Elemental Sulfur - PDF document

Elemental Sulfur Chemistry Department, University of Washington, Seattle, Washington lnorganic Materials Research Division, Lawrence Berkeley Laboratory, University of Berkeley, California 94 720 Received March 1975 (Revised Manuscript Received Contents I. Introduction A. General Background B. Nomenclature Molecular Variety Bond Energy 111. Solid Sulfur A. General B. Allotropes of Cyclic Molecules C. Allotropes of Polymeric Sulfur D. High-pressure Allotropes E. Low-Temperature Solids Melt below A. General B. Individual Species II. The Sulfur Bond Liquid Sulfur V. Sulfur Vapor VI. Solutions VII. Conclusion V111. References 1. Introduction A. General Background 367 367 369 369 369 369 370 372 373 373 373 378 378 379 379 380 38 1 382 382 383 384 386 last 10 years 12 sulfur rings have been the structure the third solid clooctasulfur allotrope been determined, Elemental sulfur and used for thousand years. Until 1880, the most important source of in- dustrial sulfur the volcanic Sicily. Since patents of permitted mining ever-increasing vol- of very pure in North tons of Frasch the University dress. study sulfur the recovery of high-purity elemental sulfur almost universally ultrapure sulfur been quite success- Most chemists now aware of sulfur, and of solid sulfur are dependent on nomenclature of sulfur 367 368 Chemical Reviews, Beat Meyer TABLE I. Guide to Nomenclature Molecular species Designation used Section in this review or ref Name Synonyms CY (alpha) Rhombic, orthorhombic, Muthmann’s I Monoclinic i, Muthmann’s I I, prismatic I I Monoclinic Ill, Muthmann’s IV, y-monoclinic Aten, rhombo- 5th monoclinic, Korinth 4th monoclinic, Korinth Tetragonal, Korinth Erametsa Erametsa Cycloocta-S Or thorh om bic-(Y 1II.B Monoclinic-fl 1II.B Cycl oocta-S Cycl oocta-S Monocl i nic-y 1II.B 6, 7 6 (delta) Cycloocta-S Allotropes of S, E (epsilon) Cyclohexa-S Rhom bohedral 6, 7 Allotrope of S, Cycl oocta-S 6, 7 Cycl oocta-S Cycl oocta-S Allotrope of S, Allotrope of S, Allotrope of S, Allotrope of S, Cycloocta-S, Solid or liquid Polymeric-S Solid polymeric Allotrope of S, Allotrope of S, Frozen liquid 6, 7 6, 7 6 6 6 1II.C 1V.B 1II.C 6, 7 6, 7 1V.A 6 1II.B Insoluble, white, supersu bl imation Triclinic I.1 Mixture Cycloocta-S Cycloocta-S Ring mixture Cyclohexa-S Cycloocta-S Mixture Polycatena-S Mixture Mixture Mixture Cyclohexa-S Allotrope of S, Fibrous Fibrous Po I y mer ic Fibrous Polymeric III.C, D I I I.C, D 1II.C III.C, D I1I.C Cycloocta-S Allotrope of S, Solid Polymeric Rhombohedral Solid, Polymeric Rhombohedral 6 1II.C m n Aten Braun Engel Korinth Muthmann Schmidt Amorphous Cubic See E, p See p See E, p See 8, n, 8, t See CY, P, Y, 6 See orthorhombic-S,, High pressure cubic $’, Q, phase II “Crystex,” super- sublimated Phase I, white, w, p, x High pressure metallic Erametsa’s red P plastic Cyclohexa-S Mixture Cyclohexa-S Cycloocta-S Cycloocta-S Cyclododeca-S Mixture 1II.B 6 1II.B 6 6 111.8 1II.C 111.0 Solid, polymeric High pressure forms Fibrous Insoluble Fibrous Insoluble Catenapoly-S Mixture III.C, D 1II.C Laminar Meta I1 ic Catenapoly-S ? La mi nar High pressure forms Photosulf ur Quenched liquid Trapped vapor Trapped vapor Trapped vapor Trapped vapor Trapped vapor Trapped vapor Trapped vapor Allotrope of S, III.C, D 1II.D Photosulfur Black ? ? Mixture Mix tu re Mixture VI 1II.E Brown Green Orange Purple Red 1II.E 1II.E 6 1II.E 1II.E 1II.E 1II.E 6 6 Mixture Mixture Mix tu re Mixture Mixture Elemental Sulfur Chemical Reviews, 369 Orbital Ionization Configuration Orbital I,, E" X S~D~DD P 12.4 2.4 7.4 . .. (sp3) 2 (5 p 3) 25 p3s p3 5P3 15.5 4.8 10.1 (sp2)2(sp2)*5p277 SP2 16.3 5.4 10.9 2.8 7.7 a Reference 32. short guide names and syn- cussed, and the present experimental knowledge with the solid, liquid, and while section nonpolar and solvents. The latter includes a negative elemental ions. and the sparse will be the discus- 6. Nomenclature reasons for the confusing multitude nomenclatures which use. Several tropes were discovered at a the molecular struc- the nature not yet under- stood. As a matter and many were known that sulfur an element. by various authors nomenclature have failed, as they third modi- of Muthmann, for example, of Korinth, and is also many types a system- yields long and is between clumsy or ambiguous names, al names likely remain use. Table chosen for this lists some doubt, the the most name is will be sparingly as remain the designations for the three fully identified solid allotropes for identifying cyclohexasulfur are one allotrope exists. different class of compounds. These used as comprehensive terms for identifying well-known, but characterized mixtures: refers to all the sulfur is a fibrous solid allotrope, obtained quenching liquid polymeric Electronic Structure The sulfur same number valence electrons as oxygen. have physical those of and ozone. has a ground state of excited electronic levelsz9 correspond those of a well-known a bent structure, analogous most likely is the the chemistry sulfur has a pronounced tendency for most frequently based on electron structure the atom. e band +::- Electronic structure derived from Q for various low-lying unoccupied it is that the and 3d of sulfur the participation of 2s and 2p discussion of the wave-mechanical calculations the ionization states of ionization potentials, electron Mulliken's electronegativity for various Considerations regard- wave functions,33 radial functions field calculations, orbital energies al energies are only about of the p-orbital energies32) justify the belief initio calculation arrives at conclusion.35 Recent calculations by Miller and catena-S6 that the d orbitals of divalent sulfur compounds37 is negligible, but they served that of energy d orbitals energies, and that calculated with d orbitals observed spectra bet- than those without d orbitals. The same entire series of sulfur and chains, and ions. The d orbitals the presence as was shown by Craig and Zauli3' for phase and the atomic orbitals been given has described sulfur chains a one-electron and obtained energy levels compatible with those of Palma.42 Muller developed a three-dimensional yields good various sizes. charge distribution and ions has been cal- a Huckel model Meyer, Peter, and 6. Bond Geometry observed geometry The sulfur probably the bond geometry. This Chemical Reviews, TABLE Ill. Representative S-S Bond Distances t r.095 LJ- -LJ the sulfur three atoms three turns. ic sulfur. a torsion Figure 2, terminal atoms, suggest the of further bonds. and others have proposed that dihedral angle is determined between lone pairs on adjacent atoms. bond distances has cussed by bond distances bonding. Partial bonds shorter than single bond value of 2.08 using the basis for bonds. His be discussed unstrained bond probably about 2.06 bond distance to the ratio hybridization. Torsion around the bond is restricted. energy of shows the and right-handed of fibrous sulfur, which atoms. Table indicates the conformations some other sul- fur compounds. Some species can exist in two different con- depending on the cation the sulfur chain shares solid phase. The observed bond data are listed Molecular Variety controversy whether sulfur various phases rings, chains, the chain ends within bond- distance.49 The necessary are known to for molecules with six or considerable distortion the normal divalent sulfur bond occur before a be formed. Table chain configurations, and the distance between terminal atoms for some short chains. on a and are based on bond as- bond angle supplement of an internal tational angle bond distance good enough for mental ionization patterns5' and thermodynamic consider- Molecule S-S bond length. .4 Re€ 52 S2F2 s20,2- Me252 s, (C FA2 S,(Me)2 580 Diphenyl disulfide s 7-2 &-Cystine 8 hydrochloride si * H2S2 56 s* S, sn0,2- n = 3 n=4 n=5 n=6 s20,2- s20,2- s,o,z- S4N4 1.887 1.89 2.00 2.03 2.03 2.038 2.065 2.04 2.04 2.05 2.04-2.20 2.053 2.055 2.057 2.060 2.066 2.15 2.02 2.04; 2.12 2.04; 2.10 2.15 2.209 2.389 2.58 29 a b d e f C g 106b 105 h 137 7 7 145 j k 1 m n P 4 1 0 (1965).?H. J. Trans. Faraday J. Haase, 891 (1963). Dreizler, and H. D. Rudolph, Z. Naturforsch. E. Stanley, Acta Crystallogr., 9, 897 (1956). TABLE IV. Conformations of Catena Sulfur Compounds Conformation0 Compound Di-2-iodoethyl trisulfide t r i Dimethanesulfonyl trisulfide barium pentathionate pentathionate hydrate barium pentathionate +- Cyanogen trisulfide dihydrate hexasulf ide trans-Dichlorodienylcobalt (I I I) hexa- thionate monohydrate +-+ Potassium barium hexathionate @The slgn corresponds to the sign of the internal rotational angle: and --corresponds unfavorable distances and contain inequivalent atoms, exist in all Obviously, the larger the chain, the greater is the probabili- that some ring clo- neither thermodynamic nor increases with size. Experiments show that the most stable molecules, because of symmetry considerations and because Elemental Sulfur Chemical Reviews, 1976, Vol. 76, No. 3 371 Favoring Ring Distance No. of between atoms terminal in atoms, chain A Chain conformations 5 6 7 8 9 10 11 12 20 3.59 2.06 1.01 3.68 1.56 2.94 2.43 3.26 3.71 1.07 3.24 3.42 3.70 3.72 3.75 2.01 2.23 2.27 2.54 2.59 2.63 0.20 0.53 0.66 0.91 1.57 +-+ +-+- +-+-+ ++-+- +-+--- ++-+-+ +i-+-+- +-+-+-+ +++-+-+ ++-i-+-- +-+-+--+ +-+--++- +-++--+- ++-+--++ +++-+-+- ++-+-+--+ i-++---+-+ ++-i--++- +++-i-+-- ++--+-++- ++--++---+ +-I-----++-+- -++-+--++-+ +I---+-++-- ++--++--+i +--f----+-+ ++ t--+ + i---+ + t i+t a See ref 49. atoms across Experimental bond data of these molecules are well established, chair structure, thermodynamic stable and was used his predecessors basis for determining dure constituted the best possible approach at that successful, even differs from other rings, erable cross-ring the amazingly stable that Pauling's assumptions even though his unnecessarily assumptions about bond conformations led to forbid Large rings are unstable because higher order bond contributions, are awkward, have unshielded atoms. Furthermore, the have low-lying the resulting chains undergo quick -s,- - s8 f -sx-8- large rings rings, whenever the chains are long Schmidtg showed that large rings can be made, that they and that such rings, once formed, are stable than been anticipated. chains having given number atoms have very different structure. Sulfur chains, except while rings have fully paired electrons. rings correspond chains with nal groups, such as, example, sulfanes ference between chain shows chemical reac- as well physical properties, for example, a 2.060a 0 Figure 4. The structures of S8 and SI*, a b S 18 C s20 d Figure 5. Views of (a) Ss, (b) S,, (c) S18, and (d) S20. Molecular data are summarized in Table XI, the structures of the solids in Table XIII. known rings exhibit yellow hues.58 spectra, as far as transition energies38 are shown the corresponding yellow the 280-nm region. With increas- ing chain length, the absorption shifts to the sequence tion energies calculated60 with one-electron model, as those obtained with extended Huckel elemental free-radical chains absorb the visible. served absorption short chain members the calculated transitions are also shown predict that with increasing chain length the transition converges toward 850 nm, transition energies intrinsic property homologue series. thoroughly discussed for the alkane free-radical chains interaction be- terminal atoms, via the chain, becomes negligible. the sulfanes6* of the 372 Chemical Reviews, 1976, Vol. 76, No. 3 Beat Meyer S” 800 6oo nm 200 400 First allowed sulfur chains, CIS -0.14 P branched D3h non sym 8 tetrahedron Td Figure 7. Six isomers of S4. The numbers on the terminal atoms in- dicate the electronic charge. chains indicate atoms are sufficient to interrupt intra- chain communication between terminal long chains act intermediate chain members a large ring. Sulfur chains,63 absorbing the visible, are deeply colored, according to these considerations, be black. However, polymeric sulfur is in the seen in Figure 14b. polymeric sulfur,58 the eluci- the structure, S4 and bond geom- etry prevents formation ring. However, this molecule, several other structures are feasible. Fig- shows six possible isomers Huckel cal- on spectroscopic suggest that the the branched comparable stability. All others, including the pla- ring, are calculated to be significantly less stable. shown later experimental evidence determine the structure S4, even evidence seems to favor Summary of ~~ Atoms Ionb per molecule Ringa Chaina +2 -1 -2 1 2 3 4 5 6 7 8 9 10 11 12 18 r that the indicate solid, liquid, and ions occur indicates chain, r indicates whether sulfur forms branched molecules raised periodically, but always been rejected, largely with experimental evidence regarding larger mole- cules. However, the idea of a branched not as might seem for chains, because isovalent with charge, indicat- terminal atoms in all pears also reasonable. It should the S42+ ion, has quite a different electron config- that of to that observed for Se42+. liquid. Table a nonsymmetric would be stressed. A require a quite unnatural bond the solid, a diradical chain a structural elucidation isoelectronic with bring further information. species which have been number of different sulfur molecules exist. According to their properties can be four groups. observed rings containing up atoms can be isolated as pure solids. Small molecules the vapor. Large, polymeric molecules occur in the while ions are formed only ionic solution. Each group the phase normally exists. Energy and energy have measured for data for various allotropes. dissociation energy’8i69.70 the position gaseous sulfur cules, believed to be were calculated by Berkowitzi2 a three-dimensional electron model reliable absolute values, but are quite reliable The average bond Chemical Reviews, 373 Elemental Sulfur TABLE VII. v, of Ten Sulfur Allotropes Species v,, crn-' T, K Ref 718 (9) s3 590 (m) 5.4 668 (m) S,Z+ 584 '6 471 (s) s, 481 (s) sx 559 (9) s, 475 (s) SI 2 459 (s) SCO 456 (I) (5) 880 650 20 300 880 300 300 30 200 400 3 00 206 199 199,200 64, 65 206 91 103 111, 114 140 109 192 TABLE VIII. Thermal Data for Phase Transitions well-established solid allotropes containing cyclo- hexa-S, cyclododeca-S, other sulfur sulfur compounds. Another sulfur compounds hot liquid comprises insoluble long helices polymeric sulfur, which is easily prepared, has well-known bulk proper- ties. However, its structure is incompletely character- it contains helices mixed with identified form sulfur, which identical with pressure allotropes which been reported. Transition Process or reaction AH, AS, cal/ kcal/g-atorna deg,g-atom T, K Ref P Sublimation 01 E P Fusion (Y P h, Polymerization a-S,(s) -+ P-S,(S) a-S,(s) + cyclo-S,(g) P-S,(S) -* cYClo-S,(g) E-S,(S) -+ cyclo-S,(g) cU-S,(s) - cycio-S,(I) + ?b P-S,(S) - cyclo-S,(I) + ?b cyclo-S,(I) - catena-S,(I) catena-S,(I) + cyclo-S,(I) 368.46 i 0.1 368.5 368.5 300 383C 392.9c 43 2 442.8 0.096 2.979 2.883 4.02 0.507 0.3842 0.396d 119, 128 8.38 6 2.88 71, 119, 122, --* catena-S,(I) Vaporization Si(l1 * Si(S) 717.824e 2.5 3.5 6 = 444.674 "C the melt CSee also temperature reference point on the International Practical Temperature Scale, ref 201. except for rings are the chains. bond energies polysulfides with activation energies for are deduced in the case can totally alter processes56 such as bond ducing ionic reactions which proceed and by than the homolytic scission. problem is which is not satisfactorily explained. Since reliable for the preparation pure sulfur allotropes have available, accurate thermochemical data can be pected within the next few is reflected in the Raman and ir frequencies also provide valuable information. lists the stretching for nine elemental molecules for which assignment seems reliable. Except for which the for which branched struc- ture is possible, the trend follows is clearly to the bond strain, discussed above. be discussed connection with the of x-ray spectra in terms of bonds has Ill. Solid Sulfur A. General sulfur is orthorhombic a-sulfur cycloocta-S molecules. converts into monoclinic 0-sulfur, which melts at 119.6 can be obtained from monoclinic y-sulfur these allotropes be discussed following sec- tions which are the molecular species solid is composed. data74 for the con- forms are given in is well cycloocta-S allotropes are separately viewed in sulfur, as is recently re- are listed in Most are close to those observed West,74 except for 0-sulfur, for which thermal conductivity decreases from it is ranks with best thermal insulators. Recent have been reviewed Mechanical properties solid allotropes been reviewed reference sources list different are listed in be discussed on sulfur Cyclic Molecules the formula occur in equilibrium with chains in liquid sulfur near the melt- the fraction called rings have been found spectrometer in the vapor.'* pure solid allotropes are not very During the seven new metastable allotropes with the 374 Chemical Reviews, Beat Meyer u r (cal/g-at om Sulfur species T, “K @-s(s) ~-S(S)~ Liquid Vapor 10 15 20 25 40 50 60 100 150 200 298.15 368.54 400 433 440 460 717.75 1000 0.103 0.348 0.608 0.868 1.465 1.795 2.089 3.090 3.990 4.650 5.430 5.778 0.163 0.412 0.906 1.490 1.808 2.101 3.077 4.072 4.81 7 5.551 6.053 7.579 5.569 11.930 10.800 9.925 7.694 5.252 5.137 a Reference 74. b Reference 75. Reference 176 gives 7.423. of Elemental p, T, a p, T, Torr “C atm OC 10-5 39.0 1 444.61 10- 58.8 2 495 10-3 81.1 5 5 10-2 106.9 10 644 lo-’ 141 20 721 1 186 40 800 10 244.9 50 83 3 100 328 100 936 760 444.61 200 1035 West and Men~ies.’~ average values; and Rau.96’203 for example,a0 prepared from reaction sulfanee’ with atoms, and chlorosulfane with H2S, + S(j2-n)C12 - Si2 + 2HCI With this reaction, are best prepared titanium dicyclopentadienyl pentasulfide using procedures to those 1903 prepared (NH4)2PtS15r known to contain three chains forming six- membered rings with central platinum molybdenuma5 contains sulfur atoms five-membered ring: posed the synthesis of has not yet been synthesized, but Schmidt52 pro- (C&i5)2M0S4 + SC12 + S5 + (C~H~)~MOCI~ Schmidt reports this allotrope been explained above that expected to unstable, because TABLE XI. Structural Parameters of Sulfur Molecules Torsion angle, angle, Molecule length, A deg deg Ref 52 1.889 29 0.7 98.3 1.4 86.1 102 (93) cyclopenta-S becomes from the separa- terminal atoms47 V). Semiempirical indicate that the chain isomer be thermodynamically most stable. 8.60 eV. 2. S6, Cyclohexasulfur by the reac- concentrated hydrochloric acid with saturated solu- Atena7 identified the rhombohedral proposed their presence liquid sulfur. Kellasaa liquid constituent responsible polymerization. However, most chemists ignored mined the chair form, shown bond angle are comparable torsion angle than that of bond length torsion angle and Raman been recorded by and Bromelsgl are listed amplitudes at 300 K, coordinate analysis. been published the equilibrium liquid,95 believed to occur as been calculated with and by calculated transition energies charge distribution for the catenahexa-S. times faster with nucleophil- agents” than decomposesg7 forming best method to prepare Dilute solutions ether are combined to form cyclohexa-S H2S4 + sic12 + sg + 2HCI orange-red solid can be purified from toluene been measured rhombohedral crystals have the highest density of Obviously, the are very in the unit group R3-C3?. constants are Chemical Reviews, and Raman Four Sulfur Allotropes sa (30 K) S,2+ (planar) s.5 (8 fundamentals) SI (15 fundamentals) (1 1 fundamentals) Designa tiona s12 (20 fundamentals) A,, V, = 548 A,, V, = 471 v, = 262 v, = 481 v, = 236 A,, v, = 313 v, = 274 A,, v,=390 v, = 397 V? = 530 E, v, = 463 V, = 180 v, = 516 v, = 180 E, v,=460 E, v,=448 v8= 202 v8 = 145 B,, V, = 330 Ref 64, 65 Ref 9 1-93 Ref 103 v,, = 356 v,, = 274 a R = Raman active, I = inactive, and Ir = infrared active. Solid Allotropes v, = 475 v, = 218 a, R v, = 459 V, = 243 b, Ir v, = 471 v, = 191 e, lr v, = 465 v, = 475 v8 = 152 e, R v, = 425 v, = 266 v, = 86 v,= 62 v,, = 437 v,, = 248 e3 R Ref 107-115, 121 Ref 140 V, = 411 bl I Mole- Unit cule Space group cella a b C P, de, Color gm/cm3 dec, OC Ref R3-CSi2 3-18 ? 16-112 Fddd-D,,” 16-128 P~/c-C,,,~ 4-32 P2 , fa-c,), 6-48 Pnnm-D,h l 2-24 P2,2,2,-D24 4-72 Pbcn-DZh14 4-80 C~rn2,-C,,,~~ 16Ob 10.818 21.77 10.4646 10.778 8.442 4.730 21.152 18.580 13.8 cla = 0.3956 20.97 12.8660 10.844 13.025 9.104 11.441 13.181 4 X 8.10 4.280 6.09 24.4860 10.924 9.356 14.574 7.581 8.600 9.25 Orange-red Yellow Yellow 95.80 Yellow 124” 98’ Light yellow Pale yellow Lemon yellow Pale yellow Yellow 2.209 2.090 2.069 1.94 2.19 2.036 2.090 2.016 2.0 1 50-60 39 94 (112) 133 - 20 148 128 104 124-125 three turns. solid allotropes but under mass spectrometer,12 molecules vaporize energy5’ of 3. S7, Cycloheptasulfur the reaction (C5H&TiS5 + S2C12 - S7 + (C5H5)2TiC12 has the structure shown Figure 5b. The Zahorszky.’’l Not x-ray structure’02 and this fact. are expected. light yellow have a density of group of this allotrope, Sixteen molecules, chains, but not Allotropes of Cyclooctasulfur symmetry is structure is Cycloocta-S has the crown shape shown in Figure 4a. The bond angle molecule occurs the solid, liquid, and configuration at cross-ring resonance. 131, 132 146, 149 a chair configuration might exist such a chair diagonal sulfur above and structure. A similar been found by Steudel for for mixed and by Weiss Above 150 observed. The is estimated to be molecule is sensitive visible light.55 this is at 280 nm, or whether has a triplet absorption have been and discussed Clark,41 Miller,36 Gibbons4’ discussed the en- except for recent have studied confirmed the assignment of the measured by von A normal coordinate’ sis has been by Cyvin, and mean square have been Cycloocta-S can crystallize different lattices; the structure three solid allotropes Orthorhombic a-Sulfur accurate structure established the and Pawley and Rinaldi118 and measured the termolecular distances. molecular packing is com- plex.’17 Figure 8a shows a “crankshaft” structure’ this allotrope, by many contain coaxially stacked rings. 376 Chemical Reviews, Beat Meyer C solid allotropes a-sulfur, (b) views are the unit and the density crystal growth by Thackray’ tals with only very few dislocations can be of CSp, frequency at has long been for a not easily sulfur. Instead, they and Raman Ward,Io8 Ozin,Io9 These data observed crystals at TABLE XIV. Melting Point of Allotropes Allo- trope M~,~c Remarks Ref a-S 112.8 Single crystal 115.11 Microcrystal &S 114.6 “Natural” 119.60 and obsd 120.4 133 yS 106.8 108 108.6 6-S 106.0 WS 77;90; 160 104 s, 75 104 s, (50- 1 s, (39-1 S,, 148 S,, 128 S,, 124 Microcrystal “Ideal” calcd Classic Optical, DTA Mi crocrysta I Microcrystal Optical, TDA, DTA Optical Classic Decornpositi on Decomposition Decomposition Decornposi t ion Decomposi t ion 122, 174 a Thermodynamic melting point. which are resolved at the Ward’” published crystal splitting effects. electronic spectrum has been discussed by and Gibbons.40 latter concludes that ductivity of contributions: (a) hole mobili- ty, which has a value cm2/V.s, and negative temperature coefficient, port in band (Figure strong vibrational conductivity. Gibbons gives a trons are located an individual molecule for several vibra- tions, i.e., that ions can be rate in was measured by Hampton and Sherwood.’20 The conductivity was measured Spear and and Kuramoto and a-sulfur is the heat of b. Monoclinic &Sulfur structure of @-sulfur was determined by atoms, occupy the A view of the along the Figure 8b. @-Sulfur forms melts at analysis has been conducted described formation Thackrayllg the solid. Thermal data for transitions are in VIII. The smaller than that of been some controversy about a phase caused by evaporation of water Recently new values have been by Mont- anomaly at been described by c. y-Monoclinic Sulfur y-sulfur, first described Elemental Sulfur Chemical Reviews, 377 been determined by Watanabe,13’ 1974, who “sheared penny by de Haan132 (Figure allotrope can its melt, but the best prepare the light yellow which slowly decompose pyridine.I3’ y-Sulfur large needles. constants are the struc- different choices of axes. has been molecules occupy one density of this allo- is 2.19 g/cm3, i.e., higher than d. Other Allotropes of Cyclooctasulfur years about 24 allotropes containing cyclo-S8 have Some of these are listed doubtful whether other than y-sulfur are Instead, most of the other letter allotropes are constitute merely crystal forms. progress has been made7 developing reliable no new information we can omit discussion of confusing list for details 5. S9, Cycloenneasulfur cyclo-Sg by the reaction Wilhelm133 prepared needles of (C5H5)2TiS5 + S4C12 -+ (C5H&TiC12 f Sg this compound has not yet been published. the vapor. not published. 6. Slo, Cyclodecasulfur Schmidt and Wilhelmg9 prepared yellow-green by the as their solubilities differ substantially.100 better yield can be the following is conducted at (C5H5)2TiS5 4- 2SO2C12 + Slo + 2S02 + (C5H5)2TiC12 structure has been published, and indicated limited stability. solid must 7. SI 1, Cycloundecasulfur tion Wilhelm134 prepared cyclo-S1l by the properties have not yet of sulfanes and proper chain Schmidt and Wilhelm52i’33 prepared by the 2HzS4 + 2S2C12 + Si2 + 4 HCI H2S8 S4C12 + SI2 + 2HCI TABLE XV. Preparation Methods for Metastable Allotropes Species Reagents (C,H,),MoS, + SCI, (a j HS,O,- + HCI (b) S,CI, + H,S, (C,H,),TiS, + S,CI, CuSSCOC,H, + pyridine (C,H,),TiS, + S,CI, (a) H,S, + S,CI, (b) (C,H,),TiS, + SO,CI, (C,H,),TiS, + S6CI, H,S, + S,CI, H,S8 + SIOCI2 H,S,O + S,OCI, 82 86 74, 99 82, 85 131 133, 134 79 138 135, 136 141 141 first reaction135 second,136 discov- later, has reaction is dilute solution the reagents have intermediate chain H-S- - -Sx-S-CI complete ring before reaction with Schmidt and since prepared eight other with similar the structure137 shown the solid slightly distorted The bond bond length These values to those for i.e., the unperturbed bond value. discussed above, this fits prediction for the bond value of sulfur even though did not molecule from metastable allotropes. However, experiments show that stable than It is a decomposition product and benzene has been discussed by The mass studied by at 148 were determined by Hellner and The unit 24 atoms. Mixed crystals Raman and was studied by Steu- the 20 fundamentals. 9. Sl8, Cyclooctadecasulfur mixtures141 of the average formula has been sulfane and H2S8 + S1oC12 Si8 2HCI starting materials cannot pure form, shorter sulfanes prevent formation and other a pro- lemon colored allo- a mixture with and must be sepa- This unexpectedly stable 378 Chemical Reviews, 1976, Vol. 76, No. 3 Beat Meyer at 128 dark for several a noticeable the x-ray diffraction pat- bond angle properties are those of and similar those of fibrous sulfur lattice constants’42 are 72 atoms, form a and the density prepared intermediates S~hmidt’~~-’~~ prepared S20 by combination of carefully H2S10 + S1oC12 + S20 + 2HCI at 124 already decomposes solution at pale yellow density of structure of the molecule, bond values’42 are: bond length bond angle those of and fibrous rameters are molecules, with C. Allotropes of Polymeric Sulfur the allotropes described below contain molecule forms a section a left- Three turns the helix contain bond length bond angle These values believed that they represent unperturbed values of the Solid polycatenasulfur rubbery sulfur, sulfur, laminar sulfur, and insoluble supersublimation, white, of these forms are or less well-defined helices, cyclo-S8, and other depending on are made. Their with time. impurities are occur within less than 1 month. some allotropes, the helices of the nonpolymeric fraction other solvents. Donohue7 summarized the structural information helices are stacked or curled. forms are which helices are mainly parallel during their are at a “cross-grained” a “plywood” 6oo t liquid 0 20 40 60 pressure (kbar) Ward and have been quenched is 1. Fibrous Sulfur 1931, and data by Geller’46 indicate contains 160 The structural conducted on diffraction patterns prepared at 27 kbars. density of brous sulfur Lind and Geller’46 believe that their preparation of phase yields single been indexed along the axis, and 72 axis. Thermal tran- sition, polarization, and other 2. Lakinar Sulfur Laminar sulfur first described to be with Geller’s and 20 kbars (Figure been discussed by Dono- incomplete; it is similar to, with, insoluble w-sulfur and “second fibrous” sulfur of of freshly drawn fibrous along the High-pressure Allotropes Various sulfur allotropes can obtained by heating sulfur pressure. Figure shows some the high a melting curve which a curve Ward and published an- melting curve which matches melting curve is also Figure 9, but not indicated, not yet been firmed. However, Figure shows the laminar and fibrous sulfur’49 have been quenched. sulfur zone well with the molar volume and Schneider162 report the optical properties pressure, and Kuballa and Elemental Sulfur Chemical Reviews, 379 report a differential thermal analysis pressure. Block plain the divergent observations reported for pressures above 24 kbars temperatures above 250 this point for 3 without observing equilibrium, plain this the poor thermal con- authors using different equipment techniques observe phases! The best established high-pressure forms are In contrast, has not likely that contain interesting E. Low-Temperature Solids stored below below quickly quenched hot liq- hot sulfur contains a variety metastable solids.63-165 have shown these solids contain other allotropes. the next be shown that these quenched phases been repeatedly determine the composition of hot fraction. However, great experimental a significant fraction species is to is such a poor thermal conduc- exchange is slow. molecules formed in quenched solids are bination products. are discussed in the next sections together with those the starting absorption spectra various molecules in frozen low-temperature solution in Figure matrices from sulfur vapor photolysisi67 of is best prepared from the vapor, gentle photolysis of S3CI2 in frozen solution. prepared from in matrices. can be studied in frozen solution. Polymeric sulfur can be quenched a thin is yellow. it contains small molecules which recombine the species be discussed the section in the in which they are IV. Liquid Sulfur the molecular composition sulfur differ three distinct temperature discussed separately. A. The Melt below 150 OC different melting points sulfur have gives a selection for the ing points @-sulfur, which constitutes the stable solid melting point, freezing point is influenced the melt, high-purity sulfur was not until 1942, most old are unreliable; the freezing points are too Small droplets sulfur can particles with a diameter at 25 observed nucleation a microscope, determined a crystalliza- tion rate have observed formation ing crystallization molten sulfur. best present TABLE XVI. Freezing Point Depression Av no. TA - Tf, "C Equilibrated of at T, "C Calcd Obsd atorns/ring 120 4.6 13.8 130 5.7 4.9 14.6 140 7.1 5.9 15.7 150 9.1 7.6 17.6 174, 179, the melting point is 119.6 observed melting indicates that the ideal melting point might freezing point equilibrated melt been called "natural" melting best value to be 1.9 cal/g-atom while the higher values are valid the liquid at 120 the expansion the viscosity and Fanellii68 reported in that the melting point freezing point recognized that this was not to impurities, which could various temperatures. Aten178 proposed 1913 that the freezing temperature autodissociation of sulfur, forming a the concentration of which determined freezing point depression. Krebsg5 ar- 1953 in the existence small rings in liquid determined the concentration x-sulfur in liquid it, extracting the solid with which precipitates from the extract cooling to problem with this fraction is that it its composition for several insulator such the solid preserve the composition the liquid. physical chemists familiar with paper tended to distrust explanations based on that at least seven can be found in all solidified melts. 1967 Krebs another very studyg5 on extracted with a mixture these solvents, a distribution while the coefficient is 0.11. After 700 distribution steps, separated into three fractions, having a molecular fraction containing rings, with lyzed the freezing point depression the con- determined for this molecule the polymerization higher temperature. the freezing point depression average number ring calculated with the help a polymer model. the equilibrium composition melt is not yet "natural" thermodynamic melting point is determine because slow kinetics. It now assumed this point, solid monoclinic equilibrium with a liquid mixture "ideal" melting points can the slow of the cyclooctasulfur "ideal" melting point for the transition 380 Chemical Reviews, 1976, Vol. 76, No. 3 2- I_ 8 8 Beat Meyer 100 200 300 400 T Temperature sulfur components small species, computed from liquid cyclooctasulfur is not yet history reproducibly freezing point indicates not only various hot sulfurs proceeds via dif- paths and leads different metastable tures of metastable species, probably mostly rings. temperature dependence of the equilibrium composi- various ob- served and estimated is shown observed an unusual molar polarization proposed a cyclo-S8 a chair configuration as a further of the liquid. It is now certain other than Whether, and much, ca- tena-S8 or other is not yet es- tablished. However, Miller,36 and show that is sufficient complexes. Such complexes have been small concentration sulfur at This observation detail by melting point the samples. also depend because of slow kinetics.164 B. Polymerization at Tx = 159.4 OC fer a discontinuity. Figure for example, the change.182 Points close to the transition were observed after of equilibration. were interpreted as singularity, a which is of sound,IB3 the surface tension,‘65 and many other proper- have been measured temperature region. However, the most striking called temperature, is the sudden change Eotvosse and long ago described the well-known of the liquid, as did Schenk. Hammick5 and Schenk5 determined the weight Bacon and the influence viscosity, and showed can be 1 I I I I I I -002 0 0.02 004 T-Ti Figure 11. Density of liquid sulfur at 159 OC (after Pate11e2). 1 II Ill 2.3 160 c E F b 220- U m Y I- 1 2.10 20 40 100 200 300 T OC 180 200 Tme (sed Temperature dependence polymerization rate (after KlementlQ‘). the viscosity again, as did and Tobolsky and Eisenberg7’ developed a explains the viscosity cyclo-S8 r; catena48 (1) (11) catena-S8 + cyclo-S8 - catena-S8x2 This theory has been discussed and thoroughly small modifications other than possible the of other The thermodynamic of the West and measured the heat (Table balla, and conducted a differential thermal used laser Raman polymerization, which sity changes of bands at 456, 416, and discussed the of the a chain-end interchange terchange, and concluded that bond interchange viscous sulfur. of equilibration have been in- vestigated by atures, and found the observed as early as 1909 that the equilibrium is on the degree Wiewiorowskiig5 for CS2, of the causes sulfur the melting point. showed that lowers the polymerization for over reduces the average chain length of pressure on polymerization Elemental Sulfur Chemical Reviews, 381 I I a, I\ LI~UK Absorption Edge of Liquid Sulfur I I 'b -12 I IJ I I I I 100 200 300 400 500 Tqerature "C computed from concentration from used the visible absorption sulfur was Gardner and signal, but suggested that the hot liquid, but by Poulis and small free concentrations. Koningsberger'81 pleted a study of of pure sulfur, and and of sulfur doped He obtained the free Figure 13. Koningsberger spin concentration the polymer concentra- which can be polymer P length, and reported a similar temperature dependence. Figure that the almost identical computed by thermodynamic considerations. this question not raise doubts regard- validity of the polymerization theory; it merely raises small species uid at the polymerization temperature, is pale yellow. corresponding absorption spectrum is shown Figure 14a. an organic same figure, edge. At but the absorption is now of the that of Liquid polymeric and has an edge at sulfur, obtained by a thin sulfur at while cyclo-S8 turns snow white, and has the spectrum indicated Figure 14b. not dark red, as impure liquid organic impurities. will be the next section; the absence of deep dark sulfur at remains a because the free-radical to all colored. Several pos- been proposed. organic impurity is sufficient explanation revives the 3CO 400 530 600 nrn Figure 14. Visible absorption edge of liquid sulfur at (a) 120, (b) 250, and (c) 500 OC. The absorption of individual components was ob- tained for Sa at -70 OC in EDTA, for polymeric sulfur at -196 OC, as a thick film; SB, S4. and Ss in EDTA at -196 OC; and Sp at -253 OC, in a rare gas matrix (after ref 58 and 63). long intertwined rings. However, charge complex situation best. Sulfur is known to form indicate that Liquid Sulfur high temperature, the viscosity sulfur de- creases rapidly, the color then brown, extremely reac- in all except most pure color effect is obscured by irreversible darkening due organic impurities. boiling has the as the equilibrium vapor.63 The the spectrum obtained on hot thin liquid films, liquid nitrogen. Such films retain the absorption curve the spectra gas phase, or low temperature else is about hot reactive toward almost every chemical. plained by the presence thiozone, and point201 of sulfur, Davy, and by Dumas, transition is a primary, point, according IUPAC.201 The of sul- is at 1040 indicate that liquid thermodynamic considerations that just below the has a the vapor.96,203 seen whether However, the composition the vapor12.50,51~96 liquid" (Figure 10) extrapolate smoothly data (Figure 15). small molecules hot liquid Reviews, 1976, Beat Meyer TABLE XVII. Critical Data I' 'I Quantity Value TC 1313 K = 1040 "C 5 96 Pc 179.7 atm 203 200 atm 20 2 VC 158 cm3/mol dC 0.563 g/cm3 VC 2.8 at o ms/m olecu le 8 I I I Tc 1040% nl 4 \ J I I 400 800 1200 T "C of atoms at the casts light on unknown phase, surely must Sulfur Vapor vapor pressure of sulfur from room measured by and Meyer.15 Some vapor pressure are listed published high-pressure temperatures reported by Baker and Baker's critical pressure is larger than that reported by Rau (Table heat and other thermal the vapor have been summarized for and others. Preuner and Schupp204 Braune and vapor pressure and that an absorption observed the spectrum now but as- The 100- that vapor even detected photoionization yields one ionization ports earlier thermodynamic reasoning12 that species occur as rings. depends upon which it LaMer produced169 with particle to 75 for several and have times equilibrium vapor pressure. Berkowitz12 has by mass that vapor cyclohexa-S contains preferential vaporization of sulfur used by Berkowitz5' and Rickert's electrolytic overcome problems identifying the molecular ionization a mass - E e Y 0 2 0 -2 -4 -6 -8 I I I 1 1 0 400 800 1200 Temperature T sulfur: the pressure curve constructed from in ref 96, were estimated 12 and 1.0 p I I I I I1 X 01 001 I I I I 400 Boo 1200 % saturated vapor, be- in ref unravel the molecular composition a large temperature range. At the vapor, make up the rest; and the vapor absorptiong3 of the ter- Upon heating, the concentration vapor steadily decreases, and the vapor the small species. is the most abundant species. At the contains mainly the intermedi- temperature range Figure 17. goes through a at about accounts for than about the total pressure, while at about the same temperature. relative concentration species increases unsaturated pressure. for over corresponding value This vapor vapor was already studied by Torr and This vapor is cherry red.63 Sulfur atoms are not present below the as transient various sulfur species are Sulfur Chemical Reviews, 1976, TABLE XVI II. Thermodynamic Data of Gaseous Sulfura Molecule Equilibrium B(S-S), kcal/mol AH“ f, s“, kcal/rnol cal/mol .deg Obsdb CalcdC 52 2S(S) * S,(g) 31.20 54.40 s3 2S,(g) 2 3S2(9) 33.81 64.39 55.8 48.7 s4 S,(g) 2 2S,(9) 34.84 74.22 s, 2S,(g) 2 5% (4) 26.14 73.74 60.0 58.3 S6 3/4S,(g)’2 S,(g) 24.36 84.60 s, 1/8s8(g) 2 S,(g) 27.17 97.41 62.2 63.6 S8 S8(g) e 4S2(g) 24.32 102.76 12, 96, Reference 12. Reference 43. Heat Capacities C,, cal/deg.rnol A B C 300 K 1000 K 8.54 12.854 19.092 25.558 31.580 37.038 42.670 5.268 0.28 1.04 0.783 0.253 0.120 0.613 0.860 6.121 4.79 -1.554 -2.820 -3.771 -4.400 -4.723 -5.1 10 -0.816 0.8 1.8 1.3 1.9 1.4 1.9 1.5 1.9 1.6 1.9 1.6 1.9 1.7 1.9 5.4 5.6 aSee ref 203. bSee also Table IX. heats have been calculated makes available computer program which allows com- of vapor hot sulfur vapor rapidly quenched solids can be obtained,ls5 which contain mixture of various vapor components, together with recombination products which are formed during condensation. trap individual vapor species diluting the gas. This method’66 has been sulfur compounds has made solutions containing chains of these systems B. Individual Species are not present equilibrium pressure below atoms can as an and by Gunning and preparative methods, using energy levels31 sulfur are well known. Photolysis produces atoms state, as the excited state short wavelengths the also obtained yield.21 Excited atoms carry excess energy a sufficient lifetime Strausz and have ex- plored extensively the chemistry sulfur atoms molecules. Some inorganic reactions have been Donovan.213 With themselves, the atoms stream containing about Thermodynamic properties215 the atom, such heat of entropy have been constitutes nearly the equilibrium vapor202,203 can also be ob- TABLE XX. Dissociation Energy of S, Do0(S,), kcal/mol Ref Method 100.69 i 0.01 spectroscopya 224 101.0 f 0.2 Photoionization 12 5101.0 i 0.8 Photoionization 223 101.7 i 2.9 Kn u dsen-t orsi on 222 2.5 Thermochemistry 97t 5 Mass spectrometric 221 predissociation at 35 590 cm-’. Year 1969 1969 1968 1966 1964 or by organic glass, via recombination been discussed reactions yield nificant fraction first electronic excited 1A.218 the most stable the small sulfur molecules. However, the levels exhibit Recently, the yielded accurate ground-state ground-state frequency, observed long been recognized. Seventeen energy levels are violet color lowest allowed transition energy of observed whenever sulfur compounds are reducing flame. This emission, which the transition well determined, for quantitative by gas energy levels have been reviewed who has made most the original observations. has been an extended controversy about the disso- energy of can now be considered solved, thermochemical considerations,221 Knudsen-torsion effusion measurements,222 mass ization, listed agree on the possible values. yields the most accurate value; 35 590 value for confirms the earlier value electron impact. photoelectron spectrum50 of expected, to have been conducted trapping vapor, absorption spectrum shows progression with at 20 to yield vibrationally state atoms. sion spectrum Raman band at 720 have also been reported. band165 at and is Beat Meyer 384 Chemical Reviews, 1976, Vol. spectrum can be observed be discussed the section on ionic solu- called thiozone Erdmann,226 1908, sumed that liquid sulfur. d'Or published the spectrum of it to most physical chemists re- jected the existence 1964, when it in the mass It has since been proven sulfur vapor199 and in liquid sulfur.63 point, thermodynamic measurement indicates best conditionsE3 found at 10 Torr and makes up about the vapor, it its characteristic deep, cherry red color. shows ex- tensive rotational structure. 465 cm-', as vibrational shifts bands of cate a stretching frequency excited state. from a using sulfur atom parameters. not only isoval- also with and has bent structure, carries on the minal atom show that can be prepared glasses199 by careful photolysis other similar 585, 490, Raman227 frequencies recombination products have been recently attributed experimental photoionization5' Two ions are known. which can be easily recognized pale blue color, will the trisulfide ion, forms and gaseous 530 nm been repeatedly occurs together with Its vapor can be and 20 sumed to for about of the forms between 24 suggest that can occur Semiempirical Huckel trans chain branched molecule similar stability, are far more stable the pyramid, the form, including the ring. quite possible the branched S03-type structure, with terminal atoms, can exist, low temperature. careful photolysis sulfide, or, much easier, at 688, 483, 320, trapped discharges227 through assigned to been published yet. electron impact method" yielded value of 10.4 Three ions have been reported: be planar,65 salt melts, the stable tetrasulfide ion, occurs in aqueous pH, as been studied the in a solid.52 Thermodynamic considerations it is a ring.12.50,5' the chain, stereochemical considerations. be preserved, even the most favorable unstrained chain conformation rings, values of about are computed. Except for which accounts for about energy5' of 8.60 eV, isoelectronic with fairly well known.22a be pointed out the most small paramagnetic striking similarity between centration computed the concen- species observed might not be accidental. found in that they rings are likely present the curious melting phenomena of sulfur. However, these molecules temperature, where can be much more easily are discussed with solid al- the liquid perhaps also in the 12, can also occur transient equi- librium species. These should be easily recognizable deep color which they must VI. Solutions rings dis- room temperature without decomposition. Represen- values of solvents are indicat- Distribution factors in eight mixtures can be 230. Binary systems sulfur have including liquid have been Recently, fugaci- similar systems have been above 130 thermal dissociation homolytic scission free-radical reac- usually hydrogen Thus, many the re- ported systems suffer slow chemical reaction, recognizable color change. excellent solvent, and lends ir studies does with iodine,lEi chlorine,i74 and at temperatures with into insoluble photosulfur, which partly redis- ionic solutions, suffers nucleophilic colored solutions, Geitner,236 which contain molecular ions, usual- Molecular Ions. charged negative are quite stable pH, and solid salts. Singly salt melts at high temperature, doubly charged are observed are formed chain scission, which rapid chain or by yielding chains S-S = 2.048 A S-S-S = 107' 53' s-s-s-s = 900 compounds are pale yellow, equilibrate rapidly with each other, yielding with well-established free sulfanes Elemental Sulfur Chemical Reviews, 385 Solubility of Solubility (wt %) g of SjlOO Solvent g solvent T, "C Ref CCI, CHCI, CHBr, CHI, Ethanol-(CH,),SO (1: 1) Acetone-(CH,),SO (1: 1) H,O-(CH,),SO (1~1) NH,-(CH,),SO (1~1) "3 ("412s S*CI2 Pyridine Aniline Benzene Ethanol Ethyl ether Acetone Hexane 0.14 0.005 1.3 0.0078 0.039 0.46 4 35.5 55.66 0.148 0.86 1.94 1.2 3.64 0.003 0.37 0.45 42 15 38.6 21 37 17 97 10.5 19.2 46 2.1 17.5 0.066 0.283 2.7 0.25 2.8 7.3 -60 0 80 25 60 140 -80 25 -2 4 25 60 60 5.6 85 60 60 60 -20 30 20 -9 21 110 85 100 130 25 100 25.3 23 25 20 100 233 234 234 234 5, a b b b b C d a 27 27 5, a 27 27 Kubota, and the spectra59 sulfanes, and their uv spectra6' a one-electron model. energies. Figure increasing chain length the transition 320 nm. the structure some alkaline earth di- and distancez4' varies between SrS3 and singly charged have been ob- sulfur or sulfur compounds occur in ultramarines is explained ions, but neutral sulfur ion absorbs24' 400 nm, lies between Nal and KBr, depending of about at 610-620 has Raman242 and an has been estimated4' spectrum has bond angle smaller than this ion,248 i.e., its the ionic media, is not yet have been Gillespie identified cations are parts known best.Io5 Its structure established. The chair structure, and bond dis- tances are somewhat shorter' than wise, nonbonding are closer is intermediateIo6 to that of is quite Se42+ ion, frequency252 has been measured u1 = 584 cm-' Ai, u3 = 460 cm-' E, studied the u2 = 530 cm-l Big u4 = 330 cm-' Bug Reaction Products Allotrope Reagents __~._____ Phase Molecular species Well-established Inconclusive or mixture Solid sulfur Solid sulfur Liquid sulfur Sulfur vauor Solution, containing Solution, containing cyclo-s, compounds (a) Stable (STP): (b) Metastable: Catena-Si, 16 i 10' Charge transfer complex c yc I 0- S, -ca t e na- Si- Cyclo-S, as a-SS Cycl o-s,a cyclo-s, CYCIO-S~, 6 24 (?) Catena-Si, 3 i los Catena-Si, 2 i 5 CYCIO-S,, 6 [I 12 Cycl 0- s, Sb (Geller II) Laminar, w, orange, me- tallic, Vezzoli, Geller, Bask dary fibrous, K! §, O,T, 6, u, w, nl-n2, red (E, F, G), orange (I, K, L, M) an = 6. 7, 8. 9. 10, 11, 12, 18, 20. 386 Chemical Reviews, Beat Meyer have also been identified circular di- A third species, Gillespie's also formed in super properties are VI/. Conclusion been learned the mo- lecular structure elemental sulfur. lt is now known types of rings are sufficiently metastable temperature for several days. It high temperature, the equilibrium composition allows for a comparable concentra- it is boiling point the liquid contains small species with three, five atoms. Now that these species can be isolated become possible the relative ty of different allotropes. Such knowledge make possi- ble selective reactions, which open simpler cheaper paths to synthesize sulfur compounds, industrially important useful com- about the ferent pure allotropes. Bartlettg4 Davisg8 have reported with triphenylphosphine chloride. Knipps- reacts with times faster quick reaction which reacts only very slowly with by The these reactions conclusively established, the reaction ion, and plausibly explained.25 work of his group256 radioactive marking can greatly help determination of the kinetics. with various organic compounds has been recently reviewed Such reactions are very sensitive bases.25 Reactions sulfur with sulfide and many been investigated reacts with aromatic other hydrocarbons, speciess3 formed at this temperature. irreversible darkening the purest molten high temperature species has only been superficially explored.260 However, the now well to the work conclusion, a summary of allotropes which have been most widely described. A the pure allotropes which yield better better models for the bonds would most valuable. 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