“…As a first step, we were interested in modeling the relative ground-state stabilities of analogues of 1 and 2a . As mentioned above, all previous structural determinations for the former were acyclic while the latter portray the thionosulfite functionality as part of a five-membered ring (e.g., 2c − e ). − ,,, ,,, A common conformational feature of two crystal structures containing the latter ring 8,9 is the presence of a 40°−45° twist in the τ(O−C−C−O) dihedral angle accompanied by unequal τ(C−O−S−S) angles of 95.6° and −127.4°. We hypothesized that obligating the ring to adopt another geometry, perhaps with greater conformational flexibility, might influence the relative ground-state stabilities of cyclic isomers of 1 and 2a .…”
Section: Resultsmentioning
confidence: 96%
“…Of particular interest is the fact that not only do compounds with the formula S 2 F 2 and R 2 S 2 O 2 exist in their branched configurations but also in the latter case they commonly exist as their unbranched dialkoxy disulfide isomers, 1 vs 2a . − In the oxygen-containing family, the known branched isomers are universally embedded in a five-membered ring, 2b . − ,, ,, We sought to answer two questions: (1) What are the factors that govern the isomeric preference between dialkoxy disulfides 1 and thionosulfites 2a ; and (2) with this knowledge, can features be designed that broaden the structural base of each of the thionosulfite and dialkoxy disulfide families? …”
Isomeric preference between cyclic dialkoxy disulfides and thionosulfites is governed by the ring size of the heterocycle. Rings smaller than seven atoms prefer the thionosulfite connectivity, whereas larger rings or acyclic analogues favor the unbranched dialkoxy disulfide structure. Density functional calculations were employed to predict the crossover point at which both constitutional isomers are of comparable stability. Follow-up synthesis provides the previously unknown eight-membered ring dialkoxy disulfide 14 and seven-membered ring thionosulfite 15 from the same reaction. X-ray crystallography for all but one of the reaction products and complementary NMR analysis furnishes insights into both solid-state and solution conformations. A long-standing issue regarding the concerted vs catalyzed isomerization pathway between XSSX and X(2)S=S has been addressed for X = RO and shown to be acid dependent.
“…As a first step, we were interested in modeling the relative ground-state stabilities of analogues of 1 and 2a . As mentioned above, all previous structural determinations for the former were acyclic while the latter portray the thionosulfite functionality as part of a five-membered ring (e.g., 2c − e ). − ,,, ,,, A common conformational feature of two crystal structures containing the latter ring 8,9 is the presence of a 40°−45° twist in the τ(O−C−C−O) dihedral angle accompanied by unequal τ(C−O−S−S) angles of 95.6° and −127.4°. We hypothesized that obligating the ring to adopt another geometry, perhaps with greater conformational flexibility, might influence the relative ground-state stabilities of cyclic isomers of 1 and 2a .…”
Section: Resultsmentioning
confidence: 96%
“…Of particular interest is the fact that not only do compounds with the formula S 2 F 2 and R 2 S 2 O 2 exist in their branched configurations but also in the latter case they commonly exist as their unbranched dialkoxy disulfide isomers, 1 vs 2a . − In the oxygen-containing family, the known branched isomers are universally embedded in a five-membered ring, 2b . − ,, ,, We sought to answer two questions: (1) What are the factors that govern the isomeric preference between dialkoxy disulfides 1 and thionosulfites 2a ; and (2) with this knowledge, can features be designed that broaden the structural base of each of the thionosulfite and dialkoxy disulfide families? …”
Isomeric preference between cyclic dialkoxy disulfides and thionosulfites is governed by the ring size of the heterocycle. Rings smaller than seven atoms prefer the thionosulfite connectivity, whereas larger rings or acyclic analogues favor the unbranched dialkoxy disulfide structure. Density functional calculations were employed to predict the crossover point at which both constitutional isomers are of comparable stability. Follow-up synthesis provides the previously unknown eight-membered ring dialkoxy disulfide 14 and seven-membered ring thionosulfite 15 from the same reaction. X-ray crystallography for all but one of the reaction products and complementary NMR analysis furnishes insights into both solid-state and solution conformations. A long-standing issue regarding the concerted vs catalyzed isomerization pathway between XSSX and X(2)S=S has been addressed for X = RO and shown to be acid dependent.
“…Over 50 years ago, Foss 2 reviewed the evidence for the existence of the sulfur branched-bond and underscored the role that thiosulfoxide structure 3 might play as an intermediate in the extrusion of elemental sulfur. Study of the sulfur monofluoride structures 3 4 and 5 in 1964 and the discovery of thionosulfites 4 (e.g., compound 6, Figure 1b) further supports the existence of the sulfur branched-bond. This structural arrangement for the thionosulfites was conclusively demonstrated in 1980 with an X-ray single-crystal structure analysis of 7.…”
The relationship of cyclic dialkoxy disulfide 11, its thionosulfite isomer 12, and the related sulfoxylate 13 has been examined. Investigations demonstrate an interconversion between thionosulfite 12 and sulfoxylate 13. This sequential transformation brings evidence that a branched-bond sulfur structure is likely involved in sulfur extrusion.
Sulfur vapor has been investigated by Raman spectroscopy in a wide temperature and pressure range (300 to 900°C, 0.05 to 0.5 MPa for overheated vapor and 0.007 to 1.65 MPa for saturated vapor). At least eight different species have been observed through their characteristic Raman lines. Well separated Raman lines have been selected for the observation of the change in vapor composition with temperature: symmetrical bending modes for the cyclic species Sg, S7, and S6 and stretching modes for the chain-like species SJ and S2. There are indications for the presence of two conformational isomers of S4, and in addition a new species has been discovered by its resonance Raman line at 635 em -1 which is tentatively assigned to chain-like molecules S. (n > 4) of helical conformation. Several relationships between the thermodynamic properties of the vapor and the Raman measurements are proposed, e.g., the determination of the temperature at which the molar fraction Xi is maximum for the overheated vapor at constant pressure; estimation of the x.lx; ratios using solution measurements of the Raman cross section for the molecules Sg, S7, S6. A careful comparison of these results with the values deduced from Rau's model results in good agreement for the behavior of most of the species apart from S7 which is underestimated in Rau's model. S7 is the dominating species in saturated sulfur vapor above 600 K until S2 takes over at ca. 1000 K.
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