Coupled-cluster study of the electronic structure and energetics of tetrasulfur, S4

Abstract

Ab initio electronic structure calculations are reported for S4. Geometric and energetic parameters are calculated using the singles and doubles coupled-cluster method, including a perturbutional correction for connected triple excitation, CCSD(T), together with systematic sequences of correlation consistent basis sets extrapolated to the complete basis set limit. The geometry for the ground state singlet C2v structure of S4 is in good agreement with the microwave structure determined for S4. There is a low-lying D2h transition state at 1.6 kcal/mol which interchanges the long S-S bond. S4 has a low-lying triplet state (3B 1u) in D2h symmetry which is 10.8 kcal/mol above the C2v singlet ground state. The S-S bond dissociation energy for S4 into two S2(3Sigma*g) molecules is predicted to be 22.8 kcal mol(-1). The S-S bond energy to form S3+S(3P) is predicted to be 64 kcal/mol.

Recombination reactions as a possible mechanism of mass-independent fractionation of sulfur isotopes in the Archean atmosphere of Earth

“…Those proceed through formation of an intermediate tetratomic complex (e.g., S + SSQ ↔ SSSQ* ↔ SSS + Q) and are also expected to be barrierless, nearly isoergic, and fast processes. This assumption is supported by ab initio calculations (33,34) that showed at least two paths for atom exchange: either through rectangular isomer on the triplet surface, 3 B 1u S 4 (D 2h ) that correlates with the ground-state 3 P sulfur atom, or through the "boat" isomer on the singlet surface, 1 A 1 S 4 (C 2v ) that correlates with photo-excited 1 …”

“…Proper inclusion of atom-exchange processes is shown to be important. This model predicts significant and equal depletions driven by reaction stoichiometry for all rare isotopes: 33 S, 34 S, and 36 S. Interestingly, the ratio of capital Δ values obtained within this model for 33 S and 36 S is −1.16, very close to the mass-independent fractionation line of the Archean rock record. This model may finally offer a mechanistic explanation for the striking mass-independent fractionation of sulfur isotopes that took place in the Archean atmosphere of Earth.…”

“…The atom-triatomic recombination reactions (5a-5e) are likely to be less important for production of S 4 in the ground electronic state 1 A 1 , because they either require an excited 1 D sulfur atom as reagent or they need an additional nonadiabatic pathway for quenching of S 4 from the electronically excited triplet state 3 B 1u to the ground electronic state (34). Thus, one should focus on diatom-diatom recombination, discussed next.…”

“…In what follows the symbol S is used for the most abundant sulfur isotope, 32 S, and Q is used for one of the rare isotopes ( 33 S, 34 S, or 36 S). It is assumed that those are present in trace amounts, and each can be treated independently from other rare isotopes.…”

“…Unfortunately, experimental studies of gas-phase sulfur chemistry and photochemistry are complicated, and their results are often not entirely certain. Moreover, sulfur has four stable isotopes ( 32 S, 33 S, 34 S, and 36 S), which leads to a multitude of possible isotopic combinations of reagents and products in the recombination reactions listed above. It is unlikely that all these processes will be characterized experimentally at the required level of detail any time soon.…”