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. 2022 Jul 30;13(1):4425.
doi: 10.1038/s41467-022-32170-x.

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

Antonio Francés-Monerris  1 , Javier Carmona-García  2   3 , Tarek Trabelsi  4 , Alfonso Saiz-Lopez  3 , James R Lyons  5 , Joseph S Francisco  6 , Daniel Roca-Sanjuán  7
Affiliations

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

Antonio Francés-Monerris et al. Nat Commun. .

Abstract

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.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Time evolution and photolysis yields of (SO)2 isomers.
a cis-OSSO and b trans-OSSO non-adiabatic molecular dynamics (NAMD) were computed with the multi-state complete-active-space second-order perturbation theory (MS-CASPT2) method (SHARC2.1/OpenMolcas),, whereas the excited-state dynamics of the other systems (c–f) were run using the time-dependent density functional theory (TD-DFT) method (SHARC2.1/Gaussian 16),. Photodynamics of cis-OSSO and trans-OSSO are clearly dominated by photocleavage to 3SO + 3SO, while 3S production arises also as another relevant light-induced decomposition product in cyclic-OS(=O)S, S=SO2, cis-OSOS, and trans-OSOS. Note that the spin multiplicity of the photoproducts corresponds to their respective ground-state, assumed to be reached at the end of the photodissociation process. The sensitivity of the yields to the number of trajectories is analyzed in Supplementary Table 4. See time evolution of relevant bond distances along the simulations in Supplementary Figs. 6, 7, 9, and 18–23. Red and yellow balls represent oxygen and sulfur atoms, respectively.
Fig. 2
Fig. 2. 3SO + 1S2O → 1SO2 + 3S2 reaction energy profile.
Two approaches of multiconfigurational quantum chemistry (CASPT2) are shown, with a wavefunction based only on the lowest-lying electronic state (1 State) or a wavefunction allowing the interaction between the 3 lowest-lying nearby states (3 States). Benchmark analyses demonstrate the latter to be of higher accuracy, while the former is used as sensitivity test (see section 4.1 in the SI). A two-step process is found via an intermediate adduct 3(SOSSO) involving energy barriers heights of at most ~2 kcal mol−1 for both approaches to access the transition states for the adduct formation (TS1) and S–O bond cleavage from the adduct (TS2). The values of the imaginary frequency that characterize the TSs are also shown. Red and yellow balls represent oxygen and sulfur atoms, respectively.
Fig. 3
Fig. 3. Estimated steady-state profiles for several key sulfur species.
a S2 profiles for several possible model conditions. “S2 Pinto” is derived from photolysis of cis-OSSO as in Pinto et al.. Turning off this reaction and using a fast 3S + 3S rate coefficient yields “S2 Nic”, and using a slower 3S + 3S rate coefficient yields “S2 Du”. In both cases 3S2 is dramatically reduced. b With the Pinto et al. S2 formation mechanism turned off, reaction of 3SO with 1S2O becomes the dominant pathway for 3S2. 3S2 and 1S2O profiles are shown for a rate constant of 1 × 10−10 cm3 s−1 (solid lines) and for a rate constant of 1 × 10−14 cm3 s−1 (dashed lines). The higher rate constant is consistent with ab initio calculations presented here and yields substantial 3S2.
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