摘要:

QUARTERLY REVIEWS THE CHEMICAL SOCIETY PATRON HER MAJESTY THE QUEEN President SIR ALEXANDER TODD M.A. D.Sc. F.R.S. Vice-presidents who have filled the office of President H. J. EMELBUS C.B.E. M.A. D.Sc. SIR CHRISTOPHER INGOLD D.Sc. SIR CYRIL HINSHELWOOD O.M. M.A. SIR ERIC RIDEAL M.B.E. M.A. E. L. HIRST C.B.E. D.Sc. LL.D. SIR ROBERT ROBINSON O.M. D.Sc. F.R.S. F.R.I.C. F.R.S. Sc.D. F.R.S. D.Sc. F.R.S. F.R.S. LL.D. F.R.S. Vice-presidents F. BERGEL D.Sc. F.R.I.C. F.R.S. E. J. BOWEN M.A. D.Sc. F.R.S. E. R. H. JoNEs,D.S~.,F.R.I.C. F.R.S. J. M. ROBERTSON M.A.,D.Sc. F.R.S. M. STACEY Ph.D. D.Sc. F.R.S. Honorary Treasurer J. W?. BARRETT Ph.D. A.R.C.S. F.R.I.C. Honorary Secretaries K. W. SYKES M.A. D.Phil. J. CHATT M.A. Sc.D. F.R.S. A. W. JOHNSON Sc.D. Ph.D. A.R.C.S. Ordinary Members of Council C.B. AMPHLETT Ph.D. D.Sc. A. K. HOLLIDAY Ph.D. D.Sc. G. 0. AsPINALL,D.SC.,P~.D.,F.R.I.C. F.R.I.C. L. J. BELLAMY B.Sc. Ph.D. G. E. COATES M.A. D.Sc. F.R.I.C. W. COCKER M.A. D.Sc. F.R.I.C. D. P. CRAIG M.Sc. D.Sc. D. H. EVERETT M.B.E. M.A. H. T. OPENSHAW M.A. D.Phil. 1. J. FAULKNER Ph.D. F.R.I.C. R. A. RAPHAEL Ph.D. D.Sc. W. GERRARD D.Sc. Ph.D. F.R.I.C. C. H. HASSALL M.Sc.,Ph.D.,F.R.I.C. R. N. HASZELDINE M.A. Sc.D. W. A. WATERS Sc.D. F.R.I.C. Ex Oficio J. W. COOK D.Sc. F.R.I.C. F.R.S. (Chairman of the Chemical Council) D. H. HEY D.Sc. F.R.I.C. F.R.S. (Chairman of the Publication Committee) E. D. HUGHES D.Sc. F.R.I.C. F.R.S. (Chairman of the Joint Library Committee) J. HONEYMAN Ph.D. D.Sc. A. H. LAMBERTON Ph.D. J. W. LINNETT M.A. D.Phil. F.R.S. M.R.I.A. E. A. MOELWYN-HUGHES D. Phil.D.Phi1. D.Sc. Sc.D. H. M. PowELL,B.SC. M.A. F.R.S. J. C. ROBB D.Sc. Ph.D. F.R.I.C. F.R.I.C. F.R.S. General Secretary J. R. RUCK KEENE M.B.E. T.D. M.A. Librarian R. G. GRIFFIN F.L.A. QUARTERLY REVIEWS VOL. XV 1961 Publication Committee Chairman D. H. HEY D.Sc. F.R.I.C. F.R.S. G. 0. ASPINALL D.Sc. F.R.S.E. C. KEMBALL M.A. Ph.D. F.R.I.C. J. A. KITCHENER D.Sc. Ph.D. WILSON BAKER M.A. D.Sc. F.R.S. G. KOHNSTAM Ph.D. J. W. BARRETT Ph.D. A.R.C.S. H. T. OPENSHAW M.A. D.Phi1. L. N. OWEN Ph.D. D.Sc. F.R.I.C. R. P. BELL M.A. F.R.S. R. A. RAPHAEL D.Sc. A.R.C.S. D. M. BROWN Ph.D. F.R.I.C. G. M. BURNETT Ph.D. D.Sc. A. G. SHARPE M.A. Ph.D. F.R.I.C. I. G. M. CAMPBELL B.Sc. Ph.D. J. C. SPEAKMAN M.Sc. Ph.D. D.Sc. N. CAMPBELL O.B.E. D.Sc. Ph.D. K. W. SYKES M.A. D.Phi1. N.B.CHAPMAN M.A.,Ph.D.,F.R.I.C.J. C. TATLOW B.Sc. Ph.D. F.R.I.C. J.CHATT M.A.,Sc.D.,F.R.I.C.,F.R.S. SIR ALEXANDER TODD M.A. D.Sc. T. COTTRELL B.Sc. D.Sc. F.R.I.C. D. P. CRAIG Ph.D. D.Sc. F.R.I.C. H. J. V. TYRRELL B.Sc. M.A. D. D. ELEY Sc.D. Ph.D. B. C. L. WEEDON D.Sc. A.R.C.S. D. F. ELLIOTT Ph.D. A.R.C.S. F.R.I.C. D. H. WHIFFEN M.A. D.Phi1. R.H.HALL,P~.D.,A.R.C.S.,F.R.I.C. M. C. WHITING M.A. Ph.D. T. G. HALSALL Ph.D. M.A.,A.R.I.C. A.R.C.S. C. H. HASSALL M.Sc. W. WILD Ph.D. A. W. JOHNSON Sc.D. Ph.D. A.R.C.S. G. WILKINSON Ph.D. A.R.C.S. F. R. I. C. F.R.I.C. F.R.S. A.R.I.C. Editor R. S. CAHN M.A. D.Phil.Nat. F.R.I.C. Deputy Editor L. C. CROSS Ph.D. A.R.C.S. F.R.I.C. Assistant Editors A. D. MITCHELL D.Sc. E.R.I.C. I. J. CANTLON Ph.D. L O N D O N T H E C H E M I C A L S O C I E T Y CONTENTS SYNTHETIC GEMSTONES.By E. A. D. White . . . . . THE CHEMISTRY OF COMPOUNDS CONTAINING SULPHUR-FLUORINE BONDS. By H. L. Roberts . . . . . . . . . THE GIBBERELLINS. By John Frederick Grove . . . . NUCLEAR FISSION. By G. N. Walton . . . . . . By A. V. Kiselev . . . . . . . . . . SURFACE CHEMISTRY ADSORPTION ENERGY AND ADSORPTION EQUILIBRIA. THERMODYNAMIC PROPERTIES OF ORGANIC OXYGEN ~OMPOUNDS. By J. H. S. Green . . . . . . . . . . THE HISTORY AND CHEMISTRY OF MUSCARINE. By S. Wilkinson HALIDES OF THE PHOSPHORUS GROUP ELEMENTS (P As Sb Bi). By D. S. THE THEORY OF CHARGE-TRANSFER SPECTRA. By J. N. Murrell . MECHANISMS OF ELECTRON TRANSFER AND RELATED PROCESSES IN SOLUTION. By J. Halpern . . . . . . . . THE PRODUCTION DETECTION AND ESTIMATION OF ATOMS IN THE GASEOUS PHASE. By K. R. Jennings . . . . . . . . ALKALOID BIOSYNTHESIS. By A. R. Battersby . . . . . MOLECULAR ELECTRONIC ABSORPTION SPECTRA. By S. F. Mason. . AN OUTLINE OF RHENIUM CHEMISTRY. By A. A. Woolf . . . PHOTOCHEMICAL REARRANGEMENTS AND RELATED TRANSFORMATIONS. By P. De Mayo and S. T. Reid . . . . . . NITROSATION DIAZOTISATION AND DEAMINATION. By J. H. Ridd. . THE ACTINIDE OXIDES. By L. E. J. Roberts . . . . . CUMULATIVE INDEXES . . . . . . . . . Payne. . . . . . . . . . . . PAGE 1 30 56 71 99 125 153 173 191 207 237 259 287 372 393 418 442 461

ISSN:0009-2681    

DOI:10.1039/QR96115FP001

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

THE CHEMISTRY OF COMPOUNDS CONTAINING SULPHUR- FLUORINE BONDS By H. L. ROBERTS (RESEARCH DEPARTMENT IMPERIAL CHEMICAL INDUSTRIES LIMITED SULPHUR has the electronic configuration (3~)~(3px)~(3py)(3pz) in the ground state and as would be expected is commonly bivalent in its compounds with other elements. However its 3d-orbitals are available and there is considerable evidence1 that these play an important role in its chemistry. This is particularly evident for sulphur-fluorine compounds where sulphur is normally either quadri- or sexi-valent. For sulphur to increase its valency in this manner one or two electrons must be promoted to the 3d-shell to permit the formation of more bonds whose energy must be sufficient to make up for the energy of promotion. The formation of strong bonds depends on the overlap of the orbitals of the two atoms form- ing the bond.A study of overlap integrals shows that for sulphur the 3d-orbitals in the free atom are too weakly bound and diffuse to contribute significantly to the bond energy. Craig and Magnusson2 have made a theoretical study of this problem and described a model in which the ligand atoms in a sexivalent sulphur compound are replaced by six point positive changes in octahedral disposition. In such a model the d-orbitals are contracted and for a ligand such as fluorine effective overlap of sp3d2 hybridised orbitals becomes possible. The contraction is most effective for small electronegative ligands and is therefore in accordance with the fact that sulphur has its highest covalency when bonded to oxygen and fluorine. In addition to this ligand effect on the d-orbitals the low dis- sociation energy of the fluorine molecule (F2 38; Cl, 58; H2 104 kcal.mole-l) means that for a given bond strength a fluorine compound will be more exothermic than a chloride or hydride. It is therefore not surprising that sulphur forms a stable hexafluoride but no hexachloride and that only a limited replacement of fluorine in sulphur hexafluoride by other atoms or groups is possible. This contraction of d-orbitals which permits the formation of sulphur hexafluoride also means that the 2p- electrons of oxygen can overlap effectively with the 3d-orbitals of sulphur so that in sulphuryl fluoride SO,F the oxygen atoms are linked to sulphur by a double bond and the sulphur atom is thus sexivalent in this compound also. In this case where the oxygen atoms play an important role there are chlorine and bromine analogues of similar structure.In sulphur tetrafluoride where the co-ordination number of the sulphur atom is four it is still necessary to promote one electron to a d-orbital and even here there is no chlorine analogue as sulphur tetrachloride exists ALKALI DIVISION NORTHWICH CHESHIRE.) Cilento Chern. Rev. 1960 60 147. Craig and Magnusson J. 1956 4895. 30 ROBERTS COMPOUNDS CONTAINING S-F BONDS 31 only in condensed phases where it is present as SCl,+Cl-. Thus it is only in certain oxyfluorides and oxychlorides that there is any similarity between the fluorides and chlorides of sulphur. While it is usual for there to be a marked difference between the fluorides and chlorides of a given element there is probably no other element where this is quite so striking as with sulphur.One interesting feature of sulphur-fluorine chemistry is that it provides a series of compounds in which both the maximum covalency and the co-ordination number of six are simultaneously reached. In this respect it is similar to carbon in the first row of the Periodic Table where covalency and co-ordination number have a maximum of four. This leads to several interesting parallels between the derivatives of sulphur hexafluoride in which one or two fluorine atoms are replaced by other atoms or radicals and those of carbon tetrafluoride. Thus both have a stability to hydrolysis which is kinetic rather than thermodynamic but the range of possible substituents in the case of sulphur hexafluoride is much more restricted than with carbon tetrafluoride.The known sulphur-fluorine compounds are classified in this Review under five headings (1) Sulphur monofluoride S,F2 sulphur difluoride SF, and their derivatives of the type R-SF where R is a univalent element or radical. (2) Sulphur tetrafluoride SF and its derivatives RmSF,. (3) Sulphur hexafluoride SF6 and its derivatives R-SF and SF,R,. (4) Fluoro-oxy-acids and oxyfluorides of sulphur. ( 5 ) Sulphur-nitrogen- fluorine compounds. This classification is one of several possible and is chosen for clarity of presentation rather than for its chemical significance. Sulphur Monofluoride and Sulphur Difluoride and Their Derivatives The preparation of sulphur monofluoride has been reported by several worker^^*^*^ by the action of silver(1) fluoride or mercurous fluoride on sulphur.The compound is said to attack glass; although it was stated to be impure a boiling point of -99" and freezing point of -120.5" are given3 while other workers4 give -38" as the boiling point of S,F2. Sulphur monofluoride is said4 to decompose when heated giving sulphur and sulphur difluoride but this product is also poorly characterised. More recently both S,F and SF2 have been reported as present in the products of electrical breakdown of sulphur hexafl~oride,~~~ but in this case the identification given depends on unpublished infrared spectra. It must be concluded that neither of these compounds has been adequately charac- terised. Centnerszwer and Strenck Ber. 1923 56 2249; 1925 58 914. Trautz and Ehrmann J. prakt. Chem. 1935 142 79. Matutano and Otero Anales real Soc.espafi. Fis. Quim. 1955 51 B 223. Schumb Trump and Preist Ind. Eng. Chern. 1949 41 1348. Edelson Bieling and Kohrnann Ind. Eng. Chein. 1953 45 2094. 32 QUARTERLY REVIEWS The only derivative of sulphur difluoride which has been claimed is trichloromethanesulphenyl fluoride CCl,.SF which was obtained* by the action of mercuric fluoride on the corresponding chloride. However no attempt was made to establish the structure of the product which could have been CFCl2.SC1. The existence of fluorides of bivalent sulphur is therefore not established. Sulphur Tetrafluoride and its Derivatives In 1950 Sidgwickg described reports of sulphur tetrafluoride as " almost certainly wrong". Brown and Robinson,lo who review the earlier literature on the lower fluorides of sulphur were however able to isolate and characterise sulphur tetrafluoride as a result of reaction of fluorine with a thin film of sulphur at -75".Sulphur tetrafluoride is also obtained by the action of chlorine trifluoridell or iodine pentafluoride12 on sulphur. Sulphur dichloride sulphur monochloride or sulphur dibromide may be used in place of sulphur in any of these reactions. This has the advantage that the reagent being a liquid can be more readily cooled to remove the heat of reaction and minimise the tendency to form sulphur hexafluoride but the product is then contaminated with chlorine which can be troublesome to remove.12 All these reactions involve the use of elementary fluorine or a halogen fluoride and thus need special apparatus which is not available in labora- tories that do not specialise in fluorine chemistry.A ' more convenient preparative reaction12 is the fluorination of sulphur dichloride in the presence of acetonitrile at 70-80° by sodium fluoride 3SCI + 4NaF -+ SF + S,CI + 4NaCI This remarkable reaction which proceeds readily to give a high yield of sulphur tetrafluoride may occur through the unstable SCl which arises13 from the equilibrium 3Scl2 + SCl + S2C12 the function of the catalyst being to stabilise the SCl,. In the absence of acetonitrile or other catalyst the reaction proceeds only at elevated temperatures.12 Sulphur tetrafluoride is a colourless gas (b.p. -40") which condenses to a colourless liquid whose Trouton constant (27-1) indicates that it is associated. It solidifies at - 121 ". Its critical temperature is 91 O.The structure of the sulphur tetrafluoride molecule by analogy with other halides of Group V VI and VII elements may be expected to be derived from a trigonal-bipyramidal arrangement of valency electrons in 3s3p33d-orbitals around the sulphur atom1 For sulphur tetrafluoride this Sidgwick "The Chemical Elements and their Compounds," Oxford 1950 p. 943. * Kober J. Arner. Chern. Suc. 1959 81 4810. lo Brown and Robinson J. 1955 3147. l1 B.P. Appln. 16,404/1959. l2 Tullock Fawcett Smith and Coffman J. Amer. Cliem. SOC. 1960 82 539. l3 Rosser and Whitt J. Appl. Chem. 1960 10 229. l4 Gillespie and Nyholm Quart. Rev. 1957 11 339. 33 ROBERTS COMPOUNDS CONTAINING S-F BONDS leaves two possible structures in which the unshared pair of electrons occupy either an equatorial or a polar position.A study of the infrared and Raman spectral5 strongly suggested that the lone pair occupied an equatorial position a result which was confirmed by nuclear magnetic resonance s p e c t r o ~ c o p y . ~ ~ ~ ~ ~ The structure is therefore intermediate between those of phosphorus pentafluoride and chlorine trifluoride the former being a trigonal bipyramid with a fluorine atom at each corner and the latter having two lone pairs in equatorial positions. It is of interest that in chlorine trifluoride the two polar fluorine atoms are bent towards the single equatorial fluorine on account of the repulsion of the lone pairs; a similar effect might arise in sulphur tetrafluoride although here there are only one lone pair and two equatorial fluorine atoms. By a study of the variation of the fine structure of the nuclear magnetic resonance spectrum with temperature a value of 4.5 -j= 0-8 kcal.mole-1 is obtained for the activation energy of fluorine exchange. The heats of hydrolysis18 and hydrogenati~nl~ of sulphur tetrafluoride have been measured and lead to values of 176 and 172 kcal. mole-l respectively for the heat of formation of gaseous sulphur tetrafluoride. The sulphur-fluorine bond energy in sulphur tetrafluoride can be cal- culated from these values and known thermochemical data20 to be 78-79 kcal. mole-l. Sulphur tetrafluoride is very readily hydrolysed by aqueous media to thionyl fluoride which although it can be hydrolysed further to sulphur dioxide is less reactive than the tetrafluoride This ready hydrolysis makes it difficult to handle sulphur tetrafluoride in glass apparatus unless special precautions to avoid moisture are taken.SF + H,O -+ SOF + 2HF l6 Dodd Woodward and Roberts Trans. Faraday SOC. 1956,52 1052. l6 Cotton George and Waugh J. Chem. Phys. 1958 28 994. l7 Muetterties and Phillips J . Amer. Chem. Soc. 1959 81 1084. l9 Muetterties 17th Cong. Pure Appl. Chem. Munich 1959. Nicholls Ph.D. thesis Durham University 1958. Cottrell “Strengths of Chemical Bonds,” Butterworths Scientific Publications 2nd edn. London 1958. 2 34 QUARTERLY REVIEWS In conjunction with the near-identity of the boiling points of sulphur tetrafluoride and oxychloride it may account for much of the confusion in the early literature concerning the lower fluorides of sulphur. The reaction of sulphur tetrafluoride with water has a parallel in its reaction with other compounds containing a hydroxyl group and there is some correlation between the acidity of this group and the yield of product.Carboxylic and sulphonic acids are converted in high yield at room temperature into acid fluorides While alcohols do give the corresponding fluoride the appropriate ether is always a major by-product. In addition to reacting with hydroxyl groups sulphur tetrafluoride reacts with organic compounds containing a carbonyl group,21 such as aldehydes ketones carboxylic acids and their derivatives the oxygen atom being replaced by two fluorine atoms R.C02H + SF4 -+ R+COF + SOF 50" 35% 110" 60% CHSCHO + SF4 -+ CHS.CHF2 + SOF2 CH,*C0.CH3 + SF4 -+ CH,*CFz*CH + SOF 1 20" C,H,.COF + SF4 -+ C,H,*CF + SOF2 41 % The reaction is very specific; for example unsaturated acids may be used 120" CH=CCO,H + 2SF4 -+ CHGCCF + 2SOF + HF Certain ketones and all esters react in this manner only with difficulty but even in these cases reaction will proceed in the presence of a catalyst such as hydrogen fluoride boron trifluoride or arsenic pentafluoride and other fluorides which are Lewis acids.A possible mechanism21 for this reaction is shown in the annexed scheme. The initial step may be the co-ordination of the fluoride XF, which may be SF in the uncatalysed I I s+ s- s+ L- F-SF3 SCO + X G - -C-0-XF -C-O-XFn 4 -C-O-sF f X$ 'x i F I i I I -5-F + SOF )-t - 5 - 0 7 F F F -XF,,- + XF \ / reactions with the carbonyl group and the reaction can then proceed as depicted. That sulphur tetrafluoride is a weak Lewis acid is shown by its interaction with tertiary amines to form a complex in which the nitrogen is the donor.22 Sulphur tetrafluoride has also been shown to be a useful 21 Hasek Smith and Engelhardt J.Amer. Chem. Soc. 1960 82 543. 22 Muetterties U.S.P. 2,897,055. ROBERTS COMPOUNDS CONTAINING S-F BONDS 35 reagent for the preparation of other inorganic It reacts with most inorganic oxides and sulphides to give the corresponding fluorides or oxyfluorides I,O + 5SF4 -+ ZIF + 5S0F2 With oxides thionyl fluoride is produced but with sulphides sulphur is the usual by-product SF + SnS -> SnF + 3s This reaction is of little value for the readily obtainable alkali-metal or alkaline-earth metal fluorides but the production of iridium pentafluoride selenium and tin tetrafluoride and molybdenum and tungsten hexa- fluoride from the corresponding oxides or sulphides provides the most con- venient route to these compounds.With certain oxides such as nitrogen dioxide and chromium trioxide which are strong oxidising agents sulphur tetrafluoride is oxidised to thionyl tetrafluoride. Sharpz4 has extended the scale of this reaction to the preparation of complex fluorides by carrying out the reaction of sulphur tetrafluoride with oxides sulphides and carbonyls in the presence of alkali-metal fluorides. Thus with molybdenum hexacarbonyl or molybdenum dioxide the following reaction takes place P205 + 3SF4 -f ZPOF + 3soF s F4 NaF + Mo(CO) -4 NaMoF 205 O SF4 NaF + MOO -+ NaMoF 350' Mo(CO) + SF4 -+ MoF,,SF 205 O In each case the final product contains MoV although the starting com- pounds contained Moo and MoIV.Other oxides studied were boron trioxide tantalum and niobium pentoxide cerium and titanium dioxide and silica ; in each case a complex fluoride with the metal in the same valency state as in the oxide was obtained. This method of preparation can probably be extended to the preparation of complex fluorides of Group VIIl metals. Sulphur tetrafluoride forms an interesting series of solid complexes with certain inorganic fluorides which can accept fluoride ions. These complexes BF,,SF, AsF,,SF, and SbF,,SF, written in ascending order of stability were first discovered by Bartlett and Robinson25 who postulated that they were simple Lewis acid-Lewis base complexes SF,+BF,. This formulation was criticised on theoretical grounds by Cotton and George.26 They pointed out that sulphur tetrafluoride would be a poorer electron- donor (because the extra fluorine atom would decrease the lability of the 23 Oppegard Smith Muetterties and Engelhardt J .Arner. Chem. Soc. 1960 82 24 Sharp J. personal contribution. 25 Bartlett and Robinson Proc. Chem. Soc. 1957 230. 2G Cotton and George J . Inorg. Nuclear Chem. 1958 7 397. 3835. 36 QUARTERLY REVIEWS lone pair) than phosphorus trifluoride which has been shown not to react with boron trifluoride. Further while boron trifluoride can accept fluoride ion giving BF,- and can function as general Lewis acid arsenic and antimony pentafluoride are known to be very strong fluoride-ion acceptors but are not known to be good general Lewis acids. These authors therefore prefer the formulation BF,-SF,+. More direct evidence for this structure is given by Seel and D e t ~ e r ~ ' who have been able to obtain an infrared spectrum of a thin film of BF,,SF on a rock-salt plate.This shows a strong band at 1050 cm.-I which is typical of BF4- and of greater significance because the BF,- could arise by reaction of the fluorides with sodium chloride a doublet at 908 and 940 cm.-l which by comparision with phosphorus trifluoride is assigned to the SF3+ ion. In the gas phase the complexes are completely dissociated and from a study of the vapour pressure-temperature relationship a value of 25-5 kcal. mole-l is deduced2' for the heat of dissociation of BF3,SF,. Sulphur tetrafluoride reacts with organic chlorides in presence of boron trichloride to give fluorides and a mixture of sulphur dichloride and chlorine; no compounds of the type SF3CI SF2C12 or SC1,F were detected and in view of the low activation energy required to produce isotope exchange in sulphur tetrafluoride it is likely that such compounds would disproportionate very readily.With fluorine sulphur tetrachloride readily gives sulphur hexafluoride; with chlorine trifluoride reaction is slow at room temperature but at 200" gives an equimolar mixture of sulphur hexafluoride and sulphur chloride pentafluoride ; Somewhat higher temperatures are needed before reaction takes place with chlorine monofluoride but at 380" reaction is rapid and gives sulphur chloride pentafluoride.28 With bromine trifluoride26 and iodine penta- fluoride12 there is no reaction below 200". Sulphur tetrafluoride does not react with oxygen alone but in the presence of nitrogen ZSF + CIF + SFJI + SF it is oxidised to thionyl tetrafluoride 2SF + 0 + 2S0F4 Derivatives of Sulphur Tetrafluoride.Several derivatives of sulphur tetrafluoride of the type RsSF are known. Trifluoromethylsulphur TABLE 1. Sulphur tetrafluoride and its derivatives. Compound M.p. B.p. -121" - 40.4 O -110 - 96.5 - 51- 60'15 mm. 7 35 26 27 Seel and Detmer 2. anorg. Chem. 1959,301 113. 28 Roberts B.P. Appln. 2543/1959. 49 Smith and Engelhardt J. Amer. Chern. SOC. 1960,82 3838. ROBERTS COMPOUNDS CONTAINING S-F BONDS 37 trifluoride CF,.SF, was prepared by Tyezkowski and Bigelow30 by reaction of carbon disulphide vapour with fluorine highly diluted with nitrogen; and SF,CF2-SF has been isolated from the electrochemical fluorination of carbon di~ulphide.~~“ These are similar to sulphur tetra- fluoride in ease of hydrolysis.In contrast to this 2,4-dinitrophenylsulphur trifl~oride,~~ obtained by fluorination of the corresponding disulphide in solution in anhydrous hydrogen fluoride is stable in air and can be recrystallised from acetone or ethanol. A general method for the preparation of aromatic trifluoro- sulphur derivatives has been published32 in which aromatic disulphides dissolved in the inert 1,1,2-trichloro-l,2,2-trifluoroethane are treated with silver difluoride. Phenylsulphur trifluoride is a colourless liquid which slowly attacks glass and like sulphur tetrafluoride can be used in place of sulphur tetrafluoride to replace carbonyl groups by CF,. As it is a high-boiling liquid the reaction can be carried out in ordinary laboratory equipment.Authenticated derivatives of this series are listed in Table 1. Sulphur Hexafluoride and its Derivatives Sulphur hexafluoride is a colourless gas which was first isolated and characterised in 1900 by Moissan and L e b e a ~ . ~ ~ It was prepared by burning sulphur in an atmosphere of fluorine and removing lower fluorides by washing the products with aqueous alkali and this is still the most convenient method of making it;34 it is also produced in vigorous fluorination of organic compounds which contain sulphur. Alternative methods which have been suggested for the preparation of sulphur hexa- fluoride are reaction of metal fluorides on sulphur in the presence of an excess of chlorine35 at 500° 6NaF + S + 3CI -f 6NaCI + SF and pyrolysis and electrochemical fluorination of sulphur tetrafluoride;29 but neither of these methods gives high yields.Sulphur hexafluoride is a colourless odourless non-toxic gas which at atmospheric pressure condenses to a white solid at -63.8”. Its physical properties have been studied in great detail on account of its value as a symmetrical molecule in checking the predictions of various theories of the liquid Chemically sulphur hexafluoride is almost totally inert being un- affected by aqueous or fused alkali ammonia or oxygen; and even alkali 30 Tyezkowski and Bigelow J. Amer. Chem. SOC. 1953 75 3523. 30u ClifFord El-Shamy Emelkus and Haszeldine J. 1953 2372. *l Chamberlain and Kharasch J. Arner. Chem. SOC. 1955 77 1041. 3a Sheppard J. Amer. Chem. SOC. 1960 82,4751. 33 Moissan and Lebeau Compt. rend. 1900,130 865 984.34 Dodd and Robinson “Experimental Inorganic Chemistry,” Elsevier Amsterdam 35 Muetterties B.P. 805,860. 36 Cady “Advances in Inorganic Chemistry and Radiochemistry” (ed. Emeltus and 1955. Sharpe) Academic Press New York 1960. 38 QUARTERLY REVIEWS metals react appreciably only at elevated temperat~res.~~ This lack of reactivity and its high dielectric strength have led to the use of sulphur hexafluoride as an insulating atmosphere for high-voltage electrical equipment,38 although when breakdown does occur a very corrosive atmosphere containing lower fluorides of sulphur and fluorine is pro- duced.' The extreme chemical stability of sulphur hexafluoride must be kinetic rather than thermodynamic as the S-F bond energy differs little from that in sulphur tetrafluoride. The heat of formation of sulphur hexafluoride has recently been redetermined by Hayaman Gross and L e ~ i ~ ~ who give a value of 288.9 kcal.mole-l which is 27 kcal. mole-l higher than the previously accepted value.4o The S-F bond energy20 in sulphur hexafluoride is therefore 76 kcal. mole-l. The hydrolysis of sulphur tetrafluoride which is rapid and that of sulphur hexafluoride which does not proceed at a detectable rate under ordinary experimental conditions have free energies that may be calculated and compared viz. SF,(g) + 2H,O(g) -f SO,(g) + 4HF(g) SF,(g) + 3H20(g) -+ SO,(g) + 6HF(g) Several explanations have been advanced for the high activation energy of hydrolys of sulphur hexafluoride. Direct attack of water or hydroxyl ion on the sulphur atom in the covalently saturated sulphur hexafluoride could only take place by extensive electronic rearrangement and may be ex- pected to be difficult whereas attack could take place easily on sulphur tetrafluoride.This theory receives support from the ready fluorine-isotope exchange in sulphur fetrduoride,l6J7 compared with the lack of such exchange in sulphur he~afluoride.~~ The other factor which must also be important is the failure of a water molecule or a hydroxyl ion to co- ordinate on to a combined fluorine atom because when one fluorine in sulphur hexafluoride is replaced by chlorine to form sulphur chloride pentduoride (see below) attack by hydroxyl ions is rapid. The structure of sulphur hexafluoride is octahedral as would be expected for sp3d2-hybridisation.14 This has been established by electron diffrac- tion,42p43 the S-F bond length being 1.56 & 0.02A.The infrared,44 R a ~ a n ~ ~ and nuclear magnetic resonance17 spectra can be fully assigned on the basis of an octahedral structure. They have been used to afford thermodynamic functions for sulphur hexafluoride over a range of temperature^.^^ Derivatives of SuIphur HexaJEuoride. The similarity between the chemistry of sulphur hexafluoride and carbon tetrafluoride suggests that 37 Cowan Riding and Warhurst J. 1953 4168. Bucchner van de Graaf Spertuto Burril McIntosh and Urquhart Phys. Rev. Hayaman Gross and Levi Internat. Symp. Fluorine Chemistry Birmingham 1959. A G O = -58 kcal. mole-' AGO = -48 kcal. mole-1 1946 69,692. 40 Nat. Bur. Stand. Tables Circular No. 500. 41 Rogers and Katz J . Amer. Chem. SOC. 1952,74 1375. 42 Braune and Knoke 2.phys. Chem. 1933 B 21 297. 43 Brockway and Pauling Proc. Nut. Acud. Sci. 1933 19 68. 44 Lagemann and Jones J . Chem. Phys. 1951 19 534. 45 Gaunt Trans. Faraduy SOC. 1953 49 1122. ROBERTS COMPOUNDS CONTAINING S-F BONDS 39 there might exist a homologous series of the general formula SnF4n+2 similar to that of the fluoroalkanes CnF2n+2 and that an SF group may show properties similar to those of the CF group. It is to be expected however that the range of possible substituents on an SF5 group will be somewhat limited as the condition necessary for sulphur to form six bonds is that the ligand atoms shall be both small and electronegative.2 Also there will always be a tendency for SF,X to lose XF and give sulphur tetrafluoride a possibility not present with CF,X.At present only S,F1 is known of the series SnF4n+2 while the range of SF compounds is confined to SF,C1 SF,.OF SF5.0-SF, SF,.O.O.SF, and R-SF where R is an alkyl substituted alkyl aryl or fluoroalkyl radical. The number of R,SF4 compounds is even more restricted only perfluoroalkyl derivatives being known with any certainty. Disulphur DecaJEuoride. Disulphur decafluoride S 2F10 was first isolated and characterised by Denbigh and Wh~tIaw-Gray~~ as a by-product from the formation of sulphur hexafluoride by the action of fluorine on sulphur. These authors give a melting point of -92" and a boiling point of 29" but later workers4' showed that the melting point was very sensitive to small traces of impurities and give a melting point of -55" a vapour pressure curve indicating a boiling point of 29" and a Trouton constant of 21.3.It is more conveniently prepared by the photochemical reduction of sulphur chloride pentafluoride with hydrogen :48 2SF,CI + H2 +. S,F, + 2HCl Disulphur decafluoride is a colourless liquid which like sulphur hexa- fluoride is stable to aqueous alkali. It is however rapidly absorbed by molten potassium hydroxide and reacts vigorously with molten sodium. In marked contrast to sulphur hexafluoride disulphur decafluoride is extremely and its use as a war-gas has been suggested. Thermal decomposition of disulphur decafl~oride~~ to sulphur hexafluoride and sulphur tetrafluoride is quite rapid at 200" and is thought to proceed by a preliminary fission of the S-S bond to two SF,. radicals. The structure of disulphur decafluoride is the expected one of two SF groups linked by an S-S bond.Electron-diffraction meas~rements~l show that the S-F bond length 1.56 &- 0.02 A is similar to that in sulphur hexafluoride but that the S-S bond 2.21 & 0.03 A is considerably longer than the 2.08 A observed for disulphides containing bivalent sulphur.52 This structure is supported by the infrared and Raman spectra53 which can be assigned completely on this model. Force-constant calculation^^^ show 46 Denbigh and Whytlaw-Gray J. 1934 1346. 47 Hollies and McIntosh Canad. J. Chem. 1951 29 494. 48 Roberts B.P. Appln. 30,908/1960. 49 Lester and Greenberg Arch. Ind. Hyg. Occup. Med. 1950,2 348. 6o Trost and McIntosh Canad. J. Chem. 1951 29 508. 51 Harvey and Bauer J . Amer. Chem. SOC. 1953 75 2840. 62 Sutton "Interatomic Distances," Chem.SOC. Special Publ. No. 11 London 1958. 63 Dodd Woodward and Roberts Trans. Faraday SOC. 1957 53 1545. 64 Woodward and Roberts Trans. Faraday SOC. 1957 53 1557. 40 QUARTERLY REVIEWS that the S-F stretching force constant is the same as that for sulphur hexafluoride while the S-S stretching force constant has an unusually low value. Although the SF group is very electronegative it is not small and it is likely that the substitution of a second SF in place of a fluorine atom is not possible. Sulphur Chloride PentaJuoride SF,Cl. The replacement of a fluorine atom in sulphur hexafluoride by another halogen has been ac- complished only in one case sulphur chloride pentafluoride SF,Cl. This compound was first isolated from the products ~ b t a i n e d ~ ~ ~ by fluorination of liquid sulphur dichloride at -lo" but the yield was very low.Some sulphur chloride pentafluoride is also obtained5' when disulphur decafluoride reacts with chlorine at 200" and when chlorine trifluoride reacts with sulphur.ll The best preparative method however is the reaction of chlorine monofluoride with sulphur tetrafluoride at 380" which gives an almost quantitative yield:28 SF + CIF -+ SF,CI Sulphur chloride pentafluoride is colourless and has b.p. -21" and m.p. -64". The latent heat of vaporisation is 4560 cal. mole-1 and the Trouton constant (18.2) is somewhat lower than the usual value (21) for an unassociated liquid. The critical temperature is 1 15 0.58 The structure of sulphur chloride pentafluoride is that expected for a monosubstituted octahedron. This has been established by a study of the microwave which leads to a bond length of 1-58 & 0.01 A for S-F and 2.030 & 0.002 A for S-C1.These values are larger than those quoted for sulphur hexafluoride and disulphur di~hloride,,~ but the difference is less than the probable experimental error. The infrared and Raman spectra are also consistent with this structure.60 The heat formation of sulphur chloride pentafluoridesl has been deduced from its heat of hydrolysis to be 245 kcal. mole-l. Toegther with the S-F bond energy deduced from sulphur hexafluoride this leads to a value of only 45 kcal. mole-1 for the S-C1 bond energy which is considerably lower than the value of 61 kcal. mole-1 found for the much less stable disulphur dichloride. It is probable therefore that the S-F bond energy is less in sulphur chloride pentafluoride than in the hexafluoride.This is consistent with the theoretical work of Craig and Magnusson2 who suggest that the effect of replacing fluorine by a larger atom would be to weaken all the bonds in the molecule. Thermally sulphur chloride pentafluoride is somewhat more stable than disulphur decafluoride. It is recovered unchanged after being heated 65 B.P. Appln. 31,320/1958. 66 Roberts and Ray J. 1960 665. 68 Leach unpublished work. 6B Kewley Murty and Sugden Trans. Faraday Soc. 1960,56 1732. 6o Cross Roberts Coggin and Woodward Trans. Faraday SOC. 1960 56 945. 61 Leach and Roberts J. 1960,4693. George and Cotton Proc. Chem. Soc. 1959 317. ROBERTS COMPOUNDS CONTAINING S-F BONDS 41 to 350” in an inert container,5s but at 400” it is decomposed to sulphur hexafluoride sulphur tetrafluoride and chlorine Hydrolytically it is less stable being rapidly hydrolysed by aqueous sodium hydroxide although it is not attacked by neutral or acidic media SF,CI + 8NaOH -+ Na,SO + 5NaF + NaCl + 4H,O It liberates iodine rapidly from aqueous potassium iodide and bromine from potassium bromide but appears not to react with alkali-metal chlorides.Calcium chloride however is slowly attacked and its use as a drying agent for sulphur chloride pentafluoride should be avoided. The ease of hydrolysis of sulphur chloride pentafluoride is in marked contrast to the stability of disulphur decafluoride and other compounds containing the pentafluorosulphur group and suggests that attack is by the hydroxyl ion on the chlorine atom. As such a reaction does not easily take place with chlorotrifluoromethane this indicates that the chlorine in sulphur chloride pentafluoride is positive and a possible mechanism for the hydrolysis is 2SF,CI -f SF + SF + CI 6- s+ SF,CI + OH- -+ SF,- + ClOH SF,- 3 SF4 + F- SO,2- + HOCl -+ SO,2- + H+ + CI- the overall result being that given in the equation above.The positive nature of the chlorine atom receives support from the observation that sulphur chloride pentafluoride is a powerful oxidising agent. In addition to the halogen displacements mentioned it oxidises alcohols and aldehydes to carboxylic acids and aromatic amines to azo- compounds. Organometallic reagents such as phenyl-lithium or Grignard reagents react vigorously and reduce sulphur chloride pentafluoride to sulphur tetrafluoride.With benzene under Friedel-Crafts conditions it gives chlorobenzene again in agreement with the positive nature of the chlorine atom:s2 SF + 60H- -+ SO:- + 4F- + 3H,O SF,CI + C&& -f C,H,CI + [SF,H] 5. SF + HF The presence of positive halogen in sulphur chloride pentafluoride suggests that the properties of this molecule may be similar to those of trifluoromethyl iodide. Trifluoromethyl iodide can add across the double bond of olefins to give trifluoromethyl compounds the reaction taking place readily under the influence of ultraviolet light :63 CFSI + CZH4 + CF3.CH,*CH,.I 62 Case Ray and Roberts unpublished work. 63 Haszeldine “Fluorocarbon Chemistry,” Roy. Inst. Chem. Monograph No. 1 1957. 42 QUARTERLY REVIEWS This reaction always proceeds through primary attack of the CF,.radical on the olefin and usually takes place at a CH group where this is available or with fluoro-olefins at a CF group:g4 With sulphur chloride pentafluoride a similar series of reactions can take place;65 with olefins and acetylenes the reactions are similar to those of trifluoromethyl iodide. Thus with propene either under the influence of ultraviolet light at one atmosphere or at 90" in an autocalve 2-chloro- propylsulphur pentafluoride is produced Additions of this type have been carried out with ethylene propene butadiene and cyclohexene but with less reactive olefins lsuch as 1,2- dichloroethylene no reaction takes place and isobutene and styrene poly- merise in the presence of sulphur chloride pentafluoride. With fluoro- olefins which are less reactive thermal reactions must be catalysed by a free-radical initiator such as benzoyl peroxide but when this is done they proceed in a manner analogous to those of trifluoromethyl iodide by a mechanism involving attack of SF,.radicals on the olefin. In the case of addition to tetrafluoroethylene initiated by benzoyl peroxide CF,I + CH,CH:CH 4 CH,CHI*CH,-CF CH,.CH:CH + SFSCI + CH,.CHCI.CH,.SF5 SFSCI + C6H5. + SF,. f C6H5CI SF5 + CF2:CFz + sF,.CF2*CF2- SF,CF2CF2* + C2F4 -f SF5-[CFz]4. (propagation) SF,CF2.CF2- + sF,cI -f SF,*CF2-CF2CI + SF,. (chain transfer) products of the form SF,.[ CF2CF,] ;C1 are obtained. trifluoromethyl radical With unsymmetrical olefins the point of attack is the same as that of a SF,CI + CHF:CF2 -+ SF6-CHFCF2CI SF,CI + CFz:CFCI + SF5.CF2*CFC12 In photochemical reactions the primary step is the photolysis of sulphur chloride pentafluoride hv SF,CI -+ SF,.+ CIS Both these radicals then react with the olefin e.g. (a) CIS + CH2:CH2 + CICH2.CH2* (b) SF + CH2:CH2 4 SF,CHz*CH2- Both these radicals can now abstract a chlorine atom from sulphur chloride pentafluoride to liberate another SF,. radical. Reaction (b) therefore is self-sustaining but reaction (a) goes over to (b) after one step and thus the 64 Haszeldine and Steele J. 1957 2800. 65 Case Ray and Roberts J. in the press. ROBERTS COMPOUNDS CONTAINING S-F BONDS 43 SF,.CH,CH2C1 is present to the virtual exclusion of ClCH,CH,Cl. With unsaturated fluorocarbons of the two reactions (a) CIS + CF2:CF -+ CICF2-CF2. (b) SF,. + CF,:CF2 -+ SFs.CF2*CF2. (a) is very much the more rapid and the compound ClCF,*CF,Cl is found in addition to the SF,.[CF,-CF2] ,Cl produced in the thermal reactions. The mixture of products is difficult to separate and the method is of little preparative value. The reaction of sulphur chloride pentafluoride with acetylenes has been studied in two cases only. In these addition of only one molecule takes place CH:CH + SF,CI 3 SF,.CH:CHCI CH,-C:CH + SF,CI -+ SF,CH:CCI*CH In contrast to sulphur chloride pentafluoride itself these organic sulphur pentafluorides are stable to aqueous alkali. However the compounds in which hydrogen and chlorine are present on adjacent carbon atoms react with alcoholic potassium hydroxide to give alkenylsulphur penta- fluorides KOH-EtOH SF6*CH2*CH&I - 3 SF,*CH:CH A similar reaction with 2-chloropropenylsulphur pentafluoride leads to decomposition but if powdered potassium hydroxide in dry light petroleum is used prop- 1-ynylsulphur pentafluoride is produced SF,*CH:CCI*CH -+ SF5C i CCH The compound SF,.CHF.CF,Cl like SF,CHF, is decomposed by aqueous or alcoholic potassium hydroxide but with solid potassium hydroxide gives trifluoroviriylsulphur pentafluoride SF,CF :CF,.This instability to aqueous media of the group SF,-CFH is rather curious as SF,CH,R SF,CF2R and CHF are all inert to aqueous alkali. Compounds produced in these reactions are included in Table 2. Arylsulphur pentafluorides cannot be produced by this type of reaction but they have been obtained by reaction of silver difluoride with aryl- sulphur trifluorides :32 C6H,*SF3 + 2AgF2 -+ C6H5.SFs + 2AgF The SF group has a strongly deactivating influence on the benzene ring and nitration gives a m-nitro-derivative as would be expected for a strongly electronegative substituent.Perjluoroalkyl Derivatives of Sulphur Hexafiuoride. A number of perfluoroalkylsulphur pentafluorides and di(perfluoroalky1)sulphur tetra- fluorides have been obtained by oxidative fluorination of aliphatic thiols and bis-sulphides. The method most commonly used is electrochemical 44 QUARTERLY REVIEWS TABLE 2. Pentafluorosulphur compounds obtained by addition of SF5CI to unsaturated compounds. Compound M.p. SF5CH2CH2Cl SF5.(CH2*CH&Cl SF5.CH2.CHCl.CH3 SF5CH2CHCICH :CH2 2-Chlorocyclohexylsulphur pentafluoride SF5.CH :CCl.CH SF5CH2CHCI2 SF5*CH2CHClCH2-CHC12 SF5CH CH2 SF5CH CHCH SF,.CH,.CH :CH2 1 C yclo hexen ylsulp hur SF,-CrCMe SF,CH CHCl SF5CHClCH2Cl SF,.CF,.CF&I - 113" pentafluoride SF5-(CF2*CF.J2Cl -90 SF5.(CF2*CF.J3CI -5 SF,CF2CFCl SF5CF CF2 SF,CHFCF2Cl SF5CFCI-CF2CI B.p.92" 171-172 109 78-80/26 mm. 188-1 90 92 108 72'19 mm. 41 8&82 161 66 111 47 99 142 80-83 19 79-5-42 nto d20 1.3590 1.64 1.3900 1.3686 1 -4320 1.4783 1.3760 1,3840 1 -4269 1.4282 1 -3052 1.3079 fluorination in which the aliphatic compound is dissolved in anhydrous hydrogen fluoride and electrolysed under conditions such that hydrogen but no fluorine is evolved although other methods have been used.66-s* The scope of this type of preparation is indicated by the compounds listed in Table 3. These fully fluorinated compounds are chemically very inert being quite similar to sulphur hexafluoride and aliphatic fluorocarbons. At high temperatures trifluoromethylsulphur pentafluoride has been shown to act as a fluorinating agent.Thus at 500" it reacts with phosphorus nitride P,N5 to give a mixture of trimeric and tetrameric phosphonitrilic fluoride (PNFc3 and (PNFc34,69 while with hexafluoropropene it affords perfluoro- neopentane and other fluorocarbons.6e The structure of these perfluoroalkylsulphur pentafluorides and bisper- fluoroalkylsulphur tetrafluorides is that expected for derivatives of sulphur hexafluoride. The 19F nuclear magnetic resonance spectra indicate that 66 Lovelace Rausch and Postelnek "Aliphatic Fluorine Compounds," Amer. Chem. SOC. Monograph 138 Reinhold New York 1958 Chapter 13. 67 Dresdner and Young J . Amer. Chem. SOC. 1959 81 574. 68 Dresdner Reed Taylor and Young J. Org. Chem. 1960 25 1464.Mao Dresdner and Young J. Amer. Chem. SOC. 1959 81 1020. ROBERTS COMPOUNDS CONTAINING S-F BONDS TABLE 3. Fluorocarbon derivatives of sulphur hexafluoride. Compound M.p. B.p. n t5 CF3*SF5 -20" to -21" C2F5-SF5 11.3 n-C,F,SF 42 n-C4F,.SF5 70-5 n-C,F,,. SF 118.2 ClCF,.SF 16.5 -31 " SF5CF2COF (C2F5)2.NCF2CF2-SF5 CF,*SF4-CF3 C2F,- S F4 - C2F5 CYC~O- CGFI ,.SF5 C,F,*SF,*CF C3F7-SF4C3F7 C4Fs.SF4-C4Fs \ 17.2 1 24 110 20.5 47.1 70 116 154 80.3 88 112 82"/61 mm. 144.5 60 60 65 90-91 126"/37 mm. 71-72 65"/1 mm. 1 -2594 1.2710 1 ~2829 1.2921 1.3041 1 -2674 1 *2753 1.2856 1 -2943 1.3041 1.3010 1.3258 1-2884 1 -2598 45 d25 1.801 1.8541 1.8910 1.86 1.9530 1.803 1 -875 1 *865 1 -903 1.9031 2.041 1-772 the fluorine atoms in the SF group form a square pyramid while the C-S-C bond angle in compounds containing the group -SF,- may be either 90" or 180" as expected for octahedral co-ordination of the sulphur atom.70 Oxygen-containing Derivatives of Sulphur HexaJuoride.The two compounds bispentafluorosulphur oxide and peroxide SF5.0.SF5 and SF,.O.O.SF, are by photochemical oxidation of sulphur chloride pentafluoride. This reaction is formally analogous to that of trifluoromethyl iodide with sulphur to give first CF,.S.SCF and on further irradiation CF3.S*CF,.72 The probable mechanism is hu 2SF,CI + 0 --+ SF,*O*O.SF hu SF,*O*O*SF6 -+ SF,.0*SF6 + *02 this is supported by the fact that continuous removal of the products from 'O Muller Lauterbur and Svatos J. Amer. Chem. Soc. 1957 79 1043. 72 Haszeldine and Kidd J. 1953 3219. Roberts J. 1960,665. 46 QUARTERLY REVIEWS the reaction enhances the yield of the peroxide with respect to the oxide while irradiation of the peroxide leads to formation of the oxide.Bispentafluorosulphur oxide is a colourless liquid of b.p. 31". It is stable to aqueous alkali and appears to be quite inert. The peroxide b.p. 49" is also stable to alkali but reacts with quite a variety of organic compounds sometimes violently. Bispentafluorosulphur oxide like disilyl ether has the possibility of interaction of the lone-pair electrons on the central oxygen atom with vacant d-orbitals on the outer atom. With disilyl ether this leads to a large increase in the Si-0-Si angle from that observed in diethyl ether,73 which causes it to obey the spectroscopic selection rules for a linear molecule. A study of the infrared and Raman spectra of bispentafluoro- sulphur oxide while leading to no definite conclusions concerning the exact symmetry of the molecule does eliminate any possibility that the molecule has a linear S-0-S skeleton.74 Bispentafluorosulphur peroxide has been studied by electron diffra~tion~~ and has a structure similar to that of hydrogen peroxide the two SF groups replacing the hydrogen atoms. The S-F bond length 1-56 &- 0.02 A is normal and the S-0 bond length is 1.66 & 0.05 A. In addition to the two compounds SF,.O.SF and SF,-O-O.SF, there exists also pentafluorosulphur hypofluorite SF,.OF in which the SF group is bonded to oxygen. This compound is obtained by fluorination of thionyl fluoride over a silver difluoride catalyst :76 SOF + 2F2 -+ SF,.OF As would be expected for a compound containing a fluorine-oxygen bond it is very reactive.It attacks mercury and dissolves in aqueous alkali with the evolution of oxygen SF,*OF + 6OH- -+ $02 + SF- + S0,F- + 3H20 In this it is similar to trifluoromethyl hypofluorite. Its structure has been confirmed by electron diffra~tion;~' the SF g.roup is octahedral with the S-F bond length 1.53 & 0.04 A the same within experimental error as for other SF compounds which have been studied. The S-0 bond length 1.66 It is evident that where they exist SF compounds are similar to the CF compounds. There are however important differences. The SF group is reduced by organometallic compounds the CF group is not and the extensive chemistry based on metal and metalloid compounds of CF is therefore not likely to be reproduced by SF compounds.There is also a limit to the extent to which other atoms can replace fluorine in 0-05 A is similar to that in SF,.O.O.SF,. 73 McKean Taylor and Woodward Proc. Chem. Soc. 1959 321. '* Goggin Thesis Oxford 1960. 75 Harvey and Bauer J. Amer. Chem. SOC. 1954,76 859. '6 Dudley Cady and Eggers J. Amer. Chem. Soc. 1956 78 1553. 77 Crawford Dudley and Hedberg J. Amer. Chem. SOC. 1959 81 5287. ROBERTS COMPOUNDS CONTAINING S-F BONDS 47 sulphur hexafluoride which is not present for carbon tetrafluoride. Thus all members of the series CF, CF,Cl CF,C12 CFCl and CC14 are stable while from sulphur hexafluoride only sulphur chloride pentafluoride has been prepared and even the dichloride tetrafluoride is likely to be unstable. Neither pentafluorosulphur bromide nor the iodide has been reported.With hydrogen even the replacement of a single fluorine in the hexafluoride to give SHF, is unlikely. Despite these limitations however there is here a wider field of chemistry to be developed. Fluoro-oxyacids and Oxyfluorides of Sulphur 1 he known fluoro-oxyacids and oxyfluorides of sulphur are shown along with their parent oxyacids in Table 4. Fluorosulphurous Acid. Fluorosulphurous acid may be regarded as the monoacid fluoride of sulphurous acid and like sulphurous acid itself does not exist as a definite compound at room temperature though it gives rise to a series of salts with alkali-metal and ammonium cations. These salts are obtained by the reaction of sulphur dioxide on the ap- propriate fluoride; a reaction which proceeds more readily for larger cations such as K+ Rb+ and Cs+ than for the smaller Na+ and does not proceed at all with Lif.The fluorosul phites so formed have a measurable dissociation pressure even at room temperature KS0,F + SO + KF but X-ray investigation shows that they are distinct chemical compounds structurally similar to the isoelectronic chlorates and are not molecular complexes. The stability of the fluorosulphites and their solubility in sulphur dioxide also increase with increasing size of the cation. When heated at 170-180" in the presence of excess of sulphur dioxide the fluorosulphites disproportionate to fluorosulphates and sulphur. 2KSOzF + SO -f 2KS0,F + S Thus as where sulphur tetrafluoride yields sulphur hexafluoride and sulphur on pyrolysis this reaction indicates that sexivalent have higher stability than quadrivalent sulphur compounds.The fluorosulphites are very useful as reagents for replacement of chlorine by fluorine. The reactions take place under mild conditions and usually proceed cleanly and in high yield. Sulphuryl chloride can be converted successively into the chloride fluoride and the phosphonitrilic chlorides can be converted into fluorides with the same degree of poly- merisation (PNCI,) + 6KS0,F + (PNF,) + 6KCI + 6S0 Other compounds prepared from the corresponding chlorides by this reaction are benzoyl fluoride sulphuryl fluoride phosphorus trifluoride, 48 QUARTERLY REVIEWS TABLE 4. Fhoro-oxyacids and oxycfluorides of sulphur. Oxyacid OH / o=s 'OH Sulphurous acid 0 OH \s/ 0' \OH Sulphuric acid 0 0-OH Xs/ 0' \OH Permonosulphuric acid 0 0 II II H 0-S-0-S-0 H I1 I1 0 0 Disulphuric acid 0 0 II II HO-S-0-0-S-OH II II 0 0 Perdisulphuric acid Peroxydisulphuric acid 0 0 0 /I II II HO-S-0-S-0-S-OH 11 II II 0 0 0 Trisulphuric acid { Fluoro-oxyacid F / o=s 'OH Fluorosulphurous acid O F B.p.162-6" XS/ 0' 'OH Fluorosulphuric acid 0 0 I1 II II I/ 0 0 HO-S-0-S-F Fluorodisulphuric acid Oxyfluoride F / M.P. -1 o= s \ B.P. -43.7" F Thionyl fluoride 0 F M.P. -120" \s/ / \F B.P. -55.4" 0 Sulphuryl fluoride 0 0 . F \-/ M.P. -158.5" 0 /-\F B.p. -31.3" 0 Fluorine fluorosulphate (sulphuryl fluoride hypo- fluorite) 0 0 /I !I M.P. -48" F-S-0-S-F I/ I1 B.p. 51" 0 0 Disulphuryl difluoride 0 0 II 11 M.P. -55.4" F-S-0-0-S-F II 11 B.p. 67.1' 0 0 Peroxydisulphuryl difluor- ide 0 0 0 II It II F-S-0-S-0-S-F II II II B-P. 120" 0 0 0 Trisulphuryl fluoride F M.p.-99.6" o=s B.P. -49.0" F I'F Thionyl tetrafluoride ROBERTS COMPOUNDS CONTAINING S-F BONDS 49 and arsenic trifluoride ; complex-forming fluorides such as boron trifluoride are isolated as the salt of the fluoro-acid BCI + 4KS0,F -+ KBF + 3KCI + 4S0 This reaction cannot however be used to prepare sulphur difluoride or disulphur difluoride as no reaction takes place between fluorosulphites and the sulphur ~ h l o r i d e s . ~ ~ ~ ~ ~ Thionyl Fluoride. Thionyl fluoride which is the diacid difluoride of sulphurous acid is a colourless gas with a pungent and irritating smell. It is obtained by a variety of reactions usually involving the replacement of chlorine by fluorine.36 The most convenient small-scale preparation is the reaction of thionyl chloride with antimony trifluoride which can be con- veniently carried out in ordinary Pyrex vessels but on a larger scale the reaction of anhydrous hydrogen fluoride with thionyl chloride in steel vessels is better.The product can be freed from traces of sulphur dioxide by distillation.80 It is also the first product of hydrolysis of sulphur tetrafluoride. Structurally thionyl fluoride is similar to thionyl chloride both mole- cules having a trigonal pyramidal structure based on the tetrahedral arrangement of four a-bonding pairs of electrons one of which is a lone pair.14 The microwave spectrum of thionyl fluorides1 indicates the following structural parameters rso = 1.412 i- 0.001 A YSF = 1-585 i- 0.001 A LFSF = 92” 49’ & 5’ LOSF = 106” 49‘ 5‘. The bond length rso is therefore shorter than the value of 1.43 found for sulphur dioxide and sulphuryl chloride;52 YSF however is similar to that in sulphur hexa- fluoride and other sulphur-fluorine compounds.The infrared and the Raman spectras2 of the thionyl halides have been studied and are consistent with the above structure. It is of interest that the bond-stretching force constant kso is 11.0 x dynes cm.-l for thionyl fluoride and 9.69 x dynes cm.-l for thionyl chloride. These variations in bond length and force constant are consistent with the expected increase in the pr-dr- double-bond character of the S-0 bond due to the increasing electro- negativity of the halogen. Thionyl chloride fluoride SOClF is also known but apart from its preparation and characterisation has not been further studied.s3 Chemically thionyl fluoride is somewhat less reactive than the cor- responding chloride.It is only slowly hydrolysed by water to give sul- phurous and hydrofluoric acid and with ammonia gives the amide SO(NH&2.36 Unlike sulphur tetrafluoride it does not react with carbonyl Seel and Riehl 2. anorg. Chem. 1955 282 293. Seel and Langer 2. anorg. Chem. 1958,295 316. Brauer (ed.) “Praparative Anorganische Chemie,” Ferdinand Enke Verlag Ferguson J. Amer. Chem. SOC. 1954 76 850. Stuttgart 1960. 82 Cotton and Horrocks Spectrochim. Acta 1960 16 358. 83 Jonas 2. anorg. Chem. 1951 265 273. 50 QUARTERLY REVIEWS groups but with carboxylic acids under forcing conditions it gives acid fluorides R-C02H + S 0 F 2 -+ R-COF + HF + SO Fluorosulphuric Acid. Fluorosulphuric acid is the monoacid fluoride of sulphuric acid. It can be prepared in a variety of ways which are all essentially the reaction of hydrogen fluoride with sulphur trioxide or the fluorination of chlorosulphuric acid.Industrially it is prepared by com- bining sulphur trioxide with anhydrous hydrogen fluoride :36984 SO + HF -f HSO,F The anhydrous acid is a colourless liquid which fumes in moist air and dissolves in water with evolution of much heat. Rather remarkably it can be distilled at atmospheric pressure in Pyrex apparatus (b.p. 162-6”) a property which makes it a convenient laboratory fluorinating agent. The acid is isoelectronic with perchloric acid and as would be expected is a strong acid in aqueous solution. The free acid in water is to some extent hydrolysed an equilibrium being established HSO,F + H20 + H,SO + HF If however the solution is made alkaline it is possible to drive off the water and isolate the alkali-metal fluorosulphates.This method cannot be used for the alkaline-earth or other metals whose sulphates or fluorides are insoluble but these fluorosulphates as well as those of the alkali metals can be prepared by the reaction of sulphur trioxide on the metal fluoride at 200”.85 CaF + ZSO -+ Ca(SO,F) This reaction is similar to the formation of fluorosulphites from fluorides and sulphur dioxide but in this case is not confined to the alkali-metal and ammonium salts. When strongly heated the alkali-metal and calcium fluorosulphates decompose by reversing their formation reaction and so give fluorides and sulphur trioxide but strontium and barium fluorosulphate give sulphates and sulphuryl fluoride Ba(SO,F) -+ BaSO + S02F2 This difference in behaviour on pyrolysis may be due to some minor variation in the structure of the solid The fluorosulphates of the elements of Groups other than IA and IIA can also be prepared both by the action of fluorosulphuric acid on chloridess6 and by the reaction of sulphur trioxide on fluoride~.~~,~’ ** Lange in “Fluorine Chemistry” (ed.J. H. Simons) Academic Press New York 86 Hayeck Puschmann and Czaloun Monarsh. 1954 85 359. 13’ Clark and Emeleus J. 1957 4778. 1950 Vol. I pp 167-182. Muetterties and Coffman J. Amer. Chem. SOC. 1958 80 5914. ROBERTS COMPOUNDS CONTAINING S-F BONDS 51 These compounds are frequently double salts such as AlCl(SO,F), TiCl(SO,F), ZrF2(S03F)2,86 WF2(S03F), NbF,(SO,F), and TaF (SO,F),. They are salt-like in nature and are hydrolysed readily by water.Fluorosulphates of non-metallic elements are also known. Nitrosyl fluorosulphate which is a white hygroscopic solid can be obtained by reaction of dinitrogen trioxide with fluorosulphuric acid :36988 N,O + 2HS0,F -f 2NO+SO,F- + H20 Nitronium fluorosulphate is obtained in a similar manner from dinitrogen pentoxide and fluorosulphuric acid :36989 N20 + HS0,F -f NO,+SO,F- + H,O Phosphorus does not appear to form a fluorosulphate as phosphorus trifluoride and sulphur trioxide do not react,85 but phosphorus penta- chloride and fluorosulphuric acid give phosphorus oxyfluoride.86 In contrast with this arsenic trifluoride does react with sulphur trioxide giving a product of b.p. 142" whose empirical formula corresponds to 2AsF3,3SO,. A study of the AsF,-SO system by nuclear magnetic reson- ance ~pectro~copy~~ indicates that the interaction is complex and that several compounds are formed.The structure (I) is proposed for the 2 3 constant-boiling mixture. Antimony trifluoride behaves more as a metal fluoride with sulphur trioxide and gives a white solid thoughts5 to be a trisfluorosulphate S b( SO ,F) ,. The behaviour of the halogens in forming fluorosulphates is also inter- esting. Fluorine reacts with sulphur dioxide in the presence of a silver difluoride catalyst to give a compound F.SO,.OF fluorine fluoro- ~ulphate.~~ This compound is related to fluorine perchlorate and contains a fluorine-oxygen bond and it can also be formally regarded as the diacid fluoride of peroxymonosulphuric acid. It is highly reactive and in bulk is liable to explode without warning.Its structure has been confirmed by nuclear magnetic resonance spectros~opy.~~ Chlorine itself appears not to form a fluoro~ulphate,~~ but chloryl fluoride with sulphur trioxide gives a red liquid chloryl fluorosulphate ClO,.SO,F. 94 Bromine forms two 88 Lange Ber. 1927 60 967. 8 9 Goddard Hughes and Ingold J. 1950 2559. Gillespie and Oubridge Proc. Chem. Soc. 1960 308. 91 Dudley Cady and Eggers J. Amer. Chem. SOC. 1956 78 290. 92 Dudley Schoolery and Cady J. Amer. Cltem. SOC. 1956 78 568. 93 Roberts and Cady J. Amer. Chern. SOC. 1960 82 352. 94 Woolf J. 1954,4113; Schmeisser and Ebenoch Angew. Chem. 1954,66 230. 52 QUARTERLY REVIEWS distinct fluorosulphates BrSO,F a red liquid and Br(SO,F), a solid of m.p. 59 O by reaction with peroxydisulphuryl diflu~ride;~~ iodine forms only I(SO,F) in a similar reaction.Another compound IF,(SO,F) is formed from fluorine fluorosulphate and iodine. Thus with fluorine a rather weak 0-F bond is formed with chlorine the 0-CI bond would be thermodynamically unstable with respect to CI, and bromine and iodine both form bonds to the fluorosulphate rather more readily giving structures in which the halogen is positive. Sulphuryl Fluoride. Sulphuryl fluoride the diacid difluoride of sulphuric acid is readily obtained by fluorination of sulphur dioxide with elementary fluorine,s5 by replacement of chlorine in sulphuryl chloride by means of a variety of reagents,78 or by heating barium fluorosulphate.80 A careful studys6 has been made of its thermodynamic properties from 1 2 " ~ to its boiling point at 217.78"~ (-5538"c) and from this study a standard entropy of 62-66 e.u.at the boiling point has been obtained. The structure of sulphuryl fluoride has been established by microwave ~pectro~copy.~~ The bond lengths are YSF = 1.530 & 0.003 A (SF, 1.58 A 0.02 A) and rso = 1.405 & 0.003 I$ (SO, 1.43 & 0.02); the bond angles are LOSO = 123" 58' & 12' and LFSF = 96" 7' & 10'. The structure is therefore a distorted tetrahedron with LOSO greater than the LFSF as would be expected owing to the greater electron density in the S-0 bonds. The S-F and S-0 bonds are significantly shorter than in sulphur hexafluoride and trioxide respectively. The infrared and Raman spectra are consistent with this ~tructure.~~ The spectroscopic entropy at the boiling point is 63.24 e.u.which is greater than the calorimetric value and it is suggested that this difference may be due to disorder in the crystals at O'K owing to the close similarity in size between the bonded oxygen and the fluorine The heat of formation of gaseous sulphuryl fluoride has been estimated as 205 kcal. mole-' by combining the appearance potentials of the ions SO2+ and S02F2+ with the known heat of formation of sulphur dioxide and the dissociation energy of molecular fluorine.s8 If the S-F bond energy derived from sulphur hexafluoride (76 kcal. mole-l) and the S-0 bond energy from sulphur trioxide 2o (104 kcal. mole-l) were used the heat of formation would be only 147 kcal. mole-'. The actual bond energies are higher than this as would be expected from the observed shortening of the S-F and S-0 bonds in sulphuryl fluoride compared with those in sulphur hexafluoride and trioxide respectively.Chemically sulphuryl fluoride is inert. It is hydrolysed slowly by aqueous sodium hydroxide to sodium fluorosulphate and sodium fluoride SO,F + 2NaOH + NaS0,F + NaF + H,O O5 Moissan and Lebeau Compt. rend. 1901 132 374. O6 Bockhoff Petrella and Pace J . Chem. Phys. 1960 82 799. O7 Hunt and Wilson Spectruchim. Acta 1960 16 570. sg Reese DibeIer and Franklin J. Chem. Phys. 1958 29 880. ROBERTS COMPOUNDS CONTAINING S-F BONDS 53 It does not react further with fluorine even at 400° whereas sulphur trioxide gives fluorine fluorosulphate under similar conditions. Fluorodisulphuric Acid and Disulphuryl Dijluoride. There are no reports in the literature of fluorodisulphuric acid but a calcium salt of this acid is thought to be an intermediate in formation of disulphuryl difluoride from calcium fluoride and sulphur trioxide :85 + 2s0 CaF + 2S0 -+ Ca(SO,F) - -f Ca(S,O,Fd H,O in cow.hSSO -+ CaSO + H2S04 + S2O,F2 Disulphuryl difluoride is also produced by decomposition of certain fluorosulphates reaction of fluorosulphuric acid with arsenious and fluorination of the appropriate It is a colourless liquid which hydrolyses slowly to fluorosulphuric acid and is therefore formally the anhydride of this acid. Peroxydisulphuryl Dzyuoride. Peroxydisulphuryl difluoride may be regarded as the diacid fluoride of peroxydisulphuric acid. It is obtainedgg by reaction of fluorine with an excess of sulphur trioxide at 250° or at a lower temperature over a silver difluoride catalyst 2S0 + F -+ FSO,.O*O*SO,F The liquid (b.p.67") is quite stable and can be distilled without decom- position. Its reactions with the halogens have already been described. It also reacts with sulphur dioxide to give trisulphuryl fluoride:91 (FSO,*O) + SO -+ FS02*O*S02*O*S02F Thionyl Tetrafluoride. In addition to the sulphur(vr) oxfluorides containing the group SOzF there exists a compound thionyl tetrafluoride SOF4 which is unique in that the sulphur atom has a co-ordination number of five. It is obtained by the reaction of fluorine on thionyl fluoride :SO SOF + F -f SOF or by oxidation of sulphur tetrafluoride by nitrogen dioxide:27 SF + NO -+ SOF + NO It is probable that its structure is similar to that of sulphur tetrafluoride being a trigonal bipyramid with the oxygen in an equatorial position similar to that occupied by the lone pair of electrons in sulphur tetrafluoride.The nuclear magnetic resonance spectrum of thionyl tetrafluoride shows only a single peak at the temperature at which it was studied but it would probably show a similar pattern to that of sulphur tetrafluoride16J7 at lower temperatures as fluorine exchange would then be slower. O 9 See1 and Simon Angew. Chem. 1960,72 709. 54 QUARTERLY REVIEWS It is hydrolysedjby aqueous media to sulphuryl fluoride SOF + H,O -+ SO,F + 2HF but less rapidly than sulphur tetrafluoride and it can be handled in glass apparatus with only the normal precautions to exclude moisture. With ammoniag9 it reacts to give the ammonium salt NH FSN [:I which when heated affords ammonium fluoride and a polymeric sulpha- murylfluoride ( - := N ) which is isoelectronic with phosphonitrilic \ / n fluoride (PNF,),.Like sulphur tetrafluoride it forms complexes with boron trifluoride and with arsenic and antimony pentafluoride,26 but these are much weaker. The boron trifluoride complex has a vapour pressure of one atmosphere below O" whereas the arsenic and the antimony pentafluoride complex have measurable decomposition pressures at room temperature. See1 and Detmer2' regard these complexes as ionic e.g. SOF,+AsF,-. If this is the case the ion formed SOF,+ is isoelectronic with the very stable sulphuryl fluoride S02F,. No other reactions of thionyl tetrafluoride have been reported. Sulphur-Nitrogen-Fluorine Compounds In contrast to the other compounds containing sulphur-fluorine bonds no compounds in which sulphur is sexivalent and bonded to both nitrogen and fluorine have been fully characterised but it is probable that this is due more to the lack of experimental work on this type of compound than to any chemical principle.The first compound of this type to be prepared was tetrathiazyl tetrafluoride S4N4F4 which is obtainedlOO by cautious fluorination of tetrasulphur tetranitride with silver difluoride. The SIN ring remains loo Glemser Schroder and Haeseler Z. anorg. Chem. 1955 279 28. ROBERTS COMPOUNDS CONTAINING S-F BONDS 55 intact and the fluorine atoms are bonded to sulphur. This is shown by its hydrolysis by aqueous sodium hydroxide to give sodium sulphite which means that the sulphur atom is quadrivalent. The equivalent trimeric compound has been obtained by fluorination of the trimeric chloride.lol A more vigorous fluorination of tetrasulphur tetranitride leads to a mixture of two compounds SN,F and SNF whose structures have not been determined but probably do not involve S-F bonds; this mixture reacts furtherlo with silver difluoride to give the compound SF,=NF fluoroiminosulphur difluoride.This compound is similar in physical and chemical properties to thionyl fluoride. It is a colourless gas b.p. -23" and with dilute sodium hydroxide gives ammonia sodium sulphite and sodium fluoride FN=SF + 5NaOH -+ Na,SO + 3NaF + NH + H,O A series of compounds the iminosulphur difluorides derived from this by replacement of the fluorine bonded to nitrogen can be obtained by reaction of sulphur tetrafluoride with nitriles isocyanates and thiocyan- ates :lo3 R*C= N + SF +- RCF,*N=SF R.N=C=O + SF + R*N=SF + COFa NaOCN + ZSF -+ CFa*N=SF + NaF + SOF The compound trifluoromethyliminosulphur difluoride is also obtained by fluorination of methyl thiocyanate,lo3 although it was originally thought that an isomer pentafluorosulphur cyanide was formed.lo4 On further fluorination this compound gave a product thought then to be SF,CF,-NF2 but more likely to be CF,.NF.SF,.If this is the case it is the only known compound containing a pentafluorosulphur group bonded to nitrogen. Rather surprisingly this compound is unstable.lo4 The unstable compound SN2F mentioned above decomposes on being heated; it gives the com- pound NSF isoelectric with sulphur dioxide which it resembles closely in its chemical and physical properties.lo5 It boils at 4.8" and is hydrolysed by sodium hydroxide to ammonia sodium sulphite and sodium fluoride which indicates that its structure must be NGS-F.In addition to these compounds fluorination of tetrasulphur tetra- nitride produces a variety of unstable compounds which have not been isolated and further study of this reaction is needed if it is to be com- pletely understood. It is also probable that a range of compounds con- taining the group SF,.N could be prepared. lol Schroder and Glemser 2. anorg. Chem. 1959 298 78. lo2 Glemser and Schroder 2. anorg. Chem. 1956,284 97. Io3 W. C. Smith Tullock R. D. Smith and Engelhardt J. Amer. Chem. SOC. 1960 l o 4 Attaway Groth and Bigelow J. Amer. Chem. Soc. 1959 81 3599. lo5 Glemser and Haeseler Z. anorg. Chem. 1956,287 54. 82 551.

ISSN:0009-2681    

DOI:10.1039/QR9611500030

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

THE GIBBERELLINS By JOHN FREDERICK GROVE (IMPERIAL CHEMICAL INDUSTRIES LIMITED AKERS RESEARCH LABORATORIES THE FRYTHE WELWYN HERTS.) 1. Fungal gibberellins THE discovery of the gibberellins originated from an investigation of a soil-borne disease of rice caused by the fungus Gibberella fujikuroi. Infected plants eventually wilt and die but at an early stage of the disease called “bakanae” in Japan the leaves and stems of some seedlings elongate more rapidly than those of healthy plants. In 1926 Kurosawal showed that cell-free filtrates from cultures of the fungus produced in healthy seedlings the elongation symptons characteristic of the disease and eventually in 1938 Yabuta and his collaborators2 succeeded in isolating from such culture fluids a crystalline active material which they named gibberellin A.The chemistry and plant-growth promoting properties of this material were described in a series of papers from the University of Tokyo these papers have been re-assembled and re~iewed.~ More recently in an attempt to repeat the isolation of gibberellin A from cultures of G. fujikuroi,* a group of workers at the Akers Research Laboratories of Imperial Chemical Industries Limited obtained a new active metabolite gibberellic acid,4 which differed from gibberellin A in its physical and chemical properties. The same compound was also isolated at the North Regional Research Laboratories Peoria U.S.A. where5 it was named gibberellin X until the identity with gibberellic acid was established.6 The American workers obtained a mixture of two compounds gibberellic acid and gibberellin A, which was similar to gibberellin A.These results led to a re-examination of the crude gibberellin produced by various Tokyo University strains of G. fujikuroi and gibberellic acid (gibberellin A3) gibberellin A, and a third active metabolite gibberellin A, were eventually obtained.’ Gibberellin A, another component of the mixture was described later,8 and the exact composition of the gibberellin A obtained in 1938 remains obscure. Modification of the fermentation conditions used to produce gibberellic acid has led to the isolation by the Kurosawa Trans. Nat. Hist. SOC. Formosa 1926 16 213. 2Yabuta and Sumiki J. Agric. Chem. Soc. Japan 1938 14 1526; Yabuta and Hayashi J. Agric. Chem. SOC. Japan 1939 15 257. Stodola “Source Book on Gibberellin 1828-1957,” U.S. Dept. of Agriculture 1958.* Curtis and Cross Chem. and Ind. 1954 1066. Stodola Raper Fennell Conway Sohns Langford and Jackson Arch. Biochem. Cross J. 1954,4670. Takahashi Kitamura Kawarada Seta Takai Tamura and Sumiki Bull. Agric. * Takahashi Seta Kitamura and Sumiki Bull. Agric. Chem. SOC. Japan 1957,21,396 1955,54 240; Stodola Nelson and Spence Arch. Biochem. 1957,66,438. Chem. Soc. Japan 1955 19 267. 56 GROVE THE GIBBERELLINS 57 group at Imperial Chemical Industries Limited of gibberellin A,9 and gibberellin A9.10 Gibberellic acid has been produced in much greater yieldll than the other gibberellins and a more extensive investigation of its chemistry and biological activity has been carried out than was possible with gibberellin A. The structure and some of the stereochemistry of gibberellic acid has been elucidated and the structures of the other gibberellins have been related to gibberellic acid.This contribution from the group at Imperial Chemical Industries Limited has been reviewed in a comprehensive treatise.12* Despite widespread searches there is no well-authenticated example of the production of a gibberellin by a fungus other than G..fujikuroi. 2. Occurrence of gibberellins in higher plants The fungal gibberellins were found to promote many normal processes of plant-growth development and this led to the discovery that compounds similar in chemical structure and physiological properties were widely distributed in higher plants. Mitchell Skaggs and Anderson13 had shown in 1951 that ether extracts of immature dwarf-bean seeds contained a substance which stimulated the elongation of seedling epicotyls to a degree much exceeding that observed after auxin application-an effect now recognised as gibberellin-like.More recently many w o r k e r ~ ~ * - ~ ~ have obtained similar extracts from the seed roots and shoots of a wide variety of plants.7 From immature seed of the runner bean Pl~aseolus multiJEorus MacMillan and his collaborator^^^ isolated gibberellin A in a yield of 2 mg./kg. fresh weight of seed together with three new compounds gibberellins As,,' A6,I8 and West and Phinney19 have also obtained gibberellins A and A (bean factor 11) from seed of the French bean Phaseolus vulgaris and gibberellin Al has been isolated from young shoots of Citrus reticulata Blanco var. unshiu.20 The presence of gibberellic acid * When this treatise was written all the available evidence favoured a trans-fusion of rings A/B antipodal to that normally found in diterpenes and an a-orientation for the ring A lactone bridge the latter is now believed to be fl-oriented (see Section 6) but no ring A stereochemistry has been adopted here since no definite conclusions have been reached.t For a more extensive bibliography see references 12 and 23. Cross. Galt. and Hanson. Tetrahedron Letters. 1960. No. 15. 18. lo Cross; Galt and Hanson; Tetrahedron Letters 1960 No. 23 22. l1 Brit. P. 838033. l2 Brian Grove and MacMillan Progr. Chem. Org. Nat. Prod. 1960,18,350. l3 Mitchell Skaggs and Anderson Science 195 1 114 159. l4 Phinney West Ritzel and Neely Proc. Nat. Acad. Sci. U.S.A. 1957 43 398. l5 Radley Nature 1956,178 1070; Ann.Bot. 1958,22 297. l6 Lona L'Alteneo Purmense 1957,28 11 1. l7 (a) MacMillan and Suter Natiirwiss. 1958 45 46; (b) MacMillan Seaton and Suter Proc. Chem. SOC. 1959,323; (c) Tetrahedron 1960,11 60. l8 MacMillan Seaton and Suter unpublished work. l9 West and Phinney J. Amer. Chem. Soc. 1959 81 2424. 2o Kawarada and Sumiki Bull. Agric. Chem. SOC. Japan 1959 23 343. 58 QUARTERLY REVIEWS in green maltz1 and in several higher plantsz2 has also been claimed on the basis of chromatographic evidence but the isolation of gibberellic acid from these sources has still to be achieved. 3. The significance of the gibberellins in plant physiology The discovery of the gibberellins in plant tissues implies that they are natural hormones with a regulating function in many aspects of plant growth and development.Only very small quantities of some gibberellins are available and little biological work has been done with them; but so far as is known they all have a qualitatively similar action. In most circumstances gibberellic acid is the most active followed by gibberellin A,.24 Exceptionally members of the Cucurbitaceae respond most readily to gibberellins A and A,.25 In the simple case of plants whose growth is not rigorously determined by day-length or temperature a gibberellin increases the length of the stem internodes without altering their number and only those internodes actually extending at the time of application are affected.26 In bushy plants a gibberellin enhances apical dominan~e.~' In general genetically dwarf plants show the greatest response and treated plants are similar to the tall varieties.Increases in both cell-division2* and cell-size2' are involved in gi b berellin-induced s t em-ex t ension. In the more complex case of plants whose development is determined by day-length or temperature or both the effect of a gibberellin is more intricate. Many biennials which normally require a period of cold treat- ment can be induced to bolt and flower by gibberellin applicati~n.~~ A gibberellin will similarly replace the long-day requirements of some plants30 and will terminate the dormancy induced by short-days in deciduous shrubs and trees;31 and the development of autumn foliage colours and leaf-fall can be Although the gibberellins promote cell elongation they differ from the auxins in many ways. For example they have little or no action on root which is inhibited by auxins; neither do they initiate roots on 22 Adler Medwick and Johl 138th Meeting Amer.Chem. SOC. New York Sept. 23 Phinney and West Ann. Rev. Plant Physiol. 1960 11 411. 24 Bukovac and Wittwer Nature 1958 181 1484. 25 Lockhart and Deal Naturwiss. 1960 47 141 ; Brian and Hemming Nature 1961 26 Brian and Hemming Physiol. Plant. 1955 8 669. 27 Brian Elson Hemmmg and Radley J. Sci. Food Agric. 1954 5 602; Brian 28 Sachs Bretz and Lang. Amer. J. Bot. 1959 56 376. 2 9 Lang Naturwiss. 1956 43 257 284. 3O Bukovac and Wittwer Quart. Bull. Mich. Agric. Exptl. Stn. 1957 39 650. 31 Lockhart and Bonner Plant Physiol. 1957 32,492. 32 Brian Petty and Richmond Nature 1959 183 58. 33 Brian Hemming and Radley Physiol. Plant. 1955 8 899. Lazar and Dahlstrom personal communication.ll-l6th 1960 Abs. p. 25A. 189 74 Hemming and Lowe Physiol. Plant. 1959,12,15. GROVE THE GIBBERELLINS 59 cuttings an effect which is promoted by auxins. Exogenous auxins have a large effect on the extension of isolated plant tissues but little effect on intact plants; the gibberellins on the other hand induce large responses in intact plants but have relatively little effect on shoot or coleoptile sections unless auxins are added. This synergism between the gibberellins and auxin^,^^-^^ and the nature of the interaction between the gibberel- lins and other growth regulators such as the kin in^,^' is not fully under- stood. The failure of the gibberellins to simulate the effects of cold treatment or long days in a number of cases (reviewed elsewhere12) may be a consequence of such hormonal interactions in which hormones other than the gibberellins are limiting.The practical uses of the gibberellins in agriculture have been reviewed.38 The gibberellins are used to stimulate swelling of fruits e.g. grapes and tomato; and they are more effective than auxins in inducing partheno- carpic fruit setting in tomato. They can also be used to break dormancy of seeds of e.g. lettuce peach and Douglas fir and to accelerate the germination of barley. Growth of bacteria and fungi is unaffected2' by gibberellic acid which has negligible mammalian 4. Nomenclature The systematic nomenclat~re~~ is based on the trivial name gibbane for the fully saturated tetracarbocyclic system (I) numbered as shown. The 8,9-bridge in (1) is /3 (absolute configuration) and the ring system derived from gibbane by inversion at positions 7 and 9a is called 7a-gibbane.Gibberellic acid (2 ; R = OH) is thus 2,4a,7-trihydroxy-l-methyl-8- methylenegibb-3-ene- 1,10/3-dicarboxylic acid 1 +4a-lactone. Catalytic reduction (steric control) of an 8-methylene group gives epimeric pairs of methylgibbanes in which the absolute configuration at position 8 is not known with certainty. Only one epimer is obtained by chemical reduction (thermodynamic control) of gibberellic acid and is arbitrarily called an 8-methylgibbane the epimer is called an 8-epimethyl- gibbane. The 8-methyl compounds may be expected to have the less 34 Brian and Hemming Nature 1957 179 417; Ann. But. 1958 22 1. 36 Purves and Hillman Physiul. Plant. 1958 11 29. 36 Galston and Warburg Plant Physiul.1959 34 16. 37 Wickson and Thimann Physiul. Plant. 1958 11 62. 38 Wittwer and Bukovac Ecun. But. 1958 12 213. 39 Peck McKinney Tytell and Byham Science 1957 126 1064. 40 Grove and Mulholland J. 1960 3007. 60 QUARTERLY REVIEWS hindered configuration and in this event will be 8cc-methyl compounds. The term gibberellic acid nor-ketone is used to describe the 8-ketone obtained by oxidative removal of the 8-methylene substituent. Trivial names e.g. allogibberic acid (18) and gibberic acid (8) are retained for degradation products in which ring A is aromatic. Within this class the prefix epi is reserved for those compounds in which the 4b- hydrogen atom is /3-oriented. Degradation products in which ring D of gibbane has been opened are named as derivatives of fluorene e.g.9p-carboxy-4ba,5,6,7,8,8a-hexa- hydro- 1 -methyl-7-oxofluorenyl-8a~-acetic acid (24; R = H). The gibberellins are defined as a group of naturally occurring plant hormones containing the tetracyclic system (1). As new gibberellins are isolated they are allotted trivial names in the series gibberellin A . . . . A,. This procedure is adopted because the names gibberellin B and gibberellin C were given by the Japanese workers to allogibberic acid (18) and the 7a-gibbane (34; X = H OH; R = H) respectively of which the former has no plant-growth promoting proper tie^.^^ 5. The gibberellins and their structural relationships The structures and physical properties of the gibberellins are listed in the Table. All are gibbane-10P-carboxylic acids with a 1+4a lactone bridge. Gibberellic acid and gibberellin A have an 8-methylene substituent Compound Structure Formula M .P . ~ [a]; SourceC Gibberellic acid 2; R=OH C1gH2206 233-235” - T92 F Gibberellin A 4; R=R’=OH R”=H c,QH,o,( :::-258 +36 F Gibberellin A2 5 Gibberellin A4 4; R=R”=H R’=OH C,QH,O~ 21k215 - 3 Gibberellin 6 Gibberellin A 2 ; R = H C19H2205 202 f20 F Gibberellin A8 4; R=R=R=OH C1gH240 210-215 +30 P c1QH2606[:&237 + 12 F Gibberellin A 3 C ~ Q H & ~ 260-261 -77 P 222-225 Gibberellin A 4; R=R’=R=H CigH2404 208-211 -12 F aDecomposition point. The alternative values given are for polymorphic forms. dLater work shows this to be a 2,3-epoxide CleH220,. ethanol or methanol. cisolated from fungus (F) or higher plant (P). and d3 double bond in gibberellin A the latter is replaced by a A 2 double bond. Gibberellins A, A, A, and A have the 8-methylene substituent as the only unsaturated centre in gibberellin A 2 this has been saturated by the addition of the elements of water.With the exception of 41 Brian Grove Hemming Mulholland and Radley Plant Physiol. 1958 33 329. GROVE THE GIBBERELLINS 61 gibberellins A, A, and A all have a 2(ax)-hydroxyl group while gib- berellic acid and gibberellins Al A, A, and A8 have a 7(eq)-hydroxyl substituent. Gibberellin A is unique in having a hydroxyl group at posi- tion 3. The position of one hydroxyl group in gibberellin A has still to be located (see however the Table). The relations between the gibberellins were elucidated as follows (for practical convenience the methyl esters were frequently used). Reduction of the 3,4- double bond in gibberellic acidg2 and gibberellin A,g gave gibberellin A and with simultaneous reduction of the 8- methylene substituent dihydrogibberellin A respectively.Reduction with zinc and acetic anhydrideg3 of the 7-hydroxy-8-ketone obtained from gib- berellin A by ozonolysis gave gibberellin A nor-ketone (as its acetyl derivative). The relation between gibberellins A and A has been con- firmedlO by an alternative route. Treatment of gibberellin A with dilute mineral acidg4 gave gibberellin A,; and the action of collidine on the 2-toluene-p-sulphonyl derivative of gibberellin Al gave gibberellin The 7-methyl-8-0~0-7a-gibb-2-ene derived from gibberellin A by the acid-induced rearrangement of rings C/D (see p. 65) proved to be a key compound in relating gibberellin A to gibberellin A and to A,.'' Hydroxy- lation with osmium tetroxide of the 2,3- double bond gave the correspond- ing 7-methyl-8-ketone obtained from gibberellin A,; and catalytic reduction of the 2,3- double bond gave a 7-methyl-8-0~0-7a-gibbane obtained from gibberellin A6 by a reaction sequence in which after rearrangement of rings C/D the remaining hydroxyl group was replaced by halogen and the halogen removed by treatment with Raney nickel.Collidine treatment of the toluene-p-sulphonyl derivative of gibberellin A nor-ketone methyl ester followed by catalytic reduction gave gibberellin A nor-ketone methyl ester.1° 6. The chemistry of gibberellic acid The evidence for the structure and stereochemistry of gibberellic acid is discussed first in paragraphs (a)-(d) and some of the reactions commonly encountered with the gibberellins are mentioned in paragraph (e).Gibberellic acid (2; 'R = OH) formed mono- and di-acetyl derivatives each of which yielded a monomethyl ester as did the acid itself.6 One of the hydroxyl groups is secondary since in the reduction products of gibberellic acid it was oxidised to a ketone :45 the other was considered to be tertiary from the difficulty of acylation. When gibberellic acid was kept with excess of alkali at room temperature a second equivalent was con- sumed this together with the fact that the infrared spectra of the acid and 4 2 Grove Jeffs and Mulholland J. 1958 1236; Takahashi Seta Kitamura and Sumiki Bull. Agric. Chem. SOC. Japan 1957 21 327. 43 Kitamura Takahashi Seta Kawarada and Sumiki Bull. Agric. Chem. SOC. Japan 1959,23 344. 44 Grove unpublished work.45 Cross J. 1960 3022. 62 QUARTERLY REVIEWS its derivatives showed a strong band near 1780 cm.-l indicated the presence of a y-lactone ring. Microhydrogenation revealed the presence of two ethylenic double bonds. This evidence6 showed that gibberellic acid was a tetracarbocyclic dihydroxylactonic carboxylic acid. Further information about the ring HO m~ W O H -OH 1 M . O H CH2 H02C CH2 H02 (2) R'j$-&. Hoc$+oH Me H0* CH2 CH2 H 02C R H02C (4) (5) (See however the Table p. 60.) system was derived largely from the study of two products of acid hydro- lysis allogibberic acid (18) and gibberic acid (8). Selenium dehydrogenation of both allo- gibberic and gibberic acid has earlier given46 a hydrocarbon gibberene which from its ultraviolet spectrum was regarded correctly as a substituted fluorene but was incorrectly given the formula C16H16 and 4-ethyl-5- methylfluorene was suggested as a possible structure.By degradation to the known fluorene- 1,7-dicarboxylic acid Mulholland and Ward47 showed that gibberene was 1,7-dimethylfluorene (11; R = Me) Cl5HI4 and this was confirmed by unambiguous synthesis47 from 2-amino-5-methylbenzoic acid and o-tolylmagnesium bromide via the intermediate benzophenone (b) Structure of gibberic acid. Treatment of gibberellic acid or allo- gibberic acid with boiling dilute mineral acid gave6 gibberic acid (8) C18H2003 m.p. 153-154" or 174-175" [ M I D -7" together with an isomer epigibberic acid (23) m.p. 227-230" or 252-255" [RID + 131". The formation of an ester an oxime and an oxime ester showed that gibberic acid was a keto-acid and a band at 1741 cm.-l in the infrared spectrum indicated that the ketone group was present in a five-membered ring.Microhydrogenation showed the absence of ethylenic double bonds but the ultraviolet spectrum (Amax265 274 mp; log E 2-56 2-47) revealed the presence of a benzenoid ring. The presence of the hexahydrofluorene nucleus in the tetracarbocyclic system (8) was established by stepwise d e g r a d a t i ~ n ~ ~ ~ ~ via the a-diketone (9) and the tricarboxylic acid (12) to 1,7-dimethylfluorene and by oxidation of gibberic acid to benzene- 1,2,3- tricarboxylic The substitution pattern of the five-membered ring (a) Structure of gibberene. (17). 46 Yabuta Sumiki Aso Tamura Igarashi and Tamari J. Agric. Chem. SOC. Japan 47 Mulholland and Ward J. 1954 4676. 48 Cross Grove MacMillan and Mulholland J.1958 2520. 1941 17 975. GROVE THE GIBBERELLINS 63 containing the ketone group was deduced from ultraviolet absorption studies on the cc-diketone (9) which was found to have no enolisable hydrogen atom. The position of the carboxyl group was establi~hed~~ by dehydrogenation of the methyl ester of (9) to methyl 1,7-dimethyl- fluorene-9-carboxylate identical with a synthetic specimen prepared by carboxylation of the 9-lithium derivative of 1,7-dimethylfluorene followed by methylation. 1. at? Me 2 __c 3 c- h - - - I (91 0 0 { 3 M e H02C C0,H 0 2) AcHN 07) Me CH,-Ci.H CMe I C q M e C0,Me (16) Me CHART 1 Reagents 1 KMnO,. 2 SeO,. 3 Pd-C. 4 Se. 5 H,O,-NaOH. 6 CrO,. 7 KMn0,-Mg (NO,),. The position of the -CH,CO- bridge followed from a second stepwise degradati~n~~ in which the key compound was a ketone gibberone C1,Hl,O (10) obtained directly from gibberic acid by dehydrogenation over palladium-charcoal or indirectly by decarboxylation of dehydro- gibberic acid (7) (Amax.260 269 mp; log E 4.14 4-09) a permanganate oxidation product of gibberic acid. Oxidation of gibberone with chromic oxide gave the 1-oxoindanespirocyclopentanone (13) which was stable to hydrolysis and was not therefore a 1,2’-diketone. Further oxidation of the indanone (1 3) gave 3-methylphthalic acid (14) and /3-methyltricarballylic acid (15); and opening of both five-membered rings by second-order Beckmann rearrangement ofthe 2’-oximino-compoundgave after hydrolysis 64 QUARTERLY REVIEWS and methylation two diastereoisomeric tetramethyl esters (16) m.p.83- 84" ( a ) D -6' and m.p. 47-48" (a)D +12". The structure (16) of the esters was confirmed by the unambiguous synthesis of their race mate^.^^ This synthesis completed the elucidation of the structure of the indanone (13) and consequently of gibberone (10). It followed that the methylene carbonyl bridge in gibberic acid must be attached as in (8). Racemic gibberone has recently been synthe~ised.~~ (c) Structure and stereochemistry of allogibberic acid. With cold dilute mineral acid both gibberellic acid6v41 and the intermediate gibberel- lenic acid (19)51 gave allogibberic acid (18) C1SH2003 m.p. 201-203" ( a ) D -84" and 1 mol. of carbon dioxide With hydrazine hydrate gibberellenic acid gave both allogibberic acid and an isomer epiallo- gibberic acid (20),40 m.p. 244" ( a ) D +87".Allogibberic acid (Amax- 266 274 mp; log 6 2.50 2.35) contained a benzenoid ring and an ethylenic double bond present in an exocyclic methylene grouping since ozon01ysis~~ gave formaldehyde and a nor-ketone Cl7Hl8O4 (21). The carboxyl group was attached in the same position as in gibberic acid since the methyl ester was isomerised with acid to methyl gibberate. The third oxygen atom was present as a hydroxyl group (vmax. 3460 cm.-l) considered to be tertiary because of the difficulty of acylation and the failure to oxidise dihydro- allogibberic acid to a ketone. These facts together with the following evidence established allogibberic acid as a tetracarbocyclic hydroxy-acid (18) the presence of an a-ketol system in a five-membered ring in the nor-ketone (21) was shown by the infrared spectrum (vmax.1742 cm.-l) and by oxidation with sodium bi~muthate~~ to a tricyclic dibasic keto-acid C17Hls05 (24; R = H) in which the carbonyl group was contained in a saturated six-membered ring. The position of this carbonyl group and hence of one point of attachment of the five-membered ring in allogibberic acid was ascertained by selenium dehydr~genation~~ to 8-methylfluoren-2- 01 (1 1 ; R = OH) whose structure was proved by ~ynthesis.~~ With aqueous alkali the keto-ester (24; R = Me) gave the acid (24; R = H) together with a new dibasic keto-acid C17H1805 epimeric at position 9. With acetic anhydride both dibasic acids gave the same cis- anhydride but hydrolysis of the anhydride regenerated only the acid (24; R = H) derived directly from allogibberic acid.The 10-carboxylic acid substituent and 8,9-two-carbon bridge are therefore cis in allogibberic acid.40 Since catalytic redwtion of dehydrodihydroallogibberic acid (26) a 4b(5)-ene related to dehydrogibberic acid (7) will take place from the less-hindered side of the molecule opposite to the 10-carboxyl substituent *O Morrison and Mulholland J. 1958 2536. 5 0 Loewenthal Proc. Chem. SOC. 1960 355. 61 Moffatt J. 1960 3045. 5a Mulholland J. 1958 2693. 53 Morrison and Mulholland J. 1958 2702. GROVE THE GIBBERELLINS 65 and 8,9-bridge7 and since this process regenerated the original stereo- chemistry at position 4b,40 it followed that rings B/C were trans-fused in allogibberic acid. The absolute configuration (1 8) followed from measure- ments of optical rotatory dispersion on the keto-ester (24; R = Me).40754 In addition to racemisation at position 9 the ester (24; R = Me) on CHART 2 Reagents 1 H+.2 N2H4,H20. 3 0,. 4 NaBiO,. 5 OH-. treatment with base underwent*O an intramolecular Claisen-type condensa- tion at position 6 this was followed by fission of the 6,7-bond and hydro- lysis liberating the 4ba78aa,9cc-isomer (25) of the acid (24; R = €€). Epiallogibberic acid which gave epigibberic acid (23) on acid treatment was chemically similar to allogibberic acid and yielded the 4b/3,8a/3,9p- enantiomer (22) of (25) on ozonolysis followed by fission of the resulting cc-ketol. It followed that epiallogibberic acid differed from allogibberic acid only in configuration at position 4b.40 The acid-catalysed rearrangement of these acids takes place by a Wagner- Meerwein m e c h a n i ~ r n ~ ~ .~ ~ via the cation (27) and in the resulting gibberic acids the 8,9-bridge has the opposite configuration (a) to that which it occupied in the allogibberic acids. The racemate of the ester (28) a reduction product of the ketone (22) has been ~ynthesised.~~ 54 Stork and Newman J. Amer. Chem. Soc. 1959 81 3168. 55 Grove MacMillan Mulholland and Turner J. 1960 3049. 56 House Paragamian and Wluka J. Amer. Chem. Soc. in the press. 3 66 QUARTERLY REVIEWS (d) Structure of gibberellic acid. The position of the carboxyl sub- stituent in gibberellic acid was the same as in allogibberic acid since the methyl ester with acid gave methyl gibberate. An early suggestion5' that the two acids had the same B/C/D structure was by the ozono- lysis of methyl gibberellate which took the same course as the ozonolysis of methyl allogibberate giving formaldehyde and ultimately a keto-acid (32; R = H).The methyl ester (32; R = Me) of the latter gave the ester (24; R = Me) with acid and the formation of allogibberic acid from gibberellic acid therefore involves only the aromatisation of ring A. Ring A of gibberellic acid must accommodate the methyl group which appeared at position I in the fluorene degradation products the saturated y-lactone ring and the secondary hydroxyl group which was shown to be allylic by oxidation with manganese of methyl gibberellate to the d3-2-one (29) (Amax. 228mp; log E 3-99). Catalytic hydrogenation of the ketone (29) afforded the 2-ketones (3 l) epimeric at position 8 also obtained HO w methyl gi bberellate 0 MeCqR 4 (R=H) Me02C (30) 0 1 CHART 3 Reagents 1 MnOz.2 Hz/Pt. 3 H2/Pd. 4 Cr03. 5 03. by oxidation with chromic oxide of methyl tetrahydrogibberellate and its 8-epimer. The position of the allylic hydroxyl group was e ~ t a b l i s h e d ~ ~ ~ ~ by selenium dehydrogenation of the ketones (35; R = H X = H2 Y = 0) and (34; R = Me X = 0) both derived from gibberellin A, to l-methyl- fluoren-2-01 and 1,7-dimethylfluoren-2-01~~ respectively. The position of 57 Cross Grove MacMillan and Mulholland Chem. and Id. 1956 954. 58 (a) Seta Takahashi Kitamura Takai Tamura and Sumiki Bull. Agric. Chem. SOC. Japan 1958 22 61 ; (6) Seta Takahashi Kitamura and Sumiki Bull. Agric. Chenz. SOC. Japan 1958 22,429. 59 Cross and Melvin J. 1960 3038. GROVE THE GIBBERELLINS 67 the lactone bridge was deduced from the high yield of acidic hydrogen- olysis products on catalytic reduction of methyl gibberellate which suggested an allylic lactone system and from the formation of a hetero- annular conjugated diene gibberellenic acid51 (19) (Amitx.253 mp; log E 4.35) from gibberellic acid in aqueous solution. It was concludedGo that 0 0 gibberellic acid has structure (2; R = OH) and this conclusion was supported by nuclear magnetic resonance studiesG1 An alternative ring A structure (36) has been s~ggested~~b but is inconsistent with much of the foregoing evidence. The quasi-axial nature of the 2-hydroxyl substituent in gibberellic acid was deduced from the base-catalysed isomerisation of gibberellin Al to the more stable 2(eq)-hydro~y-epimer.~~ The rotary dispersion curvc s for the keto-esters (35; R = Me X = 0 Y = H,OH) and (24; R = Me) were almost identicalG3 and significantly different from that for the methyl ester of the keto-acid (22) obtained from epiallogibberic acid.Gibberellic acid therefore probably has the same trans-^/^ ring fusion as allogibberic acid but more evidence on this point would be desirable. Oxidation and de- carboxylation of the 8-epimethyl acid (30; R = H) obtained by hydrogeno- lysis of methyl gibberellate gave the keto-ester (33) the rotatory dispersion curve of which showed a negative Cotton effect,63 but this fact is in itself in- sufficient to determine unequivocally the nature of the A/B fusion in the acid (30; R = H). The a~signment,~~ from nuclear magnetic resonance studies of a trans-orientation to the hydrogen atoms at positions 10 and 10a now appears to be unwarranted.61 Some controversy arose over the interpretation of the physical evidence for the orientation of the 1+4a lactone bridge.64 The chemistry of the ester (30; R = Me)65 indicated that the 2-hydroxyl substituent was equatorial and that consequently hydrogenolysis of methyl gibberellate involved inversion of configuration at position 4a.This chemical evidence is decisively in favour of a /%orientation for the lactone bridge but a rigorous proof of the configuration at position 10a is still required. Cross Grove MacMillan Moffatt Mulholland Seaton and Sheppard Proc. Chem. SOC. 1959,302. G1 Sheppard J. 1960 3040. G2 Cross Grove and Morrison J 1961 in the press. 63 Cross Grove McCloskey Mulholland and Klyne Chem. and Ind. 1959 1345. 64 (a) Stork and Newman J .Arner. Chern. SOC. 1959,81,5518; (6) Edwards Nicolson Apsimon and Whalley Chem. and Ind. 1960 624. G5 Grove and McCloskey unpublished work. 68 QUARTERLY REVIEWS (e) General chemical reactions of the gibberellins. Some of the more important reactions encountered with the gibberellins are indicated below. (i) 2(ax)-Hydroxygibbane 1 +4a-lactones are epimerised in dilute alkali to the more stable 2(eq)- hydroxy-compounds and a retroaldol mechanism via the intermediate (37) has been suggested.62 Under the same conditions62 2(ax)-hydroxygibb-3-ene 1+4a-lactones undergo an allylic type of rearrangement to gibb-4-ene 1 +3-lactones without concomitant epimerisation of the hydroxyl substituent. 0 (ii) In the presence of a l+4a-lactone bridge catalytic reduction of a 3-ene precedes that of an 8-methylene substituent ; but the 8-methylene group is reduced before a 4-ene group in gibb-4-ene 1+3-lactones.The latter are difficult to reduce and hydrogenolysis of the lactone pre- dominates. 62 (iii) 2-Hydroxyl groups are smoothly oxidised to 2-ketones only in gibbanes in which an 8-methylene substituent has been reduced or eliminated.45 The 2-ketones are reduced by alkali-metal hydrides to the 2(eq)-hydroxy-~ompounds.~~ (iv) In gibbanes 2(ax)-hydroxy-substituents are readily eliminated,17c either directly under the influence of nucleophilic reagents or via the toluene-p-sulphonate giving gibb-2-enes. (v) Compounds containing the C/D partial structure (38; R = OH) undergo Wagner-Meerwein rearrangement with acid to give ketones of partial structure (39).55 Under the same conditions the elements of water are added to the 8-methylene group in compounds (38; R = €3).(vi) Among degradation products in which ring A is aromatic compounds having a 10-methoxycarbonyl substituent in the less stable configuration undergo base-catalysed racemisation at this centre. Only compounds in which the 4b-hydrogen atom is trans to the 10-carboxylic acid substituent are oxidised by permanganate to 4b(5)-enes in some cases neighbouring groups cause steric inhibition of this r e a ~ t i o n . ~ ~ * ~ ~ The configuration at position 10 also determines the steric course of the catalytic reduction of 4b(5)-enes hydrogenation occurring trans to a 10-carboxyl or 10-methoxy- carbonyl substituent. 7. Biogenesis of the gibberellins Inspection of structure (2; R = OH) showed that it could have arisen by a variant of the processes leading to the tricyclic diterpene skeleton (40) in which (i) the 17-carbon atom had been lost (ii) contraction of ring B to GROVE THE GIBBERELLINS 69 a five-membered ring had occurred with extrusion of a carboxyl group and (iii) formation of the phyllocladene-type of bridged-ring structure had occurred from ring c and its substituents according to the scheme suggested by Wenkert.6s The correctness of these speculations was e~tablished~~ by the degradation of gibberellic acid obtained from G.fujikuori grown on 0 0 [carboxy-14C] acetic acid [*indicates labelled atom in (40)] and [2-14C] mevalonic lactone (42) ; the results of the degradation were consistent with the labelling pattern (41) expected from the usual mode of incorporation (40) of these precursors in a tricyclic diterpene.The stereochemical implications of this important work have been the subject of much peculation.^^^^^ Two points are clear however first oxidation at position 7 in the gibberellins is subsidiary to the main biogenetic process and this may also be true of the hydroxylation of ring A. Secondly since the 17-carbon atom is lost and contraction of ring B is likely to involve an intermediate 10-0x0-derivative it is not necessary to postulate that gibberellic acid is derived from other than the normal (1 1 a) type of trans-anti-trans-hydrophenanthrene precursor (40). 8. The gibberellins in diterpene chemistry Many terpen~ids~~ and more commonly compounds with isoprenoid side chains70 are known among fungal metabolic products but few isolated.Of these only the groups related diterpenoids have been OH (4 4) to rosenon~lactone~~ (43) and gibberellic acid have been extensively investi- gated. Few diterpenes have the 2-hydroxy-substituent which occurs frequently 66 Wenkert Chem. and Ind. 1955 282. 67 Birch Ricjcards and Smith Proc. Chem. SOC. 1959 192; Birch Rickards Smith 68 Djerassi Cais and Mitcher J. Amer. Chem. SOC. 1959 81,2386. 69 (a) Haagen-Smit Progr. Chern. Org. Nat. Prod. 1955 12 1 ; (b) Jones and Halsall 'O Birch English Massy-Westropp and Smith J. 1958 369. 71 Harris Robertson and Whalley J. 1958 1799 1807; Freeman Morrison and Harris and Whalley Tetrahedron 1959 7 241. ibid. p. 44. Michael Biochem. J. 1949,45 191. 70 QUARTERLY REVIEWS in the gibberellins. Among those possessing this feature several e.g.dar~tigenol~~ (44) and andr~grapholide~~ (45) also have the abnormal antipodal trans-A/B stereochemistry which however is not invariably associated with- the presence of a 2-hydroxyl group e.g. eperuic (46) and cassaic (47). The axial configuration of the 2-hydroxyl substituent in the gibberellins is also unusual although similarly situated axial hydroxyl groups have been recorded in the triterpenoids e.g. in the mould product polyporenic acid A.69b The 8,9-two-carbon bridge in ring c carrying an 8-methylene substituent is found in the tetracarbocyclic diterpenes related to phyllocladene (48).76 The structure of steviol been amended to (49) and the steviol -+ isosteviol rearrangement is now recognised as analogous to the acid- induced conversion of allogibberic into gibberic acid.Contraction of ring B in a diterpene skeleton such as (40) may be assumed to involve a 9,lO-dioxygenated intermediate and xanthoperol (51)78 provides an example of a diterpenoid 9,lO-diketone. Benzilic acid rearrangement of the enantiomer of methyl-9,1 O-dioxopodocarpa- 5,7,13(14)-trien-16-oate has been shown to give the hydrofluorene- carboxylic acid (51).79 OH The gibberellins differ from all other tri- and tetra-cyclic diterpenoids in that selenium dehydrogenation gives substituted fluorenes instead of substituted phenanthrenes. Apart from this fact the more characteristic reactions (see Section 6e) are associated with the 2-hydroxygibb-3-ene 1 +4a-lactone and 2-hydroxygibbane 1 +4a-lactone systems for which there are no exact analogies among natural products. 72 Diara Asselineau and Lederer Bull. SOC. chim. France 1959 693. 73 Cava and Weinstein Chem. and Ind. 1959 851; Chan Haynes and Johnson 74 Barltrop and Bigley Chem. and Ind. 1959 1447. 76 Turner Herzog Morin and Riebel Tetrahedron Letters 1959 No. 2 7. 78 Briggs Cain Davis and Wilmshurst Tetrahedron Letters 1959 No. 8 13. 77 Dolder Lichti Mosettig and Quilt J. Amer. Chem. SOC. 1960 82 246. 78 Bredenberg Acta Chem. Scand. 1960,14,385. Chem. and Ind. 1960,22. Grove and Riley J. 1961 1105.

ISSN:0009-2681    

DOI:10.1039/QR9611500056

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

NUCLEAR FISSION ByG. N. WALTON (UNITED KINGDOM ATOMIC ENERGY AUTHORITY HARWELL) BY nuclear fission is meant the division of a heavy atomic nucleus into two parts of about equal mass. The phenomenon is a physical one and a particular concern of nuclear physics but nuclear fission was discovered by chemists and followers of that discipline have contributed greatly to our present knowledge of the subject. Although this knowledge is now considerable there are still many aspects of fission for which no satis- factory theoretical explanation is known or is likely to be known until more experimental evidence is available. The chemist with his ever- widening range of techniques is still in the best position both to extend old and to make new observations in this field. Bearing this in mind the inten- tion in writing this review is to describe the part that chemistry has played and will continue to play in its development.Discovery of Fission Unlike most other subjects the enquiry into nuclear fission is not recorded in -a continuous sequence of publications because in 1940 the possible application of nuclear reactions to military purposes led to a withholding of information. Consequently the work by which fission was discovered is not well known. However before 1940 work had been carried out largely in academic laboratories and was published in the normal way. The early chemistry much of which was done under the leadership of Otto Hahn in Berlin appears mainly in German publications. It is often thought that a chance observation led Hahn and Strassmann in 1939 to postulate that the uranium nucleus could divide into two parts but in fact it was the culmination of an exceedingly tantalising investiga- tion lasting over several years and pursued in a number of laboratories.This started in 1934 soon after the discovery of the neutron when Fermi suggested in an article to Nature1 that the bombardment of uranium with neutrons might lead by ,&decay of the initial product to new trans- uranic elements with atomic numbers higher than 92. From the nuclear charge the electronic structure of the first new element (atomic number 93) would be expected to be homologous with those of manganese technetium and rhenium. This last prediction was made before the possibility of filling the 5f electron shell to give elements homologous with the rare-earths had been appreciated.Neutron capture in uranium might also be expected to lead by proton release [(n,p) reactions] or a-particle release [(n,or) reac- tions] to elements of lower atomic number. These processes if they occurred would give protactinium (,,Pa) thorium (,,Th) perhaps actinium * E. Fermi Nature 1934 133 898. 71 72 QUARTERLY REVIEWS (89A~) and radium (8,Ra). All these predictions were rapidly but for the most part mistakenly confirmed. Fermi’s original note describes how by precipitating manganese dioxide from a solution of uranium irradiated with neutrons from a radium- beryllium source /3-activity with a 13-minute half-life could be separated which did not appear to belong to U Pa Th Ac Ra Fr Rh Bi or Pb and might therefore be due to a new element. His suggestion was criticised by Noddack who pointed out that manganese dioxide would adsorb a great variety of elements and she intimated even at this early date that light elements might be present.The 66Eka’S-elements.-In 1935 Hahn and Meitner published their first paper on the subject in which they confirmed that materials with 13- minute and 90-minute activities coprecipitated with manganese from irradiated uranium solutions and they proved rigorously that the activity was not from uranium protactinium or thorium. However subsequent work by Hahn published during 1936 and 1937 failed to confirm the 13-minute and 90-minute half-lives but disclosed a p-active nuclide with a 23-minute half-life which was correctly assigned to uranium; this is now known to be uranium-239 one of the precursors of plutonium.Resolution of the p-active materials coprecipitated with manganese and with rhenium was continued as these were considered to be the new transuranic atoms. Osmium gold and platinum homologues were also found. In each case careful work was done with the intention of showing that the new elements were not isotopes of lower atomic number than uranium. For this purpose the new active element was mixed with natural products such as UZ the 6-6-hour protactinium isotope “:Pa and a separation by precipitation was made. The separation was proved by following the half-lives of the separated precipitates and matching them with the half-lives before mixing. By this means new transuranic elements were recognised and named eka-rhenium eka-osmium eka-iridium and eka-platinum. Their proper- ties were reviewed in a paper2 by Hahn Meitner and Strassmann in 1937 and the following observations are typical of those by which they were identified.Eka-rhenium and rhenium Comparisons (a) Neither electrochemically separated from noble metals in acid solution. (b) Neither separated by coprecipitation with metallic bismuth from alkaline solution. (c) Both quantitatively precipitated by hydrogen sulphide from moderately concentrated hydrochloric acid but not from strong acid. 0. Hahn L. Meitner and F. Strassmann Ber. 1937 70 1374. WALTON NUCLEAR FISSION 73 (d) They formed mixed crystals on precipitation of rhenium (e) As chloride and oxide both were volatile at high (a) Eka-rhenium was not volatile from sulphuric acid Inspection of the original curves of j3-active decay by which the new elements were recognised shows that they were in fact mixtures of fission- product elements including probably technetium (technetium- 101 has a 14-minute half-life and was probably mistaken as the 13-minute eka- rhenium) ruthenium and rhodium.These fission products are still notorious for their ubiquity in analytical separations and could easily give rise to the observations quoted. In 1938 a long review3 appeared on the transuranic elements in which over 90 references were quoted. This makes curious reading because the conclusions from the work described are now known to be nearly all wrong. The evidence for the “eka”-elements was accepted. The possibility that the new elements were formed by filling of the 5felectron shell as originally discussed4 by A. von Grosse in 1935 is dismissed because of the chemical similarity between the “eka”-elements and the transition elements.However the review pointed out many inconsistencies in the evidence and in this way must have contributed to the discovery of fission in the follow- ing year. The “Radium” Isotopes.-The second part of Fermi’s original suggestion was also mistakenly confirmed. Elements were found in the product of neutron irradiation of uranium that behaved like radium and actinium but as their radioactivity differed from that of natural radium and actinium they were thought to be isotopes or isomers of the natural elements. These species are now known to belong to the fission products namely barium and the rare-earths but a great deal of effort was yet to be expended in trying to attribute them to the products of (n,p) or (n,a) reactions on uranium as Fermi had suggested.In Italy in 1935 D’Agostino and E. Segrk turned their attention to the irradiation of thorium with neutrons. This also gave rise to the active elements that behaved like radium and actinium. It was hoped that the (4n + 1) transmutation series would be found. The 4n (natural thorium) series the (4n + 2) (natural uranium) series and the (4n + 3) (natural actinium) series had been recognised but in 1935 the (4n + 1) series was not known. In the confusion that followed the attempt to identify members of the (4n + 1) series amongst the fission products the isotope thorium-233 with a 23-minute half-life which is one of the precursors of uranium-233 was correctly identified. Interest in the natural transmutation chains with nitron.temperatures. solution in a stream of hydrogen chloride. Contrast L. L. Quill Chem. Rev. 1938 23 87. A. von Grosse J. Amer. Chem. SOC. 1935,57 440. 74 QUARTERLY REVIEWS brought I. Curie and Savitch into the field in 1937 and a year later5 they came very close to the correct interpretation. They investigated closely a 3.5-hour activity which was supposedly due to an actinium isotope. This material they mixed with natural lanthanum and with MsTh(rr) (the 6.1-hour actinium isotope actinium-228 obtained from natural thorium). Lantha- num oxalate was then fractionally crystallised ; the 3.5-hour activity followed the lanthanum rather than the actinium. Curie and Savitch stated that the 3-5-hour activity from irradiated uranium (now known to be 3.8-hour lanthanum-141) was more like lanthanum than actinium.Unfortunately however they found and this must have misled them that the new activity could also be fractionated from lanthanum. Probably the new activity was not due to pure lanthanum-141; their material doubtless contained yttrium which also has an isotope with a 3.5-hour half-life (yttrium-92). In the meantime physicists were studying the radiation emitted when uranium is bombarded with neutrons. They were looking for high-energy a-particles from the expected (n,cc) reactions and might well have been expected to note the pulses caused by recoiling fission fragments which would be much greater than a-pulses. In Germany von Droste observed these pulses which he attributed to 9-Mev cc-particles. The large pulses due to the fission fragments probably escaped notice because the uranium sources were often thinly covered to shield the ion chambers from the low-energy natural a-radiation and this covering would be sufficient to stop the heavy fission fragments.S . Curran has said that at the Cavendish Laboratory the large pulses were observed but were attributed to electronic faults . In 1938 Hahn and Strassmann repeated Curie and Savitch’s work but concentrated on the supposed radium isotopes. Four of these had been recognised; “Ra-I” 1 min. “Ra.11” 14 min. “RaIII” 86 min. and “Ra.IV” 10-13 days. All had active daughter-products and there were too many to fit in with the known isotopes of radium and actinium. The supposed radium isotopes were eventually proved to be 18-min. barium-141 85-min. barium-139 and 12.8-day barium-140.The “Ra.IV” was mixed with natural Th-X (3.6-day radium-224) and barium carrier added. Barium bromide was then fractionally crystallised to effect a separation of barium and radium. The “Ra-IV” followed the barium fraction and the radium-224 followed the radium. Similarly as in Curie and Savitch’s work “Ac.11” (the 3.5-hour daughter of Ra.11) followed lanthanum and not actinium when MsTh(rr) (6.1-hour actinium-228) and lanthanum oxalate were fractionally crystallised. This was the evidence that led Hahn and Strassmann in 1939 to publish the celebrated paper,6 “The detection and behaviour of alkaline-earth metals formed in the irradiation of uranium with neutrons.” They suggested that the uranium I. Curie and P. Savitch Compt. rend. 1938 206 906. 0. Hahn and F. Strassmann Nafurwiss.1939 27 11 89. WALTON NUCLEAR FISSION 75 nucleus had divided and pointed out that the masses of barium-138 and masurium- 101 (technetium-101 familiar from the “eka” elements) added up to 239 the mass of uranium plus a neutron. Subsequently they pointed out correctly that “Ra.111” (86 min.) had a half-life correspond- ing to that of barium-139 (85 min.) which is formed by the neutron irradia- tion of natural barium and that “Ac.IV” (35 hours) had a half-life corresponding to that of lanthanum-140 (40 hours) which is formed by the neutron irradiation of natural lanthanum. Thus they reasoned that “Ra-IV” must be identical with barium-140. At this time the lighter elements strontium and yttrium were also recognised amongst the uranium products. By blowing air through irradiated solutions of uranyl nitrate xenon and krypton isotopes were removed and their casium and rubidium daughter-products were identified.Following Hahn and Strassrnann’s paper it was immediately realised by Meitner and Frisch,’ and independently by Joliot,8 that the fission process would release a large amount of energy and was therefore feasible. This energy would appear as the kinetic energy of the two fragments recoiling from each other by the coulombic repulsion of their nuclear charge. This was confirmed experimentally when the effect of the energy was observed in the large pulses from an ionisation chamber and in the visible tracks several centimetres long in a cloud chamber. Much of this early work is described in an important review9 by Turner produced just before government classification prevented all further publication.The remarkable work carried out by a large team of chemists in the U.S.A. during the War on the detailed analysis of the fission products was admirably reportedlo by Coryell and Sugarman in 1951. There was a break of twelve years before the next comprehensive account by Whitehouse,ll and since then a number of reviews have been written. Thus Walton12 has discussed the recoil effect and Halpern13 paid particular attention to the angular distribution effects. A recent comprehensive note14 by E. Hyde reviews the whole field of nuclear fission giving special atten- tion to the measurements of fission yields and neutron emission. There are a number of review papers in the proceedings15 of the two Geneva Con- ferences and a book “The Physics of Fission,” has been translated from the Russian.16 L.Meitner and 0. R. Frisch Nature 1939 143 239. F. Joliot Compt. rend. 1939 208 341. L. A. Turner Rev. Mud. Phys. 1940 12 1. lo C. Coryell and N. Sugarman National Nuclear Energy Series “The Fission Pro- l1 W. J. Whitehouse Prug. Nuclear Phys. 1952 2 120. l2 G. N. Walton Prog. Nuclear Phys. 1957 6 192. l3 I. Halpern Ann. Rev. Nuclear Sci. 1959 9 245. l4 E. Hyde Univ. California Reports U.C.R.L. 9036 9065 1960. l5 Proc. Internat. Conf. Peaceful Uses of Atomic Energy Geneva 1955 Vol. 7; l6 “The Physics of Fission,” trans. J. E. S. Bradley Pergamon Press London 1958. ducts,” 195 1 McGraw-Hill New York. 1958 Vol. 15. 76 QUARTERLY REVIEWS Description of Fission The term fission is normally used for processes in which a nucleus divides into two main fragments of comparable mass.It is distinguished from “spallation” a term used for a reaction in which a nucleus looses a number of nucleons but otherwise remains intact and from “fragmen- tation” in which small and large fragments not necessarily in the same yield are produced in high-energy reactions. A few fissions lead to a ternary process in which an a-particle or a tritium nucleus is produced as well as the two main fragments. Ternary and quaternary fissions giving three or four particles of comparable size have been reported but these events are not well substantiated. As first observed fission was initiated by the capture of neutrons but it is now known to be a general process which can be initiated in a variety of ways. Soon after its discovery an explanation of the process was put forward by Bohr and Whee1er.l’ Using the analogy of a charged liquid drop for the nucleus they showed that as the nucleus became larger it would become less stable.This is because the repulsive forces of the positive charges increase as the protons become more numerous in a manner proportional to Z2/A* 2 being the number of protons and A* being a measure of the distance between nucleons (A the total number of nucleons determines the volume of the nucleus). At the same time the surface tension holding the drop together increases as the area of the surface i.e. as A+. By introducing the numbers that show how the mass or binding energy of a nucleus varies with 2 and A they were able to predict quantitatively that a small distortion could cause nuclei in the region of uranium to become unstable.The energy necessary to give the small initial distortion is called the fission threshold energy or potential barrier for fission. The energy level of this barrier for a number of nuclides is illustrated in Fig. 1 .18 From the same reasoning it follows that the products of fission will be more stable than the parent nucleus because primarily their surface to volume ratio is larger. As can be seen from Fig. 1 a nucleus can be in such a state that distortion energy is unnecessary and it undergoes spontaneous fission. As would be expected this occurs more readily the heavier the nucleus. A slow spon- taneous rate with a half-life of 10l6 years has been detected for uranium-238 but for californium-254 the half-life is about 56 days.When a nucleus captures a thermal neutron that is one that has shared its kinetic energy with its surroundings the excitation energy introduced is equal to the difference between the ground-state mass of the capturing nucleus plus the mass of the neutron and the ground-state mass of the compound nucleus. For instance as shown in equation (l) 2tiU acquires an excitation energy of 0-0069 of an atomic mass unit which is l7 N. Bohr and J. A. Wheeler Phys. Rev. 1939 56,426. l8 E. Segrh “Experimental Nuclear Physics,” Chapman & Hall London 1953. WALTON NUCLEAR FISSION 77 2 3 5 u + on 1 -+ 2:lU + 0.0069 . . . . . (1) 92 235- 1 170 1 -0090 236.1 191 equivalent to 6-4 MeV. As Fig. 1 shows this energy is sufficient to overcome the fission barrier. Because even-even nuclei like “;U have relatively low ground-state energy levels their excitation energies when they are formed from odd nuclei are liable to be high.If the above calculation is performed for the change 238U+239U the excitation energy (4.8 MeV) is found to be too low to cause fission and this explains why uranium-235 undergoes fission with thermal neutrons while uranium-238 does not. By this means the susceptibility of all nuclei to undergo fission by thermal neutrons can be predicted. 0 5 0 I00 Distance apart of nuclei centres ( 10 -I3crn.) FIG. 1. Potential energy of a nuclear drop as a function of distortion. The distortion is here represented by a singZe parameter some measure of effective distance of charge separation. (Reproduced with permission from E. Segr& “Experimental Nuclear Physics,” Chapman and Hall London 1953.) The distortion energy for fission can also be provided by fast particles and by photons.For instance when fast neutrons of 1-6 Mev bombard uranium-238 the excitation of the nucleus is increased from 4.8 Mev (as shown in the previous paragraph) to about 6-4 MeV and fission becomes possible for uranium-238. If the particles are charged they must have 78 QUARTERLY REVIEWS sufficient energy to overcome the coulomb barrier which prevents them reaching the nucleus. This barrier for heavy ions (e.g. 6C on 92U) is about 90 MeV and for protons (lH on 92U) it is about 14 MeV. With fast particles of sufficient energy all heavy nuclei can undergo fission and it has been claimed to happen with elements as light as silver. However the probability of fission relative to other reactions such as y-ray or neutron emission varies greatly from nucleus to nucleus.Particle emission leads to a variety of other nuclei. Each of these may undergo fission so that the fission products arising from the incidence of particles of high energy can come from a variety of different nuclei. By an examination of the fission yields by chemical separation of the products the different processes can be distinguished to some extent. Proton irradiation of thorium provides an example which has been extensively investigated.lg Fission Neutrons.-If as originally observed one of the fragments of 2EU is 56Ba the other element must be 36Kr in order to conserve the total number of protons. The heaviest stable isotope of barium is barium-138 and of krypton it is krypton-86 giving a total mass of 224 which is some 11 mass units less than that of the uranium.The uranium atom is thus very rich in neutrons relative to its stable fission products. The excess of neutrons is lost in a variety of ways; some are emitted as prompt neutrons before or during the fission process and some are emitted as delayed neutrons from certain of the fission fragments after formation. The re- mainder stay with the fragments to form neutron-rich isotopes the radio- active fission products which decay by 18- and y-emission leading ultimately to the stable elements. Mass Distribution.-The energy balance for fission can be expressed by an equation of the following form A A A zM =,;MtZ:M+v(n+En) + E M + E . . . . . A Here Z1 + 2 = 2 Al + A + v = A where ,M represents the mass in energy units of the original fissile nucleus and k M 2 M represent the masses of the two initial fragments.v is the number of neutrons of mass n and energy En emitted EM is the kinetic energy of the fragments andE is the energy emitted as prompt y-rays. A table of nuclear masses will show that there are many combinations of masses and charges represented by the first two terms on the right-hand side of equation (2) which give nearly the same total. Considerable variations in EM and v are also observed so that a large number of different divisions of mass and charge are possible and are in fact observed. From the simple liquid-drop model of fission it would be expected that symmetrical fragments would be more readily formed than unsymmetrical fragments whereas experiment has shown that l9 B.J. Bowles F. Brown and J. P. Butler Phys. Rev. 1957 107 751. WALTON NUCLEAR FISSION 79 in uranium and the heavier elements the formation of heavy and light fragments is more probable than the formation of equal fragments. An explanation for the asymmetry of fission has not yet been found. The precise nuclear masses of the highly unstable nuclides formed in fission are not yet well established but by extrapolation from known masses it has been suggested that the energy associated with a combination of unsymmetrical fragments favours them in comparison with the energy associated with a pair of symmetrical fragments. The theory has been developed by Fong20 in analogy with the statistical theory of chemical equilibria. This statistical theory of fission has been criticised on several grounds.Some advances have been made with the liquid-drop model by cohsidering the energy levels of the “cold” deformed nucleus.21 This has served to explain some features of the angular distribution of the fragments emitted from a fissioning nucleus but again no complete explanation of fission asymmetry has been‘ found. A great deal of radiochemical work has been done and is still being done to establish precisely the shapes of the curves of mass yield for various types of fission. Fig. 2 shows a recent curve for the spontaneous Mass number Yield-mass curves for spontaneous fission of curium-242 (solid line) and for fission of plutonium-239 induced by pile neutrons (broken line). Circles represent observed yields and triangles estimated total chain yields.(Reproduced with permission from E. P. Steinberg and L. E. Glendenin Phys. Rev. 1954 95 431.) FIG. 2. fission of curium-242 compared with that for neutron fission of plutonium- 239. The ordinate represents the percentage of fissile atoms which give rise to fission products of the mass shown; the total yield of atoms sums 2o P. Fong Phys. Rev. 1956 102 434. 21 A. Bohr Proc. Internat. Conf. Peaceful Uses of Atomic Energy Geneva 1955 2 151. Paper P/911. 80 QUARTERLY REVIEWS 4 9 Q - b) 2.0- 0% W 0 - .- s- c 0 n IL .- .- 0 9 / to 200%. As seen in Fig. 2 yields in the region of mass 132-134 always appear to predominate. This may be attributed to the stability of the 82- neutron shell and 50-proton shell in the product nuclei. The yields of the light fragments change slightly as the mass of the fissioning nucleus increases an effect which brings the light peak closer to the heavy peak.There is also a tendency for the trough in the curve to become more shallow and the peaks to widen as excitation energy increases.22 Both effects can be seen in Fig. 2. The slight irregularities which are called “fine structure” in the yield curves have been firmly established. An excessive yield at mass 134 in uranium-235 fission has been attributed to neutrons lost from mass 135 or gained in mass 133 but this is not the complete explanation since the yields of the adjacent masses are not correspondingly low and further- more the yield of the complementary mass 100 shows some enhance- ment. The extent and cause of the “fine structure” is a source of active controversy.The curve of mass yield for the fission of bismuth and lighter elements is different from that for thorium and all the heavier elements in that it shows a single symmetrical peak. Unfortunately between bismuth and thorium there are no elements that can be obtained readily in bulk for irradiation. Radium has recently been irradiated with protons and the triple-peaked curve shown in Fig. 3 has been reported.23 No satisfactory I - \ \ ‘\ - / / I I I I I I WALTON NUCLEAR FISSION 81 charge on the individual fragments i.e. every mass arises initially as a set of isobars (nuclides with the same mass number) such as those shown in Fig. 4.24 The distribution of nuclear charge is difficult to determine because the nuclei start to undergo transmutation by p-decay within seconds of their formation.However in favourable cases measurements can be made; for instance early in some chains there is a long-lived or stable isobar and the next isobar of higher charge is said to be “shielded”. For example in the 134 chain shown in Fig. 4 czsium-134 is “shielded” by the stable isobaric nuclide xenon- 134. Accordingly any czsium- 134 found among the fission products must have been formed directly and could not have been formed by decay of xenon-134. The direct yields are called the “independent yields”. The sum of the “independent yields” of all the isobars of any given mass gives the total fission yield for the mass in question. In many cases the independent yield can be obtained by making measurements sufficiently quickly that is before the precursors have had time to decay.An example is the yield of lanthanum-140 which has been measured before barium-140 (12.8 days) has decayed.25 The independent yield of lanthanum-140 is 0-07 % of the chain yield and experiments have to be carried out rapidly before an amount of the order of 0.07% of the barium-140 has decayed i.e. within 12 minutes of the irradiation. The “cumulative yield” so obtained is the amount formed both independently and by the decay of precursors. A series of observations is made on the cumulative yield of lanthanum- 140 from the barium-140 after different decay periods and these are back-extrapolated to the time of termination of the irradiation to find the amount of lanthanum-140 initially present. Correction is still necessary for the amount that has grown in during the period of the irradiation.This type of work involves a series of carefully timed rapid operations which is unique in chemical work. Special pro- cesses such as the co-precipitation and centrifugation of very small quantities and the use of ion-exchange columns on a micro-scale have been developed to meet the demands of this type of e~periment.~~ Another type of measurement takes advantage of the properties of the rare-gases krypton and xenon. Thus Wah12‘j irradiated uranium foil covered with a layer of barium stearate. This material retained all the fission products except the rare gases. These were allowed to escape into an evacuated space where they deposited their long-lived daughter-products. Comparison of the amounts of the daughter-products found in the evac- uated space with the same products which remained in the barium stearate layer gave the fractional cumulative yields of the rare-gas fission products.Measurements of independent yields have shown that the nuclear charge for any given mass is normally distributed about a most probable charge Zp. In the thermal fission of uranium the width of this distribution appears 24 J. 0. Blomeke Report O.R.N.L. 1783. 25 W. E. Grummitt and G. M. Milton J. Znorg. Nuclear Chem. 1958 5 92. 26 E. A. C. Crouch and I. G. Swainbank Proc. Internat. Symp. Micro-Chemistry Pergamon Press London 1958 p. 220. FIG. 4. TjpicalJission product chains.24 Mass 140 z 54 55 56 57 58 16-sec. I4OOXe -+ 66-sec. 140Cs -+ 12.8-day 140Ba -+ 40-2-hr. 140La -+ stable 140Ce A 1 5 131 3.4-min. lnlSn -3- 23-min. 131Sb 3.2-hr. 134mC~ stable l:j4Ba I 0.20 8-05-day 1311 I 7 A / 134 45-sec.134Sb -f 44-min. 134Te -+ 52-5-min. 1341 + stable 134Xe 2-4-year 134Cs z 51 52 53 54 55 56 WALTON NUCLEAR FISSION 83 to be similar for all masses and the experimental observations on inde- pendent yields can all be plotted on one curvez7 (Fig. 5 ; in this diagramZ-Zp 2-Zp FIG. 5. Variation of independent yield with nuclear charge. 233U ; 0 235U ; Q 23Spu (Reproduced with permission from I. F. Croall M.Sc. thesis Manchester 1959.) is the difference between the charge of the isobar plotted and the most probable charge for its mass). From an empirical curve of this type it is possible to calculate the value of 2 from any observed independent yield. When the values of Zp so obtained are plotted against the mass Fig. 6 is obtained.z8 Superimposed in Fig.6 are values of ZA which is the most stable charge for any given mass. Fig. 6 shows that the values of Zp for a light fragment and for its complementary heavy fragment are equally far away from ZA a hypothesis that was originally put forward by Glendenin and C ~ r y e l l . ~ ~ As suggested by Fong the precise distribution of the points shown in Fig. 6 is probably closely connected with the energy balance as given in equation (2). Unfortunately the values of 2 M and khl for the highly unstable most probable fission products are not precisely known. Extrapolations can be made from the masses of the stable products and investigations are in progress to see whether the most probable charges 27 I. F Croall M.Sc. thesis Manchester 1959. 28 A. C. Wahl J. Inorg. Nuclear Chem.1958 6 263. 29 L. E. Glendenin C. Coryell and R. R. Edwards National Nuclear Energy Series “The Fission Products,” McGraw-Hill New York 1951 Paper 52. 84 QUARTERLY REVIEWS of the fission fragments are predicted by any of the various mass equations (see for instance references 30 and 31) proposed for the calculation of the masses of unstable nuclei. A (heavy) 110 10s 100 95 90 85 80 75- A (lfght) FIG. 6. Empirical plot of the value of the most probable nuclear charge (Zp) for diferent masses formed in fission. (Reproduced with permission from A. C. Wahl J. Inorg. Nuclear Chem. 1958,6,263.) Heavy fission products (left-hand scale). Light fission products (right-hand scale). Broken lines show the values of the most stable charge (ZA) for diflerent masses. Also shown are values of the charge for closed-shell structures of 50 and 82 neutrons in the diferent masses.In fission caused by high-energy particles the values of the independent yields change sharply with energy. Fig. 7 shows recent results obtained by B o w l e ~ ~ ~ for the change in independent yield of bromine-82 silver-1 12 and barium-135m with proton energy of fission of plutonium. Interpreta- tion of these ’results is again uncertain but it appears that in high-energy fission more neutrons are “boiled-off” before fission than in low-energy fission. This would be expected to cause the values of Z p to approach more closely those of ZA. The independent yields of isobars with charges lying between ZA and Zp thus increase with energy as is in Fig. 7. The distribution of the values of Z about Z p also becomes wider than that shown in Fig.5.s3 There is also some indication that the charge splits in 30 A. G. W. Cameron Proc. Internat. Conf. Peaceful Uses of Atomic Energy Geneva 1955 15 425 Paper P/198. 31 T. D. Newton Proc. Symp. Physics of Fission Paper D1 Report CRP-642-A 1956 Chalk River Ontario Canada. 3a B. J. Bowles to be published. 33 B. D. Pate Canad. J. Chem. 1958 36 1707. WALTON NUCLEAR FISSION 85 the same ratio as the mass splits so that Zp is closer to a stable value in the heavy fragments than it is in the light fragments.34 As is seen in Fig. 4 the fission chains exhibit great variety. Sometimes neutron emission causes the isobars to move from one chain into another as in 88Br+87Kr. The neutrons so produced are called “delayed neutrons” because they originate a few seconds after the initial fission at times corresponding to the half-life of the isobar from which they are emitted.Although there are only a few delayed neutrons by comparison with the number of “prompt neutrons,” which are emitted in the fission process itself they play an important part in the control of criticality in nuclear reactors. Details of the numbers and delay times of the “delayed neutrons” are well known but the chemical identification of the nuclides from which they originate has not been well established. Born bard i ng energy (Mev) The change in independent yields in the fission of plutonium-239 as a function of the energy of the bombarding particle. 82Br lI2Ag and lssmBa represent independent yields and 83Br l13Ag and laQBa represent total chain yields.Another source of variety in the fission chains is the formation of nuclear isomers by branching. Many nuclides can emit radiation in more than one way depending on the stability of the energy levels which the nucleus can occupy. The term isomer was given originally by analogy with the use of the term in organic chemistry but it is a misnomer because nuclear isomers do not arise from different arrangements of the nucleons but from the different energy levels as in the metastable states of excited ions or atoms. According to selection rules which can be closely linked with the shell structure of the nucleus an isomer may decay by y-ray FIG. 7. 34 R. H. Goeckermann and I. Perlman Phys. Rev. 1949,76 628. 86 QUARTERLY REVIEWS emission to the ground state of the nucleus or it may decay directly by /%particle emission to the next isobar without going through the ground state.As shown in the decay chain for the production of iodine-131 in Fig. 4 both processes occur in the 30-hour isomer of tellurium-131. To establish unequivocally the details of the decay chain of a fission product is exceedingly difficult and involves extensive physical and chemical studies. In many cases the decay chains and especially the values of the branching ratios which govern the amounts of radioactive decay in any direction are not well established and are subject to continual revision. Anisotropy in Fission.-Considerable interest was aroused when it was discovered in 1952 that in fission by collimated beams of photons neutrons or charged particles the fission products were not emitted isotropically from the compound nucleus but were emitted preferentially in certain directions.In fission of uranium-235 no anisotropy was detected but in fission of thorium-232 the anisotropy was very marked and moreover it changed sharply with the energy of the bombarding particles. Much of this information has been obtained by chemical methods. The fissile target is surrounded by a cylinder of foil which catches the fission fragments emitted when the target is irradiated. The foil is cut up into strips which are separ- ately analysed and the anisotropy of the fission is measured from the distribution of the fission fragments in the various strips. A reasonable explanation of the anisotropy of fission fragments was given by A. Bohr at the 1955 Geneva Conference.21 He suggested that in a highly deformed nucleus at the moment of fission the bulk of the energy content would be associated with deformation and only a few low-lying and widely separated quantum states would be available for excitation.Each of these should give rise to different angular distributions of the fragments as observed. This suggestion led to extensive experimental investigation which has been reviewed by Ha1~ern.l~ Very recently measurements have been made on the distribution of fission fragments from fissile nuclei aligned at very low temperatures but only preliminary results have been reported.35 Attempts to associate changes in the anisotropy of fission fragments with changes in the asymmetry of fission have not so far been very fruitful. Kinetic Energy of Fission and the Range of Fragments The energy of fission arises as shown in equation (2) from the difference between the mass of the initial fissile nucleus and the sum of the masses of the products including the neutrons.As calculated when fission was first discovered the amount of energy released is about 200 Mev per fission. The precise value depends upon which pair of fission products are con- sidered and there is therefore a wide distribution in the energy. Table 1 shows the different forms the energy may take. EY and En are the energies of the y-rays and kinetic energies of the neutrons emitted in the fission 35 J. W. T. Dabbs L. D. Roberts and G. W. Parker Bull. Amer. Phys. SOC. 1959,11 4 373. WALTON NUCLEAR FISSION 87 process and EB E’ and E the energies emitted in the radioactive decay of the fission products by the emission of 18- and y-rays and By far the largest term in Table 1 is EM the energy of the fission fragments.TABLE 1. Energy release in fission ( M ~ v ) ~ ~ E M fragments fragments Isotope Light Heavy Ey vE Ea+E’ E Total 233u 97 66 -7 5-0 -9 -7 191 235u 98 67 7-8 4.9 9 7-2 194 2*U 95.3 67.4 5.2 2 3 9 h 100 72 -7 5.8 -9 -7 201 This arises as kinetic energy of motion due to the coulomb repulsion between the positive charges of the nuclei at their formation. A simple calculation of the repulsion C of two spheres with uniformly distributed charges Z,e and Z2e distant ro(A1k + A,*) apart is given by C = 2,Z2e2/r0 (A,* + A29 . . . . . . . . . . (3) where e is the charge of the proton and r is the effective volume per nucleon in the nucleus. When ro has the value 1.5 x l&13cm.andA,=A,= 236/2 and Z1 N 2 x 92/2 these being values appropriate to the uranium nucleus C is by equation (3) equal to 208 MeV. The actual mean kinetic energy of fission fragments is 165 Mev which suggests that fission occurs usually from a rather more elongated shape than from two contiguous spheres. The mean kinetic energy of fission fragments is approximately the same whatever the initial excitation of the compound nucleus. It increases with the charge and size of the nucleus in the manner suggested by equation (3). The energy of fission fragments has been measured by the ionisation produced in a gas by the density of tracks in photographic emulsions by the velocity as measured by the time of flight over long distances in a vacuum and recently by scintillations in a casium iodide-thallium crystal spectrometer.The values are consistent and agree well with those obtained by the calorimetric method in which the heat generated on stopping the fragments is The range and energy of fission fragments are of interest in chemical work because they determine the distance that the fragments can pass From one phase to another during their formation and the amount of chemical decomposition or radiation damage that can be produced in fissile materials and their surroundings. The range of fragments is most easily measured by the stacked-foil technique. Layers of thin absorbers are placed over a thin film of uranium. After irradiation the absorbers are separated and analysed radiochemically 36 Argonne National Laboratory Report No. A.N.L.5800 1959. 37 R. B. Leachman and W. D. Wafer Canad. J. Phys. 1955,33 357. 88 QUARTERLY REVIEWS for their content of fission products. By plotting the amounts passing the foils against the thickness of foil traversed the range and the distribution in range can be estimated. Fig. 8 shows the extrapolated range of fission 4.4 4-2 4.0 c 5 3.8 A 3.6 \ m rn c 0 3.4 a 3.2 3.c 1 I I 1 I I 80 90 I 0 0 110 120 130 140 150 Masr number FIG. 8. Range of fission fragments in aluminium. fragments in aluminium plotted against the mass of the fragment^.^^ The apparent fall in range for fragments of mass about 1 15 has been previously observed in air. Fragments of this mass are formed in symmetrical fission and their low range is of particular interest because it implies that the kinetic energy is also low for this type of division although the simple coulomb effect would suggest that the repulsion is greatest when the charges are equal.Very recently measurements on the range of fragments of the same or nearly the same mass but of different nuclear charge have been made.39 Fig. 9 shows the range of czesium-136 compared with the range of czsium-137. This czsium isotope is formed from iodine-137 and xenon-1 37 and represents the range of fragments with the most probable charge for mass 137. Czsium-136 is shielded by stable xenon-136 and is formed in low yield about 2.4 units of charge away from the most probable charge. Here again as for symmetrical fission the nuclide formed in lower yield has the shorter range. As already mentioned the symmetrical fission products of bismuth are formed in high yield and it will be of interest to find out if these products have high or low ranges.It is possible that a 38 L. J. Le Roux and G. N. Walton to be published. 39 F. Brown and B. H. Oliver Third Symp. Nuclear and Radiochem. 1960 Paper 21. WALTON NUCLEAR FISSION 89 9 8 7 .- 4 6 = 5 L 4 + J 3 n L 2 “ C * 0 L .- 0 “ I r 4 0 .- 7 a 6 5 4 3 2 1 C .- > 3 0 ’ 136 cs 1 -30 4 0 5 0 60 Sample number FIG. 9. Diference in the range in aluminium of a nuclide with an improbable charge c~sium-136 and a nuclide representing the most probable charge cesium-137. The mean range and the most probable range are shown in both cases. The sample number represents the depth of penetration of the fragments. (Reproduced with permission from F. Brown and B. H. Oliver Third Symp.Nuclear and Radiochem. 1960.) fall in kinetic energy is associated with enhanced neutron emission but further work is required to establish this. When a fission fragment is formed initially it is of sufficient energy to be partly stripped of its electrons. The moving particle interacts mainly with the electron cloud of surrounding atoms and behaves as though it were being bombarded with electrons moving with its own velocity. The 90 QUARTERLY REVIEWS ionic charge (which must be distinguished from the nuclear charge) has been measured by the deflection of the fragments in a magnetic field. In gases the light fragment tends to be stripped initially of about 16 electrons while the heavy fragment is stripped of about 14 electrons; numerous measurements have been made of the stripping in different gases and at different pressures.Fragments emitted from solids into a vacuum tend to be more highly charged than fragments moving in gases. Bohr and Lindhard attribute this effect to the excitation energy of the moving ion which in gases at low pressures may be lost between collisions while in solids the excitation energy is retained and promotes the stripping process.4o More recently moving fragments have been introduced into a mass spectrometer and separated according to the ratio of their charge to mass. By carrying out radiochemical analysis on different regions of the collector of the spectrometer it has been possible to study the charge-to-mass ratio for a particle of known mass.41 Although results so far obtained have been only approximate the technique should in principle give extensive information on the binding of electrons in partially stripped atoms.Effects on the Medium in which Fission Occurs Experimental Observation.-A great many empirical observations have been made on the effects of fission on the medium in which it occurs mainly in order to assess the mechanical stability of the fuel used in nuclear reactors. However comparatively little systematic work has been done. In general two effects have to be considered; on the one hand each fissile atom such as uranium is replaced by two fission-product atoms and on the other hand a large amount of energy mainly due to bombard- ment by fast moving particles is dissipated. In the first effect the rare-gas fission products for instance tend to accumulate as bubbles which can cause dimensional changes in metalsg2 Caesium and other fission products tend to collect at grain boundaries in metals.In oxide and graphite fuels the rare-gases and other volatile products can escape to a greater or less extent causing pressure build-up in reactor-fuel assemblies. Although the effects of accumulation of fission product can only be studied after long irradiation whereby sufficient atoms have undergone fission the second effect of damage by fission fragments can often be observed in brief irradiations. The effect of individual fission fragments was observed by D. A. Young by an etching technique. Single crystals of lithium fluoride were cleaved along the (100) plane and exposed to fission fragments from a uranium film at 1 mm. distance by irradiation in a low thermal neutron flux for a few minutes at room temperature.The surface of the crystal was then etched with a mixture of hydrofluoric acid and acetic 40 N. Bohr and J. Lindhard Kgl. danske Videnskab. Selskab Mat. fys. Medd. 1954 41 B. L. Cohen A. F. Cohen and C. D. Coley Phys. Rev. 1956,104 1046. 42 Barnes et a/. Proc. Internat. Conf. Peaceful Uses of Atomic Energy Geneva 28 7. 1958 5 543. Paper P/81; S. F. Pugh U.K.A.E.A. Report A.E.R.E.-R 3458 1960. WALTON NUCLEAR FISSION 91 acid. Microscopic etch pits appeared in the surface the number and distribution of which corresponded to the number and distribution of the fission fragments entering the surface.43 Similar observations have been made by Silk who has photographed the tracks made in mica by using an electron micro~cope.~~ Tracks of fission fragments appear very plainly in photographic emulsions.The blackening along the length of the track has been investigated by Mathieu and Demers and correlated with the energy deposi t i ~ n . ~ ~ In covalent materials the decomposition caused by recoiling fission fragments is often very marked. The decomposition caused by fission fragments recoiling into solid potassium nitrate is about three times that caused by y-rays depositing an equivalent amount of energy,46 while in potassium iodate47 the factor is about 150. The enhanced decomposi- tion is attributed to the density of energy deposition along the radiation track which is some lo4 times greater for fission fragments than for y-rays. The dense energy deposition causes neighbouring molecules to decompose so that recombination effects can occur which do not lead back to the original molecules.These observations have been made on materials in which fission fragments recoil from one phase into another. Experiments have also been carried out on single-phase systems such as uranyl oxalate and uranyl iodate in which the fission fragments originate inside the crystal lattice on neutron irradiati~n.~~ In aqueous solution the enhanced effect of fission fragments compared with y-rays is much less marked in causing the decomposition of water. The energy efficiency and the composition of the products are very similar to those obtained with a-particle irradiation. Fission fragments appear to be less efficient than y-rays in causing the oxidation of ferrous sulphate in aqueous As would be expected fission fragments promote corrosion of metal surfaces in contact with an aqueous solution.The effect has been closely investigated for the corrosion of “Zircaloy” in water. In the absence of irradiation a protective oxide film forms; exposure to irradiation by fission fragments appears to promote the transport of material through the film so that it is no longer protective and the cor- rosion continues indefinitely instead of reaching a steady condition.50 This effect has been the main difficulty in designing nuclear reactors in 43 D. A. Young Nature 1958 182 375. 44 E. C. H. Silk and R. S . Barnes Phil. Mag. 1959,4 970. 46 R. Mathieu and P. Demers Canad. J. Phys. 1953 31 97. 46 D. Hall and G. N. Walton J. Inorg. Nuclear Chem. 1959 10 215. 47 U. Bertocci R. Jacobi and G. N. Walton Proc.Symp. Chemical Effects of Nuclear Transformations Prague 1960 Paper C.E.N.T/17. 48 D. A. Young U.K.A.E.A. Report A.E.R.E-R 2429 1959; D. Hall J. Inorg. Nuclear Chem. 1958 6 3 . L. Ehrenberg and E. Saeland Jener Publications No. 8 Joint Establishment for Nuclear Research Norway 1954; J. W. Boyle et al. Proc. Internat. Conf. Peaceful Uses of Atomic Energy Geneva 1955,7 576. P/741. B. Cox K. Alcock and F. W. Derrick U.K.A.E.A. Report-R 2932; W. C. Yee U.S.A.E.C. Report O.R.N.L. 2742,1959. 92 QUARTERLY REVIEWS which the fuel is held in the core as an aqueous solution of a uranium salt and as yet no material has been reported which can adequately resist the corrosion due to fission. In gases irradiated with fission fragments many reactions have been observed and the possible fixation of nitrogen by this means has been studied.51 At the 1958 Geneva Conference it was reported that the deposi- tion of fission energy in silica gel through which reactant gases were passed promoted the efficiency with which the energy could be accepted by the gases.The possibility of using such processes has been disc~ssed.~~ Theory.-Much theoretical work has been carried out on the processes occurring in the track of a fission fragment. The fragment dissipates its energy in a solid by interacting with both electrons and atoms. As a charged ion the fragment undergoes coulomb interaction with the electrons. It can be estimated that some 80-85 % of the energy is lost in this manner producing electronic excitation and ionisation in the atoms of the solid. The remainder of the energy is lost in collisions with lattice atoms and it is this process that produces most of the lattice damage in crystals.In metals this is the only portion of the energy that need be considered because electronic excitation and ionisation do not persist in materials in which the electrons are mobile and in which energy is immediately dispersed as heat. In non-metallic materials both processes have to be considered. The heating effect along the track of a fission fragment a “thermal spike” as it is termed can be considered by analogy with the normal laws of heat conduction from a hot wire. The distribution of temperature round a thermal spike would then be given by T = (Q/4mDt)exp(-r2/4Dt) where r is the radial distance from the track T the temperature rise at time t after the passage of the fragment Q the energy loss by the fragment per unit length of track c the heat capacity of the medium and D the thermal diffusivity.For a fragment of 80 Mev energy and range of 5 microns Q is of the order of 6 x cal./cm. Such a particle moving through uranium metal of thermal capacity 0.72 ~al./cm.~ and thermal diffusivity of the order cma2/sec. would form a thermal spike of about 0.01 micron radius and 5 microns long in which the effective temperature would rise above 2000”~ for a period of 3 x 10-lo second. The spike is evidently exceedingly evanescent and it is doubtful whether the concept of temperature mentioned is very meaningful especially as no allowance is made for loss of radiant heat which momentarily must be significant. Atomic displacements caused by recoiling fission fragments have been considered theoretically by a number of authors of whom Brinkman 51 P.Harteck and S. Dondes J. Chem. Phys. 1957,27 546; J. K. Dawson et al. Ind. Chemist 1959 June 269. WALTON NUCLEAR FISSION 93 has given the most recent summary.52 It is estimated that a recoiling fragment produces a comparatively small number of “primary knock- ons” each of which carries away a considerable fraction of the initial energy and possesses a significant range. The reality of this effect can be seen in cloud-chamber photographs in which branch spikes appear along the track of the fragment. Each primary knock-on produces many dis- placements so that the total number of displaced atoms in uranium metal 30001 10 lo2 lo3 lo4 105 2 5 2 5 2 5 2 5 2 FIG. 10. Number of primary knock-on atoms n( E) having energy in excess of c produced in uranium metal by light and heavy fission fragments.Curve A heavy fission fragment; curve B light fission fragment. has been estimated to be of the order 5 x lo4 per fission. Fig. 10 shows the number of primary knock-on atoms calculated to be produced in uranium metal. There is little experimental evidence which directly supports the theoretical picture of fission spikes. The volume and time of duration of the spikes has had some support from work by Konobeevsky in which he measured the electrical resistance of uranium-molybdenum alloys which had a eutectoid micro-str~cture.~~ The resistance changed sharply in the first few hours of irradiation and more slowly over the subsequent period. The fast effect was attributed to the period during which the whole volume of the alloy became subjected to the conditions of a thermal spike and the slow effect was attributed to homogenisation of the eutectoid promoted by the displacement processes.In experiments on the same alloys as used by Konobeevsky and also on uranium-niobium alloys Bleiberg Jones and 52 J. A. Brinkman Report N.A.A.-S.R.-4164 1960 Atomics International P.O. Box 309 Canago Park California. G. J. Dienes and G. H. Vineyard “Radiation Effects in Solids,” Interscience London 1957. F. Seitz Discuss. Faraday SOC. 1949,5 271. 53 S. T. Konobeevsky et al. Proc. Internat. Cod. Peaceful Uses of Atomic Energy Geneva 1958 5 574. P/2192; S. T. Konobeevsky J . Nuclear Energy 11 1956 3 356. 94 QUARTERLY REVIEWS Lustman showed that irradiation induces a phase transformation froin the stable to the metastable form at temperatures much lower than those necessary to transform the alloys thermally.54 Transformation as judged by electrical resistance hardness density and X-ray diffraction from a lamellar structure to the homogeneous y-phase occurred during irradiation below 200° whereas the y-phase normally only begins to be stable for the 9% molybdenum alloy for instance above 560".The phase reversal was explained on the basis of hornogenisation followed by rapid quenching within the volume of each thermal spike. From all these changes of properties induced by irradiation estimates of the volume and duration of the spike could be made. These show good agreement with theory although all authors admit that the agreement must be considered largely fortuitous in view of the uncertainties in the theoretical assumptions.Chemical State of Fission Products Little progress has been made in understanding the factors governing the chemical state of fission products after their formation. Experimental work has served mainly to emphasise the multiplicity of factors involved. In outline it can be presumed that a fission fragment must go through some or all of the following stages (i) It starts as a fast moving highly charged positive ion which when arrested may have one or more positive charges or it may become neutral or pick up an additional electron and become negative. (ii) Its velocity is reduced by electrons and other atoms in accordance with their density. The fragment is thus necessarily stopped inside a phase as an isolated atom and is not stopped preferentially at a surface or phase boundary.In a pure fissile material which has been very highly irradiated so that an appreciable fraction has undergone fission the fragments will have a chance of stopping in adjacent positions but this condition is difficult to achieve in controlled experiments in most nuclear reactors. (iii) The fragment next undergoes a series of transmutations by p- and y-decay to the element in the form of which it is finally observed. As discussed previously the initial nuclear charge is distributed over several values and in general some of the atoms of the element measured will have passed through several transmutations whereas some will have arisen directly from fission. Furthermore each isotope of the element measured will have arisen by a different path.(iv) When the fission-product element is soluble as a monatomic species in the medium in which it is stopped it is likely to behave as would a dilute solution of the-natural element in the same medium. If it is 64 M. L. Bleiberg L. J. Jones and B. Lustman J. Appl. Phys. 1956 27 1270. WALTON NUCLEAR FISSION 95 insoluble however its condition is unique because isolated atoms do not normally disperse except by dissolution. Thermo- dynamic considerations applicable to bulk material can hardly be used for isolated atoms and ionic or atomic properties such as electron affinity size and polarisability are more likely to be relevant to its state. After the fragment has been stopped the possibilities of diffusion or migration by other processes arise and coagulation at surfaces or grain boundaries is possible.In this process impurity atoms in the stopping medium are likely to have a dominant role and it is difficult to conduct controlled experiments in which traces of impurities such as oxygen are not present in much greater amounts than the fission products themselves. It is interesting to review the experimental work with reference to these stages. No evidence has been found to suggest that the initial electron- stripping process of the first stage is important in determining the final chemical state of a fragment. As shown in Fig. 11 Hall and Walton found 1 1 I I 1 I i 1 FIG. 1 1. State of oxidation of iodine isotopes formed in the neutron irradiation of uranyl iodate. The diflerent points refer to diflerent experimental conditions and the broken lines show theoretical values.that iodine-135 formed by fission in the irradiation of uranyl iodate prefers to exchange isotopically with reduced forms of iodine whereas iodine-1 3 1 prefers the oxidised forms.55 Iodine-135 has a high independent yield in fission whereas iodine-131 is formed from precursors. If the electron stripping played a dominant role it might be expected to promote the oxidation of iodine-135 rather than iodine-131 whereas the reverse is observed. Again many observations have been made on the krypton and xenon atoms emitted in fission but it has never been suggested that they behave in any way different from that expected of a neutral rare-gas atom. The effect of the recoil process described under (ii) is evident in many experimental observations.Stubbs Russell and Walton showed that the 55 D. Hall and G. N. Walton J. Inorg. Nuclear Chem. 1960 in the press. 96 QUARTERLY REVIEWS emission from a thin film of U308 depends upon the volume and density of gas in which the recoils are stopped.5s A number of workers have studied the migration of fission products in graphite and it has been shown that there are two distinct processes a fast movement through the pores of the graphite and a very slow movement from the grains or crystallites in which fragments are stopped.57 Erschler and Lapteva have measured the amount of material released from a surface by a recoiling fission fragment and found for instance that about 1000 atoms of uranium metal are volatilised per fission.58 A number of investigations have also been carried out on the recoil of fission fragments during irradiation of small particles of uranium dioxide or other fissile material suspended in liquids or as a The influence of precursors mentioned in (iii) appears to be one of the more dominant effects in controlling the condition of fission products.In the graphite work it was found that the amount of cerium-141 moving into the graphite was some 200 times greater than the amount of cerium- 144 an effect that was attributed to movement of the 18-min. barium-141 through which most of the cerium-141 arises.57 The more oxidised condi- tion of iodine-131 compared with iodine-135 shown in Fig. 11 is also attributed to the fact that the lower isotope arises through the readily oxidised tin and antimony isotopes whereas iodine-135 arises without these precursors.The emission of fission products from leaking fuel elements in nuclear reactors and the radioactive “fall out” from atomic weapons are both largely controlled by the precursor effect. There is considerable experimental evidence to show that fission products which form true solutions with the medium in which they are stopped display the thermodynamic properties of the normal element. In the Brookhaven laboratory extensive research has been carried out on the use of a solution of uranium in molten bismuth as a nuclear fuel. After irradiation fission-product elements that dissolve in the bismuth can be extracted by equilibration with fused salts. Thus molten magnesium chloride extracts a rare-earth fission product such as cerium as follows ge1.59 3MgC1 + 2Ce (in Bi) + 3Mg (in Bi) + 2CeC1 The standard free-energy change involved in this reaction has been found from experiments on the natural materials and the same constants have been found for extraction of the fission-product elements from irradiated 56 F.J. Stubbs P. Russell and G. N. Walton U.K.A.E.A. Report A.E.R.E.-C/R 2983 1959. 57 G. J. Hunter N. R. Large e f ul. U.K.A.E.A. Report A.E.R.E.-R 3290 1960; J. R. Findlay U.K.A.E.A. Report R 2683 1959. 68 B. V. Erschler and F. S. Lapteva J. Nudear Energy 11 1957 4 471. 69 R. Wolfgang J. Inorg. Nuclear Chem. 1956 2 180; M. E. A. Hermans Nuclear Science and Engineering 1957 2 224; B. S. Hickman Australian Atomic Energy Com- mission Report A.A.E.C./X 2; R. G. Sowden B. R. Harder and K. E. Francis U.K.A.E.A. Report A.E.R.E.-R 3269 1960; P.J. Kreyger et a/. Proc. Internat. Conf. Peaceful Uses of Atomic Energy Geneva 1958 9,427. P/1828. WALTON NUCLEAR FISSION 97 fuel.60 However the volatilisation of fission products from molten irradi- ated uranium and other reactor fuels has shown little quantitative correla- tion with theory as expected vapour pressures are closely associated with the access of oxygen which is very difficult to Comparatively little theoretical progress has been made on the condition of fission products which are insoluble in the medium in which they are stopped. Attention has been mainly given to the behaviour of the rare gases in metals. In practice the rare gases coagulate rapidly into bubbles in the metal but the rate at which the coagulation occurs and the diffusion processes that accompany it are not well understood.The condition of rare-gas atoms in metals was discussed by Le Claire and by Rimmer and Cottrell and recent reviews of the subject have been given by Barnes et a!. and by Pugh.42~62 The diffusion of the radioactive rare-gas fission products during their formation has been measured by Stubbs and his co-workers.63 Fission-product elements other than the rare gases such as iodine ccesium strontium and barium have been investigated but little progress has been made.64 For a graphite medium the relative migration rates of fission products through the graphite lattice have been measured and discussed in terms of the ionisation potentials and ionic radii.57 In many of the observations impurities tend to confuse the issue. However some work has been carried out on iodine as a fission-product element in the atmosphere of a closed building where the atmospheric impurities could be controlled to some extent.It was here found that iodine vapour at very low concentrations was rapidly associated with nuclei in the atmosphere and that the diffusion behaviour and the efficiency of adsorbers for removing iodine from the air was highly dependent on the concentration of nuclei present.65 Conclusion It appears inevitable that in the future nuclear fission will be applied increasingly to industry and for military purposes. This article has attempted to describe some of the chemical problems associated with the fission process itself and its initial products. There are in addition many problems associated with the dispersion of the fission products in geological 6o W.S. Ginell Ind. Eng. Chem. 1959,51 185; R. H. Wiswall et al. Proc. Infernat. Conf. Peaceful Uses of Atomic Energy Geneva 1958,17,421. P/1434. slG. E. Creek W. J. Martin and G. W. Parker U.S.A.E.C. Report O.R.N.L. 2616; R. K. Hilliard U.S.A.E.C. Report H.W.60609 1959 Richland Washington U.S.A. 62 A. D. LeClaire and A. H. Rowe U.K.A.E.A. Report A.E.R.E.-M/R 1417; D. E. R i m e r and A. H. Cottrell Phil. Mug. 1957 23 1345. 63 F. J. Stubbs and G. N. Walton Proc. Internat. Conf. Peaceful Uses of Atomic Energy Geneva 1955,7 163. P/435. 64 G. N. Walton B. Bowles and I. F. Croall Proc. Internat. Conf. Peaceful Uses of Atomic Energy Geneva 1955,7 155. P/436; M. T. Robinson Nuclear Sci. and Eng. 1958 4 263 288; A. E. Greendale and N. E. Ballon U.S. Naval Radiological Defence Laboratory Report U.S.N.R.D.C.-436 1954. 65 A. C. Chamberlain to be published. 98 QUARTERLY REVIEWS and biological systems. lnvestigation of these problems is only just beginning. They open up new fields especially in the distribution and beha- viour of trace elements the exploration of which will be of increasing importance in the future. The assistance of Professor P. L. Robinson in reading the script and making many suggestions is gratefully acknowledged.

ISSN:0009-2681    

DOI:10.1039/QR9611500071

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

SURFACE CHEMISTRY ADSORPTION ENERGY AND ADSORPTION EQUILIBRIA By A. V. KISELEV (ADSORPTION LABORATORY CHEMISTRY DEPARTMENT M. V. LOMONOSOV STATE UNIVERSITY OF Moscow U.S.S.R. SURFACE CHEMISTRY GROUP OF THE INSTITUTE OF PHYSICAL CHEMISTRY The Theory of Adsorption Equilibria and Adsorption Energy FOR a long time adsorption was interpreted mainly by empirical and semi- empirical methods. Thus Polanyi’s theory by its very nature does not permit derivation of adsorption potentials or even of the shapes of adsorption isotherms. The adsorption potential is determined by purely empirical means from the experimental isotherm. From the Langmuir and the B.E.T. theory for localised adsorption on homogeneous surfaces the shape of the adsorption isotherm can be found theoretically but the equili- brium constants of adsorbate-adsorbent interactions are again determined from the experimental adsorption isotherm.These theories are therefore semi-empirical. The same is true of the modifications to the Langmuir and the B.E.T. theory which take into account adsorbate-adsorbate interactions. The simplest equations of this type are the approximate relationships suggested for unimolecular localised adsorption by the present author :l OF THE U.S.S.R. ACADEMY OF SCIENCES) 8 or ~ = Kl + KlK,8 8 K,(1 - 8) (1 + Kn8) h - h(l - 8) and for multi-molecular adsorption by the author and D. P. Poshkus:l where 8 is the surface coverage h = p/ps is the relative vapour pressure and Kl and K are the respective equilibrium constants of adsorbate- adsorbent and adsorbate-adsorbate interactions. As in the Langmuir and B.E.T.equations which are particular cases of equations (1) and (2) when K = 0 only the shape of the 8-h plot is found theoretically while the constants Kl and K are determined from the experimental adsorption isotherms.2 Similarly Hill’s and deBoer’s e q ~ a t i o n ~ ~ ~ for non-localised unimolecular adsorption,* exp [q(i - e) - ~ ~ e ] (3) 8 Kl(1 - 8) h = * Also the appropriate Hill’s equation for multimolecular adsorption. A. V. Kiselev Doklady Akad. Nauk S.S.S.R. 1957 117 1023; Kolloid Zhur. 1958 20 338; A. V. Kiselev and D. P. Poshkus Izvest. Akad. Nauk S.S.S.R. Otdel. khim. Nauk 1958 1520. a A. V. Kiselev N. V. Kovaleva V. A. Sinitsyn and E. V. Khrapova Koffoid. Zhur. 1958,20,444. * J. H. de Boer “The Dynamical Character of Adsorption,” Oxford 1953. T. L. Hill.J. Chem. Phys. 1946. 14 441. 4. 99 100 QUARTERLY REVIEWS gives only the shape of the adsorption isotherm eqn. 3 is found by means of the Gibbs equation from the empirical two-dimensional equation of state while the constants Kl and K are found from the experimental i s o t h e r m ~ . ~ ~ ~ Thus equation (3) is also semi-empirical. The same is true of many attempts to obtain an adsorption-isotherm equation by statistical thermodynamics in these cases only the general shape of the 0-p curves is usually found while the equilibrium constants containing the partition function of a given adsorbate on a given adsorbent are not calculated theoretically but are found from the experimental isotherms. All this renders it impossible to surmount the limits of purely experi- mental investigation each time a new adsorbent-adsorbate system has to be studied new experiments have to be made and the adsorption equilibria found experimentally.This state of affairs is further complicated by the very scanty information on the structure of adsorbents so that to take their irregularities into account one must again proceed from the experimental adsorption isotherms and adsorption heats All this makes a rather dismal picture showing how the theory of adsorption equilibria lags behind the present-day theory of chemical and many bulk-phase equilibria. However considerable progress has been made recently; this gives grounds for greater optimism. First adsorbents have been obtained with surfaces sufficiently homogeneous to enable verification of the simplest theories and sufficiently large to make possible accurate measurement of adsorption isotherms as well as adsorption heats and specific heats of adsorption systems.We are referring primarily to graphitised carbon bla~ks,~-l~ especially thermal carbon blacks heated to -3000" whose surface consists mainly of basal faces of graphite crystals.5* 9*13-15 Secondly a method has been devised for calculating the potential energy of adsorption @ of molecules on homogeneous crystal faces starting from S. Ross and W. Winkler J. Colloid Sci. 1955 10 319 330. R. A. Beebe J. Biscoe W. R. Smith and C. B. Wendell J. Amer. Chem. Sac. 1947,69,95; R. A. Beebe M. H. Polley W. R. Smith and C. B. Wendell ibid. p. 2294; R. A. Beebe and D. M. Young J. Phys. Chem. 1954,58 93. ' N. N. Avgul G. I. Beresin A. V. Kiselev and I.A. Lygina Zhur. fiz. Khim. 1956 30,2106; Izvest. Akad. Nauk S.S.S.R. Otdel. Khim. Nauk 1956,1304; 1957,1021 ; 1959 787; A. V. Kiselev N. N. Avgul G. T. Beresin T. A. Lygina and G. G. Muttik J. Chim. phys. 1958 197. * A. V. Kiselev Proc. 2nd Tnternat. Congress on Surface Activity Vol. 11 London 1957 p. 168. M. H. Polley W. D. Schaeffer and W. R. Smith J. Phys. Chem. 1953 57 469; C. H. Amberg W. B. Spencer and R. A. Beebe Canad. J. Chem. 1955,33,305; R. A . Beebe and R. M. Dell J. Phys. Chem. 1955,59,746,754; W. B. Spencer C. H. Amberg and R. A. Beebe ibid. 1958,62,719. lo J. W. Ross and R. J. Good J. Phys. Chem. 1956,60 1167. l1 E. L. Pace J. Chem. Phys. 1957,27 1341. l2 N. N. Avgul G. I. Beresin A. V. Kiselev and A. Ya. Korolev Kolloid. Zhur. 1958 l3 A. V. Kiselev and E. V. Khrapova Kolloid.Zhur. in the press. l4 N. N. Avgul A. V. Kiselev and I. A. Lygina Kolloid. Zhur. in the press; Izvest. l5 A. A. Isirikyan and A V. Kiselev Kolloid. Zhur. in the press. 20 298. Akad. Nauk S.S.S.R. Otdel. Khim. Nauk in the press. KISELEV SURFACE CHEMISTRY 10 1 information on only the structure and physical properties of the adsorbent and adsorbate. It is true that this method encounters two difficulties even in the simplest cases when adsorption occurs under the influence of only electrokinetic (dispersion) attractive forces. It is based on using the Kirkwood-Miiller formula16 to calculate the energy constant C1 of electrokinetic attraction in the term Clr6 (r is the distance between inter- acting centres) and similar f o r m u l ~ e ~ ~ - ~ ~ for the subsequent terms C2r8 and C3r-l0.However these quantum-mechanical formulze are not strictly accurate. Moreover the exponential repulsion constant p in the 6 exp (r/p) type potential is usually found empirically from compressibility experiments while the constant b is excluded when the condition of equilibrium (minimum @ at a certain selected distance from the surface) is introduced. The constant b is then replaced by a different constant related to the equilibrium distance between the closest interacting centres which is calculated from the constants of the respective lattices taken separately i.e. under potential-field conditions which differ somewhat from such conditions for the adsorbate-adsorbent system. However with due regard for these difficulties which have not as yet been overcome by present-day quantum mechanics we can obtain the value of the adsorp- tion potential from the physical properties of the adsorbate and the adsor- bent.The numerous calculations carried out so far by this method have resulted in -@ values satisfactorily close to the most reliable measured heats of a d ~ o r p t i o n . ~ J ~ J ~ - ~ ~ Thirdly definite progress is noticeable also in the statistical-ther- modynamic treatment of adsorption equilibria. The simplest cases were examined by Hill,27 Pace,ll and Fisher and McMillan.21~28 Hill made a direct attempt to calculate the equilibrium constant of the B.E.T. equation for certain of the simplest cases confining himself to an “ideal” adsor- bent. In this connection mention should also be made of Kemball’s paper29 which contains attempts to calculate the adsorption entropy by an examination of various models of adsorbate-molecule movement at the adsorbent surface.Proceeding from a theoretical calculation of @ and l6 J. G. Kirkwood Phys.Z. 1932,33,57; A. Muller,Proc. Roy. Soc. 1937 A 161,476. A. V. Kiselev Vestnik Akad. Nauk S.S.S.R. 1957 No. 10 p. 43. 18N. N. Avgul A. A. Isirikyan A. V. Kiselev I. A. Lygina and D. P. Poshkus lo A. V. Kiselev and D. P. Poshkus 2hur.fi.z. Khim. 1958,32,2824. 2o R. M. Barrer Proc. Roy. SOC. 1937 A 161 476. 21 B. B. Fisher and W. G. McMillan J. Chem. Phys. 1958,28,562. 32 W. J. C. Orr Proc. Roy. SOC. 1939 A 173 349. 23 A. D. Crowell and D. M. Young Trans. Faraday Soc. 1953 49 1080; A. D. Crowell J. Chem. Phys. 1954,22 1397; 1957,26 1407. 24 T. Hayakawa Bull. Chem. SOC. Japan 1957,30 236.25 R. A. Pierotti and D. Halsey J. Phys. Chem. 1959 63 680. 26 E. L. Pace and A. R. Siebert J. Phys. Chem. 1959,63 1398. 27 T. L. Hill J. Chem. Phys. 1948,16 181 ; J. W. Drenan and T. L. Hill ibid. 1949 B. B. Fisher and W. G. McMillan J. Chem. Phys. 1958,28 555. 29 C. Kemball Proc. Roy. SOC. 1946 A 187 73 ; 1947 A 190 1 17. Izvest. Akad. Nauk S.S.S.R. Otdel. khim. Nauk 1957 1314. 17 775. 102 QUARTERLY REVIEWS the entropy values obtained by Kemball the present author and D. P. Poshkus30 attempted to calculate the equilibrium constant of the B.E.T. equation for the adsorption of benzene on graphite and on magnesium oxide. However the entropy values used in these calculations were ob- tained by Kembal129 for models which were selected rather arbitrarily. Therefore selection of the models of adsorption-complex movement based on independent facts is a very important problem.These facts must be obtained by spectral and magnetic investigations. Only preliminary attempts have been made in this direction as yet.31s32 Such in general terms is the state of the problem of theoretical calcula- tion of adsorption energies and adsorption equilibria. It shows that at present this question should be posed in all its aspects to draw it to the attention of both experimentalks and theoreticians. There are grounds for hope that despite inevitable disappointments the properties of adsorp- tion systems will with increasing frequency become theoretically predic- table from their nature. This Review deals with several examples of the effect of the nature of adsorption systems (Le.the surface chemistry and molecular structure of the adsorbate) on adsorption properties mainly the adsorption energy but partly the adsorption equilibria. Adsorption Heats Heats of Adsorption of Vapours of Non-polar Substances on Graphitised Carbon Blacks. The heats of adsorption of the simple gases and a number of hydrocarbons on graphitised carbon blacks have been dete~mined.~-~~@ Preliminary calculations of the adsorption potential of non-polar molecules on graphite reported by the present authorS at the Second International Congress of Surface Activity in London in 1957 were soon afterwards refined in the works of N. N. Avgul I. A. Lygina D. P. Poshkus and the present a ~ t h o r . ’ ~ ~ ~ These refinements involved account- ing for the later terms in the equation for dispersion energy introduction of more accurate values for the exponential repulsion constant p and summation over a large number of graphite carbon atoms.The adsorp- tion energy was computed by the formula @ = -Cil L’r,j-s - Ci2Crij-s f - Ci3Pij-l0 + B ‘ P p ( - rij/p) where i is the force centre of the adsorbate molecule; the constant Cil of electrokinetic interaction of this centre with the carbon atom j of the graphite was calculated by the Kirkwood-Miiller formula and the con- stants Ciz and Ci3 by similar f o r r n ~ l a e . ~ ~ ~ ~ ~ The contribution of the term - Ci2pij-8 was about 10% of @ and of the term Ci3Pij-l0 about 1 % 30 A. V. Kiselev and D. P. Poshkus Proc. 2nd Internat. Congress on Surface Activity Vol. TI London 1957 p. 202. 31 G. L. Kington “The Structure and Properties of Porous Materials,” p.247 ed. D. H. Everett F. Stone London 1958. 35 N. N. Avgul A. V. Kiselev I. A. Lygina and D. P. Poshkus Zzvest. Akud. Nauk S.S.S.R. Otdel. khim. Nauk 1959 1196. (4) A. V. Kiselev and V. I. Lygin Kolloid. Zhur. in the press. KISELEV SURFACE CHEMISTRY 103 (therefore owing to the approximate nature of the calculation the latter term may be neglected). The constant B' was found from the condition of minimum energy of interaction with the entire lattice @ at an equilibrium distance from the outer phase. The value p = 0.28 A resulted in @ values close to those obtained in c a l ~ u l a t i n g ~ ~ ~ ~ ~ ~ ~ ~ the contribution of the repulsion potential by the Lennard-Jones exponential formula B" Crij-12. i / / X / I 1 I I I I - - r - - - i h 3 4 5 6 7 8 9 Values of n - I4 7r n -12 a -.3 -10 ; - 8 -6 1 FIG. 1. Experimental values of standard diyerenrid heats of adsorption Quo (points) and theoretical energies - @O calculated by equation (4) (lines) for normal alkanes (jhll line) and olejins (broken line) with n carbon atoms on graphitised carbon black. References are 0 7 8; D 10; 0 6; A 6; 8 34; x 34; V 15 39. Fig. 1 is a cornparism of the refined calculation of @ according to equation (4) (with approximate allowance for the adsorbate-adsorbate interaction energy for standard surface coverage 6 = 0.5 ) with the experi- mental values of standard differential adsorption heats of a number of n-alkanes and olefins on graphitised carbon black.* Table 1 lists the results * Measurements of the differential adsorption heats Q of n-hexane vapour on T-1 thermal carbon black graphitised at 3000" showed15 that the region of initial Q.drop due to residual heterogeneity of the specimen had greatly narrowed. But the region of increase of Q. on filling of the monolayer by adsarbate-adsorbate interaction had become wider and the height of the maximum had increased in comparison with Spheron-6 1700" carbon b l a ~ k . ~ ~ ~ For adsorption of benzene complete monolayer coverage of the surface involves hardly any increase in the heat of adsorption. This is perhaps due to partial compensation of the weaker electrokinetic adsorbate-adsorbate attraction by electrostatic repulsion of the H-C dipoles in the ring plane and of the quadrupoles formed by the system of n-electrons and residual charges on the carbon a toms of benzene.35 4- 104 QUARTERLY REVIEWS for several other adsorbates particularly isoalkanes for which the different distances of the separate links of the molecule from the graphite surface were taken into account.' These results show that calculation by equation (4) gives first the correct sequence of the adsorption energy values (for example - @O as Qao for benzene are smaller than for hexane) and secondly satisfactory quantitative agreement,l* The calculations and experiments of Crowell and Young23 and of Pacell for argon and those of Pace and Siebert26 for hydrogen and deuterium suggest the same conclu- sion.TABLE 1. Graphite adsorption energies - Qi0 calculated by us,18*34 and experimental diflerential adsorption heats Q ao on the surface of graphitised carbon blacks at standard coverage 8 = 0.5 (kcal./mole).Adsorbate Hydrogen Deuterium Neon Argon Krypton Nitrogen Propane n-Butane n-Pentane n-Hexane n-Heptane n-Octane 2,ZDimethylbutane 3-Methylhexane 2,2,4-Trimethylpentane Cyclopentane Met hylc yclopen t ane Propene Benzene Toluene - @O 0.90* 0.95" 1.0 2-6 3.7 2-6 6.8 8.5 10.4 12.4 14.2 16.1 10-2 13.2 12.6 9.6 10.4 6.2 10.3 12.0 Q ao 0.9 1 0.95 1.0 2-7; 2.6; 2.3 3.5; 4.5 2-8; 2.3 6.5 8.7; 8.3 10.2 12.1; 12.5 14-0 16.0 10.0 12.7 12.7 8.9 10.2 6-2 10.0; 10-3 12.1 Ref. 26 26 34 6 11 37 5 9 6 37 34 6 10 7 8 7 8 15 778 7 8 7 8 7 8 7 8 7 8 7 8 34 7 8 38 7 8 Heats of Adsorption of Alcohols and Water on Graphitised Carbon Black. The adsorption energy of alcohols differs from that of the cor- responding hydrocarbons in that the dispersion interaction of the hydro- * After Pace and Siebert,26 with a 10 % correction introduced by us for the contribu- tion of the term - Ci2 L'rijm8.34 A. G. Bezus V. P. Dreving and A. V. Kisclev Kolloid. Zhur. in the press. 35 A. V. Kiselev and D. P. Poshkus Doklady Akad. Nauk S.S.S.R. 1958,120,834. 36 J. G. Aston and J. Greyson Proc. 2nd Intcrnat. Congress on Surface Activity 37 S . Ross and W. W. Pultz J. Colloid Sci. 1958 13 397. 38 A. A. Isirikyan and A. V. Kiselev J. Phys. Chem. in the press. j Vol. 11 London 1957 p. 39. KISELEV SURFACE CHEMISTRY 105 carbon part of the molecule is supplemented by hydroxyl interaction namely the weak electrokinetic interaction of the hydroxyl oxygen atom with graphite and electrostatic polarisation of the graphite carbon atoms by the OH dipole. Besides in this case a strong adsorbate-adsorbate interac- tion comes into play involving formation of a hydrogen bond between the hydroxyl groups of the alcohol molecules.14 \V I 2 4 6 8 10 O( mole / m.*) FIG.2. Dependence of diferential heat of adsorption Q. of butan-1-01 vapour on the amount adsorbed per unit area a by graphitised thermal carbon blacks. The horizonta broken line here and in other fisirres represents the heat of condensation. Fig. 2 shows the dependence of the differential heat of adsorption Qo of butan-1-01 on thermal carbon black graphitised at 3000" on a (the amount adsorbed per unit area of surface) as obtained by N. N. Avgul and I. A. Lygina. Residual surface heterogeneity hinders determination of the initial heat of adsorption of isolated alcohol molecules in the region of coverage of the more homogeneous part of the surface (where the curve begins to rise) the heat of adsorption of butan-1-01 is already considerably higher than that of n-butane.Thus at 8 = 0-5 Quo for butan-1-01 is 14.4 and for butane is 8.4 kcal./mole; the difference between these values 6-0 kcal./mole is close to the energy of the hydrogen bond in alcohols. In the region of predominant second-layer adsorption Qu again passes through a weak maximum. Fig. 3 presents the Qa-a curves for adsorption of a number of alcohols on a carbon black with a less homogeneous surface (Spheron-6 calcined at 2800°).14 The increment per CH group in the Qao values of the lower alcohols (1 -5 kcal./mole) is smaller than the corresponding increment for 106 QUARTERLY REVIEWS FIG. 3. Dependence of the differential heat of adsorption Qa on the amount adsorbed per unit area a of normal alcohol vapours on Spheron-6 2800" carbon black.L's are the corresponding heats of condensation. Here and below solid points indicate desorp- tion. 1 Methanol; 2 ethanol; 3 propan-1-01; 4 butan-1-01. -2' 10 \ E O 1 & O O . 10 2 0 3 0 40 5 0 o( (166 rnole/g.j FIG. 4. Dependence of differential heat of adsorption Qa of water vapour on the amount adsorbedper unit area a and a by Spheren-6,2800' carbon black. KISELEV SURFACE CHEMISTRY 107 n-alkanes (1 -90 kcal./mole) ; this is because in hydrogen-bonded alcohols the hydrocarbon part of the alcohol molecule closest to the hydroxyl group cannot assume the most favourable orientation. Fig. 4 shows the &-a relation for adsorption of water on the same carbon black.14 Measurement of the heat of adsorption at low p/ps is greatly complicated in this case owing to the exceedingly small adsorption values.Nevertheless it can be confidently concluded that the heat of adsorption Qa of water vapour on the surface of the graphitised carbon black is smaller than its heat of condensation L. 0 0 E - 0 . 0 5 0.1 5 10 15 Plps P (mm.) FIG. 5 . Adsorption isotherm of ( 1 ) butan-1-01 and (2) n-butane on Spheron-6 2800° carbon black. Fig. 5 shows the adsorption isotherms of butane and butan-1-01 in relation to p and p / p s . At identical values ofp the adsorption of butan-1-01 is greater than of butane and at equal p/p8 the reverse. This agrees with the fact that the full heat of adsorption Qa of butanol is larger than that of butane while the net heat of adsorption Qa-L of butanol is less than that of butane.14 Fig.6 shows the adsorption isotherms of water and of alcohols. These isotherms are described satisfactorily by equations (1) and (2) for localised adsorption in conformity with the fact that in this case the adsorbate-adsorbate interaction cannot be neglected in com- parison with the adsorbate-adsorbent interactions.* Heats of Adsorption of Hydrocarbons on Magnesium Oxide. The dependence of the heats of adsorption of n-hexane and benzene on the surface coverage of magnesium oxide calcined at 1000" is illustrated in * At the transition to graphitised thermal carbon blacks with more homogeneous scrfaces the imtherms of adsorption of lower alcohols are represented better by the isotherm equations for non-localised adsorption.14 This is probably connected with the increase in the regions of free migration for these molecules.108 d Q I QUARTERLY REVIEWS 0. I 0.2 0.3 0.4 0.5 P h FIG. 6 . Adsorption isotherms of (1) water vapour (2) nzethanol ( 3 ) ethanol (4)proparz-I - 01 and ( 5 ) butan-1-01 on Spheron-6 2800" carbon black. The curves are calculated bv means of equation (2). Fig. 7.39 At medium monolayer coverage the heat of adsorption depends little on 6 and therefore the Qao values at 6 = 0.5 can be accepted as the standard values of the differential heat of adsorption. Table 2 lists these values in comparison with the values of - @O calculated from equation (4) after allowance for polarisation of the adsorbate molecules and correction for the energy of adsorbate-adsorbate interaction at 8 = 0-5,1s~19 The energy of induction attraction is very small here because ions of opposite signs alternate on the (100) face of the magnesium oxide.It can be seen from the Table that the calculated energies of adsorption agree satis- factorily in this case with the measured heats of adsorption. The agreement between the calculated and the measured energies of adsorption of nitrogen krypton and methane on sodium bromide has been pointed out by Fisher and McMillan.21 A. A. Isirikyan and A. V. Kiselev Zhur. fiz. Khim. 1960 34 2817. KISELEV SURFACE CHEMISTRY 109 TABLE 2. Calculated energies of adsorption - @jO on the (100) face of magnesium oxide and experimental diferential heats of adsorption Q a0 on magnesium oxide at 8 = 0.5 (kcaLlmole.) Absorbat e - @O Q a' Ref.n-Hexane 9.7 9.4 39 n-Heptane 11.1 11.3 39 n-Octane 12.6 12.4 39 Benzene 8.6 9.1; 9.2 39,40 Toluene 10.1 10-3 40 h I - I I I I 0-4 0 - 8 2 I I I I I I I 0.2 0-4 0.6 o( (IO-~ mole /g> FIG. 7. Dependence of diferential heat of adsorption Q. of (1) benzene (2) n-hexane and ( 3 ) n-octane vapours on surface coverage of magnesium oxide. *O S. D. L. Shreiner and C. Kemball Trans. Faraday Soc. 1953 49 1080. 110 QUARTERLY REVIEWS Heats of Adsorption of Hydrocarbons on Magnesium Hydroxide. Investigation of the energy of adsorption on hydroxides is of major interest as identical and identically oriented hydroxyl dipoles project from the surface in this case. They create a more homogeneous though weaker electrostatic field than does the (100) face of magnesium oxide and similar ionic lattices.41 This causes an increase in the energy of adsorption of molecules that have markedly non-uniform electron-density distribution.Such distribution occurs when a molecule has either a dipole or a large quadrupole moment formation of a strong or weak hydrogen bond then being possible if the orientation is favourable. Interaction arises (supple- mentary to the dispersion energy) more for benzene than for hexane. In the benzene molecule the 7-electron clouds increase the electron density on both sides of the hexagon of carbon atoms,35 causing a weak hydrogen bond with the hydroxyl groups on the outer surface of the magnesium hydroxide. The energy contribution of the electrokinetic interaction with this lattice as in the case of graphite is higher for hexane than for benzene.41 In conformity with this are the experimental Qao values:42 for n-hexane 9-0 and for benzene 9.6 kcal./mole.Complete calculation of the energy of interaction of the benzene molecule with magnesium hydroxide is difficult owing to the absence of reliable data on the charge distribution in the hydroxyl of Mg(OH),. Therefore the problem has been solved so far only as functions of the degree of covalency of the bond and of the value of the OH dipole moment .43 Heats of Adsorption on Silicas with Hydrated and Dehydrated Surfaces. Adsorption on amorphous hydroxides with large surfaces-silica gels and aerosols-is of great practical importance. By drying silica gels after their formation at not more than 150° or by hydrothermal treatment of de- hydrated silicas completely hydrated silica surfaces can easily be ~ b t a i n e d .~ ~ ~ ~ Sufficiently prolonged evacuation at ordinary temperatures (not above 150”) makes it possible to free these surfaces almost completely from adsorbed water retaining a dense coating of hydroxyl groups of the silica proper. This is demonstrated by the reversibility of the isotherms and by the heats of adsorption of the water vapour as well as by a study of the infrared spectrum in the region of valency vibrations of the hydroxyl in Si-OH or %-OD and in the region of deformation of the water mole- c u l e ~ . ~ ~ Fig. 8 shows first the infrared spectrum of an evacuated silica gel with a hydrated surface. There is a major absorption by silica-hydroxyl in the region of valency vibrations with an indication of a weak hydrogen 41 D.P. Poshkus and A. V. Kiselev Zhur. $2. Khim. 1960 34 2640. 42 A. V. Kiselev and D P. Poshkus Kolloid. Zhur. 1960 22,403. 43 D. P. Pcshkus and A. V. Kiselev Zliur. 3 2 . Khim. 1960 34 2646. 44 A. V. Kiselev Kolloid Zhur. 1936 2 17. 45 A. V. Kiselev “The Structure and Properties of Porous Materials,” ed. D. H. 46 A. V. Kiselev and V. I. Lygin Kolloid. Zhur. 1959,21 581 ; 1960,22,403. Everett F. Stone London 1958 p. 195. KISELEV SURFACE CHEMISTRY 10- , 3 .'as..._ .:. I [ . . . . . . . . I '00 1600 Id 111 lo W a v e n u m b e r s (cm?) FIG. 8. Infrared absorption spectra in the system silica gel-water. Left Region of va1enc.y vibrations of silica gel hydroxyl groups. Right Region of' deformation vibrations of water hydroxylgroups. (1 1 After evacuation at 200"; (2) afer adsorption of monolayer of water ( 3 ) after capillary condensation.bond between them.*' In the region of deformation vibrations of the water molecules there is no absorption and therefore there is practically no water on the surface in the form of adsorbed molecules. Secondly Fig. 8 shows the corresponding spectrum after adsorption of approximately a monolayer of water. It now displays a hydrogen bond between the water molecules and the silica gel hydroxyls and considerable absorption in the region of deformation vibrations in the water molecules. Finally after capillary condensation of water both these effects became stronger almost reaching the normal effects in ordinary liquid water.46 The presence of a hydroxyl coating on the surface of hydrated silica results not only in strong adsorption of molecules capable of forming hydrogen bonds with the silicic acid hydroxyl groups (water alcohol *' A.V. Kiselev and V. I. Lygin Proc. 2nd Internat. Congress on Surface Activity Vol. 2 London 1957 p. 204; V. I. Lygin Vestnik Moskovskovo Universiteta 1958 No. 1 223. I12 QUARTERLY REVIEWS e t ~ . ~ ~ ~ ~ * ) but also as in the case of magnesium hydroxide in a sharp in- crease in the adsorption of molecules non-polar on the whole but with a very non-uniform electron-density distribution. With favourable orienta- tion of the quadrupole moment (parallel to the hydroxyl axis) these molecules are adsorbed much more strongly than are molecules of similar dimensions having the same or even higher polarisability but a smaller quadrupole moment. D. P. Poshkus and the author demon~trated~~ that calculation of the Coulomb contribution to the energy of electrostatic interaction of the benzene molecule with the surface hydroxyl groups (quadrupole-dipole electrostatic interaction) results in energy values which make it possible to account for the f a ~ t ~ ~ 9 ~ ~ that the heat of adsorp- tion of benzene on a hydrated silica gel surface is greater than that of hexane (Fig.9).* This conforms with the shifts of stretching frequency in the infrared absorption spectra of the surface hydroxyl groups observed by A. N. Terenin and V. N. phi limo no^^^ in the overtone region during the adsorption of benzene ( d v = 236 cm.-l) and n-hexane ( d Y = 70 cm.-l>.t This also accounts for the sharp lowering of the heat of adsorption of benzene (in contrast to hexane) on dehydration of the silica ~ u r f a c e ~ ~ - ~ (see Fig.9) and the corresponding decrease in the adsorption of whose molecules possess a rather large quadrupole moment. The higher adsorption of nitrogen on the hydrated surface is in conformity with the relatively high values of the shift in vibration frequency of the silica hydroxyls. 55 9 56 Fig. 10 shows plots of (Qa - L) against a for n-hexane and benzene,4g and Table 3 gives the values of their differential adsorption heats at 8 = 0.5 on a series of adsorbents (both with and without hydroxyl groups) coated on their surface. It can be seen from the Figure and the Table that in passing from the non-polar adsorbent-graphitised carbon black to silica gel which has a highly hydrated surface the difference of the heats of adsorption of these hydrocarbons changes its sign.* For the same reasons the standard heat of adsorption on a hydrated silica gel surface is higher for propene than for propane60 (7.4 and 5.0 kca.l./mole) and higher for but-1-ene than for butane.51 7 For the main spectral region V. I. Lygin et recently obtained the values 90 and 26 cm.-l for the stretching frequency shifts of groups OD in a silica gel surface coated by adsorption of benzene and hexane. 48 A. V. Kiselev Proc. 2nd Internat. Congress on Surface Activity Vol. 2 London 1957 p. 179. 49 L. D. Belyakova and A. V. Kiselev in the collection “Obtaining the Structure and Properties of Sorbents,” Leningrad 1959 p. 180 (in Russian). 5 0 A. G. Bezus V. P. Dreving and A. L. Klyachko-Gurvich Kolloid. Zhur. in the press. 51 W. R. Smith and R.A. Beebe Ind. Eng. Chem. 1949,41,1431. 62 A. N. Terenin in the collection “Surface Properties of Chemical Compounds and their Role in Adsorption Phenomena.” ed. A. V. Kiselev Moscow State Univ. Press 1957 p. 206 (in Russian); V. N. Philimonov Optika i Spekroskopiya 1956,1,490. 63 A. V. Kiselev and V. I. Lygin Kolloid. Zhur. in the press. 64 A. V. Kiselev and E. V. Khrapova Kolloid. Zhur. 1957 19 572. 56 R. McDonald J. Amer. Chem. SOC. 1958 79 850. 66 G. J. C. Frohnsdorff and G. L. Kington Trans. Faraday Soc. 1959,55 1173. KISELEV SURFACE CHEMISTRY 4 I I I I I I I I I I 1 I I I I I I I I I I I I I I I I I I I I I m a 114 QUARTERLY REVIEWS I l l 1 2 3 4 5 3 FIG. 10. Dependence of differential net heat of adsorption Pa-L on adsorption a for benzene (solid curves) and n-hexane (broken curves) on surfaces of silica with mean surface concentration of hydroxyl groups aOH = 12.5 p-moleslm.2; 2-ditto with = 10.8; 3-ditto with aOH = 3.5; 4-ditto with aOH as.follows (1) 12.5 (2) 10.8 (3) 3.5 and (4) 1.0 x mole/m.2; and ( 5 ) on magnesium oxide amd (6) on Spheron-6 1700" carbon black.Standard heats of adsorption Qao of n-hexane and benzene Adsorbent Hexane Benzene Hexane - Benzene TABLE 3. vapours on various adsorbents (kcal./mole). Graphitised carbon black 12.5 10.3 +2.2 Magnesium oxide 9.4 9.2 +0*2 dehydrated surface 8.6 8.5 +0.1 Silica gel with strongly Magnesium hydroxide 9.0 9-6 -0.6 Silica gel with strongly hydrated surface 8.8 10.4 -1.6 Heats of Adsorption on Silica with its Surface Chemically Modified by Trimethylsilyl Grours. In this case trimethylsilyl groups are chemically bonded to tke silica s ~ r f a c e .~ ' ? ~ ~ Fig. 11 shows the scheme of this bonding 57 H. W. Kohlschiitter P. Best and G. Wirzing 2. anorg. Chem. 1950 285 236. 58 A. V. Kiselev N. V. Kovaleva A. Ya. Korolev and K. D. Shcherbakova Doklady. Akad. Nauk S.S.S.R. 1959 124 617; 1. Yu. Babkin V. S. Vasilyeva J. V. Drogalyeva A. Ya. Korolev and K. D. Shcherbakova ibid. 1959,129,131. KISELEV SURFACE CHEMISTRY 115 FIG. 1 1. (a) Possible combinatio:i (6) Model of close-packed nierhj (c) Tridinzite structiire. 0 4 m (v 0 - ~~ 0 I 0 u of triinethylsilyl group with silicon-oxygen tetrahedroti . GI group layer with (above) adsorbed n-hexane molecule. and a simplified model of the modifying layer.5g After sufficient modifica- tion of the surface the adsorbate moves away from it causing the adsorp- tion potential to drop sharply especially in the case of molecules adsorbed b g I.Yu. Babkin and A. V. Kiselev Dokludy. Akad. Nauk S.S.S.R. 1959 129 357. .- cn I Q 116 QUARTERLY REVIEWS not only by electrokinetic but also by electrostatic interactions (polar quadrupole) with the hydrated silica surface. A calculation of the energy of adsorption on such a modified surface made by I. Yu. Babkin and the resulted in values for n-hexane and benzene of about 4. and 4. kcal./mole. i.e. much smaller than the heats of condensation. Measurements of differential heats of adsorption carried out by us with aerosol specimens modified in A. Ya. Korolev’s FIG. 12. amount SiMe, I 2 3 O( mote /m2) Dependence of diferential heat of adsorption Qa of benzene vapour on the adsorbed per unit area a by modified aerosols.Surface concentration of o,O; n-60%; A-80%; v-90%; 0 100%. l a b ~ r a t o r y ~ ~ showed that despite the residual surface heterogeneity the net heats of adsorption of hydrocarbons on sufficiently modified specimens were actually negative (Fig. 1 2).60 Incipient capillary condensation at the contact points between the aerosol particles results in excessive Qa values 6o 1. Yu. Babkin A. V. Kiselev and A. Ya. Korolev 1961 136 373. Doklady Akad. Nauk S.S.S.R. KISELEV SURFACE CHEMISTRY 117 and therefore the true values for the pure adsorption interaction are still smaller i.e. still closer to those calculated in ref. 59. Fig. 13 presents the corresponding adsorption isotherms. Increasing the degree of modification often lowers the adsorption value by dozens of times.58~so~61 4 n hl € \ W 0 - 2 I .% Is 0.2 0.4 0.6 0.8 PIPS FIG.13. Adsorption isotherms of benzene vapour on modified aerosols. Surface concen- tration of SiMe, 0 0; 0 - 60q/ ; A - 80% ; v w 90% ; 0 N 100%. The lowest isotherm is on a larger scale. The sharp drop in the energy of universal electrokinetic interactions and the possibility of obtaining very homogeneous surfaces and of bonding a modifying layer (which screens the adsorbent) of new active functional groups to the surface make chemical modification a good method of controlling the properties of fillers for various media pigments and adsorbents for chromatography particularly for the application of gas chromatography to mixtures of heavy hydrocarbons and their derivatives and associated compounds.62 For instance by hydrothermal treatment of silica gel and chemical modification of its surface by trimethylsilyl groups it was possible to separate benzene and hexane vapours even at room temperat~re.~~ O1 A.V. Kiselev A. Ya. Korolev R. S. Petrova and K. D. Shcherbakova Kolloid. Zhur. 1960 22 671. O2 A. V. Kiselev and K. D. Shcherbakova “Gas-Chromatographie. Material zum 2. Symposium iiber Gas-Chromatographie in Bohlen,” Oktober 1959 ed. R. E. Kaiser A. G. Struppe p. 198. O3 V. S. Vasilyeva I. V. Drogaleva A. V. Kiselev A. Ya. Korolev and K. D. Shcher- bakova Doklady Akad. Nauk S.S.S. R. in the press. I18 QUARTERLY REVIEWS Adsorption Equilibria An Attempt at Complete Statistical-thermodynamic Calculation of Adsorption Equilibria. As was pointed out in the first section the chief problem in this field is to calculate isotherms for adsorption on a homo- geneous surface proceeding only from the structure and physical properties of the adsorbent and the adsorbate.Several papers dealing with this trend were mentioned above. D. P. Poshkus and the present author6* carried out a statistical-thermodynamic calculation of the change in chemical potential when argon is adsorbed on a graphite basal face for low 8 according to the formula derived on the basis of the expression given by Hill2' for the equilibrium constant. Here h and k are the Planck and Boltzmann constants m is the mass of the adsorbate molecule o, is the area it occupies in a dense monolayer j is the partition function for internal degrees of freedom of the adsorbate molecule in the gas; 8 = Naw,,/s and fa' = fa/s (fa is the partition function of the adsorbate molecule over the entire surface s and Nu is the number of adsorbed molecules).As Hill2' did when calculating fa we used the appro~imation~~ f =fclassv** Here we have the fully classical partition function (n = number of degrees of freedom of the molecule H = Hamiltonian for the adsorbate molecule and Y** = f (harm. oscill. quant.)/f (harm. oscill. class.) (8) where f (harm. oscill. quant.) andf(harm. oscill. class.) are the partition functions of harmonic oscillators in the quantum-mechanical and classical form respectively calculated from the shape of the potential-energy surface of the adsorbed molecule in the vicinity of the minimum). The potential energy in the Namiltonian was calculated for the adsorbed atom by means of equation (4) as a function of the co-ordinates x y and z (xu is the plane passing through the carbon atom of the graphite basal 64 A.V. Kiselev and D. P. Poshkus Doklndy Akad. Nauk S.S.S.R. 1960 132. 872; K. S. Pitzer and W. D. Gwinn J . Chem. Phys. 1942 10 428. KISELEV SURFACE CHEMISTRY 119 face and z is the direction normal to this plane). Substitution into equation (7) and integration with respect to the impulse components p gives:27 where (3a22/3)/2 is the area of the hexagon formed by the carbon atoms of the basal face (a = 1.418A is the graphite lattice constant) erf - - ( :y2 is the probability integral. Integration with respect to x and y in equation (9) is carried out within the limits of a single hexagon. The value of the integral was calculated graphically.Integration was carried out for those values of z for which the potential energy of the adsorbed atom @(x y z ) The value of exp [(- @ (x y z)/kT] for argon on graphite with z/z deviating from unity (2 being the equilibrium distance depending on x y ) falls rapidly. Since in the limits of z/zo within which integration was carried out the value of the factor erf (- @/kT)1/2 is substantially equal to unity we neglected it. For an argon atom located above a graphite surface the quantum- mechanical factor v** = hv - - x [ l -exp (- hvx E)] hv E[1 -exp (- hv zf)] -l kT kT exp (-%)I-' where v, vv and v are the frequencies of vibration of the adsorbed argon atom parallel to the corresponding axes near the potential minimum (above the centre of the hexagon for z = z,).Calculating the frequencies of oscillation 1 K 1 % 1 Fa v = 2~ -4- m v y = 2.rr J v z = 2n -J- m we estimated the constants K, Ky and K analytically according to the formula K = (&:)3 x = o y = o z = zo K = (g)7 x = o y = o z = zo x = o y = o z = zo In this case v = 3.35 x loll v y = 5.8 x lo1' and v z = 1-5 x 10l2 set.-' whence v** = 2.00. 1 20 QUARTERLY REVIEWS The value om depends on the packing of the argon atoms over the surface. The partition functions j,. for the internal degrees of freedom of the argon atom were assumed to be the same in the adsorbed as in the gaseous phase. Substituting the corresponding expressions and value into expression ( 5 ) we obtained the relation between d p and 8 during the transition of argon from a gas atpo = 760 mm.H g on to the surface of the graphite basal face at T = 7 7 . 8 " ~ (this relation being almost independent of corn) d p = - 1.25 + 0.35 log, 8 (kcal./mole) (13) Fig. 14 compares the initial regions of the experimental and the calculated plots of - d p against 8. The experimental curve was calculated from the adsorption isotherm measured in expression ( 5 ) by the thermodynamic 2 " i O F 0.05 ,g 0-10 FIG. 14. Dependence of change in chemical potential - A p of argon on coverage 8 of graphite surface. (1) Calculated. ( 2 ) Experimental. formula d p = RT log, [p(8)/760]. The calculated curve lies fairly close to the experimental one.* Thus on the basis of theoretical calculations of the adsorption energy the change in chemical potential of argon on passing from the gaseous phase to the adsorbed state can be calculated at low graphite surface coverages by a statistical method.An Attempt to Base the Model of the Adsorbate Molecule Movement on Spectral Data. A proper choice of the model for adsorption complexes is very important because the model affects the calculation of the partition * Analysis shows that the difference between the calculated and the experimental curve lies within the limits of error of the calculation.66 At higher values of 8 interaction between the adsorbed molecules has an effect on the experimental curve. We have not taken this into account in the calculated curve (the energy of this interaction was calculated in ref. 11). E6 D. P. Poshkus and A. V. Kiselev Zhur. fiz. Khim. in the press. KISELEV SURFACE CHEMISTRY 121 functions and entropies of the complexes.Very important in this connec- tion are the spectral investigations of chemical compounds and adsorption complexes formed on a surface. To solve these problems systematic and detailed investigations of infrared spectra must be available. V. I. Lygin and the present tried to estimate the entropy of water adsorbed on a hydrated silica surface with allowance for the frequencies of the valency and deformation vibrations of the hydroxyl groups of the silica and the adsorbed water molecules. The calculation was carried out for two localised models a water molecule bonded (i) to one surface hydroxyl group by a single hydrogen bond (which allows free rotation around this bond) and (ii) to two surface hydroxyl groups by two hydrogen bonds (retarded rotation libration).Estimates were made of the values of rotational vibrational and configurational entropies. Their sum amounted to about 7.5 e.u. for the first model and 4-5 e.u. for the second. The experimental result6' indicates higher values. Possibly this is related to the necessity of taking into account the oscillations parallel to the surface and to non-uniformity of distribution of the hydroxyl cover over the surface. This estimate of the entropy of water-hydroxyl adsorption complexes on silica is very approximate because so far the models have been selected on the basis of only average adsorption characteristicsg5 and frequencies-on the basis of spectral data obtained only in the main region of the spectrum where they give information chiefly on the internal vibrations of the atoms in the molecules.In the future a study must be made of the vibrational and rotational spectrum in the far infrared region for the molecule as a whole with respect to its surface. It is essential also that the frequencies of the retarded rotation should make it possible to estimate the height of the potential barriers. Semiempirical Calculation of Adsorption Equilibrium on a Homogeneous Surface. Formula 1-3 ~how,l-~ as was pointed out in the first section the type of functional relation between 8 and p orp/ps the constants Kl and Kn in (I) (2) or Kl and K2 in (3) being found from the experimental isotherm. Since 8 = &/am where 01 is the absolute adsorption at 8 = I to calculate 8 from experimental data we must know the value of the area w occupied by an adsorbate molecule in a dense monolayer (01,)~ = l/owL).This value of can be found independently of adsorption from the van der Waals dimensions of the adsorbate molecule and their probable orientation and packing.39 At high K and low Kn i.e. when the adsorbate-adsorbate interaction can be neglected with respect to the adsorbate-adsorbent interaction this equation describes an isotherm with a convex* beginning and one inflexion i.e. approaches the B.E.T. isotherm. When these interactions are commensurate equation Equation (2) describes three types of * In this context convex means dvldx is increasing. A. G. Foster J. 1945 360. 1 22 QUARTERLY REVIEWS (2) describes an isotherm with a concave beginning and two inflexions (for example alcohols on graphitised carbon blacks see Fig. 6). Finally when the adsorbate-adsorbent interaction is very weak the isotherm is concave throughout (water on graphitised carbon14).The values of the constants Kl and K, though of but relative importance are nevertheless interesting because they make it possible to compare conveniently the adsorption isotherms of various substances on the same adsorbenP or of the same vapour on different adsorbent~.~~ Table 4 lists the rounded values of the ratios of these constants for adsorption of a number of vapours on graphitised carbon blacks at temperatures close to room temperature. TABLE 4. Ratio of equilibrium constants of adsorbate-adsorbent K1 and adsorbate-adsorbate K interactions for adsorption of various vapours on graphitised carbon black. Adsorbate KVtIKl Shape of isotherm* Benzene 0.003 Convex with one inflexion C yclopentane 0.12 Concave at first with two inflexions Sulphur dioxide 7.0 Concave at first with two inflexions Methylamine 13 Concave at first with two inflexions Methanol 350 Concave at first with two inflexions Water (50,000) Concave throughout Of major interest are the wave-shaped isotherms of unimolecular and multimolecular adsorption of vapours.For example for nitrogen vapour at -195” on graphitised thermal carbon black the isotherm is at first concave and then passes through several inflexion points. Each wave of this isotherm can be describedI3 by an equation of the same type namely equation (2) with the corresponding (different for different waves) pairs of values of the constants K and K,. Fig. 15 gives the experimental points and curves calculated with the aid of three pairs of Kl and K constants found by the three successive waves in co-ordinates of equations (1) and (2).A complete theoretical calculation of the equilibrium constants in this case requires statistical-thermodynamic treatment of multimolecular adsorption with allowance for adsorbate-adsorbate interactions. Several steps in this direction have been made by Pace.ll Empirical Accounting for Geometrical and Chemical Surface Hetero- geneity. Discussion of these methods is not the object of this Review and therefore we shall only indicate their possibility. In studies of the dependence of experimental adsorption on the pore size and degree of * In this context convex means dyldx is increasing. 68 A. V. Kiselev “Adsorption Brief Description of Exhibits on the Stand at Soviet Science Technology and Culture Exhibition in New York 1959,” U.S.S.R.Academy of Sciences Moscow 1959. KISELEV SURFACE CHEMISTRY 123 lo4 p/ps (Lower curve) FIG. 15. Adsorption isotherm of nitrogen vapour on thermal carbon black graphitised at 3000". The curves are calculated by means of equations (1) and (2). chemical modification of the surface (for example the degree of hydration of a silica gel surface) adsorption can be expressed as a function of these factors. Comparing the corresponding graphs for a given adsorbate- adsorbent system at various p/ps the adsorption isotherms can be found from them by knowledge of the pore size and the degree of chemical modification of the surface. Examples of the application of this method to the adsorption of benzene vapour on silica gels are given in ref.45. This group of empirical methods of isotherm calculation includes also the 124 QUARTERLY REVIEWS Polanyi theory as developed by M. M. Dubinin and his co-worker~.~~ The equations of the adsorption isotherms are derived in these works the type and constants being found experimentally. This method is convenient for finding the adsorption isotherms of various substances at different temperatures from a single isotherm for one substance. Conclusion The examples discussed above illustrate the value of different methods for describing and interpreting adsorption equilibria and heats of adsorp- tion; these methods should be developed in parallel. However as methods of calculating adsorption energies and partition functions im- prove as the models of the movement of adsorption complexes introduced into the calculations become better founded on spectral and other physical methods and finally as more accurate information on the structure of adsorbents accumulates so the absolute calculation of heats of adsorption and adsorption equilibria purely from the structure and physical proper- ties of the adsorbent and the adsorbate should acquire greater importance.And though this method will long remain inferior in accuracy to direct measurement it has already the notable advantage that it makes it possible to determine not so much what the adsorption properties of the system in question are as why they are what they are. 69 M. M. Dubinin and E. D. Zaverina Zhur. fit. Khim. 1949 23 1129; K. M. Nikolayev and M. M. Dubinin Izvest. Akad. Nauk S.S.S.R. Otdel. Khim. Nauk 1958 1 165 ; M. M. Dubinin “Industrial Carbon & Graphite,” Pergamon Press London 1958 p. 219.

ISSN:0009-2681    

DOI:10.1039/QR9611500099

出版商:RSC    

年代:1961

数据来源: RSC

摘要:

CUMULATIVE INDEXES VOLUMES I-XV (1 947-1 961) CUMULATIVE INDEX OF AUTHORS Abrahams S. C. 10,407 Abrikosova I. I. 10,295 Addison C. C. 9 115 Ahrland S. 12 265 Albert A. 6 197 Allen G. 7 255 Amphlett C. B. 8 219 Anderson J. S. 1 331 Angyal S. J. 11 212 Arnstein H. R. V. 4,172 Atherton F. R. 3 146 Avison A. W. D. 5 171 Bacon R. G. R. 9 287 Baddeley G. 8 355 Baddiley J. 12 152 Badger G. M. 5 147 Bagnall K. W. 11 30 Baker W. 11 15 Baltazzi E. 9 150 Barker S. A. 7 58 Barltrop J. A. 12 34 Barnartt S. 7 84 Barrer R. M. 3 293 Barton D. H. R. 3 36; 10,44; 11 189 Bassett H. 1 247 Bateman L. 8 147 Battersby A. R. 14 77; Baughan E. C. 7 103 Baulch D. L. 12 133 Bayliss N. S. 6 3 19 Bell R. P. 1 113; 2 132; 13 169. Eentley R. 4 172 Bergel F. 2 349 Bethell D. 12 173 Bevington J. C. 6 141 Birch A.J. 4 69; 12 17 Bircumshaw L. L. 6 Bockris J. O’M. 3 173 Bolland J. L. 3 1 Bond G. C. 8 279 Bourne E. J. 7 58 Bowen E. J. 1 1 ; 4 236 Bradley R. S. 5 315 Braude E. A. 4 404 Bremner J. G. M. 2 1 Brink N. G. 12,93 Brown B. R. 5 131 Brown R. D. 6 63 15 259 157 Buchanan J. G. 12,152 Buckingham A. D. 13 Bu’Lock J. D. 10 371 Bunnett J. F. 12 1 Burkin A. R. 5 1 Burnett G. M. 4 292 Burton H. 6 302 183 Cadogan J. I. G. 8 308 Caldin E. F. 7 255 Carrington A. 14 427 Challenger F. 9 255 Chatt J. 12 265 Coates G. E. 4 217 Collins C. J. 14 357 Collinson E. 9 31 1 Cook A. H. 2 203 Cook J. W. 5 99 Cookson R. C. 10 44 Cooper C. F. 13 71 Cottrell T. L. 2 260 Coulson C. A. 1 144 Cowdrey W. A. 6 358 Cox E. G. 7 335 Crawford V. A. 3 226 Croft R. C. 14 1 Crofts P. C. 12 341 Crombie L. 6 101 Cross A. D.14 317 Cruickshank D. W. J. Curran S. C. 7 1 Cuthbert J. 13 215 14 378 7 335 Dainton F. S. 12 61 Dalgliesh C. E. 5 227 Davies A. G. 9 203 Davies D. S. 6 358 Davies M. 8 250 Davies N. R. 12 265 Davies R. O. 11 134 Dawton. R. H. V. M. De Heer J. 4 94 de la Mare P. €3. D. 3 de Mayo P. 11 189; Derjaguin B. V. 10,295 Dickens P. G. 11 291 462 9 1 126 15 393 Doyle W. T. 14 62 Dubinin M. M. 9 101 Duncan J. F. 2 307; Dunning W. J. 9 23 12 133 Eastham J. F. 14 221 Eley D. D. 3 181 EmelCus H. J. 2 132 Errede L. A. 12 301 Evans,. M. G 4,. 94 6 186 Evans R. M. 13 61 Fensham P. J. 11 227 Ferrier R. J. 13 265 Foster A. B. 11 61 Freidlina R. Kh. 10 Gascoigne R. M. 9,328 Gaydon A. G. 4 l Gee G. 1 265 Gent W. L. G. 2 383 Gibson D. T. 3 263 Gillespie R. J. 2 277; Gilman H. 13 116 Glenn A. L. 8 192 Goehring M.10 437 Gold V. 9 51; 12 173 Gowenlock B. G. 12 321,14 133 Gray P. 9 362 Green J. H. S. 15 125 Greenwood N. N. 8 1 Grfith J. S. 11 381 Grove J. F. 15 56 Gundry P. M. 14 257 Gunstone F. D. 7 175 Gutmann V. 10 451 330 8,40; 11 339 Halpern J. 10 463; 19 207 Hamer F. M. 4,327 Hardy D. V. N. 2 25 Harman R. E. 12 93 Harris M. M. 1,299 Hartley G. S. 2 154 Hassel O. 7 221 Hawkins E. G. E. 4 25 1 CUMULATIVE INDEX 463 Hawkins J. D. 5 171 Haynes L. J. 2 46; 14 Heaney H. 11 109 Hey D. H. 8 308 Hickling A. 3 95 Hochstrasser R. M. 14 Hodson H. F. 14,77 Holt R. J. W. 13 327 Hughes E. D. 2 107; 5 245; 6 34 Hush N. S. 6 186 292 146 Ingold C. M. 6 34; Irving H. M. 5 200 Ivin K. J. 12,61 11 1 Jacobs P. W. M. 6,238 Jain A. C. 10 169 Janz G. J. 9 229 Jeffrey G. A. 7 335 Jenkins E. N. 10 83 Jennings I<.R. 12 116; Jones D. G. 4 195 15 237 Kapustinskii A. F. 10 Katritzky A. R. 10 Kenyon J. 9 203 Khorana H. G. 6 340 Kipling J. J. 5,60; 10 1 Kiselev A. V. 15 99 Kitchener J. A, 13 71 283 395; 13 353 Lagowski J. J. 13 233 Lamb J. 11 134 Lamberton A. H. 5 75 Law H. D. 10 230 Lea F. M. 3 82 Leech H. R. 3 22 Leisten J. A. 8 40 Levy N. 1 358 Lewis E. S. 12 230 Lewis J. 9 11 5 Lifshitz E. M. 10 295 Linnett J. W. 1 73; 11 291 ; 12,116 Lister B. A. J. 2 307 Lister M. W. 4 20 Livingston R. 14 174 Long L. H. 7 134 Longuet-Higgins H. C. 11 121; 14 427 Loudon J. D. 5 99 Liittke W. 12 321 Lythgoe B. 3 181 Maccoll A. 1 16 McCoubrey J. C. 5 MacDiarmid A. G. 10 McGrath W. D. 11 87 McKenna J. 7 231 McLaughlin E. 14,236 Maddock A. G. 5 270 Maitland P. 4 45 Manners D. J. 9 73 Marsh J.K. 1 126 Martin F. S. 13 327 Martin R. L. 8 1 Mason S. F. 15 287 Megson N. J. L. 2 25 Millar I. T. 11 109 Millen D. J. 2 277 Morgan K. J. 8 123; Morrison A. L. 2 349 Murrell J. N. 15 191 Musgrave W. K. R. 8 364; 11 87 208 12 34 33 1 Nancollas G. H. 14 Nelson Smith R. 13 Nesmeyanov A. N. 10 Newth F. H. 13 30 Norrish R. G. W. 10 Nyholm R. S. 3 321; 402 287 330 149 7,377; 11,339 Ollis W. D. 11 15 Orgel L. E. 8 422; 11 Orville-Thomas W. J. Overend W. G. 11 61; Owston P. G. 5 344 Page J. E. 6 262 Paneth. F. A.. 2. 93 381 11 162 13,265 Parsonage N. 'G. 13 306 Pauson P. L. 9 391 Payne. D. S.. 15. 173 Pepper D. C. 8 88 Percival E. G. V. 3 369 Phillips F. C. 1 91 Pliminer J. R. 14 292 Pople J. A. 11 273 Porter G. B. 14 146 Praill P. F. G. 6 302 Pritchard H. O. 14 46 Reid C. 12 205 Reid S. T.15 393 Richards R. E. 10 480 Ridd J. H. 15 418 Riddiford A. C. 6 157 Riley H. L. 1,59; 3 160 Roberts H. L. 15 30 Roberts L. E. J. 15,442 Rose J. D. 1 358 Rowlinson J. S. 8 168 Satchell D. P. N. 9 51 Saxton J. E. 10 108 Schofield K. 4 382 Selman S. 14 221 Seshadri T. R. 10 169 Sexton W. A. 4 272 Sharpe A. G. 4 115; Shchukina L. A. 10 Sheldon J. C. 14,200 Shemyakin M. M. 10 Sheppard N. 6 1 ; 7 19 SillCn L. G. 13 146 Simes J. J. H. 9 328 Simons P. 13 3 Simpson D. M. 6 1; 7 Smales A. A. 10 83 Smith B. C. 14 200 Smith H. 12 17 Smith J. A. S. 7 279 Smith M. L. 9 1 Springall H. D. 10 230 Stacey M. 1 179 213 Staveley L. A. K. 3 65; Stern E. S. 5 405 Stone F. G. A. 9 174 Sutton L. E. 2 260 Swallow A. J. 9 31 1 Symons M. C. R. 12 230; 13 99; 14 62 11,49 26 1 26 1 19 13 306 464 QUARTERLY REVIEWS Synge R.L. M. 3 245 Szwarc M. 5 22; 12 301 Taylor A. W. C. 4 195 Tedder J. M. 14 336 Thomas S. L. 7 407 Thomson R. H. 10,27 Thrush B. A. 10 149 Tipper C. F. H. 11 3 13 Tonipkins F. C. 6 238; Topley B. 3 345 Trapnell B. M. W. 8 Trotman-Dickenson A. Truter E. V. 6,390 14,257 404 F. 7 198 Turner E. E. 1 299 Turner H. S. 7 407 Ubbelohde A. R. 4 356; 5,364; 11,246 Ulbricht T. L. V. 13,48 Uri N. 6 186 Vainshtein B. K. 14 105 Walsh A. D. 2 73 Walton G. N. 15,71 Warburton W. K. 8,67 Warhurst E. 5 44 Waters W. A, 12 277 Weedon B. C. L. 6 380 Wells A. F. 2 185; 8 380 Wells R. A. 7 307 Whiffen D. H. 4 131 ; White E. A. D. 15 1 Whytlaw-Gray R. 4 Wilkinson S. 15 153 Wilson H. N. 2 1 Wittenberg D. 13 116 Woodward L. A, 10 Woolf A. A. 15 372 12,250 153 185 Yoffe A. D. 9 362 Zakharkin L.I. 10,330 CUMULATIVE INDEX OF TITLES Absorption spectra molecular elm- Arrhenius factors (frequency factors) tronic 15 287 in unimolecular reactions 14,133 Acetylenes as natural products 10 Aspects physicochemical of some 371 recent work on photosynthesis 14 Acetylenes infrared and Raman 174 spectra of 6 1 Association of carboxylic acids 7,255 Acid use of the term 1 113 Asymmetry the non-conservation of Acids carboxylic anodic syntheses parity and optical activity 13 48 with 6 380 Atoms in the gaseous phase produc- Acids carboxylic association of 7 tion detection and estimation of 255 15 237 Acids straight-chain fatty natural and Attraction molecular direct measure- synthetic recent developments in the ment of between solids separated by preparation of 7,175 a narrow gap 10,295 Acids tetronic 14 292 Acid-base reactions simple rates of 13 169 Actinide oxides 15,442 Addition free-radical of Benzilic acid and related rearrange- olefinic systems 8 308 Adsorption enerm adsorption equili- Biological reactions r8k of phosphoric bria and surface chemistry 15 99 Adsorption of non-electrolytes from Biosynthesis alkaloid 15 259 solution 5,60 Affinities relative of ligand atoms for Bond chemical in crystals applica- acceptor moleales and ions 12 265 tion Of electron diffraction to the Base use of the term 1 113 ments 14 221 esters in 59 371 Bond aromatic 5 147 Age geological determination of by Bonding chemical and nuclear quad- Aldehydes polymerisation of 6,141 Bonds interpretation Of properties Of Alkaloid biosynthesis 15 259 Alkaloids of calabash-curare and Borazo1es7 the 149 200 Strychnos species 14 77 Boron hydrides chemistry of 9 174 Alkaloids ergot 8 192 Boron hydrides and related com- Alkaloids indole excluding harmine pounds 2 32 and strychnine 10 108 Boron trifluoride co-ordination com- Alkaloids steroidal 7 23 1 pounds of 8 1 Alkaloids veratrum 12 34 Alkanes infrared and Raman spectra Carbides of iron 3 160 Alkanes tetra- and tri-chloro- and Carbohydrate epoxides 13 30 Carbohydrate phosphates 11 61 Analgesics synthetic 2 349 Carbohydrate sulphates 3 369 Analysis conformational principles Carbohydrates newer aspects of stereochemistry of 13 265 Analysis inorganic applications of Carbon amorphous and graphite 1 59 Analysis radioactivation 10 83 Carbon-carbon bonds oxidative Anionotropy 4,404 hydrolysis of in organic molecules Anodic syntheses with carboxylic 10 261 acids 6 380 Carbon-carbon double bonds geo- Antibiotics newer chemistry of 12,93 metrical isomerism about 6 101 study Of 14 105 rupole coupling 11 162 2 260 radioactivity 7 1 Aliphatic compounds saturated inter- Bonds d~ssociation energies of 5 22 action of free radials with 14 336 of 7 19 related compounds 10,330 of 10 44 solvent extraction to 5 200 465 466 QUARTERLY REVIEWS Carbon-hydrogen bond polarity of 2 383 Carbon-hydrogen bonds mechanism of breakage of 12 230 Carbon-oxygen surface compounds of 13 287 Carbon-phosphorus bonds com- pounds containing 12 341 Carbonitrides of iron 3 160 Carbonium ions structure of 12 173 Carbons active study of porous structure of by a variety of methods 9 101 Carbons adsorbent properties and nature of 10 1 Carbonyls of metals chemistry of 1 33 1 Catalysis by metals specificity in 8 404 Catalysis of reactions involving hydro- gen mechanisms of 3,209 Catalysis and semiconductivity 11,227 Catalysts redox initiation of poly- Cations organic reactions of 6 302 Charcoals active study of porous structure of by a variety of methods 9 101 Charge-transfer spectra theory of 15 191 Chemisorption of gases on metals 14 257 Chromatography inorganic 7 307 Chromium mechanisms of oxidation Collisions in gases energy transfer in Colloidal electrolytes state of solution Colour and constitution 1 16 Colour centres in alkali halide crystals 14 62 Combustions slow in the gas phase elementary reactions in 11 313 Complex compounds stabilities of 5 l Compounds containing sulphur- fluorine bonds chemistry of 15 30 Conductance ionic in solid salts 6 23 8 Configuration of flexible organic molecules 5 364 Conformational analysis principles of 10 44 Conjugated compounds free-electron approximation for 6 319 merisations by 9 287 by compounds of 12 277 11 87 of 2 154 Co-ordination compounds of boron trifluoride 8 1 Crystal structure and melting 4 356 Crystal structures of salt hydrates and complex halides 8 380 Crystalline transition-metal com- pounds electron resistance in 14,427 Crystals alkali halide colour centres in 14 62 Crystals chemical bonds in applica- tion of electron diffraction to the study of 14 105 Crystals location of hydrogen atoms in 10 480 Crystals ionic lattice energy of 10 283 Cyanine dyes 4 327 Cyclohexane stereochemistry of 7 22 1 Deamination nitrosation and di- azotisation 15 41 8 Decarboxylation thermal mechanism of 5 131 Degradation biological of trypto- phan 5 227 Densities limiting 4 153 Diazotisation nitrosation and de- amination 15 418 Dielectric absorption 8 250 Dihalogen compounds Grignard and organolithium compounds derived from 11 109 Disproportionation in inorganic com- pounds 2 1 Diterpenoids chemistry of 3 36 Dyes effect of light on 4 236 Dyes cyanine 4 327 Dyes organic and their constitution 1 16 Earth distribution of the elements in the 3 263 Electrode processes in aqueous solu- tions mechanism of 3 95 Electrolytes effects of ultrasonic waves on 7 84 Electrolytes colloidal state of solu- tion of 2 154 Electromagnetic separation of stable isotopes 9 1 Electron correlation and chemical consequences 1 1 29 1 Electron resistance in crystalline tra.ns- ition-metal compounds 14 427 CUMULATIVE INDEX 467 Electron transfer and related processes in solution mechanism of 15 207 Electronic absorption spectra mole- cular 15 287 Electrons structures of molecules deficient in 11 12 1 Elements terrestrial distribution of 3 263 Elements heavy radioactivity of 5 270 Elements of Group VIII recent stereochemistry of 3 321 Elements of Groups IVB and IVY comments on the thermochemistry of 7 103 Elements of the rare-earth series separation of 1 126 Elements transuranic chemistry of 4 20 Energy adsorption and adsorption equilibria in surface chemistry 15 99 Energy transfer of in gaseous collisions 11 87 Enzymes degradation of polysacchar- ides by 9 73 Enzymes synthesis of polysaccharides Epoxides of sugars 13 30 1 ,2-EpoxidesY naturally-occurring the chemistry of 14 3 17 Equilibria adsorption and adsorption energy in surface chemistry 15 99 Equilibria hydrolytic quantitative studies of 13 146 Equivalent-orbital approach to mole- cular structure 11 273 Ergot alkaloids structure of 8 192 Esters carboxylic and related com- pounds alkyl-oxygen heterolysis in 9,203 Exchange reactions of hydrogen iso- topes in solution principles of 9 51 Extraction liquid-liquid in inorganic chemistry 13 327 Ferrocene and related compounds 9 391 Fission nuclear 15 71 Flames emission spectra of 4 1 Flash photolysis and kinetic spectro- Flavones nuclear methylation of 10 Fluorine-sulphur bonds compounds by,.7 58 scopy 10 149 169 containing 15 30 Fluorescence and fluorescence quench- ing 1 1 Fluorine laboratory and technical production of 3 22.Fluorine compounds general aspects of the inorganic chemistry of 11,49 Fluorine compounds laboratory and technical production of 3 22 Fluorine compounds organic reac- tions of 8 331 Foaming current concepts in theory of 13 71 Force constants 1 73 Forces intermolecular and the pro- perties of matter 8 168 Free-electron approximation for con- jugated compounds 6 319 Friedel-Crafts reaction modern aspects of 8 355 Furans some aspects of the chemistry of. 4. 195 Gases adsorbed infrared spectra of 14 378 Gases chemisorption of on metals 14 257 Gases elementary reactions in slow combustions in 11 313 Gases energy transfer in collisions in 11 87 Gemstones synthetic 15 1 Gibberellins 15 56 Graphite and amorphous carbon 1 Graphite lamellar compounds of Grignard reagents derived from di- 59 14 1 halogen compounds ll 109 Halide alkali crystals colour centres in 14 62 Halides of the phosphorus group elements (P As Sb Bi) 15 173 Halides reactions of in solution 5 245 Halides complex crystal structures of 8 380 Halogens kinetics of thermal addition of to olefins 3 126 Heats of formation of simple in- organic compounds 7 134 Heteroaromatic compounds infrared spectra of 13 353 Heterogeneous reactions transport control in 6 157 468 QUARTERLY REVIEWS Heterolysis alkyl-oxygen in carb- oxylic esters and related compounds 9 203 Hydrocarbons infrared and Raman spectra of.Part I acetylenes and olefins 6 1. Part 11 paraffins 7 19 Hydrogen molecular homogeneous reactions of in solution 10 463 Hydrogen atoms location of in crystals 10,480 Hydrogen catalysis mechanisms of 3 209 Hydrogen isotope exchange reactions in solution principles of 9 51 Hydrogen peroxide its radicals and its ions energetics of reactions involving 6 186 Hydrogenation catalytic and related reactions mechanism of 8,279 Hyperconjugation 3 226 Ice structure of 5 344 Immunochemistry aspects of 1 179 21 3 Indole alkaloids excluding harmine and strychnine 10 108 Induction asymmetric and asym- metric transformation 1,299 Infrared spectra of adsorbed gases 14 378 Inorganic chemistry and magnetism 7 377 Inorganic compounds disproportiona- tionin,2 l Inorganic compounds Raman spectra of 10 185 Inorganic compounds stereochemistry of 11 339 Inorganic compounds simple heats of formation of 7 134 Inositols 11 212 Insecticides synthetic structure and activity in 4 272 Interaction of free radicals with saturated aliphatic compounds 14 336 Interhalogen compouiids and poly- halides 4 11 5 Intermolecular forces and some pro- perties of matter 8 168 Iodine compounds inorganic some reactions of 8 123 Ion association in aqueous solution thermodynamics of 14 402 Ion exchange 2 307 Ionisation potentials and far ultra- violet spectra their significance in chemistry 2 73 Iron carbides nitrides and carbo- nitrides of 3 160 Isoflavones 8 67 Isomerism geometrical about carbon- carbon double bonds 6 101 Isotopes exchange of between different oxidation states in aqueous solution 8 219 Isotopes synthesis of organic com- pounds labelled with 7 407 Isotopes tracer techniques involving 4 172 Isotopes stable electromagnetic separation of 9 1 Lactones physiologically active un- Lamellar compounds of graphite Lanthanons separation of 1 126 Lattice energy of ionic crystals 10 283 Ligand atoms relative affinities of for acceptor molecules and ions 12 265 Ligand-field theory 11 381 Light abscrption of and photo- chemistry 4,236 Liquids transitions in 3 65 Liquids transport properties of in relation to their structure 14 236 Liquids ultrasonic analysis of relaxa- tion processes in 11 134 saturated 2 46 14 1 Magnetic resonance absorption nuclear 7 279 Magnetism and inorganic chemistry 7 377 Manganese mechanisms of oxidation by compounds of 12,277 Manganese dioxide oxidations by in neutral media 13 61 Mass spectrometry application of to chemical problems 9 23 Mass Spectrometry of free radicals 13 21 5 Mechanisms of electron transfer and related processes in solution 15 207 Melting and crystal structure 4 356 Meso-ionic compounds 11 15 CUMULATIVE INDEX 469 Metal-amine solutions reduction by; applications in synthesis and deter- mination of structure 12 17 Metal-ammonia solutions reduction of organic compounds by 4,69 Metal-transition compounds crystal- line electron resistance in 14 427 Metals chemisorption of gases on 14 257 Metals nature of solutions of 13 99 Metals specificity in catalysis by 8 Methyl radicals reactions of 7 198 Methylation biological 9 255 Methylation nuclear of flavones and related compounds 10 169 Molecular electronic absorption spectra 15 287 Molecular-orbital approach to mole- cular structure 11 273 Molecular-sieve action of solids 3 293 Molecules determination of structure of by X-ray crystal analysis modern methods and their accuracy 7 335 Molecules molecular-orbital and equivalent-orbital approach to structure of 11 273 Molecules electron deficient struc- tures of 11 121 Molecules electronically excited reac- tions of in solution 13 3 Molecules flexible organic configura- tion of 5,364 Molecules organic oxidative-hydro- lysis of carbon-carbon bonds in 10 261 Molecules simple representation of by molecular orbitals 1 1 4 4 Morphine synthetic approaches to structure of 5,405 Muscarine history and chemistry of 15 153 404 Nitrogen Compounds heterocyclic nitration of 4 382 Nitrogen dioxide-dinitrogen tetroxide system structure and reactivity of 9 362 Nitrosation diazotisation and de- amination 15 418 C-Nitroso-compounds structure and properties of 12 321 Nitrosyl group chemistry of 9 115 Non-electrolytes adsorption of from Non-electrolytes theories of solutions Nuclear chemistry quantitative 12 Nuclear fission 15 71 Nuclear magnetic resonance absorp- tion 7,279 Nuclear quadrupole coupling and chemical bonding 11 162 Nucleation in phase changes 5 315 Nucleotide coenzymes recent develop- ments in biochemistry of 12 152 solution 5,60 of 13 306 133 Oceans salt deposits from 1 91 OIefinic systems free-radical addition reactions of 8 308 Olefins infrared and Raman spectra of 6 1 Olefins kinetics of oxidation of 3 1 Olefins kinetics of thermal addition of halogens to 3 126 Olefins oxidation of 8 147 Optical activity and non-conservation of parity 13,48 Orbitals molecular approach to mole- cular structure through 11 273 Orbitals molecular and organic reactions 6 63 Orbitals molecular representation of simple molecules by 1 144 Organic compounds action of ionising radiations on 9 3 1 1 Organic compounds behaviour of in sulphuric acid 8 40 Organic compounds estimation of thermodynamic properties for 9 229 Organic compounds isotopically labelled synthesis of 7 407 Organic compounds polarography of 6 262 Organic compounds reduction of by metal-ammonia solutions.4.69 Nitramines some aspects of the Chemistry of 5 75 Nitration of aromatic compounds 2 277 NWation of heterocyclic nitrogen compounds 4 382 Nitrides of iron. 3. 160 Nitro-compounds 'aliphatic 1 358 Nitrogen active 12 116 .~ I , 47 0 QUARTERLY REVIEWS Organic oxygen compounds thermo- dynamic properties of 15 125 Organic reactions and molecular orbitals 6 34 Organolithium reagents derived from dihalogen compounds 11 109 Organometallic compounds of the first three periodic groups 4 21 7 Organosilylmetallic compounds for- mation and reactions of 13 116 5-Oxazolones chemistry of 9 150 Oxidation by compounds of chro- mium and manganese mechanisms of 12 277 Oxidation of olefins 3 1 ; 8 147 Oxidation-reduction potential of quinones relation of to chemical structure 4 94 Oxides actinide 15 442 Oxides of metals structures of 2 185 N-Oxides aromatic heterocyclic chemistry of 10 395 Oxygen compounds organic thermo- dynamic properties of 15 125 Oxygen and carbon surface com- pounds of 13,287 Parity non-conservation of 13 48 Penicillins chemistry of 2 203 Peptides methods of synthesis and terminal-residue studies of 10 230 Peptides structural investigation of 6,340 Peptides naturally occurring 3 245 Perfluoroalkyl derivatives of metals and non-metals 13 233 Peroxides organic and their reactions 4,251 Phase changes nucleation in 5 315 Phenols tautomerism of 10 27 Phosphates of carbohydrates 11 61 Phosphates condensed 3 345 Phosphoric esters r61e of in biological reactions 5 171 Phosphorus group elements (P As Sb Bi) halides of 15 173 Phosphorus oxyacids some aspects of the organic chemistry of derivatives of 3 146 Photochemical rearrangements and related transformations 15 393 Photochemistry and light absorption 4,236 Photography cyanine dyes in 4 327 Photo-oxidation primary processes in 14 146 Photopolymerisation 4 236 Photosynthesis physicochemical aspects of some recent work on 14 174 Pinacol rearrangement 14 357 Polarity of the carbon-hydrogen bond 2 383 Polarography of organic compounds 6,262 Polonium chemistry of 11 30 Polyhalides and interhalogen com- pounds 4 115 Polymerisation initiation of by redox catalysts 9 287 Polymerisation of aldehydes 6 141 Polymerisation addition some thermodynamic and kinetic aspects of 12 61 Polymerisation induced by light 4 236 Polymerisation ionic 8 88 Polymerisation radical rate constants Polymers based on silicon chemistry of 2 25 Polymers high thermodynamic pro- perties of and their molecular interpretation 1 265 Polysaccharides enzymic degradation of 9 73 Pol ysaccharides enzymic synthesis of 7 58 Portland cement constitution of 3 82 Processes primary in photo-oxidation 14 146 Production detection and estimation of atoms in the gaseous phase 15 23 7 Proteins structural investigation of 6 340 Pteridines 6 197 Purines some aspects of the chemistry Pyrans some aspects of the chemistry Pyrimidines some aspects of the Pyrrole pigments biogenetic origin of in.4 292 of 3 181 of 4 195 chemistry of 3 181 4 45 Quadrupole coupling nuclear Tad chemical bonding 11 162 Quadrupole moments molecular 13 183 Quenching of fluorescence 1 1 CUMULATIVE INDEX 47 1 Quinones relation between the oxida- tion-reduction potential and chem- ical structures of 4 94 Radiations ionking action of on organic compounds 9,3 1 1 Radicals free electron resonance spectroscopy of 12 250 Radicals free interaction of with saturated aliphatic compounds 14 336 Radicals free mass spectrometry of 13 215 Radioactivation analysis 10 83 Radioactivity determination of geo- logical age by 7 1 Radioactivity of the heavy elements 5 270 Reactions unimolecular Arrhenhis factors (frequency factors) in 14 133 Rearrangements aromatic 6 34 Rearrangements benziIic acid and related 14 221 Rearrangements photochemical and related transformations 15 393 Rearrangement pinacol 14 357 Redox potentials of quinone relation of to chemical structure 4 94 Reduction by metal-amine solutions; applications in synthesis and deter- mination of structure 12 17 Reduction by metal-ammonia solu- tions of organic compounds 4 69 Relaxation processes molecular in liquids ultrasonic analysis of 11 134 Rhenium chemistry 15 372 Salt hydrates crystal structures of 8 Salts basic structure of 1,247 Salts deposits of from oceans 1 91 Salts solid ionic conductance in 6 Sandmeyer reactions 6 358 Semiconductivity and catalysis 11 Sesquiterpenoids recent advances in Shock waves 14,46 Silicon chemistry of polymers con- Silyl compounds 10 208 Sodium “flame” reactions 5 44 380 23 8 227 chemistry of 11 189 taining 2 25 Solids molecular-sieve action of 3 293 Solids thermal transformations in 11 246 Solids transitions in 3 65 Solids separated by a narrow gap direct measurement of molecular attraction between 10 295 Solution aqueous thermodynamics of ion association in 14,402 Solutions aqueous mechanism of electrode processes in 3 95 Solutions of non-electrolytes theories of 13 306 Solvation ionic 3 173 Solvent extraction and its applications to inorganic analysis 5 200 Solvents ionising non-aqueous reac- tions in 10 451 Specificity in catalysis by metals 8 404 Spectra charge-transfer and related phenomena 8,422 Spectra charge-transfer theory of 15 191 Spectra emission of flames 4 1 Spectra far ultraviolet ionisation potentials and their significance in chemistry 2 73 Spectra infrared and Raman of hydrocarbons.Part I acetylenes and olefins 6 1. Part 11 paraffins 7 19 Spectra infrared of heteroaromatic compounds 13 353 Spectra Raman of inorganic com- pounds 10,185 Spectra rotation 4 131 Spectroscopy electron resonance of free radicals 12 250 Spectroscopy kinetic and flash photo- lysis 10 149 Stabilities of complex compounds 5 1 Stereochemistry of cyclohexane 7,221 Stereochemistry of elements of Sub- group VIB of the Periodic Table 10 407 Stereochemistry of elements of Group VIII of the Periodic Table 3 321 Stereochemistry of inorganic com- pounds 11,339 Steric hindrance 2 107; 11 1 Steroidal alkaloids 7,23 1 Structure of liquids in relation to their transport properties 14 236 472 QUARTERLY REVIEWS Substitutions aromatic nucleophilic mechanism and reactivity in 12 1 Sugar epoxides 13 30 Sulphur-fluorine bonds compounds containing 15 30 Sulphur nitride and its derivatives 10 437 Sulphuric acid behaviour of organic compounds in 8 40 Surface chemistry adsorption energy and adsorption equilibria 15 99 Surface compounds the chemistry of carbon-oxygen 13,287 Sydnones 11 15 Synthetic gemstones 15 1 Tautomerism of phenols 10 27 Tetronic acids 14 292 Theory of charge-transfer spectra 15 191 Thermodynamics of ion association in aqueous solution 14,402 Thermochemistry of the elements of Group IVB and IV comments on 7 103 Thermodynamic properties estima- tion of for organic compounds and chemical reactions 9 229 Thermodynamic properties of high polymers and their molecular inter- pretation 1 265 Thermodynamic properties of organic oxygen compounds 15 125 Tracers radioactive preparation of 2 93 Transformation asymmetric and asymmetric induction 1 299 Transformations thermal in solids 11 246 Transformations related and photo- chemical rearrangements 15 393 Transition-metal compounds crystal- line electron resistance in 14 427 Transitions in solids and liquids 3 65 Transport control in heterogeneous reactions 6 157 Transport properties of liquids in relation to their structure 14 236 Triplet state 12 205 Triterpenes tetracyclic 9 328 Tropolones 5 99 Tryptophan biological degradation of 5 227 Ultrasonic analysis of molecular relaxation processes in liquids 11 134 Ultrasonic waves effects of on electrolytes and electrolytic pro- cesses 7 84 Veratrum alkaloids 12 34 Wool wax constitution of 5 390 X-Ray crystal analysis modern methods of determination of mole- cular structure by and their ac- curacy 7,335 p-Xylylene chemistry of and of its - - analogues and polymers 12 301

ISSN:0009-2681    

DOI:10.1039/QR9611500461

出版商:RSC    

年代:1961

数据来源: RSC