摘要:

QUARTERLY REVIEWS VOL. I 1947 Committee of Publication Chairman C. IS. HINSHELWOOD M.A. Sc.D. D.Sc. F.R.S. N. K. ADAM M.A. Sc.D. F.R.S. M. P. APPLEBEY M.B.E. M.A. D.Sc. W. BAKER M.A. D.Sc. F.R.S. G. M. BENNETT M.A. Sc.D. F.R.S. E. G. Cox D.Sc. F. P. DUNN B.Sc. F.R.I.C. H. J. EMEL~US D.Sc. F.R.S.. M. G. EVANS D.Sc. F.R.S. C. S. GIBSON O.B.E. M.A.. Sc.D. D. L. HAMMICK M.A. D. H. HEY Ph.D. D.Sc. F.R.I.C. C. K. INGOLD D.Sc.,F.R.S. F.R.I.C. D. J. G. IVES D.Sc. A.R.C.S. E. R. H. JONES D.Sc. F.R.I.C. F. E. KING M.A. DSc. Ph.D. D.Phil. G. A. R. KON M.A. D.Sc. F.R.S. F.R.I.C. F.R.I.C. A.R.C.S. F.R.S. F.R.I.C. Editor F.R.I.C. 4. E. DRIVER M.A. Ph.D. M.Sc. R. P. LINSTEAD C.B.E. D.Sc. F.R.S. H. W. MELVILLE Ph.D. D.Sc. R. G. W. NORRISH B.A. Sc.D. S. G. P. PLANT D.Phil. M.A. B.Sc. M. POLANYI Ph.D.M.D. F.R.S. H. RAISTRICK Sc.D. F.R.S. F.R.I.C. E. K. RIDEAL M.B.E. M.A. D.Sc. F. L. ROSE B.Sc. Ph.D. F.R.I.C. B. C. SAUNDERS M.,4. Ph.D. J. L. SIMONSEN DSc. F.R.S. D. W. G. STYLE Ph.D. S. SUGDEN D.Sc. F.R.S. H. W. THOMPSON M.A. B.Sc. A. R. TODD M.A. D.Sc. F.R.S. W. WARDLAW D.Sc. F.R.I.C. F. G. YOUNG DSc. Ph.D. F.R.I.C. F.R.S. F.R.S. Ph.D. F.R.S. F.R.I.C. F.R.I.C. D.Phil. F.R.S. Assistant Editor A . D. MITCHELL D.Sc. F.R.I.C. Indexer MARGARET LE PLA BSc. LONDON THE CHEMICAL S O C I E T Y CONTENTS FLVORESCESCE AND FLUORESCENCE QUENCHIKG. By E. J. BO\VES . C‘OLOCR .%SD (’OXSTITUTION. By A. hlACCOLL . AMORPHOVS CARBON -4313 GIXAPHITE. By H. L. RILEY . FORCE C‘OSST-~NTS. By J . \\.. LINXETT . Ocsasrc S ~ L T DEPOSITS. By F. C. PHILLIPS . THE USE OF THE TERMS .. -ACID ” AND .. BASE ”. BJ’ R. P. BELL THE SEPARATION OF THE LANTHAKOSS (RARE-EARTH ELEMENTS). By J . I<. MARSH . REPRESESTATION OF SIMPLE NOLECULES BY MOLECULAR ORBITALS. U y C. A. COULSON _ASPECTS OF I ~ l S I U S O C H E ~ I I S T I ( Y . 11. STACEY . Bas~c SALTS. By H. BASSETT . SOME THEKSIODYS;.+MIC PROPERTIES OF HIGH POLYMERS AND THEIR 3TOLECUL.lR ISTERPRET.%TIOT. EJ’ G. GEE . -~SYXMETRIC TR.~NSFORM.ITIOS ANII SYMMETRIC INDUCTION. By 1:. E. TURNER and SI. 31. HIRHIS . C’HEMISTHY OF THE ATETAL C’AIIBOS\7LS. I3y J. S. ANDERSON . r’.mE 1 16 59 73 91 113 126 144 179 246 265 299 33 1 T H E ALIPH.ATIC ~lTRO-C‘OMPOUYDS. By S. LEI-Y and J. D. ROSE 358

ISSN:0009-2681    

DOI:10.1039/QR94701FP001

出版商:RSC    

年代:1947

数据来源: RSC

摘要:

COLOUR AND CONSTITUTION By A. MACUOLL M.Sc. (I.C.I. RESEARCH FELLOW UNIVERSITY OF LONDON) 1. Introduction THE birth of the organic dye industry with the discovery of mauve by Perkin in 1856 made possible attempts at the correlation of the colour of an organic compound with its chemibal constitution. The first observation was made by C. Graebe and C. Liebermam twelve years later. These writers pointed out that all the coloured compounds known a t that time became colourless on reduction. Their conclusion was that the colour of a compound was in some way due to a close linking between the C 0 rand N atoms contained in it. 0. N. Witt in 1876 introduced the con- ceptions that have formed the basis of all subsequent theories. According to his theory before colour can appear in a compound two conditions have to be satisfied.First the molecule must possess a potentiality for colour ; secondly there must be a salt-forming group present for developing the colour potentiality. A group of atoms possessing the potentiality for colour is known as a chromophose the molecule containing it being a chromogen. The group responsible for developing the colour is known as an auxochrome. Typical chromophores are -NO -NO, -N=N- C=C and C=O and auxochromic groups are represented by -OH -NH, -NHR and -NR,. Some chromogens are themselves coloured for example nitrosobenzene (I) fulvene (11) and diacetyl (111). But it is of NO CH2 I ti 0 A nil 0 0 II I1 CH3-C-C-CH3 (1.) (11.) (111.) interest to note that none of the chromogens possesses dyeing properties. Thus Witt’s concept of the function of an auxochrome was not only to develop the colour latent in the chromogen but also to develop its dyeing propensities.H. E. Armstrong in 1888 advanced the view that many organic dyes either contained the quinonoid grouping or could be modified in such a manner as to contain it. This view was probably arrived at from a con- sideration of the recently discovered phenomenon of tautomerism. That the quinonoid theory was not sufficient to account for all the facts is seen 1 An excellent account of the development of dolour theory up to 1918 is to be found in Watson’s ‘‘ Colour in Relation to Chemical Constitution,” Longmans Green & Co. London 1918. Ber. 1868 1 106. Ibid. 1876 9 552. Proc. 1885 4 37. 16 MSCCOLL COLOUR -4ND CONSTITUTION 17 from a consideration of iminoquinone (IV) and di-iminoquinone (V) both (IV.) of which possess the quinonoid structure but are colourless.Then again a number of coloured substances are known to which a quinonoid structure cannot possibly be assigned. However the quinonoid theory was of great importance both because of the impetus it gave to experimental work and because of its practical application in the hands of dye chemists. In attempting to correlate the data on the triphenylmethane dyes R. Nietzki in 1879 postulated that the colour of a dye may be deepened by adding groups so as to increase t'he molecular weight the deepening being roughly proportional to the increase in molecular weight. M. Schutze 6 in 1892 subjected this rule to a careful scrutiny and was able to point out many exceptions. He was able to show that the chemical nature of the added group was of importance and introduced the terms bathychromic and hypsochromic.The former refers to a group the addition of which t o a molecule gives rise to a deepening of the colour the latter referring to a group which has the opposite effect. The term deptlh of colour will be defined later. Although the colour of organic compounds is one of their most striking properties yet it was realised early in this century that it is unsatisfactory to attempt t o base a theory of colour merely upon visual observation. The human eye is only sensitive to the spectral region from about 4000 A. t o 7500 A. and this physiological fact places a severe limitation upon the range of observation. The recognition that the ultra-violet region is merely a continuation of the visible made possible an explanation as to how an apparently small change of structure might convert colourless substance into a yellow one.This view was well expressed in the Annual Reports for 1907 " It is now generally recognised that a more precise meaning must be given to the idea of colour than has often been the caee. The production of physiological colour due to the occurrence of absorption in the visible spectrum is more or less an accidental circumstance. Absorption bands may occur in the ultra-violet of equal importance with those in the visible spectrum. I n some cases a change in the frequency of the absorbed rays may cause a band to move from the ultra-violet into the visible region without any change of form. A colourless substance may therefore be converted into a coloured one without any real change in constitution having taken place.. . . The quantitative study of the absorbing power of a substance . . . makes an exact comparison of Merent derivatives possible and relationships are made evident which would escape notice if the ex- amination were confined to visual observation of colour. '' This realisation of the importance of objective measurements of colour enabled the basis of the physicochemicsl theory of colour to be laid. However this theory 6 Verhandl. Vereins Bef6rd. Gewerbefleisses 1879 58 231. 2. physikal. Chem. 1892 9 109. B 18 QUARTERLY R;ICVIEWS is dependent upon quantum concepts and did not develop until some thirty years later. The colour of an organic compound is due to its having one or more absorption bands in the visible region of the spectrum.The relationship between the wave-length of the absorbed light and the colour of a compound is shown in Table I. This table is strictly only applicable to a substance TABLE I Absorption Wave-length and C o h ~ Wave-length A. 4000-4350 4350-4800 4800-4900 4900-5000 5000-5600 5600-5800 5800-5950 5950-6050 6050-7500 Colour absorbed. violet blue green-blue blue-green green yellow-green yellow orange red yellow-green yellow orange red violet blue green-blue blue-green Purple which has a single narrow absorption band between 4000 and 7500 A. High intensity of absorption often implies a wide absorption band which may invalidate conclusions drawn from the table. Also colour is a subjective phenomenon. I n view of these complications the desirability of representing the colour of a compound by an objective method becomes obvious.The term depth of colour which has been used in a number of senses may now be defined. I n the following one substance will be said to have a deeper colour than another if the wave-length of maximum absorption is greater for the former than the latter. As the absorption moves towards the red the colour is said to deepen. He noted that the introduction of an amino-group into fuchsonimine (VI) to give Doebner’s violet (VII) led to the development of colour and he attributed An important observation was made by A. Baeyer in 1907. ’ A m d e n 1907 354 162. MAccOI& COLOUR AND CONS!lXTUTION 19 this to an oscillation of the quinonoid condition between the two benzene nuclei. However here again not all coloured compounds could be classified as of this type.R. Willstattm,e as a result of his observations on the quinhydrones came to a similar conclusion namely that colour is due to an oscillation by which two benzene nuclei alternately become quinonoid. Following investigations on the colour of azo-compounds J. T. Hewitt and H. V. Mitchell in 1907 concluded that the colour of a substance is deeper the longer the conjugated chain. This view was modified in 1916 by A. C. Sircar,l* who suggested that the important factor was the length of the conjugated chain in the part of the molecule containing the auxo- chrome. In 1913 E. R. Watson 11 propounded the view that dye molecules. which are quinonoid in all possible modifications will have a deep colour Thus indamine (VIII) is blue while 4 4’-diaminoazobenzene (IX) is yellow.Two years later E. R. Watson and D. B. Bleek l2 suggested that the col’oa is deeper the longer the conjugated chain reversed in the tautomeric change. These authors were led to distinguish two cases. The first was that in which the substance was ionised and the oscillation consisted merely “ in a rhythmic rearrangement of strain within the molecule,” which wag responsible for the deeper and more intense colour ; thus for Michler’s hydro1 (X) the oscillation was represented by. The second case was concerned with hydrogen atom tautomerism. Another phase of colour theory WM initiated in 1910 by P. Pfeiffer,ls who from a study of free aryl radicals concluded that the tervalent carbon atom was the essential chromophore. W. Dilthey lP in 1920 went further and regarded chromophoric properties as being associated with atoma rather than with groups of atoms as Witt had postulated.The roots of chromo- phoric activity in atoms lay in their “ co-ordinative unsaturation.” Thus while the carbon atoms in diamond or in a paraffin have all their valencies satisfied in ethylene benzene and graphite this is not so. Thus the carbon atoms in the latter substances can act as chromophores. Similarly in the triphenylmethyl cation (XI) the carbon atom marked with a dot (in Dilthey’a Ber. 1908 41 1458 3245. J. 1907 91 1251. lo J. 1916 109 757. l 1 Proc. 1913 29 348; J . 1914 105 759 l * J . 1915 107 1567. l 3 Annalen 1910 376 292. l4 Ber. 1920 53 261. 20 QUARTERLY REVIEWS nomenclature) possemea chromophoric activity. Dilthey also observed that in non-polar compounds chromophoric activity ie often weak but becomes strengthened on conversion into an ionic form.(XI.) R. K. Wizinger l6 in 1926-27 emphasised the fact that the action of auxochromes was most observable in kationic chromogens. This led t b the concept of anti-auxochromes which function in the opposite sense to the auxochrome in that they behave in an anion in the same way as do auxochromes in a cation. Such anti-auxochromes are -NO -NO, -N=N- and >C=O. ,The colour of electrically neutral molecules in the Dilthey-Wizinger theory is due to the presence of an intramolecular ionoid state between auxochrome and anti-auxochrome. In the case of p-nitro-p’-methoxystilbene (XI) the chromophore auxochrome and anti-auxochrome are respectively the >C=C< -OMe and -NO groups.- L M~o-~-cH=cH-C-)-NO (XII.) J. Stieglitz l6 approached the problem of colour from the viewpoint of oxidation-reduction. Reduction of a dye as noticed by Graebe and Liebermann usually destroys its colour while gentle oxidation will restore it. Strong oxidation on the other hand may completely destroy it. The concept of a dye being in an intermediate state of oxidation led Stieglitz to identify chromophores and auxochromes with groups capable of oxidation and reduction respectively. The colour of a compound is then due to a type of intramoleculaT oxidation-reduction. The importance of the intensity as well as the wave-length of maximum absorption was emphasised by N. Q . Chak0.l’ He pointed out that a number of the bands which were responsible for the colours of organic compounds were relatively weak and by applying the classical theory of the light absorption process he was able to calculate a fundamental measure of the intensity of absorption in the so-called oscillator strength.Some interesting relationships between the oscillator strength and structure of related compounds emerged although no readily interpretable dependence of band width the other constant of the classical theory and structure came out. l5 “ Organische Farbstoffe,” Ferd. Dummlers Verlag Berlin u. Bonn 1933. 16 Proc. Nut. Acad. Sci. 1923 9 303. 17 J . Chern. Physics 1934 2 644. MACCOLL COLOUR AND CONSTITUTION 21 The development of quantum mechanics in the years following 1926 made possible a more detailed theory of colour in terms of molecular energy levels.C. R. Bury l* suggested that the basis of the colour of an organic compound might lie in the quantum-mechanical resonance phenomenon and he was able to translate many of the older ideas on colour into modern terms. Thus Baeyer had postulated that the colour of Doebner’s violet was due to an oscillation between (XIII) and (XIV) whereas Bury pointed NH,CI=<=)=CC>-NH + I - Ph (X1II.I Ph (XIV.) out that the oscillation was only structural resonance theory providing a mechanism for the “rhythmic rearrangement ” suggested by Watson and Meek. The function of the auxochrome then was to realise the ~H,-/-)=C(T>-NH 1- + NH2-C)-P={=>=&H2 - Ph Ph possibility of resonance. The same type of mechanism holds for the case of acidic dyes such as benzaurin (XV). However Bury was at a lorn to Ph Ph (XV.) provide the mechanism for neutral dyes like indigo (XVI).I1 II 0 0 (XVI.) + NH NH I1 0 I 0- (XVII.) Before 1937 quantum-mechanical calculations had aimed a t determin- ing the energy of the ground state of a molecule and thus its resonance energy. However in that year A. L. Sklar l9 realised the importance of the excited states of a molecule for the light-absorption process and was able to calculate the absorption bands of a number of unsaturated hydro- 18 J . A m r . .Chem. SOC. 1935 57 2115. J . Chem. Phyeice 1937 5 669. 22 QUARTERLY REVIEWS carbons. In the next year this work was extended by Th. Forster.20 The attack on the quantum-mechanical problem of the colour of dyes was sketched out by L. Pauling 21 in 1939. As well as outlining the general solution of the acid and basic dyes problem Pauling was able to account qualitatively for the absorption bands of neutral dyes such as indigo in terms of resonance between the classical valency structures and dipolar structures of higher energy such as (XVII).This problem was also con- sidered by F. Arndt and B. Eistert.22 Since 1939 this work has been followed up by R. S. Mulliken Th. Forster A. L. Sklar K. F. Herzfeld and others. It is of interest to note that whereas the older theories were concerned with complex molecules and were inadequate to deal with simple systems quantum mechanics has reversed this position and having started with the simple systems is now developing methods of attack for the more complex. G. N. Lewis and M. Calvin 23 in 1939 set out “ t o re-examine the data of light absorption and to see whether by applying the more inductive methods of chemistry together with such general results of quantum theory as are applicable to all systems we may obtain a better understanding of the data.Starting with the idea that in the absorption of light the energy is taken up by electronic oscillations we have considered these oscillations as analogous to classical oscillations but subjected to the rules of simple quantisation.” This quasi-classical theory has correlated a great deal of data and has stimulated some extremely interesting experimental investigations. From this point this article will be primarily concerned with a more careful scrutiny of the development of the theory of colour that has taken place in the last decade or so. 2. The Clmsical Theory of Light Absorption 24 The classical theory of the interaction of light with matter due to Drude is based upon Maxwell’s electromagnetic theory and the concept of electrons.The former theory shows that a beam of light may be con- sidered as consisting of transverse waves the oscillating entities being the electric vector E and the magnetic vector H these two vectors being per- pendicular to each other and t o the direction of propagation. The plane through the electric vector and the direction of propagation is known as the plane of polarisation. Explicit solution of Maxwell’s wave equation shows that a plane monochromatic wave of frequency Y travelling in the z direction in a medium of refractive index a and polarised in the x direction is given by E = S,~~IZ~V(~-UZ/C) H = j ~ y e 2 n i r 3 ( t - ~ ~ / ~ ) .. (2.1) ao 2. physikal. Chem. 1938 B 41 287. e a Ber. 1939 72 202 860. a 4 (a) The treatment of this section closely follows that of Chako (ref. 17). a1 Proc. Nat. Acad. Sci. 1939 25 577. 23 Chenz. Reviews 1939 25 273. Com- pare also ( b ) Th. Forstor 2. Elektrochem. 1939 45 551 ; (c) R. A. Morton Ann. Reports 1941 38 7 ; ( d ) R. S . Mulliken and C. A. Rieke Rep. Prog. P h y s k 1941 8 231. MACCOLL COLOUR AND CONSTITUTION 23 where i and j are unit vectors along the x and y axes and c is the velocity of the waves in @ vacuum. In order to investigate the interaction of radiation with matter some hypothesis as to the structure of matter has to be made. It is satisfactory for the present purpose to assume that in a gaseous medium containing N molecules per C.C.each molecule containsjj electrons bound by an elastic force to an equilibrium position and having a frequency of oscillation vj. The electric force acting on a given electron in the molecule is the product of the electric vector and the charge e of the electron. It can be shown that the magnetic force is negligible. The oscillating electric field of the light wave will cause forced vibrations of the electrons the equations of motion of the latter being of the form where w = 2nv wj = 2nvi ; vj is the proper frequency of the electron and is determined by the elastic force binding the electron to its equilibrium position. A solution of (2.2) together with the solution of Maxwell's equations suffices to evaluate a the refractive index usually represented by n.The result is mh + w;mz = eE&W(t-uZ/c) . (2.2) an equation in accord with the experimental results. It can be seen that for w <aj the refractive index increases with the frequency of the light a phenomenon known as normal dispersion. If the frequency of the light coincides with one of the electronic proper frequencies the refractive index becomes infinite. The analysis may however be carried through if it is assumed that the electrons are subject to a " damping force," proportional to their velocities. The eqhation of motion then becomes mx + mgjx + dmx = eE,&w(t-44 . . (2.4) The refractive index may be determined by the same procedure as before and is given by aa = + T & a - 0 2 3 + ~ i w * . (2.5) Thus the motion of the light wave in the medium is governed by a complex refractive index.Writing a = n(1 - i ~ ) . . (2.6) and separating real and imaginary parts we have 4nezN' 3 . (2.8) The refractive index thus shows a marked change in the neighbourhood of an electronic proper frequency. If it is assumed that there is a single 24 QUrnTERLY BEVIEWS electronic freqnency then (2.7) shows as the frequency of the light approaches that of the electron the refractive index increases until a maximum is reached after which it begins to decrease reaching a minimum when it again increases. The existence of more than one electronic frequency will complicate this phenomenon which is known as anomdous dispersion. To interpret K the ratio of the energy of the wave after having traverqd a distance x in the medium to its value on entering the medium has to be determined.This treatment leads to I = Ioeb-/c . . (2.9) where I, and I are the entrant and emergent intensities. While this law has been derived for the gaseous state most of the relevant data refer to solutions. For this reason (2.9) wil’l be written in a form appropriate to solutions and the corrections which are necessitated by this procedure will be discussed later. Thus since K is proportional to N’ (2.9) mcby be expressed as I IQO-E’Cd * . (2.10) E’ being a new constant c being the concentration of the medium and d the distance through which the light has travelled. Formula (2.10) combines Beer’s and Lambert’s laws and has received experimental confirmation. By the um of (2.8) (2.9) and (2.10) the following expression is obtained for E’ ( 2 . i l ) where yj = gj/2n and N is Avogadro’s number.F’rom (2.11) it follows that if Y is markedly different from v, E’ is essentially zero. The curve of E’ against v is bell-shaped E’ reaching a maximum at Y = vj. The substance is said to have an absorption band at vj. The integrated intensity over the whole band may be obtained in the following way. If the band in question is a weak one removed from the strong bands that are responsible for dispersion then its contribution to n may be neglected and for n may be written the value due to all other bands at Y. In addition if the band is sufficiently narrow it can be shown that neaN x 10-5 cmn d.dv = (2.12) Up to the present it has been assumed that the absorbing substance is in the gaseous state. To test the effect of a solvent which does not absorb in the region in question the Lorentz-Lorenz forces acting on the absorbing molecule due to the polarisation of surrounding molecules have to be taken into account.e’.dV = . C17t . ( L ) j j 9n Q - . (2.13) This treatment leads to nelN x 10-3 (na 4- 2)’ I MACCOLL COLOUR AND CONSTITUTION 25 for a dilute solution no being the refractive index of the solvent at fre- quency Y . This constitutes a correction to the integrated intensity as measured in solution t o give the vdue for the gaseous state. Chako haa tabulated fi 5 and also f” which is obtained from (2.12) by setting n = 1. With regard to the solvent correction Chako came to the conclusion “ that it is impossible t o account for the influence of the solvent through the Lorentz-Lorenz forces.” This problem of solvent correction has also been considered by R.S. M ~ l l i k e n . ~ * ~ Some interesting data which throw light on this question have been recorded by V. Henri and L. W. Pickett 25 for cyclohexadiene and by L. W. Pickett E. Paddock and E. Sackter *6 for cyclopentadiene. For cyclohexadiene the vapour and the solution values agreed very well but for cyclopentadiene the integrated absorption was about 20% higher for the vapour than for the solution. This effect is the opposite t o that which would be predicted from a consideration of the Lorentz-Lorenz forces On such grounds as these Mulliken concluded that in the absence of empirical data as to the ratio of vapour to solution intensity the best procedure is t o neglect the correction given by (2.13) and to assume n = 1 which would be true for the substance in the gaseous state.The importance of the correction of f values derived from measurements in solution t o f values for the gaseous state lies in the fact that the latter quantities may be calculated by quantum-mechanical methods as will be shown in Section 3. - From (2.11) it follows that I 2e2N x 1 0 - 3 fj (n = 1) CmYi E max. = or by using (2.12) (2.14) If y is constant throughout the band dv = 2yj . . (2.16) where dv is the distance between the points at which E’ = 1 / 2 ~ ’ ~ ~ ~ . . The “ damping constant ” y is equal to the half-width of the band. Both (2.15) and (2.16) were used by Chako to evaluate yj. However no readily interpretable relationship was found between y j and the chemical constitution of the substance or the nature of the solvent. Equation (2.14) allows an estimate to be made of the magnitude of For if it is assumed that fj = 1 i.e.that there is one electron per molecule concerned in the absorption process and that y - 2000 cm.-l then substitution in (2.12) shows E ’ ~ ~ ~ . to be of the order of 100,000. How- ever for many molecules Experimental results are usually expressed in terms of E or E defined by 7 = 1 l O - E . . (2.17) E = ECd . . (2.18) is much smaller. a 5 J . Chew. PhysicR 1939. 7 439. 2 6 J . Anzer. Chem. SOC. 1941 63 1073. 26 QUARTERLY REVIEWS E is known as the extinction while E the extinction for unit concentration and chtance is known as the molecular extinction coefficient c being the molar concentration. The data are conveniently represented by plotting E or E or their logarithms as abcissa the ordinate being either the wave- length or the frequency.In the former case either hgstrom units (A. ; 1 A. = cm.) or millimicrons (mp ; 1 mp = lo-’ cm.) are used while in the latter the frequency (Y) in sec.-l or the wave-number.(Y” = AL1 = Y / C ) in cm.-l is employed. The frequency is conveniently expressed in fresnels one hsnel being equal to a frequency of lo1 sec.-1. To determine the f value of a band the method usually employed is to measure the area under the curve obtained by plotting e as a function of 5 ; f can then be calculated from (2.12) allowance being made for the change of base of the logarithms. Thus f - 4.31 x lo-’ &.dG . . (2.19) While Lambert’s law-that the proportion of light absorbed by a sub- stance is independent of the incident intensity but directly proportional to the logarithm of the distance travelled-always holds Beer’s law is not of such universal applicability.Beer’s law states the proportionality be- tween the light absorbed and the number of molecules of the absorbing substance through which the light passes. It does however apply where chemical reaction either between solute molecules or between solute and solvent molecules does not occur. A more detailed picture of the electronic proper frequencies and the oscillator strengths is afforded by Bohr’s adaptation of the old quantum theory to the discussion of the energy states of atoms and molecules. Bohr showed that atoms and molecules exist in radiationless stationary states of constant energy known as energy levels. When the electron jumps from a stationary state of energy El to one of energy E, light is absorbed or emitted according as El 2 E, the frequehcy being J (2.20) where h is Planck’s constant.The electronic proper frequencies are the values of Y given by (2.20) and E - El is the transition energy. In the molecular case the vibrational-rotational energy of the molecule as a whole is quantised as well as the electronic energy. As the order of magnitude of these energies is usually Erot.@Evib.&Eelec. the internal energy of the molecule (Le. excluding translational energy) may be written E = Erot. + Evib -t- Eelec. Three types of spectra can be distinguished viz. rotational spectra occurring in the far infra-red vibration-rotation spectra in the near infra-red and electronic spectra in the visible and ultra-violet. Molecular spectra are distinguished from atomic spectra in that the latter consist of a number of h e e whereas the former consist of bands except at very high resolution MACCOLL COLOUR AND CONSTITUTION 27 in the gas phase when the bands are seen to consist of closely and regularly spaced lines.The bands arise from the fact that concurrently with an electronic transition there are vibrational and rotational transitions. The spectrum of a molecule in the gas phase is thus very complex and a great deal of work has been published dealing with the interpretation of the fine structure of molecular spectra. However this lies outside the scope of the present review.27 In silution owing to inter molecular interact ions the fine structure of the electronic bands is largely lost and smoothed off regions of relatively large width appear.Vibra- tional structure may still remain as is shown for benzene in Fig 1 . 2 8 The vibrational energy levels obtained from the electronic absorption spectra will be those of the excited state since most of the absorbing molecules will be in the lowest vibrational level of the normal electronic state. Because of the compli- cations produced by vibrational struc- ture it is not surprising that Chako found no correlation between the half- widths of absorption bands and the nature of the absorbing molecule or of the solvent. The old quantum theory also pro- vides an explanation of the low values of f (- 0.0001) which are sometimes observed for molecules. On the classical theory the explanation would be naive namely that only one in 10,OOO molecules was of a kind capable of absorption.The old quantum theory I . ._ I moo 2500 d A. FIG. 1 The absorption spectrum of benzene in ethyl-alcoholic solution showing the fine structure. interprets f as the probability of transition for a given molecule. This view makes possible fractional f numbers but no further information as to their magnitude could be obtained apart from the fact that zlfj was equal to the total number of electrons in the molecule. On the basis of dispersion theory the j’ are known as oscillator strength. The procedure adopted in obtaining the f value of a band is equivalent to comparing it with a single line in an atomic spectrum. Where the band shows vibrational structure the analogous case would be an atomic line showing fine structure. The justification for this procedure has been examined by R.S. M~lliken.~~ $7 This field is reviewed by H. Sponer and E. Teller Rev. Mod. Physics 1941 13 76. 28 W. V. Mayneord and E. M. F. Roe Proc. Roy. SOC. 1935 A 152 299. 4D J . Chem. Phyeice 1939 7 14. 28 QUARTERLY REVIEWS 3. The Quasi-classical Theory of Lewis and Calvin 3Q This theory is based on the concept that the energy absorbed during the interaction of a molecule with radiation is taken up by electronic oscil- lations within the molecule. The oscillations are regarded as being analogous to classical oscillations but quantum methods are used to determine a set of energy levels. One of the simplest molecules that can be considered is ethane which shows continuous absorption below 1600 A. Indeed it is characteristic of most simple molecules that absorption commences only in the far ultra-violet.The oscillating unit is taken to be the pair of electrons constituting the C-C bond. Since electronic motion is very rapid compared with nuclear motion no difficulty arises from taking the nuclei as being fixed in space. The electronic oscillations may be interpreted in terms of the following resonance structures for ethane f - H,C :CH H,G-CH H86 6H It is not necessary to assume a complete transition from (XVIII) to (XX) an approach towards these structures from (XIX) being all that is required. If now a restoring force proportional to the displacement of the electron pair from its equilibrium position is assumed the energy levels of the system are given by En - (n + 1 / 2 ) h v n - 0 1 2 . . . . (3.1) where Y is the frequency of the electronic oscillation related to the restoring force/unit distance (k) by (XVIII.) (XIX.) (=.I . (3.2) and n is the quantum number specifying the energy level. Absorption then corresponds to a change in n from 0 to 1 the frequency of the light absorbed being Y as given by the Bohr frequency condition (2.20). Even when the oscillator is in the state n = 0 there is a certain energy associated with it the zero-point energy. The authors next consider a series of double bonds c=s \ / --N=N- \ C=N- \ / \ / \ / c=c arranged in order of decreasing force constant by the following argument. Simple compounds containing a carbon-carbon double bond absorb in the region of 2000 A. those containing a carbon-oxygen double bond a t about 2800 A. while the absorption characteristic of the azo-group lies at about 3500 A.Although the spectra of simple compounds containing carbon- nitrogen or carbon-sulphur double bonds have not been studied the positions of these groups can be asaigned from their behaviour in more complex compounds. Assuming that the double bond is the essential chromophore 30 Reference 23. MACCOIL COLOUR AND CONSTITUTION 29 k.e. neglecting the lone pairs on the nitrogen sulphur and oxygen atoms it follows that the force constant diminishes in the series from >C=C< to >C=S. The extreme structures t o which the eleotronic oscillations tend are represented by (XXI) and (XXII) (XXIII) being the clttrssical + - - + A- :B A :-B A=B (=I.) (XXII.) (XXIII.) structure. If the oscillations are simple harmonic a series of parabolae representing the potential energies of the electron pair in the various nuclear frameworks can be plotted.For this type of potential function tho selection rules lay down that An = & 1 i.e. the oscillator can only jump from a given state to an adjacent one. However if the potential function becomes non-parabolic a t large displacements the transition An = 2 becomes allowed the intensity varying with the degree of departure from the parabola.31 In the case of a conjugated molecule as well as there being a possibility of overtones there is also the possibility of the molecule possessing a band which is relatively insensitive to the structure of the molecule as a whole. These considerations led Lewis and Calvin to propose the following classifica- tion of absorption bands. ( A ) Fundamental Bands (1) First order n changes from 0 to 1.(2) Second order n changes from 0 to 2. (B) Bands of Partial Oscillation. Whereas type A are characteristic of the molecule as a whole type B are. the result of a 1oca.lised oscillation. This classification may be illustrated by the compounds (XXIV) (XXV) and (XXVI). Although the absorption (XXIV.) CH= I t Et Et (XXVI.) 31 The second-order bands aro further dodt with by G. N. Lewis a.nd J. Bigeleisen J . Amer. Chew. SOC. 1943 65 2107. 30 QUAB-Y BBVIEWS curma of (XXrV) md (XXV) are closely similar that of (XXVI) while showing a band similar to that of (XXV) also shows an entirely new band at longer wave-length. The former band can be considered aa due to partial oscillation the latter being characteristic of the molecule as a whole. A further classification comes from a consideration of %he geometry of the molecule.In the case of a diphenylpolyene such as (XXVII) the D-CH=CH-CH=(XXVII.) 0 0 I (XXVIII.) oscillation may be thought of as following an approximately linear path. However in a molecule such as crystal-violet (XXVIII) the positive charge can be placed on any of the three nitrogen atoms and so the oscillation can be resolved into two component oscillations at right angles. I n this case the oscillation frequencies in the two directions will be equal but if there is a marked dissymmetry in the molecule this degeneracy will be removed. Similarly if the absorbing molecule possesses extension in three dimensions there will be the possibility of a third component oscillation. In this way the fundamental bands of a molecule can be classified as NMe (a) x Bands.One-dimensional oscillator. ( b ) x and y Bands. ( c ) x y and x Bands. Two-dimensional oscillator. Three-dimensional oscillator. By convention the band of longest wave-lcngth characteristic of the molecule as a whole is taken to be the x band.32 The nature of the electronic oscillations may be enquired into in further detail. If as a first approximation the effect of the end groups is neglected these systems can be represented by the extreme structures being R-CH-[CH =CHI,-,-CH-R and R-CH-[CH =CH].-,-CH-R The polyenes form one extreme class. R-[CH =CHJ.-R + - - 4- .. .. 33 Additional evidence for y bands is afforded by Lewis and Bigeleisen ibid. p 2102.' MACCOLL COLOUR AND CONSTITUTION 31 The normal state of the molecule will be non-polar.However in the presence of an electric field of strength E a moment ( p ) will be induced in each > CH-CH < unit given by ,u = e x = x E . . (3.3) x being the displacement of the charge and a the polarisability of the molecule. When the electrons are displaced the eIectric force acting on them must be balanced by the restoring forces. Thus eE = k z . . (3.4) From (3.3) and (3.4) it follows that the force constant is given by k = e 2 / x . . (3.6) If the molecule now be placed in the field of a light wave whose wave- length is long compared with the dimensions of the molecule individual oscillations are set up in the units of the chain the displacements being the same for each unit. On the assumption that in the excited states of the molecule the oscillations are of the same character as the induced oscillations discussed above the dependencc of frequency on chain length can be investigated.If rn is the mass of the effective electrons in each unit then for a chain of n units the vibration frequency will be determined by the force constant k and the mass nm. It is given by v = & J n G . k . (3.6) or 1 2 = k'n . . (3.7) k' being a new constant This treatment predicts a linear relationship between lb2 and the number of units in the chain. However the effect of the end groups cannot be predicted to this approximation. In the above treatment it is assumed that while the individual units vibrate in phase there is little interaction between them. Even in the excited state of the molecule the electrons may never get far from their mean positions.However a second extreme type of oscillator is illustrated by the carbocyanines the essential structures being + + > N =CH-[CCH = ClI],,-X < > N-[CH =CH],-CH =N < I n this case even in the normal state the charge is distributed through tho whole molecule. The restoring force on each electron pair is dependent on the position of the other electrons. Lewis and Calvin liken the oscil- lations of such a system to the longitudinal oscillations of a stretched string and conclude that the wave-length of absorption should be proportional to the length of the chain or A = k"n . . (3.8) The two types of molecules just discussed represent extreme cases. in between these extremes intermediate types of behaviour may be expected. The determining factor is the mobility of the electrons within the molecule.When the normal state of a molecule can be represented by a formula without any formal charges the mobility will be small. However as the 32 QUARTBRLY REVIEWS normal state of the molecule departs from the classical formula so will the mobility increase becoming greatest when two structures can be written down differing only in the position of the charge. 4. The Quantum Theory of Light Absorption 33 Although the Bohr theory led to a general understanding of the light- absorption process it was left to quantum mechanics to elucidate the details. The quantum-mechanical treatment of organic molecules goes back t o 1927 when Heitler and London put forward their theory of the hydrogen molecule. Heisenberg had shown in 1926 that if a number of structures can be written down for an atomic or molecular system of equal or nearly equal energy then the actual state of the system is more stable than any of the hypothetical states represented by the structures.The system is said to be stabiliaed by resonance. The principle of indeterminacy expressed the fact that electrons could not be rigidly located and Heitler and London were able to account for the stability of the hydrogen molecule by casting off the concept of Iocalised electrons. The resonance phenomenon rather than being fundamental is a legacy from classical chemistry which thought in terms of particle electrons and rigid bonds and which guided the choice of structures used in the quantum-mechanical calculations. Before considering in more detail the Heitler-London approach t o the hydrogen molecule it is necessary to consider the quantum-mechanical description of the hydrogen atom.The starting point is the Schrodingur equation Hy -Ey . . (4.1) where H is a differential operator E the energy of the system and ly a function c the co-ordinates which specifies the state of the system ; H can be obtain3d from the classical mechanical expression for the total energy of the aystem by substituting certain differential operators for the co-ordinates and momenta of the particles comprising the system. If now it is stipulated that y shall be single valued and continuous and such that . (4.2) where fy* is the conjugate complex of y d t being tt generaliid volume element then these conditions can only be satisfied for certain valuea of E. These values of E are known as eigenvalues the corresponding y’s being the eigenfunctions or wave functioh.The “space )’ of which dz is an infinitesimal element is not ordinary three-dimensional space except in the The following papers review the quantum mechanical treatment of molecules (a) “ The Quantum Theory of Valence,” J. H. van Vleck and A. Sherman Rev. Mod. Physics 1935 7 167. ( b ) “ The Quantum Mechanics of MOl0~d08,” G. J. Kynch and W. G. Penney Ann. Reports 1936 23 37. (c) “ Gnrndziige der “heorie ungesiittige und aromatischer Verbindungen,” E. Hiickel 2. Ekktrochem. 1937 43 762. ( d ) “ The Theory of Molecular Structure,” W. G. Penney Rep. Frog. Phy& 1939 6 212. ( e ) ‘‘rille Quantum Theory of the Chemical Bond,” C. A. Cod~on Prm. Roy. Sm. Edin. 1941-43 A 61 114. (f) C. A. Coulson this vol.to appear. MACCOLL COLOUR AND CONSTITUTION 33 case of a ayatem comprising a single particle but has dimensions equal to the number of co-ordinates required to describe the system. The quantisa- tion of the energy of the system then arises as the result of placing certain limitations on the solution of (4.1). A justification of the foregoing con- ditions is provided by the interpretation of y namely that y*ydr . . (4.3) measures the probability of the system being in the " volume element " dz. The normalising condition (4.2) ensures that the probability of finding the system somewhere in " space " is unity. In the case of a single electron may be interpreted as the probability of finding the electron in the element of volume dz in the neighbourhood of ( r 8 4) or if multiplied by e the quantity of charge in an infinitesimal region surrounding the point.As well as being a function of the co-ordinates the wave function of an electron in a spherically symmetrical field depends upon four parameters known as -quantum numbers ; Y - Yn I m AT 8 $1 The first n is known as the principal quantum number and determines the energy of the electron. The second and third I and m are known as the azimuthal and magnetic quantum numbers and determine respectively the total orbital angular momentum and the orbital angular momentum in the direction of an external magnetic field not strong enough to affect the energy of the electron. The fourth quantum number s the spin quantum number describes the spin momentum of the electron which can be orientated parallel with or antiparallel to the magnetic field.All the quantum numbera except s are integers and are subject to l g n - 1 - l < r n < l The spin quantum number can only have the values & 1/2. The Pauli exclusion principle stipulates a condition on the quantum numbers describing the electrons in an atom. It states that no two electrons in an atom can have the same four quantum numbers. Thus the maximum number of electrons in an atom with specified n I m is 2 ; the number with specified n I is 2(2I + l) while the number with a given n is 2n2. A more general statement of the exclusion principle is that the wave function of an atom must ghange sign (be antisymmetric) when the co-ordinates of two electrons are interchanged. While the individual angular momenta of the eleotrons in a many-electron atom are not constants the total angular momentum of dl the electrons is.As the total angular momentum is related to the values for the individual electrons these latter still provide a useful means of clrtssification. A wave function specified by n I m is known as an atomic orbitul (A.O.). By the exclusion principle each orbital can contain a maximum of two electrons and these must have their spins opposed. For purposes of classification it is sufficient to specify the electrons of an atom in terms of n 1. Electrons with 1 = 0 1 2 . . . are known as 8 p d . . . electrons an electron with n = 2 1 = 1 being represented by 2p. In this Y*(T 8 qb)~(r 8 +)dr C 34 QUBRTERLY REVIEWS way the electron structure of the carbon atom may be represented by 1 ~ 2 2 ~ 2 2 ~ 2 the superscripts indicating the number of electrons of the given type* Once the Schrodinger equation has been solved all information concern- ing the state of the system can be obtained.I n general the mean value of a property of a system represented by the operator F for a system in the state yn is given by F = v*nFvndt . (4.4) However most of the systems of chemical interest are too complex to admit of a solution of the Schrodinger equation. Recourse has then to be had to approximate methods. It usually happens that the wave-equation may be solved for the system if some simplifying assumptions are made. The wave functions thus found can be taken as a starting point for an improved calculation. A theorem of very great utility states that the energy of a system calculated with an approximate wave function is always greater than the true energy the difference decreasing as the approximate wave function approaches the true one.If a system were considered as possibly existing in n structures represented by yl y . . . yi,& an approximate wave function of the form '1' = ulyjl -t a2y2 + . . + a,,yj . . (4.5) The a are then determined so as to make the energy of the This niinimising process gives rise to the quantum - I could be used. system a minimum. mechanical secit1a.r equation (4.6) where The energies of the structures are %i E7.Z being the interaction energies sii ASlij between these structures ; E is the approximation to the energy of the system. This equation is an algebraic equation of the nt,h degree the n roots El E, . . . En bcing the approximations to the energy levels.The approximations to the states of the system are found by substituting the values of ai corresponding to the required energy level into (4.5). I n the case of the hydrogen molecule two possible structures present themselves (XXIX) and (XXX) ; (XXTX) for example represents elec- MACCOLL COLOUR AND CONSTITUTION 35 This was It is essentially the theory of the tron 1 being attached to nucleus a and electron 2 to nucleus b. the method used by Heitler and London. / * l /*b a. ' 2 (XXIX.) (XXX.) homopolar molecule ionic structures like (XXXI) and (XXXII) playing no part. 1 1 i *b a*\ i (XXXI.) (XXXII.) The wave functions representing (XXIX) and (XXX) are and the approximate The wave-mechanica,l tions are Y A A A [ 1 Y ( 1 ) " b ( 2 ) - Wa(2)?#3(1)1 - . (4.8b) the corresponding energies being E and E,.These quantities are functions of I? the internuclear distancq and computation shows that whereas E,(R) possesses a minimum E,(R) does not. Hence (4.8a) corresponds to the formation of a stable molecule. The subscripts S and A refer to symmetry or asymmetry with respect t o the interchange of electrons. When electron spin is taken into account four spin wave functions present themselves. If a(1) represents electron 1 with s = $ and p(2) represents klectron 2 with s = - 4 they are a(l)a(2) a(l)p(2) a(2)/3(1) /3(1)@(2). These four wave functions can be combined to give four new functions one of which is antisymmetric and the other three of which are symmetric with respect t o interchange of electrons. I n order to satisfy the exclusion principle (4.h) must be multiplied by the antisymmetric spin' function and (4.8b) by the symmetric spin function to give the complete wave function of the molecule.As E represents the stable state and as the antisymmetric spin function is made up from a( 1)p(2) and a(2)/3( l) it follows that the electrons forming a chemical bond have opposed spins. 36 QUABTERLY RBVrEWS The second method of treating molecular structure wan developed by Hund Lennard-Jones Mulliken and Huckel. It involves the buMing up of a molecular orbital (M.O.) from the wave function of a single electron moving in the potential framework of the nuclei and the other electrons. The energy of their molecular orbitals may in principle be determined by a rigorous solution of the Schrodinger equation. Each molecular orbital can contain two electrons and so to determine the molecular energy the elec- trons are fed two at a time into the orbitals of lowest energy.Rather than rigorously solve the Schrodinger equation the L.C.A.O. method assumes that the M.O. is given by a linear combination of atomic orbitals. The molecular orbital for the hydrogen molecule to this approximation would be y = a,Yu(l) + %Yb(1) * . (4.9) The correct linear combinations are found to be Y4 - 47[Yu(l) + Wb(1)l - * (4.9a) Yu = Au[Ya(l) - Yb(1)I * . (4.93) The subscripts g and u indicate symmetry and asymmetry with respect to inversion of the orbital in the centre of symmetry. Quanta1 calculation shows (4.9a) to be the orbital of lowest energy (y is said to be a bonding orbital while yN is antibonding). Hence the molecular wave function is - Ag*[lUu(1) + Yb(l)I[Wa(2) + Yb(2)I * .(4.10) If this expression is multiplied out it is seen to contain two terms correspond- ing to (4.8a) together with two additional terms of the type Yo( 1 )Yu(2) Y b ( l ) Y b ( 2 ) * . (4.11) which are the wave-mechanical transcription of (XXXI) and (XXXII). Thua the molecular orbital treatment gives as much weight to the ionic structures as to the purely homopolar ones considered in the Heitler- London treatment. Both of these methods when generalised so as to apply to complex organic molecules suffer from limitations the Heitler-London or valence-bond (V.B.) method on account of its neglect of ionic structures the M.O. method because of its treatment in terms of a single electron with consequent neglect of interelectronic interaction.However both methods may be modified to overcome these deficiencies. The Heitler-London treatment of the hydrogen molecule shows that the pair of electrons forming a chemical bond have their spins opposed. This suggests that the valency of an atom is equal to the number of electrons with unpaired spins that it possesses. Thus carbon in the state ls22s22p2 would be bivalent the two p electrons being capable of forming bonds. It is of interest to enquire into the directional properties of valence bonds. To do this requires a qualitative discussion of the nature of the s and p electron distributions. The solutions of the Schrodinger equation for an electron in a spherically symmetrical field shorn that 'while the charge distribution of s electrons is spherically symmetrical those of p d .. electrons exhibit maxima in certain directions. In the case of the p elec- trons the maxima are mutually perpendicular and so they can be classified as pz p y or p,. If the formation of a chemical bond is regarded as the MACCOLL COLOUR AND CONSTITUTION 37 building up of a charge between bonded atoms then an atom with two p electrons would tend to form bonds directed at right angles with two other atoms. Iteturning to the case of carbon in order to obtain a quadrivalent state it seems reasonable to assume that a 29 electron is promoted to the 2p state giving the structure ls22s2p3 in which case there me four electrons with unpaired spins. If however these orbitals are used for bond formation a model comprising three mutually perpendicular bonds with the fourth symmetrically disposed with respect to the other three would be obtained.On this basis methane would not possess the symmetry of a regular tetra- hedron a structure which has been amply confirmed by physicochemical studies. However as has been seen a better approximation to the system can be obtained by taking four linear combinations of the four orbitals. This was the method used by Pauling who took as his criterion in the formation of linear combinations the principle of maximum overlapping. This assumes that a bond between two atoms will be stronger the greater the degree of overlapping of the orbitals forming the bond. Pauling defined the strength of bonds formed by s orbitals as 1 and of those formed by p orbitals as 3112. By linear combination of the 2s and the three 2p electrons he was able to form four bonds directed tetrahedrally with the maximum strength of 2.Similarly the strongest bonds that can be formed from the configuration sp2 are inclined at 120". The latter process gives rise to the trigonal bonds of great importance in the discussion of conjugated systems. This process involving the mixing of s and p electron wave functions is known as hybridisation. 34 Huckel was the first to apply these ideas to unsaturated molecules. In the case of benzene for example he assumed that the 9 p, p electrons were hybridised to give the trigonal bond system already referred to the three cr orbitals being used in forming the C-C and C-H bonds. He was then able by both methods to determine the energy of the pz or z electrons and hence the energy over and above that of the framework bonds.While the L.C.A.O. treatment of conjugated systems is relatively straight- forward the valence-bond treatment introduced several new concepts. The latter method employs the fact that for a conjugated system the electron spins of the various p electrons may be coupled in a number of different ways corresponding to different structures or bond diagrams. A useful theorem due to Rumer limits the number of bond arrangements that have to be considered. He showed that for a system of n bonds (2n electrons) any conceivable structure can be represented in terms of a fundamental set of structures. These are known as a canonical set. For benzene the now famous structures are (2n) ! (n)! (n + l)! (4.12) / \ A . B. c. D. E. For a more complete discwion of this process see Reference 33 (f).38 QUARTERLY REVIEWS Structures C D E containing a para-bond are said to be excited. The number of canonical structures increases very rapidly with the number of bonds ; for anthracene there are 429. The number of structures cont'aining formal bonds (bonds between non-adjacent atoms) also increases with the number of bonds. A structure containing no formal bonds is said to be unexcited with one formal bond first excited and so on. It is unfortunate that the same term " excited '' is used to refer both to the structures and to the actual states of a molecule. However the context usually makes clear the sense in which it is used. Largely as the result of work by Slater Pauling and Eyring simplified methods were evolved for setting up the secular equation in terms of the canonical structures.These methods were then employed to calculate the resonance energies of a large number of conjugated molecules. Before 1937 attention had been focused upon the lowest root of the secular equation i.e. on the ground states of molecules. However in that year Sklar 35 calculated the differences in energies between the ground and the first excited states of a number of molecnles and was able to show that this energy differenco or transition energy corresponded to their long wave-length absorption bands. Rather than to use explicit expressions for the atomic orbitals his method was to determine the energy difference between the states in terms of a quantum-mechanical parameter a. He obtained the value of this parameter from purely thermochemical data.His results for benzene are given in Fig. 2(a) arid these are to be compared with the experimentally determined values s1ion-n in Fig. 2(c). With a value of a equal to 1.92 v.e. the long wave-length absorption band of benzene is found to lie at 2470 A. compared with t'he experimentally determined band 2470 J MOO 2470 2700 1500 t The electronic energy levels of benzene by (a) the valence bond (b) the antisymmetrbed molecular orbital method ( c ) experimental. The allowed transitions are shown with an asterisk. 35 Reference 19. MACCOLL COLOUR AND CONSTITUTION 39 a t 2600 A. The attempt to calculate the position of the shorter wave-length bands by this method led to results in poor agreement with experiment. Sklar attributed this breakdown to the neglect of excited ionic structures such as (XXXIII) (XXXIV) and (XXXV).Because oE the large number +<3- (XXXIII.) (XXXIV.) (XXXV.) of these structures he was only able to incorporate the twelve (XXXIII). The calculations show that while the ionic structures only slightly with the lower states of the molecule they become importance when the highly excited states are considered. of type of great interact The problem may also be approached from the molecular orbital view- Calculation shows that the energies of the various molecular orbitals w = 28 cos (2,2/6)1 . . (4.13) where is the resonance integral which may be evaluated empirically. The energy of the orbitals increases with I I I. Thus two electrons are placed in the orbitals with l = 0 and four in those with \ I I = 1. The total energy is then 8/3.In the first excited state one electron is raised from I I I = 1 to 1 I / = 2. This state is fourfold degenerate since the odd electron in the 1 I I = 1 level may have l = & 1 while the electron in the I I I = 2 level may have I = 3 2. The energy of the first excited level is 6/3 the energy difference corresponding to the long wave-length band being 28. Since estimates of by different workers differ considerably all that can be said of this calculation is that it gives a value of the right order of magnitude.36 In order to refine the molecular orbital method M. Goeppert-Mayer and A. L. Sklar 37 utilised a modification suggested by Mulliken. It is known as the method of antisymmetrical molecular orbitals. One of the main sources of error in the L.C.A.O. method (molecular orbitals formed as a linear combination of atomic orbitals) is the neglect of the interelectronic repulsions.These interactions are responsible for removing the degeneracy previously discussed. But as has been seen the molecular wave function which is the product of the appropriate molecular orbitals allows many electrons to congregate around a gieen nucleus. Because of this the intro- duction of the interelectronic repulsion may lead to results ih worse agree- ment with experiment. However if the molecular wave function is multiplied by an appropriate spin function and the whole antisymmetrised in the co-ordinates of the electrons all terms which represent an accumula- tion of more than two electrons on a single nucleus vanish. By means of this refinement the limitations of the molecular orbital method are largely overcome.In addition the expression for the energy levels may be obtained *6 R. S. Mulliken and C. A. Riske Rev. Mod. Physics 1942 14 259. 17 J . Chem. Physics 1938 6 645; A. L. Sklar and R. H. Lyddane ibid. 1939 7 374; F. London discusses some of the approximations involved in the method (ibid. 1945 13 396). point. for benzene are given by (I = 0 f 1 3 2 3) 40 QUARTERLY REVIEWS in terms of certain integrals which are calculable analytically the only empirical data used being the internuclear C-C distance. In this way the results shown in Fig. 2(b) were obtained. The calculakd values are seen to be in good agreement with experiment. In 1939 R. S. M ~ l l i k e n ~ ~ in a series of papers developed methods for calculating the intensity of a band quantum mechanically.The transition (electric) moment is defined as (er)mn = jy*n(zeri)ymdr . . (4.14) where the summation is taken over all the electrons and ri is a vector defining the position of the ith electron. The theory of radiation now relates the tramition moment or the dipole strength ( T ) ~ ~ ~ with the oscillator strength of dispersion theory. The relationship is i fj=(T)*; . (4.15) where the subscript j refers to the transition m -+ n. It is often convenient to consider the components of q namely xj yj zj. If it so happens that xj yj zj = 0 then t o this degree of approximationfj = 0 and the transition is said to be forbidden. Mulliken’s method may be illustrated by his treatment of the hydrogen molecule. From the L.C.A.O. viewpoint the ground state is represented by y N = Ys(l)lyd2) - .(4.16) while the first excited state to which transition is allowed is given by . (4.17) The transition corresponding to absorption is that of an electron from a bonding M.O. to an antibonding one. If the molecular axis is taken as the z direction then xj = yj = 0. 1 Y E = -“f!‘~(l)Yu(2) + Y&2)%(l)1 - 4 2 Expression (4.14) thus reduces to ( z ) N B = Y E ( z l + z,)YAVdr . (4.18) This expression on substitution for YJE and !Pa becomes after reduction s (4.19) Now yazyaCi’r is simply the average value of z for atomic orbital a and as 5 the internuclear distance R = zA - zB R (Z)N,E = ~ * d2(1 - 8 2 ) (4.20) The factor [2(1 - s2)]-4 is related to A and A of (4.9a) and (4.9b). the V.B. point of view it can be shown that From . (4.21) SR ( 4 N E = - 1 / 1 - 8 4 SaJ.Chem. Physics 1939 7 14 20 121 339 353 364 570. MACCOLL COLOUR AND CONSTITUTION 41 the transition being that from an essentially non-polar ground state to an essentially ionic excited state. Substitution of the known values for R and S gives f = 0.68 from (4.20) and 0-49 from (4.21). The agreement between the two values is excellent in view of the approximations involved. The theory of groups can be applied to simplify the calculation of the energy states of a molecule and to decide the question as to whether a given transition is allowed or not. For benzene which has the symmetry of a plane regular hexagon it can be shown that transitions from the ground state 2 A. FIG. 3 The absorption spectrum of solid hexantethylbenzene (1) electric vector in the plane of the ring ; (2) electric vector perpendicular to the plane of the ring.to the two lowest excited states are forbidden. This is in qualitative agree- ment with the experimental data which show a weak band at 2500 A. a stronger one at 2000 A. and a very intense band at 1800 A. The appearance of the weak long wave-length band is due t o a distortion of the symmetry of the molecule by vibrations. The intensity of the forbidden band at 2000 A. is probably related to its proximity to the allowed transition at 1800 A.39 An interesting study has been the effect of the distortion of the benzene symmetry by the gentle method of replacing the hydrogen atoms H. Sponer G. Nordheim A. L. Sklar and E. Teller J . Chem. Phyeics 1939 7 207, 42 QUARTERLY REVIEWS by deuterium40 or by the more drastic method of replacing them by F,41 OH,42 NH2,43 or CH,.44 For the conjugated molecules to be discussed in the present review it is assumed that the energy levels of the 7c electrons are concerned in the long wave-length spectra.That ultra-violet absorption can arise in other ways is shown by satarated molecules such as ethane for which absorption starts a t about 1600 A. The recent work of G. Scheibe St. Hartwig and R. Miiller 45 strikingly demonstrates the part played by the n electrons in the long wave-length spectrum of hexamethylbenzene. This molecule possesses a layer structure in the solid state and so a known orientation of a crystal implies a known orientation of the molecular planes.46 The absorption spectrum can then be determined by using polarised light with the electric vector parallel and perpendicular to the plane of the molecule.If the n electrons are responsible for the absorption the extinction in the latter case should be very small. The extinction wit’h the electric vector perpendicular to the plane of the molecule is a tenth of that with the electric vecbor parallel thus confirming the r6le of the ?t electrons. Other examples of the directional properties of light absorption are discussed by R. A. RIorton.47 The results are shown in Fig. 3. 5. Some Qualitative Applications of the Resonance ThRory 48 The problem of the relationship between the colour of an organic com- pound and its chemical constitution has been approached from two directions. The first was along the lines of classical organic chemistry and consisted in the examination of a series of related molecules in order t o find the effect upon the absorption maximum of the parent substance caused by changes in constitution.Unfortunately much of the early work is marred by the neglect of “subsidiary ” changes in constitution which may have had a greater effect than the change which was t o have been investigated. The second approach is the calculation of the wave-length and intensities of ab- sorption by quantum-mechanical methods. The quantal treatment however is limited to relatively simple molecules although qualitatively the theory of resonance is capable of providing much valuable information. The present section will deal with the applications of the resonance concept and the last section with the results of the quantal calculation. The relationship between the resonance energy of a compound and its light absorption is of interest.I n Section 4 it has been shown that if more than one structure can be written for a molecule interaction between the 40 C. A. Beck and H. Sponer J . Chem. Physiccl. 1942 10 575. 41 S. H. Wollman ibid. 1946 14 123. 4 2 F. A. Matsen N. Ginsberg and W. W. Robertson ibid. 1945 13 309. 43 N. Ginsberg and F. A. Matsen ibid. p. 167. 4 4 N. Ginsberg W. W. Robertson and F. A. Matsen ibid. 1946 14 511. 4 5 2. Elektrochem. 1943 49 372. 48 K. Lonsdale Proc. Roy. SOC. 1929 A 123 494. 47 Reference 24 (c). 48 A discussion of the colour of organic compounds from the M.O. viewpoint is given by E. J. Bowen Ann. Reports 1943 40 12. MACCOLL COLOUR AND CONSTITUTION 43 structures will give rise to a set of states which will be better approximations to the actual molecular states.The energy difference between the lowest state found in this way and the structure of lowest energy is the resonance energy of the molecule whereas the energy difference between the two lowest molecular states is equal t o the energy corresponding with the long wave-length absorption band provided of course that the transition is allowed. In the diphenylpolyenes (XXVII) the resonance energy increases with addition of each successive vinylene group while the absorption maxima move towards the red. However an increase in the resonance energy does not always imply a bathychromic effect as a consideration of the isomeric molecules naphthalene and azulene (XXXVI) shows. For while naphtha- lene has a greater resonance energy than azulene the former molecule absorbs at a much shorter wave-length than the latter.Another example comes from the work of G. Schwarzenbach et U Z . ~ ~ who have studied the spectra of indicators capable of undergoing several colour changes. These authors concluded that the most highly coloured form of the indicator was the most stable. However chemical evidence would suggest that malachite- green (XXXVII) has a smaller resonance energy than crystal-violet (XXXVI.) c (XXXVII.) (XXVIII) while the wave-lengths of absorption are 6230 and 5900 A. respectively. 50 The bathychromic effect together with the increase in resonance energy observed in the case of the polyenes as the series is traversed can be understood on the following basis. The successive addi- tions of vinylene groups cause a rapid increase in the number of excited structures there being only one unexcited structure corresponding to the classical formula in each case.Thus on proceeding up the series the ground state of the molecule is lowered corresponding with an increased resonance energy but the first excited state is still further lowered corresponding with a shift of the absorption towards the red. Although the prediction of the relationship between the resonance energy and light absorption may safely be made for a related series of molecules yet great care has to be exercised when the argument is extended to molecules outside the series. For closely related molecules the greater the extent of the resonance 4D Helu. Chim. A& 1937 20 1591. Reference 23. 44 QUARTERLY REVIEWS system the longer the wave-length of absorption.This is clearly shown by the phenomenon of insulation of chromophores. The spectrum of 6 15-dihydrohexacene (XXXVIII) is almost identical with the sum of the (XXXVIII.) spectra of naphthalene and anthracene.61 The extent of the resonance system is reduced and the spectrum of the molecule reverts to the spectra of the two systems formed by the insulating effect of the two methylene groups. The insulating effect may be achieved by more subtle methods as has been shown by the work of L. W. Pickett G. F. Walter and H. France on some substituted diphenyl~.~~ The long wave-length band of diphenyl is very similar in position to that of benzene but has a greatly increased intensity 18,000 compared with 200 for benzene). Yet the spectrum of 2 2’ 4 4’ 6 6‘-hexamethyldiphenyl for example approximated to that of mesitylene.This phenomenon is understandable in terms of reson- ance theory. The structures for diphenyl are the four classical structures and excited ones such as (XXXIX) and the corresponding ones with a +<=x=>- - - -<=x>+ - - a a7 (XXXIX.) formal bond between the pp’ positions. The question of the relative import- ance of the ionic and the formal bonded structures will be discussed in Section 6. For the maximum interaction between structures of this type the two rings must be coplanar.53 This follows from the principle of maximum overlapping since the n orbitals of atomB a and a’ have their maxima perpendicular to the plane of the rings. Thus any effect which tends to force the rings far out of the coplanar condition will decrease the contributions of the above structures and the spectrum of the molecule will revert to that of the corresponding benzene derivative.So far the discussion has been mainly concerned with chromogens. According to Bury the function of an auxochrome is to provide a greater possibility of resonance. Thus in aniline as compared with benzene 51 E. Clar Ber. 1942 75 1283. 5 2 J . Amer. Chem. Soc. 1936 58 2296. 6s For a discussion of the structure of diphenyl see J. Karle and L. 0. Brockway s4 Reference 18. ibid. 1944 66 1974. MACCOLL COLOUR AND CONSTITUTION 45 utmctures such as (XL) participate as well as the two Kekul6 and the p-bonded types. Confirmation of this view comes from a comparison of the spectrum of aniline with that of the anilinium ion for which structures of the type represented by (XL) become highly improbable.The two ; H a &H2-==>- h2<=> (XL. 1 spectra are very merent that of the anilinium ion being practically identical with that of benzene. The anti-auxochromes of Wizinger can be inter- preted in a similar fashion. Thus in p-nitro- enhanced limiting structures such as (XLI) Some very interesting results have been re- - aniline the possibility of resonance becomes further &H2=r>=yB playing a part. (XLI.) corded showing the relationship between the spectra of members of a series of compounds. In particular may be mentioned the work by K. W. Hausser et aZ.55 on certain polyenes and the attack on the cyanine dyes initiated by N. I. Fisher and F. M. Hamer,56 and extended by L. G.S. Brooker and his c o - ~ o r k e r s . ~ ~ Other data have been obtained by A. E. Gillam and D. H. Hey 58 for two series of polyphenyls and by E. Clar et U Z . ~ ~ for certain polycyclic aromatic compounds. Only the first two series will be discussed here as they are illustrative of tho general method of approach. The polyenes are members of R vinologous series which can be repre- sented by the general formula (XLII). The compounds considered were of the following types R,R’ = CH, CHO ; CH, CO,H ; ( 11 CHO ; and <I> a. Of particular importance is the tabulation of the integrated intensities of the bands for as was shown in Section 2 the integrated intensity or the related f value constitutes the best measure of the intensity of absorption. The tabulation of Amax. and E ~ ~ ~ .is unsatis- O r + - R-[ CH=CH] TL-R’ R-CH-[CH=CH],- 1-CH-R (XLII. ) (XLIII. ) factory since f is a function of both E,,,. and y the half-width of the band. In Table I1 are given the results obtained for the diphenylpolyenes the data referring to benzene solution. 5 5 2. physikal. Chcin. 1935 B 29 363 371 378 384 391. b6 Proc. Roy. SOC. 1936 A 154 703 ; 1937 A 163 138. b7 Rev. Mod. Physics 1042 14 275. 58 J. 1939 1170. 6o E. Clar “ Aromutische Kohdonwasserstoffe,” Springer Vorlag Berlin 1941 ; see also R. N. Jones Chem. Reviews 1943 32 1. 46 QUARTERLY REVIEWS TABLE I1 The dip hen ylpolyenes I -I-- -I__ -~ A,,,. . . 2515 3190 3520 3770 f . . . 0.41 1 0.58 ~ 0.77 1 1.26 The other series show the same general type of behaviour. In terms of the resonance theory the structures are the classical structure (XLII) and excited ones such as (XLIII).There are of course many other contributing structures involving smaller separation of charge. If it is assumed that structures of the type (XLIII) are of main importance for the excited state then a bathychromic effect would be expected upon ascending the series and this is observed. Since all the polyene series examined show the same type of dependence of absorption maxima upon n the end groups may as a first approximation be neglected. Fig. 4 shows the resonance energy over I T -m- - I n = O n = I n=2 n=3 FIG. 4 The resotiuiLce eriergies (t - - -+) and transitioii energies (-4) of some diphenylpo1yene.u. and above that of the four phenyl groups and the transition energy as a function of n. The increase in intensity with increase in n may be accounted for qualitatively since the transiiion moment will be greater the longer the chain.This series of compounds formed the first type of linear oscillator in the sense of Lewis and Calvin. The relationship derived (3.7) indicates that a plot of A 2 against n should give a straight line. This is shown in Fig 5 the agreement being excellent. This series also affords evidence of the existence of a “ second-order band,” as the polyenes all show a second band at shorter wave-lengths. The ratio of 3L1 to A 2 approaches 2 as the length of the chain increases as is to be expected for a second-order band. The light absorption of an important series of polyenes the carotenoids has been discussed by L. Zechmeister L. Pauling et Of particular 6o J . Amer.Chem. SOC. 1943 65 1941 ; see also R. S. Mulliken J . Ghem. Physics 1939 7 364. MACCOLL COLOUR AND CONSTITUTION 47 8 % e h 0 interest is the existence of steric isomers due to the possibility of dieerent arrangements around the double bonds. Quanta1 calculations show that for a polyene in the all-trans-configuration transitions are allowed from the ground to the first and the third excited states but forbidden to the second. I I 1 I I I I I ' h e 75 - 10 - ?* 8 co 5 - 2 A. Fra. 6 - - - - equilibrium mixtiire of isomers. If however the molecule possesses a cis-arrangement about some of the double bonds transition to the second excited state becomes allowed. This accounts for the third peak characteristic of the cis-carotenes. Fig. 6 shows the development of the " cis-peak '' in y-carotene.The development of the '' cis-peak " i n carotene - a11 trans-carotene ; 48 QUARTERLY REVIEWS The cyanine aeries investigated by N. I. Fisher and F. M. Hamer can be represented by the general formula (XLIV) TI T, T, T being atoms in a heterocyclic molecule. Molecules of this type form another example TI \+ N==C/H-[ CH=CH],-N n. of a vinologous series. The cyanines may be divided into two classes the symmetrical cyanines for which the two end groups are the same and the unsymmetrical ones in which the end groups differ. Three general relation- ships emerge from this work. In the fist place there is a bathychromic shift as n increases and secondly the intensity of the long wave-length band increases with n. The third conclusion that may be drawn is that in many cases the wave-length of absorption of an unsymmetrical cyanine can be calculated as the mean of the wave-lengths of absorption of the two related symmetrical compounds.The colour of dyes has been extensively investigated by L. G . S. Brooker and his collaborators. The first point of interest lies in a comparison of the spectra of a series of cyanines (XLV) with those of the related anhydro- I I I I I I Et Et (XLV.) 0 . . . . 1 . . . . 2 . . . . 3 . . . . I I Et (XLVI.) bsscs (XLVI). alcohol. The data are given in Table 111 the solvent being methyl TABLE 111 Comparison of the Spectra of a Series of Cyanines with those of tJx Corresponding Anhydro-bases 4 P d x . . emax x 1 0 - 4 . 4 m x em,,. x lo-'. 4230 8.43 3060 5.85 5575 14.8 4580 5.65 6500 22-9 4900 6.4 7.380 2 i . 6 5100 6.8 I - I Cynniney.1 Anhgdro-bases. M-4CC'OLL COLOUR AND CONSTITUTION 49 For the cyanines Amax is approximately linearly related to n the behaviour to be expected from " linear oscillators of the second type '' in the Lewis and Calvin sense. The increase in intensity on ascending the series is also to be expected on this view. However an entirely different state of affairs exists for the anhydro-bases. In the first place E,,,. is approximatlely constant for all members of the series and secondly whereas a change in n from 0 to 1 causes a bathychromic shift of 680 A . yet the change in n from 2 to 3 only causes a shift of 200 A. The marked difference between the behaviour in these two cases enables Brooker to classify the spectra of series of compounds on the grounds of convergence or non-convergence.The former series is non-convergent while the latter is convergent. This author attributes the difference in behaviour in the two cases to the degeneracy or non-degeneracy of t,he extreme structures. This term will be used to signify structures with the charge placed at the end of the chain. Whereas in the cj-anines there exist two extreme structures of equal energy for the anhydro-hases there is it single low-encrgy classical structure together with two extreme structures such as (XLVII). In both there are additional intermediate strnctures (XLT'TI.) but it is not necessary to considcyr thew for the present argument. For the cyanine series a degeneracy csists among t hc extreme structures whereas in the anhydro-bascs the classicd struct uro differs widely i n energy from the ionic structures.This tlegcncracy nccorcling t o Brooker is ;t prerequisite for the non-convergencc of a series. of a sjmimetrical cj-aninr of' give11 chaiu leiigth with alterations in the end groups can be explaiiied by the consideration of intermediate structures such as (SLVITT). Pauling 1133 pointtxl out the The variation in i Et I Et (XLVIII.) necessity for taking into account structures of this type since the extreme structure representcd by (XLV) and the corresponding one with the charge on the left-hand nitrogen atom will not by themselves interact. Brooker concludes that the greater the energy difference between (XLV) and (XLVIII) the Icw t Iir resonance splitting of the degenerate levels and tlhe P 4 s P. 9 found that it decreased with n. If it is assumed that the process involves the addition of a proton to the chain and thus the loss of resonance stabilisa- tion the decrease in this quantity with increasing n is verified.Non-degeneracy may also be observed in the unsymmetrical cyanines if the basicity of the two heterocyclic nuclei are different. In this case there would be a stabilisation of the extreme structure with the positive charge on the more basic nitrogen atom. For the compound (XLIX) there is a marked convergence the absorption maxima for n = 0 1 and 2 being 5045 6170 and 6800 A. respectively. I--$--I I I I I I I I n-0 n = I n=2 n=3 I Et (XLIX.) The non-convergence makes itself apparent in another fashion. It had been noted by Fisher and Hanier that the wave-length of absorption of an unsymmetrical cyanine could be calculated from the mean of the values for the two related symmetrical compounds.This relationship must obviously break down if the series of the symmetrical cyanines are non-convergent while that of the unsymmetrical compound is convergent. Brooker has used the deviation 3Lcalc. - lobs. as a measure of the relative basicity of heterocyclic nuclei. For a given value of n one of the end groups is kept constant while the othor is varied. The greater the deviations in 61 L. G. S. Brooker “ Resonance and Orga,nic Chemistry,” “ Frontiers in Chemistry,” Vol. 3 Interscience Publishers Inc. New York 1945. MACCOLL COLOUR AND CONSTITUTION 51 such a series of compounds the greater is the difference in basicity between the two end groups. Consistent results for the order of basicities have been obtained by Brooker from measurements both in the cyanine and in the styryl dye series.G . N. Lewis 6 2 has recently published a classification of dyes based upon the concepts of the Lewis and Calvin theory together with rules for estimst- ing their long wave-length absorption bands. The parent molecule con- sidered is (L) and the various families of dyes are obtained by specifying X” X”’ and the X” = N X”’ zz= 0 auxochromic groups X and X’. the oxazine dyes are obtained. For example with X”’ may be absent as in the triphenylmethyl dyes. The positive charge formally represented as. residing at X” is distributed through the molecule owing to resonance. The “ electronic oscillation ” responsible for the colour is supposed to take place between X and X‘ the auxochromic groups.The simplifying assump- tion is made that the effects due to alterations in X X’ X” and X”’ act independently and the direction of these effects is calculated by the use of the following rule. “ I f the colour of a compound is associated with an oscillation of positive charge along a given path the frequency of absorption will be increased by anything that diminishes the amount of positive charge and decreased by anything that increases it.” Thus the bathychromic effect of X,X‘ = NR2 as conipared with X,X‘ = OH and the hypsochromic effect of X”’ = 0 as compared with a molecule in which X‘” is absent can be accounted for on this basis. Malachite-green being taken as the standard molecule the effect of constitutional changes upon the colour can be deter- mined empirically as additke effects.Lewis has considered a large number of dyes and the’agreement between lobs. and ILcalc. is very good the mean error being 30 A. In the acridine family X” = CR X”’ = N the agree- ment is very poor and this is interpreted as indicating that the long wave-length band of such compounds corresponds to a “ vertical oscillation.” The foregoing are typical of the arguments used in the application of resonance theory to a correlation of the absorption spectra of organic molecules. The value of such treatment will be discussed at the end of the next section. 6. The Application of Quantul Methods The calculation of the colour of an organic compound besides having an intrinsic importance also has a utilitarian importance outlined by Sklar 63 “ The determination of the structure of a natural product perhaps J .Amer. Chem. SOC. 1945 67 770. 6 8 Reference 19. 52 QUARTERLY REVIEWS a hormone is a matter of many years' work in which many guesses are made and later rejected. One could calculate the spectrum of a proposed structure check it against the experimental one and so keep on the right track." Although at the present time this desirable vista is still distant considerable progress has been made towards it. Sklar considered the four molecules butadiene benzene fulvene and azulene and was able to calculate the transition energy for each molecule by the valence-bond method in terms of 8 quantum-mechanical parameter x . Rather than evaluate a analytically recourse was had to thermochemical data which led to an empirical value for the constant.The results of the calculations together with some results obtained by other authors are shown in Table IV. TABLE I V Application of the Valence-boftd Method Molecule. ButadieneO . . . . Hexatriene . . . . Octatetraeneb . . . Fulvene". . . . . Benzene . . . . Styrened . . . . . cycZoOctatetraene . . Azulene" . . . . Naphthalene c . . Diphenyld . . . . hthracone C . . . . Phenanthrene . . . Naphthacenec . . . PyroneC . . . . . Pentaceno . . . . Calc. (Sklar). 1900 2570 3120 3650 2470 2570 3320 6910 2680 2570 - - - - - Calc. (Farster). - 2450 3860 7800 2950 3650 3000 4500 3450 5450 - - Obs. 2100 2600 3020 3650 2600 2850 - 4000 7000 2750 2515 3700 4600 3300 5800 3-100 d a A. L. Sklar Reference 19. &I. Kovner Actn Physicochiw. U.R.S.$. 1914 19 385; cf. Conipt. rend. Acad. Sci. U.R.S.S.1942 35 54. C Th. Forster Reference 20. d G . W. Whelrtncl "The Theory of Resonance," Wiley New York 1944. A. BIuccoll Nature 1946 157 695. The difference botwoen the values quoted in the table and those in the reference is duo to the choosing in the present case ofvalues of the parameter a to conform with the values med by Sklar and Forster. Unless explicit references are given tho soux~cos of tho oxperimental data are taken from the papers which rocord the resuits of the calculations. I n setting up the secular equations for butadiene benzene and fulvene Sklar included all the canonical structures. Since the number of these rapidly increases as the number of electrons increases the met hod becomes unmanageable for a inolecule such as azulene I n this case onlj- thc two unexcited structures and the first excited ones were coiisidercd.This can be justified on the grounds t'hrtt t'he highly excited structures will riiake onljr MSCCOLL COLOUR -4ND CONSTITUTION 53 a small contribution to the ground and the first excited state. Even with this approximation the work involved in the calculations for complex mole- cules becomes very great and so Forster 64 in 1938 went a step further and neglected all but the unexcited structures in setting up the secular equation. This method is obviously only applicable to molecules for which more than one unexcit,eci structure exists. Forster was able to treat molecules such as pent,acene (LI) and pyrene in this way. &I.) As well as the nssuinption discussed above a number of others of a more fundamental nature are made in the treatment of the states of molecules by the V.B.method. A justification of these assumptions is indicated a posfiori by the success of the method in calculating the resonance energies of molecules. However a much more serious criticism of the results obtained comes from a consideration of tho intensities of the calculated absorption hands. Kovner 6 5 has shown in the case of butadiene that the valence-bond method lends to the conclusion that the long wave-length transition is for- bidden for the model with a trans-a,rrangement abont the single bond (LII). (LII.) (LII I. ) As this transition becomes allowed for thc model with a &-arrangement (LIII) Kovner concluded that butadiene must possess the s-cis-arrangement. (This term has been introduced by Mulliken 66 to denote cis- and trans- isomerism about a single bond.) Kovner’s conclusion is in direct conflict with the work of i l l ~ l l i k e n ~ ~ using the molecular orbital method.Even if butadiene possessed the s-cis-configuration it is doubtful whether a sufficiently large transition moment could be obtained by considering as Kovner did only homopolar structures. The same type of argument applies to many of the molecules listed in Table IV. That this argument is correct is borne out by the calculations of Sklar 6 8 for benzcnc where in order to obtain a reasonable value for the position of the first allowed transition ionic structures had to be taken into account. There exists the anomaly that while the positions of the absorption bands of molecules listed in Table IV show good agreement with the experimental values there is little reason to assume that the treatment would lead to agreement in the inten- sities.The case of cyclooctatetraene is hardly a fair test insofar as a planar configuration was assumed and the effects of strain neglected. A great deal of theoretical work in recent years has been concerned with the bond lengths in conjugated systems. Thus Pauling et aZ.89 and 6 4 Reference 20. 66 Rev. Mod. Physics 1942 14 266. 67 Reference 24 (d). 69 L. Pauling L. 0. Brockway and J. Y. Beach J . Am?. C h m . Soc. 1935 57 2705. 6 5 Reference b Table IV. 68 Referenoe 19. 54 QUARTERLY REVIEWS W. G. Penney 70 have adapted the valence-bond method and J. E. Lennard- Jones71 the molecular orbital method to deal with such problems. The simple valence-bond treatment assumes in the first plam that all the bond lengths are the same in a conjugated system and secondly that the bond lengths in the excited state of such a system are the same as those in the ground state.Both these limitations have been removed by G. J. Kynch and W. G. P e n n e ~ ~ ~ who have taken explicit account of the variation of the quantum-mechanical parameter in calculating the light absorption of buta- diene hexatriene and benzene. Results calculated on this basis show good agreement with experiment and in particular the high value of the reson- ance energy of benzene which is implied by Sklar’s treatment is brought into line with the observed value. Mulliken however has criticised this treatment in particular the identification of the calculated absorption band with the observed.F. S. Shifrin 73 has examined the valence-bond treatment of the colour of organic compounds and has pointed out that one of its limitations is the restriction of the calculations to hydrocarbons. He extended the calcula- tions to certain heterocyclic molecules by assuming that a nitrogen atom in a heterocyclic molecule contributes one n-electron and that the nitrogen atom can be considered as a CH group. On this basis he arrives a t the results shown in Table V. The calculated values have been altered from those in TABLE V I Benzene. 1 Pyridine. I Naphthalene. I - _.____________- I Amx.(calc.) ’ 2450 1 2450 2950 2950 1 3650 3650 3650 I Amax.(obs.) I 2600 1 2650 1 2750 1 3150 I 3700 1 3600 1 3750 ~ I the paper referred to in order to take account of the fact that the values for benzene and pyridine are obtained by Sklar’s method whereas the remainder were obtained by Forster’s method.The case of naphthalene is not straight- forward as it seems likely that the band observed at about 3100 A. should be used for the comparison. Evidence against the treatment of nitrogen atoms simply as CH groups comes from a study of the gaseous absorption spectra of benzene pyridinc and pyrimidine. The observed 0-0 transitions are respectively 2600 2900 and - 3100 A. indicating a bathychromic effect upon the replacement of a CH group by it nitrogen atom.74 Shifrin has also considered the case of pyrrole indole and carbazole pointing out that the NH groups in these compounds possess a lone pair of electrons which may interact with the ring electrons. On this basis the spectra of the above-mentioned compounds would be expected to resemble benzene naphthalene and anthracene respectively.70 Proc. Roy. SOC. 1937 A 158 306. 7 2 Ibid. 1941 A 179 214. 73 Compt. rend. Acnd. Sci. U.R.S.S. 19.10 29 27. la A. Maccoll J. 1946 670. 7 1 Ibid. p. 280. MACCOLL COLOUR AND CONSTITUTION 55 The calculation of the intensities of transitions for conjugated systems by the M.O. method has been investigated by R. S. M~lliken.'~ The transi- tions arise from the transfer of a n-electron from a bonding molecular orbital to a non-bonding one corresponding in valence-bond nomenclature to the transition from a mainly homopolar ground state to a largely ionic upper state. The calculations are made by the method outlined in Section 4. As the L.C.A.O. method leads t o values for 5 in disagreement with experi- ment Mulliken calculated a " semi-theoretical " f.This was done by using the experimental value for ? in equation (4.15). Some of the results obtained are shown in Table VI. The agreement is seen to be reasonably satisfactory TABLE VI f Values Calculated by the M.O. Treatment ~ Model. Ethylene . . . . . Butadiene . . . . . #l-Carotene . . . . . Benzene. . . . . . Diphenyl . . . . . Stilbene . . . . . 1 4-Diphenylbutadiene . 1 8-Diphenyloctatetraene ~ - I . I . trans Cis . ' tram I intermediate - . I - . ' trans . trans . I traits I cis 61,000 47,700 - 22,000 56,000 4 1,000 32,000 37,000 30,000 26,000 - f (calc.). 0.30 0-52 0.43 7.0 4.8 0.71 0.4 1 0.55 0.33 0.85 1.57 f (obe.). - 0.53 2.69 - - - 0.41 0.62 0-39 0.84 1.42 although it must be borne in mind that the values listed under f (calc.) were obtained from the theoretical values by multiplication by an empirical correction factor.In general the method leads to results which are greater than the experimental values; this being probably connected with the overemphasis that the L.C.A.O. treatment places on ionic structures. The antisymmetric molecular orbital method has been employed by Sklar 76 to investigate the intensities of the long wave-length transitions in substituted benzenes. The study of the directing power of substituents in a benzene ring as regards further substitution has led to two mechanisms whereby the sym- metrical charge distribution of benzene may be altered in an.isolated mole- cule. The first is known as the inductive effect and arises from the electro- static influence of the substituent upon the n-electrons of the benzene ring ; the second is the mesomeric effect which is brought into play by a migration of charge from the substituent into the ring or vice Groups such as C1 OH NH, which possess a lone pair of electrons are capable of trans- ferring an electron to the ring ; whereas groups such as NO, CHO CO,H which are electron deficient can accept an electron from the ring.It is found experimentally that a substituent which has a strong directing power also exerts a marked intensifying effect on the forbidden transition of benzene 7 5 Reference 24 (d). i 6 J . Client. Physics 1939 7. 984; 1942 10 135. 'I7 C. K. Ingold Chem. Reviewa 1934 15 225. 56 QUARTERLY REVIEWS at 2600 A. Whereas the NH group in aniline possesses a lone pair of elec- trons capable of migration into the ring this possibility no longer exists for the anilinium ion.This means that the only effect left to the ion is the inductive effect. Experimentally the absorption curves of H+ H the anilinium ion and benzene are almost superimposable c ' /H whereas in aniline the shape of the curve is markedly altered II and the intensity of the long wave-length band greatly increased. On these grounds Sklar assumes that migration effects will be mainly responsible for intensity changes. 0 - The case of toluene is of interest insofar as the migrating (LIV.1 electrons must be conceived as coming from a C-H bond. This effect is known as hyperc~njugation.~~ On the valence- bond view the effect can be represented in terms of structures such as (LIV).That this effect is negligible for the anilinium ion is explained on the basis of the increa.sed ionisation potential of NH3. The transition moment was calculated in terms of a parameter which determined the extent of migration of a radical electron into the benzene ring. The parameter was then determined by minimising the energy. The results obtaincd are shown in Table VII in which the f values are uncor- rected for the solvent effect. The value for fluorobenzene is estimated from the work of S. H. W0lleman.7~ On the whole the agreement is good con- sidering the uncertainty in the solvent correction. It follows from the treat- ment that a radical will produce a large intensification effect if it has a low ionisation potential a lone pair of electrons and not too great a ring-radical distance.Sklar has also dealt with polysubstituted benzene derivatives. + TABLE VII Migration Moments of Mofzosu bstit uted Ben- &enes f (theor.). f (oh.). 0.00 13 0.0246 0.0197 Fluorobenzeno . N 0.001 The colour of organic dyes has been investigated by Th. Forster,Bo along the lines suggested by Pauling. 81 Using the V.B. method he considered two idealised systems (LV) and (LVI) representing the cyanine and triphenyl- ineth yl type dyes respectively. In both calculations show that an increase in chain length or an increase in the strength of the auxochrome groups will cause a bathychromic shift, such as is found experimentally. System (LVI) is related to (L) which G. N. Lewis took as the basis of his classification of dyes. The empirical relationships The A groups represent ausochromes.MACCOLL .COLOUR AND CONSTITUTION 57 observed by this author receive a quantum-mechanical foundation on the basis of Forster's work. A I CH CH I I I C+ TI T4 ./ \ \+ -I- + + +/ C'H CH \ N-CH- -[CH-C'H),-N \C'H \ A T3 / \ rJ-z (L\-II*) A (LYI.) K. F. Herzfeld and A. L. Sklar,82 independently of Forster have exam- ined the cpnine dyes in greater detail by both the V.B. and the L.C.A.O. method. The structures used in the IT.B. treatiiient have already been discussed in Section 5 . There will be (2n -i 3) of them for the system repre- sented by (SLIV) corresponding t o the (212 + 3) atoms on which the positive charge may be placed. The secular equation is of degree (212 + 3) and the difference betwecn the two lowest roots is the energy corresponding with the long wave-length transition of the molecule.On the L.C.A.O. view the probloin is that of finding the energy of an electron in the field of t'ho framework (LVII) and of the ( 2 n f 2) othcr electrons. The secular equation gives the energies of ( 2 r ~ f- 3) molecular orbitals and the (2n + 2) electrons are fed into the (11 + 1 ) molecular orbitals of lowest energy. The long wave-length transition then corresponds with the removal of an elect'ron from the highest filled to the lowest unfilled molecular orbital. The secular equations obtaiiied by these two methods are formally similar although the various terms occurring in them have a different interpretation in the two eases. On carrying out the calculations which involve a number of approximations it is found that both treatments predict that the absorp- tion wave-lengths of the members of the series will go to infinity as ?t increases.However only the L.C.A.O. method giws a liiicar relationship between Amax. and n as is observed experimentally. For the unsymmetrical ions the V.B. method is superior predicting the convergence of the lb,n:,x. values as n increases. The present state of affairs in the field of colour and constitution may now be examined. Dealing first with the Lewis and Calvin theory it is found that the simple concept of the two types of linear oscillators is not sufficicnt to explain the behaviour of certain series of compounds. Thus with the p-polyphenyls 83 the convergence is too rapid to give it linear plot of 1.2 against n. This behaviour may be accounted for a p o ~ t i o r i but such a state of affairs is not very satisfactory.It niust be pointed out, however e2 Rev. Mod. Phys?ks 1942 14 294 ; J . Chew. Physicrr 1942 10 508 521. R 3 Reference 58. 58 QUARTERLY REVIEWS that the authors regarded the two types of oscillators as limiting cases but much of the attractiveness of the quantitative treatment is lost when subsidiary assumptions have to be made. Perhaps the greatest achievement of the theory is the broad correlation it has made possible yielding a panoramic view of the field rather than LL “close-up.” The same conclusions can be drawn from an examination of the results of the qualitative application of the theory of resonance. Here again a broad correlation has been achieved,-not only in the field of colour but in the many other aspects of chemistry that can be accounted for on the resonance concept.The theory has aEorded a correlation on the grand scale of the apparently divergent hypotheses introduced by the early works. More specifically a chromogen may be thought of as a resonating system the absorption of which does not extend into the visible. Auxochromes and anti-auxochromes are groups capable of extending the resonance system of the chromogen in such a way that the absorption moves into the visible. The importance of the quinonoid structure in producing colour and in particular the oscillation of the quinonoid condition become intelligible with the aid of resonance concepts. The limitations of the early theories were largely due to their derivation in terms of specific systems ; the power of the resonance treatment lies in its general applicability.Finally there remain the quantal calculat’ions. Here again the position leaves much to bc desired. On the one hand the V.B. treatment has been very successful in predicting the position of the absorption bands for a number of compounds but a t present there exists no guarantee that the calculated absorption bands refer to those actually observed. Only by an investigation of the intensities of the transitions can this point be settled and such an investigation would have to take account of ionic structures which greatly complicate the calculations. Very little evidence can be adduced from the literaturc as to the importance of ionic structures. On the other hand the L.C.,4.0. method has had reasonable success in computing the intensities of absorption bands but only with the aid of the observed wave-length of absorption.It is still to be shown that the bands for which the intensities are calculated are the observed bands. In both these methods the approximations made are rather disconcerting although some justifica- tion may be obtained from the success of similar approximations in the treatmcnt of the ground states of molecules. The most satisfying treatment is with the us0 of antisymmetrical molecular orbitals but here the labour involved restricts a t present the treatment to relatively simple molecules. Many loose ends are apparent in the quantal treatment and only the passage of time will show how successfully they may be tied. The author wishes to acknowledge the award by the University of London of an Impcrial Chcmical Industries Fellowship during the tenure of which this rovinw was written. He also wishes to thank Professor C. K. Ingold F.R.S. for the interest he has shown in this review and Dr. C. A. Coulson and Mr. D. P. Craig for discussions on the subjects under consideration.

ISSN:0009-2681    

DOI:10.1039/QR9470100016

出版商:RSC    

年代:1947

数据来源: RSC

摘要:

AMORPHOUS CARBON AND GRAPHITE By H. L. RILEY D.Sc. F.R.I.C. (PROFESSOR OF INORGANIC AND PHYSICAL CHEMISTRY KING’S COLLEGE NEU-CASTLE-UPON-TYNE) GEOMETRICAL factors p1ay an important r d e in the molecular architecture of solids molecules atoms and ions all possess definite size and shape ; homopolar valency bonds are directed in space and it is therefore difficult to conceive hon- a solid particularly one in which homopolar bonds play a major part can liavc a completely random crystal structure. The examina- t ion b\- S-rag diffraction inethocls of substances which show no obvious crJ-stnlliiic cliaracterist ics nud t)rc\-iously were considered amorphous e.g. vit reoils niid highly dispersed solids niiimnl and vegetable fibres soaps ctc. has shoi\-~i thiit t hc great ninjoritjr posscss structural characteristics which are t>-picnl of thc cr\-stnlline state.As n rcsnlt of the X-ray examina- tion of a nuni1)cr of specimens P. De1jJ-e and P. Scherrer coiiclucled that amorphous carbon is J I ~ C I T ~ J - gra1)liite in a state of sub-tlivision so fine that it coultl iievcr lm rcachctl 11)- incc~liniiici~l means. P. P. ven Weimarn and T. Hagiivara mailit iiine(1 froin cslwrimeiit s on the precipitation of barium sulphate that ewii 1~1leii X-ray tiiffr action photographs indicate that a substance is aniorl)lious tlii.3 niii\t not be taken as cz proof that it is so. Glassy or vitreous so1 ids inalio 1111 ail iiiiportant group of the so-called amorphous substances. -1 cornparison of X-ray powder photographs of silica wollastonitc sodium ciiboratc selenium potash and soda felspars boric oxicle complcx silicates glucose aiitl sucrose in the vitreous and the crystalline state led J.T. Randall H. Y . Rooksby and B. S. Cooper to put forward the “ cr\-stallitc ” theory of tlic vitreous state. They showed for example that the main featiires of the diffraction pattern of vitreous silica can be accouiitecl for by the assumption that it consists of exceedingly minute crystals of crj-stoljnlite wit 11 avcragc linear dinicnsions of the order of 15 A . and lattice coiistants sonic G.G% grcater than those of large crysto- balite crystals. pointed out that this suggestion leads t o discrcpaiicies bctwcen the observed and the calculatcd densities and is not in accord with tlic cliwacteristic mechanical and thermal properties of the glasses he suggested that the ultimate condition for the formation of a glass is that the substance can form czii extended three-dimciisional network of atoms lacking periodicity with an energy content comparable with that of the corresponding crystal network.put forward what is perhaps a compromise betwccn tlic above views namely that thc formation of a glass is due to the presence of large or irregular groups of atoms too cumber- some for direct addition to the lattice. RI. L. Huggins Kuan-Han Sun and A. Silverman6 have pointed out that a solid will readily assume the If7. H. Zacliariasen G. Haigg Physikal. Z. 1917 8 291. 2 KriBt. 1930 75 196. I . Anier. Chem. SOC. 1932 64 3841. J . Chem. Physics 1935 3 42. .J. Anier. Cerclni. SOC. 1943 26 393. 59 2 Kolloid-Z. 1926 38 120 ; Kolloidclicnr. Ueih. 1927 23 400.60 QUARTERLY REVIEWS amorphous state if the regular network structure (crystalline) has practically the same energy content as the irregular in certain glasses besides homo- polar valency bonds hydrogen bonds also probably have an important function in building up the irregular network. In an attempt to show how far our knowledge of the structure of the so-called " amorphous " carbons is compatible with the above views we shall first of all consider the crystal structure of graphite then the crystal chemistry of amorphous carbon and of its transition into graphite and fhally discuss its relationship with other amorphous substances. Throughout we shall use the term " amorphous carbon " in its usual sense viz. for those forms of black carbon other than macro-crystalline graphite which possess no obvious crystalline characteristics.The Graphite Lattice.-The graphite crystal structure which first received wide acceptance was that deduced by J. D. Bernal and by 0. Hassel and H. Mark 8 and later confirmed by C. Mauguin and H. Ott lo it consists of carbon atoms arranged in flat honey- comb-like layers which are stacked parallel to and equidistant from each other in such a way that half the atoms in one lnyerlie normally above half the atoms in the layer beneath while the other half are normally above the centres of the hexagons of the layer beneath altcrnate layers lie atom for atom normally above each other. (a) (61 The C-C spacing in the layers is 1-42 A. whilst adjacent layer-planes are 3.35 A. apart.ll D. S. Laidler and A. Taylor,f2 however pointed out that this structure does not account for the presence of certain faint lines in the X-ray powder photographs of both natural and artificial graphites these lines had previously been reported by G.I. Finch and H. Wilman l3 in electron-diffraction photographs of graphite. To explain the presence and intensity of these additional diffractions H. Lipson and A. R. Stokes l4 suggested that many graphites have a cornyosite structure made up of about 80% of the ordinary (Bernal) structure 6% of a disordered structure (turbo- stratic) and 14% of the structure originally suggested by Debye and Scherrer (Zoc. cit.) in this structure the flat honeycomb planes are stacked parallel to each other as in the Bernal structure but instead of alternate layers being normally above each other the layers follow an abcabc sequence the third layer being symmetrically related to the planes above and below (see Fig.1 a and b ) . These two ways of stacking hexagon layer-planes are FIG. 1 reminiscent of the manner in which close-packed planes of spheres can be Proc. Roy. Soc. 1924 A 106 749. Bull. Soc. franp. Min. 1926 49 32. 8 Z . Physik 1924 25 317. lo Ann. Physik 1928 85 81. l1 J. B. Nelson and D. P. Riley Proc. Physical SOC. 1945 67 477. l2 Nature 1940 146 130. l 4 Ibid. 1942 A 181 101. l a Proc. Roy. SOC. 1936 A 155 345. RILEY AMORPHOTTS CARBON AND GRAPEUTE 61 13.9 2.45 11.7 ' 2.28 10.8 j 1.99 11.5 2-02 13.2 1 2-02 stacked to give either the close-packed hexagonal or the face-centred cubic structure according to whether the stackings follow the ababab or the abcabc sequence respectively.The phenomenon may have some connection with the metallic character of the inter-planar valency bonding in graphite. The lattice structure of graphite however is not yet fully elucidated J. Gibson l 5 has recently obtained powder photographs of ash-free graphites both natural and artificial which show some 16 faint lines which cannot be indexed on the basis of either of the two structures described above. In addition to the crystallographic there is also physical and chemical evidence in support of the layer-lattice structure of graphite. Its pro- nounced electrical inagnet'ic and mechanical anisotropy is in keeping with this structure so also are the close parallelism between the chemical properties of graphite and those of the triarylrnethyls,ls and the lamellar reactions of graphiteel' Amorphous Carbon.-Certain types of amorphous carbon show crysto- chemical reactions similar to those of macrocrystalline graphite ; e.g.0. Ruff and 0. Bretschneider 18 state that norit an active carbon freed from oxygen and water vapour by ignition reacts with elementary fluorine at 280" to form a solid monofluoride containing 57.3% of fluorine [theory for (CF)n 61.27y0] which they suggest has fluorine atoms intercalated between hexagon layer-planes of carbon atoms as in the compound made from macrocrystalline graphite. U. Hofmann and A. Frenzel19 have shown that certain apparently amorphous carbons give good yields of graphite oxide when oxidised with potassium chlorate in the presence of concentrated sulphuric and nitric acids (see Table I). Sugar charcoal however when treated similarly was 93.6 I 92.3 83.3 87.6 20.2 TABLE I Composition and Yield of Graphite Oxide Carbon used.Ceylon graphite . . . . . . Acheson graphite. . . . . . Carbonmonoxidecarbon . . . Retort carbon . . . . . . Petroleum soot . . . . . . 1 Percentage composition. Percentage yield of 1 graphite oxide 1 i 1 calculated on I 1 C. i H,O. 1 C 0. carbon present. 53.4 53.3 ~ 52-3 oxidised practically completely to gaseous and soluble colloidal products no graphite oxide being formed. This reaction has been used as a means of lb Nature 1946 158 752. l6 H. L. Riley J . Inst. Fuel 1937 10 149. l7 Detailed summaries of the lamellar reactions and compounds of graphite have been given by W. Ritdorff Wien. Chem. Ztg. 1944 47 172 and H. L. R,iley FveZ 1946 24 1. 1* 2.anorg. Chem. 1930 217 1. l b Ber. 1930 B 63 1248. 62 QUARTERLY REVIEWS differentiating amorphous from graphitic carbon. M. Berthelot in 1870 defined graphite as “toute varikt6 de carbon susceptible de fournir par oxydation un oxyde graphitique.” W. A. Selvig and W. C. Ratliff 2o used the Brodie reaction (oxidation with fuming nitric acid and potassium chlorate) to determine the proportion of graphitic carbon in coals and cokes and from their results concluded that the former contain no graphitic carbon whilst the latter contain only a small amount (< 1%). Hofmann and Frenzel (Zoc. cit.) on the basis of their X-ray results (see below) however argue that these differences in graphite oxide yield are due t o differences in crystallite size and packing rather than to any fundamental difference in the crystal structures of graphitic and amorphous carbons the much smaller crystallites of the chars and cokes are more readily oxidised to soluble products than the larger crystallites in graphitic carbons.It is significant however and we shall return to this point again that the amorphous carbons which give relatively large yields of graphite oxide are those made by deposition from the gas phase. K. Fredenhagen and G. Cadcnbach 21 found that potassium rubidium and czsium react not only with macro-crystalline graphite but also with soot pure arc carbon wood charcoal and active carbon ; whereas sodium reacts with soot but not with graphite. From more detailed investigations on the interaction of potassium and a soot made by the incomplete com- bustion of a tar oil K.Fredenhagen and H. Suck 2 2 concluded that under the same conditions graphite and soot take up practically the same amount of potassium the establishment of equilibrium with the soot required much longer than with the graphite ; the pretrcatrnent of the soot played some part in determining the velocity of the absorption and desorption of the potassium. These results and those of the X-ray study of (C,K) and (CI6K),$ by A. Schlecde and 31. Wellmann 23 suggest that lamellar structures are present in soot. The intercalation of bromine between the layer-plancs of the graphite crystal lattice has been investigated by W. R t i d ~ r f f ~ ~ who found that graphites take up about 80% of their weight of bromine whatever their state of subdivision active carbon takes up about 170% of its weight of bromine probably because of its morc highly developed crystnllite surface.H. H. 1,owvry and S. P. Jloigan 25 prepared graphite in a highly dispersed state by first oxidising it to graphite oxide and then subjecting the product to thermal decomposition and repeating this treatment on the resulting graphite the final product possessed from one-third to one-quarter of the adsorptive power of the best active charcoal. E. Herl K. Andress L. Rein- hardt and W. Herbert 26 convliicled from X-ray and electrical conductivity measurements that there is 110 truly amorphous carbon in active carbons its properties differ from thosc of graphite because of the size and mesomor- phous character of its crystallites ; the elccbrical conductivity of samples 20 J. Physical C‘/ic111.1!125 29 1103. 21 Z. anorg. Chevt. 1926 158 549. 2 2 Ibid. 1929 178 353. 23 2. physikal. Chem. 1932 B 18 1. 2 4 2. a,Lorg. C’/ic)ti. 1941 245 383. 26 2. p k y s i l i ~ l . Chem. 1932 158 273. 25 J . Physical Chem. 1925 29 1105. RILEY AMORPHOUS CARBON AND GRAPHITE 63 prepared a t high temperatures way approach that of macrocrystalline graphite. The above properties of amorphous carbon are qualitatively in keeping with the DebyeScherrer hypothesis that they consist of exceedingly minute graphite crystallites. There are however amorphous carbons which possess properties which this hypothesis cannot explain. Cokes and chars always contain an appreciable proportion of chemically combined foreign matter ; e.g. carbonisation of carbohydrates up t o 1000" gives chars containing 1-2% of combined oxygen which is not eliminated by prolonged heating in a vacuum at the same or a lower temperature ; only prolonged heating a t much higher temperatures eliminates this oxygen.That it plays an important r6le in the structure of amorphous carbon is indicated by studies of nitrogenous and sulphurous carbons. Amorphous carbon does not take up nitrogen when strongly heated in the gas ; if' how- ever it is heated in gaseous ammonia hydrogen cyanide is formed and the carbonaceous residue may contain up to 3% of nitr~gen.~' E. Terres 28 studied the carbonisation of nitrogenous organic compounds such as glycine asparagine etc. and obtained nitrogenous cokes. W. Hook 29 and H. E. Blayden J. Gibson and H. L. Riley 30 have contrasted the analysis of chars prepared at various temperatures from carbohydrakes on the one hand and from glycine on the other it is significant that the former even those prepared a t lOOO" all contain appreciable quantities of oxygen ; the latter all contain nitrogen but oxygen is present only in the chars prepared a t the lower temperatures.The 1200" glycine char contains 3.64% of nitrogen but no oxygen. Sulphurous chars have been prepared by heating sucrose char with elementary s ~ l p h u r ~ l by carbonising organo-sulphur compounds 32 and by heating semi-coke filter-paper and wood in a current of sulphur dioxide.33 The sulphur in these products cannot be removed by exhaustive extraction with sulphur solvents and in some cases amounts to several per cent. I n addition to oxygen nitrogen or sulphur cokes and chars even when prepared at high temperatures always contain appreciable amounts of hydrogen These foreign atoms all probably play an important part in disordering the atomic arrangement in amorphous carbons.The X-ray powder photographs of amorphous carbons are strikingly different from those of macrocystalline graphites not only are the diffrac- tions much broader but they are also fewer in number most specimens giving only two or three highly diffuse haloes instead of the numerous sharp diffractions in the graphite X-ray photograph. A much deeper insight into the crystallographic character of amorphous carbon has been obtained from 27 W. G. Mixter Amer. J . Sci. 1893 45 363. 28 J . Qaibekucht. 1916 59 619. 2B Carnegie Schol. Mem. Iron and Steel Imt. 1936 25 81. so " The Ultra-Fine Structure of Coals and Cokes," B.C.U.R.A.London 1944 s1 J. P. Wibaut Rec. Trav. chim. 1919 38 159 ; Proc. K . Akad. Wetensch. Amster- 32 R. Ciusa Gazzetta 1922 52 130 ; ssF. Fischer and A. Pranschke Brennstoff-Chem. 1928 9 361. 176-231. dam 1921 24 92. 1925 55 385. 64 QUARTERLY REVIEWS the detailed study of these X-ray powder photographs. The diffracting units are of colloidal dimensions and therefore because of their low resolving power give diffuse diffractions there is a quantitative relation between this diffuseness and the size of the diffracting unit. The first explanation of the disappearance of all the hkl diffractions and most of the high-order diffrac- tions in bhe powder photographs of amorphous carbons was put forward by Berl Andress Reinhardt and Herbert 26 and by H. Arnfelt 34 it is briefly as follows.I n these finely crystalline or so-called amorphous carbons the hexagon layer-planes are stacked parallel t o each other in the individual crystallites with a spacing approximately equal to or somewhat greater than the corresponding spacing in the graphite crystal ; the individual hexagon layer-planes however are otherwise orientated in a completely random manner. J. Biscoe and B. E. Warren 36 have coined the word turbostratic to describe such a crystallographic structure. It can give rise to only 001 and hk0 (more correctly hk) diffractions the hk bands are cross- lattice diffractions formed by the individual hexagon layer-planes acting as two-dimensional diffraction gratings. Such diffractions are not symmetrical but show " tails " towards greater values of 0 .B. E. Warren 36 has discussed in detail this type of diffraction and derived a formula relating the broadening of the cross-lattice band with the linear dimension of the diffracting net-plane ; this is of the same form as the Schemer formula for three-dimensional crystallites but the numerical constant instead of being approximately unity is 1-84 1,843 Linear dimension of crystallite L = ___ p cos 8 where 3 is the wave-length of the X-radiation p the breadth of the difiaction a t half its peak intensity (half-peak width) and 0 the Bragg angle. Warren has also shown that with very small crystallites there is a small displacement of the diffraction maximum of the cross-lattice band towards a larger value of 8 0.163. d(sin0) - - L If a correction is not made for this displacement the calculated C-C spacing within the hexagon layer-planes will be too The assessment of the average.size and shape of the cryatallites present in amorphous carbons from the broadening of the diffraction bands in X-ray powder photographs has been attempted by several workers.Unfortunately differeiit niet hods of calculation have been employed pa&icularly in correct- ing the iticasured hdf-pcilk u-idth for the width of a diffraction from a similar specinien made up of large crystals the results reported by dieerent workers t herefow arc often not strictly comparable. Nevertheless the results reported for similar carbons are of the same order of magnitude. Because 3 4 A r k . Nat. Ast. Fys. 1932 23B No. 2. 37 Cf. for examplo H. E. Blayden H. L. Riley and A. Taylor J .Arner. Chem. S O C . 19.10 62 180 ; .J. H. de Boor Rec. Trav. chim. 1940 59 828 ; U. Hofmann XL'atrrrwis.y. 1944 260. J . Appl. Physics 1941 9 492. 313 Physical Rev. 1942 49 693. RILEY AMORPHOUS CARBON AND GRAPHITE 65 usually only two diffraction bands are of sufficient intensity for accurate measurement it has become customary to express the results of measure- ments of this kind in terms of a hypothetical average cylindrical crystallite the diameter La of which is termed the a dimension and the height L, the c dimension. Some typical results 38 are shown in Table 11. These and the results of other workers indicate that the original Debye-Schemer theory of the crystallographic nature of amorphous carbon requires modifica- tion in a t least two respects 'uiz. the small crystallites in amorphous carbon differ from macro-crystalline graphite in that (1) they are turbostratic and TABLE I1 Crystallite Dimensions (Hofmann and Wilm 38) Carbon.Ceylongraphite . . . . . Carbon monoxide carbon 700" . 9 9 , 550" . 9 9 1 , 420" . Y f Y , 400" . Acetylene soot (explosive decom- position) . . . . . . . Retort carbon . . . . . . Acetylene soot. . . . . . s t , calcined . . . , activated . . . Actiyemrbon AKTIV . . . 9 7 , , , calcined. 9 , , , activated Supranorit . . . . . . . , calcined . . . . , recalcined 30 hrs.. . ) activated . . . . Carboraffi calcined . . . . , aotivated . . . . Gas-mask carbon calcined . . 9 7 , act.iveted . . Sugar carbon . . . . . . 7 , activated . . . C. 99.1 94.9 95.0 91.8 - 99.7 96.9 95.0 99.1 98.5 88.5 96-5 94.4 89.3 97.3 97.7 96.2 93.0 97.4 96.5 98.1 98.0 L Analysis (jo.H. 0.0 0.1 0.1 0.1 - 0.3 0.3 0.8 0.3 0.0 1.7 0.35 0.0 0.6 0.15 0.0 0.6 0.0 0.6 0.0 0.7 0.0 - Ash. 0.3 4 3 2 - 0.0 0.6 0.3 0-3 0-5 1.1 1.1 2.0 0.3 0.4 0.9 1 . 9 3.3 0-5 1.0 0.0 0.0 - Layer- plane spacing C/? A. 3-35 3.4 3.4 3.4 3.45 3.43 3.45 3.55 3.55 3-6 3.5 3.45 3.65 3.5 3.45 3.55 3.7 3.G 3.7 3-55 3.6 3.6 3.6 Crystallite tliriiension A . L O . a 120 150 60 40 45 45 21 26 22 17 25 18 18 26 26 24 13 19 20 17 21 18 &. ca. 200 180 160 70 35 50 40 13 14 16 11 10 8 8 7 7 7 10 7 10 8 9 7 (2) the hexagon-layer spacing is not constant a t 3.35 A. but increases as the number of layer-planes in the crystallite decreases. Even when allowance is made for the uncertainty in the fundamental accuracy 39 of these results the extreme minuteness of these crystallites is striking in the smaller crystallites the individual hexagon layer-planes are not much larger than certain polynuclear aromatic molecules of known structure and the c dimensions indicate that the crystallites can be built up from a.s few a's three or four layer-planes.38 U. Hofmann and D. Wilm 2. EEektrochem. 1936 42 504. 89 a. H. Cameron and A. L. Patterson Amer. SOC. Testing Mat. Symposium on Radiography and X-ray Diffraction 1937. E 66 QUARTERLY REVIEWS It would appear from the above that the process of charring or carbonisa- tion must involve the progressive transformation of the more or less complex organic matter into the polynuclear aromatic ring systems and their subse- quent stacking and growth. Some such process is indicated by the experi- ments of R.C. Smith and H. C. Howard 40 who charred pure cotton cellulose at temperatures ranging from 190" to 400" and subjected the products t o exhaustive oxidation with alkaline permanganate the magnitude of the yields of aromatic acids from the various chars indicated progressive aromatisation as the charring temperature increased. X-Ray investiga- tions have shown however that this simple picture is far from complete. Fig. 2(a) summarises the results of crystallite-size determinations on chars 40 J . Amer. Chem. SOC. 1937 59 234. RILEY AMORPHOUS CARBON AND URAPHITE 87 prepared from cellulose at various temperature^.^^ Results of an identical character have been reported by U. Hofmann and F. Sinkel 42 who worked with sucrose chars. The striking feature of these results is the constmcy of the average height ( c dimension) of the crystallites (experimentally the half-peak width of the 002 diffraction band) over a wide range of carbonising temperature.Crystallites built up merely from polynuclear aromatic layer-planes held together by relatively weak van der Waals forces would not be expected to behave in such a manner but rather to vary their degree of packing with the speed and temperature of carbonisation. This variation would be reflected in changes of the c dimension. Other carbohydrates lignin and wood i.e. organic compounds containing a relatively high propor- tion of oxygen all give chars containing crystallites with c dimensions between 9 and 10 A. Carbonising conditions can be varied between wide limits with respect to both speed and temperature (up to lZOO") without any consequential change in the half-peak width of the 002 difiaction of the resulting char.Cellulose carbonised in an atmosphere of ammonia and also glycine behave in a similar manner but the constant c dimension is somewhat greater viz. between 12 and 13 A. This constancy of the c dimension is highly suggestive and appears to indicate some kind of cross- linking between the layer planes of the crystallites. This hypothesis appears more probable when we consider the X-ray crystallographic behaviour on carbonisation of certain organic compounds which have a relatively small oxygen content. Fig. 2(b) shows the c-dimen- sion curve obtained with cokes made over a range of temperatures from that part of a bituminous coal-tar pitch which is soluble in carbon tetrachloride Similar c-dimension curves 43 are given by pitches certain bitumens bituminous coals the pyridine-chloroform soluble extracts of bituminous coals and certain pure polynuclear aromatic compounds e.g.dibenzanthrone Caledon jade-green etc. Such c-dimension curves appear to be character- istic of the presence in the parent carbonaceous matter of large thermally stable lamellar aromatic molecules. Aromatic hydrocarbons of this type are volatile and apparently the presence of some oxygen and/or nitrogen in the parent molecule is necessary for a large carbon yield the proportion however must not be too large otherwise charring occurs at a low tempera- ture. These c-dimension curve8 have been interpreted as follows. The ascending part of the curve between room temperature and about 50O0 indicates a unidimensional crystallisation brought about by the increased packing of thermally stable aromatic molecules under the influence of thermal vibration.The maximum at about 500" reflects the conversion of a system consisting of stable independent organic molecules held together by weak dispersion forces (molecular lattice) into a " carbon " crystallite [rigid three- &%ensionally cross-linked ( ?) lattice]. Dibenzanthrone after being heated slowly up to 475" and cooled still gives its characteristic X-ray difiaction I1 H. E. Blayden H. L. Riley and A. Taylor J. 1939 67. Ia 2. a w g . Chem. 1940 245 85. u H. E. Blayden J. Gibson and H. L. Riley J . Inst. FueZ 1945 War Time Bulletin 117. 68 QUABTERLP BEVIIOWS pattern made up of several more or 106s sharp diffkactione; after heatixq to 500" the pattern characteristic of the dibenzanthrone crystal lattice haa completely disappeared and been replaced by two diffuse haloes character- istic of amorphous carbon.This change molecular lattice -+ carbon occurs over a relatively narrow range of temperature the height of which appears to be a function of the proportion of oxygen in the parent carbonaceous material. Substances like cellulose form rigid (cross-linked ?) carbon crystallites at temperatures little above 200" whereas certain coking cod bitumens retain their molecular identity up to temperatures as high as 550". The interpretation of the descending portion of the c-dimension curve between 550" and 900" [Fig. 2(b)] is not quite so clear. The phenomenon is however of general occurrence whenever a carbonaceous substance on being heated to some temperature below 550" gives a product which contains crystallites having an average c dimension greater than about 12 A.then a further increase of temperature brings about a decrease in the c dimension (experimentally a broadening of the 002 diffraction). The increasing dis- order over this temperature range may be due to the evolution of oxygen as oxides of carbon and a consequent reduction in the extent of tho cross- linking between the hexagon layer-planes. The subsequent increase in the c dimension above 900" is perhaps facilitated by the remnants of the ordered structure which the system possessed at 550". A further type of c-dimension curve is shown in Fig. 2(c) it is character- ised by an abrupt increase in the average c dimension in the temperature range 800-1000".Crystallite growth of this kind is shown by peats to a less extent by brown coals and lignites and also by " bituminised " cellulose and lignin Le. cellulose or lignin which has been heated in an autoclave to about 300" with an excess of aqueous alkali.43 In the case of peat the crystallite growth of a part of the char between 800" and 1OOO" is so great as to suggest some profound atomic rearrangement perhaps a kind of polymorphic change. Although the true nature of this crystallographic change is still not clear J. Gibson 11. Holohan and H. L. Riley44 have pointed out that besides the diamond and graphite lattices there is a third possible spatial arrangement of carbon atoms which may play an important part in the atomic architecture of amorphous carbon.This arrangement can be pictured as follows. Imagine a graphite layer-plane to be made up of discrete hexagons connected together by valency bonds as in Fig. 3(a) ; break the necessary bonds and rotate the hexagons A,B,C etc. inwards through 60" [Fig. 3(b)] ; it will be found that the valencies so freed can be attached without strain to those of similarly tilted hexagons in adjacent planes above and below This can be seen in Fig. 3(c) which shows the spatial relationship of 4 hexagons. The structure 80 formed is a three- dimensional network of the " skeleton lattice '' type full of large holes and long channels and reminiscent of the structure of certain zeolites. The disappearance of the flat layer-planes suggests that the resonance energy of this structure would be less than that of the graphite lattice and its existence therefore less probable.The new structure however contains an orderly 44 J. 1946 458. 69 network of conjugated double linkages running throughout the whole three- dimensional lattice and its resonance energy would therefore be of an appre- ciable magnitude. The existence of such a structure is made more probable by the discovery by I. L. Karle and L. 0. Brockway,45 by means of electron- diffraction measurements) of the non-co-planarity of o-tetraphenylene [see Fig. 3(c)]. The suggested structure is a three-dimensional repetition of o-tetraphenylene residues. RILEY AMORPHOUS CARBON AND GRAPHITE \ FIG. 3 A consideration of carbonising processes which would possibly tend to give some structure other than the flat honeycomb network of carbon atoms led Gibson Holohan and Riley to prepare carbon by (a) carbonising hexa- iodobenzene and (b) the interaction of hexachlorobenzene and sodium amalgam ; (a) gave a carbon which showed practically no coherent scattering of X-rays and ( b ) gave a carbon the X-ray powder photograph of which although highly diffuse showed new features which can be explained by the existence of small volumes of the three-dimensional network described above.The constancy of both the a and the c dimensions [Fig. 2 (d)] when a carbon prepared by method ( b ) (temperature of preparation did not exceed 300") was heated to higher temperatures provides additional evidence of a rigid cross-linked lattice structure These X-ray investigations leave little doubt that the idea that black a,morphous carbons are merely graphite in an extremely fine state of sub- d5 J .Amr. C h . SOC. 1944 66 1974, 70 QUARTERLY REVIEWS division is inadequate and needs further qualification. It is evident that the structure of a particular sample may be greatly influenced both by the nature of the parent carbonaceous material and by the method of carbonisa- tion. It is difficult to conceive for example how a system consisting of relatively free and independent lamellze could fail to develop some degree of order under the influence of thermal vibration and yet carbons can be made which give no coherent scattering of X-rays. Before outlining a modified view of the crystallographic nature of amor- phous carbons we must consider the process of graphitisation i.e. the transition amorphous carbon + graphite.Graphitisation.-The high thermal stability of pure amorphous carbon is shown by the results of Biscoe and Warren 35 (Table 111) who studied the TABLE I11 C'rystallite Dimensions of Carbon Black (Biscoe and Warren) Time and temp. of heating. - 2 hrs. at 760" 2 hrs. at 1040" 2 hrs. at 1500" 2 hrs. at 2000" 2 hrs. at 2800" 2 hrs. at 2000" 2 hrs. at 1040" Layer-plane spacing A. 1 Crystallite dimension A. Frorn002. 1 From001. 3.55 3.53 (3.65) 3.48 3.46 3.45 3,47 3.55 3 4 1 3-43 3.47 3-45 3-44 3.44 3.43 3.46 La. 20.0 22.6 28.0 44.2 55.8 65.2 60.5 29.8 Lc * 12.7 14.4 14.9 24.9 32-3 40.0 35.5 14.9 effect of high temperature on the crystallographic character of a commercial carbon black. Similar results were obtained by P. Corriez 46 who studied the effect of high temperature on the crystallite dimensions of sucrose chars.I n the case of pure carbons it is only a t extremely high temperatures that pronounced thermal recrystallisation occurs. U. Hofmann A. Ragoss and F. Sinkel 47 have shown by means of X-ray diffraction electron-microscope and adsorption studies that the small polycrystalline spherical particles of carbon black on heating for 24 hrs. a t 3000" undergo thermal recrystallisa- tion to an extent limited by the size of the original spherical particle i.e. the indiyidual polycrystalline particles probably become single crystals. This thermal stability of the amorphous carbon crystallites is due of course to the high resonance energy of the hexagon network planes of carbon atoms. It is only when these networks are broken down by some chemical mechanism that pronounced crystal growth can occur at temperatures below 2000".This was realised by E. G. Acheson long before the crystal structure of graphite was known in his original patent on the manufacture of graphite 4% Cornpt. rend. 1935 201 1189. 47 Kolloid-Z. 1941 96 231 ; s00 also D. Wilm and U. Hofmann ibid. 1935 70 21 ; A. Ragoss U. Hofmann and R. Holst ibid. 1943 105 118 ; M. von Ardenne and U. Hofmann 2. physikal. Chenz. 1941 B 50 1. RITAEY ttJIORPHOT7S CARBOX ,IN D GR~IP~IITE 71 he states 4 8 " I hnvc also discoverctl that in order to produce pure graphite from carbonaccous materials thcre is nri indirect conversion and that the act of formation of the graphite is morc in thc nature of an act of dissociation of the carbon from its combination with other materials than a conversion of the ordinary carbon into graphite and that as a preliminary step the carbon has to be combined chcniically with some other material.Thus I have found that if the carbonaccous material or carbon used in the process contains a considerable proportion of mincral matter or if it is mixed with a certain proportion of oxide or oxides such as silica clay alumina mangan- ese lime or oxide of iron and subjected to treatment as hereinafter set forth (high temperature) the yield of graphite is enormously increased and the product is most satisfactory." It is significant too that processes which involve the formation of carbon by deposition from the gas phase often give at relatively low temperatures products which are highly crystalline soe for example the crystallite dimensions of the carbons formed on a catalyst by the reaction 2CO = C + CO, quoted in Table 11.The growth mechanism of the carbon crystallites formed in such reactions apparently involves the initial formation of single carbon atoms which subsequently attach themselves to pre-formed graphite nuclei. Other reactions are known which give rise to highly crystalline graphite at temperatures far below that of thermal recrystallisation they all involve the liberation of individual carbon atoms e.g. the graphitisation of austenite,49 and the decomposition of carbides 5O and calcium cyanamide. 51 Carbons deposited from hydrocarbon gases in the iieighbourhood of lOOO" are not so highly crystalline as those deposited from carbon monoxide a t the same temperature the presence of hydrogen appears to inhibit crystal growth presumably by saturating the valencies of the carbon atoms at the surface of the crystallites.It is reasonable t o suppose however that carbons formed by any process which involves the participa- tion of single carbon atoms will have the most stable structure i e . the flat layer-plane graphite-like structure rather than any cross-linked structure of rather hlgher potential energy. JI. Holohan and R. Iley 52 have shown that in spite of' their highly dispersed state carbon blacks and carbon deposited from the gas phase on vitreous silica a t lOOO" have ignition temperatures which approach and may even exceed those of natural and artificial graphites. Tho X-ray-scattering power of carbons of this type is 48U.S.P. 568,323 29.9.1890 quoted by F.-4. J. Fitzqcrald J . SOC. Chem. Id. 1901 20 443 ; see also H. Ditz Chem. Z t y . 1904 28 167 and W. C. Arsem Met. Chem. Eng. 1911 9 536. 48 F. Wust and C. Geiger StuhZ zl. Eisen 1905 25 1134 1196 ; I<. Iokibe Sci. Rep. Tdhoku Inip. Unip. 1920 9 273 ; L . Sorthcott J . Iron Steel Inst. 1923 108 491. 5 0 H. Ishikawa Elect. Rev. Japan 1931 19 419 493 690 726 824 884 ; C. Hahn and A. Strutz Metallurgie 1906 3 72.3 ; Chem. Ztg. 1907 31 Rep. 33 ; J. West- becker Metullurgie 1904 1 137 ; 2. Elektrochenz. 1904 10 837 ; A. Frank Versamm. Ges. deutschen Naturforscher u. Aerzte Sept. 1905 ; Chem. Ztg. 1905 29 1044 ; V. M. Weaver U.S.P. 1,576,883 16.3.26. 61 E. Collett and M. Eckardt E.P. 5713 7.3.1911 ; A. Reme16 and B. Rassow Z . angew. Chem. 1920 33 139; N. Kameyama J.Chem. Ind. Japan 1921 24 1131. 5a Unpublished work of the Northern Coke Research Laboratory. 72 QUARTERLY REVIEWS much greater than that of chars of comparable crystallite size sulphuroue chars (see above) have a still lower scattering power. Although these Wer- ences in scattering power have as yet only been studied semi-quantitatively they are of such a magnitude that there is little doubt of their si@cance. Conclusion.-The above considerations lead to the conclusion that amorphous carbons are built up from two types of structure vix. (I) the turbostratic lamellar graphite-like structure and (11) the disordered three- dimensionally cross-linked structure (Fig. 3). Amorphous carbons which contain a preponderance of structure I are usually much purer than those made up largely of I1 it is possible that the foreign atoms hydrogen together with oxygen nitrogen or sulphur play some part in stabilising the type I1 disordered structure.Such a view of the crystallographic character of amorphous carbon appears to offer a reasonable basis for the explanation of the great diversity of its properties e.g. the highly specific nature and range of its adsorptive properties the great variation in its chemical reactivity the dif€erences in its mechanical properties and scattering power for X-rays etc. Carbons deposited from the gas phase which may contain little or no combined oxygen will be made up largely of type-I structures whilst cham and cokes in general will consist of intimate associations of both types. A large proportion of oxygen in the parent carbonaceous matter appears to favour the formation of structure 11 whereas carbonaceous matter containing little oxygen and a large proportion of benzenoid carbon e.g.pitches bitumens etc. will give carbons with a higher proportion of structure I. Both struc- tures possess high thermal stability structure I1 appears to persist in Borne specimens at temperatures well above 1000". No mention has been made of the importance of the secondary structure i.e. the cohesion and degree of agglomeration of the crystallites in deter-. mining the properties of amorphous carbon. Carbon blacks used in the rubber industry have been investigated in detail ; 53 little is definitely known however of the secondary structure of chars and cokes their accessible ~urface can be measured but the open lattice present in both structures I and I1 makes the interpretation of such measurements ditlicult.Such is the diversity of the crystal structure of solids that no single comprehensive theory of the amorphous state is possible. Metals and salts can be so disordered by mechanical strain or dispersion that they give diffuse X-ray diffractions. Crystals which are made up of long homopolar chains exist in an amorphous condition when the chains become tangled together e.g. vitreous sulphur and selenium rubber borate-glass etc. Systems in which the molecules are large lamellae can assume the amorphous (or meso- morphous) state because of the ditliculty these cumbersome units have in packing into an ordered lattice e.g. pitches bitumens polynuclear aromatic compounds. Finally there is the disordered three-dimensional network present in certain glasses and vitreous solids.Amorphous carbons should probably be placed in a class intermediate between the disordered lamella and the disordered cross-linked lattice. 68 Columbian Carbon Company " The Partiole Size and Shape of Colloidal Csrboa w Revealed by the Electron Microscope," New York 1940. AMORPHOUS CARBON AND GRAPHITE By H. L. RILEY D.Sc. F.R.I.C. (PROFESSOR OF INORGANIC AND PHYSICAL CHEMISTRY KING’S COLLEGE NEU-CASTLE-UPON-TYNE) GEOMETRICAL factors p1ay an important r d e in the molecular architecture of solids molecules atoms and ions all possess definite size and shape ; homopolar valency bonds are directed in space and it is therefore difficult to conceive hon- a solid particularly one in which homopolar bonds play a major part can liavc a completely random crystal structure.The examina- t ion b\- S-rag diffraction inethocls of substances which show no obvious crJ-stnlliiic cliaracterist ics nud t)rc\-iously were considered amorphous e.g. vit reoils niid highly dispersed solids niiimnl and vegetable fibres soaps ctc. has shoi\-~i thiit t hc great ninjoritjr posscss structural characteristics which are t>-picnl of thc cr\-stnlline state. As n rcsnlt of the X-ray examina- tion of a nuni1)cr of specimens P. De1jJ-e and P. Scherrer coiiclucled that amorphous carbon is J I ~ C I T ~ J - gra1)liite in a state of sub-tlivision so fine that it coultl iievcr lm rcachctl 11)- incc~liniiici~l means. P. P. ven Weimarn and T. Hagiivara mailit iiine(1 froin cslwrimeiit s on the precipitation of barium sulphate that ewii 1~1leii X-ray tiiffr action photographs indicate that a substance is aniorl)lious tlii.3 niii\t not be taken as cz proof that it is so.Glassy or vitreous so1 ids inalio 1111 ail iiiiportant group of the so-called amorphous substances. -1 cornparison of X-ray powder photographs of silica wollastonitc sodium ciiboratc selenium potash and soda felspars boric oxicle complcx silicates glucose aiitl sucrose in the vitreous and the crystalline state led J. T. Randall H. Y . Rooksby and B. S. Cooper to put forward the “ cr\-stallitc ” theory of tlic vitreous state. They showed for example that the main featiires of the diffraction pattern of vitreous silica can be accouiitecl for by the assumption that it consists of exceedingly minute crystals of crj-stoljnlite wit 11 avcragc linear dinicnsions of the order of 15 A .and lattice coiistants sonic G.G% grcater than those of large crysto- balite crystals. pointed out that this suggestion leads t o discrcpaiicies bctwcen the observed and the calculatcd densities and is not in accord with tlic cliwacteristic mechanical and thermal properties of the glasses he suggested that the ultimate condition for the formation of a glass is that the substance can form czii extended three-dimciisional network of atoms lacking periodicity with an energy content comparable with that of the corresponding crystal network. put forward what is perhaps a compromise betwccn tlic above views namely that thc formation of a glass is due to the presence of large or irregular groups of atoms too cumber- some for direct addition to the lattice.RI. L. Huggins Kuan-Han Sun and A. Silverman6 have pointed out that a solid will readily assume the If7. H. Zacliariasen G. Haigg Physikal. Z. 1917 8 291. 2 KriBt. 1930 75 196. I . Anier. Chem. SOC. 1932 64 3841. J . Chem. Physics 1935 3 42. .J. Anier. Cerclni. SOC. 1943 26 393. 59 2 Kolloid-Z. 1926 38 120 ; Kolloidclicnr. Ueih. 1927 23 400. 60 QUARTERLY REVIEWS amorphous state if the regular network structure (crystalline) has practically the same energy content as the irregular in certain glasses besides homo- polar valency bonds hydrogen bonds also probably have an important function in building up the irregular network. In an attempt to show how far our knowledge of the structure of the so-called " amorphous " carbons is compatible with the above views we shall first of all consider the crystal structure of graphite then the crystal chemistry of amorphous carbon and of its transition into graphite and fhally discuss its relationship with other amorphous substances.Throughout we shall use the term " amorphous carbon " in its usual sense viz. for those forms of black carbon other than macro-crystalline graphite which possess no obvious crystalline characteristics. The Graphite Lattice.-The graphite crystal structure which first received wide acceptance was that deduced by J. D. Bernal and by 0. Hassel and H. Mark 8 and later confirmed by C. Mauguin and H. Ott lo it consists of carbon atoms arranged in flat honey- comb-like layers which are stacked parallel to and equidistant from each other in such a way that half the atoms in one lnyerlie normally above half the atoms in the layer beneath while the other half are normally above the centres of the hexagons of the layer beneath altcrnate layers lie atom for atom normally above each other.(a) (61 The C-C spacing in the layers is 1-42 A. whilst adjacent layer-planes are 3.35 A. apart.ll D. S. Laidler and A. Taylor,f2 however pointed out that this structure does not account for the presence of certain faint lines in the X-ray powder photographs of both natural and artificial graphites these lines had previously been reported by G. I. Finch and H. Wilman l3 in electron-diffraction photographs of graphite. To explain the presence and intensity of these additional diffractions H. Lipson and A. R. Stokes l4 suggested that many graphites have a cornyosite structure made up of about 80% of the ordinary (Bernal) structure 6% of a disordered structure (turbo- stratic) and 14% of the structure originally suggested by Debye and Scherrer (Zoc.cit.) in this structure the flat honeycomb planes are stacked parallel to each other as in the Bernal structure but instead of alternate layers being normally above each other the layers follow an abcabc sequence the third layer being symmetrically related to the planes above and below (see Fig. 1 a and b ) . These two ways of stacking hexagon layer-planes are FIG. 1 reminiscent of the manner in which close-packed planes of spheres can be Proc. Roy. Soc. 1924 A 106 749. Bull. Soc. franp. Min. 1926 49 32. 8 Z . Physik 1924 25 317. lo Ann. Physik 1928 85 81. l1 J. B. Nelson and D. P. Riley Proc.Physical SOC. 1945 67 477. l2 Nature 1940 146 130. l 4 Ibid. 1942 A 181 101. l a Proc. Roy. SOC. 1936 A 155 345. RILEY AMORPHOTTS CARBON AND GRAPEUTE 61 13.9 2.45 11.7 ' 2.28 10.8 j 1.99 11.5 2-02 13.2 1 2-02 stacked to give either the close-packed hexagonal or the face-centred cubic structure according to whether the stackings follow the ababab or the abcabc sequence respectively. The phenomenon may have some connection with the metallic character of the inter-planar valency bonding in graphite. The lattice structure of graphite however is not yet fully elucidated J. Gibson l 5 has recently obtained powder photographs of ash-free graphites both natural and artificial which show some 16 faint lines which cannot be indexed on the basis of either of the two structures described above.In addition to the crystallographic there is also physical and chemical evidence in support of the layer-lattice structure of graphite. Its pro- nounced electrical inagnet'ic and mechanical anisotropy is in keeping with this structure so also are the close parallelism between the chemical properties of graphite and those of the triarylrnethyls,ls and the lamellar reactions of graphiteel' Amorphous Carbon.-Certain types of amorphous carbon show crysto- chemical reactions similar to those of macrocrystalline graphite ; e.g. 0. Ruff and 0. Bretschneider 18 state that norit an active carbon freed from oxygen and water vapour by ignition reacts with elementary fluorine at 280" to form a solid monofluoride containing 57.3% of fluorine [theory for (CF)n 61.27y0] which they suggest has fluorine atoms intercalated between hexagon layer-planes of carbon atoms as in the compound made from macrocrystalline graphite.U. Hofmann and A. Frenzel19 have shown that certain apparently amorphous carbons give good yields of graphite oxide when oxidised with potassium chlorate in the presence of concentrated sulphuric and nitric acids (see Table I). Sugar charcoal however when treated similarly was 93.6 I 92.3 83.3 87.6 20.2 TABLE I Composition and Yield of Graphite Oxide Carbon used. Ceylon graphite . . . . . . Acheson graphite. . . . . . Carbonmonoxidecarbon . . . Retort carbon . . . . . . Petroleum soot . . . . . . 1 Percentage composition. Percentage yield of 1 graphite oxide 1 i 1 calculated on I 1 C. i H,O. 1 C 0. carbon present. 53.4 53.3 ~ 52-3 oxidised practically completely to gaseous and soluble colloidal products no graphite oxide being formed.This reaction has been used as a means of lb Nature 1946 158 752. l6 H. L. Riley J . Inst. Fuel 1937 10 149. l7 Detailed summaries of the lamellar reactions and compounds of graphite have been given by W. Ritdorff Wien. Chem. Ztg. 1944 47 172 and H. L. R,iley FveZ 1946 24 1. 1* 2. anorg. Chem. 1930 217 1. l b Ber. 1930 B 63 1248. 62 QUARTERLY REVIEWS differentiating amorphous from graphitic carbon. M. Berthelot in 1870 defined graphite as “toute varikt6 de carbon susceptible de fournir par oxydation un oxyde graphitique.” W. A. Selvig and W. C. Ratliff 2o used the Brodie reaction (oxidation with fuming nitric acid and potassium chlorate) to determine the proportion of graphitic carbon in coals and cokes and from their results concluded that the former contain no graphitic carbon whilst the latter contain only a small amount (< 1%).Hofmann and Frenzel (Zoc. cit.) on the basis of their X-ray results (see below) however argue that these differences in graphite oxide yield are due t o differences in crystallite size and packing rather than to any fundamental difference in the crystal structures of graphitic and amorphous carbons the much smaller crystallites of the chars and cokes are more readily oxidised to soluble products than the larger crystallites in graphitic carbons. It is significant however and we shall return to this point again that the amorphous carbons which give relatively large yields of graphite oxide are those made by deposition from the gas phase.K. Fredenhagen and G. Cadcnbach 21 found that potassium rubidium and czsium react not only with macro-crystalline graphite but also with soot pure arc carbon wood charcoal and active carbon ; whereas sodium reacts with soot but not with graphite. From more detailed investigations on the interaction of potassium and a soot made by the incomplete com- bustion of a tar oil K. Fredenhagen and H. Suck 2 2 concluded that under the same conditions graphite and soot take up practically the same amount of potassium the establishment of equilibrium with the soot required much longer than with the graphite ; the pretrcatrnent of the soot played some part in determining the velocity of the absorption and desorption of the potassium. These results and those of the X-ray study of (C,K) and (CI6K),$ by A.Schlecde and 31. Wellmann 23 suggest that lamellar structures are present in soot. The intercalation of bromine between the layer-plancs of the graphite crystal lattice has been investigated by W. R t i d ~ r f f ~ ~ who found that graphites take up about 80% of their weight of bromine whatever their state of subdivision active carbon takes up about 170% of its weight of bromine probably because of its morc highly developed crystnllite surface. H. H. 1,owvry and S. P. Jloigan 25 prepared graphite in a highly dispersed state by first oxidising it to graphite oxide and then subjecting the product to thermal decomposition and repeating this treatment on the resulting graphite the final product possessed from one-third to one-quarter of the adsorptive power of the best active charcoal.E. Herl K. Andress L. Rein- hardt and W. Herbert 26 convliicled from X-ray and electrical conductivity measurements that there is 110 truly amorphous carbon in active carbons its properties differ from thosc of graphite because of the size and mesomor- phous character of its crystallites ; the elccbrical conductivity of samples 20 J. Physical C‘/ic111. 1!125 29 1103. 21 Z. anorg. Chevt. 1926 158 549. 2 2 Ibid. 1929 178 353. 23 2. physikal. Chem. 1932 B 18 1. 2 4 2. a,Lorg. C’/ic)ti. 1941 245 383. 26 2. p k y s i l i ~ l . Chem. 1932 158 273. 25 J . Physical Chem. 1925 29 1105. RILEY AMORPHOUS CARBON AND GRAPHITE 63 prepared a t high temperatures way approach that of macrocrystalline graphite. The above properties of amorphous carbon are qualitatively in keeping with the DebyeScherrer hypothesis that they consist of exceedingly minute graphite crystallites.There are however amorphous carbons which possess properties which this hypothesis cannot explain. Cokes and chars always contain an appreciable proportion of chemically combined foreign matter ; e.g. carbonisation of carbohydrates up t o 1000" gives chars containing 1-2% of combined oxygen which is not eliminated by prolonged heating in a vacuum at the same or a lower temperature ; only prolonged heating a t much higher temperatures eliminates this oxygen. That it plays an important r6le in the structure of amorphous carbon is indicated by studies of nitrogenous and sulphurous carbons. Amorphous carbon does not take up nitrogen when strongly heated in the gas ; if' how- ever it is heated in gaseous ammonia hydrogen cyanide is formed and the carbonaceous residue may contain up to 3% of nitr~gen.~' E.Terres 28 studied the carbonisation of nitrogenous organic compounds such as glycine asparagine etc. and obtained nitrogenous cokes. W. Hook 29 and H. E. Blayden J. Gibson and H. L. Riley 30 have contrasted the analysis of chars prepared at various temperatures from carbohydrakes on the one hand and from glycine on the other it is significant that the former even those prepared a t lOOO" all contain appreciable quantities of oxygen ; the latter all contain nitrogen but oxygen is present only in the chars prepared a t the lower temperatures. The 1200" glycine char contains 3.64% of nitrogen but no oxygen. Sulphurous chars have been prepared by heating sucrose char with elementary s ~ l p h u r ~ l by carbonising organo-sulphur compounds 32 and by heating semi-coke filter-paper and wood in a current of sulphur dioxide.33 The sulphur in these products cannot be removed by exhaustive extraction with sulphur solvents and in some cases amounts to several per cent.I n addition to oxygen nitrogen or sulphur cokes and chars even when prepared at high temperatures always contain appreciable amounts of hydrogen These foreign atoms all probably play an important part in disordering the atomic arrangement in amorphous carbons. The X-ray powder photographs of amorphous carbons are strikingly different from those of macrocystalline graphites not only are the diffrac- tions much broader but they are also fewer in number most specimens giving only two or three highly diffuse haloes instead of the numerous sharp diffractions in the graphite X-ray photograph.A much deeper insight into the crystallographic character of amorphous carbon has been obtained from 27 W. G. Mixter Amer. J . Sci. 1893 45 363. 28 J . Qaibekucht. 1916 59 619. 2B Carnegie Schol. Mem. Iron and Steel Imt. 1936 25 81. so " The Ultra-Fine Structure of Coals and Cokes," B.C.U.R.A. London 1944 s1 J. P. Wibaut Rec. Trav. chim. 1919 38 159 ; Proc. K . Akad. Wetensch. Amster- 32 R. Ciusa Gazzetta 1922 52 130 ; ssF. Fischer and A. Pranschke Brennstoff-Chem. 1928 9 361. 176-231. dam 1921 24 92. 1925 55 385. 64 QUARTERLY REVIEWS the detailed study of these X-ray powder photographs. The diffracting units are of colloidal dimensions and therefore because of their low resolving power give diffuse diffractions there is a quantitative relation between this diffuseness and the size of the diffracting unit.The first explanation of the disappearance of all the hkl diffractions and most of the high-order diffrac- tions in bhe powder photographs of amorphous carbons was put forward by Berl Andress Reinhardt and Herbert 26 and by H. Arnfelt 34 it is briefly as follows. I n these finely crystalline or so-called amorphous carbons the hexagon layer-planes are stacked parallel t o each other in the individual crystallites with a spacing approximately equal to or somewhat greater than the corresponding spacing in the graphite crystal ; the individual hexagon layer-planes however are otherwise orientated in a completely random manner. J. Biscoe and B. E. Warren 36 have coined the word turbostratic to describe such a crystallographic structure.It can give rise to only 001 and hk0 (more correctly hk) diffractions the hk bands are cross- lattice diffractions formed by the individual hexagon layer-planes acting as two-dimensional diffraction gratings. Such diffractions are not symmetrical but show " tails " towards greater values of 0 . B. E. Warren 36 has discussed in detail this type of diffraction and derived a formula relating the broadening of the cross-lattice band with the linear dimension of the diffracting net-plane ; this is of the same form as the Schemer formula for three-dimensional crystallites but the numerical constant instead of being approximately unity is 1-84 1,843 Linear dimension of crystallite L = ___ p cos 8 where 3 is the wave-length of the X-radiation p the breadth of the difiaction a t half its peak intensity (half-peak width) and 0 the Bragg angle.Warren has also shown that with very small crystallites there is a small displacement of the diffraction maximum of the cross-lattice band towards a larger value of 8 0.163. d(sin0) - - L If a correction is not made for this displacement the calculated C-C spacing within the hexagon layer-planes will be too The assessment of the average.size and shape of the cryatallites present in amorphous carbons from the broadening of the diffraction bands in X-ray powder photographs has been attempted by several workers. Unfortunately differeiit niet hods of calculation have been employed pa&icularly in correct- ing the iticasured hdf-pcilk u-idth for the width of a diffraction from a similar specinien made up of large crystals the results reported by dieerent workers t herefow arc often not strictly comparable.Nevertheless the results reported for similar carbons are of the same order of magnitude. Because 3 4 A r k . Nat. Ast. Fys. 1932 23B No. 2. 37 Cf. for examplo H. E. Blayden H. L. Riley and A. Taylor J . Arner. Chem. S O C . 19.10 62 180 ; .J. H. de Boor Rec. Trav. chim. 1940 59 828 ; U. Hofmann XL'atrrrwis.y. 1944 260. J . Appl. Physics 1941 9 492. 313 Physical Rev. 1942 49 693. RILEY AMORPHOUS CARBON AND GRAPHITE 65 usually only two diffraction bands are of sufficient intensity for accurate measurement it has become customary to express the results of measure- ments of this kind in terms of a hypothetical average cylindrical crystallite the diameter La of which is termed the a dimension and the height L, the c dimension.Some typical results 38 are shown in Table 11. These and the results of other workers indicate that the original Debye-Schemer theory of the crystallographic nature of amorphous carbon requires modifica- tion in a t least two respects 'uiz. the small crystallites in amorphous carbon differ from macro-crystalline graphite in that (1) they are turbostratic and TABLE I1 Crystallite Dimensions (Hofmann and Wilm 38) Carbon. Ceylongraphite . . . . . Carbon monoxide carbon 700" . 9 9 , 550" . 9 9 1 , 420" . Y f Y , 400" . Acetylene soot (explosive decom- position) . . . . . . . Retort carbon . . . . . . Acetylene soot. . . . . . s t , calcined . . . , activated . . . Actiyemrbon AKTIV .. . 9 7 , , , calcined. 9 , , , activated Supranorit . . . . . . . , calcined . . . . , recalcined 30 hrs.. . ) activated . . . . Carboraffi calcined . . . . , aotivated . . . . Gas-mask carbon calcined . . 9 7 , act.iveted . . Sugar carbon . . . . . . 7 , activated . . . C. 99.1 94.9 95.0 91.8 - 99.7 96.9 95.0 99.1 98.5 88.5 96-5 94.4 89.3 97.3 97.7 96.2 93.0 97.4 96.5 98.1 98.0 L Analysis (jo. H. 0.0 0.1 0.1 0.1 - 0.3 0.3 0.8 0.3 0.0 1.7 0.35 0.0 0.6 0.15 0.0 0.6 0.0 0.6 0.0 0.7 0.0 - Ash. 0.3 4 3 2 - 0.0 0.6 0.3 0-3 0-5 1.1 1.1 2.0 0.3 0.4 0.9 1 . 9 3.3 0-5 1.0 0.0 0.0 - Layer- plane spacing C/? A. 3-35 3.4 3.4 3.4 3.45 3.43 3.45 3.55 3.55 3-6 3.5 3.45 3.65 3.5 3.45 3.55 3.7 3.G 3.7 3-55 3.6 3.6 3.6 Crystallite tliriiension A . L O . a 120 150 60 40 45 45 21 26 22 17 25 18 18 26 26 24 13 19 20 17 21 18 &.ca. 200 180 160 70 35 50 40 13 14 16 11 10 8 8 7 7 7 10 7 10 8 9 7 (2) the hexagon-layer spacing is not constant a t 3.35 A. but increases as the number of layer-planes in the crystallite decreases. Even when allowance is made for the uncertainty in the fundamental accuracy 39 of these results the extreme minuteness of these crystallites is striking in the smaller crystallites the individual hexagon layer-planes are not much larger than certain polynuclear aromatic molecules of known structure and the c dimensions indicate that the crystallites can be built up from a.s few a's three or four layer-planes. 38 U. Hofmann and D. Wilm 2. EEektrochem. 1936 42 504. 89 a. H. Cameron and A. L. Patterson Amer. SOC. Testing Mat. Symposium on Radiography and X-ray Diffraction 1937.E 66 QUARTERLY REVIEWS It would appear from the above that the process of charring or carbonisa- tion must involve the progressive transformation of the more or less complex organic matter into the polynuclear aromatic ring systems and their subse- quent stacking and growth. Some such process is indicated by the experi- ments of R. C. Smith and H. C. Howard 40 who charred pure cotton cellulose at temperatures ranging from 190" to 400" and subjected the products t o exhaustive oxidation with alkaline permanganate the magnitude of the yields of aromatic acids from the various chars indicated progressive aromatisation as the charring temperature increased. X-Ray investiga- tions have shown however that this simple picture is far from complete.Fig. 2(a) summarises the results of crystallite-size determinations on chars 40 J . Amer. Chem. SOC. 1937 59 234. RILEY AMORPHOUS CARBON AND URAPHITE 87 prepared from cellulose at various temperature^.^^ Results of an identical character have been reported by U. Hofmann and F. Sinkel 42 who worked with sucrose chars. The striking feature of these results is the constmcy of the average height ( c dimension) of the crystallites (experimentally the half-peak width of the 002 diffraction band) over a wide range of carbonising temperature. Crystallites built up merely from polynuclear aromatic layer-planes held together by relatively weak van der Waals forces would not be expected to behave in such a manner but rather to vary their degree of packing with the speed and temperature of carbonisation.This variation would be reflected in changes of the c dimension. Other carbohydrates lignin and wood i.e. organic compounds containing a relatively high propor- tion of oxygen all give chars containing crystallites with c dimensions between 9 and 10 A. Carbonising conditions can be varied between wide limits with respect to both speed and temperature (up to lZOO") without any consequential change in the half-peak width of the 002 difiaction of the resulting char. Cellulose carbonised in an atmosphere of ammonia and also glycine behave in a similar manner but the constant c dimension is somewhat greater viz. between 12 and 13 A. This constancy of the c dimension is highly suggestive and appears to indicate some kind of cross- linking between the layer planes of the crystallites.This hypothesis appears more probable when we consider the X-ray crystallographic behaviour on carbonisation of certain organic compounds which have a relatively small oxygen content. Fig. 2(b) shows the c-dimen- sion curve obtained with cokes made over a range of temperatures from that part of a bituminous coal-tar pitch which is soluble in carbon tetrachloride Similar c-dimension curves 43 are given by pitches certain bitumens bituminous coals the pyridine-chloroform soluble extracts of bituminous coals and certain pure polynuclear aromatic compounds e.g. dibenzanthrone Caledon jade-green etc. Such c-dimension curves appear to be character- istic of the presence in the parent carbonaceous matter of large thermally stable lamellar aromatic molecules. Aromatic hydrocarbons of this type are volatile and apparently the presence of some oxygen and/or nitrogen in the parent molecule is necessary for a large carbon yield the proportion however must not be too large otherwise charring occurs at a low tempera- ture.These c-dimension curve8 have been interpreted as follows. The ascending part of the curve between room temperature and about 50O0 indicates a unidimensional crystallisation brought about by the increased packing of thermally stable aromatic molecules under the influence of thermal vibration. The maximum at about 500" reflects the conversion of a system consisting of stable independent organic molecules held together by weak dispersion forces (molecular lattice) into a " carbon " crystallite [rigid three- &%ensionally cross-linked ( ?) lattice].Dibenzanthrone after being heated slowly up to 475" and cooled still gives its characteristic X-ray difiaction I1 H. E. Blayden H. L. Riley and A. Taylor J. 1939 67. Ia 2. a w g . Chem. 1940 245 85. u H. E. Blayden J. Gibson and H. L. Riley J . Inst. FueZ 1945 War Time Bulletin 117. 68 QUABTERLP BEVIIOWS pattern made up of several more or 106s sharp diffkactione; after heatixq to 500" the pattern characteristic of the dibenzanthrone crystal lattice haa completely disappeared and been replaced by two diffuse haloes character- istic of amorphous carbon. This change molecular lattice -+ carbon occurs over a relatively narrow range of temperature the height of which appears to be a function of the proportion of oxygen in the parent carbonaceous material. Substances like cellulose form rigid (cross-linked ?) carbon crystallites at temperatures little above 200" whereas certain coking cod bitumens retain their molecular identity up to temperatures as high as 550".The interpretation of the descending portion of the c-dimension curve between 550" and 900" [Fig. 2(b)] is not quite so clear. The phenomenon is however of general occurrence whenever a carbonaceous substance on being heated to some temperature below 550" gives a product which contains crystallites having an average c dimension greater than about 12 A. then a further increase of temperature brings about a decrease in the c dimension (experimentally a broadening of the 002 diffraction). The increasing dis- order over this temperature range may be due to the evolution of oxygen as oxides of carbon and a consequent reduction in the extent of tho cross- linking between the hexagon layer-planes.The subsequent increase in the c dimension above 900" is perhaps facilitated by the remnants of the ordered structure which the system possessed at 550". A further type of c-dimension curve is shown in Fig. 2(c) it is character- ised by an abrupt increase in the average c dimension in the temperature range 800-1000". Crystallite growth of this kind is shown by peats to a less extent by brown coals and lignites and also by " bituminised " cellulose and lignin Le. cellulose or lignin which has been heated in an autoclave to about 300" with an excess of aqueous alkali.43 In the case of peat the crystallite growth of a part of the char between 800" and 1OOO" is so great as to suggest some profound atomic rearrangement perhaps a kind of polymorphic change.Although the true nature of this crystallographic change is still not clear J. Gibson 11. Holohan and H. L. Riley44 have pointed out that besides the diamond and graphite lattices there is a third possible spatial arrangement of carbon atoms which may play an important part in the atomic architecture of amorphous carbon. This arrangement can be pictured as follows. Imagine a graphite layer-plane to be made up of discrete hexagons connected together by valency bonds as in Fig. 3(a) ; break the necessary bonds and rotate the hexagons A,B,C etc. inwards through 60" [Fig. 3(b)] ; it will be found that the valencies so freed can be attached without strain to those of similarly tilted hexagons in adjacent planes above and below This can be seen in Fig.3(c) which shows the spatial relationship of 4 hexagons. The structure 80 formed is a three- dimensional network of the " skeleton lattice '' type full of large holes and long channels and reminiscent of the structure of certain zeolites. The disappearance of the flat layer-planes suggests that the resonance energy of this structure would be less than that of the graphite lattice and its existence therefore less probable. The new structure however contains an orderly 44 J. 1946 458. 69 network of conjugated double linkages running throughout the whole three- dimensional lattice and its resonance energy would therefore be of an appre- ciable magnitude. The existence of such a structure is made more probable by the discovery by I. L. Karle and L.0. Brockway,45 by means of electron- diffraction measurements) of the non-co-planarity of o-tetraphenylene [see Fig. 3(c)]. The suggested structure is a three-dimensional repetition of o-tetraphenylene residues. RILEY AMORPHOUS CARBON AND GRAPHITE \ FIG. 3 A consideration of carbonising processes which would possibly tend to give some structure other than the flat honeycomb network of carbon atoms led Gibson Holohan and Riley to prepare carbon by (a) carbonising hexa- iodobenzene and (b) the interaction of hexachlorobenzene and sodium amalgam ; (a) gave a carbon which showed practically no coherent scattering of X-rays and ( b ) gave a carbon the X-ray powder photograph of which although highly diffuse showed new features which can be explained by the existence of small volumes of the three-dimensional network described above.The constancy of both the a and the c dimensions [Fig. 2 (d)] when a carbon prepared by method ( b ) (temperature of preparation did not exceed 300") was heated to higher temperatures provides additional evidence of a rigid cross-linked lattice structure These X-ray investigations leave little doubt that the idea that black a,morphous carbons are merely graphite in an extremely fine state of sub- d5 J . Amr. C h . SOC. 1944 66 1974, 70 QUARTERLY REVIEWS division is inadequate and needs further qualification. It is evident that the structure of a particular sample may be greatly influenced both by the nature of the parent carbonaceous material and by the method of carbonisa- tion. It is difficult to conceive for example how a system consisting of relatively free and independent lamellze could fail to develop some degree of order under the influence of thermal vibration and yet carbons can be made which give no coherent scattering of X-rays.Before outlining a modified view of the crystallographic nature of amor- phous carbons we must consider the process of graphitisation i.e. the transition amorphous carbon + graphite. Graphitisation.-The high thermal stability of pure amorphous carbon is shown by the results of Biscoe and Warren 35 (Table 111) who studied the TABLE I11 C'rystallite Dimensions of Carbon Black (Biscoe and Warren) Time and temp. of heating. - 2 hrs. at 760" 2 hrs. at 1040" 2 hrs. at 1500" 2 hrs. at 2000" 2 hrs. at 2800" 2 hrs. at 2000" 2 hrs. at 1040" Layer-plane spacing A. 1 Crystallite dimension A.Frorn002. 1 From001. 3.55 3.53 (3.65) 3.48 3.46 3.45 3,47 3.55 3 4 1 3-43 3.47 3-45 3-44 3.44 3.43 3.46 La. 20.0 22.6 28.0 44.2 55.8 65.2 60.5 29.8 Lc * 12.7 14.4 14.9 24.9 32-3 40.0 35.5 14.9 effect of high temperature on the crystallographic character of a commercial carbon black. Similar results were obtained by P. Corriez 46 who studied the effect of high temperature on the crystallite dimensions of sucrose chars. I n the case of pure carbons it is only a t extremely high temperatures that pronounced thermal recrystallisation occurs. U. Hofmann A. Ragoss and F. Sinkel 47 have shown by means of X-ray diffraction electron-microscope and adsorption studies that the small polycrystalline spherical particles of carbon black on heating for 24 hrs. a t 3000" undergo thermal recrystallisa- tion to an extent limited by the size of the original spherical particle i.e.the indiyidual polycrystalline particles probably become single crystals. This thermal stability of the amorphous carbon crystallites is due of course to the high resonance energy of the hexagon network planes of carbon atoms. It is only when these networks are broken down by some chemical mechanism that pronounced crystal growth can occur at temperatures below 2000". This was realised by E. G. Acheson long before the crystal structure of graphite was known in his original patent on the manufacture of graphite 4% Cornpt. rend. 1935 201 1189. 47 Kolloid-Z. 1941 96 231 ; s00 also D. Wilm and U. Hofmann ibid. 1935 70 21 ; A. Ragoss U. Hofmann and R. Holst ibid. 1943 105 118 ; M. von Ardenne and U.Hofmann 2. physikal. Chenz. 1941 B 50 1. RITAEY ttJIORPHOT7S CARBOX ,IN D GR~IP~IITE 71 he states 4 8 " I hnvc also discoverctl that in order to produce pure graphite from carbonaccous materials thcre is nri indirect conversion and that the act of formation of the graphite is morc in thc nature of an act of dissociation of the carbon from its combination with other materials than a conversion of the ordinary carbon into graphite and that as a preliminary step the carbon has to be combined chcniically with some other material. Thus I have found that if the carbonaccous material or carbon used in the process contains a considerable proportion of mincral matter or if it is mixed with a certain proportion of oxide or oxides such as silica clay alumina mangan- ese lime or oxide of iron and subjected to treatment as hereinafter set forth (high temperature) the yield of graphite is enormously increased and the product is most satisfactory." It is significant too that processes which involve the formation of carbon by deposition from the gas phase often give at relatively low temperatures products which are highly crystalline soe for example the crystallite dimensions of the carbons formed on a catalyst by the reaction 2CO = C + CO, quoted in Table 11.The growth mechanism of the carbon crystallites formed in such reactions apparently involves the initial formation of single carbon atoms which subsequently attach themselves to pre-formed graphite nuclei. Other reactions are known which give rise to highly crystalline graphite at temperatures far below that of thermal recrystallisation they all involve the liberation of individual carbon atoms e.g.the graphitisation of austenite,49 and the decomposition of carbides 5O and calcium cyanamide. 51 Carbons deposited from hydrocarbon gases in the iieighbourhood of lOOO" are not so highly crystalline as those deposited from carbon monoxide a t the same temperature the presence of hydrogen appears to inhibit crystal growth presumably by saturating the valencies of the carbon atoms at the surface of the crystallites. It is reasonable t o suppose however that carbons formed by any process which involves the participa- tion of single carbon atoms will have the most stable structure i e . the flat layer-plane graphite-like structure rather than any cross-linked structure of rather hlgher potential energy.JI. Holohan and R. Iley 52 have shown that in spite of' their highly dispersed state carbon blacks and carbon deposited from the gas phase on vitreous silica a t lOOO" have ignition temperatures which approach and may even exceed those of natural and artificial graphites. Tho X-ray-scattering power of carbons of this type is 48U.S.P. 568,323 29.9.1890 quoted by F. -4. J. Fitzqcrald J . SOC. Chem. Id. 1901 20 443 ; see also H. Ditz Chem. Z t y . 1904 28 167 and W. C. Arsem Met. Chem. Eng. 1911 9 536. 48 F. Wust and C. Geiger StuhZ zl. Eisen 1905 25 1134 1196 ; I<. Iokibe Sci. Rep. Tdhoku Inip. Unip. 1920 9 273 ; L . Sorthcott J . Iron Steel Inst. 1923 108 491. 5 0 H. Ishikawa Elect. Rev. Japan 1931 19 419 493 690 726 824 884 ; C. Hahn and A. Strutz Metallurgie 1906 3 72.3 ; Chem.Ztg. 1907 31 Rep. 33 ; J. West- becker Metullurgie 1904 1 137 ; 2. Elektrochenz. 1904 10 837 ; A. Frank Versamm. Ges. deutschen Naturforscher u. Aerzte Sept. 1905 ; Chem. Ztg. 1905 29 1044 ; V. M. Weaver U.S.P. 1,576,883 16.3.26. 61 E. Collett and M. Eckardt E.P. 5713 7.3.1911 ; A. Reme16 and B. Rassow Z . angew. Chem. 1920 33 139; N. Kameyama J. Chem. Ind. Japan 1921 24 1131. 5a Unpublished work of the Northern Coke Research Laboratory. 72 QUARTERLY REVIEWS much greater than that of chars of comparable crystallite size sulphuroue chars (see above) have a still lower scattering power. Although these Wer- ences in scattering power have as yet only been studied semi-quantitatively they are of such a magnitude that there is little doubt of their si@cance.Conclusion.-The above considerations lead to the conclusion that amorphous carbons are built up from two types of structure vix. (I) the turbostratic lamellar graphite-like structure and (11) the disordered three- dimensionally cross-linked structure (Fig. 3). Amorphous carbons which contain a preponderance of structure I are usually much purer than those made up largely of I1 it is possible that the foreign atoms hydrogen together with oxygen nitrogen or sulphur play some part in stabilising the type I1 disordered structure. Such a view of the crystallographic character of amorphous carbon appears to offer a reasonable basis for the explanation of the great diversity of its properties e.g. the highly specific nature and range of its adsorptive properties the great variation in its chemical reactivity the dif€erences in its mechanical properties and scattering power for X-rays etc.Carbons deposited from the gas phase which may contain little or no combined oxygen will be made up largely of type-I structures whilst cham and cokes in general will consist of intimate associations of both types. A large proportion of oxygen in the parent carbonaceous matter appears to favour the formation of structure 11 whereas carbonaceous matter containing little oxygen and a large proportion of benzenoid carbon e.g. pitches bitumens etc. will give carbons with a higher proportion of structure I. Both struc- tures possess high thermal stability structure I1 appears to persist in Borne specimens at temperatures well above 1000". No mention has been made of the importance of the secondary structure i.e.the cohesion and degree of agglomeration of the crystallites in deter-. mining the properties of amorphous carbon. Carbon blacks used in the rubber industry have been investigated in detail ; 53 little is definitely known however of the secondary structure of chars and cokes their accessible ~urface can be measured but the open lattice present in both structures I and I1 makes the interpretation of such measurements ditlicult. Such is the diversity of the crystal structure of solids that no single comprehensive theory of the amorphous state is possible. Metals and salts can be so disordered by mechanical strain or dispersion that they give diffuse X-ray diffractions. Crystals which are made up of long homopolar chains exist in an amorphous condition when the chains become tangled together e.g.vitreous sulphur and selenium rubber borate-glass etc. Systems in which the molecules are large lamellae can assume the amorphous (or meso- morphous) state because of the ditliculty these cumbersome units have in packing into an ordered lattice e.g. pitches bitumens polynuclear aromatic compounds. Finally there is the disordered three-dimensional network present in certain glasses and vitreous solids. Amorphous carbons should probably be placed in a class intermediate between the disordered lamella and the disordered cross-linked lattice. 68 Columbian Carbon Company " The Partiole Size and Shape of Colloidal Csrboa w Revealed by the Electron Microscope," New York 1940.

ISSN:0009-2681    

DOI:10.1039/QR9470100059

出版商:RSC    

年代:1947

数据来源: RSC

摘要:

FORCE CONSTANTS By J. W. LINNETT M.B. D.PHIL. ( U m DEMONSYXUTOR IN CHEMISTRY AND FELLOW OF THE QUEEN’S COLLEGE OXFORD) Introduction THB vibration frequencies of a molecule having a particular shape depend (u) on the mame of the atoms in the molecule and (b) on the restoring foroee that come into play when the molecule is distorted from its equilibrium configuration. Therefore a study of the vibration frequencies supplies information about these restoring forces. Since it may be assumed t h a t the atomic masses are known the problem of aocounting for molecular vibration frequencies is one of finding out about the redoring forces. The most marked advances have been made by thinking of the molecule in terms of its valency bonds and supposing that when any valency bond (or angle) is distorted a restoring force comes into play to restore the bond (or angle) to its equilibrium value.It is found that the restoring force may be assumed to be proportional to the distortion and the proportionality constant relating the force and the distortion is called the force constant. However this simple picture of thinl$ng of the molecule only in terms of the tendency of its valency bonds to return to their origins1 lengths and positions when distorted is not a complete one. The result ie that much work has bean and is being done to try to discover how thh simple valency picture is best modified to account for the obser- vatione. When a suitable force field has been selected and tested for a given molecule we obtain from the measured vibration frequencies the numerical values of the force constants of its valency bonds and angles.The bond force constants have been used to assess the strength of the bonds and it has been found that the force constant of a bond between a given pbir of atome depends on the nature of the bond-or bond order. Force oonstanta have been used to assess bond order. It has also been found that other properties of the bond (such as the equilibrium length) may be related to its force constant and work has been done on these relationships. There has been much less development m our understanding of bending form constants and there is as yet little theory to account for their mrgnitudes. Vibratwn Frequency Calculations To obtain an expression for the frequencies in terms of (a;) and (b) it ia necemary to write expressions for the kinetic and potential energies of the molecule.To do this we have to choose a co-ordinate system to represent the positions of the atoms in the molecule. Various co-ordinate ryefsmr may be aelected but because under ordinary conditions a molecule dwbp mrmrincr cllose to its equilibrium form it is eaaieet to represent the 73 74 QUARTERLY REVIEWS internal configuration in terms of displacement co-ordinates which measure the displacements of the atoms from their equilibrium positions. It is simplest to use Cartesian displacement co-ordinates so that if there are N atoms in the molecule 3N co-ordinates are required xl yl xl x2 . . . xs. To make our formulz less lengthy it is convenient for us to represent these 3iV Cartesian co-ordinates by the symbols ql qz . . . qn (where n = 3N q1 = xl q2 = y1 .. . and qn = xAV). The kinetic energy of the system may then be written as T = Z$miqiz (Qi = dqi/dt) . - (1) where mi is the mass of the atom associated with the qi co-ordinate. We have in (1) an expression t o take account of the effect of (a;) on the internal movement,s of the molecule-the atomic masses are the coefficients in the kinetic energy (K.E.) expression. Now we must consider how ( b ) the restoring forces affect the internal motions. It is easiest to represent this by writing the potential energy rather than the restoring force as a function of the distortions. The potential energy and restoring force are intimately related since the force along any co-ordinate qi is given by - aV/aqi. The most general form of this potential energy (P.E.) function will be where the summation terms cover all combinations of the co-ordinates including squared terms.The zero of any potential-energy scale is arbitrary so we will choose our zero so that the potential energy is zero at the equili- brium configuration (i.e. V = 0 when all the qi’s are zero). Therefore Vo = 0. Also the equilibrium configuration must be a minimum of potential energy so aV/aqi = 0 for all qi’s when all the displacement co-ordinates are zero. Thirdly under ordinary conditions molecules are never distorted far from their equilibrium configurations (i.e. the qi’s are always small). This means that we may to a first approximation presume that the terms involving qiqjqk (product of three small numbers) are always small compared with terms involving qiqj (product of two small numbers).Equation (2) may therefore be simplified to V = Ziaijqiqj . (3) This means that the variation of the potential energy along any one co- ordinate is parabolic in form for if all the co-ordinates except qi are zero we have V = &z&-i.e. the potential energy is proportional t o the square of the displacement which is the equation of a parabola having its minimum a t the equilibrium configuration. Moreover since the force along the ith co-ordinate Fi = - aV/aqi = - uiiqi when all the other qj’s are zero the force is proportional to the displacement. The success that has attended the application of (1) and (3) to the calculation of molecular vibration frequencies implies that (3) is an adequate form for the P.E. function and that the proportionality between restoring force and displacement is in h c t a good approximation.Therefore all the Ai’s are zero. LINloETT FOECB CONSTANTS 76 The fact that reshoring force and displacement are proportional to o m another means that the mation will be simple harmonic. By equathg the product of and acceleration of each atom to the reeforing force act% on that atom as deduced from (3) and doing this for all the atoms in the molecule one can obtain an expression for the vibration fhquencies which ia of the form 1 4 . (4) where Y is the vibration frequency. The values of a, as etc. are dependent on and can be calculated from the atomic masses mi and the constants in the potential-energy function ail. Equation (4) is satisfied by n values of 9. Some of these are zero and correspond to the small translations and rotations of the molecule which have a “ vibration frequency ” of zero since translations and rotations do not involve any change in the potential energy.For a non-linear molecule (4) is satisfied by n - 6 (i.e. 3N - 6) positive non-zero values of 9. The positive values of 2/? give the vibration frequencies of the molecular system. For molecules which possess some symmetry (e.g. methyiene dichloride CH,Cl, which has two planes and a two-fold axis of symmetry) the algebraic equation (4) factorises. The reason for this is that if a molecule has for instance a plane of symmetry the fundamental vibrations must be either symmetric or antisymmetric to the plane and (4) factorises into two-one factor giving the frequencies of the symmetric vibrations and the other factor those of the antisymmetric vibrations.In methylene dichloride there are four symmetry types (i) symmetric to both planes ; (ii) anti- eymmetric to both; (iii) symmetric to the CH plane and antisymmetric to the CCl plane ; (iv) vice versa from (iii). The equation (4) in this case factorises into (i) a quartic (ii) a linear and (iii) and (iv) two quadratic equations in v 2 which give the nine vibration frequencies of the methylene dichloride molecule. This factorisation simplifies the calculation problem (two quadratics may be solved more quickly than a quartic). To calculate the fundamental vibration frequencies of a given molecule of a known configuration we have to know the constants in (1) and (3). The constants in (1) are the atomic masses which are always known. However to know the aij in (3) we have to make some assumptions regarding the way in which V varies with the distortion of the molecule.That is we have to make some assumptions regarding the force field existing within the molecule. That is knowing the atomic masses we use the vibration frequencies to obtain information about the potential-energy function of the molecule. However we find that the available data are inadequate for us to determine all the constants in (3) -i.e. all the aij’s. In methylene dichloride 1 E. T. Whittaker “ Analytical Dynamics,” Cambridge Univ. Press. * G. Henberg “ Infra-red and Raman Spectra of Polyatomic Molecules,” Van v a - a1.v2(n-1) + as .v%n-2) - . . . ~ - 1 .Y* + = O In practice the process is of course reversed. An example will show this. Nostrand Co. E. B. Wilson J . Chem. P h y a k 1939 7 1047.4 M. A. El’yashevich Compt. rend. A&. ScL U.R.S.S. 1940 28 604. rle QUARTERLY REVIEWS the numbr of possible independent constants for the most general tspe of the potentid-energy function (3) is seventeen. The number is redud compared with m uneymmetric pentatomic molecule by the symmetry. But methylene dichloride has only nine fundamental vibration hquenciea. One cannot determine more than nine comtants from nine observed quan- tities. In this case a way out of the difEculty is to examiqe the nine fundamental frequenciee of CHDClz and the nine of CD,Cl,-one could also use^ the chlorine isotopes but the percentage change of mass is smaller and the eflFect on the frequencies is therefore smaller. There are now a suEcient number of observed frequencies (27) to calculate the 17 constants.Also there is an internal check on the reliability of the treatment because there are more obeerved frequencies than adjustable constants. However it is not always possible to determine all the fundamental frequencies of a molecule and it is not always possible to make use of isotopes. Then the other way out of the difficulty is used. Instead of determining more vibration frequencies assumptions are made regarding the molecular force field (potential-energy function) which reduce the number of independent constants in (3). The two most important simplifications are the " simple valency force field " (S.V.F.F.) and the " simple central force field " (S.C.F.F.). SpeciaE Force Fields The assumption of the valency force field is that the only forces in the molecule are those associated with valency bonds.Though we know that in molecules non-bonded atoms do exert forces on one another it is reason- able to suppose that these may be neglected relative to the bond forces. The S.V.F.F. assumes that if a bond alters its length there is a force tending to restore it to its original length which is proportional to its change in length. It likewise supposes that if the angle between two bonds alters there is a force proportional to the change which tends to restore it to its original value. This means that on the basis of the S.V.F.F. the P.E. function (3) may be written as where AR is the change in the length of the bond " a " from the equilibrium value and Aa is the distortion in the angle " m ". The summations are over all bonds and angles.The constants k are called the force constants. Since - dV/dAR is the restoring force along the " a " bond it will be seen that when all the distortions other than dR are zero the restoring force according to (5) is - kaARa-i.e. the restoring force is proportional to the displacement. By geometry the dR and Aa may be represented in terms of the Cartesian displacement co-ordinates qi and so ( 5 ) may be converted into the form of (3) (i.e. V as a function of the $8) which may be used in conjunction with (1) to obtain (4). Let us see how this would help us with methylene dichloride which has fwo C-H bonds two C-C1 bonds one CleCl angle one HCH angle and four ClCH angles. Both C-H bonds must be the same and have the V = ZikaARi + E&kmA& . - ( 5 ) LINNETT FORCE CONSTANTS 77 same force conetant.Therefore the potential-energy function of methylene dichloride according to the S.V.F.F. is obtained in terms of $ve force constants. If all the nine fundamental frequencies have been found then the five constants may be determined. More important still because the nine frequencies are obtained in terms of less than nine constants a relation between the observed frequencies which is independent of the actual values of the force constants may be obtained. This may be used to check the reliability of tho initial assumption (a S.V.F.F.). Let us turn to a simpler example than methylene dichloride namely methane. On a S.V.F.F. this has one bond-stretching constant kcH and one angle constant kHCH. It has four fundamental frequencies which are observed in the Raman and the infra-red spectra to be 2914 1526 3020 and 1306 cm.-1.5-' If the force field in methane were really of the S.V.F.F.type it can be shown that the following relation would hold For methane the left-hand side (L.H.S.) is 0.89 and the R.H.S. 0.94 which is fairly good agreement showing that the S.V.F.F. is quite a good approxi- mation for methane. However for CCl the L.H.S. is 2-44 while the R.H.S. is 2.92 which is not such good agreement the difference having risen to about 20%. Other tetrahalides AX, are rather like carbon tetrachloride. Before discussing the merits of the S.V.F.F. we will discuss the S.C.F.F. This assumes that the only forces in molecules are those between atoms along the line joining them whether the atoms are bonded or not. It supposes that there are no angle forces like those of the S.V.F.F.The forces are supposed proportional to the displacement and the potential energy is therefore proportional to the squares of the displacements where ARAB is the change in the distances between atoms A and B from the equilibrium value and kAB is the force constant. The summation is made over all pairs of a,toms. Thus in methylene dichloride there are five constants For the two C-H bonds for the two C-C1 bonds for the H-H separation for the CI-Cl separation and for the four C1-H separations. If we return to the tetrahedral molecules the S.C.F.F. gives an expression similar to (6) which may be used to :eat the validity of the assumptions v - Z ~ C ~ ~ A R ~ ~ . - (7) For methane the L.H.S. is imaginary because 4v is greater than v ao the S.C.F.F.gives a poor account of methane. For carbon tetrachloride the L.H.S. is 7.96 and the R.H.S. 2.92. For SnBr, the L.H.S. is 2-14 and the R.H.S. 1.57 and this last gives the best agreement of all the tetra- hedral molecules. The general conclusion which is supported by much R. G. Dickinson R. T. Dillon and F. Rasetti Physical Rev. 1929 34 682. J. P. Cooley Astrophys. J. 1926 62 73. 6 G. E. MacWood and H. C. Urey J . Chem. Pihysics 1936 4 402. 78 QUARTERLY REVIEWS other evidence is that the S.V.F.F. is closer to the truth than the S.C.F.F. (see W. G. Penney and G. B. B. M. Sutherland for triatomic molecules and J. B. Howard and E. B. Wilson It is noteworthy that on the whole in the tetrahedral molecules the S.V.F.F. becomes less successful as we pass from the hydrides to the halides (e.g.CH to CCl,) while the S.C.F.F. becomes more successful. This suggests that the forces between the non-bonded halogen atoms are more important than those between the non-bonded hydrogen atoms as would be expected. A further objection to the S.C.F.F. is that it does not account for the bending vibrations of linear molecules since no interatomic distances change in such vibrations. Also it fails to account for the out-of-plane vibrations of planar molecules (e.g. CH,O). However in this last case the S.V.F.F. only accounts for such vibrations if some rather more complicated bending terms are included. The general conclusion of this phase of the study of molecular force fields .has been that we should base any improvements or refinements on fields of the S.V.F.F.type rather than on those of the S.C.F.F. type Before leaving our consideration of the S.V.F.F. it is worth mentioning that some workers have made use of the unmodified S.V.F.F. in the following way. The symmetrical non-linear triatomic molecule AB (H,O type) has on the S.V.F.F. two force constants. It has three fundamental vibration frequencies. The expression for these frequencies depends on the atomic masses which may be taken as known the two force constants and the BAB angle. Because there are three frequencies the two force constants may be eliminated and an expression obtained for the BAB angle in terms of the three frequencies. So the angle may be calculated from the observed vibration frequencies.10 However the reliability of the result depends on the reliability with which the S.V.F.F.may be applied to the particular molecule. Because the S.V.F.F. is known t o be only an approximate representation of the force field for most molecules the author considers that this is a dangerous method to use to determine bond angles in molecules. The S.V.F.F. has been extended by including in the P.E. function “ cross terms ” so that (5) becomes for molecules of the AX type) V = ZikJR; + aZ&kmA& + C&k,bARaARb + Z+krn,tAamAan + -WbmARaAarn . * (9) The P.E. function is still quadratic and will lead to an equation of the form of (3) when the dR’s and A x ’ s are replaced by q displacement co- ordinates. One may suppose that the cross-term ARa.Aarn is included in the P.E. function to account for the fact that when the bond a changes its length the equilibrium value of the bond angle a is affected.For example it may be that when the length of the C-C1 bond in CH,Cl changes the conBguration of the methyl group which has a minimum potential energy also changes. Such eEects can be allowed for by including these cross-terms and much use has been made of them. However by reason * Proc. Roy. Soc. 1936 A 156 654. lo D. M. Simpson Trans. Faraday SOC. 1945 41 209. J . Chern. Physics 1934 2 620. LMNETT FORCE CONSTANTS 79 of the scarcity of our data (number of frequencies available) we cannot usually introduce all the possible cross-terms. Thus in the molecule AB (H,O type) we could have besides the two S.V.F.F. constants two constants associated with the two types of cross-terms one between the two bonds and the other between the bond and the angle.So the most general form of the valency force field requires the use of four constants. If we cannot make use of isotopes we have only three fundamental frequencies and so can determine only three constants. Therefore in most cases it is necessary to limit the number of cross-terms that we introduce into the V.F.F. potential energy function. The difficulty is then to know which cross- terms to include and which to omit. It must be admitted that there is no sure way of doing this. The only course to adopt is to include those cross-terms which seem to account best most reasonably and most easily for the departures from the S.V.F.F. but it does seem sometimes that there is little physical explanation for some cross-terms that are introduced and that the introduction of these cross-terms has resulted in the V.F.F.system becoming rather artificial It must be stressed that it is never satisfactory to use as many unknown constants as there are observed fre- quencies because there is no check then whether the right set of cross-terms has been selected. That is there is no check on the reliability of the P.E. function that has been used and a careful check is very important as we do not yet know with any exactness the true nature or relative importance of the various interatomic forces that may occur in molecules. Ethylene formaldehyde and similar molecules have been treated by P.E. functions of this kind by H. W. Thompson and J. W. Linnett.ll3 l2 Z. I. Slawsky and D. M. Dennison,13 and J. W. Linnett l4 have treated the methyl halides CH3X in this way and a difference between the two methods of approach brings out an important point.15 Both treatments used the P.E.function (10) which it is to be noted had been arrived at quite independently I‘ = zikCHd&H + *kCXd&X + z@HCHdakCH + z*kHcxdaLc + Z#’dRcxdaHCX (10) There are four S.V.F.F. squared terms and one cross-term involving the constant E’. Slawsky and Dennison supposed that kcH and EHcH were the same in the four methyl halides as in methane. So they calculated kcH and En, from the frequencies of methane. Then for each methyl halide they adjusted the remaining three constants to give the six observed fre- quencies as well as possible. The check on the P.E. function was in 1311 cases quite good. Linnett on the other hand concluded from a study of ethane that a P.E. function of the above type was a satisfactory one to use.He then deduced separately for each methyl halide the five con- stants in (10). Because four of the six frequencies are determined almost entirely by four of the constants there is very little check on the P.E. l1 J . 1937 1376. l2 J. 1937 1384. 13 J . Chem. Physics 1939 7 509. l5 D. M. Dennison Rev. Mod. Physics 1940 12 175. l4 Ibid. 1940 8 91. 80 QUARTERLY REVIEWS function for each CH,X molecule separately. The reliability of the method depends on whether the successful test with ethane is enough. However in this case the two approaches do agree quite well. Linnett h d s that kHcH is virtually constant throughout the series as assumed by Slawsky and Dennison. However he finds that the consiant kCH increases from 4.71 to 5-00 x lo5 dynes/cm.on passing from methyl fluoride to the iodide. The study of the methyl halides therefore raises the question To what extent is it justifiable to use a force constant determined in one molecule for an apparently identical bond in another molecule ? One would have said that the C-H bonds in the methyl halides were all very nearly the mme but the above treatment suggests that their force constants do in fact change not inconsiderably from one halide to another (6%). The conclusion would therefore seem to be that we must be very careful in transferring a constant determined in one molecule to another even if the two bonds appear similar though the results for the kHCH constant show that in certain cases such a transfer is satisfactory. F. Stitt l6 treated ethane with a V.F.F.of the ty-pe represented by (9). By using the determined frequencies of both C,H and C,D Stitt was able to introduce a large number of cross-terms (6) into his P.E. function and dealt with this molecule most completely and very satisfactorily. B. L. Crawford and S. R. Brinkley l7 studied together acetylene ethane methyl- acetylene dimethylacetylene hydrogen cyanide methyl cyanide and the methyl halides. As far as possible they transferred both squared and cross-term constants from molecule to molecule. For example they used the force constants they found necessary for the methyl group in ethane for that group in all the other molecules studied-and similarly for the acetylenic and cyanide groupings. That is they went a stage further than Slawsky a1 4 Dennison in transferring cross-term as well as squared-term constants f:*om molecule to molecule.The agreement they obtained between calculated and observed frequencies was most satisfactory. They calculated for all the molecules 84 frequencies with 31 constants which implies that the procedure adopted namely transfer of constants was reliable. Lin- nett l8 treated methyl- and dimethyl-acetylene and methyl cyanide with a much simpler force field based on the one he had successfully used for ethane. He used an entirely new set of constants for methyl- and dimethyl- acetylene and calculated the 25 frequencies of these two molecules satis- factorily using 11 adjustable constants. Linnett obtained a value for the C-C bond force constant in these acetylene derivatives which was dieerent from that obtained by Crawford and Brinkley and showed that his value was more in agreement with the observed bond length the correctness of Douglas Clark's empirical relationship (see later) being assumed.He therefore questioned whether Crawford and Brinkley were justified in transferring constants so liberally from one molecule to another. The transfer of constants in the series of chlorinated derivatives of methane was studied by H. H. Voge and J. E. Rosenthal,19 but the number of frequencies l6 J . Chem. Physics 1939 7 1115. l7 Ibid. 1941 9 69. l8 J . Chem. Physics 1936 4 137. Trans. Faraday SOC. 1941 37 469. LINNET"! FORCE CONSTANTS 81 only exceeded the number of adjustable constants by one so that the test cannot be regarded as very convincing. A quite different but very realistic potential-energy function has been used by H.C. Urey and C. A. Bradley 2o for a number of molecules of the CCl type (tetrahedral). They superimposed on the S.V.F.F. (one bond and one bending constant in this case) a repulsion potential of the form V' = a/- between the non-bonded atoms. Using the four frequencies of the tetrahedral molecules they are able to calculate the two S.V.F.F. constants and a and n. In fact they assumed n to be 7 throughout and found that they could account for the four frequencies each of CCl, SiCl, TiCl, SnCl, CBr, and SnBr with the three adjustable constants (a and the two S.V.F.F. constants for each molecule). They found that the repulsions which had to be assumed between the non-bonded atoms were of the same order as those required for similar repulsions in other circum- stances.Urey and Bradley also considored the ions SO; and C10; and found that by adding an additional Coulombic repulsion force between the oxygen atoms the observed frequencies could be accounted for. Rosenthal has treated the tetrahalides with a more general potential-energy function and has considered her results in the light of Urey and Bradley's assumptions. This section has shown repeatedly that the problem in'obtaining force constants is How is one to choose a sufficiently general force field and yet not have too many unknown constants for a particular molecule ? The ideal method of approach is to obtain as many frequencies of a given structure as possible by using isotopic molecules. For instance I?. Miller and B. L. Crawford 21 have been able to deduce and check the most general form of the P.E.function for the non-planar distortions of the benzene molecule by using the vibration frequencies of benzene and a number of its deuterated derivatives. However this method of approach is not always possible and we are often forced to use simplified forms. For such purposes the modified valency force systems are the most satisfactory. Moreover the chemist always finds such systems more useful than any other even the more general ones because it gives him results in terma of valency bonds with which he is accustomed and equipped to deal. It is the determination of the force constants of individual bonds in the molecule which the chemist particularly requires for with these he may assess similarities and differences between bonds in different molecules.However these bond-force constants must be obtained by using a P.E. function (a force field) whose reliability for the molecule or molecules in question has been satisfactorily tested. G. Glockler and J. Y. Tung 22 have suggested that it is convenient in the case of for example triatomic molecules of the water type which on the general V.F.F. have four force constants (see p. 79) to plot three of the force constants against the fourth. One can then see quickly and easily what are the possible sets of values of the four constants though which four is the correct set cannot be decided with any certainty. Glockler *O Physical Rev. 1931 38 1969. 21 Ibid. 1945 13 388. 21 J . Chem. PhyeiCS 1946 14 282. F 82 QUARTERLY REVIEWS and Tung in addition suggest an arbitrary method of deciding which set of force constants is the correct one.However there does not seem to be any sound basis for the method suggested and the author agrees with D. M. Simpson 23 that “ the information obtained by using it should not be considered entirely reliable.” Nature of Experimental Data The vibration frequencies that are used in force-constant calculations are obtained mostly from infra-red 24 and Raman spectra,25-28 though ultra-violet fluorescence and resonance spectra may be used to a limited extent. In what has been said in previous sections it has been presumed that all the vibration frequencies of the molecule under consideration had or could be determined. Except for the simpler molecules this by no means represents the position. It is often difficult to decide for instance which infra-red bands are fundamentals and which combinations or over- tones.In addition when a molecule has some symmetry it is important to assign each observed fundamental frequency to its proper symmetry class. This is necessary so that we may know which frequencies are to be accounted for by a given factor of the algebraic equation (4). The assignment of frequencies to their proper symmetry classes is made possible by the application of selection rules which tell us which vibrations are forbidden to appear in the infra-red spectrum and which in the Raman spectrum.2 The polarisation of the scattered radiation in the Raman effect may also be used to help us make the correct assignment. The contours or fine structure of the infra-red bands may be used similarly. But even with all these aids it is often impossible to decide what the correct assign- ment is.For water,29 acetylene,30 ethylene,31 carbon dioxide,32 the methyl hallides,2 and many other molecules of a similar complexity all the frequencies are known and correctly assigned. For ethylene oxide cy~Zopropane,3~ the vinyl halide~,~4 p r ~ p y l e n e ~ ~ and even ozone 109 37 no complete and certain assignment has been made. The present position is conveyed by a few examples. 23 J . Chem. Physics 1946 14 294. 24 R. B. Barnes R. C. Gore U. Liddel and V. Z. Williams “ Infra-red Spectro- 25 J. H. Hibben “ The Raman Effect and its Chemical AppliCations,” Reinhold 86 G. B. B. M. Sutherland “ Infra-red and Raman Spectra,” Methuen. 27 G. Glockler Rev. Mod. Physics 1943 15 111. 28 D. M. Dennison ibicl.1931 3 280. ae B. T. Darling and D. M. Dennison Physical Rev. 1940 57 128. 30 G. Herzberg and J. W. T. Spinks 2. Physik 1934 92 87. 81 G. K. T. Corm and G. B. B. M. Sutherland Proc. Roy. Soc. 1939 A 173 172. a2 A. Adel and D. M. Dennison Physical Rev. 1933 43 716. 83 J. W. Linnett J. Chmn. Physics 1938 6 692. 34H. W. Thompson and P. Torkington Trans. Far&y SOC. 1945 41 236. 35 E. B. Wilson and A. J. Wells J. Chem. Physics 1941 9 319. 96 K. S. Pitzer ibid. 1944 12 310. 37 A. Adel and D. M. Dennison ibid. 1946 14 379. scopy,” Reinhold Publishing Gorp. Publishing Corp. LINNETT FORCE CONSTANTS 83 The accuracy of the determination of the observed frequencies is variable. Those obtained from the Raman spectra are accurate to 1 cm.-l if a reason- ably good spectrometer has been used.In the infra-red if the fine structure can be accounted for the centre of the .band may be placed even more accurately. However some bands have very complex contours and it is often difficult t o place the centre of such bands accurately. The same is even more true if bands overlap. Other difficulties such as resonance between vibrational energy levels may also make it difficult to determine the classical fundamental frequencies. The theory of molecular vibrations considers the isolated molecule. However the Raman spectrum is very often obtained by using the liquid and it is uncertain how much the internal vibration frequencies are aEected in this state by the intermolecular forces. In some cases the effect seems to be slight. Thus the totally symmetric vibration of the benzene ring gives a Raman shift of 993 cm.-l if the gas and 994 cm.-l if the liquid is used.On the other hand the observed value of the C-C1 valency vibration in CH3C1 changes from 732 to 709 cm.-l on passing from the gas to the liquid.2 So far it has not been found possible to assess what the change is likely to be for a given vibration but in most cases it is probable that the percentage error from this source is small. The assumption in (3) that the potential energy is a quadratic function of the displacement co-ordinates is an approximation. This means that the motion is not strictly simple harmonic and the observed frequencies are different from the values they would have if the displacements were infinitesimal. The assessment of this anharmonicity correction necessitates the observation of a large number of overtone and combination frequencies and is hardly ever possible.So we are forced t o use the uncorrected frequencies which are observed as if they were the fundamental frequencies for S.H.M In doing this we introduce an error into the derived force constants which may be considerable. For instance for the water molecule the anharmonicity correction averages 4%. z9 This means that the force constants derived by using the positions of the infra-red bands will be 8% different from the true force constants derived for vibrations involving infinitesimal displacements. For nitrous oxide the error in the force constants would be less-about 374.2 It must always be realised that the force constants derived from observed frequencies are subject to this error.When comparing force constants of similar bonds in different molecules it is presumed that the anharmonicity correction is likely to be very much the same in all cases. Thus for the C-H valency vibration in HCN the anharmonicity correction to the frequency is 4% (3312 to 3452) whereas for the 0-H vibration in water it is 43% (3652 to 3825). For all bonds between the same pair of atoms (e.g. all C-H bonds) it is even more likely to be the same. Uses of Force Constants Before considering the uses of force oonstants we will 8e0 what is the The bond-force constants are order of magnitude of these quantities. 84 QUARTERLY REVIEWS iisually measured in dynes/cm. and measure the restoring force that would come into play if the bond were stretched 1 cm. if the law of force assumed persisted t o such large separations.The force constant of the C-C bond in ethane is 4.5 x 105 dynes/cm. and constants as low as 2 x lo5 and as high as about 20 x 105 are known. If the C-C bond in ethane whose equilibrium length is 1-55 A. is stretched by 0.1 A. the restoring force exerted (- kAR) is 4.5 x 10-4 dyne and the potential energy (@AR2) is 2-25 x This energy per molecule corresponds to 3260 cals. per g.-mol. and if the force of N molecules in the same configuration could be added together the force would correspond to 2-73 x loll tons weight per g.-mo1.-nearly a million million tons weight. Angle-bending constants are measured in dpe-cm./radian (Le. ergs/radian). The bending constant of the HcH angle is about 0-5 x 10-11 dyne-cm./radian. So if the angle is distorted by 0.1 radian (5.73") the potential energy increases by 0.25 x erg.This is equivalent to 362 cals. per g.-mol. The force on the hydrogen atom which is a t the end of the C-H bond whoselength is 1.09 A. is k.Aa/R and this is 0.46 x loF4 dyne. We see that the forces necessary to distort a molecule by bending the boiids is of the order of a tenth of the force necessary to alter the length of the bonds by a similar amount. Because of this the vibrations which involve changes in a bond length have a higher frequency than those which involve mainly a bending of that bond. Much more interest has naturally been focused on the bond-stretching constants than on the bending constants and we will consider the former now. The following table records a number of these force constants. To show the variation that may obtain in the results for the force constants of a particular bond one may quote the figures for the C-Cl bond in methyl chloride these are 3-44,38 3 ~ 6 1 ~ ~ 4.42,13 3.35,14 3.64,l7 and 3.37 x lo5 dynes per It has been pointed out that force constants are related to the positions of the atoms in the Periodic Table.Thus the force constants of the bonds B-H C-H N-H O-H and F-H are 3.6 5.0 6.5 7.6 and ca. 9.0 x lo5 (all uncorrected for anharmonicity). Reference to the table will show other series. This relation to the Groups the atoms occupy in the Periodic Table appears in the empirical formulze of Douglas Clark Linnett and Gordy. Reference to the table shows that force constants vary with bond order. For those bonds which involve carbon as one member the force constants of single bonds are about 5 x lo5 or rather less those of double bonds about 10 x lo5 and those of triple bonds about 15 x 105 dynes per cm.or rather more. The variation of force constant with bond order for bonds between a given pair of atoms means that the force constant may be used to assess bond character in the way that bond lengths have been so widely employed. An example of this is provided by carbon dioxide. In carbon monoxide the CO force constant is 18.5 x lo5 dynes per cm. and in form- aldehyde it is about 12.3 x lo5. In carbon dioxide it is 15.5 x lo5. This 38 G. B. B. M. Sutherland and D. M. Dennison Proc. Roy. SOC. 1935 A 148 250. 89 H. D. Noether J . Chem. Physics 1942 10 664. erg per molecule greater than the equilibrium energy. The last is probably the best.LINNETT FORCE CONSTANTS 85 TABLE This table lists the bond-stretching force constants in dynes per cm. x 10-6 ; i.e. a force constant of 5.0 x lo5 is recorded as 5-0. The constants for diatomio molecules are corrected for anharmonicity but those for polyatomio moleaules are not. The probable correction is that 8% should be added to the uncorrected constants of those bonds of which one member is hydrogen and 2 or 3% should be added in other o m s (see text). Bond. H-H Li-Li Na-Ne K-K Rb-Rb cs-cs Li-H Na-H Rb-H K-H CS-H F-H CI-H Br-H I-H N-N N-N 0-0 0-0 c-0 c-0 N- 0 c-s P-N P-P s-s c1-c1 Br-Br 1-1 I-c1 I-Br B-H G H G H C-H G H C-H C-H C-H C-H C-H G H N-H Molecule. H2 Na2 KZ Rb cs LiH NaH K H RbH CsH HF HC1 HBr HI N2 N; 0 0,' co co + NO cs P N P Z Sa c1 Bra I ICl IBr CH CH,F CH ,C1 CH,Br CH,I C2H6 CZH HCN CH,O NH Li B2H6 c2H4 k x dynes/cm.5.76 0.25 0.17 0.10 0.08 0.07 1-03 0.78 0.56 0.51 0.47 9.7 5.2 4.1 3.1 22.8 20.0 11.8 16.5 18.9 19-7 15.9 8.4 10.1 5.5 5.0 3.3 2-5 1-7 2.4 2.1 3.6 5.0 4.7 4.9 4.95 5.0 5.1 5.1 5.9 5.9 4.4 6.5 Bond. N-H N-H N-H O-H O-H B-H 8e-H P-H Si-H c-c c-c c-c c-c c-c c-c c-c c-c C-C c-c C-C c-c c-c C-N C-N N-C C-N C-N C-N C-N c-0 c-0 c-0 c-0 c-s C-F c-Cl C-Br c-I c-CI C-Br GI G N Molecule. CH,*NH NH? NH,*CO*NH H2O CH ,*OH H2S H,Se PH3 SiH C2Hz C2H6 C2H4 C6H6 CH ,C$H CH ,-CC*CH CH,.C:CH CH ,*CE*CH CH,*CN CH,:CO CH,:C:CH C2N2 HCN CH,*CN CH,*NF CH ,*NC C2N2 ClCN BrCN ICN CH,O CH,:CO COZ cs c302 c30Z CH 3F CH,C1 CH ,Br CH,I ClCN BrCN ICN k x lo-' dynes/cm. 6.3 5.4 6.4 7.6 7.6 3.1 3.1 2.7 4.5 9.8 15.6 7.6 5.5 5.5 15-3 15.3 5.3 9.8 9.7 14-9 6.7 18-1 17.5 16-3 5.45 1'7.5 16.7 16.8 16.8 12-3 12.3 15.5 14.2 7.5 5.6 3.4 2.9 2.3 5.3 4-2 2.9 4.0 shows a t once that the C-0 bond in carbon dioxide is intermediate between that in formaldehyde and that in carbon monoxide.12 It may therefore be said fo be more than a double but less than a triple bond.A similar 86 QUARTERLY REVIEWS example is provided by cyanogen chloride,40 ClCN in which the C-Cl bond has a force constant of 5.3 x lo5 as against 3.4 x lo5 for the C-C1 bond in methyl chloride. This shows that the C-Cl bond in cyanogen chloride is stronger than a single bond an observation which can be accounted for by supposing that the real electronic structure is intermediate between and Several other similar examples of the use of force constants in this way may be quoted (c30,,41 C2N2,'* C6H6,42 methyl- and dimethyl-acetylenes 18) but in fact the method has not been used as much as it might have been because of the difficulty of finding a satisfactory tested P.E.function for all but the simplest molecules. It is to be hoped that as our knowledge of molecular-force fields increases this method of assessing bonds will be used more and more. The force constant of the C-H bond can be determined with fair accuracy in a large number of molecules. This is possible because the C-H bond vibration frequency is so much greater than all the other vibration frequencies of many organic molecules that it can be treated ~eparately.~ Linnett 43 calculated the force constants of a number of C-H bonds and found that they varied from 4.4 x 105 in aldehydes to 5-9 x lo5 dynes per cm.in acetylene. It is interesting and surprising that the force constant of this bond which is in all cases written as C-H varies over a range of ca. 30%. It appears that three factors affect the force constant ( a ) The nature of the bond orbital -whether sp sp2 or sp3 hybridised ; ( b ) the electrostatic state of the bond this being affected by neighbouring groups 44 (cf. Gordy) ; ( c ) resonance with various ionic structures. Other M-H bonds were studied with similar results but the treatment of the N-H and O-H bonds has been improved by R. E. Richards.45 Another important feature of force constants is the relation they bear to bond lengths. R. M. Badger 46 first pointed out from a study of diatomic molecules that as the bond between a given pair of atoms becomes stronger (i.e.the force constant bigger) its equilibrium length becomes shorter and he proposed the empirical relationship k = A / ( R - B)3 * ( 1 1 ) where R is the equilibrium bond length and A and B are constants. He found that their values depended on the positions in the Periodic Table of the atoms forming the diatomic molecule and gave a table of suitable values for A and A second relation was proposed by C. H. Douglas CIark,4s who suggested that \ / /cl=c=N \ I 1 - C l - C ~ - - One special case may be mentioned. kR = C - (12) ~~ 40 H. W. Thompson and J. W. Linnett J . 1937 1399. 4 2 R. C. Lord and n. H. Andrews J . Physiccll Chenz. 1938 41 149. 4 3 J. W. Linnett Traw. Furaday SOC. 1045 41 223. 4 4 H. C. Longuet-Higgins ibid. p. 233. 4fi J . Chem. Physics 1934 2 128.48 Phil. Mug. 1934 18 459; 1936 22 1137. I d e m ibid. p. 1291. 4 5 Trans. Famdny SOC. in the press. 47 Ibid. 1935 3 510. LINNETT FORCE CONSTANTS 87 The value of the constant C depends also on the position in the Periodic Table of the atoms forming the molecule and Douglas Clark and K. R. Webb 49 have given formulae from which C may be determined. Othef relations have been suggested by H. S. Allen and A. K. Longair,60 and by M. L. Huggim61 Linnett 52 suggested a potential-energy function for diatomic molecules which with various empirical assumptions led to a relation between k and Re.53 The author considers that the most useful of these relationships is that of Douglas Clark because it combines sufficient accuracy with great simplicity. For instance it gives better results than Badger’s equation and it has the great advantage that there is only one constant (C) instead of two ( A and B).G. B. B. M. Sutherland 54 has given an explanation of the Douglas Clark relationship and this has been discussed by Linnett.62 These relationships were developed for diatomic molecules but have been tested for individual links in polyatomic molecules. Badger found that his relation was quite successful with C-H C-0 C-S and S-0 links in triatomic molecules and H. W. Thompson and J. W. Linnett 55 found that both it and Douglas Clark’s relation gave good results for C-H G O and C-C links in a variety of molecules. J. J. Fox and A. E. Martin 66 pointed out that for C-C links in polyatomic molecules kRi was more nearly constant than kR:. It seems to the author that a modified Douglaa Clark relation IcR; = C may be useful n and C being fixed by reference to perhaps three molecules for which k and Re are both known.Then in other molecules k may be used to calculate Re for bonds between the same pair of atoms. There is no doubt that this possibility of deducing Re from k is very valuable as it can be used to check results obtained by more direct methods (spectroscopic and electron diffraction) and there are cases (e.g. some G-H bonds) where the direct methods cannot be employed. Badger considered also the application of his equation to molecules of the AX and the BX type in which repulsion between the non-bonded atom is ~onsiderable.~’ He found that he could account for his results if he supposed that the repulsions between the X atoms caused a considerable stretching of the AX (or BX) bond.Thus for CCl he concluded from the force constants obtained by J. E. Rosenthal 57 that the C-cl bond was stretched by 0.19 A. from the value it would have had if there had been no repulsion between the X atoms. This is a surprising result because it implies that the GC1 bond in methyl chloride in which such stretching must be very small and that in CCl have about the same length only because of a balancing of various quite large factors. 49 Trans. Faraday SOC. 1941 37 293. 61 J . Chem. Physics 1935 3 473; 1936 4 308. 5 a Trans. Faraduy Soc. 1940 36 1123. 63 J. W-. Linnett ibid. 1942 38 1. 64 Proc. I d . Acad. Sci. 1938 8 341. 6 6 J.,. 1937 1396. 66 J. 1939 884. ~57 Physical Rev. 1934 46 1934. Phil. Mag. 1935 19 1032. 88 QUARTERLY REVIEWS W.Gordy 68 has recently introduced a relation between the force con- stant bond order ( N ) bond length (Re) and the electronegativities (zA and zB) of the bonded atoms. The relation is of the form k = aN(zA.zB/Ri)f + b . * (13) where a and b are constants (for most pairs of atoms 1.67 and 0.30 respec- tively). Gordy shows that (13) may be applied widely to bonds in diatomic and polyrttomic molecules when the bond is not distorted by forces be- tween non-bonded atoms. He gives a table of electronegativities and uses measured values of Re to predict k when N is known. In other cases he predicts Re from k when N is known and in a few cases he determines the bond order when k and Re are both known (e.g. BrCN). He also con- siders the effect of a charge being located on the atoms and shows that a positive charge by affecting the electronegativity terms will cause an increase in the force constant if N remains the same.There has been no detailed work on the relation between the heat of dissociation of a bond and its force constant. Fox and Martin 56 for C-C bonds and G. Glockler and G. Matlack 59 for 0-0 bonds have shown that the graphs of both k and D against Re are smooth curves. This implies a smooth relation between k and D. For C-C bonds Fox and Martin pointed out that kE/D was a constant quantity. Linnett 53 used the P.E. function he had suggested to calculate the heats of dissociation of some diatomic molecules from the observed force constants but the data available were not accurate enough to provide a satisfactory test. It may also be noted that the C-H bond in methane has both a higher force constant and a higher diesociation energy than the C-H bond in ethane.Force constants have been used to calculate unobserved frequencies. It may happen that for a given molecule one or two vibration frequencies cannot be determined. In such circumstances it may be possible to test a P.E. function with the frequencies that have been observed use them to calculate the force constants and with these to calculate the unobserved frequency or frequencies. Before Stitt’s l6 complete treatment of ethane this approach was employed for that molecule but the frequency predicted by this method was later found to be quite wrong. This use of force constants will become increasingly valuable when we know more about molecular force fields for very often it is impossible to determine all the vibration frequencies experimentally.Yet for certain purposes such as the calculation of thermodynamic quantities it is necessary to know all the molecular frequencies. The possibility of calculating the frequencies of one molecule by using force constants obtained from another similar one is very liable to error because of the uncertainty of transferring force constants (see p. 80). Eventually it may be hoped that we shall be able to predict the values of force constants in molecules thaf have not been examined experimentally and so calculate their vibration frequencies. It was pointed out on p. 79 that cross-terms probably account for J . Chem. Physics 1946 14 305. 59 Ibid. p. 503. LINNETT FORCE CONSTANTS 89 the effect of a change of one part of the molecule on the configuration of another part.It has been suggested that the large cross-term constant for the interaction between the two bonds in CO is to be explained by the resonance in this molecule. When one C-0 bond lengthens it favours one of the single-triple bond structures (0-0) with the result that a shorten- ing of the other G O bond more readily accompanies a lengthening of the first C-0 bond. Bending Force Constants Up to the present less consideration has been given to bending-force constants than to valency-stretching force constants. The reason for this is that it has been impossible to account for the changes that occur in these constants. For instance although it is found that the HcH angle has a bending constant which is constant throughout the methyl halides (0.5 x dyne-cm./radian),l' yet the HeH angle in ethylene has a much smaller bending constant (0.36 x 10-11).l1 Admittedly the electron distribution in the two cases is Werent because the bond hybridisation is different but the change is nevertheless surprisingly big. Again the C - d H bending constant is 0-66 x 10-11,14 the C=C/ 0.6 x 10-11,11 but the CEC-H is 0.24 x 10-l1.l7 It is surprising that the change from the double to the triple bond is so much greater than the change from the single to the double. Also it might have been expected that the change would have been in the other direction and that the angle involving the triple bond would have been the most rigid. This change in the rigidity of the above angles may possibly be explained by the fact that in ethane there are four atoms round the carbon in ethylene three and in acetylene only two.Regularities can nevertheless be observed in bending constants. Thus the X - d H constants in methyl fluoride chloride bromide and iodide are 0.9,0-7,0.62 and 0.55 x 10-11. Where both bonds are multiple as in carbon dioxide the bending constant is 0.75 x It is interesting that this is smaller than that of F-dH which involves only single bonds. On comparing similar bond arrangements round the same atom it is found that the bending constant of C-C-H is 0.25 and of C-=C-C 0-35 x dyne-cm./radian. l7 Some of the above irregularities in bending constants certainly arise because some are obtained by using unsatisfactory force fields. The results obtained for bending constants are because they are small relative to the stretching constants much more dependent on the type of cross-terms that are introduced and because of this it is probable that mmy of the values obtained are unreliable.When a given type of force field is used throughout a series of molecules (e.g. the methyl halides) it is found that similar angles (e.g. HeH angles) have a single value for their bending constants throughout the series and also that a graded series of angles (e.g. X d H ) have a steadily varying value of the bending constant. This suggests that we H 90 QUARTERLY REVIEWS may hope for advances in the interpretation of the bending constants. This may be easier when more is known of the forces between non-bonded atoms. Summary In this report as much space has been spent on an examination of the force fields that have been employed as has been spent on the uses of the force constants obtained.Unfortunately this represents the position. More time has been spent since 1930 in examining the merits of various force fields than in using the constants obtained for the elucidation of chemical problems. Moreover the position today is still that for many polyatomic molecules we are by no means sure what sort of force field is best. It is in this direction that development must come first and only after this can we hope to extend the chemical applications of force con- stants. This development will bc aided by the investigation of the spectra of isotopic molecules (13C 15N etc. being used as well as deuterium) since these will provide for any given molecular system more measured vibration frequencies. The extension of our Itnowledge of force fields in molecules may then increase our understanding not only of the valency bond but also of the other forces that are exerted between the component atoms of a molecule. In conclusion I wish to thank Professor C. N. Hinshelwood and Mr. R. P. Bell for the helpful advice they have given me.

ISSN:0009-2681    

DOI:10.1039/QR9470100073

出版商:RSC    

年代:1947

数据来源: RSC

摘要:

OCEANIC SALT DEPOSITS By F. C . F’HILLIPS M.A. Ph.D. (GEORGE HERDMAN PROFESSOR OF GEOLOUY UNIVERSITY OF LIVERPOOL) THE rocks classified by the geologist as sedimentary are formed by the deposition from transporting agents of the products of disintegration and chemical decomposition of previously-existing rock masses. Material carried in suspension by rivers is laid down to form clastic sediments- sandstones clays etc.-but many of the products of chemical weathering pass into solution and are carried down int,o lakes or eventually to the sea. Most of these dissolved product>s are only recovered when laid down as chemical deposit’s as a consequence of tlic evaporation of the aqueous solvent. Of such sediments produced by evaporation the oceanic salt deposits or mariue evaporites are geologically and chemically by far the most important.Though the exact circunistnnces have been and to some extent still are a matter for disputation it has long been realiscd by geologists that at several periods during the course of past geological ages large bodies of sea-water have been evaporated sufficiently far to cause at least partial crystallisation of the dissolved salts. The waters of the present-day oceans the hydrosphere in volume about 3 x 108 cubic miles have been found to display a remarkable degree of constancy in the relative proportions of the more important ions present though the absolute concentration may vary to some extent. Early analytical work by Forchhammer and others was superseded by the deter- minations made by W. Dittmar on 77 samples of ocean waters collected during the “ Challenger ” expedition ; it is a tribute to Dittmar’s work that later investigations have effected few important modifications of the values which he gave.The complexity of the solution and the difficulty of separation of certain related substances present awkward analytical problems. Even the total salt content cannot be accurately determined by direct evaporation for it is difficult to drive off all traces of moisture without loss of other constituents. For these reasons oceanographers use two defined quantities in the discussion of the chemistry of sea-water. The “ chlorinity ” is determined by precipitation of the halogens with a silver salt and is essentially the chlorine equivalent it being assumed that the bromine and iodine have been replaced by chlorine.(For more precise discussion chlorinity has been redefined 2 in terms of the weight of silver precipitated and hence is independent of any changes in accepted values of atomic weights.) The “ salinity ” is also a defined quantity slightly less than the total weight of dissolved solids and can be calculated from 1 Reports of “ Challenger ” Expedition Physics and Chemistry 1884 1 1-251. a J. Jacobsen and M. Knudsen Assoc. d’oceanog. phys. Union geodes. geophya. intern. 1940 Publ. Sci. No. 7. 91 92 QUARTERLY REVIEWS the chlorinity or determined from a measurement of the density. Both chlorinity and salinity are customarily expressed as g. per kg. of sea-water using the symbol o/oo (per mille). of the proportions of the major constituents of sea-water for a chlorinity of 190/,, is given below A recent tabulation Na+ Mg++ Ca+ + Kf Sr+ + I Ion.j c1 = 19~oo~/o,. 1 %. 11 Ion. j c1 = 19~ooo/oo. I %. 10.556 1.272 0.400 0.380 0.013 I I 30.6 1 3.69 1-16 1.10 0.04 c1- HCO,- Br - F- so,= ' 0.026 Total 34.482 I /1 The waters of the open oceans thus contain about 34 parts per thousand of dissolved salts of which about 55% by weight is chlorine and 31% sodium. The salinity varies only slightly except near land masses where it may be greatly reduced by the influx of river-water. We shall have occasion later to discuss further whether the salinity of sea-water has remained sensibly constant for long geological periods ; it is interesting to note here that the contributions received a t the present time from river-waters are of quite a different composition from the salt content of the oceans Ca++ constitut- ing about 20% and CO,= about 35% but much of this material is soon abstracted by animals or plants and involved in a biological cycle.One of the earliest attempts to study the order of separation of salts on the evaporation of sea-water was made by J. U~iglio,~ who nearly one hundred years ago carried out a long series of experiments by evaporating samples of water taken from the Mediterranean. He was able to demon- strate a general succession calcium carbonate and sulphate being followed by sodium chloride and the sulphates and chlorides of magnesium and potassium. The problem however was much too complex for such a direct analytical approach to be successful and the scientific study of the crystallisation of oceanic salts really began with the classic studies of J.H. van't Hoff and his associates. These studies were directed towards obtain- ing a closer understanding of the conditions of formation of the salt deposits of the Stassfurt region a t that time the most important potash-producing area in the world. Instead of experimenting as did Usiglio with actual sea-water van't Hoe undertook a systematic study of the solubility relationships of all the salts in question. By working initially under atmospheric pressure and at a single defined temperature (25") solubilities were determined first in pure water and then in the presence of other salts full precautions always being taken to maintain saturation by the presence H. U. Sverdrup M. W. Johnson and R. H. Fleming " The Oceans," New York Ann. C'him. Phys. 1849 27 92-107 172-191.1942 p. 166. PHILLIPS OCEANIU SALT DEPOSITS 93 of the solids as “ Bodensalze,” and to ensure the attainment of equilibrium -the slowness of attainment of equilibrium in some instances was one of the reasons why the early experiments of Usiglio couId never have been completely successful. A second series of investigations was carried out at a temperature of 83” rtnd other particular temperatures corresponding to the appearance or disappearance of individual compounds were deter- mined. “ It is hard to say how far such researches could have been carried by chemists unacquainted with the phase rule. They would have had no guide to the apparently chaotic results obtained on evaporating mixed salt solutions. The meager results obtained before the problem was taken up by van’t HOE show that little progress could have been made by the older methods of experimentation in which the results rtnd the guiding principles of modern physical chemistry had no place.” A long series of separate contributions were conveniently summarised in extended accounts dealing first with the chlorides and sulphates of sodium potassium and magnesium and secondly with the calcium salts and boron compounds.Further work by later investigators notably J. d’Ans,6 has established in detail the conditions of equilibrium in the systems involving the chlorides and sulphates of sodium potassium magnesium and calcium over a tempera- ture range of 0-120”. The necessary preliminary work on the simpler systems whilst having no direct application t o the complex problem of the natural deposits has proved of immense importance in providing the data on which to base the subsequent extraction and refinement of the vayious salts.The following table lists the more important salts which may occur in marine evaporites Chlorides and sulphates of sodiuni potassium and magnesium Halite (rock-salt) . . NaCl Sylvine . . KC1 Bischofite . . MgC12,6H20 Carnallite . . KCl,MgCI,,6H2O Thenardite . . Na2S0 Mirabilite (Glaubersalz) . . Na2S04 10H20 Glaserite (aphthitalite) . . (K,Na),SO Kieserite . . MgSO,,H,O Hexahydrite . . ruZgS0,,6Hz0 Reichardtite (epsomite Bittersalz) . MgS0,,7H,O Vanthoffite . . 3Na,SO,,MgSO Astrakanite (bloedito) . . NazS0,,MgS04,4H,0 Loewite . . 2Na2S0 , 2MgSO ,,6H,O Langbeinite . . K2S0,,2MgS0 Leonite . . K,S0,,MgS0,,4H20 Schoenite (picromerite) .. K2S0,,MgS0,,6H,0 Kainite . . KC1,MgS04,3H,0 Qe W. A. Gale Ind. Eng. Chem. 1938 80 867. 5 J. H. vm’t Hoff “ Die Bildung ozeanischer Salzablagerungen,” I and 11 Bruns- wick 1905 1909 ; “ Untersuchungen uber die Bildungsverhiiltnisse ozeanhher Salz- ablagerungen,” Leipzig 19 12. 6 “ Die Losungt3gleichgewichte der Systeme der Salze ozeanischer Salzablagerungen,” Berlin 1933. 94 QUARTERLY REVIEWS Calcium salts Anhydrite . . CaS04 Gypsum. - . CaS0,,2H,O Glauberite . . CaSO,,Na,SO Syngenite - . CaSO4,K2SO4,H,O Polyhalite . . 2CaS0,,MgS04,K,S0,,2H,0 Tachhydrite . . CaC1,,2MgCl2,12H2O Boron compounds Boracite . . 5MgO7MgC1,,7B,O Pinnoite . . Mg0,B20,,3Hz0 Ascharite . . 2MgO,B,O,,H,O Sulphoborite . . 2MgS04,4HMgB0 7H,O Lueneburgite . 3MgO,B,O ,,P206 8H20 Iron salts Rhmeitc . . NaC1,3KCl,FeC12 Douglasite .. 2KC1,FeC12,2H,0 Ccrtairi of these names are applied by systematic mineralogists only to massive material of the kind usually found in marine evaporites other names (some of which are given above as alternatives) being applied to well-developed crystals but we shall find it convenient to continue to use those names which are well established in the literature of oceanic salt deposits. It is a fortunate circumstance that amongst so many possible solid phases there is only a very limited degree of isomorphous replace- ment ; glaserite shows a limited substitution of potassium by sodium and very small amounts of sodium may be taken up by leonite and of potassium by astrakanite. Upon evaporation of sea-water the first salt to separate is calcium carbonate. The surface layers of ocean waters may in fact be considerably supersaturated with calcium carbonate,’ and in some regions of shallow water .such as the Great Bahama Bank where the shallowness reduces circulation precipitation of carbonate is already taking place a t the present time.As evaporation proceeds it is probable that dolomite may be directly precipitated though it must be admitted that the conditions of formation of dolomite are not yet fully understood and it is often not possible to dcterniine how far a particular dolomite may have been produced by t’he later alteration of a limestone by saline waters rich in magnesium. The actual quantities involved are small-from a 1000-m. depth of water a limestone only a few cm. thick would be formed if there were no considerablc degree of supersaturation.The conditions determining the separation of gypsum or of anhydrite were studied by van’t Hoff but it has recently been pointed out 8 that his determinations were based on the incorrect assumption that gypsum dissociates directly to form anhydrite. Actually the hemihydrate CaSO,,+H,O is always formed first and gypsum and anhydrite only coexist at a four-phase equilibrium point. In saturated aqueous solutions of gypsum and anhydrite this point lies at * E. Posnjak Amer. J . Sci. 1938 35 A 247-272. As evaporation proceeds calcium sulphate appears. H. Wattenberg Portschr. Min. 1936 20 192. PHILLIPS 00EANIO SALT DEPOSITS 95 a temperature of 42'. Investigating the efFect on this transition temperature of ealt solutions of approximately the composition of sea-water E.Posnjak 9 showed that a t a temperature of 30" gypsum will begin to separate when the salinity has been increaaed by evaporation to 3.35 times the normal value and that nearly one-half of the total amount of calcium sulphate present will be depoaited as gypsum before the concentration is reached at which anhydrite becomes stable. As evaporation is continued further halite eventually separates when the water content has been reduced to less than one-tenth of the original and anhydrite and halite continue to separate together until the field of stability of polyhalite is reached. The degree of evaporation neceasary to precipitate gypsum or anhydrite and halite has been attained at various periods in past geological ages and important deposits of rock-salt are worked in many parts of the world.In this country the aalt industry of Cheshire and Durham is based on occurrences in the Permian and Triassic rocks and the associated gypsum and anhydrite are also extensively worked but practically no potassium salts have been found though polyhalite has recently been recorded. lo Only when the evaporating body of sea-water has been reduced to 1.57% of the original volume do the salts of magnesium and potassium begin to appear. Such a high degree of evaporation has only infrequently been reached and the Permian deposits of Germany still remain the best-known example of natural potash resources. The earliest workings were around Stassfurt after which the deposits are still popularly known but subsequent exploration involving the sinking of over two hundred shafts and some thousands of boreholes revealed the wide extent of the deposits both northwards towards Hanover and south of the Harz on both flanks of the Thuringer Wald.The Werra district between Eisenach and Fulda has recently become the most important producer. National requirements for potash during two world wars induced extensive exploration in other countries. In the United States of America isolated records of potassium salts had been known from the Permian Salt Basin of the Texas-New Mexico area since 1912 and after an intensive programme of exploration the &st shaft was sunk in 1929 and production begun in 1931." In the U.S.S.R. deposits closely resembling those of Germany were discovered in 1916 in the region around Solikamsk in the province of Perm and less important deposits are known also in Alsace Poland and Spain.The detailed study of the further course of crystallisation in the complete system (Na,K,Mg,Ca),(Cl,SO,) presents considerable practical difficulties inherent in the diagrammatic representation of a system which with water has six independent components. The concentration of calcium at this stage is so low that it may conveniently be leff out of account for the present but we still have to deal with a quinary system. Since further crystallisation occurs always in the presence of excess of sodium chloride which forms no binary or ternary compounds with any of the other salts a diagrammatic @ Ibid. 1940 238 659-668. lo C. E. Tilley Min. Mag. 1943 H lvii. 11 J. W. Turrentine " Potaeh in North America," Reinhold 1943 p. 27. 96 QUARTERLY REVIEWS representation can be effected in terms of MgCl, Na,SO, and KCI and van't Hoff constructed various types of isothermal model.He also made extensive use in his discussions of plane " paragenetic diagrams " which showed the combinations of salts in equilibrium with halite and saturated solutions a t various temperatures but possessed no quantitative significance. In devising further simplifications E. Janecke has been particularly active. Van't Hoff expressed his results in terms of the solubility of the various salts in a constant amount of water but Janecke proposed an alter- native method with a variable water content expressing the amount required to produce a saturated'solution. The water content is thus treated separ- ately from the composition of the salt mixtures so that any inaccuracies in determination of the water content are not carried over t o statements about the salt contents.The device also leads t o a simple graphical pro- cedure. Expressing the composition in the form mH,O,sMg,yK,,( 100 - z - y)SO,,sNa, since only neutral salts are in question all possible compositions can be plotted on a triangular diagram in terms of K, Mg and SO, by using the values of x and y and an isothermal representation in a plane diagram is thus achieved. I n such a diagram the paths of crystallisation can be traced as in a simple ternary system the usual relationships of congruent and incongruent fields still hold and quantitative information can be derived by application of the centre-of-gravity principle. For a given temperature a further quantity such as the water content m or the associated amount of sodium chloride can be plotted as ordinate whilst if the various isothermal diagrams are set above each other the equilibrium conditions throughout a range of temperatu-es can be conveniently displayed in a triangular prism.Certain features o?. the courses of crystallisation can be studied also in projections on a face of the prism. Fig. 1 illustrates such a prism constructed by Janecke l 3 in the form of a wire model. Within the prism between 0" and 120° there are 33 invariant points at which four salts (in addition to halite) co-exist in equilibrium with solution. *4mongst these three important types may be distinguished. At the first of the type S + S + S + S4 + Solution a mixture of three salts gives way on rise of temperature to the appearance of a new solid phase.As examples may be quoted the reactions Mirabilite + Reicliardtittt f - Schoenite + Astrakanite + Solution marking the appearance of astrakanite a t 4-5" and Mirabilite + Astrakanite + Glaserite + Thenardite + Solution corresponding to the formation of thenardite above 13.5" l2 See '' Handbuch der Mineralchemie," C. Doelter and H. Leitmeier 1929 IV 2 l 3 2. Elektrochent. 1934 40 741. 86-91 1253-1258 (a convenient summary of numerous earlier papers). PHILLTPS OCEANIC SALT DEPOSITS FIG. 1 Solubility of oceanic salts between 0" and 120' (Junecke). Many of the invariant points are of the type S + S + S + S4 + Solution Carnallite + Kainite t Kieserite f Sylvine + Solution indicating a change of paragenesis. is one example of this kind taking place at a temperature of 72".The important reaction 97 98 QUARTERLY REMEWS A third type 4 + S + S + S + Solution marks the disappearance of a particular solid phase a t higher temperatures as with schoenite at 26" and kainite a t 83". Fig. 2 shows the temperature ranges of formation between 0" and lOO" of the various salts in the presence of sodium chloride. It will be seen that whilst certain salts can crystallise throughout this range others have a much more limited field the highly hydrated salts giving place t o less hydrated or t o anhydrous compounds a t higher tem- peratures. Van't Hoff distinguished three stages ; below 37" schoenite reichardtite and hexahydrite disappear between 37" and 55" loewite langbeinite and vanthoffite appear whilst above 55" astrakanite leonite and kainite in succession cease t o form.It should perhaps be emphasised that Fig 2 refers only t o temperatures of formation of the various salts FIG. 2 Temperature ranges of formation of oceanic salts. within the system under consideration ; the salts themselves are stable under suitable conditions over much wider ranges and langbeinite for example will remain unchanged indefinitely in a dry atmosphere at room temperatures. If we turn next to the course of crystallisation of a solution of the composition of normal sea-water the possibilities are much restricted. For isothermal evaporation at 25" the point marking this composition falls just within the field of aatrakanite (Fig. 3) which is therefore the first salt to crystallise. The path of crystallisation following a straight line directly away from the point marking the composition of this salt soon reaches the boundary of the reichardtite field.Reichardtite begins to separate and the further course depends upon whether the previously- PRILLIPS OCEANIC SALT DEPOSITS 99 separated astrakaaite remains in contact with the solution. If it does it will be resorbed as the reicherdtite continues to crystallise ; under natural conditions however it is likely that the salts already separated will become crusted over and thus be protected from the action of the solution. In either event the path of crystallisation eventually passes across the field of reichardtite to its boundary with that of kainite. From here the path lies along the boundary with the two salts crystallising together. There follow in succession the pairs hexahydrite-kainite kainite-kieserite kieserite-crtrnallite and finally the three salts kieserite-carnallite-bischofite Hexahgdrite Reichwdtite FIG.3 Stability $fields of oceanic salte at 25'. separating together until the evaporation is complete; in this mixture of salts the bischofite is of coume greatly predominant in amount since the composition is now so close to the Mg vertex. Throughout this later part of the crystallisration halite is still separating together with small further amounts of calcium salts. The fields of stability of these latter can be delineated in a similar Mg K, SO triangle and in many of the published figures the two isothermal diagrams are superposed (me for example Fig. 1). The actual amount of calcium present is so small that its effect on the course of crystallisation is negligible and the path of crystallisation outlined above can be applied directly to the double scheme.100 QUARTERLY REVIEWS The polyhalite which is stable when the crystallisation of magnesium salts begins gives way finally to anhydrite during the course of separation of kainite. We can thus construct a theoretical profile of the salt succession to be expected from this course of crystallisation at 25" Sylvine with kieserite and I halite Kieserite carnallite bischofite Kieserite carnallite Kieserite kainite Hexa.hydrite kainite Reichardtite kainite Reichardtite Astrakanite Polyhalite Anhydri te Gypsum Carbonates Halite with kieserite and Halite with kieserite and carnalli te sylvine h.2 a ___- Halite with langbeinite Halite with loewite Halite with vanthoffite Bischofite zone Carnallite zone Kainite zone magnesium sulphate zone Polyhalite zone Anhydrite zone Gypsum zone Basal limestone and dolomite I A comparison of this theoretical profile with the successions in natural deposits reveals in most areas a general correspondence a t least up to a certain stage.Marine limestones or dolomites passing up into anhydrite and halite with or without polyhalite are developed for example in Germany in the Texas-New Mexico field and in this country. The char- acteristic calcium sulphate of most marine evaporites is anhydrite however rather than gypsum. In the succeeding zones less complete agreement with the theoretical profile is revealed and in many areas there is profound disagreement. Salts such as astrakanite reichardtite kainite or hexa- hydrite are rare or absent ; vanthoffite loewite langbeinite and sylvine are found instead.For parts of the German field a succession can be tabulated l4 i Carnallite succession. ~ Hartsalz succession. i j Older potash beds Transition beds I I I I Halite with polyhalite Older rock-salt Halite with glauberite Halite with anhydrite Sylvine which does not appear in the theoretical profile a t 25" (in which kainite is the chief carrier of potassium) is a constituent of two of the l4 E. Fulde " Zechstein," Berlin 1935 47 139. PHILLEE'S OCEANIC SALT DEPOSITS 101 most important ores in most areas-the " Hartsalz " of the German miners a mixture of sylvine with kieserite and halite and the rich " Sylvinite," a mixture of sylvine with halite. The latter is of chief economic importance in the Texas-New Mexico district,15 in the upper parts of the German succession and in the Solikamsk region.Whether a primary bischofite layer was ever developed is a question which will be referred to later; certainly nothing resembling the thick bischofite zone of the theoretical profile has ever been encountered but its absence can be readily accounted FIG.. 4 Stability fields of oceanic salts at 55". for on the supposition that in most natural occurrences the evaporation of the mother liquor did not proceed to completion. The first suggestion to arise in an attempt to explain these discrepancies may be that the evaporation took place a t higher temperatures and it was to pursue this suggestion that van't Hoff carried out a second series of experiments a t 83" a t which the field of kainite disappears.In Fig. 4 is reproduced the isothermal diagram for a temperature of 55"; loewite is the first salt to separate from normal sea-water at this temperature and langbeinite also is found in the succession. Sylvine associated with kieserite would need still higher temperatures since below 72" the field of kainite intervenes. Such high original temperatures appear most unlikely t o most 16.J. W. Turrentine "Potash in North America," Reinhold 1943 p. 24. 102 QUARTERLY REVIEWS present-day students of the marine evaporites. Many lines of evidence such as the characteristic association with “ red beds,” indicate that most oceanic salt deposits of past ages were laid down in an evaporating basin under am arid continental climate but E.Fulda is almost alone amongst recent authorities in believing that as the rapidity of evaporation decreased with increasing concentration the temperature may have risen sufficiently high to allow the direct crystallisation of these higher-temperature associa- tions. Much of his evidence in support of the contention that the present- day profiles are essentially primary is of a geological character but reference may be made here to experiments by S. Lowengart 17 on the evaporation of water from the Dead Sea. When the solution had reached a density of 1.35 evaporation came almost to a standstill whilst irradiation served merely to effect a rise of temperature. Some modifications of the normal profile might be expected if other conditions during the evaporation apart from the temperature were different from those which we have assumed.It is possible that extensive resorption of earlier-formed salts may have occurred a t a later stage of crystallisation if they remained as a porous mass permeable by the solution. During the final stages only kieserite carnallite bischofite halite and anhydrite are in equilibrium together and any earlier products now brought into contact with the magnesium-rich liquor will be resorbed and pseudomorphed. It is also possible that it layering of solutions of different concentration may have arisen in the containing basin and the crystalline products separating from the more concentrated but hotter surface layer may have been resorbed on settling through tho underlying layers of differing concentration. A further effect of the setting up of currents of solutions of different con- centrations has been specially stressed by H.Borchert.18 In the course of an investigation of the reasons for the impoverishment of salt-deposits by lateral passage into less rich ores he has developed a “ dynamicrpoly- thermal ” study in contrast with the purely static considerations of van’t Hoff and d’Ans. Such features as the configuration of the floor of the basin and different rates of evaporation in different areas may set up tem- perature gradients and circulating currents of solutions of different con- centrations. Those compounds which tend to separate in the regions of higher temperature in such a circulatory system are termed thermophile whilst the cryophile salts will be precipitated in the colder regions. The fields gf formation of some salts under these conditions are considerably modified in comparison with the static system.Sylvine and langbeinite for example show much wider possibilities of formation; a t high tem- peratures the langbeinite area is considerably extended towards the Mg apex and a langbeinite-carnallite paragenesis may then be possible. It has so far been assumed that the deposition of marine evaporites of past geological periods must be explained in terms of %he evaporation of l6 2. deut. geol. Qes. 1924 76 ; Monatsber. 7-30. l7 2. pr. Qeol. 1928 36 86-89. la Kali 1933 27 97-100 105-111 124-127 139-141 148-150 ; 1934 28 290-296 301-305; 1935 29 1-5; Arch. Lagerstforsch. 1940 No. 67. PHILLIPS OCEANIC SALT DEPOSITS 103 a solution similar in composition to present-day sea-water.It is of course possible that the composition may have been very different and the question of the origin of the salt content of the sea is an interesting geochemical problem. On the assumption that the sea was originally fresh and has gradually acquired its present content of salts from the contributions brought down to it by- rivers geologists have in the past even attempfed to derive an estimate of geological time. The salt content of rivers however must have been derived from the chemical weathering of previously-existing rocks and it is particularly difficult to accept that the high chlorine content of the sea can have been produced in this way. V. M. Goldschmidt Is has examined the availability of a large number of elements in the earth’s crust. It would appear that the concentrations of sulphur chlorine bromine and boron in sea-water are such that these elements must already have been present in the primeval ocean.2o E.J. Conway 21 has recently discussed the probable course of the chemical evolution of the ocean both on the hypothesis that the halogen content was derived from the original atmo- sphere and also on the alternative assumption that all the halogen has come from volcanic sources. The probable differences of salt content between the Permian oceans and those of the present day thus revealed are quite insufficient to effect any radical change in the course of crystallisation which we have traced. Some salt deposits are believed to have been laid down from solutions which derived their salt content not directly from the ocean but by re-solu- tion of previously-existing marine evaporites.The conditions of deposition then approach those of non-marine evaporites the formation of which can be studied at the present day in natural salt lakes. In contrast with the uniform composition of oceanic waters the waters of such lakes show a wide variety of chemical characteristics. In particular it may be suspected that beds of rock-salt without an associated layer of gypsum or anhydrite below them are of secondary origin. The discrepancies between the theoretical profile and the natural succes- sions are not merely of a qualitative kind. A difficult quantitative problem is encountered in the vast thicknesses of gypsum anhydrite or halite often recorded. A bed of halite 100 m. thick would correspond to the evaporation of a column of water some thousands of metres in depth whilst thicknesses of anhydrite have been recorded which would involve the evaporation of an appreciable fraction of the whole present hydrosphere.Even if these large original volumes in the evaporating basin could be accepted there should be evidence of marked shrinkage of the ocean as evaporation proceeded; in actual fact the beds of anhydrite and halite maintain their thicknesa close to the margins of the area of deposition. These difficulties find at least a partial explanation in the accepted picture of the conditions under which evaporation in the basin took place. Most geologists adopt in a more or lB J . 1937 667. 20 H. U. Sverclrup M. W. Johnson and R. H. Fleming op. cit. p. 221 ; C. H. White a1 Proc. Roy. Irish A d .1943 48 B 9 161-212. Arner. J . Sci. 1942 240 714-724. 104 QUARTERLY REVIEWS less modified form the “ bar ” theory put forward by Bischof and developed by C. Ochsenius in the middle of the laat century. Salt deposition ia pictured as taking place from the waters of an-enclosed lagoon behind a permanent bar. So long as a constant or intermittent connection with the waters of the open ocean was maintained an inflowing cuprent of water of normal salinity across the bar would augment the supply of salts to the evaporating wafers of the lagoon. In this manner thick beds of salt could be formed but the high degree of concentration necessary to precipitate the more soluble salts must eventually have allowed the concentrated mother- liquor to collect in the more depressed parts of the basin.Even at this stage there may have been an occasional influx of new solutions either from the FIG. 5 D@pO8itiOn.d rhythm in two salt projELee (Lotze) 0 Deposition of non-saline sediments. 1 Formation of anhydrite. 3 Formation of potassium salts. 2 Formation of halite. sea itself from one basin to another or as a result of rainfall which dissolved and washed down the salts which had separated earlier on the marginal portions of the area and had been left exposed by the retreat of the water. Indisputable evidence of such periodic additions on both a major and a minor sc&le can be found in the deposits themselves. In many parts of the German field a succes8ion beginning with anhydrite and passing upwards through the deposition of halite to the stage of formation of potash salts is overlaid by clastic sediments (often a red saline clay) above which follows a more or less complete further cycle.A diagrammatic section by F. Lotze 23 reveals three such cycles (Fig. 5 ) in one area. In this country two partial cycles from limestone through anhydrite to salt followed by a third reaching as “Die Bildung der Steinsalzlager . .,” Halle 1877. “ Steinsdz und Kalisalze,” Berlin 1938 p. 151. PHILLIPS OCEANIC SALT DEPOSITS 105 the stage of deposition of anhydrite have been distinguished.24 Periodicity on a minor scale is seen in the so-called annual rings (" Jahresringe ") a series of thin layers or streaks of anhydrite or of polyhalite in halite. Their name is derived from the belief still accepted by some geologists that they represent changes in solubility of calcium sulphate consequent upon annual fluctuations of temperature but they may equally have arisen from periodic influx of further supplies of sea-water which reduced the concentration below the point of saturation for halite.A similar fine banding by layers of clay particles is sometimes observed and seems to speak conclusively in favour of the influx of muddy water possibly consequent upon increased rainfall due to climatic oscillation. The considerations which we havexo far advanced may help to explain in part the features presented by the natural profiles but it is generally accepted that they provide only a partial clarification and that an important branch of the study of marine evaporites involves the examination of changes effected subsequently to their deposition by the heat and pressure to which they have been subjected and by reaction with circulating solutions.The salts of the theoretical profile many of them highly hydrated will be specially susceptible to rise of temperature consequent upon burial under later over- lying sediments-salt deposits have been described as " the liveliest and most temperamental of rocks "-and the importance of subsequent therm&l metamorphism was stressed especially by F. Rinne 25 and Janecke.26 That many of the present mineral components of the salt deposits are the products of secondary changes seems to be abundantly clear. Lateral changes such as carnallite passing into Hartsalz have been frequently demonstrated and it has been found possible in some weas to trace a number of successive guide-horizons of beds of almost pure halite of great lateral extent which pass unchanged between different zones of potash salts.27 Examination of the detailed studies by K. Weber 28 of the Stassfurt region or by W. T. Schaller and E. P. Henderson 29 of the succeasive replacements in the Texas-New Mexico deposits will convince most readers that whatever may have been the ori@nal succession the salts now found are the products of profound alteration. A purely thermal metamorphism Consequent upon burial would bring about the successive '' melting " of various salts with the generation of solutions of various compositions. Under a depth of burial of about 3000 m. the change from gypsum to anhydrite would release almost pure water. If a primary bischofite layer were present this would melt at a depth of about 4200 m.t o an almost pure magnesium chloride solution. Jsnecke has traced these changes in detail and put forward the following scheme 24 S. E. Hollingworth Proc. Geol. As~oc. 1942 68 145. p6 See summary amount in " Handbuch der Mineralchemie," C. Doelter and H. Fmtschr. Min. 1920 6 101-136. Leitmeier 1929 IV 2 1283-1290. A. Tinnes Arch. Lagerstfur8ch. 1928 No. 38. asKali 1931 25 17-23 33-38 49-55 66-71 82-88 97-104 122-123. ae Geol. Surv. United States 1932 Bull. No. 833. 106 QUARTERLY REVlEWS Theoretical profile. Geothermally changed proflle. Bischofite zone + Yields MgC1 solution Carnallite zone Kainite zone + Hartsalz hone} Potwh-free magnesium 8ulphatc.i ___+ Polyhalite zone -p Polyhalite zone Glauberite zone Anhydrite zone] Anhydrite zonej Anhydrite zone Gypsum zone A specially important change is that by which Hartsalz would be derived Carnallite or -7 Hartsalz zone Loewite-Vanthoffite zone -+ Kieserite zone zone - { at 72" viz.Kainite + Carnallite + Kieserite + Sylvine + Solution but much of the Hartsalz shows an apparently primary lamination and would thus appear to be an original formation rather than the product of thermal metamorphism of previously-existing salts. If the solutions generated during these reactions remained in contact with the salts they would be available for a reversal of the reaction on declining temperature. Usually however they will have been pressed away to other regions changing their composition by further reactions such as the abstrac- tion of MgCl from carnallite and effecting further modifications in the composition of the various zones.The proponents of extensive metamorph- ism of this kind believe that such residual solutions (" Restlaugen ") have exercised a profound influence on the generation of the present-day profiles of marine evaporites. To those such as Fulda however who reject com- pletely the theory of a thermal metamorphism the only solutions which have been active are the connate solutions ('.' Urlaugen ") which represent portions of the mother-liquor enclosed with the deposits a t the time of their formation and percolating ground water the action of which will be con- sidered shortly. The details of Janecke's presentation have been criticised by H. B ~ r c h e r t ~ ~ who points out that the compositions of residual solutions postulated will in fact be reached only by stages and it is probably true to say that decreasing reliance is placed on a pure thermal metamorphism at the present time to explain the features of the actual profiles of marine evaporites.Consideration of the changes effected during deformation and earth movement and of the significance of the plasticity of salt deposits would lead into a purely geological field but we may note here that earth movements have .probably been responsible for producing the " Hasel- gebirge," an intimate mixture of salt gypsum and clay which is the common salt-producing rock of the Alps. By reason of their ready solubility the salt deposits are likely aIso to suffer further changes in the upper parts of the succession under the action of circulating ground water unless effectively sealed 08 by overlying clays.Here belong a number of reactions classified by German geologists as " Hutsalzbildung " t h e formation of an altered " cap " analogous with the secondary changes often found close to the stdace in other types of ore 30 Kali 1938 32 132-135 143-146 169-172. PHILLIPS OCEANIC SALT DEPOSITS 107 deposits. A re-formation of kainite is especially chsracteristic of them later changes KCl,MgCI2,6H,O + MgSO,,H,O + zHZO = KCl,MgSOa,3H,O + (Ma + yH,O) Carnallite + Kieaerite + Water = Iceinite + Solution The sylvine and kieserite of Hartsalz may combine to yield kainite KCI + MgS04,H20 + 2H20 = KCl,MgS04,3H,0 Sylvine + Kieserite + Water = Kainite Carnallite not accompanied by kieserite may yield sylvine (which with the associated halite is the sylvinite ore) its MgCl content passing into solution.Posthumous kainite and sylvinite thus formed are usually massive and unlaminated. Kainite itself may suffer a further extraction of its chlorine yielding schoenite 2(KCI,MgS0,,3H,O) + zHZO = KzSO,,MgS04,6H,O + (MgCl2,yHSO) Kainite + Water = Schoenite + Solution In the earlier days of the mining industry in Germany large quantities of these rich kainite rocks from the shallower zones were worked and sold for use as an artificial fertiliser ; the name has tended to linger on in application to a product which is now often a mixture of salts deriving its potassium content mainly from sylvine and kieserite. Anhydrite brought within the influence of circulating ground water will be converted into gypsum and the resultant increase of volume is usually accepted as the cause of the folded and contorted appearance (“ enterolithic structure ”) of many gypsum beds intercalated in salt deposits.This structure is occasionally found in beds which are now composed of anhydrite but it is clear that in the course of the complex history of some.of these deposits an anhyclrite rock converted into gypsum at one stage may have been again dchydratecl later in its history. In their studies of metamorphic changes in rocks geologists have for long been staunch supporters of the axiom “ Corpora non qunt nisi jluida,” looking always for the prcscnce of a solvent t o act as a medium for recrystal- lisation. During the last two decades however more attention has been paid to the possibility of reaction by diffusion in the solid state and this aspect of the study of oceanic salt deposits has been explored particularly by Leonharclt and his associate^.^^ Experiments on mixtures of powdered salts which were compressed and subsequently heated showed that even in the absence of solutions compounds such as langbeinite and vanthoffite may begin to form a t a temperature of 80° and this type of reaction ma-y well be important during metamorphism related to earth movements.Thus far wc have considered only those elements present in normal sea- water in relatively high concentrations. Amongst those of bedium concen- tration considerable interest has centred on boron. Reckoned as H,BO it ranks fifth amongst the anions with a concentration of 27 mg./kg. and as we have noted it must apparently be accepted 8 s an original constituent of the primeval ocean possibly derived from the presence of BC1 in the 31 J.Loonhnrdt Fortschr. Min. 1935 19 37-39; H. Ide Kdi 1935 29 83-86 93-96 103-105. 108 QUARTERLY REVIEWS original atmosphere.32 The work of W. Biltz and E. Marcus 33 showed that the boron content of the commoner minerals of the German deposits varies widely. Sporadically however a concentration is reached sufficient t.0 allow the formation of boron minerals of which the most important is boracite. When found as well-developed crystals boracite has every appearance of being a primary product (though this conclusion has been questioned),54 but it presents an interesting genetic problem. The external habit of the crystals is in agreement with cubic symmetry but in section the crystals are seen to be composed of doubly-refracting lamella? with the symmetry of an orthorhombic structure.When heated in the laboratory the crystals become truly cubic only at 265" ; it seems unlikely that such a temperature could ever have been reached during burial and metamorphism of the salt deposits and the explanation must be accepted at present that the crystals are pseudo-cubic polysynthetic twins. Boracite is found also in a massive form originally named " stassfurtite," either interbedded with carnallite or as concretionary nodules and most of this material is clearly secondary. Although stassfurtite is readily changed further by the action of circulating ground water giving rise to other boron minerals such as ascharite kali- borite and pinnoite which are found in the Hutsalz magnesium chloride being carried away in solution yet the well-crystallised boracite seems to be much more resistant to such changes.Lueneburgite another rare mineral in the German deposits has been recorded also from the Texas-New Mexico district .35 Of other rare minerals occasionally found in marine evaporites it will suffice to mention two examples. Tachhydrite may arise from carnallite as a secondary product under the action of solutiona containing calcium chloride 2(KC1,MgCI2,6H20) + CaCI = CaC1,,2MgC12 12H,O + 2KC1 in agreement with the observed paragenesis tachhydrite-sylvine-carnallite- halite. The calcium chloride solution may arise from the action of magnesium chloride solutions on anhydrite CarnaUite Tachhydrite Sylvine and tachhydrite has beer recorded in association with kieserite. Rinneite NaC1,3KCI,FeC12 is likely to form only where there is a high local concen- tration of iron.The small amount of iron present in sea-water is commonly represented in the marine evaporites by the particles of hematite which impart to most of the carnallite and many of the other salts a characteristic red colour (good colour photographs illustrating this feature are given by G. R. Man~field).~~ A regular zonal arrangement of these particles of iron 33 H. wettenberg 2. a w g . Chem. 1938 236 355. '3 I-. 1911 72 302-312. H. Werner Kali 1930 24 129-132. 36 W. T. Schaller and E. P. Henderson Geol. Surv. United States 1932 Bull. s6 J . Chem. Educ. 1930 7 737-761. NO. 833 pp. 47-48. PHILLIPS OCEANIC SALT DEPOSITS 109 oxide within the host crystal 37 suggests that they have developed after crystallisation the iron having originally been present as an isomorphous replacement of some part of the magnesium.In the American potash field hematite as a red pigment is most abundant in the potash minerals. The insoluble residue left on dissolving away the salt has a stringy structure and it has been suggested that the hematite is here the result of bacteriologic action.38 More rarely carnallite and tachhydrite are coloured yellow by ferric chloride or the iron may be present as FeS2 (pyrite) or as Fe30 (magnetite) colouring carnallite black. During the secondary reactions which have given rise to the cap salts the iron content of the primary salts seems to pass into solution and there is often a sharp dividing line between the brightly-coloured primary salts and the colourless overlying secondary products.For the direct formation of a compound such as rinneite it is possible that the presence of organic compounds retarded oxidation. In all 4 4 elements excluding dissolved gases have been demonstrated to be present in the water of the oceans though for some the demonstration is indirect by way of the examination of the ash of marine organisms. In the marine evaporites these rarer elements are seldom or never represented by individual compounds but occur as isomorphous mixtures or as trace elements in the commoner salts. Our present knowledge of their distribu- tion is still very uneven a systematic investigation of all the minor and trace elements by modern methods having yet to be undertaken. The following table gives the concentration in ocean water of a few of these minor elements to which we may direct attention here Element.I 1 Bromine . . . Strontium . . * . Fluorine . . . Rubidium . . . Lithium . . . Mg./kg.(Cl = lQ*ooo/oo). 65 13 1.4 0.2 0.1 Element. Iodine . . . Cssium . . . Silver . . . Gold . . . . Mg./kg.(Cl = lQ.OOO/,,). 0.05 0.0003 0*000006 (0.002) I I Bromine in spite of its relatively high concentration does not give rise t o distinct bromine compounds but is found isomorphously replacing chlorine in salts such as carnallite sylvine and kainite (ionic radii Br- 1.95 a. C1- 1.81 a.). Bischofite and tachhydrite may also contain appreci- able amounts but much less replacement is found in halite. Previously to the development of methods for the extraction of bromine from sea-water Germany possessed an almost complete monopoly based on her Permian salt deposits using carnallite and sylvine with a bromine content up to 0.4%.An early investigation by H. E. Boeke 40 showed clearly that a t Stassfurt the bromine content of a given profile varied regularly with the content of carnallite and a similar relationship has been demonstrated in the Russian A. Johnsen Zentr. Min. 1909 168-173. W. T. Schaller and E. P. Henderson Zoc. cit. pp. 11 38. a@ H. U. Sverdrup M. W. Johnson and R. H. Fleming op. cit. pp. 176-177. roZ. KTiBt. 1908 45 346-391. 110 QUARTERLY REVIEWS deposits near Solikamsk where J. Moratchevsky and A. Fedorova *l found the bromine content to be independent of the depth but directly related to the carnallite content of the rock. Iodine with its still larger ionic radius (2.16 A.) does not readily replace chlorine in these salts and is found only in very small amounts in salt deposits.A detailed study of the iodine content of the German deposits has been made by J. R ~ e b e r . ~ ~ Rubidium and caesium have also been extracted from carnallite in which they replace the potassium (ionic radii K+ 1-33 A, Rb+ 1.48 A. Cs+ 1-69 A.). Rubidium has been found in the waters of the present-day oceans but the caesium content of 0.002 mg./kg. quoted in the table above is calculated on the basis of the ratio Cs Rb in carnallite.43 G. Heyne 44 found rubidium also in sylvine and langheinite but none in the kainite and polyhalite which he examined. The presence of lithium has been demonstrated spectroscopically and its occurrence has a practical application. Although the.majority of salt mines are quite dry solutions occasionally break into the workings. If these are connate waters (" Urlaugen ") enclosed with the salts a t the time of deposition they will be limited in volume and will oventually drain away harmlessly. If however surface water breaks into a salt mine through fissures the consequences are likely to be disastrous and may entail the abandonment of the mine. The observation that the harmless Urlaugen are usually notably rich in lithium may hence be put to practical 1,188.~~ The very different proportions of the alkali metals in sea-water and in the marine evaporitcs when compared with their concentrations in igneous and sedimentary rocks are to be refcrrcd to the readiness with which they are adsorbed in fine-grained sediment^.^^ Strontium with a concentrat'ion of 13 mg./kg.in sea-water is the fifth most abundant kation and in agrcenicnt with this relatively Pigh concentra- tion the sulphate celcstinc has been occasionally recorded from salt deposits both in Europe and in Amcrica. More usually strontium is found replacing calcium (ionic radii Cat+ 0.99 A . Sr++ 1.13 A . ) either in the carbonates especially aragonite or in the sulphates. The fact that a higher percentage seems to enter the anhydrite structure compared with the small amounts usually found in primary may be used to study the vexed question of the original form in which the calcium sulphate of oceanic salt deposits was laid down. Anhydrite which has resulted from the dehydration of primary gypsum may be expected to show a low strontium content in comparison with primary anhydrite which separated directly from solution.Some occurrences of gypsum and anhydrite in the Permian deposits of Russia have been investigated from this point of view.48 4 1 Abstract in Neues Jahrb. Min. Ref. 11 1029 686. 4a Jahrb. Hallesch. Verb. Erf. mitteldtsck. Bodensch. 1938 16 129-196. 4 3 H. Wattenberg 2. anorg. Chem. 1938 236 346. 4 4 Abstract in Neues Jalrrb. Min. 1913 I 365. 4 5 E. Fulda 2. pr. Geol. 1039 47 11-14. O 6 W. Noll Chenz. Erde 1831 6 573. 48 L. M. Miropolsky and S . A. Borovick Compt. rend. (Doklady) Acad. Sci. U.R.S.S. 47 Idem ibid. 1934 8 559. 1943 38 33-36; 41 382-383. PHILLIPS OCEANIC SALT DEPOSITS 111 The high figures sometimes quoted for the supposed gold content of sea- water would suggest that this element also might become strongly concen- trated in the residual liquor and thus enriched during the deposition of the oceanic salts.It would appear however that these high figures are partly due to faulty analytical methods and also that much of the gold actually present is not in solution as ions but exists as discrete particles or in organic matter.4s Such gold would be adswbed on the finer clastic sediments rather than concentrated in the salts. J. Goubeau and L. Birckenbach 5O found the highest content of precious metals amongst the salt minerals in those which were notably coloured by inclusions of fine clay particles and showed also that the average content of the associated clays was higher than that of the salt minerals themselves. An attempt has been made in this review to trace the development of the study of marine evaporites and to outline some of the geochemical problems involved. For recent more extended accounts written from various points of view reference may be made to the works of Lotze (1938) and Borchert (1940) cited above or to two further works by F ~ l d a . ~ l ID H. Wattenberg 2. anorg. Chem. 1938 236 352. 61 “ Steinsalz und Kalisalze,” Stuttgnrt 1938 ; “ Die Salzlagerstatten Deutsch- 6O Ibid. pp. 37-44. lands,” Berlin 1940.

ISSN:0009-2681    

DOI:10.1039/QR9470100091

出版商:RSC    

年代:1947

数据来源: RSC

摘要:

ENDEX 1947 Authors Of A r t k k :- Anderson J. S. chemistry of the metal carbonyls 331 Bassett H. basic salts 246 Bell RIGP. the use of the terms “ acid ’’ and base ” 113 Bowen E. J. fluorescence and fluor- escence quenching 1 Coulson C. A. representation of simple molecules by molecular orbitals 144 Gee G. some thermodynamic proper- ties of high polymers and their molecular interpretation 265 Levy N. and Rose J. D. the aliphatic nitro-compounds 358 Linnett J. W. force constants 73 Maccoll A. colour and constitution 16 Marsh J. K. the separation of the lanthanons (rare-earth elements) 126 Phillips F. C. oceanic salt deposits 91 Riley H. L. amorphous carbon and graphite 59 Stacey M. aspects of immunochemistry 179 213 Traller E. E. and Harris M. M. asymmetric transformation and asymmetric induction 299 Acceptor molecule 124 Acids definition of 113 115 Adamite structure of 253 Allergens natural 234 Allergy 231 Amalgams lanthanon 140 Amino-alcohols uses of 394 Ammonia hybridisation in 164 Amorphous substances 59 Amphiboles 26 1 Anaphylaxis 183 231 Andalusite structure of 254 Anisotropy induced 329 Anthrax bacilli polypeptide from 220 Anti-antibodies 213 Anti-auxochromes 20 Antibacteriophages 244 Antibodies 182 assay of 198 chemistry of 191 isolation and purification of 195 molecular size of 201 origin and specificity of 202 serological properties of 2 13 bacteria1,’214 223 Forsmann and Salmonella 222 from blood group substances 242 reactions of with antibodies 205 AlbnminR serum- 193 Anti-enzymes 243 Antigens 181 Antitoxins 182 235 Antivenoms 235 Asymmetric addition 330 induction 326 transformation 299 Atomic orbitals 145 Autunite structure of 261 Auxochromes 16 Azoproteins 188 Bacilli acid-fast immunology with 226 anthrax polypeptide from 220 tubercle polysaccharides from 227 Bases definition of 113 115 Bathychromic groups 17 Bending-force constants 89 Benzene absorption and fluorescence bands of 4 absorption spectrum of 27 canonical structures for 37 electronic energy levels of 38 molecular orbitals for 167 Blood groups 236 Blood group substances 238 Blood-serum proteins 192 Bond lengths relation of to force con- Bond order fractional 175 Bond-stretching forces table of 85 Boron minerals in oceanic deposits 108 Bronsted-Lowry definition of acids and Brucite structure of 262 Butadiene molecular orbitals for 166 Cadmium hydroxy-chloride structure of Canonical set 37 Carbohydrates hapten properties of 184 Carbon amorphous 59 Carbon black crystallography of 70 Carbon monoxide electronic structure of Carbonisation 66 Carbonyl halides 347 hydrides 343 Carbonyls 331 polynuclear 354 Carotenoids light absorption of 46 Cerium separation of 131 Charge distribution 176 Chars 63 Chlorinity of sea-water 92 Chlorites 263 Chloro-nitro-paraffins 379 uses of 394 Chloropicrin 379 pesticidal activity of 394 Cholera vibrios polymcharides from 220 Chromogens 16 Chromophoms 16 stants 86 bases 116 259 valence angles of 162 159 ChrySOt& &38b08tOS S t N C t U r e Of 261 3 96 INDEX 1947 397 Clay mirierals 262 Cobalt tc&racabonyl hydrolysis of 340 Cokes 63 Colour crlculation of 51 constitution and 16 theories of 17 Complerr.ent 2 10 Constituiion colour and 16 Cryophil 3 compounds 102 Crystalli:iation mutarotation and 313 Crystal-tiolet 43 structl re of 30 Cyanines colour and constitution of 48 Cyanite structure of 254 Descloisite structure of 254 Desensit sation 234 Dextran:, 219 6 lCi-Diiydrohexacene spectrum of 44 Dipheny structures for 44 Dipheny polyenes energies and structure Dipole strength 40 Dyes classification of 51 organil colour of 56 Dysentery immunochemistry of 224 Earths rare separation of 126 Eigenfun ctions 32 Eigenvalues 32 ElasticitJT of an ideal network 278 rubber-like.273 Electrolg tic dissociation theory 1 14 Electron velocity 177 Epidote 261 Ethide 394 Exclusiort principle. Pauli’s 33 Fluorapatite structure of 265 Fluoresconce 1 Force co istants 73 uses of 83 tetracc ,rbonyl hydride 345 of 46 fields 76 Gadoliniiun separation of 133 Gases solubility of in polymers 295 Globnlin:; seriun 194 d-Glucoscb crystallisstion and mutarots- tion of 313 Graphite 50 Graphiti..A t ion. 70 Grignard reaction with nitro-paraffins 378 Grignard reagents addition of to nitro- olefiiis 391 Halloysit 3 . structure of 262 Haptens 182 Hay fever. 232 Hesainet liylbcnzene absorption spectrum of. 11 Higgind e. strticture of 254 Hornblertde htructure of 261 FLybridisitioii. 3 7 148 serologr and structure of 189 Hydrargillite structure of 262 Hydrogen chloride molecular orbitals for Hydroxyapatite structure of 255 Hydroxylamine uses of 395 Hydroxyl bond 248 Hyperconjugation 56 172 Hypsochromic groups 17 Illhiurn 128 Immunity active and passive 180 Immunochemistry 179 213 Immuno-polysaccharides 2 14 Insulation of chromophores 44 Iridium carbonyls 336 Iron carbonyl halides 348 158 pent acarbonyl hydrolysis of 340 Lanthanons separation of 126 Lepidocrocite 257 Lewis definition of acids and bases 122 Libethinite structure of 253 Light absorption classical theory of 22 Lewis and Calvin’s theory of 28 quantum theory of 32 Lipoids immunology with 222 Magnesium hydroxide and silicates struc- Malachite structure of 255 Malachite-green absorption and resonance energy of 43 Mannich reaction 375 Marine evaporites 91 Metal carbonyls 331 Methane molecular orbitals for 171 Micas 263 Mineral deposits containing lanthanons 129 Molecular orbitals 36 144 localised 160 non-localised 165 heteronuclear 157 homonuclear 152 ture of 260 constitution of 351 Molecules diatomic 150 polyatomic 160 representation of by molecular orbitals 144 Monazite treatment of 130 Mullite structure of 255 Mutarotntion 313 Mycolic acid 226 Naphthalene fluorescence quenching of Neodymium separation of 133 Nitro-alcohols uses of 393 Nitro-compounds aliphatic 358 Nitro-dienes 354 Nitroform 380 Nitrogen valence angles of 161 Nitro-olefins 383 Nitro-paraffins 358 13 physical properties of 368 398 INDEX 1947 Nitro-paraffins uees of 393 plyNitro-paratltins 380 Nitrosocarbonyls 356 Nucleic acid immunology with 229 Nucleoproteins immunology with 229 Oceanic salt deposita 91 salts solubility of 97 O l e h nitration of 362 Olivenite structure of 253 Optical activity in tin compounds 306 Oscillator strengths 27 OBmium carbonyls 336 Osmotic pressure of polymers 286 Oxygen molecular orbitals for 156 valence angles of 161 Paraffins nitration of 360 365 Peat carbohtion of 68 Phosphorescence 6 Pnemnwoccus polysaccharides 2 14 2 1 7 Polymers crystallisation of 266 high thermodynamics of 265 solubility of in liquids 286 swelling of in liquids 290 Polysaccharides bacterial immunology of complex related t o blood group sub- 215 8tances 242 Polythene crystallisation of 269 Praseodymium Reparation of 132 Proteins conjugation of 187 Pseudo-acids 119 Pseudo-bases 119 Pyrophyilite structure of 263 Pyroxenes 261 Quantum numbers 33 Quenchers of fluorescence 1 I Quenching of fluorescence 6 Racemates resolution of 326 Racemisation asymmetric catalytic 31 2 X-Ray structure of inorganic compounds Resonance theory qualitative applica- Rhenium carbonyls 337 Rhesus factor 242 Rhodium carbonyls 337 Rubber crystallisation of 266 elastic ext;ension of 275 QR-S solubility of in mixed liquids 288 solubility of gases in 298 vulcanisates swelling of in liquids 291 reactions of 184 246 tions of 42 Rubrene fluorescence quenching of 13 Ruthenium carbonyls 336 Salt deposits oceanic 91 Salts basic 246 oceanic solubility of 97 Samarium separation of- 141 Schrodinger equation 32 Sea-water chemistry of 91 Sediments 91 Sensitisation 23 1 Sillimanite structure of 254 Solid solutions fluorescence of 14 Solids reaction by diffusion of 107 Solvent systems of acids and bases 115 Species specificity 236 Specificity chemical basis of 183 Spectra absorption and colour 18 vibration frequency from 82 Steel elastic extension of 274 Sugars crystallisation and mutation in types of 250 evaporation of 94 313 Talc 263 Tetranitromethane 38 1 Tetryl dermatitis caused by 232 Thermodynamics of high polymers 265 Thermophile compounds 102 Tin compounds optical activity in 306 Topaz structure of 255 Toxins 182 Transformation equilibria fmt-order 322 Transition moment 40 Tremolite structure of 261 Tubercle bacilli polysaccharides from 227 Tuberculostearic acid 226 Valence free 176 Vapours absorption of by polymers 282 Vermiculites 262 Vi-antigens 22 1 Vibration frequency calculation of 73 Viruses immunology of 244 Vitreous solids 59 Wassermann substance 222 Water hydridisation in 164 molecular orbitals for 161 ocean 91 sea- chemistry of 91 evaporation of 94 Wood swelling of 293 Yttrium separation of 135

ISSN:0009-2681    

DOI:10.1039/QR9470100396

出版商:RSC    

年代:1947

数据来源: RSC