摘要:

QUARTERLY REVIEWS THE CHEMICAL SOCIETY PATRON HER MAJESTY THE QUEEN President C. K. INOOLD D.Sc. F.R.I.C. F.R.S. Vice-Presidents who have filled the office of President F . G. DONNAN C.B.E. D.Sc. LL.D. W. H. M m s M.A. Sc.D. F.R.S. F.R.S. SIR E R I ~ RIDEAL M.B.E. M.A. LL.D. F.R.S. Sc.D. F.R.S. LL.D. F.R.S. SIR IAN HEILBRON D.S.0.p DSSc. D.Sc, F.R.S. Sm CYRIL HINSHELWOOD &I.& SIR ROBERT ROBINSON O.M. D.Sc.7 Vice-Presidents G. R. CLEIIO D.Phil. D.Sc. F.R.S. R.D. WAWORTH,D.SC.,P~.D.,F.R.S. F.R.S. D. H. HEY Ph.D. D.Sc. F.R.I.C. E. L. HIRST D.Sc. LL.D. F.R.S. SIR. JOHN SINONSEN D.Sc. F.R.I.C. M. STICEY Ph.D. D.Sc. F.R.S. Treasurer SIR WALLACE AKERS C.B.E. D.C.L. D.Sc. F.R.S. Secretaries L. E. SUTTON M.A. D.Phil. F.R.S. Ordinary Members of Council G. BADDELEY WSc. Ph.D.D.Sc. WILSON BAKER M.A. D.Sc. F.R.S. E. A. MOELWYN-HUGHES D.Yhil. D. H. R. BARTON Ph.D. D.Sc. D.Sc. Sc.D. H. M. POWELL M.A. B.Sc. F.R.S. J. BELL D.Sc. Ph.D. F.R.I.C. A. ROBERTSON M.A. Ph.D. F.R.S. R. P. BELL M.A. B.Sc. F.R.S. M. A. T. ROGERS B.Sc. Ph.D. F. BERGEL D.Phil.Nat. D.Sc. A.R.I.C. F . R . I C . C. W. SHOPPEE D.Sc. D.Phil. NEIL CAMPBELL D.Sc. Ph.D. F.R.I.C. J. CHATT M.A. Ph.D. F.R.I.C. J. W. SMITH Ph.D. D.Sc. F.R.I.C. M. J. S. DEWAR M.A. D.Phi1. T. S. STEVENS B.Sc. D.Phil. I. J. FAULKNER B.Sc. Ph.D. A.R.I.C. A. R. J. P. UBBELOHDE M.A. D.Sc. S. J. GREGG Ph.D. A.R.C.S. F.R.S. A. I. VOGEL D.Sc. D.I.C. F.R.I.C. BRYNMOR JONES Sc.D. Ph.D. H. BURTON Ph.D. D.Sc. F.R.I.C. E.D.Hu~HEs,D.SC.,F.R.I.C.,F.R.S. F. B. KIPPINQ M.A. Ph.D. F.R.I.C. F.R.I.C. F.R.I.C. F.R.I.C. Ex-Ofilcio Members of Council A.FINDLAY C.B.E. D.Sc. LL.D. F.R.I.C. (Chairman of the Chemical Council) W. WARDLAW C.B.E. D.Sc. F.R.I.C. (Chuirman of the Joint Library Com- Gcenerd Secretary Librarian Telephone Numbers Regent 0676/6 mittee) 6. R. RUCK KEENE M.B.E. T.D. B.A. A. E. Cm~ms. Printed in Great Britain by Butler & Tanner Ltd. Frome and London QUARTERLY REVIEWS Committee of Publication Chairman SIR CYRIL HINSHELWOOD M.A. Sc.D. F.R.S. SIR WALLACE AKERS C.B.E. D.C.L. J. S. ANDERSON 1\1.Sc. Ph.D. F.R.S. D. H. R. BARTON Ph.D. D.Sc. C. E. H. BAWN B.Sc. Ph.D. F.R.S. D. J. BELL Sc.D. Ph.D. R. P. BELL M.A. B.Sc. F.R.S. E. J. BOWEN M.A. D.Sc. F.R.S. H. BURTON Ph.D. D.Sc. F.R.I.C. A. H. COOK D.Sc. F.R.I.C. F.R.S. C. A. COULSON M.A. D.Sc. F.R.S. F. S. DAINTON M.A. Ph.D. C. W. DAVIES D.Sc.F.R.I.C. H. J. EMEL~US D.Sc. A.R.C.S. F. FAIRBROTHER D.Sc. R.D. HAWORTH D.Sc. Ph.D. F.R.S. E. D. HUGHES D.Sc. F.R.I.C. H. R. ING M.A. D.Phil. F.R.S. D.Sc. F.R.S. F.R.I.C. F.R.S. F.R.S. C. K.INGoLD,D.SC. F.R.I.C. F.R.S. H. 31. N. H. IRVING MA. D.PhI. F. G. MA” Sc.D. F.R.I.C. F.R.X. R. A. MORTON D.Sc. Ph.D. F.R.X. L. N. OWEN Ph.D. D.Sc. F.R.I.C. J. M. ROBERTSON M.A. D.Sc. H. N. RYDON D.Sc.,D.Phil.,F.R.I.C. N. SHEPPARD M.A. Ph.D. C. W. SHOPPEE D.Sc. D.Phil. W. F. SHORT D.Sc. 11.S~. R. SPENCE C.B. Ph.D. D.Sa. M. STAGEY Ph.D. DSc. F.K.S. L. E. SUTTON M.A. D.Phil. F.R.S. J. WALKER Ph.D. D.Plul. D.Sc. W. A. WATERS MA. Sc.D. F.R.I.C. T. S. WHEELER Ph.D. D.Sc. F.R.I.C. F.R.S. F.R.I.C. F.R.I.C. F.R.I.C. Editor R. S. CAHN M.A. D.Phil.Nat. F.R.I.C. Assistant Editors A. E. SOMERFIELD B.A.B.Sc. A. D. MITCHELL D.Sc. F.R.I.C. L. C. CROSS Ph.D. A.R.C.S. F.R.I.C. LONDON T H E C H E M I C A L S O C I E T Y CONTENTS PAGE THE DETERMINATION OF GEOLOGICAL AGE BY MEANS OF RADIO- THE ~XFRA-RED AND RAMAN SPECTRA or HYDROCARBONS. PART 11. PARAFFINS. By NORMAN SHEPPARD and DELIA M. SIMPSON . . 19 ENZYMIC SYNTHESIS OF POLYSACCHARIDES. By S. A. BARKER and E. J. BOURNE . 56 THE EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES AND ELECTRODE PROCESSES. By S. BARNARTT . . 84 COMMENTS ON THE THERMOCHEMISTRY OF THE ELEMENTS OF GROUPS IVB AND IV. By E. C. BAUGHAN . . 103 THE HEATS OF FORMATION OF SIMPLE INORGANIC COMPOUNDS. RECENT DEVELOPMENTS IN THE PREPARATION OF NATURAL AND By F. D. GUN- THE REACTIONS OF METHYL RADICALS. By A. F. TROTMAN- ACTIVITY. By S. C. CURRAN . . 1 By L. H. LONG .. 134 SYNTHETIC STRAIGHT-CHAIN FATTY ACIDS. STONE . . 175 DICKENSOX . . 198 STEREOCHEMISTRY OF Cyd0HEXA4NE. By 0. HASSEL . . 221 STEROIDAL ALKALOIDS. By JAMES MCKENNA . . 231 THE ASSOCIATION OF CARBOXYLIC ACIDS. By G. ALLEN and E. F. CALDIN . . 256 NUCLEAR MAGNETIC RESONANCE ABSORPTION. By J. A. S. SMITH . . 279 INORG-4NIC CHROMATOGRAPHY. By R. A. WELLS . . 307 MOLECULAR STRUCTURE DETERMINATION BY X-RAY CRYSTAL ANALYSIS MODERN METHODS AND THEIR ACCURACY. By G. A. JEFFREY and D. W. J. CRUICKSHANK. (With a MAGNETISM AND INORGANIC CHEMISTRY. By R. S. NYHOLM . 377 THE SYNTHESIS OF ISOTOPICALLY LABELLED ORGANIC COMPOUNDS. By S. L. THOMAS and H. S. TURNER . . 407 Preface by E. G. COX) . . 335

ISSN:0009-2681    

DOI:10.1039/QR95307FP001

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

THE INFRA-RED AND RAMAN SPECTRA OF HYDROCARBONS. PART II. PARAFFINS By NORMAN SHEPPARD M.A. PH.D. and DELIA M. SIMPSON M.A. PH.D. (Mrs. J. N. AGAR) (DEPARTMENTS OF COLLOID SCIENCE AND PHYSICAL CHEMISTRY FREE SCHOOL LANE CAMBRIDGE) IN Part I of this Review,l dealing with the acetylenes and the olefins attention was focused on those features of the spectra which originate from relatively small and rigid parts of on the whole much larger mole- cules. This made the discussion of the spectra a comparatively simple matter and allowed the identification of most of the expected frequencies. To some extent a similar approach is helpful in the interpretation of the spectra of the paraffins in that many compact groups such as CH, CH, CHMe, and CMe, give rise to characteristic frequencies which can be recognised and assigned in the spectra of series of related molecules.How- ever it is not usually possible to obtain in this way complete interpretations of the spectra of the paraffins because the vibrations of the different parts of a molecule are so closely coupled as to defy analysis by such a method. This is true for instance in the case of the polymethylene chain of the n-paraffins. Further most of the data available in the infra-red and Kaman spectra of the paraffins refer to the liquid or the vapour phase. As will be shown in a later section under these conditions the substances are usually examined as mixtures of different rotational isomers. This produces other complications since each isomer of a given paraffin has its own set of characteristic frequencies. It should also be noted that the spectra of even the smallest paraffins where rotational isomerism is absent (such as propane) are relatively complex so complete assignments of their frequencies are not available to assist in the interpretation of the spectra of the more complex molecules.The study and correlation of the spectra of series of related paraffins is clearly of paramount importance and must form the first stage in attempt- ing the assignment of their frequencies. Most of the correlations in the infra-red spectra of paraffins discussed in this Review were noted in the course of a programme of research carried out during the last war on the infra-red spectra of hydrocarbons in aviation fuel a t the Universities of Oxford and Cambridge under the direction of Dr. H. W. Thompson and Prof.G. B. B. M. Sutherland. The results have been published in report form and have appeared in a more general review of group frequencies.3 They were also used in a more detailed discussion of the vibrations of some Sheppard and Simpson Quart. Reviews 1952 6 1. a Fellgett Harris Simpson Sutherland Thompson Whiffen a,nd Willis Institute of Petroleum Report XI 1946. Thompson J . 1948 328. 19 20 QUARTERLY REVIEWS branched paraffins by Simpson and S~therland.~ D. C. Smith 5 carried out' an extensive analysis of the data obtained in Washington and an article by Barnes and his co-workers lists various paraffin correlations in a general tabulation of group frequencies. Since the present Review was first written a paper by McMurry and Thornton 7 has appeared which includes a dis- cussion of the infra-red paraffin correlations between 1500 and 700 cm.-l.Their analysis is presented mainly in tabular form and is particularly valu- able in that intensities are also listed. The approach is empirical with little discussion of the assignment of the various frequencies and none of the analogous Raman data. Many of the correlations in the Raman spectra of paraffins have already appeared in articles by Stepanov 8 and by Sheppard.1° It should also be ernphasised that the earlier work of Kohlrausch l1 and Mecke l2 is of considerable importance. The experimental data on the infra-red spectra used in this Review have as far as possible been taken from curves contributed by the Naval Research Laboratory to the Catalogue of Spectra published by the American Petroleum Institute; l3 in most cases no observations are available below 450 cm.-I.The Raman data employed are those published in two papers by Fenske and his collaborators.14~ l5 Together these cover more than a hundred substances including observations available on all paraffins up to the dodecanes that have hitherto been investigated. The Raman data on the paraffins up to 1939 have been reviewed by Hibben,lG and most of the references up to 1943 as well as tables of fre- quencies will be found in the most recent book by Kohlrausch; l7 further references may be obtained from an article by Glockler.ls The following papers on the Raman spectra of series of paraffins may be noted n- paraffins,19 octanes,20 hexanes and heptanes ; 21 the work of Bazhulin and his collaborators is also extensive.22 The pioneer studies by Lambert and (a) J .Chem. Phys. 1947 15 153 ; ( b ) Proc. Roy. SOC. 1949 A 199 169. 5 D. C. Smith Report on the infra-red spectra of hydrocarbons C 3274. 6 Barnes Gore Stafford and Williams Analyt. Chem. 1948 20 402. 7 Ibid. 1952 24 318. J . Phys. Chenz. U.R.S.S. 1946 20 017. 9 Acta Physicochim. U.R.S.S. 1947 22 238. 10 J . Chem. Phys. 1948 16 690. 11 Kohlrausch " Ramanspektren " Hand- und Jahrbuch der Chemischen Physik 1s American Petroleum Institute Research Project 44. Catalogue of infra-red Carnegie Institute of Technology Pittsburgh Pa. U.S.A. 1 4 Fenske Braun Wiegand Quiggle McCormick and Rank Analyt. Chem. 1947 16 " The Raman Effect and its Chemical Applications " Reinhold New York 1939. 17 Op. cit. 1943 p. 209. 19 Herz Kahovec and Wagner Monatsh. 1946 76 100.20 von Grosse Rosenbaum and Jacobson Analyt. Chem. 1940 12 191. 21 Cleveland and Porcslli J . Chem. Phys. 1950 18 1469. 22 ( a ) Bazhulin Plate Solovova and Kazanskii Bull. Acad. Sci. U.R.S.S. (SBr. Chim.) 1941 13 ; (b) Bazhulin Bokshtein Liberman Lukina Margolis Solovova and Kazanskii ibid. 1943 198 ; (c) Bazhulin Ukholin Bulanova Kopernina Plate and Kazanskii ibid. 1949 481. Naval Research Laboratory Washington D.C. U.S.A. 1948. Reeker und Erler Leipzig 1943. a,bsorption spectral data. 19 700. la 2. phys. Chem. 1937 B 36 347. l5 Braun Spooner and Fenske ibid. 1950 22 1074. l8 Rev. Mod. Phys. 1943 15 111. SHEPPAR-D AND SIMPSON SPECTRA OF HYDROCARBONS. PART I1 21 Lecomte on the infra-red spectra of paraffins should be mentioned,23 24 as well as the wartime reports published under the auspices of the British Institute of Petroleum 2 ,5 26 and the investigation of the octanes in the liquid and the vapour state by Oetjen and Randall.27 Other spectra are to be found in the collection of data published by the American Petroleum Institute,13 and further references will be given in their appropriate contexts.1. Introduction In considering the spectra of the paraffins it is convenient to divide the vibrations into two main types those involving mainly the hydrogen atoms and those concerned with the nuclear framework although in a few cases such a distinction is too drastic. it is clear that the skeletal frequencies will differ considerably from one molecule to another whereas some at least of the hydrogen vibrations may be ex- pected to remain nearly invariant.It is necessary in the first place to describe briefly the various types of hydrogen vibrations associated with the CH, CH, and CH groups. These fall into two categories (i) internal vibrations are those that would appear even if the group in question were isolated (for example the carbon-hydrogen stretching modes) ; (ii) ex- ternal vibrations (such as many of the carbon-hydrogen deformation modes) are only present if there is some mechanical interaction between the group itself and the rest of the molecule. As might be expected the former give rise to very constant frequencies whereas those corresponding to the external vibrations are much more dependent on the structure of the molecule as a whole. These externa'l vibrations may couple extensively with other carbon-hydrogen deformation modes and with the skeletal vibrations.(a) Carbon-Hy drogen Stretching Vibrations .-T he carbon - h y d r og en stretching vibrations usually appear in the range 3000-2700 cm.-l and a,re generally intense in both Raman and infra-red spectra. It is possible as shown by Fox and 29 to use the infra-red frequencies to dis- tinguish between the different modes of the CH, CH, and CH groups ; all are internal vibrations. CH Symmetrical stretching vibration 2872 cm.-l 2962 cm.-] From the earlier discussion These assignments are listed below Asymmetrical doubly degenerate stretching vibration CH Symmetrical stretching vibration Asymmetrical stretching vibration 2853 cm.-l 2926 cm.-l CH Stretching vibration (semi-schematic) 2890 cm.-l 2 3 (a) Andant Lambert and Lecomte Compt.rend. 1934 198 1316 ; ( b ) Lambert 2 4 Lambert and Lecomte Ann. Physique 1938 10 503. z 5 Sutherland and Thompson Petroleum Board Enemy Oils and Fuels Committee 26 Idem Institute of Petroleum Report X 1945. 37 Rev. Mod. Phye. 1944 16 165 and (with Anderson) 260. 28 PTOC. Roy. SOC. 1938 A 167 257. *9 Ibid. 1940 A 175 208. and Lecomte ibid. 1938 206 1174. Report 1943. 22 QUARTERLY REVIEWS The above are mean values and are to some extent dependent on the number and environment of each type of group present (see also refs. 30-32). Other weaker frequencies are found near 2934 cm.-l (infia-red) 2912 cm.-l (Raman and infra-red) and in the range 2800-2700 cm.-l (Raman). These are interpreted as overtone or combination frequencies of the internal deformation modes of the CH and CH groups and appear with enhanced intensity through interaction with the neighbouring carbon-hydrogen stretching fundamentals of the same symmetry classes.( b ) Carbon-Hydrogen Deformation Vibrations.-The deformation modes involving changes in the angles associated with carbon-hydrogen linkages occur in the range 1500-600 cm.-l. (1) The CH Group.-An isolated CH group attached to a heavy frame- work has three internal deformation vibrations (one symmetrical and two approximately doubly degenerate asymmetrical modes) which are observable as nearly constant frequencies. The two doubly degenerate vibrations appear in the range 1470-1440 cm.-l and are intense in both Raman and infra-red spectra. The symmetrical deformation mode occurs as an intense infia-red band near 1380 crn.-l but the corresponding Raman line is usually missing.As has already been noted,l the frequency of this mode shifts to somewhat higher values if the CH group is directly attached to a double or triple bond and its intensity in the Raman spectrum is then enhanced. In the paraffins splitting of some of the infra-red bands occurs when a plurality of CH groups is attached to a single carbon atom. The regu- larities which have been noted are summarised below and are quoted as average values (in cm.-l) 5 9 7 / \ CHa-C- CH H isolated 1380 isopropyl 3 3-dimethyl tert.-butyl 1385 1370 1384 1367 1397 1370 The splitting patterns in more complex paraffins with several of these groups have not yet been fully worked out. Each CH group has also 30 Sushinskii; Bull. Acad. Sci. U.R.S.S. (SBr.Phys.) 1947 11 341. 31 Saier and Coggeshall Analyt. Chem. 1948 20 812. 3a Hastings Watson Williams and Anderson ibicl. 1052 24 612. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART II 23 three external vibrations viz. two rocking (or wagging) modes and a tor- sional motion of the group about the CH,-C linkage. The frequencies of the CH rocking modes vary considerably in different classes of paraffins and may be coupled strongly with the skeletal vibrations. They appear to be confined to the range 1250-800 ~m.-1,~3 and may give rise to both prominent Raman lines and intense infra-red bands. The CH torsional mode corresponds in most cases to some sort of restricted rotation and so may be expected in the low-frequency region < 300 cm.-I. This mode seems to be weak in both Raman and infra-red spectra and usually cannot be located directly (2) The CH Qroup.-An isolated CH group attached to a heavy frame- work has four deformation modes as shown in Fig.1.l0$ 34 The bending vibration is internal in type and occurs in the narrow range 1470-1440 cm.-l (Le. it coincides with the asymmetrical CH deformation vibration) ; it is strong in both Raman and infra-red spectra. Bending Wagging Twisting Rocking FIG. 1 Approximate diagrams of the deformation modes of an bolated CH group. The three remaining CH modes are all external and so are more variable in frequency. Investigations of the spectra of the long-chain paraffins 36 have suggested that the CH wagging and twisting frequencies are to be found in the range 1350-1150 cm.-l ; this range is also indicated by cal- culations for a variety of paraffin^.^ 34* 36-41 Some of these modes give rise to prominent Raman lines near 1300 cm.-l; in general the infra-red absorption bands are only of medium or weak intensity.There has been some discussion about the exact interpretation of the various series,l0* 83-35 but no general agreement has been achieved. Further investigations of the spectra of propane and its deuterated derivatives by McMurry and Thornton 4 2 9 43 (see later) have shown that for these molecules the CH wagging vibration is coupled with other modes of the same symmetry class. It might therefore be thought unrealistic to attempt to assign frequencies to the various external modes of CH groups. However in the opinion 3 3 Torkingt,on J. Chem. Phys. 1950 18 768. 34 Rasmussen ibid. 1948 16 712.35 Brown Sheppard and Simpson Discz~ss. Faraday SOC. 1950 9 261. 3* Eliashevitch and Stepanov Conapt. rend. Acad. Sci. U.R.S.X. 1941 8S 481. 37 Stepanov Bull. Acad. Sci. U.R.X.S. (SBr. Phys.) 1947 11 357. 38 Kellner Nature 1949 163 877. 3* Idem Proc. Phys. Soc. 1951 A 64 521. 40 Simanouti b. Chem. Pkys. 1949 17 734. 4a Ibid. 1960 18 1515. 4 3 Ibid. 1951 19 1014. 4l Barrow ibid. 1951 19 346. 24 QUARTERLY REVIEWS of the Reviewers such a procedure is still useful as a first approximation since it is not impossible that the effects of interaction are less drastic in larger molecules. Such a classification has in fact been used by McMurry and Thornton in their recent analysis.; The CH rocking modes occur in the range 1100-700 cm.-l as sug- gested by theoretical treatments 37 and investigations of the long-chain paraffins.35 These vibrations cannot be identified in the Raman spectra but give rise to intense infra-red absorption bands particularly in the region 770-720 ~ m .- l . ~ ~ 45 (3) The CH Group.-The two hydrogen deformation modes of a lone CH group appear in symmetrical surroundings as a doubly degenerate pair as in isobutane 46 near 1335 cm.-l. In other circumstances splitting may occur but one component can usually be recognised as a Raman line be- tween 1360 and 1330 cm.-l; lo owing to the proximity of the intense absorption due to the CH deformation vibration the corresponding infra- red frequencies cannot usually be identified (c) Skeletal Vibrations.-The range of skeletal stretching vibrations in branched paraffins is probably 1300-650 ~ m .- l ~ ? 47 though it is more restricted for the n-paraffins. It should however be noted that some authors 3 4 7 48 still favour an upper limit near 1100 cm.-l as suggested originally by Kohlrausch and Koppl. 4g The skeletal deformation modes and the torsional vibrations of the nuclear framework occur together below 600 cm.-l. From the above summary it may be concluded that the carbon-hydrogen stretching vibrations and the internal deformation modes of the CH and CH groups are relatively insensitive to changes in structure. In the follow- ing discussion attention will be concentrated on the remaining types of vibration which occur a t frequencies below 1350 cm.-l the torsional modes about which too little is known being omitted. 2. Rotational Isomerism in the Paraflfins As already noted the interpretation of the spectra of many paraffins is complicated by the fact that such substances exist as mixtures of two or more rotational isomers in the liquid or vapour state.Thus to consider a very simple example n-butane (which may be regarded as symmetrical dimethylethane) can occur in two distinct forms shown in Fig. 2 The trans-form corresponds to the planar zig-zag configuration ; the gauche forms (spectroscopically indistinguishable) are derived from this by rotation through approximately 120" of one half of the molecule with respect to the other about the central carbon-carbon linkage. Although restricted rotation of the CH group can also occur about the terminal C-C linkages 4 4 Sheppard and Sutherland Nature 1947 159 739. 4 5 Vallance Jones and Sutherland ibid.160 567. .t6 Pitzer and Kilpatrick Chem. Reviews 1946 39 435. 47 Ahonen J . Chem. Phys. 1946 14 625. 48 Rank Saksena and Shull Discuss. Fcrrnday Soc.. 1950 9 187. 49Z. phys. Chem. 1934 B 26 209. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART II 26 the high symmetry of this group causes the resulting configurations to be indistinguishable. The existence of such mixtures of rotational isomers in paraffins was first demonstrated spectroscopically by Kohlrausch and K o ~ p 1 ~ ~ who investigated Raman spectra in the liquid state and showed that more lines than predicted were present Since then considerable evidence both spectroscopic and thermodynamic has accumulated to confirm these observations. Of major importance is the fact that the spectroscopic observations indicate that each rotational isomer of a given molecule has its own set of vibrational frequencies.Me H H Me Me trans ‘Gauche’ FIO. 2 The conjguralions of the rotational isomers of n-butane. The number of possible rotational isomers of a particular paraffin may be readily deduced. Systematic discussions have been given by Pifzer,s0 and by Simanouti and Mizushima 51 who have invented a convenient notation for the description of the different forms. Since all the rotational isomers of a given molecule may theoretically be present in the liquid substance it is obviously of considerable import- ance to know their relative stability since this will determine the propor- tions of the different forms in the mixture. A direct method of tackling this problem is based on spectroscopic measurements.The spectra (Raman or infra-red) of the crystalline solid and the liquid at different temperatures are compared. The solid material shows the frequencies of one form (often but not always the most stable isomer) ; the extra frequencies in the liquid arise from isomers of different energy content. In favourable circumstances the liquid may contain only one species other than the most stable isomer ; by comparison of the variation with temperature of the intensities of cor- responding frequencies of the two forms the energy difference involved may be determined directly. This method in the hands of Rank and his col- laborators has proved very successful for some of the 12-paraffins 52-55 (see also ref. 56). Alternatively it is possible to calculate energy differ- ences from thermodynamic data.50 At present data on the solid-state spectra are available for a rather 50 J.Chem. Phys. 1940 8 711. 5 1 Sci. Papers Inst. Phys. Chem. Res. Tokyo 1943 40 467. 5 2 Szasz Sheppard and Rank J. Chenz. Phys. 1948 16 704. 53 Rank Sheppard and Szasz ibitl. 1949 17 83. 5 1 Sheppard and Szasz ibid. p. 86. “ ( a ) Axford and Rank ibid. p. 430; (b) ibid. 1950 18 51. 56Mizushima and Okazaki J . Amer. Chem. SOC. 1949 71 3411. 26 QUARTERLY REVIEWS limited range of molecules. The isomer present in the crystalline n-paraffins has the planar zig-zag configuration? as is also known from the X-ray investi- gation of the solids ; 57 the stability of the other isomers which are formed by one or more rotations about the different carbon-carbon bonds away from the planar configuration decreases with the number of these rotations.50 Thus the gauche form of n-butane has a greater energy content than the trans-isomer.A few branched paraffins have also been studied but the interpretation of the data is still under discu~sion.55~~ 58-61 Since each rotational isomer of a particular molecule has its own distinct set of vibrational frequencies? it might be thought that in order to discuss the characteristic group frequencies of a series of related paraffins? it would be essential first of all to distinguish between the frequencies arising from each type of isomer and then to compare those of each type in turn. Such a procedure might be feasible in principle but would in practice be extremely laborious and time-consuming. Fortunately? it is found empirically that Characteristic group frequencies may be recognised in various classes of paraffin even when these substances are investigated in the liquid state.There are a number of reasons for this; it might so happen that one or more vibrations of the different isomers of a given type of molecule all appear near the same frequency as for instance for some of the CH wagging and CH rocking modes of the n-paraffins. Alternatively? if a symmetrical unit such as the tert.-butyl group forms the common feature of a series of paraffins the frequencies of the vibrations characteristic of this group may remain relatively unaffected by what is happening in the rest of the molecule so that they too are nearly the same for different isomers. Even in less favourable circumstances it is still possible to pick out frequencies charac- teristic of particular structures.Purther details of the examples already discussed and other cases of rotational isomerism will be found in the appropriate sections. 3. The n-Paraffi In the discussion of the frequencies characteristic of paraffins the spectra of the n-paraffins form an appropriate starting point for a number of reasons. In the first place most paraffins possess within their structures a carbon- carbon chain of varying length so it is necessary to know the characteristic frequencies of such an arrangement? in order that the frequencies of other groupings may be identified. Further the data on the spectra of the n-paraffins have provided most of the information on the CH and CH hydrogen vibrations discussed above and it is desirable at an early stage to show how this has been derived.Moreover as mentioned previously only in the case of the n-paraffins are extensive data available for a series 57 Muller Proc. Roy. Sroc. 1928. -4 120 437. 55 Mizushima Morino and Taketla. Sci. Papers Inst. Phys. Chon. Res. Tokyo 1941 59 Avery and Ellis J . Chem. Phys. 1942 10 10. 60 Szasz and Sheppard ibid. 1949 17 93. 6 1 Brown and Sheppard ibid. 1951 19 976. 38 437. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART 11 27 of rotational isomers of a particular type (in this case with the trans-planar zig-zag configuration) and the data on the liquid substances are also much more complete than those for any other class of paraffin. Comparison of the spectra of the trans-isomers with those of mixtures of the isomers as found in the liquids is very instructive (Fig.3). Finally much attention Infra-red spectra between 1500 and 700 cm.-l of n-octane in the liquid (A) and the crystalline solid (B) phase.87 ( Vertical lines indicate characteristic absorption frequencies listed below.) has been given to the theoretical treatment of the skeletal vibrations of chains of repeating units of which the trans-isomers of the n-paraffins form examples. Comparison of the theoretical expectations with the experi- mental results for these molecules gives some indication as to how far such treatments are likely to be helpful guides in the interpretation of spectra. (a) Methane and Ethane.-The smallest paraffin methane is in a class by itself since it has only one carbon atom and therefore no skeletal vibra- tions. The frequencies of the four hydrogen modes are well-known ; those listed by Herzberg 62 have been confirmed by more recent work.63 6 4 Extensive investigations and assignments have also been made for the various deuterated derivatives.62 The structure of ethane has been the subject of some discussion; free rotation about the carbon-carbon linkage is definitely absent and on the whole the staggered configuration with a centre of symmetry is ~referred.~5> The frequencies as given by Herzberg 6 5 have been slightly revised by the recent work of L.G. Smith.66 The CH stretching modes and the internal CH deformation vibrations give frequencies corresponding to those listed on pp. 21 22. The single skeletal mode appears as a polarised Raman line 6 2 " Infra-red and Ra,man Spectra of Polyatoinic Molecules " Van Nost,rand New Pork 1945 p.306. 63 Holden Taylor arid Johnston J. Chen~. Phys. 1949 17 1356. 64 Rank Shull and Axford ibid. 1950 18 116. 6 6 Op. cit. 1945 p. 342. 6 6 J . Chem. Phys. 1049 17 130. 28 QUARTERLY REVIEWS a t 993 cm.-l. The two doubly degenerate CH rocking vibrations appear a t 1190 cm.-l (infra-red active) and 822 cm.-l (Raman active depolarised). The frequency of the torsional mode has been located at 290 cm.-l; this is in good agreement with the thermodynamic data. The molecule C2D6 has also been investigated ; references and the generally accepted assign- ments of the frequencies may be found in Herzberg’s book.65 ( b ) Propane.-Although propane and several of its deuterated derivatives have been extensively studied,17 2 6 9 4 2 9 439 67-70 the assignment of its frequen- cies below 1350 cm.-l has proved unexpectedly trouble~ome.~~~ 3 6 9 429 439 67-72 The main reason for this difficulty is that although it is formally possible to draw up a scheme classifying the vibrations in this region as CH torsion CH rocking CH deformation and skeletal modes yet in this molecule (because of the small number of atoms) vibrations of the same symmetry type become more or less mixed so that such clear-cut distinctions become unrealistic.2 9 43 For the present purpose it is not necessary to discuss in detail the eight CH stretching modes and the six internal deformation vibrations of the CH groups. These occur in the expepted frequency regions as set out on pp. 21 22. With regard to the remaining vibrations the following seems to be the most plausible assignment on the data available (Y in cm.-l).It should be emphasised however that more experimental work is necessary before it can be considered established. In particular the polarisations of the Raman lines of propane itself are required as well as further investigations on the deuterated derivatives. B C-C stretching (asymmetrical) 1053 A C-C stretching (symmetrical) 870 A CH rocking (in-plane symmetrical) 1155 B CH rocking (in-plane asymmetrical) 923 B A CH rocking (out-of-plane asymmetrical) 940 ? CH rocking (out-of-plane symmetrical) A CH bending 1460 B CH wagging 1336 A CH twisting 1278 B CH rocking 748 A C-C-C deformation 372 The above assignment is essentially that of P i t ~ e r ~ ~ as modified by Rasrnu~sen,~~ and is based on the earlier infra-red and Raman studies of the molecule.l79 67 The two CH torsional modes have been put at 283 and 202 cm.-l by Pitzer,Y2 using indirect methods.More recent investi- gations have not modified this interpretation though some difficulties remain. The infra-red band corresponding to the A CH rocking mode b7 Wu and Barker J . Che?iz. Phys. 1941 9 487. 68 A.P.I. Infra-red Spectrograms Nos. 60 99 629 791 792 793 1141. 69 McMurry Thornton a,nd Condon J . Chem. Phys. 1949 17 918. 70 Friedman and Turkevitch ibid. p. 1012. 7 1 Wagner 2. phys. Chem. 1939-1940 B 45 69. 78 Pitzer J . Chem. Phys. 1944 12 310. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART 11 29 has a somewhat anomalous contour under low resolution the reality of the weak Raman line corresponding to the A CH rocking mode needs confirmation and the position of the B CH rocking mode has yet to be established .Infra-red data are available for a number of deuterated derivatives of propane. The results for the unsymmetrical molecules are not easy to interpret.693 70 From the investigations of the molecules CH,*CD,CH3,42 CD3*CH,*CD, and C3DB,43 it has been established that the external carbon- hydrogen deformation vibrations and the skeletal modes of the same sym- metry classes interact significantly so that in propane the A frequencies at 1155 and 870 em.- and the B frequencies a t 1336 1053 and 923 crn.-l are respectively coupled together. Although the Reviewers agree with the conclusion that it is not strictly correct to attribute a given frequency solely to one particular type of group vibration yet they feel that such a classification is valuable as a first approximation even for propane.It should also be noted that for the undeuterated molecule the separation of some of the interacting frequencies of the same symmetry class is quite large. Further it appears not impossible that owing to the large shifts in the carbon-hydrogen deformation frequencies that occur on deuteration the degree of coupling of vibrations of the same symmetry type may alter significantly and is possibly more important the more deuterated the molecule.43 (c) The trans-Isomers of the n-ParafEns.-As has already been noted. in the crystalline solids the n-paraffins CnHZn+2 assume a planar zigzag truns- configuration. This implies that molecules with an even number of carbon atoms have a centre of symmetry so that vibrations are allowed in either Raman or infra-red spectra ; on the other hand many of the vibra- tions in molecules with an odd number of carbon atoms may appear in both types of spectra.These differing selection rules have proved most valuable in assigning the various series of frequencies that are observed. Further guidance for this purpose is provided by the theoretical dis- cussions (of varying degrees of complexity) of the skeletal vibrations of a regular chain of repeating units such as that represented by the truns- isomers of the n-paraffins.12 33 35-41 47 49 50p 73-86 The results of these investigations indicate that the (n - 1) skeletal stretching modes of a n-para& CnH2n+2 are confined to a definite range of frequencies depending mainly on the magnitude of the carbon-carbon stretching force constant.73 Bartholom6 and Teller 2. phys. Chem. 1932 B 19 366. i4 Bauermeister and Weizel Physikal. Z. 1936 37 169 7 5 Barriol J. Phys. Radium 1939 10 215. 7 G Barriol and Chappelle ibid. 1947 8 8. i i Parodi ibid. 1941 2 58. 7 8 Stepanov J. Phys. Chem. U.R.S.S. 1940 14 474. Eliashevitsch and Stepenov ibid. 1943 17 146. Kellner Trans. Faraday SOC. 1945 41 217. Whitcomb Nielsen and Thomas zbid. 1940 8 143. 81 Kassel J. Chem. Phys. 1935 3 326. 8 4 Mizushima and Simanouti J. Amer. Chern. SOC. 1949 71 1320. 85 Simanouti and Mizushima J. Chem. Phys. 1949 17 1102. 86 Gates ibid. p. 393. 8 2 Kirkwood ibid. 1939 7 506. 30 QUARTERLY REVIEWS The widest possible range may be put as 1200-700 cm.-l. Further as the value of n increases the highest and lowest frequencies approach definite limiting values ; the lower limit corresponding to the most symmetrical modes should appear as highly polarised Raman lines whatever the value of n.The (2% - 5) skeletal deformation modes are probably restricted to the region below 500 cm.-l. The most symmetrical of these are expected t o give rise to prominent polarised Raman lines which do not however correspond to the upper limit. Each trans-n-paraffin has two pairs of CH rocking vibrations cor- responding t o in-plane and out-of-plane motions. The data on ethane and propane (p. 28) suggest that their frequencies should lie between 1190 and 820 cm.-l Le. they fall in the same frequency range as the skeletal stretching modes. Interaction between the CH rocking modes and the skeletal stretching vibrations of the same symmetry type may therefore be expected.It should be emphasised that the value of these theoretical treatments lies in the guidance they have provided in the interpretation of the spectra. The simple methods do not give good agreement between the calculated and the observed frequencies. More ambitious discussions can overcome this difficulty but the effort involved is considerable. Complete treatments of both skeletal and hydrogen vibrations such as those given by Stepanov for n-butane and n-~entane,~l 36 could in principle be carried out for higher members of the series but it is doubtful whether such laborious calculations would add a proportionate amount to the understanding of the spectra. Experimental data for the trans-isomers of the n-paraffins are available for all molecules up to n-decane and for some higher members of the series.The Raman spectra were obtained by Mizushima and his c o - ~ o r k e r s . ~ ~ ~ 85 Raman and infra-red spectra have been published for the lower members by Rank and his collaborators,52-55 and further work on the infra-red spectra has been carried out in 87 A fairly complete assign- ment of the frequencies of trans-n-butane has been given 35 and sufficient data are also available to permit adequate discussion of the gauche isomer. Table 1 gives the characteristic frequencies that can be picked out in the spectra of the n-paraffins in the solid state; infra-red data are not available below 700 cm.-l. The series of prominent Raman lines near 1300 cm.-l (col. 3) which have rather weak infra-red counterparts cor- responds to CH wagging or twisting modes as already indicated in the introduction.Molecules with an even number of carbon atoms should on account of their centre of symmetry give rise to frequencies that appear only in the Raman or the infra-red spectra. Since the series near 1300 cm.-l shows up consistently in both types of spectra it must correspond to at least two modes with different selection rules but coincident frequencies. Four series (cols. 4-7) occur in the region 1200-800 cm.-l appropriate to the skeletal stretching and CH rocking frequencies. Like the 1300 cm.-l series that near 1060 cm.-l (col. 6) must correspond to at least two modes of vibration for it is observed in both types of spectra. Specific assign- 87 Brown and Sheppard unpublished dat,a. TABLE l.* Characteristic frequencies (in cm.-l) of the n-parafins in the crystalline state Paraffin __- n-Butane .n Pentane 17-Hexane . n-Heptane n-Octane . n-Nonane . ?a-Decane . n-Undecane n - D o decane n-Tridecane n-Tetra- decane ?a- Hexa - decane n - Nona - decane Infra-red mtensity Raman mtensity I R R I R R IR R IR R IR R IR R IR R IR R IR R IR R IR R IR R IR R - 1350 1300 1308 1303 1307 1300 1305 1296 1303 1297 1298 1297 1303 1295 1290 vc-c or CH rockmg f 1 965 1151 f 1138 1025 1145 1031 f ' 1057 1143 f 1140 1139 f 1138 1138 1136 f 1075 - 1088 f 1094 - 1103 I 984 ' f I 732 1059 837 1 - i 1066 ' f 726 1064 1 898 1 - [lo571 910 ' 723 [I0561 1 905 1 - I 1058 f ' 722 1062 ~ 899 1 - [lo631 889 ' 721 [lo601 I 888 - 1062 f 720 1060 1 886 - 1136 j f 1137 1 1106 [lo641 1 889 1 720 <- No observations 1303 1112 1062 f 722 1297 1 1:36/ f 1061 892 - a, Ref.f 426 NI 406 f 373 NI 31 1 f 283 NI 249 f 231 NI --+ f 194 N I 1304 1 1138 1110 [lo641 893 723 - No observations ____I__ 1306 ~ f 1114 ' 1060 f 1 722 1 f P No observations --r t-------- No observations -+ 1135 f 1068 888 - 160 1300 1296 ~ 1134 1121 [lo561 890 720 NI - No observations -j I 56a 84 65b 84 56b 84 87 84 35 84 35 84 36 84 87 - 36 84 87 35 - - - 84 87 - [ ] indicates two modes with coincident frequencies * Explanation of nomenclature employed in Tables 1 4 and 6-9 API American Petroleum Institute IR Infra-red R Raman NI No data available No entry no frequency can be recognised VC-C skeletal stretchmg mode 6~ skeletal deformation mode f forbidden st strong m.st medium strong m medium w weak v.w very weak abs.absent not observed w./t. = waggmg or twwting. - r = roolung * no vibration exists 32 QUARTERLY REVIEWS ments for each of these series have been suggested,35 but only in the case of the highly polarised Raman lines near 890 cm.-l (coI. 7) is there general agreement-they correspond to the lower limiting skeletal stretching modes.10 359 84 85 Prominent infra-red absorption bands which have no observable Raman counterparts are found in the region 1100-700 cm.-1; these correspond to the CH rocking modes. All molecules for n > 5 show the intense bands near 720 cm.-l (col. 8 ) which are characteristic of this type of vibration ; these can almost certainly be assigned to the lower limiting vibrations. In addition bands which spread to higher frequencies (up to 1050 cm.-l) split off as the chain length increases ; 35 they seem to be approaching an upper limit as is also suggested by the theoretical treat- m e n t ~ .~ ~ The series of polarised Raman lines below 500 cm.-l (col. 9) cor- responds to the most spmetrical skeletal deformation vibrations.lO+ 35 g49 85 Polythene must be mentioned here since it represents an infinite paraffin chain with the [-CH,*CH,-] repeating unit. For detailed discussions reference must be made to the original papers ; 35 38-41 4 5 85 88 89 it is sufficient to note that the assignments of the polythene frequencies can be correlated in a satisfactory way with those given above for the truns-isomers of the n-paraffins. ( d ) Spectra of the n-Parafhs in the Liquid State.-Experimental data on the Raman and infra-red spectra for the n-paraffins in the liquid state are available for all molecules up to and including n-tetradecane and for some higher members.The prominent regularities already noted in the spectra of the trans-n-paraffins are also apparent in the spectra of the liquids which consist of mixtures of rotational isomers. Some modifications in intensity and activity are observed because in many cases the non-planar isomers may have identical frequencies but different selection rules as e.g. in the 1140 cm.-l series. Other series may be recognised which must be due only to non-planar isomers.49 Since all these are of some practical importance for the identification of unbranched chains they are given in Table 2 together with brief comments on their behaviour. Most of these series have effectively constant frequencies.Others in which progressive shifts are observed have'been noted among the weaker bands in the range 1320-1220 cm.-l; these must correspond to additional CH wagging and twisting modes.' I n the spectra of the liquids the region 800-700 cm.-l is somewhat complex. The intense band at 720 cm.-l is accompanied by subsidiary bands at higher frequencies which can also be arranged in shifting sequences. It may be noted in this connection that paraffins with an unbranched chain of four carbon atoms show an intense band near 740 cm.-l (compare n-butane 747 and 731 cm.-l) and those with a chain of three carbon atoms i.e. an ethyl group one usually near 770 cm.-l; these are to be attributed to CH rocking modes of the shorter chains (see also ref. 23b). From the above discussion it may be concluded that the spectra of the liquid n-paraffins are closely related if there are five or more carbon atoms 88Thompson and Torkington PTOC.Roy. SOC. 1945 A 184 3. 89 Elliott Ambrose and Temple J. Chem. Phys. 1948 16 877. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART I1 33 present. In the Raman spectra there is some alternation of frequency for 11 even and n odd but this is not distinguishable among the infra-red bands.5 The most prominent frequencies in the infra-red spectra are those associated with tlhe hydrogen deformation modes. It follows that the features that TABLE 2. Characteristic frequencies (in cm.-l) of the n-parafins in the liquid state v approx.* E l . . 13401 1305 . . . 1140 . . . 1120-965 . 1070 . . . 890 . . . 870-840 840-810) ' 720 .. . 425-150 . . Infra-red spectra Moderately prominent Moderate Moderate Moderate Difficult to Weak ; Weak ; distinguish difficult to distinguish difficult to distinguish Intense No data Raman spectra Absent Prominent Prominent Weak Prominent Prominent ; polarised Prominent ; polarised Absent Prominent ; polarised Assignments and comments Hydrogen deformation modes of non- planar isomers ; nearly constant n 2 5 CK wagging or twisting modes ; nearly constant n> 5 ; intensities of the bands increase as n increases Nearly constant n 9 4 Intensities and frequencies of the bands increase as n increases ; gives indica- tion of the chain length Nearly constant n 2 4 Lower limiting skeletal stretching modes ; nearly constant n > 5 Lower limiting skeletal stretching modes of non-planar isomers ; alter- nating frequencies n even < n odd CH rocking modes; nearly constant Symmetrical skeletal deformation modes ; decrease regularly as n in- crease s n> 6 * Characteristics frequencies found also in spectra of the trans-isomers are in italics.are valuable for recognising the n-paraffins or the presence of an extended unbra,nched carbon chain are limited in number the most reliable being the group of Raman lines between 900 and 800 cm.-l and the infra-red absorption bands in the region 730-700 cm.-l. 4. The tert.-Butyl Group (a) neoPenta,ne.-neoPentane ( 2 2-dimethylpropane) may be considered as the first member of the series of p a r a f i s having the general formula CMe,R where R is an alkyl radical. This symmetrical molecule has apart from the CH torsional modes seven distinct vibrations which should occur in the region below 1350 cm.-l two of which are inactive in both the Raman and the infra-red spectra.The substance has been extensively investi- gated 1 2 ) 25) 349 48 4 9 7 and attempts to assign the frequencies have been A.P.I. Infra-red Spectrograms Nos. 104 442 514 784. 91 Young Koehler and McKinney J . Amer. Chem. SOC. 1947 69 1410. 9 2 Rank and Bordner J . Chem. Phys. 1935 3 248. 93 Kohlrausch 2. phys. Chem. 1936 23 28 340. C 34 QTTARTERLY REVIEWS made by many authors but as yet complete agreement has not been achieved. The following are those assignments (Y in cm.-l) about which there is no controversy (see also Table 3) i z l Symmetrical skeletal stretching vibration 733 335 41 5 n2 Doubly degenerate skeletal deformation vibration n4 Triply degenerate skeletal deformation vibration Ambiguity arises over the interpretation of the 1249 and 921 cm.-l fre- quencies both of which appear as depolarised Raman lines and intense infra-red bands.The assignment proposed originally by Kohlrausch and Koppl 49 has since been favoured by a number of authors.34* 4 8 9 91 9 2 p 94 This puts the 1249 cni.-l frequency as the triply degenerate CH rocking mode and the 921 cm.-l frequency as the triply degenerate skeletal stretch- ing vibration n,; the reverse interpretation has been preferred by other investigator^.^! 109 46 95* O6 Most of these authors have attempted some TABLE 3. Relationships between the skeletal modes of neopeiztune and those of parafins with one tert.-butyl group ?a1 . . 1 a .. 2 n3 . . 3 Q1 . . 3 Paraffins with one CH,-C-K. krt.-butyl group I CH P ia dP ia dP a \ dP a \ Selection rules. Numbering Degeneracy I I Numbering v1 . . vp . . -vg . . .v* . . -vg . . .vg . . Selection ru!es Degeneracy Raman mfra-red I P dP dP dP P P p = polarised ; dp = depolarised ; a = active ; ia = inactive kind of theoretical treatment. Pitzer and Kilpatrick 46 have suggested schernat,ically that the two inactive CH rocking modes are at about 1150 cm.-l for the doubly and 950 cm.-l for the triply degenerate vibrations while Rasmussen 34 prefers 1200 and 900 cm.-l respectively. Unfortunately it does not seem possible to come t o any final decision on the data a t present available for neopentane and investigations of the deuterated derivative (CH,),C*CH,D have not been conclusive.48 (See the discussion between Rank 97 and Sheppard Simpson and Sutherland.98) For the purpose of considering the characteristic frequencies of the tert. - butyl group this is not a serious difficulty. 9 4 Wagner 2. pJbys. Cham. 1939 B 45 341. 95 Wall and Eddy J. Chem. Phys. 1938 6 107. 9.5 Silver ibid. 1940 8 919. 97 Discuss. Faradccy SOC. 1960 9 219. s8 Ibid. p. 216. TABLE 4. Characteristic frequencies (in cm.-l) of parafins with tert.-butyl groups in the liquid state * ,4. Paraffins with one tert.-butyl group. NI 364 N I 319 ~~ Paraffiii vc or CH roching voc d, 1 Refs. API Nos. 13 14 13 14 2 2-Dimethylbutane . . 2 2-Dimethylpontane . . 2 2-Dimethylhexane . . 2 2-Dimethylheptane . . 1223 1206 1212 1205 1210 2 2 3-Trimethylbutane 2 2 3-Trimethylpent,ane 2 2 3-Trimethylhexane 2 2 4-Trimethylpentane 2 2 4-Trimethylhexane 2 2 4-Trimethylheptane 2 2 4-Trimethyloctane 2 2 5-Trimethylhexane 2 2 6-Trimethylheptane 2 2-Dimethyl-3- ethylpentane ethylhexane .. . . . . . . . 2 2-Dimethyl-4- 924 720 1 310 928 745 NI 931 749 301 926 744 NI 927 744 305 IR R IR R IR TC IR R IR R IR R IR R IR R IR R IR R IR R IR R IR R IR R IR R - - (2) 1252 1258 1250 1254 1250 1251 1250 1251 - 1252 1255 1244 1245 c- 1239 1247 1254 1247 1250 t- 1252 t- 1252 1248 1257 1255 1249 1236 1236 t- 1245 (3) 1217 1221 1209 1210 1202 1204 1203 1202 929 929 926 92 8 932 928 932 930 71 1 714 74 1 746 - 746 - 750 NI 361 NI 340 NI 303 NI 283 686 686 716 717 Ins - _ f - 1205 1206 1204 1201 1218 1226 920 929 920 923 919 925 13 655 670 14 13 571 588 14 25 14 2 14 - 746 - 745 729 727 NI 302 NI NI 308 - To observations -> 1205 926 739 307 15 13 14 13 14 - 15 - 15 13 14 25 14 13 14 - 15 573 587 576 590 577 589 667 676 579 592 683 685 36 QUARTERLY REVIEWS B.Paraffins with two tert.-butyl groups. Paraffin 2 2 4 4Tetramethyl- 2 2 5 5-Tetframethyl- 2 2 3 4 4-Penta- peiitaiie hexane nicthylpentane met hgl hexane methylheptane 2 2 3 5 5-Penta- 2 2 4 6 6-Penta- Infra-red intensity . . . Raman intensity . . . . vc-c or CHa rocking vCc 6c-c Refs API Xos. IR 1244 R 1249 IR 1247 R t R t - IR f- IR 1238 R 1255 IR 1244 R 1248 st - 916 730 13 583 591 - 920 731 11 - 910 755 13 690 691 No observations + - 1214 930 748 13 693 694 No obsorvat'ions + - No observations ___+ - 1211 924 753 15 5 1209 924 758 14 1208 927 ~ - 1 m m i s t l m *For the meaning of symbols and abbreviations see the foot of Table 1.( b ) Paraffis with one tert.-Butyl Group.-Before discussing the fre- quencies characteristic of the tert.-butyl group it is helpful to consider briefly how the skeletal vibrations of ~eopentane are likely to alter when the symmetry of the molecule is reduced by the substitution of an alkyl radical R for one CH group. Table 3 shows the relationships between the modes of the parent and the derived molecules together with the degener- acies and the selection rules; these relationships are well In order t o get some idea of the magnitude of the splitting of the frequencies of n3 and n4 and the variation with the nature of R of the frequencies v1 to v6 for the molecules CMe,R it is necessary to assume a suitable model and to make calculations using force constants chosen from the values available in the literature.Two such investigations have been made. Kohlrausch and his co-workers loo were interested primarily in those mole- cules in which R is a halogen atom ; Simpson and Sutherland * were concerned with the 2 2-dimethyl paraffins. The conclusions reached in both treatments are similar. The vibrations vl v4 and vj shift steadily to lower frequencies as the mass of R increases ; Y after an initial decrease changes more slowly and appears to approach a limiting value while the modes v2 and v6 remain effectively invariant. Further it seems probable that the maximum split- ting of n for the 2 2-dimethyl paraffins is unlikely to be greater than 50 cm.-l though that of n4 is probably considerably larger.* 99 Kohlrausch op.cit. 1943 p. 168. 100 Idem ibid. p. 188. SHEPPARD AND SIMPSON SPECTRA O F HYDROCARBONS. PART I1 37 Tables 4A and 4B summarise the characteristic frequencies in the Kaman and infra-red spectra of the liquids for most of the paraffins having one or two tert.-butyl groups hitherto investigated (some very complicated molecules are not included). Figs. 4A and 4B show typical spectra for a paraffin of this class. The tert.-butyl group provides a particularly favour- able configuration for the discussion of the characteristic frequencies of such a unit because it forms a rigid group which cannot because of its threefold symmetry axis itself give rise to rotational isomerism i . e . the existence of rotational isomers in any molecule in which it occurs must be dependent on changes in the radical R.It seems plausible to suppose that such changes are unlikely to influence t'he vibrat'ions of the tert.-butyl group though direct experimental evidence on this point is not available. The various characteristic frequencies of molecules having a single tert . - butyl group will now be discussed in turn. As already noted (p. 22) there is a Splitting of the band due to the CH synimetrical deformation modes to give a doublet a t 1397 and 1370 cm.-l. The other series as given in Table 4 can all be related to frequencies in neopentane by using the theoretical conclusions outlined above. Col. 6 lists the frequencies of the symmetrical stretching vibration vl of the C unit associated with the tcrt.- butyl group (750-7 10 em. -I). These frequencies are easily recognised in the Raman spectrum on account of their strength and high degree of polarisation ; 10 the corresponding infra-red bands are not prominent and are in many cases overlapped by strong absorption due to the CH rocking modes.This series is related to the Raman line a t 733 cm.-l in the spectrum of neopentane. From the figures given it is apparent that the actual fre- quency of this mode is dependent on the nature of the radical R ; thus if R is branched next to the tert.-butyl group (as in 2 2 3-trimethylbutane) a marked lowering occurs. The second general trend is that as the inass of R increases for a given type of R the frequency of this vibration also increases. This is contrary to the theoretical predictions but as they were based on drastically simplified models it is perhaps not surprising.Col. 7 gives the frequencies of the symmetrical skeletal deformatmion vibration v5 as observed in the Raman spectra below 400 cm.-l. This mode can be picked out with fair confidence on account of its polarisation but is by no means easy to identify in the more complex molecules. The actual frequencies decrease with increasing mass of R and seem to be dependent to some extent on its structure. This vibration on account of its considerable variation is less valuable as a means of identifying the tert.-butyl group. The skeletal deformation modes v4 and v6 which should occur as depolarised Raman lines below 500 cm.-l cannot be recognised with certainty though tentative series have been proposed for these vibrations in the simple 2 2-dimethyl paraffin^.^ The two frequencies near 1250 and 1210 cm.-l (cols.3 and 4) are of the greatest practical importance in the identification of the tert.-butyl group as they almost invariably appear as fairly prominent Raman lines 8 10 and as very strong infra-red b a n d ~ . ~ j They undoubtedly arise by the splitting of the 1249 crn.-l frequency in neopentane. According to the FM. 4A FIG. 4B A. IrLfm-retl spedtu bctwceiz 1500 t o i t ? 300 ctti.-I o j some / y p i c i l 0tmtclted pwujirts i t i the liqzcid stalp. (A,flcr -4 mericcrrr J’cft CIP14)il I/,stitzite I,zfra-red Spcclropums Nos. 57 1. 588 ; 552. 554 ; 372 58s 002. ti1 1 .) ( VtJrticn/ I i n p s indicnle (*lrnracfcristic ahsorptior& frequemic.9 w~e,itiotr~tl it1 llrc tc rt.) 33. Ramatb spectra betwee,% 1500 atid 300 c m - 1 of the sume paraffh i t z the liquid state.(After B’enske et al.14) (Characteristic frequencies metrtioned in the text are indicated by *.) I 2 2-Di?neti~ylpe,ita}ze. III 3 3-Dimethy1penta;ne. 11 2-Methylhexune. IV 3-Methylhexane. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART I1 39 interpretation given for the symmetrical molecule (p. 33) they may be assigned either as CH rocking modes of the tert.-butyl group or as the two skeletal vibrations v2 and v3. As may be seen from col. 3 the 1250 cm.-l frequency remains remarkably constant whatever the nature of the radical Rt. The other series near 1210 cm.-l (col. 4) shows more variation ; there is a general lowering as the mass of R increases though the lowering is less pronounced if R is branched near the tert.-butyl group.The behaviour of the 1250 and 1210 cm.-l series is in general agreement with the theoretical predictions for v2 and Y, though this is not conclusive evidence in favour of these assignments. The relative intensities of the infra-red. bands near 1250 and 1210 cm.-l are interesting; in general the former are stronger unless R is branched near the tert.-butyl group in which case the lower frequency becomes more prominent. Col. 5 gives figures for the remaining frequency (near 930 cm.-l) that can be definitely associated with the tert.-butyl group. This appears as a prominent Raman line and a recognisable but not very strong infra-red band. Like the 1250 cm.-l frequency it remains nearly invariant whatever the nature of R but is less valuable for the identification of the tert.-butyl group because many other paraffins have frequencies in this region.The 930 cm.-l series in molecules with a tert.-butyl group is undoubtedly associ- ated with the 921 cm.-l frequency in neopentane and so may be assigned either to the invariant skeletal modes v2 or to CH rocking vibrations of the tert.-butyl group. It should be noted however that if the former interpretation is preferred it is not possible to pick out any series which could be attributed to the other skeletal vibrations Y,. Of the substances listed in Table 4A only 2 2-dimethylbutane has been the subject of detailed consideration probably because rotational isomerism is absent in this molecule. The conclusions of Kilpatrick and Pitzer lol and Sheppard lo2 are in fair agreement. Table 4A does not include the data for some ten more complicated paraffins with one tert.-butyl group. All the characteristic frequencies noted above appear consistently except that the 1210 cm.-l frequency is missing for the two molecules with a 2 2 3 3-tetramethyl grouping. It is also interesting to find that these same frequencies may readily be recognised in the spectra of tert.- butylethylene and tert.-butylacetylene (3 3-dimethylbut-1 -ene and -1 -yne). l o 2 Two somewhat weaker frequencies near 1025 and 880 cm.-l which appear to be associated with the tert.-butyl group in these molecules cannot be recognised with certainty in the spectra of the relevant paraffins. ( c ) Paraffis with two tert.-Butyl Groups.-The molecule 2 2 3 3- tetramethylbutane might be considered as the first member of the series of paraffins with two tert.-butyl groups.However both the electron- diffraction data loa and the spectroscopic evidence lo* suggest that it has a staggered configuration analogous t o that of et'hane. Because of the I o 1 Kilpatrick and Pitzer J. A ~ 2 c v . P h p i i ? . Sot. 1918 68 1066. lo? J . (:hem. Pltys. 1949 17 466. lU3 Bauer and Beach J . Amer. Chem. Soc. 1942 64 1142. lo* Cleveland Lamport and Mitchell J. Chew. Phys. 1950 18 1320. 40 QUARTERLY REVIEWS resulting centre of symmetry the skeletal vibrations of such a molecule cannot be completely related to those of other substances with a tert.-butyl group. The spectra of this paraffin have been investigated by a number of authors 2 5 9 27b 4 7 9 l o 4 9 lo5 and an assignment of its frequencies which seems satisfactory has recently been given.lo6 The data on other molecules with two tert.-butyl groups are somewhat scanty; they are collected in Table 4B. Most of the frequencies charac- teristic of a single tert.-butyl group (except the skeletal deformation mode) may be recognised without difficulty in the spectra of these more complex paraffins. 5. The isoPropy1 Group ( a ) isoButane.-isoButane may be considered as the first member of the series of paraffins having the general formula CHMe2R where R is an alkyl group ; it has nine distinct vibrations (other than the CH torsional modes) that should occur in the region below 1350 cm.-l all but one of which are active in both Raman and infra-red spectra; the remaining mode is forbidden. This molecule has been the subject of many investi- gations l 7 3 2 4 9 25 34 1°79 lo* but no definite assignment of the frequencies is generally accepted.Apart from the doubly degenerate CH deformation mode observed as a Raman line near 1335 cm.-l the only vibration about which there is no controversy is the non-degenerate skeletal stretching mode n at 795 cm.-l (see also Table 5). The two skeletal deformation frequencies occur a t 435 and 370 cin.-l. Most authors allot these respectively to the symmetrical non-degenerate (n2) and unsymmetrical doubly degenerate (n4) vibrations on the basis of Ananthakrishnan's polarisation measurements. lo8 Simpson and Sutherland have given reasons for preferring the opposite interpretation but further work will be needed before a final conclusion can be reached. There has been considerable discussion about the assignment of the unsymmetrical doubly degenerate skeletal stretching vibration n3 which has been variously located at 925,46 965 34 94 and 1170 ~ m .- l . ~ lo The infra-red spectrum of the molecule (CH,),*CD has been published but not analysed.log It is clear t'hat some of its modes of vibration are coupled together (compare discussion on propane I->. 28) ; the observations do not however allow the identification of' n3. The inactive CH rocking mode has been schematically put at 1200 cm.-l by Pitzer and K i l p a t r i ~ k ~ ~ and at 900 cm.-l by Rasn~ussen.~~ The active singly degenerate CH rocking vibration probably occurs at 1095 The two active doubly degen- erate CH rocking modes correspond to the two frequencies of those listed above which are not attributed to the unsymmetrical skeletal stretching vibration.105 A.P.I. Infra-red Spectrogram No. 444. Io6 Scott Douslin Gross Oliver and Huffmann J. Arner. C'hern. Soc. 1952 74 883 Io7 A.P.I. Infra-red Spectrograms Nos. 62 374 439 1142. Io8 Proc. Indian Acad. Sci. 1936 A 3 527. log Condon McMurry and Thornton J. Chem. Phys. 1951 19 1010. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART 11 41 It is unfortunate that the assignment of frequencies of this key molecule is so uncertain ; however it is possible to discuss the frequencies character- istic of the isopropyl group without difficulty although their exact inter- pretation is ambiguous. ( b ) ParafEns with one isoPropyl Group.-It is clear from the foregoing discussion that it is profitable to consider the relationships between the skeletal vibrations of the most symmetrical and the other members of a related series.Thus in the present instance it is desirable to know how t'he skeletal modes of isobutane are related to those of other molecules of the type CHMe,R. The general arguments are analogous to those already given for the tert.-butyl group and the relevant calculations have been carried out on similar lines by the same author^.^ loo Their conclusions are summarised in Table 5. TABLE 5. Relutionships between the skeletal modes of isobutanw uncl those of parafins with one isopropyl group H I XsoButanc C€13-C -CII Numbering Selection rules Degeneracy Raman H I wopropyl group I CH3 Paraffins with one CJI,-C-R Selection rules - Numbering Raman '::$- 1 P 1 2 dp a -v3 . A V g . /v4 2 dp a / -v6 . . Effect on frequency of Increase of mam of R Decreases fairly slowly Decrease.steadily Effectively constant Decreases fairly slowly Effectively constant Decreases steadily Tables 6A and 6B list the characteristic frequencies in the Raman and infra-red spectra for a selection of pmaffins having one or two isopropyl groups respectively ; all substances were investigated in the liquid state. Figs. 4A and 4B show typical spectra for a paraffin of this class. Unlike the tert.-butyl group the isopropyl unit is not symmetrical so that even the simplest molecules (such as 2-methylbutane) 61 can exist as mixtures of rotational isomers in the liquid state. Moreover in all the molecules listed (except 2 2 3-trimethylbutane where such isomerism is absent) the isopropyl group itself is actively concerned in the variations in configuration.Since it is probable from the work on molecules where one form has been isolated that certain frequencies of the various isomers of a given molecule may differ considerably it follows that those given in Table 6 may in fact correspond to different forms of the molecules. However it is possible to make quite reasonable assignments on the assump- tion that they all correspond to one particular type of isomer although 42 QUARTERLY REVIEWS TABLE 6. Characteristic frequencies (in crn.-l) of parafins with isopropyl groups in the liquid state * A. Paraffins with one isopropyl group. Paraffins %-R IR R 1343 I R R 1344 I R R 1345 IR R 1344 IR R 1345 IR f i G C 463 459 N I 442 N I 430 N I 434 N I KI - - BPI Sos 550 55i 55 1 55. 552 55 353 55 605 614 608 614 - 1149 1154 1147 1154 1145 1148 1144 1149 1143 1148 1142 - - 953 959 964 - 957 961 968 95 1 961 957 - ~ 920 920 918 928 919 - I - I 919 - 918 - 794 794 816 815 822 822 813 814 824 826 833 - 1176 1175 1172 1176 1171 1179 1171 1177 1167 1175 1167 2-Methyl- butane 2 -Methyl- pentane 2-Methyl- hexane 2-&hhyl- heptane 2 -Methyl - octane 2-Methyl- nonane 4-isoPropyl- heptane N I 368 XI 325 N I 311 N I 289 NI 256 XI 13 14 13 14 13 14 13 14 13 14 13 - 927 920 928 931 920 917 920 923 - 832 830 826 829 826 828 835 833 - 13 14 13 14 13 14 25 14 IR 1161 - 957 R 1331 - - - I R 1168 - 956 R - 1176 - 957 IR 1170 1121 950 R 1345 - 1125 958 IR 1170 1125 949 R - - 1125 - N I 449 N I 423 N I 407 N I - 573 587 577 589 579 592 2 2 3-Tri- met h ylbutane met hylpent an6 me thylhexane methylheptanc 2 2 4-Tri- 2 2 5-Tri- 2 2 6-Tri- B.Parfins with two isopropyl groups. 2 3-Dimethyl- 2 4-Dimethyl 2 5-Dimethyl- 2 &Dimethyl- butane pentane hexane heptane - I R R 1348 1165 I R 1170 R 1348 1164 I R 1171 R 1341’1175 I R 1170 R 1341 1171 1153 - - - - 1154 1150 1149 ~ 921 919 924 920 911 920 - - N I N I 414 N I 440 N I 423 - N I - N I 306 N I 263 N I 255 13 14 13 14 13 14 2 15 656 6iC 659 672 663 674 - 939 956 956 949 963 950 953 ‘ G 809 810 839 840 835 840 * For the meaning of symbols and abbreviations bee foot of Table 1. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART I1 43 the resulting conclusions must be considered as tentative until confirmatory experimental evidence becomes available. The various characteristic fre- quencies of molecules having a single isopropyl group will now be discussed in turn.As already noted (p. 22) there is a splitting of the band due to the CH symmetrical deformation modes of the group to give a doublet a t 1385 and 1370 cm.-l. The CH deformation vibrations give rise to a series of Raman lines near 1345 crn.-l,lO which are shown in Table 6A col. 3. Col. 8 gives the series (835-749 cm.-l) that can certainly be attributed to the symmetrical stretching vibrations vl of the isopropyl group. This is easily recognised in the Raman spectrum on account of the strength and polarisation of the lines ; the corresponding infra-red bands are usually weak.10 These frequencies correspond to the vibration observed a t 795 cm.-l in the spectra of isobutane. Prom the figures given it is apparent that the actual frequency of this mode is not strongly dependent on the nature of R though a slight shift to higher frequencies may be noted as the mass of R increases.This variation is not in agreement with the predictions of the simple theory. Col. 9 gives the series of polarised Raman lines (465-430 cm.-l) that may be assigned to the constant skeletal deformation vibrations v4. These may be recognised without much difficulty lo and could be used to detect the presence of an isopropyl group although in such paraffins there are usually a number of lines in this spectral region. Col. 10 lists another series of polarised Raman lines below 400 cm.-l. These lines may be assigned to the skeletal deformation modes Y, and diminish in frequency as the mass of R increases in accordance with the theoretical predictions.They may be identified with fair certainty in the simpler molecules but would be of little value for recognising the presence of the isopropyl group. The series of depolarised lines below 500 cm.-l corresponding to the skeletal deformation modes Y cannot be definitely identified ; a tentative inter- pretation has been proposed for the 2-methyl paraffin^.^ Smith has noted a series of weak infra-red bands between 550 and 530 cm.-l appearing regularly in the spectra of paraffins with an isopropyl group which can possibly be assigned as combination bands. The spectra in this region are however rather complex so this series is not very helpful for structural analysis. The frequencies near 1170 cin.-l listed in col. 4 are those which are of the greatest importance in the identification of the isopropyl group ; this series is always prominent as strong bands in the infra-red spectrum 49 5 that usually have corresponding Raman lo The frequencies are very insensitive to changes in the nature of R and are undoubtedly associated with the 1170 cm.-l frequency in isobutane.Col. 5 lists a fa'irly constant frequency near 1150 cm.-l which appears regularly in the spectra of the 2-methyl pa,raflfins though not generally i n the more complex molecules. This series also appears t'o be associated with the 1170 cm.-l frequency of isobutaiie. These two series may be interpreted either as CH rocking modes of the isopropyl group or as the skeletal stretching vibrations v3 and v5. 44 QUARTERLY REVIEWS The series listed in col. 6 appears as prominent Raman lines and as infra-red bands of variable intensity near 955 cm.-l; the frequencies remain almost unaffected by alteration in the structure of the radical R and may be correlated with the vibration appearing a t 965 cm.-l in the Raman spectrum of isobutane.Col. 7 shows a series near 920 cm.-l which appears as prominent infia-red bands that sometimes have corresponding Raman lines. These frequencies show very little variation; they would seem to be correlated with the 925 cm.-l absorption in isobutane. Paraffins with a tert.-butyl group also have frequencies in this region (Section 4). It should be noted that there is no real evidence for any consistent split,ting of either the 955 or the 920 cm.-l series in isopropyl compounds i.e. if either of these is assigned to the skeletal modes v3 there are no characteristic frequencies recognisable in this region that could correspond to skeletal vibrations v,.Data are available for some 25 more highly branched paraffins with isopropyl groups. Of the characteristic frequencies listed in Table 6A the Raman lines near 1345 cm.-l are usually present. The 1170 and 955 cm.-l frequencies are nearly always found in both infra-red and Raman spectra and those near 920 cm.-l as infia-red bands. A frequency corresponding to the symmetrical stretching mode v1 and another that may be attributed to the consta,nt skeletal deformation vibration v4 may be identified in the Raman spectra of about half the molecules. The behaviour of the 540 em. -l infra-red series is somewhat erratic ; the 1150 cm.-l frequency is usually absent and the deformation vibration Y cannot be distinguished with certainty.(c) Paxaffis with two isoPropyl Groups.-The data on the simpler paraffins with two isopropyl groups are summarised in Table 6B which shows that most of the characteristic frequencies of the isopropyl group may be easily recognised in these more complex molecules. The frequencies of 2 3-dimethylbutane seem a little anomalous. In the liquid this molecule is known to consist of a mixture of two rotational isomers one of which has a symmetrical structure with a centre of symmetry.61 This may possibly account for the discrepancies observed. Further the two isopropyl groups must be closely coupled together so that deviations due to their interaction may be expected. 6. Paraffins with an Internal Quaternary Carbon Atom C I C The molecules of this class possess a C structural unit C-A-C in common with those with the tert.- butyl group previously discussed (Section 4). Consequently analogies may be sought in the specha of these two classes of paraffin. Further it might be expected that the assignments made for neopentane would prove helpful in the interpretation of the spectra ; this method of approach has already been used by both Sheppard lo and Smith.5 A number of paraffins with an internal quaternary carbon atom have been SHEPPARD AND SIMPSON SPECTRA O F HYDROCARBONS PART I1 45 1216 1210 1211 1230 1229 TABLE 7. Characteristic frequencies (in cnz. -I) of pamfiIis with an tnternal qua'tern-crry carbon atom in the liquid state * - 1193 1195 1188 - Paraffin 1017 * * 1010 1018 1016 - 1000 999 1012 1015 3 3-Dmethyl- pentane hexane 3 3-Dimethyl 3 3-Dmethyl- heptane 2 3 3-Trimethyl pentane 2 3 3-Trimethyl hexane 2 3 3 4-Tetra- me thylpentane 3 3 4-Trimethyl hexane 3 3 5-Trimethyl heptane 3 3 4 4-Tetra- methylhexane 4 4-Dimethyl- heptane 2 4 4-Trmethyl hexane 658 - - NI 753 493 723 481 721 478 686 468 681 475 N I N I 706 485 - 455 684 - 3 -Ethyl- 3 - met h y lpen t ane met h ylhexane 3-Ethyl-3- 1232 - 1235 1229 3 3-Diethyl- 3 3-Diethyl- pentane hexane 1179 1192 1183 1191 Infra-red intensity .. Raman intensity - IR R IR R IR R IR R IR R IR R IR R IR R IR R IR R IR It IR R IR R IR R IR R - - - - (2) I c-c 01 CH roiking 1217 1217 1212 1208 1215 121 1 1218 1206 1210 1221 1227 1205 121 1 1211 I 1192 1200 1189 - 1187 - 1188 - 1186 1191 1183 1186 1182 1192 1186 +- No ( I 1225 I 1190 - - 1000 1007 1006 1010 1005 - - - * * * * 1006 1011 - 695 695 725 722 727 724 672 676 689 69 1 672 67 1 701 704 721 482 482 482 492 NI 478 472 470 490 483 47 1 467 482 484 - 1220 1175 1015 733 N I No observations - Hefb 13 14 13 14 2 14 13 14 13 14 13 14 13 14 13 - - 14 2 14 13 14 13 14 2 14 13 15 2 - - A H Nos 572 588 574 589 578 590 668 676 585 693 681 682 686 688 580 592 581 591 * For the meanmg of symbols and abbreviations see foot of Table 1.46 QUARTERLY REVIE\VS investigated ; Table 7 includes the relevant experimental data on the Raman and infra-red spectra of the liquids for all these except for the four which also possess a tert.-butyl group. Fig. 4A and 4B show typical spectra for a paraffin of this class. As noted on p. 22 there is a characteristic splitting of the band due to the CH deformation modes of the 3 3-dimethyl paraffins to give a doublet a t 1384 and 1367 cm.-l.Table 7 shows that corresponding to the 1250 1210 cm.-l frequencies of the tert.-butyl group two series are found consistently in the infra-red spectra of paraffins with an internal quaternary carbon atom ; the corresponding Raman lines are not always present. One series near 1190 cm.-l remains nearly constant in frequency (col. 4) and in the infra-red spectra is more intense than the other near 1210 cm.-l (col. 3) which is more variable in its behaviour. The former provides the most reliable method of distinguishing the presence of an internal quaternary carbon atom. The region 1100-1000 cm.-l which is not very helpful for structural analysis gives rise to a number of characteristic frequencies in molecules with an internal quaternary carbon atom that seem to have no counter- parts in paraffins with a tert.-butyl group.The bands in this range are somewhat complex with much splitting but regions of prominent absorption are apparent near 1100 1040 and in most cases near 1010 cm.-l and many molecules also show corresponding Raman lines. The frequencies of the first two series behave in a somewhat erratic fashion there being no obvious relationship to the structures. Tentatively they may be assigned to CH rocking modes not involving the C structural unit since this is the only type of vibration in the appropriate range common to all the molecules listed and similar frequencies are found in some other classes of paraffins (compare Table 10).The absorption bands near 1010 cm.-l (col. 5) are always prominent in paraffins with an internal quaternary carbon atom if they also have at least one ethyl group ; the Raman counterparts are usually absent. It seems plausible to suggest that these bands are to be assigned to the carbon-carbon stretching vibrations within the ethyl group analogous to the mode that appears as a Raman line a t 993 cm.-l in the spectrum of ethane. Such molecules also have intense bands near 770 cm.-l which were noticed by Smith ; these are attributable to the CH rocking modes of the ethyl groups. Characteristic frequencies analogous to those near 930 cm. -l in the spectra of the tert.-butyl compounds might be expected to appear for paraffins with an internal quaternary atom in the range 950-900 cm.-l.The infra-red bands in this region are usually complicated and on the whole not very intense ; Raman lines are found for some molecules near 930 cm.-l and in the range 920-910 cm.-l. It is difficult to distinguish any plausible series that is consistently observed for all these molecules and could be used to characterise them especially as the other groups present also have frequencies in this region. As may be seen from col. 6 all paraffins with an internal quaternary carbon atom possess a strong polarised Raman line that may be assigned SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART I1 47 to the symmetrical stretching mode of the C unit (750-685 cm.-l) ; the corresponding infra-red bands if they can be picked out from among those of the CH rocking vibrations are weak.These frequencies are analogous to those already described for the tert.-butyl compounds and correspond to the 733 cm.-l Raman line in neopentane. Moreover their variation is similar; in particular substitution near the C unit produces a marked lowering in frequency. This variation reduces the usefulness of these lines as a means of identifying the presence of an internal quaternary carbon atom. Below 500 cm.-l two series of Raman lines can be picked out that may be assigned t o skeletal deformation modes of the C unit though the exact relationship t o the corresponding vibrations of the tert.-butyl group is not very clear. One series (col. 7) has a fairly constant frequency; the other between 350 and 305 cm.-l (not listed) is polarised and the frequencies decrease as the molecule becomes larger.Neither would be very valuable as a method of identification as paraffins with an internal quaternary carbon atom possess a number of Raman lines in this region. The above discussion has not specifically taken into account the prob- ability that paraffins with an internal quaternary carbon atom exist in the liquid state as mixtures of rotational isomers. It is significant that the symmetrical stretching vibration of the C structural unit does not appear t o show any tendency to split. This suggests that the vibrations of this unit are little affected by the configuration of the rest of the molecule (just as in the analogous tert.-butyl paraffins) and provides support for the assignment of the pair of frequencies near 1200 cm.-l and the Raman lines near 485 cm.-l t o such modes.7. Paraffins with other Structural Units (a) Para5u with One Internal Tertiary Carbon Atom.-The molecules H I I of this class possess a C structural unit C-C-C in common with those c with the isopropyl group previously discussed (Section 5). The most hopeful method of approach already used by Sheppard lo and Smithy6 is to seek analogies between the spectra of these two classes of paraffin. As will be apparent from the following discussion the interpretation of the spectra of molecules with an internal tertiary carbon atom is not easy. Serious difficulties are encountered probably because the different rotational isomers have differing frequencies even for those vibrations that Can be associated with the C unit. Experimental data for a large number of molecules are available.Table 8 lists the relevant Raman and infia-red frequencies of the liquids for those paraffins which have a single C unit and no other branched grouping. Figs. 4A and 4B show typical spectra for a paraf5.n of this class, 48 QUARTERLY REVIEWS 1043 1016 1046 - 1032 1002 1036 - - 1011 1049 1012 1041 - - 1012 1044 - TABLE 8. Churactetistic frequencies (in cm.-l) of parafins with one internal tertiary curban atorn in the liquid state * - 443 819 446 817 NI 816 436 819 NI - NI - 422 819 NI 820 - Paratan 1166 1166 1166 1166 1167 3-Methylpentane . 3-Methylhexme . 3-Methylheptane 3-Methyloctane . 3-Methylnonane . 3-Methylundecene . 1047 1 1042 1 1 1044 834 NI 1044 * 830 414 1046 * 1 842 1 NI 4-Methylheptsne . 4-uethyloctane . 4Methylnonane . - 3-Ethylpentane . I-Ethylhexme .. 3-Ethylheptane . 1167 1164 1166 1166 1166 1161 Infra-red intensity. Ramanintensity . 1042 1004 832 467 1039 1007 832 447 1043 1011 821 NI 1042 - 823 434 1060 1006 836 NI 1047 - 836 467 IH R IR R IR R IR R IR R IR R IR R IR R IR R IR R IR R IR R -_ - __- 1166 1162 1165 1168 1166 1166 1166 1166 1166 t 1166 4- No observations ,-+ I I I I I I I 1 . 1146 1046 1006 860 NI 1164 1 1047 1 - 1 848 I 433 Refs. 13 14 13 14 13 14 13 14 13 6 - - 13 14 13 14 13 - 13 14 13 14 2 14 2 14 API Nos. 6011 611 602 611 603 612 606 612 609 616 604 613 607 613 610 616 667 671 660 671 For the meaning of symbols and abbreviations see foot of Table 1. In the Raman spectra of p a r a f f i with an internal tertiary carbon atom lines are observed for most molecules near 1360 cm.-l (not listed) ; these can be assigned to the CH deformation modes.Table 8 shows that a nearly constant frequency (1156 cm.-l) appears consistently in the Raman and infra-red spectra of these molecules (col. 3). This series may be readily SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART 11 49 identified and forms the most useful means of recognising the presence of an internal tertiary carbon atom. The infra-red bands are prominent (though they show some evidence of splitting) and the corresponding Raman lines are the only ones that occur in this region. There is little doubt that this series is analogous to that occurring near 1170 cm.-l in the isopropyl paraffins. The assignment is thus dependent on the final interpretation of the 1170 cm.-l frequency in the isobutane spectra. It should be noted however that the persistence of this series even when no CH groups are directly attached to the tertiary carbon atom does seem to suggest that the relevant vibration must be predominantly skeletal in character.The frequencies shown in col. 4 near 1040 cm.-l are found in the Raman and infra-red spectra of most of the molecules listed and are only slightly affected by changes in structure. This series seems analogous to that already discussed for the paraffins having an internal quaternary carbon atom (Section 6) and presumably may also be attributed to CH rocking modes not involving the C structural unit. The infia-red bands near 1010 cm.-l (col. 5) are found in all molecules having an ethyl group in agreement with the interpretation previously given. Examination of the 850-700 cm.-l region of the Raman spectra shows that paraffins with a single internal tertiary carbon atom have at least one nearly constant frequency (col.6) that can be assigned to the sym- metrical stretching vibration of the C structural unit analogous to those already described for the isopropyl compounds. The corresponding infra- red bands are insignificant. There is evidence especially in the 3-ethyl paraffins of a second Raman series at a somewhat lower frequency (770-750 cm.-l) which can be attributed to the same vibration in a second type of isomer. Analysis in the range 770-740 cm.-l is however complicated by the presence of the CH rocking modes of the ethyl and n-propyl groups which are also present. Col. 7 shows that a series (450415 em.-l) attributable to skeletal deformation modes can be dis- tinguished in the Raman spectra ; it is presumably analogous to the series already noted in the isopropyl paraffins near 435 cm.-l.The actual fre- quencies depend on whether the molecules are 3-methyl 4-methyl or 3-ethyl paraffins but are fairly constant for a particular class of molecules. Neither of these Raman series is very helpful for identifying the presence of an internal tertiary carbon atom because in both cases a number of other lines are present in the same frequency region. Besides the series shown in Table 8 which are common to all these molecules other regularities have been separately noted in the spectra of the 3-methyl 4-methyl and 3-ethyl paraffins ; these are given in Table 10. It must however be emphasised that correlations based on a limited number of molecules are sometimes misleading and are found to be invalid when more experimental data become available.The frequencies near 1130 cm.-l of the 3-ethyl paraffins are remarkably constant. The 1080 cm.-l infra-red frequencies of the 4-methyl paraffins are presumably analogous to those already discussed for molecules with an internal quaternary carbon atom D 50 QUARTERLY REVIEWS (Section 6). The various series observed in the region 970-870 cm.-l can be correlated with those found near 955 cm.-l in the Raman spectra and 920 cm.-l in the infra-red spectra of paraffins with an isopropyl group. The weak bands near 550 cm.-l in the spectra of the 3- and 4-methyl paraffins can probably be correlated with similar series found at slightly lower frequencies for paraffins with an isopropyl group.Data are also available for some twelve molecules having an internal tertiary carbon atom together with a tert.-butyl group or an internal quaternary carbon atom. The prominent features which can be attributed to the internal tertiary carbon atom can also be recognised in the spectra of these more complex molecules. ( b ) Paraffins with Two Adjacent Tertiary Carbon Atoms.-Data are avail- able for a fair number of paraffins with two adjacent tertiary carbon atoms Le. a C structural unit C-C-C-C; this may be terminal (as in the 2 3-dimethyl parafhs) or in a more limited number of cases internaL5 It is desirable to discuss whether any of the characteristic frequencies already noted for molectiles with a single tertiary carbon atom persist in these more complex paraffins.Accordingly Table 9 lists t,he relevant data on the liquid spectra of some molecules with this C structural unit but does not include any of the more complicated examples. With regard to the interpretation of the frequencies dealt with in the following discussion it should be emphasised that in these rather complex paraffins significant disturbing effects due to the presence of mixtures of rotational isomers are to be expected as in the case of 2 3-dimethylbutane already noted.61 Col. 3 gives EL series of Raman lines near 1160 cm.-l which usually have no infra-red counterparts; in paraffins with an isopropyl group the cor- responding frequencies always appear in both types of spectra. Col. 4 on the other hand shows a series of prominent infra-red bands of nearly constant frequency (near 1125 crn.-l) ; no corresponding Raman lines can be recognised ; these bands are absent in other paraffins having a tertiary carbon atom.These two series taken together provide the most reliable guide for the identification of this C structural unit. The spectra of paraffins with this C structural unit are difficult to analyse in the range 1100-900 cm.-l. Three or four Raman lines occur sporadically and the infra-red spectra show a large number of small bands. On the whole absorption is weaker if the C unit is terminal. Most mole- cules show a band near 1070 cm.-l (col. 5) ; but this feature is not usually prominent (compare Table 10). Some molecules also have somewhat vari- able frequencies near 1040 cm.-l in both Raman and infra-red spectra. These series are evidently analogous to those discussed for other paraffins with a tertiary carbon atom and can probably also be assigned to CH rocking modes.The absorption band near 1010 cm.-l shows up regularly for molecules with an ethyl group. Below this frequency there are indica- tions of a variable series near 960 cm.-l and tt more constant series of H H I I I I c c SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART I1 51 infra-red bands near 916 cm.-l (col. 6). Possibly they are analogous to those found near 955 and 920 cm.-l for paraffins with an isopropyl group. None of the series in the range 1100-900 cm.-l is very helpful in identifying this C structural unit. Col. 7 shows that it is possible to distinguish prominent polarised Raman lines (770-740 cm.-l) which by analogy with the paraffins previously discussed may be assigned to symmetrical skeletal vibrations.The fre- TABLE 9. Characteristic frequencies (in cm.-l) of parafins with two adjacent tertiary carbon atoms in the liquid state * Paraffin vcc or CH rocking 2 3-Dimethylbutane IR R 2 3-Dimethylpentam IR R 2 3-Dimethylhexane IR R 2 3-Dimethylhept'ane ' IR R 2 3-Dimethyloctane IR R 3 4-Dimethylhexane IR R 3 4-Dimethylheptane IR R 4 5-Dimethyloctane I IR R Infra-red intensity . Raman intensity. . - 1129 1066 921 1165 - - - 1166 1122 1075 918 1166 - - 915 - 1127 1075 919 1163 - - - - 1126 1080 918 1159 - - 926 - 1128 1080 915 4- No observations 1160 1166 - 1166 - t - W m (3) 1122 1071 - l i j l i 3 920 - - 1124 1077 910 - o observations st m m abs. abs. usually abs. vC-C - 754 740 747 756 765 7 60 760 770 736 738 765 751 - 480 477 464 459 457 NI 469 N I -+ - 467 47 1 NI NI - I - Refs API Nos ' 13 14 13 14 13 14 2 16 2 - 13 14 2 14 2 - 656 670 658 672 661 673 664 673 *For t,he meaning of symbols and abbreviations see foot of Table 1.quencies are moderately constant ; the corresponding infia-red bands are weak. This series gives a valuable indication of the presence of the C structural unit. It is also possible to pick out a fairly reliable series of Raman lines (col. 8) which lie in the region of the spectrum in which the skeletal deformation modes are found (480460 cm.-l). Smith ti has found evidence for a series of weak bands near 550 cm.-l similar to those mentioned for other molecules with a single tertiary carbon atom (pp. 41,47). Some eight other parafis with adjacent tertiary carbon atoms have been examined.It is satisfactory to find that all the frequencies listed 52 QUARTERLY REVIEWS in Table 9 may be found in their spectra ; those near 1125 and 755 remain prominent and so seem to provide a reliable methocl of identifying this C structural unit. (c) The Ethyl Group.-Many of the paraffins discussed in the preceding sections possess one or more ethyl groups and two characteristic frequencies have already been noted that can definitely be attributed to this radical &. an infra-red band near 1010 cm.-l which may possibly be assigned to a carbon-carbon stretching vibration and one near 770 em. -l attributable to a CH rocking mode. In this connection it is noteworthy tha,t Kohl- rausch,llO working some years ago on the Raman spectra of other types of molecule was able to recognise a Raman line near 1000 cm.-l as char- acteristic of the ethyl group.No definite evidence is available to suggest that the other CH deformation modes appear at frequencies differing from those discussed on p. 23 ; nor can any series (other than the 1010 cm.-l bands) be found which might be assigned to the two CH rocking modes of the ethyl group. Presumably they occur in the complicated range 1150-850 cm.-l; either they are not sufficiently prominent to be recog- nised or they coincide with the frequencies of other radicals. The deforma- tion skeletal mode of the CH,*CH,*C group must occur below 500 cm.-l but cannot be identified among the many Raman lines that most paraffins show in this region. 8. Conclusion We conclude with some general remarks on the preceding sections.First it must be emphasised that the larger the number of molecules of a particular class the more reliable are the characteristic frequencies. Secondly where there is evidence for disturbing effects due to the presence of mixtures of rotational isomers the interpretation of the characteristic frequencies must be considered tentative. In this connection there is no doubt that investigations of the solid-state spectra would prove as illuminat- ing for other molecules as they have been for the n-paraffins. Thirdly examination of the spectra of paraffins which have two or more structural units of the types described in the earlier sections suggests that in many cases each unit retains at least some of its characteristic frequencies. This is particularly apparent for the tert.-butyl group but in other cases some exceptions have been noted,5 and accidental coincidences in frequencies become more probable the more complex the molecule.Fourthly it must be admitted that the use of characteristic frequencies to recognise certain groupings is to some extent a matter of experience. This is particularly true of the region below 500 cm.-l in the R,aman spectra and for infra-red bands generally. The absorption spectra of the larger paraffins between 1350 and 800 cm.-l tend to degenerate into a somewhat puzzling area of bands of moderate intensity with complicated splitting patterns and only after comparison with those of smaller molecules of the same type do the regularities become apparent. Finally it is desirable to indicate those regions of the spectra which 110 Op.cit. 1943 p. 228. SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS. PART I1 53 TABLE 10. Characteristic frequencies (in cm.-l) of parafins in the liquid state (The frequencies of less well establishsd series are shown in parentheses. Fre- quencies marked with an asterisk are particularly prominent.) I Frequency range. crn.-l * 1470-1440 . * 1397 1370 . * 1385 1370 . * 1384 1365 . * 1380 . . . *1360-1330 . 1370 . . . 1355 . . . 1340 . . . * 1310-13UO . * 1260-1250 . * 1220-1200 . * 1215-1210 . * 1200-1190 . * 1175-1165 . * 1165-1150 . * 1160-1150 . * 1150-1140 . "-1140 . . * 1130-1120 . (1130) . . 1120-965 . (1100-1080) 1080-1065 . (- 1080). . * - 1070 . . (-1040). . * 1020-1000 . (- 960) . . 1045-1035 . * 960-950. . (950) . . . Infra-red intensity strong strong strong strong strong o bscurod medium medium medium medium strong strong strong strong strong weak m.strong strong medium strong active mediiim active medium active not promin. weak active strong active medium active Raman intensity strong absent absent absent' absent prominent absent absent absent prominent medium medium} E%-} I I medium medium medium medium prominent absent absent wea.k active absent absent prominent medium active medium active medium absent Classes of paraffin and comments CH asymm. 6 CH bending tert. -butyl { CH isolated CH symm. 6 $ ~ ~ ~ ~ ~ e t h y l CH deformabion n-paraffins hydrogen deformation CH wagging or twisting tert.-butyl internal quaternary ca.rho~~ atom isoprop yl adjacent tertiary carbon at>oms internal tertiary carbon atom isopropyl n -p araffins adjacent tertiary carbon atoms 3-ethyl ri -parafins internal quaternary carbon atom adjacent tertiary carbon atoms &methyl n-paraffins internal tertiary carbon atom int$errial quaternary and adja- cent tertiary carbon atoms :thy1 3 -methyl and adjacent tertiary kopropyl t-methyl carbon atoms Table 2 2 '1 Y 4 4 7 7 6 9 8 6 2 I) 3 54 QUARTERLY REVIEWS TABLE 10 (cont.) Frequency range cm.- (-930) . . * 930-925 . 920-915. . "-920 . (920-910) . (900-890) . (- 890) . . (- 880) . . * 870-840 . * 840-810 I * N 890 850-815. . * 835-795. . (770-750) . 770-740. *"770 . . * 750-710. . * 750-685. * -740 . . *-720 . . (- 550) . . (660-530) - 490-480. . 480-460. . 465-430. . 450-415. . *425-150. . 370-255. . 360-280. (350-305) . Infra-red intensity active medium m.strong medium active ao tive active active weak weak medium obscured weak s t,rong weak weak strong strong weak weak weak weak NI NI NI NI NI NI Raman intensity active medium medium absent active active active active prominent polarised medium prominent prominent medium absent prominent polarised prominent polarised absent absent weak weak medium medium prom ineri t polarised medium polarised medium polarised weak polarised Classes of paraffin and comments internal quaternary carbon atom tert. -butyl isopropyl adjacent tertiary carbon atoms internal quaternary carbon atom 4-methyl 3 -ethyl 3-methyl n-paraffins v c ~ internal tertiary carbon atom isopropyl vGG adjacent tertiary carbon atoms V C ~ CH rocking ethyl tert . - butyl vC4 internal quaternary carbon CH rocking n-propyl CH rocking 5 carbon chain w-c 3-ethy1 V C ~ atom vcx 3-methy1 4-methyl adjacent isoprop yl tertiary carbon atoms internal quaternary carbon adjacent tertiary carbon atom SC+ atoms sc-c ~opropyl 6c-c internal tertiary carbon atom n-paraffins Sc-c isopropyl Sc-c tert.-butyl &-( internal quaternary carbon atom 6~ Table 4 6 9 SHEPPARD AND SIMPSON SPECTRA OF HYDROCARBONS.PART I1 55 are most profitable for the purposes of structural identification. Table 10 gathers together all the characteristic frequencies for the liquid paraffins which have been listed or discussed in the preceding sections and includes also the hydrogen deformation frequencies common to all types of molecule. Examination of this table suggests that the infra-red bands in the range 1260-1100 cm.-l and the polarised Raman lines between 900 and 650 cm.-l are the features likely to be most valuable for this purpose since they are prominent and on the whole show little variation in frequency. In the intermediate region the series are less satisfactory since they often lack one or both of these desirable characteristics and some are common to several classes of paraffin. These series and other weaker frequencies are not in themselves sufficient to characterise the various structural groupings.

ISSN:0009-2681    

DOI:10.1039/QR9530700019

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

ENZYMIC SYNTHESIS OF POLYSACCHARIDES Rp S. A. BARKER B.Sc. PH.D. and E. J. BOURNE B.Sc. D.Sc. (CHEMISTRY DEPARTMENT THE UNIVERSITY BIRMINGHAM) Introduction SINCE the natural processes which lead to the synthesis of polysaccharides are frequently reversible under physiological conditions most of the enzymes concerned can under suitable circumstances degrade polysaccharides to simpler substances. In any comprehensive review of these enzymes it would be necessary to consider both their synthetic and their degradative functions as well as their own physical and chemical properties. Such a review could not be condensed adequately into an article of this type and so we shall lay emphasis on the mechanisms by which polysaccharides are synthesised rather than on the enzymes responsible ; further we shall consider only the conversion of saccharides into larger molecules wit'hout showing how the simpler sugars themselves arise since this aspect was reviewed recently by Avison and Hawkins Our aim will be to outline the present state of knowledge on the synthesis of each poly- saccharide in turn and then to show how a master pattern of synthesis is emerging in the field as a whole.One fundamental equation will be encountered frequently viz. Gt-0-X + H-0-Gr + Gt-O-Gr + X-0-H (1 ) where GiO and X are respectively the sugar residue and the aglycone portion (i.e. the substituent a t the reducing position) of a glycosicle (G,-0-X) which serves as the substrate for an enzyme and G,-0-H is a carbohydrate receptor molecule the products being a higher saccharide (Gt-O-Gr) and a hydroxy-compound (X-0-H).The reader will see how each step in the synthesis of a higher saccharide always involves the transfer of the group G from OX to OG,. in this series. Synthesis of a-Glucosans of the Starch Class Most natural starches contain two macromolecular components amylose and amylopectin with the former constituting some 20-30% of the whole ; a few starches such as those derived from waxy maize and waxy sorghum are exceptional inasmuch as they are practically devoid of a,mylose while in others such as wrinkled pea starch amylose is the principal constituent. Amylose (I) is a polyglucose in which the sugar residues are joined by 1 4 a-linkages to form chains several hundred units in length ; there is little or no branching of the chains. I n amylopectin (11) short chains of the amylose type averaging about 20 glucose units in length are joined a t branch points principally by 1 6-a-linkages each molecule containing more; than a thousand glucose units altogether.Quart. Reviews 1951 5 171. 56 BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 57 Amylose from Glucose-1 Phosphate.-It is appropriate that the phos- phorylase-catalysed synthesis of amylose from dipotassium glucose- 1 phos- phate should be our first consideration because this was the first enzymic s-ynthesis of a polysaccharide in vitro to be established conclusively. In 1937 Cori Colowick a,nd Cori showed that a salt of a-glucopyranose-1 (dihydrogen phosphate) was formed when a! solution of glycogen inorganic OH OH n f Redu-ing end-group i s at A (11) Meyer’s struetare for ttmylopectin phosphate and adenylic acid was incubated with a dialysed muscle extract.Subsequent investigations by the same authors and by Cori Schmidt and Cori 3 using muscle phosphorylase by Schiiffner a,nd Specht 4 and by Kiessling with yeast phosphorylase by Ostern Herbert and Holmes 6 with liver phosphorylase and by Hanes ’ with phosphorylases from peas and potatoes soon established that the reaction was reversible ; it can be represented in the following overall equation nC,H,,O,*O.PO,Kz + (C&C,,O,) + nKZHPO . (ii) Phosphorylase is now known to be very widespread in Nature ; in addition to the above sources it has been found for example in waxy maize,8 barley,g 1 4-U-glucosan 2 J . Biol. Chem. 1937 121 465 ; 1938 123 375 381. Science 1939 89 464. Naturwiss. 1938 26 494; 1939 27 195.Ibid. 1939 27 129; Biochem. Z. 1939 302 50. Nature 1939 144 34 ; Biochem. J. 1939 33 1858. Nuture 1940 145 348 ; Proc. Roy. SOC. 1940 B 128 421 ; 129 174. Porter Biochem. J . 1949 45 xxxvii. *Bliss and Naylor Cereal Chem. 1946 23 177. 58 QUARTERLY REVIEWS .Lima beans,1° jack beans,ll broad beans,12 sugar beet,13 and in the micro- organisms Neisseria perflava l4 and PoZyytomelh coeccr;. l5 At equilibrium the ratio of total inorganic phosphate to total glucose-1 phosphate is dependent on the pH value of the system but the ratio of the bivalent ions [HPO,]- -/[C,H,,O,*O*PO,]- - is independent of pH and is always constant a t 2.2.'" Thus the conversion of an unbranched 1 44- glucosan into glucose-1 phosphate can be carried to virtual completion if the polysaccharide is treated with phosphorylase in the presence of a sufficiently large excess of inorganic phosphate to ensure that the equilibrium ratio of the bivalent ions is not attained before all the polysaccharide is degraded.16-ls On the other hand only about 3 5 4 0 % of am-ylopectin can be phosphorylated in this way because phosphorylase which acts by removing successive glucose units a t non-reducing chain ends cannot break or by-pass the 1 6-linkages which constitute the branch points ; the action of the enzyme ceases when the outer chains of the main branches of the polysaccharide have been shortened to 3-6 glucose ~nits.l'-~l It is inter- esting that arsenate can replace phosphate in these degradations and that the glucose-1 arsenate so formed is immediately hydrolysed to gIucose.Because of this instability of the arsenate ester no arsenate-glucose-1 arsenate equilibrium analogous to that found in the phosphate case can be established and so the arsenolytic reaction results in the complete degradation of unbranched polysaccharides containing only 1 4-a-linkagesY even when only traces of arsenate are synthesised an amylosaccharide in vitro from glucose-1 phosphate by the agency of potato phosphorylase he recognised that the product differed from natural potato starch inasmuch as it was less soluble in water was stained more deeply blue by iodine and gave a higher yield of maltose (95-100~o compared with m.60%) when treated with ,&amylase; in fact the synthetic poly- saccharide showed a close resemblance to the " amyloamylose " (amylose) component of potato starch prepared by t'he early fractionation method of Samec and Ma~er.~4 Likewise Cori Schmidt and Cori had observed that 2 2 23 Nature of the Synthetic PoZysacchari&.-When Hanes 10Green and Stumpf J .Biol. Chem. 1942 142 355. 11 Sumner Somers and Sisler ibid. 1944 152 479. l2 Hobson Whelan and Peat J . 1950 3566. 1 3 Kursanov and Pavlinova Riokhim. 1948 13 378. 14 Hehre Hamilton and Carlson J . Biol. Chem. 1949 177 267. l5Lw0ff Ionesco and Gutmann Biochim. Biophys. Acta 1950 4 270. 16 Swanson J . Biol. Cl~em. 1948 172 805 825. 17Bourne Sit,ch and Peat J. 1949 1448. IsHestrin J . Biol. Chem. 1949 179 943. l9 Meyer and Bernfeld Helv. Chim. Acta 1942 25 399 404. ZOKatz Hassid and Doudoroff Nature 1948 161 96. 21Cori and Larner J . Biol. Chem. 1951 188 17. "2at.z and Hassid Arch.Biochem. 1951 30 272. 25 Meyer Weil and Fischer HeZv. Chim. Acta 1952 35 247. 24 KolloiWm. Beih. 1921 18 2'42. ' * For a recent detailed study of hhis equilibrium see Trevelyan BIann a i d Harrison Arch. Biochem. 1952 39 419 440. BARKER AND BOURNE ENZYMIC SYNTHESIS O F POLYSACCHARIDES 59 whereas the natural amylosaccharide of the animal body is glycogen (a polyglucose similar in structure to amylopectin but more highly branched) which is stained red-brown by iodine muscle phosphorylase synthesises i n vitro a polysaccharide giving an intense blue stain. Subsequently poly- saccharides synthesised in vitro by phosphorylases derived from a variety of sources were submitted to methylation and end-group a s ~ a y ~ ~ - ~ to determinations of molecular 29 to colorimetric assays when stained with iodine,17 309 31 to potentiometric titrations with iodine,32 and to /3-amyIolysi~.~9 27 3 3 5 34 These methods chosen because they distinguish clearly between amylose on the one hand and amylopectin and glycogen on the other proved beyond doubt tlhat the synthetic product was always an unbranched polyglucose of the amylose type.Thus it can be seen that phosphorylase is specific for both the synthesis and phosphorolysis of 1 4-a-glucosidic linkages and cannot be solely responsible for the formation of amylopectin and glycogen ; as will be shown later a supplementary enzyme is necessary in the Synthesis of each of these branched polysac- charides. Conditions and Mechanism of the Synthetic Reaction.40 far as is known a t present a-D-glucose-1 phosphate is the only substrate on which phos- phorylase can display its synthetic function; the enzyme has -no action on the P - a n ~ m e r ~ ~ or on the l-phosphates of tc-~-glucose,~~ a-~-galactose,~? 37 K- D -mannose 9 37 a- D - xylose 37 a-malt ose 37 or a-D -glucuronic acid.37a Hanes observed that there was an induction period when potato phos- phorylase was incubated with glucose-1 phosphate which had been prepared from starch by phosphorolysis; the addition of a little starch abolished this lag phase. A similar observation had been made by Cori and Cori 38 using muscle phosphorylase; the effect was more marked with the more highly purified enzyme samples. By using chemically synthesised glucose- 1 phosphate Green and Stumpf lo were able to extend indefinitely the lag phase shown by specially purified potato phosphorylase but synthesis could again be initiated by the introduction of starch or dextrins derived from starch.Thus it became apparent that a “primer” is necessary for the synthesis of amylose but tlhat unless special precautions are taken in the 26Hassid and McCready J . Amer. Chem. ~Soc. 1941 83 2171. 2 6 Haworth Heath and Peat J. 1942 56. 28Barker Bourne and Wilkinson J . 1950 3027. ?9Haworth Heath and Peat unpublished result mentioned in J . 1945 877. 30Hassid and McCready J . Amer. Chem. SOC. 1943 65 1154 1157. 31 Bear and Cori J . Biol. Chem. 1941 140 111. 3 2 Bates French and Rundle J . Amer. ClhPm. Xoc. 1943 65 142. 33Bourne and Peat J . 1946 877. 3 4 Barker Bourne Peat’ and Wilkinson J . 1950 3022. 3 5 Wolfrom Smith Pletcher and Brown J . Amer. Ghem. ~Soc.1942 64 23 ; Wolfrom Smith and Brown ibid. 1943 a 256. 36 Potter Sowden Hassid and Doudoroff ibid. 1948 70 1761. 37Meagher and Hassid ibid. 1946 $8 2136. 37a Barker Bourne Fleetwood and Stacey unpublished results. 38 J . Biol. Chein. 1039 131 397. Hassid Cori and McCready J . Biol. Cham. 1943 148 89. 60 QUARTERLY REVIEWS purification of the enzyme and of the phosphate ester there is usually sufficient primer present as an impurity to initiate the synthesis. It is now known that the primer must be a glucose " polymer ', with 1 4-a-links ; for example g l u ~ o s e ~ ~ lo fr~ctose,~ s ~ c r o s e ~ ~ lo and dextran 17 18 a9 do not function in this way. A more precise definition of the essential structural features of the primer cannot be given in a general statement because different phosphorylases have different requirements as can be seen by considering the phosphorylases of the potato jack bean and muscle.In the case of potato phosphorylase the molecular size of the primer is not critical because although poly- saccharide synthesis is not promoted by maltose,l09 l7 the higher linear homologues of maltose containing three four five or six glucose units are effective,39 40 as also are starch amylose and arnylope~tin.~~ lo l7 Com- parison of the relative efficiencies of 1 4-a-glucosans as primers for the potato enzyme has shown that there are a t least two controlling factors. First priming power is related to the number of non-reducing end groups available ; this explains (a) why amylopectin (5% of end groups) is more effective than amylose (< @5y0 of end groups),17 ( b ) why in the early stages of the acidic hydrolysis of amylose and amylopectin there is a rapid increase in priming 41 4 2 and (c) why the cyclic Schardinger dextrins which contain 6-8 glucose units linked by 1 4-a-bonds7 are devoid of priming activity.10 43 Since oxidation of the terminal aldehydic grouping has little effect on the priming ability of a short unbranched dextrin of the amylose type the presence of a reducing end group cannot be a factor contributing to the priming properties.18 Secondly the effects of acidic hydrolysis and of @-amylolysis on the ability of amylose to function as a primer for potato phosphorylase cannot be explained simply on the increased availability of non-reducing terminal glucose units and it seems bhat there is a certain length of chain a t which priming activity reaches an optimum ; this chain length is probably about 20 glucose units.l7 Muscle phosphorylase resembles potato phosphorylase inasmuch as it displays its synthetic activity only in the presence of a primer containing non-reducing terminal glucose units ; i t is probably for this reason that glycogen (9% of end groups) is a much more efficient primer for the muscle enzyme than is amylose (< 0.5% of end groups).279 389 44-46 On the other hand muscle phosphorylase requires these end groups to be supplied as part of a macromolecule as is shown by fwo facts (a) that it is not primed by higher homologues of maltose con- taining fewer than eight glucose and ( b ) that the priming power of glycogen for the enzyme is rapidly destroyed when the polysaccharide is treated mildly with acid in spite of the fact that such a treatment increases 39 Weibull and Tiselius Arkiv Kemi Min.Geol. 1945 19 A No. 19. 40Bailey Whelan and Peat J. 1950 3692. 41 Hidy and Day J. Biol. Chem. 1944 152 477 ; 1945 160 273. 49 Swanson and Cori ibid. 1948 172 815. 43 Proehl and Day ibid. 1946 163 667. 4 4 Cori and Cori Ann. Rev. Biochem. 1941 10 152. 45 Cori Cori and Green J. Biot. Chem. 1943 151 39. 46 Cori Swanson and Cori Fed. Proc. 1945 4 234. BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 61 the number of non-reducing terminal glucose units.42 46 Jack- bean phos- phorylase differs in its primer requirements from both the muscle and potato enzymes for it is primed more efficiently by amylose than by amylopectin. 47 So far as is known a t present all phosphoryla,ses catalysing the conversion of glucose-1 phosphate into amylose require the presence of a 1 4-a-glucosan primer containing non-reducing chain ends but differ as regards the most suitable molecular size for t'he primer.It is not' surprising that there should be minor differences of this sort because it is well established that the enzymes themselves are not identical chemically as can be seen from the following three examples. First muscle phosphorylase can be obtained readily in crystalline whereas potato phosphorylase has so far not been crystallised in spite of attempts by many workers to do so ; recently + CH2.OH L 0 ' O H OH (iii) a much improved method for the purification of potato phosphorylase has been devised 49 so that the chances of crystallising the enzyme have im- proved.Secondly the phosphorylases of muscle 48 and adipose tissue 50 require adenylic acid before they display their full activity whereas those of the potato lo and the jack bean 47 do not. Thirdly glucose is a com- petitive inhibitor in the case of synthesis by muscle phosphorylase 45 but not by jack-bean pho~phorylase.~~ The mechanism now generally accepted for the synthesis of amylose from salts of glucose-1 phosphate is that advanced by the Cori's and their s c h o 0 1 ~ ~ - ~ ~ largely on the basis of the part played by non-reducing end groups in the priming reactions mentioned above. Each step in the synthesis is pictured as shown above. It will be seen that this equilibrium is a special case of the general equation (i) ; one molecule of glucose-1 phosphate ((2,-0-X) reacts with 4i Sumner Chou and Bever Arch.Biochenz. 1950 26 1. 48Green Cori and Cori J. Biol. Chern. 1942 142 447. 49 Gilbert and Patrick Biochem. J . 1952 51 186. Creasey and Gray tbid. 1951 50 74. 62 QUARTERLY REVLEWS a 1 4-a-glucosan receptor molecule (H-0-G,) to. form a glucosan con- taining one additional glucose residue (G,-0-G,) together with mineral phosphate (X-0-H). Thus the function of primers is to serve as receptors for glucose residues which become attached step-wise a t the non-reducing ends ; they are not true catalysts but enter stoicheiometrically into the reaction. This mechanism explains why the average chain length of the synthetic amylose is dependent on the ratio of terminal receptor sites to glucose-1 phosphate molecules converted ; a high proportion of the ester phosphate yields a long-chain polymer and a small proportion gives a short- chain product.11 16 51 It follows that phosphorylase catalyses the sin2uZ- taneous lengthening of all pre-formed chains in the receptor molecules and does not lengthen one chain t o its full extent before dealing with the remaining chain~.~G 51 52 Before turning to other enzyme systems capable of synthesising amylosac- charides it is interesting to note the truly fantastic speed at which phos- phorylase performs its highly specific task ; Cori Cori and Green 45 have calculated that a mole of enzyme transforms 4 x lo4 moles of glucose-1 phosphate per minute under optimum conditions I Amylose from Maltose.-Monod and Torriani 53-55 have described the synthesis of an iodophilic polysaccharide from maltose by means of a cell- free extract of Escherichia coli (Monod strain ML).They have given the name " amylomaltase " to the enzyme responsible and have shown that it catalyses the following reversible overall reaction n Maltose + (Glucose) + n Glucose . (iv) It is an adaptive enzyme inasmuch as it is produced only when the organism is grown on maltose and not for example on glucose or lactose ; 54 it shows a high measure of substrate specificity being without action on methyl 01- or P-D-glucoside cellobiose lactose sucrose melibiose or glucose- 1 phos- phate. 6 3 In the forward reaction equilibrium is normally established when 60% of the maltose has been converted and a t this stage the polymeric product gives a faint red stain with iodine suggesting that the average chain length is about ten glucose If however the synthesis is conducted in the presence of notatin (glucose oxidase) the conditions of equilibrium can never be established and the conversion of maltose proceeds to ~ompletion.~~ 55 The polyglucose thus obtained is probably amylose since it gives a blue stain with iodine,53 but a full structural analysis has not yet been made.The reverse reaction proceeds when the synthetic polysaccharide is incubated with amylomaltase in the presence of glucose as is shown by the diminished intensity of the blue iodine stain given by the digest and by the appearance of a second reducing sugar (maltose ?) ; 53 56 in the absence of 61 Bailey and Whelm Biochem. J. 1952 51 xxxiii. 6 3 Bourne and Whelm hlature 1950 166 268. 53Monod and Torriani Compt.rend. 1948 227 240; 64Monod Biochem. SOC. Symposia 1950 No. 4 51. 6 6 Monod and Torriani Ann. Inst. Pasteur 1950 78 66. 66 Doudoroff Hassid Putman Potter and Lederberg J. BWZ. Chem. 1949,178 921. 1949 %3% 718. BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACC~H~RIDES 63 glucose the enzyme does not attack the polysaccharide a fact which distinguishes it from the amylases. 53 Indications have been obtained that D-xylose and D-mannose but not D-fructose D-galactose D-arabinose or 1,-arabinose can replace D-glucose in this reversal of the synthesis to yield analogues of maltose.56 Verification of this would suggest that enzyme specificity towards this particular component in the reaction is determined by the presence or absence of the structure (111). Additional evidence that amylomaltase occurs in E.coli which has been grown on maltose has been obtained from studies of the extra-cellular saccharides formed when washed resting cells of the organism are incubated with 1-1 maltose in the presence of iodoacetate and in the absence of n ~ t a t i n . ~ ~ 57 Using a mutant of E. coli (strain W-327) Hassid and his co-workers 5 6 obtained glucose un- changed maltose and a series of dextrins containing 4-6 glucose units per molecule; the presence of 1 4-a-linkages in the dextrins was strongly indicated by their susceptibility to P-amylolysis. In a similar experiment with Monod's strain (ML) of E. coli Barker and Bourne 57 fractionated the saccharides on a charcoal column and proved by both chemical and biochemical methods that they consisted of glucose un- changed maltose and higher homologues of maltose (3-5 glucose units in length).Of the glucose residues present initially in the maltose approxi- mately 29% appeared as free glucose 24% as unchanged maltose and 35% as higher saccharides. Although there are still several problems connected with amylomaltase- catalysed reactions which merit further study it seems probable 53-57 that each step in the synthesis of amylose entails the transfer of a C,H,,O unit [Gt; see equation (i)] from maltose (G,-O-X) to an amylosaccharide molecule (H--O-Gr) with the elimination of a molecule of glucose (X-0-H) as follows Maltose + Maltose + Maltotriose + Glucose Maltose + (Glucose) + (Glucose)n+l + Glucose It has not yet been established whether the glucose unit (Gt) which is transferred by amylomaltase to the receptor molecule must be furnished as maltose ; it is possible for example that equation (v) is really a speoial case and that each step in the reaction could be written in t'he more general form (where x > 1) (Glucose) + (Glucose), + (Glucose),+l + (Gluc0se)~-1 .(vi) Furthermore it is possible that more than one glucose residue can be trans- ferred at any one time. Indeed the fact that in the presence ofnotatin a very considerable increase (> 4-fold) in the chain length of the product results although the conversion of maltose is increased only from 60% t o H (m) } - (v) (first step) (later step) 67Barker and Bourne J . 1962 209. 64 QUARTERLY REVIEWS loo% may be attributable to this. Alternatively i t may be due to a greater affinity of the enzyme for longer chains.Linear Amylosaccharides from Cyclic Amy1osaccharides.-In 1905 Schardinger 58 showed that during the cultivation of B. macerans on starch non-reducing crystalline saccharides (Schardinger dextrins) are produced. It is now known that such dextrins are cycloamyloses i.e. that each dextrin molecule contains a loop of glucose units mutually linked by 1 4-a-bonds; the a- p- and y-Schardinger dextrins contain respectively six seven and eight glucose units per molecule. 59 The extra-cellular enzyme responsible for the formation of these dextrins was first isolated by Tilden and Hudson,6o and has now been obtained in an electrophoretically pure form.61 6 2 The dextrins probably arise mainly from the ainylose component of the starch substrate and from the outer chains of amyl~pectin.~~ 64 The early classification of the enzyme as an amylase was unfortunate because as Cori pointed its action is not hydrolytic since it involves the exchange of a glucosidic linkage in a polysaccharide chain for a similar one in a cyclic dextrin.The small AP which would accompany such an exchange led to the belief that the reaction should be readily reversible as indeed has been demonstrated in the following case (Glu = a glucose unit) 64 66 Glu-GI 11 / Glu \ \ Glu +Glu-Glu + Glu-[Glu],-Glu ( +Homologues) (VH) / \ / Glu- Glu In analogous reactions the maltose component can be replaced by glucose methyl a-D-glucoside sucrose cellobiose or maltobionic acid.66 More recently Norberg and French 67 have shown t’hat the activity of the Bacillus mcerans enzyme is not limited to reactions involving Schardinger dextrins but that such reactions really represent one aspect of a more general reaction.They found that the enzyme catalysed a redistfibution of the glucose residues in linear amylosaccharides ; from maltose for example they obtained a series of oligosaccharides thus . (vin) ZGlu + Glu + Glu GIu + Qlu + Glu + Glu . . . etc} ’ Overall reaction nGlu + zGlu + yGlu + zGlu + higher amylosaccharides 58 Zentr. Bakt. 11 1905 14 772 ; 1909 22 98. 59 Freudenberg and Cramer Ber. 1950 83 296. 6 * J. Amer. Chem. SOC. 1939 61 2900. 61Schwimmer and Garibaldi Cereal Chem. 1952 29 108. 3 2 Schwimmer Fed. PTOC. 1952 11 283. 6 s Wdson Schoch and Hudson J . Amer. Chem. Soc. 1943 65 1380. “Myrback and Willstaedt Acta Chem. Scand. 1949 3 91. 6s Fed. Proc.1945 4 226. 6 6 French Pazur Levine and Norberg J . Amer. Clzem. Xoc. 1948 70 3145. 67 Ibid. 1960 72 1202 1746. BARKER AXD BOURNE EXZYMIC SYNTHESIS OF POLYSACCHARIDES 85 A similar redistribution occurred with amyloheptaose the aynthesis of cyolic dextrins being more apparent of course in this case,67 The individua.1 reactions in the series proceed a t different rates as can be seen from two facts (a) amyloheptaose is converted much more readily than is maltose and ( b ) the a- 18- and y-Schardinger dextrins are formed a t different speeds.67 The formation of cyclic dextrins is facilitated by the natural tendency for a chain of 1 4-a-glucose units to assume a helical configuration and also because a 1 4-cc-linkage in such a cyclic structure is slightly more stable than is a similar bond in a linear dextrin.68 Although amylomaltase and the B.rnacerans enzyme differ inasmuch as it has been reported 5 3 7 63 that the latter but not the former degrades starch in the absence of glucose they show remarkable similarities in their actions on the lower amylosaccharides los [compare equations (v) and (viii)]. A closer comparison between the enzymes would make an interesting study; two problems which might thus be solved are ( a ) whether cyclic dextrins occur in the products of the amylomaltase-catalysed conversion of linear dextrins and ( b ) whether the B. macerans enzyme can synthesise . a polysaccharide of the amylose type from maltose in the presence of notatin. Amylopectin from Amy1ose.-Several mechanisms for the synthesis of unbranched 1 4-a-glucosans have now been discussed and we must con- sider next how amylopectin the branched component of starch might arise.In 1944 Haworth Peat and Bourne gg announced the isolation from potato juice of a,n enzyme fraction which synthesised a polysaccharide giving a reddish-purple iodine stain from glucose-1 phosphate in the presence but not in the absence of potato phosphorylase. The active principle of this fraction termed Q-enzyme was obtained later in a purer state by an improved method of isolation.70 Gilbert and Patrick 71 subsequently crystallised Q-enzyme after a carefully investigated purification procedure involving precipitation with ethanol at low temperature from solutions of low ionic strength. The nature of the polysaccharide synthesised by the joint action of these two enzymes of the potato is dependent on the relative activities of the enzymes.34 When a high proportion of Q-enzyme is employed the product is indistinguishable from natural potato amylopectin in its iodine staining properties (blue value ca.0.12) in the rate and extent (ca. 55%) of its conversion into maltose by @-amylase in its ability to prime the synthesis of amylose from glucose-1 phosphate (see p. SO) and in its average chain length (ca. 20 glucose units) as determined by methylation and end-group assay ; 289 3 3 1 3 4 9 69 it does however have a somewhat smaller molecular weight than the native poly~accharide,~~ but this is not surprising in view of the vastly different conditions attending their formation. Hydro- lysis of the methylated polysaccharide as of tri-0-methylamylopectin affords 2 3 4 6-tetra-0-methylglucose (from the non-reducing terminal units) “Myrback Arkiv Kemi Min.Geol. 1949 1 161. 6e Nature 1944 154 236. ‘OBarker Bourne and Peat J. 1949 1706. 71 Nature 1950 165 673 878; Biochem. J . 1962 61 181. E 66 QUARTERLY REVIEWS 2 3 6-tri-O-methylglucose (from units within the chains) and 2 3-di-0- methylglucose (from the branch points) ; thus the principal glucosidic linkages involve positions 1 and 4 while the branch linkages are of the 1 6 - t y ~ e . ~ ~ When the synthesis from glucose-1 phosphate is catalysed by mixtures of phosphorylase and &-enzyme containing higher proportions of the former enzyme the properties of the resulting polysaccharides are intermediate between those of amylose and amylopectin. 28 34 The mechanism of potato &-enzyme action has been determined from studies of its effect on amylose and starch.The product obtained from either of these substrates cannot be differentiated except as regards mole- cular size,7 from natural amylopectin by rigorous chemical and enzymic tests similar to those described a b ~ v e . ~ O - ~ ~ In contrast to t'he a- or P-amylolysis of amylosaccharides this ainylose -+ amylopectin conversion entails the liberation of little or no reducing sugar (< 2% expressed as maltose) ; 70-74 it is not a phosphorolysis since it proceeds equally well in the absence and presence of large proportions of inorganic phosphate,77 provided that the Q-enzyme is already fully activated by the addition of salts such as sodium acetate and ammonium chloride to the 78 Thus it seems that Q-enzyme is a transglucosidase operating by a non- phosphorolytic mechanism which converts about one in every twenty 1 4-cc-linkages of amylose into the 1 6-a-linkages which constitute the branch points of amylopectin [cf.equation (i)].75 7 7 isu It follows that the synthesis of amylopectin from glucose-1 phosphate by the joint action of phosphorylase and &-enzyme is a two-stage process consisting of (1) the phosphorylase-catalysed synthesis of amylose-type molecules from the Cori ester and (2) the conversion of these unbranched chains into amylopectin by 75 77 79 Although the &-enzyme samples used in the above studies were all obtained from the potato it is probable that the enzyme is quite widespread in Nature ; indeed similar &-enzyme samples have been obtained already from the wrinkled pea,12 the broad bean,12 green gram,sO Neisseria perflla~a,~* and Polytomella ~ a ? c a .~ ~ 8 2 It is probable that each branch point in the amylopectin molecule is establishe'd according to the following mechanism in which the arrows signify chains of 1 4-a-glucopyranose units the reducing groups being indicated by the arrow-heads and the branch points being of the 1 &type :83 5 2 Nussenbtlum and Hassid J . Riol. Chem. 1951 190 673. i 3 Bourne Macey and Peat' J. 1945 882. i 4 Peat Bourne and Barker hTatwe 1948 161 127. 7 6 Idem J. 1949 1712. 7 6 Cori and Illingworth J. Biol. Chem. 1951 190 679. 7 7 Barker Bourne Wilkinson and Peat J. 1950 93. 78 Gilbert and Swallow J. 1949 2849. 78a Hestrin Brewers' Digest 1948 23 1. 79Hobson Whelm and Peat J.1951 596. soRam and Giri Arch. Biochem. 1952 38 231. 81 Bebbington Bourne Stacey and Wilkinson J. 1952 240. 8 2 Bebbington Bourne and Wilkinson J. 1962 246. 83 Barker Bebbington Bourne and Stacey unpublished results. BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 67 .-+ + &-enzyme + (B) &-enzyme + -* (4 (B) Q-enzyme + -> (D) + A(D) +- &-enzyme . (ix) i Studies with potato Q-enzyme have shown that the amylose-type substrate (A) must contain a t least 42 glucos units before it is attacked by the enzyme.84 8 5 The initial attack probably involves fission of a 1 4-link in the substrate with the formation of an amylosaccharide (B)-enzyme com- plex and a dextrin fragment (C). The complex could then react with a second amylosaccharide molecule (D) to give the branched product (BD) together with the free enzyme.The receptor molecule (D) might be for example an intact amylQse molecule the residual dextrin (C) or a branched product formed in an earlier stage of the reaction. Evidence that the molecular size of (D) is not important a t least in the case of the Q-enzyme of PoZytonaeZh c a m was obtained recently,s6 when it was shown that the initial rate of conversion of amylose by dilute solutions of the enzyme as measured by the fall in the blue value of the substrate was markedly increased by the introduction of different amylopectins glycogen amylo- dextrins or commercial maltose but not by the cyclic Schardinger dextrins or by carbohydrates devoid of the 1 4-a-glucosidic linkage such as glucose galactose fructose cellobiose lactose sucrose dextran inulin and xylan.The function of these primers is presumably to increase greatly in the early stages of the amylose conversion the number of chains available as receptors of type (D). In the absence of such primers the reaction of the protozoal enzyme is autocatalytic since the conversion of amylose into amylopectin itself increases the number of receptor chains.86 Although alternative explanations of these phenomena could be advanced the above mechanism falls into line with polysaccharide syntheses in general. The question of the reversibility of Q-enzyme action was examined by Barker Bourne Wilkinson and Peat,77 who were unable to find any con- clusive evidence that the enzyme can break the 1 6-a-linkages of amylo- pectin or B-dextrin. It is clear that, under the experimental conditions so far employed the equilibrium favours strongly the synthesis rather than the fission of the branch points.In fact in this respect Q-enzyme seems to be complementary to the R-enzyme of beans and potatoes which can break but not synthesise the 1 6-a-linkages of amylopectin and related molecules ; 8 7 88 other 1 6-amyloglucosidases occur in muscle and in yeast.19 89-91 8 4 Bailey Peat and Whelan Biochem. J . 1952 51 xxxiv. s5Nussenbaum and Hassid J . BioZ. Chem. 1952 196 785. 8 6 Barker Bebbington and Bourne Nature 1951 168 834. Hobson Whelan and Peat Biochem. J . 1950 47 xxxix. 881dem J. 1951 1451. ssCori and Lamer Fed. Proc. 1950 9 163; J . BioE. Chem. 1951 188 17. 90Maruo and Kobayashi J . Agric. Chem. Xoc. Japan 1949 23 115 120. 91 Petrova Biokhinz.1948 18 244; 1951 16 482. 68 QUARTERLY REVIEWS In 1949 Beckmann and Roger 9 2 3 93 showed that some of the character- istics of the Q-enzyme-catalysed conversion of amylose could be simulated by the addition of a fatty acid to the polysaccharide ; they concluded that Q-enzyme was an artefact and that the “amylopectin ” produced by its agency was really an amylose-fatty acid complex. This conclusion did not take account of methylation data presented four years earlier by Bourne and Peat,33 and is a t variance with much of the later work from the same school. Moreover a method recommended by Beckmann and Roger themsel~es,~~ for distinguishing between amylopectin and amylose-fatty acid complexes has revealed very close similarity between natural amylopectins on the one hand and our amylose conversion products (with potato or PoZytomeZEa cceca Q-enzyme) on the other.81 82 This method entails measurement of the spectra (2500-8000 A) of iodine-stained solutions of the polysaccharides.Finally Nussenbaum and Hassid 7 2 have shown the synthetic amylopectin to be devoid of fatty acid and Cori and Illingworth 7 6 have confirmed by means of a specific 1 6-amyloglucosidase that 1 &branch points are indeed present and that the average chain length is 20 glucose units. It has been claimed by Bernfeld and MeutkmAdian 9 4 9 95 that amylo- pectin is produced from glucose-1 phosphate by the joint action of phos- phorylase and an isophosphorylase. This isophosphorylase was believed to synthesise 1 6-or-glucosidic linkages from the Cori ester in a manner similar to that by which phosphorylase establishes 1 4-or-links.However this claim can no longer be entertained because (a) the mechanism of synthesis advanced by Bernfeld and Meutbmkdian is at variance with certain well- established principles of phosphorylase action,77 (13) the experimental data can be interpreted quite adequately without having to postulate the existence of an isophosph~rylase,~~ and (c) neither Nussenbaum a i d Hassid 7 2 nor Meyer 97 could repeat the preparation of isophosphorylase. Glycogen from Glucose-1 Phosphate.-In view of the close relation between the structures of amylopectin and glycogen it is not surprising that they should be synthesised by similar enzymic processes. The syn- thesis of glycogen from glucose-1 phosphate again requires two enzymes phosphorylase to establish the 1 4-or-glucosidic bonds and a Supplementary enzyme (‘r branching factor ”) to form the branch points.46? 50 98-101 This supplementary enzyme analogous to the &-enzyme of the plant kingdom has been isolated from several animal organs such as the heart,46 98 the brai11,~8 the l i ~ e r ~ 6 983 993 lol and adipose loo It was first reported by Cori and Cori 98 in 1943 the year before &-enzyme was first described.sg Although the supplementary enzyme itself cannot utilise glucose- 1 phos- 9 2 Abstr.Amer. Chem. SOC. Meeting New York 1949 36c. O 8 J. Biol. Chem. 1951 190 467. Q 4 Nature 1946 162 297 616. 95 Nelv. Chim. Acta 1948 31 1724 1735. Y G Bailey and Whelan J . 1950 3873. 97 Personal communication. Q9 Hesixin Brewers’ Digest 1948 23 1. loOCreasey and Gray Biochem.J . 1950 46 ix. lol Lamer Fed. R o c . 1952 11 245. sf! Cori and Cori J . Biol. Chem. 1943 151 67. BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 69 phate as a substrate the liberation of mineral phosphate from the Cori ester by the joint action of muscle phosphorylase and the supplementary enzyme is autocatalytic and is much faster than in the ca.se of the phosphorylase alone.461 5% 98 This autocatalytic effect which is shown also by mixtures of phosphorylase and 9 lo2 a.nd of phosphorylase and ~t-amylase,~~ was attributed by Cori and Cori g8 to the fact that the supplementary enzyme by continually increasing the number of non-reducing chain ends provides more “ primers ” for the phosphorylase (see p. 60). An early observation that the supplementary enzyme is without action on amylose-type polysaccharides 98 (thus apparently differing from Q-enzyme in this most important respect) should now be re-examined because Larner lol has shown that the enzyme establishes by a non-phosphorolytic transglyco- sidase mechanism branch points in the outer chains of amylopectin to give a product closely resembling glycogen in its iodine stain.Furthermore an artificial polysaccharide prepared from glycogen by lengthening the outer chains with 14C-labelled glucose units (by means of phosphorylase and 14C-labelled glucose- 1 phosphate) was treated with the supplementary enzyme and was then found to possess radioactivity a t the new branch points.lo1 It may be that the failure of the earlier enzyme to attack amylose was due t o an insufficiency of receptor chains (see p.67). I n an independent series of researches Petrova 9l9 lo3 1°4 has studied a non-phosphorolytic enzyme fraction from rabbit muscle termed by her amylose isomerase ” which shows a very close resemblance to the supple- mentary enzyme of the Cori school inasmuch as it catalyses jointly with phosphorylase the synthesis of a glycogen-like product from glucose- 1 phos- phate. However the isomerase seems to function also in the reverse Bense as a 1 6-ol-glucosidase a property not apparently possessed by the supple- mentary enzyme. A more detailed experimental comparison between these two enzymes would be useful. An Amylopectin-type Polyslacchazide from Sucrose.-In 1946 Hehre and Hamilton lo5 3 lo6 reported that washed cells of Neisseria per-uva synthesise a polyglucose (resembling amylopectin in its behaviour towards iodine the amylases and phosphoryla,se) from sucrose but not from maltose lactose trehalose melibiose raffinose melezitose or methyl a-D-ghcoside or from a mixture of glucose and fructose.With glucose-l phosphate a trace of an iodophilic polysaccharide was produced but this synthesis which was attributed to phosphorylase was suppressed by the addition of excess of mineral phosphate whereas that from sucrose was unimpaired. The same authors lo6 found that 39 strains of Neisseria per-ava all behaved similarly. From one of these strains (19-34) Hehre Hamilton and Carlson l4 isolated a cell-free enzyme termed amylosucrase which catalysed the conversion n Sucrose + (Glucose) + n Fructose . . (x‘ < < lo2Barker Bourne Wilkinson and Peat J.1950 84. 103Petrova Biokhim. 1949 14 155 ; 1952 17 129. 104Petrovs and Rozenfeld ibid. 1960 15 309. lo6 J . Bwl. Chm. 1946 160 777. lo6 J. Bact. 1948 55 197. 70 QUARTERLY REVIEWS The amylosucrase was distinguished from the bacterial phosphorylase by its stability to heat and to gas treatment and by the fact that the synthesis was not suppressed by phosphate. The synthetic polysaccharide which was virtually free from fructose was shown to be a member of the amylopectin-glycogen class by a- and P-amylolysis by phosphorolysis by its iodine stain by potentiometric titration with iodine by its failure to give an insoluble butanol complex and by negative serological tests for dextran. Through the kindness of Dr. Hehre we were able to examine a polysaccharide produced by another strain (11-1) of Neisseria perflava and to confirm his conclusions regarding the structure methylation and end-group assay proved that chains of 1 4-a-glucopyranose units averaging 11-12 units in length were joined by branches of the 1 6-type.Since the synthetic polysaccharide possessed a branched structure it seemed probable that the amylosucrase was con- taminated with a second enzyme which was responsible for the synthesis of the branch points and this was verified when it was shown that the enzyme sample exhibited &-enzyme activity inasmuch as it converted amylose into a glycogen-type polysaccharide without the appearance of reducing sugar.14 Thus it was deduced that the function of amylosucrase itself is to convert sucrose into an unbranched polysaccharide of the amylose class by a glucose-transferring mechanism involving the exchange of the biose linkage for a 1 4-cc-glucosidic bond.14 Although there is no direct evidence that amylosaccharide primers play an integral part in the reaction as they do in the phosphorylase-catalysed synthesis of amylose it is known that sucrose is not attacked by amylosucrase in the presence of cc-amylase.Because the polysaccharide synthesis is strongly exothermic a high con- version (ca. 98%) results and the reverse reaction is difficult to demonstrate. Hehre and Hamilton log have however been able to show that a poly- saccharide with the serological properties of dextran is formed in small yield (= 1 yo of sucrose) when a mixture of starch (or glycogen) and fructose is treated with amylosucrase and dextran sucrase [the latter enzyme converts sucrose into dextran (see below)].that amylosucrase may play a part in the synthesis of an amylopectin-type polyglucose by cells of Clostridium butyricum but acceptance of this hypothesis must await the results of experiments with cell-free extracts. It has been suggested l1O1 Synthesis of a-Glucosans of the Dextran Class Dextran from Sucrose.-Dextrans are polyglucoses in which the majority They are syn- of the bonds linking the sugar units are of the 1 6-a-type. lo7Barker Bourne and Stacey J. 1950 2884. 108 Hehre Adv. Enzymology 1951 11 297. 10°Hehre and Hamilton J . Biol. Chem. 1951 192 161. 1lONasr and Baker Nature 1949 164 745. 111 Hobson and Nasr J. 1951 1856. BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 71 thesised from sucrose but not from glucose by growing cultures of such micro-organisms as Leuconostoc mesenteroides Leuconostoc dextrunicum and Betabacterium vermiforme'.They show quite large variations in molecular structure ; for example the dextran produced by Leuconostoc dextrunicum is an essentially unbranched polysaccharide having an average chain length of 200-550 glucose units whereas those from other organisms frequently possess a high degree of branching (average chain lengths 5-30 ~ n i t s ) . ~ ~ ~ - l The branch points are usually of the 1 4-type but it has been shown recently that in some cases 1 3-linkages are invo1ved.ll8 In 1941 Hehre 119 described the isolation from cultures of Leuconostoc mesenteroides of a heat-labile cell-free extract which synthesised from sucrose a polysaccharide indistinguishable from a dextran by certain chemical and serological tests.He postulated that the synthesis catalysed by dextran sucrase proceeded according t o the equation n Sucrose -j (Glucose) -1- n Fructose . . (xi) In later work by the same school improved methods for the isolation of the enzyme were developed and the optimum conditions for tbe synthesis were determined.120-123 The enzyme was obtained free from " invertase " sucrose phosphorylase and levan sucrase.123 It was shown that the above equation was obeyed stoicheiometrically and that only 0-1-1.2y0 of sucrose remained when equilibrium was reached ; in the reverse reaction no sucrose formation could be detected.123 More recently Forsyth and Webley 124 125 have confirmed that dextran synthesis is overwhelmingly favoured a t equilibrium (albeit with a final sucrose concentration of 8%) and have found also that traces of glucose are produced (the glucose was believed to arise from a hydrolytic process).Although all of the work described above was conducted with enzyme samples obtained from strains of Leuconostoc mesen- teroides other organisms,120 such as lactobacilli group H streptococci and Xtreptococcus salivarius are known to secrete dextran sucrase but the enzymes from these sources have not yet been examined rigorously. Since it has been shown that sugar phosphates are not formed as inter- mediates in the sucrose -+ dextran conversion,los and since the sucrose 112 Peat' Schluchterer and Stacey J. 1939 581. 113Daker and Stacey J. 1939 585. 114 Hassid mid Barker J. Bid. Chem.1940 134 163. 116 Stacey and Swift J. 1948 1555. I l i Jeanes and Wilham J . Amer. Chem. SOC. 1950 72 2655. 118 Barker Bourne Bruce and Stacey Chem. I n d . 1952 1156 ; Abdel-Akher Hamilton Montgomery and Smith J . -4mer. Chem. SOC. 1952 74 4970; Lohmar ibid. p. 4974. - 115 Levi Hawkins and Hibbert, J . Amer. C'hcm. Soc. 1942 $4 1959. 119 Hehre Scieizce 1941 93 237. 120Hehre and Sugg .7. Exp. Med. 1912 '75 339. 1 2 1 Sugg and Hehre J . Immunol. 1942 43 119. 122Hehre Proc. SOC. Exp. Biol. N.Y. 1943 54 18. 123Idem J. Biol. Chem. 1946 163 221. 1 2 4 Nature 1948 162 150. J. CT'en. Microbiol. 1950 4 87. 72 QUARTERLY REVIEWS substrate cannot be replaced by a mixture of glucose and fructose,11D it seems probable that each step in the synthesis of the 1 6-a-linked poly- glucose chain must involve the exchange of the glucosidic link in sucrose for one in the polysaccharide as follows Sucrose + Enzyme Glucose -I-enzyme + - - Glucose - I - enzyme 3- Fructose -+ Enzyme There is no experimental proof that a receptor molecule (primer) is required to initiate the reaction but this may be due to the fact that the enzyme has never been obtained free from associated dextran.llg l 2 O 1z3 Alterna- tively sucrose itself may serve as the primer in which case terminal fructo- furanose units should be present in the synthetic polysaccharide.The above scheme would lead of course to the formation of an unbranched dextran (a fact which has not yet been demonstrated experimentally) so that the problem of the mechanism by which the branches are established awaits solution ; it may well be that the branched polysaccharide is formed directly from the unbranched one by means of a second enzyme as is the case with amylosaccharides .In a recent paper,126 Stodola et al. reported that a new reducing disac- charide leucrose [5-O-(~-glucopyranosyl)-~-fructopyranose] is formed in about 3% yield during the synthesis of dextran by dextran sucrase isolated from Leuconostoc mesenteroides and they postulated that this sugar " plays a role in the polymerisation process ". This conclusion at variance with the above mechanism which hitherto was widely accepted would if sub- stantiated throw grave doubts also on current theories regarding the synthesis of other polysaccharides from sucrose. Further studies of the problem are imperative ; they may show that the new disaccharide arises from a side-reaction in equation (xii) involving fructopyranose liberated in an earlier stage of the synthesis namely Glucose-1 -enzyme + Fructopyranose $ Leucrose + Enzyme (xiii) Dextran from Amy1odextrins.-In an investigation of the phenomenon of '' ropiness " in beer a problem which had been studied at intervals for at least 50 years Shimwell,12' in 1947 demonstrated that cultures of Aceto- bacter viscosum and Acetubacter cupsubturn isolated from suoh beer con- verted amylodextrins into highly visoous products ; these produots were 126 Stodola Koepsell and Sharpe J .Amer. Chern. SOC. 1952 74 3202. la' J . Inst. Brew. 1947 53 280. BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 73 shown later by Hehre and Hamilton 128 to possess serological properties like those of certain dextrans.The organisms did not elaborate the slime when grown on glucose fructose sucrose or ma1t0se.l~’ Hehre and Hamilton 12* obtained cell-free extracts of Acetobacter cupsuZutum which converted amylo- dextrins into a similar viscous material and this product was studied in greater detail. log It was an amylase-resistant polyglucose which did not stain with iodine and was classified as a dextran on the basis of its serological properties its stability towards acid and its behaviour towards periodate ; the ratio of 1 6-linkages to other glucosidic linkages (as revealed by the periodate oxidation) was ca. 5 1. The application of methylation tech- niques to a sample of this polysaccharide kindly supplied by Dr. Hehre has confirmed that the principal glucosidic bonds are of the 1 6-type and has shown also that the molecules are branched and that the branches involve mainly positions 1 and 4.129 The enzyme responsible for the synthesis dextrin-dextranase cannot utilise inter alia maltose sucrose raffinose or glucose-1 phosphate ; nor are amylose amylopectin and glycogen or the higher dextrins which result therefrom by ,b’-amylolysis suitable as substrates.Indeed the enzyme seems to require open-chain dextrins containing roughly 4-10 glucose units such as are formed during the acidic hydrolysis or a-amylolysis of poly- saccharides of the starch type.lo9 From a study of t,he action of the enzyme on a purified sample of one of these dextrins amyloheptaose Hehre and Hamilton log concluded that the reaction involved the transfer of a glucose unit (in 1 4-a-linkage) from a non-reducing terminal position in an amylo- dextrin molecule to a corresponding position (in 1 6-a-linkage) in the growing dextran molecule as follows (Glu = glucose unit) Glu 1-4 Glu 1-4 Glu l....+ Glu 1-6 Glu 1 . * . * Glu 1-4 Glu 1-,.. -t Glu 1-6 Glu 1-6 G l ~ i l.... 11 . (xiv) As partial confirmation of this mechanism cycloamyloheptaose was shown to be unattacked. They deemed further study desirable before it could be decided whether or not dextran-type molecules are necessary to initiate the reaction. 1 6-or-Linked Glucosaccharides from Maltose.-Following observa- tions l30 131 that the hydrolysis of starch by fungal amylases leads to the production of non-fermentable carbohydrates Pan Andreasen and Kolachov 1322 133 found that a cell-free extract of Aspergillus niger (NRRL 337) converted maltose but not glucose into an unfermentable triose (panose) which was later obtained crystalline,134 and which was proved by 12* Proc.SOC. Exp. Biol. iV. Y. 1949 71 336. 129 Barker Bourne Bruce and Stacey unpublished resulta. 130 Stark J . Biol. Chem. 1942 142 569. 1 3 1 Pigman J . Res. hTa,t. Bur. ,Stand. 1944 33 105. 1 3 2 Science 1960 112 115. la3Arch. Biochem. 1951 30 6. 134 Pan Nicholson and Kolachov J . Amer. Chem.. SOC. 1951 73 2547. GIu 1-4 Glu + E + Glu-E + Glu Glu-E -L GZU 1-4 Glu + Glu 1-6 Glu 1-4 Glu + E Glu-E + Glu 1-6 Glu 1-4 Glu + GIu 1-6 Glu 1-6 Glu 1-4 Glu 4- E etc. Glu-E + Glu + Glu 1-6 Glu + E Glu-E + Glu 1-6 Glu + Glu 1-6 Glu 1-6 Glu + E Glu-E + Glu 1-6 Glu 1-6 Glu + Glu 1-6 Glu 1-6 GIu 1-6 Glu + E etc.> h7) BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 75 such a synthesis may shortly be achieved. Wallenfels and Bernt 13* lPO have claimed that a galactose-transferring enzyme present in Aspergillus oryxz catalyses the following transformations of lactose [cf equation (i)] . (xvi) Gal 1-4 Glu + E + Gal-E + Glu Gal-E + Lactose + Gal-Lactose + E . I n similar studies with lactases derived from Xaccharonyces fragilis and E . coli Aronson lPoa has confirmed these observations and has found that accompanying reactions are I 1 . Gal-E + H,O -+ Gal + E (not lactose) . . (xvii) Gal-E 1 G l ~ i $ Gal-Glu + E Gal-E + Gal + Gal-Gal + E As would be expected from these equations the first product is galactosyl- lactose since lactose is the only receptor molecule present in significant amount in the early stages of the synthesis.140u In the presence of large amounts of glucose lactose is transformed principally into an isomeric galactosyl-glucose.It seems that the transgalactosidation reaction involves a competition between water and receptor sugar molecules for the galactose- enzyme complex.~40a The structures of the oligosaccharides have yet to be determined. It is probable that if glucose were continuously removed from the system higher saccharides and possibly even polygnlactans would result. Indeed Caputto and Trucco 141 have obtained from the mammary glands of rats and also of cows a series of oligogalactans containing glucose ; since lactose was detected in hydrolysates of these saccharides the glucose residues must have been attached through C,,,.Synthesis of Fructosans of the Levan Class It has been known for a t least 60 years that certain micro-organisms are able to synthesise levans (i.e. polyfructofuranoses in which the principal glycosidic linkages are of the 2 &type) from sucrose. Before 1936 there were several reports that the synthesis had been effected with cell-free enzyme preparations obtained from a culture filtrate of Bacillus mesentericus ~ u l g a t u s l ~ ~ from ruptured cells of the same 0rganisrn,l4~ from spore residues of Aspergillus sydowi,lP4 from a sterile filtrate of Bacillus s ~ b t i l i s ~ ~ ~ and from Oerskov's milk baci1l~s.l~~ The fact that no clear picture of t,he mechanism of the synthesis had emerged by this time can be attributed to inadequate characterisation of the products in some of the case3 cited and to conflicting evidence concerning the nature of t'he polysaccharide precursor 140 Angew.Chem. 1952 64 28. 140a Arch. Biochern. Biophp. 1952 39 370. 141 Nature 1952 169 1061. 142 Beijerinck J . SOC. Chein. I d . 1910 29 710. 1 4 3 Owen J . I n d . Eng. Chem. 1911 3 481. 144 Kopeloff Kopeloff and Welcome J . Biol. Ghem. 1980 43 171 178. 146 Harrison Terr and Hibbert Canad. J . Res. 1930 3 449. 146 Dienes J . Infect. Dis. 1935 57 12 22. 76 QUARTERLY REVIEWS which was believed by some workers to be sucrose itself and by others to be '' nascent " fructose (fructofuranose). Principally as a result of series of investigations by Hestrin and his ~ o l l e a ~ g u e s ~ ~ ~ - 1 ~ ~ it is now generally agreed that each step in the synthesis of levan catalysed by levan sucrase involves the following fructose transfer [cf.equation (i)] Sucrose + (Fructose) = (Fructose),+l + Glucose . . (xviii) Aschner Avineri-Shapiro and Hestrin 1 4 ' 9 14* first isolated the enzyme from B. subtiZis by selective diffusion through an agar gel; they showed that it was an adaptive exocellular enzyme i.e. that it was present only when t,he bacillus was grown on a sucrose medium. An alternative method of isolation more convenient for large-scale work was based on autolysed cells of Aerobacter Zevanicum and yielded an active freeze-dried powder ; in this case the enzyme was constitutive and endocellular.14* This enzyme produced levan from sucrose and raffinose but not from invert sugar maltose lactose trehalose inulin methyl aB-D-fructofuranoside glucose- 1 phosphate fructose-6 phosphate or fructose-1 6 dipho~phate.l4~~ l 6 0 In the case o raffinose the synthesis conformed with the equation (xix 1.* - n(Ga1 1-6 Glu 1-2 Fruf) (Fruif)n + n(Ga1 1-6 Glu) (= Melibiose) Since the levan sucrase was still active when free froni phosphate and the above phosphate esters were not substrates it was deemed highly improbable that it could function by a phosphorolytic mechanism.150 There was strong inhibition of levan synthesis from sucrose by D-glucose (competitive) D-galactose D-xylose L-arabinose maltose and lactose but not by D-man- nose D-fructose or D-glucosamine ; it was concluded that the configuration a t C, of a reducing sugar was the major factor in determining its inhibitory powers.150 Some free fructose was always liberated in the synthesis from sucrose and raffinose possibly owing to the presence of a hydrolase con- taminant or possibly because water may function as the receptor of the fructose unit in (xviii).l509 152 More recently levan sucrase preparations possessing properties very similar to those described above have been obtained from Streptococcus salivarius and the spore-forming bacillus N9.1539 154 The lzvorotatory polyfructoses which were synthesised from sucrose by these preparations were classified as levans on the basis of serological 154 An inter- esting discovery made when similar tests were applied to polysaccharides produced by a variety of streptococci was that certain strains of group H It 14' Aschner Avineri-Shapiro and Hestrin Nature 1942 149 627.148 Idem Biochem. J. 1943 37 450.149 Hestrin and Avinori-Shapiro Nature 1943 152 49. 15* Idem Biochem. J . 1944 38 2. lS1 Hestrin Nature 1044 154 581. 1 5 3 Avineri-Shapiro and Hestrin Biochem. J . 1945 39 167. ls3 Hehre Proc. Soc. Exp. Biol. N.Y. 1946 58 219. 1 6 4 H e h r e Genghof and Neill J . Immunol. 1946 51 5 . BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 77 streptococci are able to synthesise dextran and levan simultaneously ; this versatility is displayed also by Leuconostoc rnesenteroides NRRL B-512. lS6 In further studies of the levan sucrase of B. subtilis Kohanyi and Dedonder 166a have shown by paper chromatography that oligosaccharides are present at intermediate stages in the synthesis while Doudoroff and O'Neal 15' have confirmed earlier observations 149t 15* that the reversibility of equation (xviii) cannot be demonstrated by treating a solution containing levan and glucose with the enzyme because at equilibrium the forward reaction is highly favoured.The reverse reaction does proceed however if the equilibrium is disturbed by yeast invertase which hydrolyses the sucrose as it is formed.157 Two important aspects of the enzymic synthesis require further study. First a comprehensive investigation by chemical methods of the structure of the polysaccharide product is desirable so that it can be shown whether the principal glycosidic linkages do in fact involve positions 2 and 6 and whether the molecules are unbranched as they should be if levan sucras~ is a single enzyme. Secondly it has not yet been possible to demonstrate that primer molecules (i.e. receptors of the transferred fructose units) are neces- sary to initiate the synthesis.It may well be that traces of levan in the enzyme fulfil this function (a possibility which was considered unlikely by Hestrin and Avineri-Shapiro,lS0 since an enzyme prepared from cells grown on glucose was still active without the addition of levan) or alternatively that the substrate sucrose is itself a primer. Support for the latter hypo- thesis is to be found in the recent work of Palmer,158 who has shown that the levan of B. subtilis contains a trace of glucose which is most probably a part of the levan molecule and not of an associated impurity. Synthesis of Fructosans of the Inulin Class Although the enzymic synthesis of inulin has not yet been achieved in vitro recent observations by several groups of workers seem t o herald an early accomplishment of this aim.In a reinvestigation of the structure of the inulin of dahlia tubers Hirst McGilvray and Percival 159 confirmed earlier reports of the presence of glucose residues (m. 6%). Hydrolysis of the trimethyl ether of the polysaccharide yielded 1 3 4 6-tetra-o-methyl- and 3 4 6-tri-O-methyl-fructofuranose (3.2 and 91yo) together with 2 3 4 6-tetra-O-methylglucopyranose (2.2%) and a mixture of tri-0- methylglucoses (3.2%). The high proportion of tetra- to tri-O-methyl- glucose suggested that the glucose residues were an integral part of the inulin molecule and did not arise from an associated polygluoosan. Since no di-0-methyl sugars were encountered a branched structure was excluded. On these grounds it was concluded 159 that a possible structure for the inulin molecule is one in which a chain of m.35 fructofuranose units (linked through positions 1 and 2) is joined through the potential reducing group 166Hehre and Neill J . Ezp. Med. 1946 83 147. ls6 Jeanes Wilham and Miers J . Biol. Chem. 1948 176 603. 16GaCompt. rend. 1951 233 1142. 158 Bwchem. J. 1961 48 389. J67 J . Biol. Chm. 1945 159 585. lSg-J. 1950 1297. 78 QUARTERLY REVIEWS (by a sucrose-type linkage) to glucose there being a second glucose residue (linked through positions 1 and 3) at some undetermined position within the fructose chain as follows Fru 2-41 Fru 21,-3 Glu 1-[l Fru 2]35-2-1 Glu It is possible of course that the tri-O-methyl- but not the tetra-o-methyl- glucose arose from an associated polyglucosan in which case the non- terminal glucose residue would be omitted.A similar structure has been suggested by Bacon and Edclman,lG0 after a study of the oligosaccharides present in extracts of tubers roots and stems of the Jerusalem artichoke (an alternative source of inulin). They found by paper chromatography a series of oligosaccharides each containing both glucose and fructose com- ponents ; sucrose was the lowest member and the others contained progres- sively higher fructose glucose ratios. A related (possibly identical) series of oligosaccharides can be extracted from barley leaves. 161 Having observed that artichoke tubers contain an enzyme (or enzymes) capable of producing the trisaccharide of the above series from mixtures of sucrose and inulin but to a markedly smaller degree from either substrate alone Bacon and Edelman 162 examined closely the course of the " hydro- lysis " of sucrose to glucose and fructose catalysed by yeast invertase.The same problem was studied simultaneously by Blanchard and Albon. 163 It was found that a t least three saccharides with R values less than that of sucrose were formed during the initial stages of the reaction and disappeared later ; all of these saccharides were shown by chromatographic procedures to contain both glucose and fructose units. 162 When separated and purified the triose fraction possessed two fructose residues and one of glucose ; since it was devoid of reducing properties it was believed to be a fructosyl- sucr0se.1~3 The oligosaccharides were produced with different concentra- tions of sucrose up to 55% and at any pH a t which the invertase was active ; l 6 3 the reaction was not modified by inorganic phosphate.162 It seems that yeast invertase functions by transferring fructose residues from sucrose to any carbohydrate receptor molecule present in the reaction mixture and also to water,164 probably via an intermediate fructose-2- enzyme c0rnplex,~~5 thus [cf.equation (i)] ( a ) Fru-Glu + E - Fru-E + Glu ( b ) Fru-E + Fru-Glu + Fru-Fru-Glu + E (c) Fru-E + Fru-Fru-Glu + Fru-Fru-Fru-Glu + E etc. * (xx) ( d ) Fru-E + H,O Such a mechanism explains the appearance of oligosaccharides during the early stages of the reaction and attributes their subsequent degradation to the non-reversibility of reaction Id). It would be interesting t o know whether a continuous removal of glucose ( e . g .by oxidation with notatin) 1 2 + Fru + E slow 160 Biochena. J. 1951 48 114. 161 Porter and Edelman ibid. 1952 50 xxxiii. lo2 Arch. Biochem. 1950 28 467. 163 Ibid. 1950 29 220. 164Bealing and Bacon Biochem. J. 1951 49 lxxv. 165Fischer KohtGs and Fellig Helv. Chim. Actcx 1951 84 1132. BARKER AND BOURNE ENZYMIC! SYNTHESIS OF POLYSACCHARIDES 79 would lead to the synthesis of products of higher molecular weight. A claim by Aronoff 1~ that the earlier workers were wrong in attributing oligo- saccharide-synthesising activity to the invertase rather than to an enzymic contaminant has been disputed by White.ls7 Recent observations by White and Secor 168 suggest that equation (xx) may be an over-simplification because these authors found a second disaccharide a second trisaccharide and another oligosaccharide on chromatograms of the products of yeast invertase action on sucrose.The disaccharide (R < sucrose) was a reducing sugar containing a fructose and a glucose unit but was not turanose (3-0-a- glucopyranosylfruct opyranose) . Both trisaccharide components contained two fructose rcsidues and one of glucose. These observations seem to parallel those of Stodola et aZ.lz6 on dexfran sucrase (p. 72). I n addition to the above studies on yeast invertase it has been shown that the invertase of Aspergillus o r y m has a similar ability to transfer fructose residues.13'~ 164 169 By the time that 80% of the sucrose has dis- appeared only 7.5% of the fructose units and 42.5% of the glucose units are present as the free sugars the remainder being in the form of non-reducing oligosaccharides which however differ chromtographically from those pro- duced with yeast in~ertase.16~ On prolonged incubation the oligosaccharides are destroyed giving only glucose and f r ~ c t 0 s e .l ~ ~ In conformity with equation (xx) the rate of oligosaccharide synthesis is reduced in the presence of glucose but that of fructose formation is n0t.164 The mould enzyme appears to differ from yeast invertase also in its ability to utilize raffinose as a substrate for the synthesis of higher saccharides.162p 169 Pazur 169 has reported that it disproportionates raffinose into a tetrasaccharide (fructosyl- raffinose) of unknown structure and the disaccharide melibiose as follows [cf. equation (i)] Gal 1-6 Glu 1-2 Fru + Gal 1-6 Glu 1-2 Fru I t } . . (xxi) Gal 1-6 Glu + Fru 2-(Gal 1-6 Glu 1-2 Fru) In addition he demonstrated with the aid of 14C-sucrose (labelled in both the glucose and the fructose portion) that the enzyme acting on a mixture of sucrose and raffinose transfers a fructose residue fiom the disaccharide to the trisaccharide.ls9 As evidence that a single enzyme from the mould is responsible for the " hydrolytic " and '' transfer " reactions Bealing and Bacon 164 have shown that preparations from different species from mycelia of different ages and from crushed sgores give rise to quantitatively similar mixtures of free sugars and oligosaccharides during their action on sucrose solutions of the same concentration.Thus the enzymic synthesis of higher saccharides by invertase prepara- tions has reached a most interesting stage of development there being several important outstanding problems.It is imperative that in future studies high priority should be given t o proof of the types of linkages present in the saccharides because it is only by this means that the current assump- 166 Arch. Riochem. Biophys. 1951 34 484. 16' Ibid. 1952 39 238. 168 Ibid. 1952 36 490. 16B Pazur Fed. Proc. 1962 11 267. 80 QUARTERLY REVIEWS tion that these substances are in fact precursors of inulin can be verified. It may well be for example that the oligosaccharides produced by invertases of yeast and Aspergillus OTZJZZ have different R values because the fructo- sidic linkages are of the inulin type in one case and of the levan type in the other. Another complexity is introduced by the fact that sucrose in which each of the sugar units is linked through its reducing group is both a fructo- furanoside and a glucopyranoside.Consequently invertases are of two types fructo- and gluco-invertases. Since those mentioned above transfer fructose residues they must belong to the first class; members of the second class would transfer glucose units thus (xxii) I Glu 1-2 Fru + E .C- Glu-E + Fru Glu-E + Glu 1-2 F r u Glu-E + Glu-Glu 1-2 Fru + + Glu-Glu 1-2 Fru + E Glu-Glu-Glu 1-2 Fru + E Glu-E + HZO + Glu + E Indeed White 16' has already mentioned briefly that the gluco-invertase of honey converts sucrose into a series of glucosaccharides. This poses yet another problem does the honey invertase differ from amylosucrase (see p. 69) or dextran sucrase (see p. 70) or both ? General Summary During the 15 years or so which have elapsed since studies of starch Erst began to yield information concerning the mechanisms of polysaccharide syntheses great adva,nces have been made.The present rapid rate of the accumulation of data can be attributed in large measure to the advent of paper chromatography but although it is undoubtedly extremely useful t'his new weapon in the chemist's armoury must always supplement and never replace entirely the older techniques involving the isolation purifica- tion crystallisation and careful characterisation of products. Extensive as current knowledge is it is still possible as the foregoing pages testify to relate all known authentic cases of the enzymic syntheses of polysaccharides to one fundamental reaction in which in the words of BelI,l70 " the energy associated with a pre-formed glycosidic link is used to form a new link by exchange of the originally substituting radical with a new one '' [cf.equation (i)]. Attention was drawn to this fact by Doudoroff Barker and Hassid,171 who suggested that an enzyme catalysing such a reaction should be termed a " transglycosidase ". It now seems to be generally accepted that each step in polysaccharide synthesis does not neces- sarily involve the direct exchange of the glycosidic link in the product for that in the substrate but that the reaction may proceed via an enzyme glycoside (for a full discussion of this point see Gottschalk 172) and so equation (i) could be expanded to the following (E =5 enzyme residue) Gt-E + H-0-G + Gt-O-Gr + Enzyme} ' (xxiii) Gt-0-X + Enzyme + Gt-E + HOX 170 Ann. Reports 1947 44 217.171 J. Biol. Chem. 1947 168 726. Adv Carbohydrate Chun. 1950 5 49. BARKER AND BOURNE ENZYMIC SYNTHESIS OF POLYSACCHARIDES 81 Now it has been shown,173 with the aid of isotopically labelled oxygen that muscle phosphorylase and sucrose phosphorylase (glucose- 1 phos- phate + fructose + sucrose + phosphate) cleave glucose-1 phosphate be- tween C and 0 and not between 0 and P so that in these cases the general substrate GtOX is split a t the Gt-0 bond. This means that the glucose residue is transferred as the glucosyl group (C,H,,O,) and not as the gluco- sidyl-group (C,H,,O,) and it was for this reason that Hehre lo8 suggested that “ transglycosidases ” should henceforth be known as ‘ I transglyco- sylases ”. It is not yet possible to decide whether this change in the terminology is justified for enzymes which utilise disaccharides as substrates.An enzyme usually displays a high measure of specificity in its choice of substrate (G,OX) ; so stringent are the structural and configurational requirements that frequently only one substance is known which will serve as a substrate for a given enzyme. Sometimes however higher homologues of the substrate are acceptable and in such cases the additional sugar units may remain in the transferred residue (G,) (e.g. in the formation of cycbamyloses) or become part of the rejected molecule (HOX) (e.g. invertase on raffinose). Other cases in which Gh contains more than one sugar residue are the conversions of amylose into amylopectin and glycogen (&-enzyme Cori’s branching factor). However a given substrate may be utilised by several different enzymes ; for example higher saccharides are formed from glucose-1 phosphate by phosphorylase and sucrose phos- phorylase and from sucrose by amylosucrase dextran sucrase levan sucrase and ‘‘ invertases ”.The receptor (HOG,) must usually conform to a certain molecular type but within wide limits may be of any molecular size ; indeed if molecular size were a critical factor polysaccharide synthesis could never result because the receptor molecule necessarily increases in length progressively as the synthesis proceeds. In certain cases (cf. the action of amylomaltase on maltose) HOG may be a second molecule of the substrate (G,OX) but where this is not permissible oligosaccharides of the appropriate molecular type (primers) must be present before the synthesis can begin (cf.the phos- phorylase-catalysed synthesis of amylose) ; to these primers the glycosyl units (Gt) are added successively a t the non-reducing ends and so the pro- portion of primer controls both the rate of synthesis qnd the chain length of the product. It has been postulated for some enzyme systems (e.g. invertase lactase) that water may function essentially irreversibly as the receptor (HOG,) and thus the apparent dual roles of these enzymes as both transferases and hydrolases may be explained ; it remains to be seen whether all enzymes hitherto recognised solely as carbohydrases fall into the same class. I n this respect “ hydrolyses ” of sugars by enzymes and by acids show a marked resemblance for it has long been known that “ reversion ” (i.e.the formation of a higher saccharide from a lower saccharide) may occur during acidic hydrolysis (cf. ref. 140a). Indeed Pacsu and Mora 17* have 173Cohn J . Bwl. Chm. 1949 180 771. 174 J . Amer. Chem. SOC. 1960 73 1046. F 82 QUARTERLY REVIEWS demonstrated that under suitable conditions polyglucoses containing some 40 units per molecule can be synthesised from glucose in this way. In the case of glucosylamine the initial product' of acidic hydrolysis is almost exclusively diglucosylamine. 175 Another point which is clear from a study of all those enzyme reactions so far discussed on which the necessary evidence is available is that the transferred residue (Gt) retains its initial ring size and anomeric link in the polymeric product ; thus an a-pyranoside always furnishes an a-linked pyranose polymer.It does not necessarily follow that the intermediate sugar-enzyme complex possesses a glycosidic link of the same anomeric type ; indeed the conversion of GtOX into Gt-E and of Gt-E into GtOG, could easily lead to Walden inversion in which case G,OX G,E and G,OG would be alternately a- and P-glycosides. Can it be that a small degree of racemisation during one of these transfers is responsible for the small per- centage of P-linkages observed recently 176 in amylose ? If so these anomalous links should be found at the non-reducing chain-ends because once a glucose residue became attached in this fashion it would not serve as a receptor for further glucose units. Although the major portion of published studies on the enzymic synthesis of polysaccharides is concerned with glucosans in no case as far as we are aware has a @-linked polyglucose been synthesised in vitro.Since such a synthesis would almost certainly require a P-glucoside (e.g. isosucrose cellobiose P-D-glucose-1 phosphate) as the substrate it is interesting that Fitting and Doudoroff 177 have reported recently an enzymic synthesis of P-D-glucose- 1 phosphate (and glucose) from maltose and mineral phosphate by means of an extract from Neisseria meningitidis. This phosphorylation is important in a second respect because i t is exceptional in that a change of the anomeric link from a t o is involved. A possible explanation of this unusual feature is that in the intermediate glucose-enzyme complex a /l-glucosyl group is attached to the enzyme or co-enzyme a t a pyrophosphate grouping as follows and that the second stage of the reaction entails scission at (B) rather than (A).Finally the energy changes during transglycosylation must be con- sidered. Hehre Io8 has emphasised that polysaccharide synthesis is favoured by an exothermic exchange of glycosidic bonds ; thus since isomaltose 176Bayly Bourne and Stacey Nature 1962 169 876. 17* Peat Thomas and Whelan J. 1952 722. Fed. Proc. 1962 11 212. BARKER AND BOURNE ENZTICZIC SYNTHESIS OF’ POLTTSACC’IfARIT7ES 83 (1 6-a-link) is more stable to acid than is maltose (1 4-0r-link),~~~ the synthesis of a 1 6-a-glucosan from a 1 4-a-linked substrate should be favoured and this is confirmed experimentally in the amylose + amylo- pectin dextrin + dextraq and maltose -+ panose conversions. How- ever the energy-rich links of glucose-l phosphate and sucrose make these substances energetically suitable as substrates for the synthesis of any glucosan.In the latter case the rejected fructose unit is liberated in the furanose form and then rapidly assumes the more stable pyranose structure (cf. Isbell and Pigman 1 y 9 ) ; consequently syntheses of glucosans from sucrose are favoured to such an extent that the reverse reactions are difficult to demonstrate unless the equilibrium is suitably disturbed by artificial means. Although it is now possible to see how the syntheses of polysaccharides proceed according to a master plan the subject is really only just emerging from its infancy and for many years to come it will continue as a fascinating field of study. Problems of immediate interest are the enzymic syntheses of pentosans p-glucosans mannans and other polysaccharides containing essentially a single sugar component ; in these cases it will be necessary to consider how both the branched a,nd the unbranched portions of the mole- cules arise.An explanation must be found too for the small percentage of anomalous linkages (e.g. P-links in amylose 1 3-links in amylopectin and in certain dextrans) which are now being revealed in such “ simple ” poly- saccharides. Then attention must be paid to the biogenesis of polysac- charides carrying substituents (e.g. chitin fucoidin) and finally the challenge of the gums and mucilages must be accepted. Will it be a decade or a century before the chemist will be able to treat samples of D-galactose D-glucuronic acid L-arabinose and L-rhamnose in a predetermined sequence with suitable specimens from his stock of crystalline enzymes and synthesise a t will a sample of gum arabic ? The authors are indebted to Professor M.Stacey F.R.S. for his interest and to the British Rayon Research Association for financial assistance to one of them (S. A. B.). 17* Wolfrom Lassettre and O’Neill J . Amer. Chena. Soc. 1961 73 595. 17@J. Res. Nat. Bur. Stand. 1938 20 773.

ISSN:0009-2681    

DOI:10.1039/QR9530700056

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

THE EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES AND ELECTRODE PROCESSES By S. BARNARTT (CHEMICAL DEPARTMENT WESTINGHOUSE RESEARCH LABORATORIES EAST PITTSBURGH PENNSYLVANIA U.S.A.) WHEN acoustic waves are propagated through a medium the particles of the medium are subjected to periodic accelerations and compressions. The pressure changes take place under practically adiabatic conditions even in the range of ultrasonic (inaudible) frequencies vix. above about 20 kilo- cycles/sec. Hence temperature fluctuations also occur. It is the changes in velocity pressure and temperature which cause the effects of acoustic waves on the properties of the medium and on reactions taking place therein. In this Review the medium is restricted to simple ionic solutions. Acoustic effects in colloidal solutions which have been reviewed by Sollner,l are largely omitted.Experimental procedures are not discussed since these have been covered in recent monographs.2 The subject of sound absorp- tion is specifically excluded. It will be clear from what follows that ultrasonic waves produce many interesting effects on electrolytes and electrode reactions. Although some of these have been known for over 15 years t'here is a striking paucity of data on them. (1) Compressibility (1.1) Compressibility from Acoustic Velocity.-The velocity of sound u in a fluid is related to its adiabatic compressibility ps = - (aV/aP),/V by t,he equation where d is the density of the fluid. If the specific heat at constant pressure C p is known then the isothermal compressibility p = - (aV/dP)T/V can he calculated from the thermodynamic formula where k = Cp/Cv is the ratio of the specific heats and a = (aV/aT)p/V is the coefficient of (cubic) thermal expansion.Conversely where iso- thermal conipressibility data are available sound velocities yield specific heats. U' = l/pad . * (1) p = kpa = pa + a2T/Cpd . * (2) 1 Sollner Chem. Reviews 1944 34 371 ; see also Alexander " Colloid Chemist>ry Theoretical and Applied " Vol. 5 p. 337 R,einhold Publishing Corp. New York 1944. 2 Bergmann " Der Ultraschall und Seine Anwendung in Wissenschaft und Tech- nik " 6th edn. S. Hirzel Verlag Zurich 1949 ; Carlin " Ultrasonics " McGraw- . Kill Book Co. Inc. N.Y. 1949 ; Richardson " Ultrasonic Physics " Elsevier Press Inc. Houston 1952 ; Vigoureux " Ultrasonics " Chapman and Hall Ltd. London 1950. 3 Markham Beyer and Lindsay Rev.Mod. Physics 1951 23 353. 84 BARNARTT EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES 85 The determination of ultrasonic velocity is the most accurate method of obtaining compressibilities of dilute solutions a t atmospheric pressure. Velocity measurements are greatly simplified at high frequencies since small samples of electrolytes are sufficient and reflections from the walls of the container can be made negligible. A resonance method is generally used wherein standing waves are produced in a column of the solution and the wave-length evaluated. Precise velocity measurements may be made with the acoustic interferometer developed by Pierce for gases and by Hubbard and Loomis for liquids. With this device a'bsolute velocities accurate to 0.06y0 are claimed.6 by an optical method originated by Debye and Sears and independently by Lucas and B i q ~ a r d .~ The optical method depends upon t,he fact that the passage of ultrasonic waves through a liquid sets up periodic density variations. The latter act as an optical grating which can be used to diffract a light beam or can be made directly visible. (1.2) Comparison of the Ultrasonic and Piezometric Methods.-Com- pressibility data from ultrasonic measurements on aqueous solutions have been shown to be in good agreement with direct piezometric determinations. Although the latter are high pressure measurements accurate compressi- bilities a t atmospheric pressure may be computed from them by invoking the concept of " effective pressure " introduced by Gibson.lo This concept follows from Tammann's hypothesis,ll that a t constant temperature the water in a given solution behaves as does the same weight of pure water under a constant effective pressure P, in addition to the external pressure.The pressure-volume relationship of pure water and of many other pure substances is given by Tait's equation where B and C are positive constants and v, is the specific volume of the substance. With Tammann's hypothesis the Tait equation applied to the water within a solution which is under a total pressure P +- P takes the form where v1 is the specific volume of the water in the solution and the constants C and B have the same values as in equation (3) for pure water. The specific Greater relative precision is obtainable - azJ,/aP = 0 . 4 3 4 3 ~ / ( ~ + P ) . ' (3) - a271/a~ = O.XMC'~(B + P 3- Y) .' (4) volume of the solution is given by v = x1v1 $- xzv2 - Proc. Amer. Acad. Arts Sci. 1925 60 271. ATature 1927 120 189 ; Phil. Mag. 1928 5 ti Freyer J . -4mer. Chem. Xoc. 1931 53 1313. 17 295. * ( 5 ) 1177 ; J . Opt Soc. Amer. 1926 i Bachem Z . Physik 1936 101 541 ; Falkenhagen and Bachem Z. Eklctrochem. 8 Debye ibid. 1932 33 849 ; Debye and Sears Proc. Nat. Acad. Sci. 2932 18 1935 41 570; Szalay Physiknl. Z. 1934 35 639. 409. Compt. rend. 1932 194 2132; 195 121; J . Phys. Radium 1932 3 464. lo J . Amer. Chem. Soc. 1934 56 4 865. l1 Z . physikal. Chern. 1893 11 676. 86 QUARTERLY REVIEWS Concn. % where x denotes the weight fraction and v2 the partial specific volume of the dissolved salt. Substitution for v1 in equation (4) leads to the Tait- Gibson equation lo6/? (bar-') NaCl KC1 1 KBr KI or in integrated form j (i) (ii) (i) (ii) (0 (ii) .40.4 40.6 41.7 41.8 43.5 43.2 . . . . 37.5 37.6 39.4 39.5 41.9 41.7 . . . . 33.4 33.2 36.2 36.3 . . . . 31-0 30.7 34.2 34.3 38.4 38.1 . . . 28.8 28.4 32.4 32.4 . . 34.8 34.6 . . . . 31.4 31.1 . . . .~ where Pat. is atmospheric pressure. At moderate concentrations and pressures the terms containing v2 are negligible. (i) (ii) 44.3 44.3 41.7 41.7 38.0 37.8 33.7 33.4 Isothermal compressibilities at 25" and 1 bar computed by two independent methods (i) ultrasonic (ii) piexometric 6 10 16 20 24 30 40 45 ~ Using equation ( 7 ) Gibson l2 determined P for various alkali halide solutions from a single compression of each solution to 1000 bars and then calculated His data are compared in the Table with those computed from Freyer's ultrasonic velocity measure- ments.6 The excellent agreement shown by these entirely independent sets of data contributes strong support for the Tait-Gibson equation and the concept of effective pressure.(1.3) Correlation with the Debye-Huckel Theory.-Acoustic velocity and compressibility data may also be correlated with the interionic attraction theory of electrolytes. For solutions of a single salt the Debye-Huckel limiting law l3 evaluates the partial molal free energy of the dissolved salt G2 = (W/an,) p at a very low but finite molarity c as a t 1 bar from equation (6). G2 - = vRT loge c - AD-IT-*(Z Yizt)ic* . * (8) where A = (n.r6N3/103k)t and Gi is a function of temperature and pressure only ; Y = C vi where vc is the number of ions of the ith species per mole- cule ; x i is the valency of the ion D the dielectric constant E the electronic cha,rge N Avogadro's number and k Boltzmann's constant.From this 12 J . Anw. Chein. ~ o c . 1933 57 284. l 3 Physikal. Z . 1023 24 185. BARNARTT EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES 87 equation the partial molal volume of the dissolved salt 7 = (aCr,/i3P)~ becomes l4 v2 - v; =avc* . (9) where The partial molal compressibility of the dissolved salt K = - aV,/aP is then evaluated l 5 to be where K - K = a & . - (10) For direct comparison with experimental determinations it is more con- The apparent venient to use the corresponding apparent molal quantities. molal volume of the dissolved salt +v is defined by where nI and n2 are the numbers of moles of solvent and solute respectively Y is the volume of the solution and 8 the molal volume of pure solvent.It follows from this definition that which may be applied to extremely dilute solutions as or in integrated form a(c+v)/ac = v2 * (13) +v=:rV2.dc 0 . Substitution for V z from equation (9) yields where Sv = $ov. where subscript zero denotes pure solvent obeys the equation where S = $aK. #lv-+;=&7c* . - (15) $K = - a+v/ap = CBV - n,BoVo>/n - (16) $ K - + ; = S X C f . * (17) d - do = ac - bc; . * (18) Similarly the apparent molal compressibility defined as Equation (15) may be rewritfen in terms of density whence l6 where a = of the solute. combining equations (15) and (17) 71ix.l' where f = 10-3(p0& - +g) and g = Chem. 1931 A 155 65. - do#$) and b = 10-3d0Xv ; M is the molecular weight A similar expression for compressibility is obtained upon #? - go = -fc + gci .' (19) - poSv). l4 See Redlich Naturwiss. 1931 19 251 ; Redlich and Rosenfeld 2. physikal. Gucker Chem. Reviews 1933 13 111. l6 Root J . Amer. Chem. SOC. 1933 55 850. 1' Gucker ibid. p. 2709. 88 QUARTERLY REVIEWS Since all these equations have been derived from the limiting Debye- For such solu- Huckel law they apply only to extremely dilute solutions. tions the relationship u2 = L/Pd may be utilised in the form (20) where Au = u - uo etc. Substitution for Ad and Ap from equations (18) and (19) gives the acoustic velocity as a function of concentration only l8 where u - u0 = hc -jG . ' (21) and For a given solvent at fixed temperature the limiting slopes SV and XK which characterise the linear relationships between the apparent molal quantities and the square root of the concentration are constants for all salts of the same valency type.Hence the limiting slopes of the plots Ad/c-c* AP/c-c* and Au/c-c* are also constants for all salts of a given valency type. Experimentally the determination of any one of the five functions c&., $= Ad/c Ap/c and Au/c a t high dilution with sufficient accuracy to test the theoretical limiting slope is difficult. Some success has been achieved with density measurements ; the results obtained for strong electrolytes tend to support the interionic attraction theory. l9 The measurement of ultrasonic velocity or compressibility in very dilute solutions with the pre- cision required t o test the limiting theory has not yet been accomplished.A striking result is obtained when the experimental values of any one of the above functions are plotted against the square root of the concentration. A linear relationship is observed in each case,2o extending over wide con- centration ranges and down to relatively low concentrations. The coefficients of these empirical relationships differ from the theoretical values given by equations (15) (17) (18) (19) and (21). Furthermore the slopes of the observed lines are not constants for salts of the same valency type. No adequate explanation of these facts has appeared. In the case of the apparent molal compressibility the empirical square- root relationship first observed by Gucker l5 17 a t high concentrations has been found to hold within experimental error down to 0.03 molar the lowest concentration studied.The same relationship has been observed for some l9 Harned and Owen " The Physical Chemistry of Electrolytic Solutions " "(1 2o Barnartt ref. (18) ; Gucker ref. (17) ; Masson Phil. Mag. 1929 8 218 ; Root Barnartt J . Ghem. Phys. 1952 80 278. edn. p. 264 Reinhold Publishing Corp. New York 1950. ref. (16). BARNARTT EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES 89 60 strong electrolytes. 21 * I n most instances the adiabatic compressibility data determined from ultrasonic velocities were used without conversion to isothermal compressibilities since neither the linearity nor the slopes of the square-root plots are appreciably affected by this substitution. 24 (1.4) Solvation from Compressibility Data,-Compressibility data may be correlated with the degree of solvation of dissolved salts.The correlation is based on the assumption that the water molecules in the immediate vicinity of an ion have modified physical properties similar to those of pure water under high pressure. An ion may be considered to be surrounded by one or two (or possibly several) shells of bound water molecules which are under such high pressure as to be virtually incompressible. As a first approximation the water of hydration about one ion is conceived as a sphere a t the boundary of which the compressibility falls abruptly from that of pure water down to zero. It follows then if ni is the number of moles of incompressible solvent in the solution that l a - B O V O v aP v * @ = - -{Vo(nl - ni)> = (nl - ni)- and therefore where s is the solvation expressed in moles of solvent per mole of solute.The study of solvation in this manner originated with Pa~synski,~5 who used the following modification of equation (23) Solvation numbers calculated from equation (24) decrease slowly with increasing concentration 25 25a and are qualitatively in harmony with Bernal and Fowler's theory of ionic hydration. 26 (1.5) Solutions of More than One Salt.-Measurements of ultrasonic velocity in solutions containing more than one salt have been confined to sea water. These measurements are important for oceanic depth studies and for sonar ranging. In an investigation of a synthetic sea water con- taining 7 salts Weissler and Del Grosso 23 found that t,he contributions of = (n1b2N - B / P o ) * * (24) 21 Bachem ; Falkenhagen and Bachem ref. ( 7 ) ; Giacomini and Pesce Ric.Sci. 1940 11 605 ; Gucker refs. (15) (17) ; Lunden Svensk Chem. Tidskr. 1941 53 86 ; 2. physikal. Chem. 1943 192 345 ; Prozorov J. Phys. Chem. U.S.S.R. 1940 14 383 391. 22 Krishnamurty Current Sci. I n d i a 1950 19 87 ; J . Sci. Iizd. Res. India 1950 9 B 215; Prakash Saxena and Srivastava h'ature 1951 168 532. 2 3 Weissler and Del Grosso J . Acoust. Soc. Amer. 1951 23 219; Lunden ref. 21. 2 4 Bachem ref. (7). 25 Acta Physicochim. U.R.S.S. 1938 8 385. 25Q Giacomini and Pesce ref. 21. 26 J. Chern. Phys. 1933 1 515. * Recent reports from India 2 2 present data on alkali halide solutions which do not obey this relationship. The ultrasonic velocity measurements however exhibit concentration dependence quite. different from that shown by the more accurate measurements of other authors,7~ 2 1 7 2 3 and must be tentat'ively considered as unreliable.90 QUARTERLY REVIEWS each salt to the sound velocity and compressibility of the solution were additive. Thus from the data on simple solutions of each of the seven salts the increments (u - uo) and (/? - Do) were obtained a t the same concentration as that at which the individual salt is present in sea water. Then the velocity and compressibility for the sea water were calculated by summing up the increments and combining the sums with the corresponding value for distilled water. The calculated values agreed with the measured values within experimental error (0.1 %). (2) Conductivity Since the conductivity of an electrolyte varies with temperature and pressure the passage of ultrasonic waves is accompanied by periodic con- ductance changes.The change in conductivity of an electrolyte dK brought about by a small adiabatic compression may be written where y = ( l / ~ ) ( a ~ / a P ) is the pressure coefficient and 6 = ( l / ~ ) ( a ~ / a T > the temperature coefficient of conductivity. Substitution for dT from the general thermodynamic relationship leads to the equation If the amplitude of the conductance change produced by an acoustic wave be measured in an electrolyte whose coefficients y and 6 are known from static measurements the pressure amplitude of the wave may be computed by means of equation (27). From the pressure amplitude p the sound intensity I is computed from where I is the average energy flow per sq. em. per second. Thus the measurement of the conductance changes in an electrolyte may be used to determine acoustic intensity.Conversely if the intensity is also evaluated experimentally then the pressure coefficient of conductivity may be calcu- lated for electrolytes whose temperature coefficients are known. In their method a filament of constant alternating current of frequency fl is passed between the ends of two fine wire electrodes brought close together. The current filament is arranged perpendicular to the direction of a travelling ultrasonic wave and may be made essentially small in comparison with the wave-length if the acoustic frequency f o is not too high. Under these conditions two side bands of frequency (fo + f l ) and (fo - fi) are produced with a voltage that depends upon the conductance changes. Measurements of the side-band voltages for sodium chloride and copper sulphate solutions showed satisfactory agreement with the voltages calculated from the known conductivity coefficients.A method of measuring the conductivity effect employing stationary dK/K = y d P + 6 dT . . (25) (aT/aP)s = aT/Cpd . * (26) dK/K = ( y -/- &T/Cpd)dP . * (27) I =p2/2ud . * (28) The conductance effect was first studied by Fox Herzfeld and 27 Phys. Review 1946 70 329. BARNARTT EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES 91 acoustic waves has been described by Lichter and Khaikin.28 A constant alternating current having the same frequency as the sound wave is applied to fine wire electrodes situated in a pressure antinode of the standing wave. Location of the electrodes in a velocity node eliminates possible complications from velocity variations.A phase difference is maintained between the applied voltage and the sound wave. The phase displacement produces a rectified voltage which is a measure of the conductivity fluctuations. Elec- trode polarisation is minimised by changing the sign of the rectified voltage periodically. This is accomplished by changing the phase of the applied voltage to the opposite phase f' times per second where f' <fo. With this method the pressure coefficient y was determined a t 20" for 0.005~- silver nitrate solution for which the temperature coefficient 6 was known. Then y being assumed to be independent of temperature 6 was determined ultrasonically over the temperature range 5-60'. These values of 6 agreed with those from static measurements within experimental error.Recently Krishnamurty 28a announced that the conductivities of nitrate solutions as measured by the usual Kohlrausch method decrease when the solution is subjected to ultrasonic vibrations. Conductance changes as high as 20% were reported for an ultrasonic frequency of 2-51 megacycles/sec. at an unspecified intensity. In addition the passage of current through each solution was accompanied by an increase in the velocity of ultrasonic waves in it and consequently by a decrease in its compressibility. These effects merit confirmatmion. (3) Space Charge (the Debye Effect) In 1933 Debye L9 predicted that ultrasonic waves create space charge in electrolytic solutions. The space charge arises because the ions owing to their inertia lag behind the solvent in the sound field.Since the positive and negative ions will usually move with different velocities the electrical potential at a given point in the irradiated solution will acquire an alternating component having the same frequency as the acoustic wave. Debye derived the magnitude of the effect to a first approximation specifically omitting diffusion and interionic attraction as being second- order considerations. The force on an ion in a plane ultrasonic wave was considered to comprise an electrical force resulting from the space charge and a frictional force resulting from the difference in velocity of the ion and the surrounding liquid. where ei pi c2 and mi are respectively the charge friction constant velocity and mass of the ion ; vo is the velocity of the solvent ; and X is the electric field int,ensity .Combination of this equation with the equation of continuity and Poisson's equation leads to the following general solution 29 The equation of motion is then eiX - p i ( ~ * i - v0) = midvi/dt . . (as) 28 J. Exp. Theor. Phys. U.S.S.R. 1948 18 661. 2aa J . Sci. I d . Res. Indin 1051 10 By 149. 29 J . C'henz. Phys. 1033 1 13. 92 QUARTERLY REVIEWS Here E is the amplitude of the potential oscillations mH the mass of the hydrogen atom a. the velocity amplitude of the solvent K the conductivity of the solution in electrostatic units D the dielectric constant of the solvent Mi the effective gram-ionic weight of the ion and m/2n the frequency. If the frequency is not too high the expression on the extreme right can be made equal to unity. Under these conditions equation (30) applied to an aqueous solution of a uni-univalent salt reduces to Thus the Debye effect provides a measure of the relative masses of ions and hence their degree of solvation.With a solution of known ionic masses the effect may be used to determine a, from which the acoustic intensity may be calculated by the equation I=+uda,2 . * (32) The order of magnitude of E may be revealed by putting p+ = p- and M - M- = 15 whence E becomes volt per unit velocity amplitude. Oka 30 extended the derivation to include interionic attraction by adding terms to equation (29) for the electrophoretic force and the force of relaxation. The added forces however applied only to dilute solutions and represented small corrections. Hermans 31 pointed out that equation (29) fails to show that the space charge must disappear when the densities of the ions and solvent are equal.He remedied this by taking into consideration the force on the ion resulting from the pressure gradient in the acoustic wave. The equation of motion then becomes eiX - pi(^< - u0) = midvi/dt - Vidoduo/dt . * (33) where Vg is the volume of the ion and do the density of the solvent. Bugosh Yeager and Hovorka 32 extended the calculations to include not only this pressure-gradient term but also the forces resulting from diffusion and interionic attraction. The extended solution has only theoretical interest at the present time however since experimental evi- dence for the Debye effect is as yet qualitative. Yeager Bugosh Hovorka and McCarthy 33 first demonstrated the existence of the effect in simple electrolytes utilising a stationary ultrasonic field.The apparatus used did not permit determination of the velocity amplitude within the test solution hence comparison of the data with the predictions of equation (31) was not possible. In agreement with theory however the measured alternating potentials were found to be roughly independent of concentration for dilute uni-univalent electrolytes and to vary with the electrolyte used. With the standing-wave technique elaborate screening precautions are necessary to prevent electromagnetic coupling between the high-frequency source and the detecting cell. This complication can be minimised by the 30 Proc. Phys. Math. SOC. Japan 1933 15 413. 31 Phil. Mag. 1938 25 426. 3 3 I b i d . 1049 17 411. 32 J . Ghem. Phys. 1947 15 592. BARNARTT EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES 93 use of pulse-modulated ultrasonic whereby the acoustical effects are separated in time from electromagnetic coupling and are measured under essentially free-field conditions.A potential amplitude of 5 microvolts per unit velocity amplitude has been obtained recently by the pulse method in 0~005~-potassium chloride solution. 35 From equation (31) this corres-a ponds to a mass difference between the potassium and chloride ions of 70 or approximately 4H20 per mole of potassium ion on the basis of Bernal and Fowler's conclusion 26 that the chloride ion is not hydrated. This result may be compared with the values 6-7H20 from compressibility data 25 and 2H,O from ionic-activity data.36 Hermans 31 and independently Rutgers 37 pointed out that the Debye effect should be much larger in colloidal solutions where the positive and negative ions exhibit greater differences in mass.Approximate theoretical treatments for this case have been presented by Hermans 313 38 and by Enderby. 39 Both theories assume that the vibration potentials in colloidal solutions arise primarily from periodic distortion of the double layer around each colloidal particle and that the relative motions of small positive and negative ions contribute very little. The distortion occurs because the heavy colloidal particle moves more slowly than the ionic charges outside it. The result,ing asymmetric charge distribution is equivalent to a dipole situated at the centre of the particle. The potential differences developed by the acoustic wave may be computed by summing up the dipole moments of all the particles.Hermans's derivation predicts that potential ampli- tudes of the order of a volt should be attainable.38 Much smaller potentials are indicated by Enderby's theory. The following simple formula for estimating the potential amplitude in colloidal solutions has been given by Vidts 4O where is the electrokinetic potential of the colloidal particles m their mass per c.c. and The first measurements reported for a silver iodide s0l,4O9 41 indicated potential amplitudes two orders of magni- tude smaller than those predicted by equation (34). More recent measure- ments in a colloidal arsenic trisulphide solution,42 however gave an amplitude of 1.5 mv as compared with 7 mv calculated from the formula. the viscosity. (4) Effects on Electrode Potentials (4.1) Unpolarised Electrodes.-When an unpolarised electrode is mb- jected to acoustic vibrations the electrode potential should acquire an 3 4 Hunter Proc.Phys. SOC. 1050 B 63 58 ; Yeager Bugosh and Hovorka ibid. 35 Yeager Dietrick Bugosh and Hovorka J. Acoust. SOC. Amer. 1951 23 627. 36 Stokes and Robinson J. Anzer. C'hem. SOC. 1948 70 1870. 37 Physica 1938 5 46. 39 Proc. Roy. Soc. 1951 A 207 329. 4* Bull Acad. roy. Belg. Classse mi. 1945 No. 3 p. 5. *l Rutgers Nature 1946 157 74. 4 2 Rutgers and Vidts {bid. 1960 165 108. 1951 By 64 83. 38 Phil. Mag. 1938 26 674. 94 QUARTERLY REVIEWS alternating component resulting from the periodic compressions. Where the electrode is composed of condensed phases only the alternating com- ponent should be very small.Moriguchi 43 observed no change in the potential of copper electrodes in dilute copper sulphate solution upon irradiation with ultrasonics. Schmid and Ehret 44 confirmed this observa- tion and also noted no change of potential for nickel electrodes in nickel chloride solution. These workers however did not attempt to reveal small alternating components. The potential of a gas electrode is much more sensitive to compression and in this case an appreciable temperature variation will occur in the gas phase if the compression is adiabatic. Schmid and Ehret 44 reported that the potential of a hydrogen electrode in dilute sulphuric acid became indefinite to & 5 mv in the presence of ultrasonics. Again no attempt was made to reveal an alternating component. A t,heoretical derivation of the alternating component has been given recently by Yeager and Hovorka 4 5 9 46 for the hydrogen electrode.If it is assumed that the irradiated electrode is reversible the change in electrode potential may be evaluated by thermodynamics. The added acoustical pressure in the gas phase Pa produces a change in potential given by the general equation where P is the time average of the gas pressure and F the Faraday of elec- tricity. With the restriction of low acoustic intensity so that Pa is a small fraction of P, the approximation log (1 + Pa/Pg) = Y,/Pg may be applied t o equation (35) to give AE = RTPa/'zFPg . (36) or E = RTp/xFP . . (37) where E is the potential amplitude and p the pressure amplitude. At low audio-frequencies where the thickness of the gas film is small compared with the wave-length the acoustic compressions in the gas phase may be almost isothermal in which case the potential amplitude would approach the value given by equation (37).At higher frequencies the compressions would be practically adiabatic and providing the electrode behaves reversibly the potential amplitude is readily shown 4 5 to be approximately where S is the molar entropy of the gas and CP its molar heat capacity a t constant pressure. Yeager and Hovorka 46 have expressed doubt whether gas electrodes can remain reversible a t ultrasonic or even a t the upper audio-frequencies. They proposed a kinetic treatment and developed a 4 3 J . Chem. SOC. Japan 1934 55 749. 4 p 2. Elektrochem. 1937 43 697. 4 6 J . Chem. Phys. 1949 17 416. 48 J . Electrochem. SOC. 1961 98 14.BARNARTT EFFECTS OF TJLTRASONIC WAVES ON ELECTROLYTES 95 general equation for the hydrogen electrode which is applicable to both unpolarised and polarised electrodes and which reduces to equation (37) at low frequencies. This electrode effect like the conductivity and Debye effects may prove useful for the absolute measurement of acoustic intensity in liquids and consequently for exploring complex ultrasonic fields. All three effects do not have a characteristic frequency as the usual crystal gauges do. They have the further advantage that the electrolyte surrounding the electrodes may be selected so that its characteristic acoustic impedance (ud) is nearly equal to that of the irradiated liquid. Since the electrodes may be made small in comparison with the wave-length reflection of the wave a t the measuring instrument can be avoided.(4.2) Polarised Electrodes.-In the case of electrodes polarised by current flow pronounced depolarising effects can be produced by the application of ultrasonic waves. Acoustic agitation is particularly violent at a liquid- solid interfa~e.~' At the electrode surface therefore the concentration changes in the diffusion layer are reduced and the products of electrolysis largely removed. From a study of the electrolysis of copper sulphate solution with copper electrodes Moriguchi 43 concluded that the diffusion layer at each electrode was eliminated by irradiation with ultrasonics. With ordinary stirring the plot of current through the cell against applied voltage deviated considerably from linearity (Ohm's law).When ultrasonic waves were substituted for the stirring however a linear plot was obtained indicating that polarisation was eliminated a t both a,node and cathode. It is possible however that appreciable polarisation remained if it were approximately proportional to the current density. Where one of the products of electrolysis is a coating over the surface of the electrode ultrasonics may give rise to considerable depolarisation by removing the coating. The surface layer tends to be disrupted and dis- persed colloidally in the liquid especially when it is brittle or does not adhere tenaciously to the substrate.4s The mechanism whereby the coating is torn off probably involves cavitation the periodic formation and collapse of cavities which occurs most readily a t interfa~eS.~8 49 Depolarisation by removal of a coating from the electrode surface was first demonstrated by Moriguchi using smooth platinum electrodes for the electrolysis of aqueous solutions.Ordinarily the liberation of hydrogen and oxygen at smooth platinum electrodes requires approximately 1.7 volts. When the anode was irradiated with ultrasonics the voltage required a t moderate current densities was reduced to about 1.2 which is close to the equilibrium value for the oxygen and hydrogen electrodes. Most of this depolarisation resulted from the removal of an anodic coating (presumably 47 Richards J . Amer. Chem. SOC. 1929 51 1724 ; Richards and Loomis ibid. 4 8 Sollner Trans. Paraday SOC. 1938 34 1170. 49 Bondy and Sollner ibid. 1935 31 835. 1927 49 3086. J . Chem. SOC. Japan 1934 55 761. 96 QUARTERLY REVIEWS platinum oxide) which was found to be dispersed in the electrolyte.At relatively high current densities the anodic film was not all removed and the marked depolarisation no longer occurred. Several other examples of ultrasonic action on electrode coatings have been described by Schmid and Ehret 50a The formation of an anodic coating on lead in sodium carbonate solution is retarded. On aluminum anodes in sodium sulphate solution on the other hand the anodic film forms more rapidly in the presence of ultrasonics ; however a thinner film and decreased polarisation are obtained. The grey layer that passivates iron in concen- trated sulphuric acid is removed and the iron continues to dissolve as long as the ultrasonic waves are applied. The reactivation of passive chromium in concentrated hydrochloric acid occurs much more rapidly.In concen- trated nitric acid the passivity of chromium is not affected by ultrasonics but that of iron is quickly destroyed. Roll 5 l has shown that the passivating film which causes pronounced polarisation of silver anodes in cyanide plating solutions at relatively low current densities does not form in an ultrasonic field until much higher current densities are applied. In intense ultrasonic fields marked depolarisation occurs at gas elec- trodes even in the absence of surface coatings on the electrodes. At constant current density there exists generally an intensity above which depolarisation suddenly increases. Similarly a t constant intensity there exists a current density above which the depolarising action rapidly dimin- ishes.Figs. 1 and 2 taken from Schmid a,nd Ehret’s data 44 show typical curves obtained for hydrogen evolution at nickel cathodes in 0*2~-sodium sulphate solution (pH 4.2) with an acoustic frequency of 284 kilocycles/sec. Curves similar to that of Fig. 1 were obtained for hydrogen deposition from sodium sulphate solution on eight other metal electrodes. Anodic deposition of chlorine on platinum from hydrochloric acid solution also gave the same type of curve. In all cases a hissing noise characteristic of cavitation began a t the intensity at which the potential jumped. That the jump in potential is produced by the cavitation was later confirmed by Polotskii and F i l i p ~ o v ~ ~ who reproduced the potential jump for both hydrogen and chlorine liberation when cavitation was brought about by superheated steam.Fig. 2 shows that the pronounced depolarisation disappears a t relatively high current densities. This behaviour was also observed by Piontelli 53 for hydrogen deposition from several other electrolytes. According to Schmid and Ehret,44 the depolarisation mechanism is a mechanical one. When cavitation occurs the gas being deposited a t the electrode is drawn into the cavities and the gas pressure at the electrode decreases. At low current densities where the gas is liberated very slowly the gas pressure a t the electrode may be reduced considerably below atmospheric pressure and the electrode polarisation may become negative when based upon the equilibrium potential for one atmosphere pressure (see Fig. 2). At high current densities only part of the liberated gas is removed by cavitation.I O U 2. Elektrochern. 1937 43 408. 5 1 2. Metallk. 1950 41 413. 62 J . Uen. Chem. U.X.S.R. 1947 17 193. b 3 Piontelli Atti Accad. Lincei Classe sci. fis. mat,. nat. 1938 27 367 681. BARNARTT EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES 97 4 6 8 I0 ( CURRENT IN OSCILLATOR CIRCUIT amp.) RE L AT1 V E I NTE N SI TY FIQ. 1 Effect of ultrasonic intensity on hydrogen deposition potential at 2.5 milliamp./crn.$ (from Schmid and Ehret Z. Elektrochem. 1937 43 597). z 2.5 5.0 7.5 10.0 CURRENT DENSITY milliump./cm2 FIG. 2 Variation of hydrogen deposition potential with current density A-in the presence of strong agitation ; B-in the presence of intense ultrasonics ; &equilibrium potential at hydrogen pressure of 1 atm. (from Sch,mid and Ehret.loc. cit.). This mechanism is in accord with the observations that the acoustic intensity required to produce the potential jump increases with current density and that the potential jump is practically independent of the nature of the cathode metal. Q 98 QUARTERLY REVIEWS Limited data on the anodic deposition of oxygen indicate somewhat different behaviour in an ultrasonic field. Schmid and Ehret,44 using a platinum anode in 0-2lur-sodium sulphate solution found no sudden potential jump rather a gradual depolarisation with increasing ultrasonic intensity. In this case the mechanism is complicated by oxidation of the platinum surface. Depolarisation a t relatively low acoustic intensities where cavitation is absent has been studied by R0ll.5~9 s4 During the simultaneous deposition of hydrogen with nickel with silver and with copper the depolarising action of ultrasonics was similar to that of strong agitation.This was true also for anodic dissolution of silver in the argentocyanide plating bath. For copper dissolution in the acid sulphate bath neither ultrasonics nor agitation produced any appreciable change in polarisation. The acoustic depolarisation is greatly dependent upon the frequency. At a given acoustic intensity level the cathodic depolarisation during nickel deposition was found to increase with decreasing frequency.55 The presence of an alternating component in the potential of a polarised gas electrode subjected to acoustic vibrations was demonstrated by Nikitin 56 a t audible frequencies and by Yeager Bugosh Hovorka and McCarthy 33 at ultrasonic frequencies.A theoretical treatment of this effect has been given for the hydrogen electrode by Yeager and H ~ v o r k a . ~ ~ Pulse techniques have been used recently to measure the alternating component for polarised hydrogen electrodes. 5' The following results were obtained.58 With platinised platinum in dilute hydrochloric acid or sodium sulphate solutions the potential amplitude increased linearly with the polarising current at low current densities but a t higher current densities the curves exhibited regions of almost constant amplitude. The amplitude was practically independent of the electrode metal although with platinised platinum the alternating component was steadiest with respect to rapid time fluctuations. At a given current density the amplitude was an increasing but not a linear function of ultrasonic intensity.Measurements in sodium sulphate solutions of different concentrations indicated that the amplitude is roughly inversely proportional to the conductivity of the solution. (5) Electrodeposition Since ultrasonic waves change the polarisation of electrodes a t which metals and hydrogen are liberated they can influence the current efficiency of metal deposition the grain growth of the depositing metal the composition of alloy deposits a,nd other important factors in electroplating. In addition the purity of the deposit can be increased. It was pointed out by K e l ~ e i i ~ ~ who first proposed the use of ultrasonics in electroplating that suspended 54 2. Metallk. 1950 41 339. b5 Roll and Schrag ibid. 1951 42 197.56 Compt. rend. Acad. Sci. U.R.S.S. 1934 4 309 ; 1936 [ Z ] 2 67 ; J . Gen. C?Lern. 57 Yeager Bugosh Dietrick and Hovorka J. Acoust. SOC. Amer. 1950 22 686. 58 Yeager Personal communication. Austrian Patent 121,986 (1931); Chem. A h . 1931 25 2926. U.S.S.R. 1936 6 1393 1401; 1940 10 97. BARNARTT EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES 99 impurities in the electrolyte would not adhere to the electrode since the relatively massive electrode will not follow the movements of $he particles. He stated also that the hydrogen content of the metal could be reduced in some cases by preventing the deposition of unstable metal-hydrogen alloys. A patent by Dutt 6* claimed that metals such as aluminum and mag- nesium may be electrodeposited from aqueous solutions in an ultrasonic field.However Schmid and Ehret 50cs did not succeed in depositing aluminum or magnesium by the method described in the patent and from the action of ultrasonics on the deposition potential of hydrogen at mag- nesium electrodes 44 they concluded that the electrodeposition of magnesium from aqueous solution appears unlikely. On the other hand these workers demonstrated that metal deposition can indeed be promoted by an intense ultrasonic field. Thus a uniform nickel deposit was obtained from a nickel sulphate solution under conditions that fail to yield nickel in the absence of ultrasonic^.^^ In the electrodeposition of metals at high current densities where hydrogen is co-deposited the agitating action of ultrasonic waves should increase the current efficiency of metal deposition just as ordinary stirring does.This effect has been demonstrated in the deposition of chromium from chromic acid solution,61 of nickel 5 4 9 5 5 and copper 51 from sulphate solution and of silver from cyanide s0lution.5~ The current efficiency increases continuously as the ultrasonic intensity is raised. 51j 54 According to Roll,54 the agitating action is unusually violent because the motion of the hydrogen bubbles a t the electrode surface is speeded up by the acoustic field. In support of this mechanism he found that air bubbles in aqueous glycerol rise more rapidly when irradiated with ultrasonics. 51 For metal deposition a t low current densities and relatively high current efficiencies the influence of ultrasonics on current efficiency cannot be explained by agitation. A reduction in current efficiency has been reported for zinc,G2 nickel,54 55 and copper 51 6 2 plating from sulphate solutions and for silver plating from nitrate solutions.63 The decrease in current efficiency may be only an apparent one however resulting from colloidal dispersion of part of the electrodeposited metal.Such dispersion is likely to occur if the deposit does not adhere strongly to the substrate 64 or if it consists of fine -grained aggregates having poor cohesion,63 particularly a t acoustic intensities above cavitation levels.65 Ultrasonic waves do not disperse solid substances of high cohesion such as glass and ductile metals 48 and large- grained ele~trodeposits.6~ The dispersion of cathodically deposited metal by ultrasonic irradiation during electrolysis is an effective method for the preparation of very finely 6 O F.P.749,007 (1933) ; Chem. Abs 1933 27 6657. 61 Miiller and Kuss Helv. Chim. Actn 1950 33 217. 6 2 Rummel and Schmitt, Korrosion u. Metullschutz 1042 19 101. 63 Levi Ric. Sci. 1949 19 887. 6 4 Clans 2. tech. Physik 1935 16 80. 6 5 Roll 8. Metallk. 1951 42 271. 100 QUARTERLY REVIEWS divided metal. The method originated with C l a ~ s ~ ~ ~ 66 who showed that the degree of dispersion obtained depends upon several factors. A smooth electrode surface low cathode current density high acoustic energy and high frequency all favour finer particle size. The most suitable cathode materials are those to which the depositing metal adheres poorly. Practi- cally all metals which separate out electrolytically can be dispersed.67 This method should find use in the preparation of metal powders sols catalysts etc.In the case of electrodeposits which are not dispersed the action of ultrasonics on grain growth is the resultant of two opposing tendencies. The violent agitation in the solution decreases the cathode polarisation and therefore promotes the growth of large grains. On the other hand the mechanical vibrations may induce prolific nucleation in the depositing metal as they do during the solidification of metallic melts.68 Rummel and Schmitt 62 noted an increase in the grain size of copper deposits from an acid sulphate bath exposed to ultrasonics. On the other hand Levi 63 found that silver crystals in deposits from irradiated silver nitrate solutions were invariably very fine. Greater hardness and tensile strength of electro- deposited copper and nickel 61 and increased hardness of chromium deposits 6 1 s 69 have been produced by ultrasonics.This is indirect evi- dence for refinement of grain size in these deposits since reduced grain size generally increases the hardness and tensile strength of electroplated metals.70 The influence of an acoustic field on grain growth should increase as the intensity of the field is increased. This has been demonstrated by Roll 71 with nickel plating. The current-density range for bright nickel plating from a sulphate solution was shifted to continuously increasing current densities as the intensity was raised. For example the maximum current density for bright deposits which was 3 milliamp./cm.2 in the unstirred solution and 9 milliamp./cm.2 with agitation was raised to 10 and 43 milliamp./cm.2 respectively with ultrasonic intensities of 0-02 and 0.3 watt/cm.2. Muller and Kuss 61 reported that the acoustic effects on copper nickel and chromium electro- deposits diminished with an increase in frequency from 16 to 320 ldo- cycles/sec. They also made the interesting observation that brass deposits had higher zinc content when plated in the presence of ultrasonics. In general the effects described above refer to electrodes situated per- pendicular to the direction of propagation of the sound. An interesting phenomenon was observed by Young and Kersten v 2 when electroplating Frequency is also an important factor. 6 6 2. tech. Physik 1935 16 202. 67 Claus and Schmidt Kolloid-Beih. 1936 45 41. 68 Schmid and Ehret 2. Elektrochem. 1937 43 869 ; Schmid and Roll ibid.1939 6@ Ishiguro and Haramai J. Centr. Aeronaut. Res. Inst. 1944 No. 3 201 ; Chm. 70 Blum and Kogaboom " Principles of Electroplating and Electroforming " 3rd 7 1 Z . ,Wetallk. 1951 42 238. 73 J . Chem. Phys. 1936 4 426. 45 769 ; Sokoloff Acta Physicochim. U.R.S.S. 1935 3 939. Ah. 1948 42 1515. edn. p. 66 McGraw-Hill Book Co. Inc. New York 1949. BARNARTT EFFECTS OF ULTRASONIC WAVES ON ELECTROLYTES 101 metals on a cathode whose surface was parallel to the acoustic beam. Rippled deposits were obtained from several plating baths. The distance between ripples was equal to half the ultrasonic wave-length. Other investigators have also observed this 6 2 Young and Kersten interpreted their results as indicating that stationary waves were set up and that the metal ions were relatively more concentrated in layers 0-5 wave- length apart.According t o the Debye effect the periodic changes in ion concentration and the potential differences arising therefrom would be minute-too small to cause gross ripples in the deposit. It is more probable that the ripples result from the depolarising action of the acoustic field which can vary considerably from the nodal positions to the antinodes. In the process of concentrating deuterium by preferential elect,rodeposi- tion of hydrogen it has been shown that ultrasonic vibrations increase the efficiency of ~eparation.'~ The explanation of this effect is based upon the assumption that some of the deuterium evolved at the cathode is not dis- charged directly but is liberated by reactions such as H + HDO + HD + H,O which are catalysed by the electrode surface. The longer the discharged hydrogen remains in contact with the cathode the greater the quantity of deuterium liberated. Ultrasonic agitation speedily removes the dis- charged hydrogen from the electrode surface and therefore diminishes deuterium evolution. 73 Mason Biddick and Boyd J. Chem. Phys. 1951 19 1551.

ISSN:0009-2681    

DOI:10.1039/QR9530700084

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

Page Line 67 10* 68 12 8,* ERRATA 1951 Vol. V No. 1 Equation should read X = s(1 - C)Y 7,* 3,* and 2" for ethylene read ethylene dibromide for X read s 1952 Vol. VI No. 4 320 reference 8 for J . Chem. phys. read J . Chim. PhYS. 1953 Vol. VII No. 3 230 reference 19 should read 0. Hassel and E. Naesha- gen Tidsskr. Kjemi 1930,10 $1 ; Zentr. 1930 111 1956 * From bottom of main text.

ISSN:0009-2681    

DOI:10.1039/QR9530700444

出版商:RSC    

年代:1953

数据来源: RSC

摘要:

Age geological by radioactivity 1 Alkaloids kurchi 248 solanum 233 steroidal 231 veratrum 241 Amylosaccharides linear enzymic syn- thesis from cyclic 64 Amylose enzymic synthesis from glucose-1 phosphate 57 ; from maltose 62 Association of carboxylic acids 255 Atomic nucleus magnetic properties 279 Benzene hexachlorides molecular struc- Biological syntheses with isotopes 426 Bond energies and heats of formation 164 Bromine isotopes in organic compounds ture 227 433 436 440 44 1 Carbon isotopes in organic compounds 415 Carboxylic acids association 255 Chlorine isotopes in organic compounds Chromatography inorganic 307 Compressibility of fluids and velocity of sound 84 Conductivity of electrolytes effect of ultrasonic waves 90 Crystal analysis by X-rays in determina- tion of molecular structure 336 Cyclitols molecular structure 227 440 Debye effect of ultrasonic waves 91 Deuterium in organic compounds 411 Diamagnetism atomic ionic and mole- Differential syntheses in molecular-struc- Dimerisation of carboxylic acids 260 cular theory 379 ture determination 357 Electrodeposition effect of ultrasonic Electrolytes effects of ultrasonic waves 84 Electron-density measurements experi- Enzymic synthesis of polysaccharides 56 waves 98 mental and theoretical 375 Fatty acids natural and synthetic straight- chain 175 naturally occurring 193 saturated synthesis 176 unsaturated synthesis 182 Fluorine isotopes 440 Fourier refinement of atomic co-ordinates 349 INDEX 1953 445 Free radicals reactions 198 Frequencies characteristic of crystalline of paraffins in the liquid state 33 42 Fructosans of levan class enzymic syn- of inulin class enzymic synthesis 77 n-parafhs 31 45 48 51 53 54 thesis 75 Galactans enzymic synthesis 74 Germanium heat of vaporisation 121 a-Glucosans of dextran class enzymic of starch class enzymic synthesis 56 Glycogen enzymic synthesis from glu- synthesis 70 cose-1 phosphate 68 Halogens isotopes in organic compounds Heats of formation and bond energies 440 164 of simple inorganic compounds 134 experimental determination 138 cycZoHexane stereochemistry 22 1 Hydrogen isotopes in organic compounds 41 1 atoms location in X-ray analysis of cryst,als 359 bonding in solid carboxylic acids 267 Infra-red spectra of hydrocarbons 19 Inorganic compounds heats of formation Iodine isotopes in organic compounds Ion exchange and chromatography 323 Isomerism rotational in paraffis 24 Isotopic synthesis of organic compounds 134 44 1 40 7 Kurchi alkaloids 248 Labelling of organic compounds by iso- Least-squares method of structure analy- topes 407 sis 364 Magnetic resonance absorption nuclear Magnetism and inorganic chemistry 377 Magnet'on nuclear 280 Methyl radicals metathetical reactions Molecular structure determination by 279 208 210 reactions 198 X-ray analysis 335 446 INDEX Nitrogen isotopes in organic compounds Nuclear magnetic resonance absorption 430 279 Organic compounds isotopically labelled Oxygen isotopes in organic compounds synthesis 407 ; purity 409 433 ParafEns infra-red and Raman spectra 19 n-Paraffins trans-isomers configuration Paramagnetic susceptibility applications Paramagnetism theory of 380 Periodic table survey of magnetic proper- ties of elements in 390 and heats of formation 142 29 386 Pharmacology of steroidal alkaloids 264 Phase problem solution of in X-ray analysis of crystal structure 342 Phosphorus isotopes in organic com- pounds 434 Pleochroic haloes 5 Polymerisation of carboxylic acids 265 Polysaccharides enzymic synthesis 56 Potentials electrode effects of ultrasonic waves on 03 Radioactivity in geological age determina- Radiocarbon method for geological age 13 Raman spectra of hydrocarbons 19 Resonance absorption paramagnetic 405 Ring systems condensed molecular struc- tion 1 ture 229 Silicon heat of vaporisation 11 6 monoxide heat of formation and dis- sociation energy 122 Solanum alkaloids 233 Solvation from compressibility data 89 Space charge creation in electrolytes by Stability in relation to valency 168 Stellar energy 17 Stereochemistry and magnetic properties 389 402 ulOrasonic waves 91 of cyclohesane 221 Steroidal alkaloids 231 Sulphur isotopes in organic compounds 437 Thermal motion effects in X-ray crystal Thermochemistry of elements of groups Tin heat of vaporisation 118 Transuranic elements magnetic propsr- Tritium in organic compounds 414 analysis 3 6 1 IVB and IV 103 ties 400 Ultrasonic waves effects on electrolytes and electrode processes 84 Valency in relation to stability 168 Veratrum alkaloids 241 Vibrations carbon-hydrogen 2 1 skeletal 24 X-Rays determination of molecular struc- ture of crystals 335

ISSN:0009-2681    

DOI:10.1039/QR9530700445

出版商:RSC    

年代:1953

数据来源: RSC