-lying electronic state of HSO 2 • along the stretching coordinates to identify where the ultraviolet–visible (UV–vis) absorption bands are for HSO 2 • with respect to HOSO • . Although recent computational studies of the photochemistry have theoretically characterized the absorption characteristics of HOSO • in the UV–vis region, no experimental UV–vis spectrum has been reported. Moreover, there are no theoretical or experimental studies of the UV–vis spectra of HSO 2 • .…”

“…with respect to HOSO • . Although recent computational studies of the photochemistry have theoretically characterized the absorption characteristics of HOSO • in the UV−vis region, 30 no experimental UV−vis spectrum has been reported. Moreover, there are no theoretical or experimental studies of the UV−vis spectra of HSO 2…”

“…• are also in good agreement with the very recent CcCR computations (Table 2). 42 Recently, the UV−vis spectrum of HOSO • in the gas phase has been computationally explored 30,43 with the highly accurate multireference methods MRCI+Q and MS-CASPT2, and three absorptions above 200 nm have been predicted. The strongest absorption is located around 250 nm with an oscillator strength (f) of 0.0184 corresponding to the combination of two excitations from the ground state (X 2 A) to the third (C 2 A) and fourth (D 2 A) excited states.…”