The Structures of Elemental Sulfur

Jack M. Miller and Gary L. Wilson The Structures of Elemental Sulfur Beat Meyer... 

Elemental phosphorus is only one of several elements whose structures are polyatomic species. Another is the structure of elemental sulfur, which consists of puckered S8 rings ... 

Figure 42 shows the energy difference curve computed for linear and bent mononuclear chains with parameters appropriate for carbon. The structures of elemental sulfur, selenium and tellurium, the only examples known of elemental chains, with x = 0.75 are in accord with the computed result. These consist of spirals of atoms, the coordination about each atom being nonlinear. Notice that the curve is indeed of the... 

Meyer B (1976) The structure of elemental sulfur. Adv. Inorg. Chon. Radiochem. 18 287... 

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

Owing to its excellent thermal and mechanical stability and its rich chemistry, alumina is the most widely used support in catalysis. Although aluminium oxide exists in various structures, only three phases are of interest, namely the nonporous, crys-tallographically ordered a-Al203, and the porous amorphous t]- and y-Al203. The latter is also used as a catalyst by itself, for example in the production of elemental sulfur from H2S (the Claus process), the alkylation of phenol or the dehydration of formic acid. 

The chemistry of elemental sulfur has been reviewed before but since that time considerable progress has been made. New and simple preparative methods for several allotropes have been developed, the molecular composition of liquid sulfur has been elucidated, X-ray structural analyses of several species have been published for the first time and new analytical techniques to detect and determine molecules S in mixtures have been worked out. In addition, several new reactions of sulfur rings have been discovered. These recent developments will be reviewed in the present paper. 

Only a few molecular orbital calculations regarding the structures, conformational changes and bonding of S molecules have been published. Fimdamental reactions of sulfur rings like interconversions are only poorly understood, and more examples for the formation of homocyclic derivatives directly from the corresponding S parent molecules are likely to be found. These few comments show that much more work has to be done before the chemistry of elemental sulfur can be regarded as well known . 

Structural proofs have been given by comparing some of the preceding condensation products with compounds obtained by reacting elemental sulfur with cinnamylidene compounds. Condensation of 5-p-methoxyphenyl-3-methylthio-l, 2-dithiolium iodide with ethyl cyanoacetate3 5 4 2 folio wed by the ethanolysis of the cyano group gave the same dithiolylidene malonate as the reaction of elemental sulfur on ethyl p-methoxycinnamylidene malonate.15... 

In this connection, a very recent paper by Allinger (8) should be mentioned. A force field has been developed to permit molecular mechanics calculations on various molecular structures of elemental sulfur. The conformational characteristics of sulfur rings containing five to 12, 14, 16, 18, or 20 sulfur atoms have been examined. Comparison with experimental data is made in all cases where such data exist, and predictions are made for other cases. Ab initio molecular orbital calculations using an STO-3G basis set were carried out for cyclohexasulfur and are consistent with the molecular mechanics calculations in indicating that the chair and the twist forms are two stable conformations, with the chair about 15 kcal/mol more stable while the boat (C2t,) is a twist rotational transition state. Calculation of possible conformations of the pro-... 

Dihydrothiazines may be prepared by the action of elemental sulfur and aziridine upon ketones " the outcome of the reaction is markedly dependent upon the structure of the ketone. Thus with symmetrical ketones of type 62, e.g., pentan-3-one, heptan-4-one, cyclopentanone, cyclohexanone, and cyclooctanone, dihydrothiazines of type 63 were obtained in high yields symmetrical thiazolidines of type 64 were also formed as minor by-products. 

Reactions of disilenes with elemental sulfur92,93 probably belong to the miscellaneous rather than radical attack category. The products from the reactions of elemental sulfur with tetramesityldisilene and ( )-l,2-di-fert-butyl-l,2-dimesityldisilene have been characterized as disilathiiranes (equation 32), and X-ray structure analysis of the former (38) has been performed93. A single isomer of the latter is formed in the reaction, presumably trans. 

In an open system some isotopic discrimination between the rubber-like polymer (PCLM) and the H2S formed is expected. This is due to two stages (1) attack to form the C-Sx bonds and (2) the extraction of hydrogens to form the H2S leading to the aromatic stable thiophenes structure. This discrimination for the open systems was lately demonstrated by us also for the reaction of elemental sulfur with unsaturated hydrocarbons (unpublished results). Nevertheless, since the system described above is totally closed (hydrocarbons and elemental sulfur in ampoule), remixing of the sulfur reservoir and its isotope ratio occurs. The above results support the suggested high temperature re-reaction of previously released reduced sulfur with no isotopic discrimination under closed vessel reactions. 

Consider the structures of elements of the third period (Na—Ar). Argon with eight valence electrons has a complete octet as a result it does not form any bonds. Chlorine, sulfur, and phosphorus form molecules (CI2, Sg, and P4) in which the atoms make one, two, and three bonds, respectively ( FIGURE 12.20). Silicon forms an extended network solid in which each atom is bonded to four equidistant neighbors. Each of these elements forms 8—N bonds, where N is the number of valence electrons. This behavior can easily be understood through application of the octet rule. 

Wynberg and co-workers have prepared helicenes composed of fused thiophene and benzene units from the photocyclization of alkene precursors [134], It was shown that the effect of substituting thiophenes for benzene rings in these structures led to a blue shift of the absorption maxima. Compound 61 can be converted to the circulene structure 62 in two steps and the latter compound represents another interesting class of fused thiophene stmctures [135]. Trithiophenes with a benzene core (63) can be prepared from diacetylene-fimctionalized bithiophenes via a five-coordinate rhodium(I) intermediate by the addition of elemental sulfur [136]. A related structure (64) was reported by Pei et al. [137], along with the elaborate helicene 65. The compounds were constracted via oxidative (FeCb) cyclization reactions of 1,2-dithienyl benzene derivatives. 

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