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Liquid Sulfur 109 ly in this work the results show many similarities to the experimental find- ings regarding the composition and thermal behavior of sulfur as described in the previous sections. To simulate the first step in the photochemical polymerization of liquid sulfur by ab initio molecular dynamics calculations one electron of an S8 ring was promoted from the HOMO to the antibonding s* orbital (LUMO) which resulted in immediate ring opening [127]. This electron configuration was then relaxed. The following adjustment in a system of ten S8 rings at 400 K revealed interesting reactions [116]. The eight-atomic diradical-chain, now in the ground state, did not recombine back to the original S8 ring nor did it react with another S8 molecule to form an S16 chain but instead it iso- merized to a novel type of structure which is best described as S7=S: SR8 !SC8 !S71⁄4S ð13Þ One atom of this structure is exocyclic while the others form an asymmet- rical seven-membered ring. Even if another S8 chain was generated by elec- tron excitation in the same system of ten S8 rings it also relaxed to S7=S rather than recombining to cyclo-S8 or reacting with a neighboring S8 ring. This S7=S structure survived for a long time in the MD simulation [116]. However, if the concentration of S8 diradical-chains was increased (by excit- ing four electrons in the system of ten S8 rings) it was found that, after the excitation was terminated, the four S8 chains recombined pairwise forming two S16 chains. The reaction of an S8 chain with the S7=S species also result- ed in an S16 chain [116]. These results indicate that diradical-chains as well as the branched rings or the other isomers of S8 shown in Fig. 4 may be im- portant intermediates in the polymerization of liquid sulfur. Numerous authors have tried to model the equilibrium composition and the related physical properties of liquid sulfur theoretically assuming certain reversible reactions and adjusting the thermodynamic functions DHo and DSo in such a way that the polymerization behavior, that is the polymer con- tent of the melt at various temperatures as known at the particular time, could be simulated or “explained”. In almost all cases, however, the melt was considered to consist entirely of S8 rings (S8R) and polymeric chain-like di- radicals (SxC). The other small and/or medium sized rings were neglected despite the fact that at the time there was already experimental evidence available for the presence of such species in liquid sulfur. Nevertheless, we will discuss some of these theories or models here to illustrate the complexi- ty of the problem. Powell and Eyring [65] in 1943 proposed the equilibrium x=8 SR8 SCx with 0PDF Image | Topics in Current Chemistry
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