摘要:

Chemical Society Reviews Vol 13 No 2 1984 Page TILDEN LECTURE Structural Studies on Bio-active Molecules By Dudley H. Williams 131 Crystal Structure Determination: A Critical View By P. G. Jones 157 LENNARD-JONES LECTURE Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures By L. A. K. Staveley 173 NYHOLM LECTURE Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education By Peter J. Fensham 199 The Royal Society of ChemistryLondon Chemical Society Reviews EDITORIAL BOARD Professor K. W. Bagnall, B.Sc., Ph.D., D.Sc., C.Chem., F.R.S.C. Professor K. R. Jennings, M.A., D.Phil., C.Chem., F.R.S.C. Professor G. W. Kirby, M.A., Ph.D., Sc.D., F.R.S.E., C.Chem., F.R.S.C.Professor G. Pattenden, Ph.D., C.Chem., F.R.S.C. Professor B. L. Shaw, B.Sc., Ph.D., F.R.S. Professor P. A. H. Wyatt, B.Sc., Ph.D., C.Chem., F.R.S.C. (Chairman) Editor: K. J. Wilkinson, B.Sc., M.Phi1. Chemical Society Reviews appears quarterly and comprises approximately 20 articles (ca. 500 pp) per annum. It is intended that each review article shall be of interest to chemists in general, and not merely to those with a specialist interest in the subject under review. The articles range over the whole of chemistry and its interfaces with other disciplines. Although the majority of articles are intended to be specially commissioned, the Society is always prepared to consider offers of articles for publication. In such cases a short synopsis, rather than the completed article, should be submit- ted to the Managing Editor, Books and Reviews Section, The Royal Society of Chemistry, Burlington House, Piccadilly, London, W 1V OBN.Members of the Royal Society of Chemistry may subscribe to Chemical Society Reviews at f15.00 per annum; they should place their orders on the Annual Subscription renewal forms in the usual way. All other orders accompanied with payment should be sent directly to The Royal Society of Chemistry, The Distribution Centre, Blackhorse Road, Letch- worth, Herts. SG6 1HN England. 1984 annual subscription rate U.K. f43.50, Rest of World $45.50, U.S.A. $87.00. Air freight and mailing in the U.S-A. by Publications Expediting Inc., 200 Meacham Avenue, Elmont, New York 11003. U.S.A. Postmaster: Send address changes to Chemical Society Reviews, Publi- cations Expediting Inc., 200 Meacham Avenue, Elmont, New York 11003. Second class postage is paid at Jamaica, New York 11431. All other despatches outside the U.K. by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. @ Copyright reserved by The Royal Society of Chemistry 1984 ISSN 0306-001 2 Published by The Royal Society of Chemistry, Burlington House, London, W1V OBN Printed in England by Eyre & Spottiswoode Ltd, Thanet Press, Margate.

ISSN:0306-0012    

DOI:10.1039/CS98413FP003

出版商:RSC    

年代:1984

数据来源: RSC

ISSN:0306-0012    

DOI:10.1039/CS98413FX005

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Chemical Society Reviews Vol 13 No 2 1984 Page TILDEN LECTURE Structural Studies on Bio-active Molecules By Dudley H. Williams 131 Crystal Structure Determination: A Critical View By P. G. Jones 157 LENNARD-JONES LECTURE Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures By L. A. K. Staveley 173 NYHOLM LECTURE Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education By Peter J. Fensham 199 The Royal Society of ChemistryLondon

ISSN:0306-0012    

DOI:10.1039/CS98413BX007

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

TILDEN LECTURE Structural Studies on Bio-active Molecules By Dudley H. Williams UN I VE RSI TY C H EM1 CA L LA BO R AT0 R Y, LEN SFI ELD ROA D, CAMBRIDGE, CB2 IEW 1 Introduction The structure elucidation of organic molecules can be regarded as falling into three areas. One area is that of X-ray crystallography; this approach requires single crystals, and therefore quantities of the substance of the order of mg or greater. It is also required that the crystals diffract X-rays satisfactorily, that they do not rapidly deteriorate in the X-ray beam, and further that the structure can be solved from the acquired data. In the absence of a heavy atom derivative, this last problem can be formidable if the asymmetric unit contains more than around 100 atoms. A second area is that of solution methods to determine the structures of the standard biopolymers, particularly DNA, RNA, and proteins (or peptides). Astonishingly sensitive and rapid methods have been developed to determine the sequence of bases (in DNA or RNA) or of amino acids (in peptides and proteins). These methods determine only the primary structure (sequence), although it is true that in a limited number of cases, secondary and tertiary structure (i.e.the whole three-dimensional picture) has been determined by the powerful method of X-ray crystallography.The third area has to attempt to encompass all the remaining problems. It includes all compounds that are not standard biopolymers and, additionally, suitable crystals cannot be obtained. Thus, we must include not only those compounds which, no matter how much is available, refuse to crystallize, but also those cases where crystallization is effectively impossible because of the small amount of substance available. This area will include an enormous number of physiologically important compounds such as hormones, vitamins, neuropeptides, toxins, and antibiotics (and their metabolites).Whereas, two decades ago, such problems had to be solved by tedious and demanding chemical degradations, recently a number of relatively rapid and powerful methods for their solution have evolved. Most notably, these methods include mass spectrometry and nuclear magnetic resonance. It is with this area of structure elucidation that the present article is concerned.The techniques will be illustrated by the solution of four quite different kinds of structures, each typifying a characteristic problem. 2 The Metabolite Problem: Structure Elucidation of 1cr,25dihydroxyvitamin D, The problem of structure determination of metabolites is often characterized by two features: the amount of metabolite available is small (< IOOpg), and the 131 Structural Studies on Bio-active Molecules metabolite is related to the known structure of the starting material by a relatively simple chemical change e.g. hydroxylation, demethylation, sulphation. Such problems are ideally suited to study by mass spectrometry since mass spectrometry is a sensitive technique (molecular weight determination in the pg to ng range).In principle, 'Hn.m.r. can be usefully applied to samples as small as 10 pg, but in practice such samples are frequently contaminated with other materials and satisfactory spectra may not be obtained. In cases such as the latter, mass spectrometry has the further advantage that a temperature gradient can be applied to the sample when it has been loaded onto the mass spectrometer probe. In this way, impurities can often be fractionated from the substance of interest, and a good spectrum obtained of a single component. By 1970, it had been established that vitamin D, (cholecalciferol) undergoes several metabolic transformations before it can affect calcium metabolism in its target tissues such as intestine and bone.-In particular, a metabolite is produced by the kidney which seemed to be obligatory for vitamin D activity. Work by the group of Dr. E. K~dicek,~at the Dunn Nutritional Laboratory, Cambridge, had shown that following intravascular injection, [4-14C,1-3H] cholecalciferol was con- verted, in the kidney, into a more polar metabolite which had lost 3H. Further- more, [4-14C,1-3H]-25-hydroxycholecalciferol (which is produced by 25-hydroxylation of cholecalciferol in liver) is converted by kidney into the same polar metabolite (again with concomitant loss of 3H). The identification of the polar metabolite from the small amount found in animal tissues (<3 ng/g) would have required extraction of several kg of tissue. However, the synthesis of the metabolite by homogenRtes of kidney provided a method which allowed isolation of 63 pg of the metab~lite.~ The molecular ions of the metabolite and its TMS ether at mi-?416 and 632, respectively, established that two additional oxygen atoms had been inserted in the cholecalciferol molecule in the form of two hydroxy groups.Cleavage of the C(24)-C(25) bond gave intense peaks at m/z 59 and 131 in the spectra of the free steroid and the TMS ether respectively (Figure l), confirming the presence of the hydroxy group at C(25). The spectrum of the free steroid has peaks at 287 and 269 corresponding to loss of the side chain, followed by the elimination of water. This indicates that the third hydroxy group [like that at C(3), originally present in cholecalciferol itself] is in the steroid nucleus.The mass spectrum of cholecalciferol, 25-hydroxycholecalciferol, and indeed all molecules so far examined with the vitamin D conjugated triene system, have intense peaks at m/z 136 and 118 (m/z 136 -H,O), and in the case of their TMS ethers at m/z 208 and 118 (m/z 208 -trimethylsilanol). The peaks at mi; 136 and 208 have been attributed to the fragment formed by the A ring plus I J. W. Blunt, H. F. DeLuca, and H. K. Schnoes, Biochemistry, 1968, 7. 3317. M. H. Haussler, J. F. Myrtle, and A. W. Norman, J. Biol. Cliem.. 1968. 243, 4055. 3 D. E. M. Lawson, P. W. Wilson, and E. Kodicek, Nature, 1969, 222, 171. D. R. Fraser and E. Kodicek, Nature, 1970, 228, 764. D. E. M. Lawson, D. R. Fraser, E.Kodicek, H. R. Morris, and D. H. Williams. Nature, 1971, 230, 228. I32 Williams 59 287 M?= 416. 269 = 287-H20 m/z 134 = 1512 -HzO 2 -2H20 131 MS M?= 632 Figure I Frugmenrution of I ,25-dih~droxyc.hole~~~l~lser~land its rri- TMS derivurive C(6), C(7), and C(19) [cleavage of the C(7)-(8) bond].6 The mass spectrum of the metabolite had intense peaks at mi-. 134 and 116, and that of the TMS ether derivative at m/z 206 (Figure 1). The mass spectrum of a mixture of the TMS ether and the deuteriated TMS ether of the metabolite had intense peaks at m/z 206 and 21 5 ('H(6)-206), and supported the interpretation that the peak at m/z 206 was due to the A-ring plus C(6), C(7), and C(19). We interpreted the difference for the metabolite and cholecalciferol (or their TMS derivatives) of two mass units be- tween m/z 134 and 136, or m/z 206 and 208, as being caused by the elimination of water or trimethylsilanol, respectively.This interpretation demands that the third hydroxy group is in the A-ring plus C(6), C(7), and C( 19).Since the U.V. absorption of the metabolite and cholecalciferol are not only similar in shape and intensity but also in ;Imax (269 nm vs. 266 nm), it is concluded that the vitamin D triene chromophore is present in both compounds. Therefore, hydroxylation in the kidney must have occurred at C(l), C(2), or C(4). Since the hydroxylation in kidney occurs with loss of tritium from C(1) (and, in particular, from C(l0r) since the T. Suda, H. F. DeLuca, H.K. Schnoes, G. Ponchon, Y.Tanaka, and M. F. Holick. Biochemistry, 1970, 9, 29 17. 133 Structural Studies on Bio-active Molecules tritium had originally been introduced with 85% stereoselectivity into the 1a-position7), it was concluded that the metabolite was 1a,25-dihydroxycholecalciferol.Note that this conclusion calls on the fact that hydroxylation of hydrocarbon chains normally occurs via oxygen insertion into the appropriate C-H bond, i.e. it occurs with retention of configuration. The structure of the metabolite has subsequently been confirmed by synthesis. The metabolite is a hormone controlling calcium metabolism. It is of clinical importance, for example when renal insufficiency causes problems in calcium metabolism. 3 The Structure Elucidation of Unusual Linear Peptides In the case of a structure which is not only unknown but also novel, it is rare that mass spectrometry alone will provide extensive structural information.This is because the fragmentation patterns of complex molecules are enormously varied, and are more often rationalized with the value of hindsight rather than being used to determine structure. A notable exception is the sequence determination of linear peptides.8 It is established that such molecules, when converted into their N-acetylpermethyl derivatives (l), fragment at each amide bond in a relatively reproducible manner. This cleavage, occurring with charge retention by the N- terminal portion is shown below; it has arbitrarily been selected between residues 3 and 4,but occurs also at the other peptide bonds.R' R2 R3 Rd RS I I I I I MeCON-CH-CO-N-CH-CO-N-CH-CO-N-CH-CO-N-CH-C02Me1 I I I IMe Me Me Me Me R' R2 R3 RL R5 I I I +.I I MeCO-N-CH-CO-N-CH-CO-N-CH-CzO N-CH-CO-N-CH-C02Me I I I I I Me Me Me Me Me This fragmentation, shown above for the case of electron impact (EI) mass spectrometry, also occurs under chemical ionization (CI) conditions. Given the masses of the twenty common amino acids, the ions corresponding to fragments containing the amino acids associated with R', R'R', R'R2R3, etc. are sufficient to sequence the peptide.' The method is limited (in EIMS) to peptides containing up to ca. 10 amino acids, and requires 225nmol of peptide. In our own P. A.Bell and E. Kodicek, Biochem. J., 1970, 116, 755. H. R. Morris, D. H. Williams, G. Midwinter, and B. S. Hartley, Biochem. J., 1974, 141, 701. I34 Williams laboratory, and others, it has been used to sequence, or partially sequence, proteins. The proteins are broken into suitable fragments by enzymic digestion, and the fragments sequenced as above. Long sequences are then generated by repeating the process with enzymes of differing specificity, and overlapping (by means of portions of common sequence) all the fragments so ~btained.~However, the same results can be achieved with comparable or greater efficiency by solution methods. The unique advantage of mass spectrometry in this area lies in studying unusual linear peptides.The unusual features include the identification of naturally occurring N-terminal blocking groups (which preclude the use of the Edman degradation). For example, an N-terminally blocked peptide was isolated via enzymic digestion of an alcohol dehydrogenase from Drosophila melunogaster N-11. It was acetylated with (CD3C0)20 prior to permethylation; the mass spec- trum of the product established the structural unit CH,CO-Ser-Phe-." Thus, the natural blocking group in the enzyme is determined to be acetyl; being already present in the enzyme, it precludes further reaction at the N-terminus with the deuterated analogue of acetic anhydride. A more complex example is found in the recent determination of the structure of an unusual peptide isolated from larvae of the sawfly.l1 The poisoning of cattle by ingestion of larvae of an Australian species of sawfly (Lophyrotoma interruptu) is a serious problem in several of the grazing areas of Queensland. 2*1 Generally, the disorder occurs in late winter or spring when there is a heavy infestation of the silver-leaf ironbark tree (Eucalyptus rnelunophloia), the principal host of the sawfly larvae. Large accumulations of dead larvae may collect on the ground. Animals eat such material, and develop a considerable liking for it. It has been suggested that it may act as a protein source. However, the end result is poisoning, and critically affected animals usually die within two days. The LDloo of the essentially pure toxin is 2 mg/kg. Amino acid analysis shows the presence of the following amino acids (molar ratios given in parenthesis): Asp (1.89), Glu (1.93), Ala (1 .OO), Val (1.16), Ile (0.82), Phe (0.92) (taking Ala = 1.OO).Initial work, relying on the principle of electron-impact sequencing outlined above established the presence of benzoyl as an N-terminal blocking group, and led to a partial sequence PhCO-Ala-Phe-Val-Ile . . . However, a typical problem of EI sequence determination, that of the higher mass ions being of low or negligible abundance (a particularly severe problem in this case), precluded extension of the sequence. Fortunately, at this period of the study, the technique of fast atom bombardment mass spectrometry (FAB MS) became available.l4 In this technique, a few pg (say 1-lOOpg, depending on the amount of sample available, and the suitability of the compound for the technique) of sample are dissolved in A.Auffret, T. J. Blake, and D. H. Williams, Eur. J. Biochem., 1981, 113, 333. lo A. D. Auffret, D. H. Williams, and D. R. Thatcher, FEBS Leff.,1978, 90, 324. I I D. H. Williams, S. Santikarn, F. DeAngelis, R. J. Smith, D. R. Reid, P. B. Oelrichs, and J. K. MacLeod, J. Cliern. Soc., Perkin Trans. I, 1983, 1869. IZ F. H. S. Roberts. Queensf. Agric. J., 1932, 37, 41. l3 L. L. Callow, Queens/. Agric. J., 1955, 81, 155. l4 M. Barber, R. S. Bordoli, R. D. Sedgwick, and A. N. Taylor, J. Chem. SOL..,Chem. Commun., 1981, 325. 135 Structural Studies on Bio-active Molecules a viscous, low-volatility, polar solvent such as glycerol or thioglycerol. The solu- tion, on a mass spectrometer probe tip, is bombarded with xenon atoms of 4-10 keV translation energy.Although the technique is not a panacea for non- polar molecules, polar molecules in the molecular weight range (say) 300-5000 daltons are projected (as ions) into the gas phase with astonishing efficiency. These ions, normally MH' in positive ion spectra, or [M -HI-in negative ion spectra (M being the molecule of interest), are then detected in the mass spectrometer in the usual way. In FAB spectra, MH+ (or [M -HI-} are normally of high abun- dance, with fragment ions being of lower abundance. The observed fragmentation processes correspond to low energy cleavages and, as for CI, give important sequence information in the case of linear peptides.For example, underivatized peptides give molecular weight information that is readily obtained up to 3000 daltons; and, with the appropriate apparatus and technique, to 5000 daltons. In favourable cases, underivatized peptides of up to ca. 15 residues have been shown to give sequence information. Some of the most useful fragmentations are given in (3). H R R R I I H(N-c"-c~)x~~-co N CH--CO-~N--CH-cO)y-~-~~-~~~H71'H I H H u '.-HH (iv) (iiil (iil (i) Where the horizontal line leaving the 'wavy' line does so to the left, these fragmentations occur with charge retention by the N-terminal fragment; where this horizontal line is to the right of the 'wavy' line, charge retention is by the C-terminal fragment.The symbol#(or<) indicates that the cleavage is associated with hydrogen migration to the charged fragment. The syrnbol'ifi'indicates that the cleavage is sometimes observed with associated hydrogen migration, and some- times without. The mass differences associated with these cleavages have been tabulated. These points are illustrated by the positive and negative ion FAB spectra of the toxin produced in Figures 2 and 3, respectively. Cleavage (ii) [see (3)] gives ions corresponding to the acylium ions (4), occurring at m/z 105, 176,323,422,535,650, 765, and 894 (and MH' at 1040, Figure 2) and indicate the sequence PhCO-Ala- Phe-Val-Ile-Asp-Asp-Glu-Gln;or the C-terminal Gln could equally be replaced by the isomeric amino acid carrying the amide on the backbone carbonyl group and the carboxy function in the side chain [iso-Gln; see (5) and (6)].D. H. Williams. C. V. Bradley, S. Santikam, and G. Bojesen, Biochen?.J., 1982. 201, 105 136 Williams .'j .. 0 U* I w,u. I -_=- _--_ - 0. c 137 Structural Studies on Bio-active Molecules 138 Williams CH,-CONH,I CH2-co2H F"2 p R~=O ~N-CH-CO,H t;-CH-CONH,H (4) (5) C -ttrminal Gh (6) C -terminal iso-Gln Fragments formed via cleavage (i) [see (3)], and corresponding to fragment ions of the general formula (7), at m/z 667, 782, and 911 (in conjunction with MH+, Figure 2) confirm the nature of the three amino acids constituting the C-terminal portion.(RCONH~)H+ (H~NR)H+ RCONH-(7) (8) (9) Those sequence ions which contain the C-terminal portion of the peptide are formed by cleavage (iv) [see (3)) Cleavage (iv) gives rise to protonated amine fragments (8), which occur at m/z 865,718, 619, and 506 in Figure 2; they indicate the sequence X-Phe-Val-Ile-Y, where the masses of X and Y are in accord with the sequence proposed above. The negative ion FAB spectrum (Figure 3) shows an extremely abundant (M-H) ion at m/z 1038. A series of sequence ions corresponding to cleavage (i) [see (3)], and most generally expressed in terms of the anion (9), occur at m/z 338, 437, 550, 665, 780, and 909. These, together with the molecular weight information, indicate the sequence X-Val-Ile-Asp-Asp-Glu-Gln (or, as before, the C-terminal residue may be iso-Glu). These data attest to the power of FAB MS not only in determining the molecular weights of relatively large polar molecules, but also in sequence determination of peptides (which is especially valuable when they are N-terminally blocked).The above established sequence might be regarded as giving a 'structure' if the peptide were a product of ribosomal synthesis of a peptide. In such cases, it can be reasonably assumed that Asp, Glu, and Gln are incorporated as such, and not as their iso-structures; and further, that all the amino acids have L-configuration. However, in our early experiments on the toxin (prior to the availability of FAB MS), an enzymic digestion with chymotrypsin had been attempted, in the ex- pectation that cleavage at the C-terminal side of the Phe residue would occur, thus giving a C-terminal fragment which might conceivably have been sequenced by EI MS.The attempted enzymic digestion failed. One possible explanation for this failure is that one or more of the amino acids in the toxin might have D-stereochemistry. Mass spectrometry cannot throw light on this question, but it can be probed using gas chromatography on a chiral column. The toxin was hydrolysed (6M-HCI at 110°C for 72h), and the resulting amino acids converted into their N- trifluoroacetyl isopropyl ester derivatives; these volatile derivatives were chro- matographed on a chiral GC column ('Chirasil-Val'). Comparison of the retention times of the components with those of authentic samples established that Val, Ile, 139 Structural Studies on Bio-active Molecules one Asp, and a Glu (or Gln) have the L-configuration; and that Ala, Phe, one Asp, and a Glu (or Gln) have the D-configuration.The ambiguity between Glu and Gln arises because Gln is converted into Glu by the total hydrolysis procedure. The above results increased the fascination and importance of the problem. Prior to this study, no peptide isolated from an animal had contained more than a single D-amino acid. In ‘higher’ organisms (the eucaryotes, including the animals), the synthesis of peptides and proteins occurs on ribosomes; in this process, only the 20 standard amino acids, in their L-configurations, are known to participate.However, in lower organisms (the procaryotes, including bacteria), peptides synthesis can, on occasions take place without ribosomes; this type of synthesis is effected on multi-functional enzymes, and can include the utilization of unusual amino acids (e.g. phenylglycine), and amino acids in their D-configuration. Subsequently, we will return to this problem of the procaryotic or eucaryotic origin of the peptide toxin although, unfortunately, we will not answer it! The above data give a further twist to the problem. The occurrence, in the hydrolytic products, of one L-As~, one D-As~, one L-Glu, and one D-GIu gives rise to no less than four combinations of stereochemistry at the C-terminus; viz., .D-Asp-L-Asp-D-Glu-L-Gln, ... L-Asp-n-Asp-D-Glu-L-Gln, .. . D-Asp-L-Asp-L-Glu-D-Gln,.. . L-Asp-D-Asp-L-Glu-D-Gln. Additionally, since the peptide is ‘non-standard’, we have to consider that Asp and Glu may be incorporated not only as they normally are in proteins [(lo); see also (5)], but as their iso-structures, in which the usual side chain becomes part of the peptide backbone [(11); see also (6)].Thus, determination of the amino acid sequence still leaves a total of 64possible structures. The determination of a unique structure will now be considered. CH CO H CH CH CO H I2 l2 --N-CH-CO--N-CH-CO-H H -ASP--G~u-COZH CO, H I I -N -CH-CH2-CO--N-CH-CH2-CH2-CO-H H -is0 -Asp--is0 -Glu-(11) The stereochemistries of the Asp residues were determined first.A possible strategy is to isolate a peptide fragment containing only one of the Asp residues, and then to determine the stereochemistry of that residue. Fortunately, it is known that acid hydrolysis of peptides occurs selectively as Asp residues, and therefore the toxin was subject to relatively mild acid hydrolysis (10% aqueous HCI, 100“C. Williams 1.5h), and the resulting peptides fragments were isolated via preparative HPLC. The molecular weights of the isolated fragments were then determined by FAB MS.' ' One fraction contained abundant ions only at m/z 493 and 247 in the positive ion FAB spectrum. These ions correspond to MH' signals from the peptides Phe-Val-Ile-Asp and Ile-Asp. Since it can be argued reasonably that one number does not guarantee securely these sequences, a few pg of the fraction was subjected to N-acetylation (Ac,O-H,O, 1:4, room temperature 4 h), and to esterification (1 O/O HCI in MeOH, room temperature, 10 h).The resulting deriva- tives, upon FAB MS analysis showed MH' peaks at m/z535 and 289; and m/z521 and 275, respectively. These peaks correspond to the anticipated mono-N-acetyl derivatives ( +42 daltons) and dimethyl ester ( +28 daltons) derivatives, and there- fore support the structure assignments. Note that these reactions occur in good yield, and the excess of reagents can be removed under vacuum, thus avoiding any tube-to-tube transfers or other manipulations in the derivatizations. Therefore, functional-group determination can be carried out on a pg scale by FAB MS; the importance and power of this technique is worthy of emphasis. The above peptide fraction, containing only the aspartic acid residue at position 5 (Asp', numbering amino acids of the toxin from the N-terminus) was then hydrolysed.When volatile derivatives (prepared as given earlier) of the resulting amino acids were analysed on the chiral GC column, Asp' was shown to have the D-configuration. Therefore, Asp6 has the L-configuration. The stereochemical assignments of Glu6 and Gln' can be determined if, prior to hydrolysis, Gln' is modified such that the product of this modification does not produce a Glu residue upon hydrolysis. A suitable modification was achieved via a Hofmann degradation,' ' which brings about the conversion RCONH, -+ RNH,.Thus, Gln will afford 2,4-diaminobutyric acid (12) upon hydrolysis, whereas iso-Gln would transiently produce 4,4-diaminobutyric acid (I 3), and hence the ninhydrin-negative 4-oxobutyric acid (14). CH,CH ,NH, CHZCHZ CO,H CH,CH,CO,HI I -ICH 6H/--\ /\ 6"H2N COZH H2N NH2 0 Following Hofmann degradation of the toxin, and acid hydrolysis of the product, amino acid analysis showed the production of (1 2). Moreover, analysis of a volatile derivative of (12) on a chiral GC column established it as possessing the L-configuration. Therefore, the C-terminal residue is Gln (and not iso-Gln) and has the L absolute configuration. It follows that the penultimate residue, producing Glu upon hydrolysis, has the D-configuration.The ambiguity between normal and iso-residues at positions 5, 6, and 7 in the toxin was resolved by successively exposing the toxin to (i) Hofmann degradation and (ii) Ac,O-pyridine-D,O. The latter treatment causes racemization of, and 141 Structural Studies on Bio-active Molecules 0 R’ 0 R’ \c/ NC/ I i f ((7H2)n R-C-N-CH-COOH H I L QI DO-D t ,(CH21nCOR’Pyridine Figure 4 The chemical basis for racemization ox and deuterium incorporation into, amino acids possessing a free carboxy group aiiached to the a-CH deuterium incorporation into, the a-position of those amino acids which have free carboxy groups attached to the a-carbon,16 i.e. those in-chain amino acids which are incorporated as iso-structures [see (1 1)], and normally constituted C-terminal amino acids [see, for example, (3,which has already been shown to be present in the toxin].The chemical basis for the racemization and deuterium incorporation is given in Figure 4. When the free carboxy group is attached to the a-CH, the mixed anhydride formed upon reaction with Ac,O is subjected to intramolecular nucleo- philic attack by the amide function incorporating the carbonyl group of the adjacent amino acid. As a result, the acetate anion is displaced, and a 5-membered ring oxazolone is formed (Figure 4).In this ring system, the hydrogen of the a-CH group is relatively acidic, being not only adjacent to a carbonyl group, but also to an olefinic double bond.In the presence of a base (pyridine) and D20, it will therefore be replaced by deuterium. Additionally, the intermediate enolate anion is sp2 hydridized at the original a-CH group, and a further consequence of deuterium incorporation is racemization at the a-carbon of the amino acid. Thus, base-catalysed hydrolysis of the oxazolone (Figure 4) produces an amino acid which is not only deuteriated at the a-position, but also racemic. Toxin treated in the above manner yielded 2,4-diaminobutyric acid which incorporated deuterium and was racemized. Such deuterium incorporation and racemization was not observed for the Asp residues, nor for the Glu residue. Therefore, these residues are not incorporated as iso-structures. The complete structure of the toxin is therefore that reproduced in Figure 5.In summary, the slowly uncovered structural features of the toxin, which dic- l6 G. N. Holcomb, S. A. Jones, and D. N. Ward, Biochemistry, 1968, 7, I2Y I. 142 Williams CH CH CH CO H CO2H CO2H CONH, ycq, CH 3, 2 CH, I IHCCH, 3,2 CH, (CH,), (CH,),CH3 iH2IIII II I C,H,CO-NCHCO-NCHCO-NCHCO-NCHCO-NC HCO-NCHCO-NCHCO-NCHCO, H H H HH H H H H 0 0 L L D L 0 L Figure 5 Structure of the peptide toxin from the sawfly Lophyrotoma interrupta tated those methods which could successfully tackle the problem are (i) the pres- ence of the N-terminal blocking group, precluding the use of the Edman degradation, (ii) the presence of D-amino acids, precluding the use of enzymic digestion, (ii) the presence of a linear peptide, allowing the application of EI and, particularly in this case, FAB MS sequencing.Note that stereochemical problems are rarely amenable to analysis by mass spectrometry, and the successful use of a chiral column required standards of known absolute configurations. Nuclear magnetic resonance (n.m.r.) was not called upon significantly in the structure elucidation because the structural sub-units were of established types. Additionally, ‘communication’ between these sub-units, although in principle possible by means of the nuclear Overhauser effect (NOE, see later), could not be successfully exploited in the above example. In the linear molecule, the con- formation was too floppy, and the molecular weight in ‘no-man’s land’, between lower and higher values which could have been exploited.The biosynthetic origins (procaryotic or eucaryotic) of the toxin remain an enigma. It has recently been established that a fungus lives symbiotically with the sawfly larvae, in the gut.” It is possible that the fungus is responsible for the production of the D-amino acids, or even later stages of the biosynthetic pathway; but this is speculation, and a definitive conclusion will have to await further experimentation. 4 The Structure Elucidation of Unusual Monocyclic Peptides The previously discussed sequencing of (unusual) linear peptides by mass spec- trometry is a useful general method. The strategy is not directly applicable to monocyclic peptides because fragmentation of an amide bond produces a linear peptide and no fragments.It might be thought that the internal energy of the linear product would be sufficient to produce fragments via further amide bond cleav- ages; but such is not generally the case in FAB spectra (low internal energy for ions). Further fragmentation often occurs in EI spectra, but the spectra are more difficult to interpret, and high resolution data are usually needed (see later). An alternative general strategy has therefore to be sought. One approach is to cleave chemically an amide bond of the monocyclic peptide, and then use EI and/or ’’ Professor H. Kleinkauf, Technical University of Berlin, personal communication. 143 Structural Studies on Bio-active Molecules FAB MS sequencing of the linear product.Such an approach was applied successfully to determine the structure of a peptide toxin from the blue-green alga, Microcystis aeruginosa. 8,1 Since the end of the last century, heavy blooms of the blue-green alga Microcystis aeruginosa have been known to cause deaths among livestock con- suming heavily contaminated water. The lethal doses (LD,,) of the purified toxins are ca. 50pg/kg for mice on intraperitoneal injection.20 The toxic peptides of M. aeruginosa are normally contained within the algal cells, and are released only when the cell is damaged. It has recently been concluded” that an increase in liver damage was indicated in a human population during the period of a bloom of M. aeruginosa in the water-supply reservoir.Although enzymatic experiments and amino acid analysis (equimolar ratios of L-leucine, D-glutamic acid, erythro-p-methyl-D-aspartic acid, L-alanine, and D-alanine)2 established the peptidic nature of the toxin, no sequence ions were seen in the FAB mass spectrum (which established a inolecular weight of 909 daltons). Additionally, it was shown that the peptide lacked a free basic N-terminus because the molecular weight was unchanged following treatment with Ac,O-H,O. The lack of fragmentation in the FAB spectrum, and the absence of a free N-terminus, together suggest the possibility of a cyclic peptide. However, it is clear that the identified amino acids fall far short of accounting for the observed molecular weight. Since the toxin was available in a quantity of ca.2mg, it was possible to throw light on the non-standard amino acid components of the toxin by high-field ‘H n.m.r. at 400 MHz. Before discussing the results of the n.m.r. study, it is necessary to outline the principle of an n.m.r. phenomenon which has assumed great importance recently. It is the nuclear Overhauser effect (NOE). Although this effect has been known for many years, and was first exploited in organic chemistry in the 1960s, it is the advent of difference spectroscopy which has resulted in its widespread use during the last few years2, When nuclei which behave like bar magnets are placed in a magnetic field, they can occupy a high- and a low-energy state. A nuclear magnetic resonance absorption signal is obtained from such nuclei when, upon supplying electro- magnetic radiation of a suitable frequency, v, there is a net passage of nuclei from the low to the high energy state.Nuclei may pass back from the high to the low energy state by a process known as relaxation. Proton nuclei are normally relaxed by a mechanism which involves neighbouring protons. The effectiveness of such neighbouring protons in bringing about relaxation depends upon Y -6, where Y is the internuclear distance between the proton being relaxed and the proton effecting the relaxation. ‘8 S. Santikarn, D. H. Williams, R. J. Smith, S. J. Hammond, D. P. Botes, A. Tuinman, P. L. Wessels, C. C. Viljoen, and H. Kruger, J. Chem. Sor., Chem. Cornmun., 1983, 652.l9 D. P. Botes, A. Tuinman, P. L. Wessels, C. C. Viljoen, H. Kruger, S. Santikarn, D. H. Williams, R. J. Smith, and S. J. Hammond, J. Chem. Soc., Perkin Trans. I, in press. *O 1. R. Falconer, A. M. Beresford, and M. T. C. Runnegar, Med. J. Ausf., 1983, 511. 2’ D. P. Botes, C. C. Viljoen, H. Kruger, P. L. Wessels, and D. H. Williams, Toxiron, 1982, 20, 1037. 22 See, for example, J. K. M. Sanders and J. D. Mersh, in ‘Progress in Nuclear Magnetic Resonance Spectroscopy’, Vol. 15, pp. 353400, Pergamon Press, Oxford, 1983. I44 Williams If the intensity of the resonance of one proton (H') is normally I, then if r1,2is relatively small (as a useful guide for our present purposes say, <0.3 nm), the effect of irradiating a second proton (H2)before recording the intensity of the resonance of H' is to change its intensity to I', i.e.I # 1'.This change in intensity is called the NOE. The effect is indicated schematically in (15)-(18). Three hydrogen nuclei in a molecule are indicated in (15). The signals which arise in the 'H n.m.r. spectrumof (1 5) in the absence of pre-irradiating any of the protons is given in (1 6). Upon pre-irradiation of H' at its resonance frequency vl, the 'H n.m.r. spectrum is modified to (1 7). Note that in (17), the resonance of H' has disappeared, because its resonance is 'saturated', i.e. the populations of upper and lower energy levels are equalized, by the pre-irradiation. The H2 resonance intensity is reduced due to the NOE. The NOE difference spectrum [NOEDS, (18)] is obtained by on-line computer subtraction of (1 7) from (1 6).H' H3 H2P (16) Note that since H3is distant from HI, no observable NOE occurs for H'. The value of the difference spectrum is evident for molecules containing many protons; the NOEDS then contains only signals due to the irradiated proton HI, and less intense signals due to those protons which are proximate to H'. Such experiments are extremely valuable in determining structures of molecules in solution. Analysis of the 'H400MHz spectrum of the toxin in D20established from chemical shift values and spin decoupling experiments the presence of the N-methyldehydroalanine (19) and of units (20) and (21). The connectivity between (20) and (21) was established to occur via the C-C bond indicated by a broken line (using NOES, determined in [2H,]DMS0 solution).Thus, pre-irradiation of the 65.53 resonance led to a 12% reduction in the intensity of the 61.66 resonance {normally expressed in the concise form [5.53]--+ 1.66 (-12%)). Similarly, [6.27]+5.48 (-15%) and [5.48] +6.27 (-15%). 145 Structural Studies on Bio-active Molecules f\NACAI II //O N CH, 0 The above combination of the two units provides an excellent example of how connectivity can be established from NOES when evidence is not available from coupling constants (because four or more o-bonds occur between adjacent protons). Note that in the above case, the n.m.r. experiment does not determine the nature of the electronegative substituents (OMe, NH, C=O) on the chain; but only that some electronegative substituent must be attached to the carbons bearing the protons at 63.42, 4.42, and 2.97.The nature of these substituents was established from fragment ions occurring in the EI mass spectrum of the toxin and/or permethylated toxin. These spectra afforded ions at m/z I35 [PhCH,CH=OMe, with or without permethylation], m/z 326 [(22), following permethylation] and m/z 272 [(23), following permethylation]. The atomic compositions of the fragments (22) and (23) are supported by high resolution measurements. When the perme- thylation procedure utilises CD31instead of CH31, the m/z values of (22) and (23) shift to 329 and 275, respectively. Thus, the nitrogen atom is methylated in the deriva tiza tion.Ph{ i OMc Me” 0 Ph+” OMc +NH Mc’ m/z 326.2112 m/z 272.2011 (calc. 326.21201 (calc. 272.2014) In view of the presumed cyclic nature of the toxin, the hydrolysis of a single amide bond in the peptide might be attempted by limited acid hydrolysis. Since the N-methyldehydroalanine unit (19) might be modified by even quite mild acid treatment (Michael addition or enamine hydrolysis), it was deemed wise to reduce this residue prior to the pulse acid hydrolysis treatment. The reduction of (19) was effected by conjugate addition of hydride ion, from NaBH,, to give N-methylalanine. This procedure has the convenience that NaBD, can alterna- tively be used, and thus provide an isotopic label for the reduced residue. The Williams reduced toxin was pulse hydrolysed (6M-HCl, 5 min, 100°C) and a product of molecular weight 812 daltons (by FAB MS) isolated.Clearly, if the toxin has been ring opened, a portion of it has also been lost. Indeed, when the two-step procedure was repeated using NaBD,, a product of molecular weight 812 daltons was again obtained. The product therefore corresponds to reduction (+ 2), ring opening ( + 18), loss of N-methylalanine ( -85) and methanol [ -32 daltons by elimination from (20)]. Micro-derivatization experiments followed by FAB MS (see earlier) show the product to contain one amino and three carboxy groups. Acetylation and permethylation of the product, followed by El MS sequencing established the sequence Ala-Leu-‘Glu’-A1a-paaf-‘Glu’,where Pas' is the novel P-amino acid (20)/(21) minus methanol; and ‘Glu’ represents Glu or P-Me-Asp (or their iso- structures), since these units are all isomeric.When the linear peptide was subjected to one cycle of the Edman degradation, N-terminal alanine was cleanly removed (as shown by FAB MS);23 and chiral column GC-MS experiments (see earlier) on the amino acids derived from the product of Edman degradation established the absence of D-Ala and the presence of L-Ala. As expected, a second Edman de- gradation removed L-Leu, but the third Edman cycle failed. Evidently, the third amino acid, in the linear peptide, is not incorporated as a standard amino acid. However, in the product of the second Edman cycle, the amino group of the third amino acid is present as a free, basic group.This group can therefore be converted into the sulphonamide by reaction with dansyl chloride. Acid hydrolysis of the product produced the dansyl derivative of /3-methylaspartic acid.’* Since this amino acid is not incorporated as a standard amino acid in the linear peptide, it must be incorporated as P-methyl-iso-aspartic acid. Note that this does not neces- sarily imply that this acid is present as the iso-structure in the original toxin; since the Asp -,iso-Asp conversion can be brought about by either mild acid or mild base, the isomerization could have occurred in the course of the conversion of the toxin into the linear peptide. Since residue -3 on the linear peptide is 8-Me-iso-Asp, it follows that the C-terminal residue is Glu (or iso-Glu).It remains to establish the points of attachment of the N-methyldehydroalanine residue which, following NaBH, reduction of the toxin, was lost during pulse hydrolysis. This was achieved” by exposing the reduced toxin to a milder pulse hydrolysis (moist trifluoroacetic acid at room termperature for 24 h). The negative ion FAB mass spectrum of this hydrolysis product showed [M -HI- at m/z 928, indicating that the dihydro-toxin (molecular weight 9 11 daltons) had undergone simple hydrolytic addition of water, without loss of an amino acid residue. More- over, N-terminal amide sequence ions [see (3), cleavage (i)] in this spectrum oc- curred at m/z 856 and 713, establishing that N-methyl-Ala is attached to a Glu (or iso-Glu) residue (24).19 23 C.V. Bradley, D. H. Williams, and M. R. Hanley, Biochem. Biophys. Res. Commun., 1982, 104, 1223. 147 4\ Structural Studies on Bio-active Molecules -OH Figure 6 Structure of the pepptide toxin BE-4 ,from Microcystis aeruginosa The presence of iso-Glu in the toxin was indicated not only by the change in chemical shift of the a-CH proton upon titration of the carboxy group of the glutamic acid residue, but also by the chemical method outlined in Figure 4.19 Similar chemical experiments on the unreduced toxin demonstrated that p-methyl-Asp is incorporated as the iso-structure in the toxin.’ Thus, a monocyclic structure for the toxin can be completed by attachment of the carbonyl of N-Me-dehydro-Ala to the amino group of D-Ala (Figure 6).Note that in the published work,I8 only a partial structure for the toxin variant BE-4 had been assigned, and that the tentative attachment’ of N-Me-dehydro-Ala as an exocyclic residue is now shown’’ to be incorrect. Experiments to determine the stereochemistry of the asymmetric centres in the /I-amino acid have yet to be carried out. As in the case of the sawfly liver toxin, note how complex details of stereo- chemistry and connectivity can be proved by use of GC, enzymic methods, and MS, so long as the sub-units have standard structures. Where a sub-unit of novel structure was involved, it was necessary to turn to high field ‘H n.m.r.; and in particular, to spin decoupling and NOES.5 The Structure Elucidation of Polycyclic Peptides; The Vancomycin Group of Antibiotics and their Mode of Action It will be clear from the foregoing two sections that attempted structure elucidation by mass spectrometry of polycyclic peptides is likely to fail. Fragmentation would require multiple bond rupture, which may not be feasible energetically. Such was our experience with the vancomycin group of antibiotics, which are a series of tri- and tetra-cyclic peptides. Thus, the r6le of mass spectrometry was largely in the determination of molecular weights; both of the antibiotics, and of fragments obtained by chemical degradation. By far the most important r6les in the structure 148 Williams elucidation of van~omycin~~-~' were played by chemical and rist~cetin'~-~~ degradation, X-ray crystallography, and H n.m.r.Most of these experiments have been reviewed previ~usly,~~ and therefore it will suffice here to state the general principles involved. The structure of vancomycin was partially determined by the characterization of fragments produced by degradati~n,~~and some of the bonds broken in hydrolytic experiments then 'reconstituted' by use of spin decoupling experiments and NOES (and other tech- nique~).~~However, the numbers of asymmetric centres of undetermined stereo- chemistry were sufficiently large to preclude a total solution of the problem by these methods. The structure of a degradation production of vancomycin, CDP-I, was determined by X-ray crystallography.26 It was initially assumed that in the mild conditions for conversion of vancomycin into CDP-I (80 "C,3 days, pH 4.2), the only chemical change was RCONH2-+RC02H.However, it has been shown subsequently that a chlorine-containing aromatic ring undergoes a rotation of ca. 180 0;27 and that an Asn residue of vancomycin isomerizes to an iso-Asp residue in CDP-I.28 The currently accepted structure of vancomycin is reproduced in Figure 7. OH -0.\ CH3 OH Figure 7 Proposed structure of vancomycin 24 G. A. Smith, K. A. Smith, and D. H. Williams, J. Chem. Soc., Perkin Trans. I, 1975, 2108. 2s D. H. Williams and J. R. Kalman, J. Am. Chem. Soc., 1977,99, 2768. 26 G. M. Sheldrick, P. G. Jones, 0.Kennard, D.H. Williams, and G. A. Smith, Nature, 1978, 271, 223. z7 M. P. Williamson and D. H. Williams, J. Am. Chem. Sor., 1981, 103, 6580. 28 C. M. Harris and T. M. Harris, J. Am. Chem. Soc., 1982, 104, 4293. 29 D. H. Williams, V. Rajananda, and J. R. Kalman, J. Chem. Soc., Perkin Trans. I, 1979, 787. 30 J. R. Kalman and D. H. Williams, J. Am. Chem. Soc., 1980, 102, 897. 31 D. H. Williams, V. Rajananda, G. Bojesen, and M. P. Williamson, J. Chem. Soc., Chem. Commun., 1976, 906. 32 C. M. Harris and T. M. Harris, J. Am. Chem. SOC.,1982, 104, 363. 33 D. H. Williams, V. Rajananda, M. P. Williamson, G. Bojesen, in 'Topics in Antibiotic Chemistry', ed. P. Sammes, Horwood, Chichester, UK, 1980, Vol. 5, Part B. I49 Structural Studies on Bio-active Molecules OH I PH HOHzC OH P Figure 8 Proposed structure of ristoceh A The structure elucidation of a second member of the vancomycin group, ris-tocetin A, has not been aided directly by X-ray crystallography because no crystals suitable for structure determination have been obtained.However, there are remarkable similarities between many of the proton chemical shifts, coupling constants, and NOES observed in the ‘H n.m.r. spectra of vancomycin and ris-tocetin. Hence, using both and n.m.r. technique^,^^.^^ it was possible to propose a structure for ristocetin A.31The first structure proposal31 has been revised by inversion of the stereochemistry at the N-terminus (as a result of chemical degradative experiment^^^), and the currently accepted structure is reproduced in Figure 8.Much definitive work on the structure of the saccharide portion of ristocetin A was carried out by Sztaricskai et al.34 Some general features of the structures reproduced in Figures 7 and 8 are (i) both are heptapeptides in which three rings are made by phenol oxidative coupling (two by C-0 bond coupling, and one by C-C bond coupling to give a biphenyl unit); a fourth ring in ristocetin A is formed by a third C-0 bond coupling, (ii) the stereochemical arrangements in the ‘left-hand portion’ (as presented) of both structures, in the aglycone unit, are the same, although ristocetin A has a methylated C-terminus which vancomycin lacks, (iii) the stereochemistries at the a-CH centres of the seven amino acids are the same, and, from the N-terminus are R,R,S,R,R,S,S.Note, 34 See, for example, F. Sztaricskai, A.Neszmelyi, and R. Bognar, TefrohedronLeu.,1980,2983, and references cited therein. 150 Williams /MurNAc // /’I TCNAC I/ o-Ala GLCNAC I D-Ala ID-Ala Figure 9 A portion of the cell wall peptidoglycan in Staphylococcusaureus however, that N-methyl-leucine and asparagine in vancomycin (Figure 7) are replaced by substituted (and cross-linked) phenylglycine units in ristocetin A (Figure 8). The p-hydroxyphenylglycine units which are incorporated into these molecules have been ~ho~n~~*~~ to be derived from tyrosine, and the m-dihydroxylated phenylglycine units to be derived from acetate. The interesting question arises as to how these molecules act as antibiotics.A number of years ago, it was shown that they inhibit the biosynthesis of the bacterial cell wall in gram-positive bacteria. In the absence of antibiotic, a key step in completing a strong two-dimensional structure for the cell wall involves a trans- peptidase enzyme. The function of this enzyme is to attach the N-terminus of a (Gly), residue to the carbonyl carbon of the D-Ala residue of (25) (indicated by underlining). During this process, the C-terminal D-Ala is di~placed,~’ and the new peptide bond between D-Ala and Gly completes the cross-linking of two parallel strands of the cell-wall polysaccharide [Figure 9, which shows one -D-Ala-(Gly),- cross-link (centre) already formed, and a -D-Ala-D-Ala residue (right) prior to cross-link formation to an adjacent strand (to the further right, and not shown)].35 S. J. Hammond, M. P. Williamson, D. H. Williams, L. D. Boeck, and G. G. Marconi, J. Chem. Soc., Chem. Commun., 1982, 344. 36 S. J. Hammond, D. H. Williams, and R. V. Nielsen, J. Chem. SOC.,Chem. Commun., 1983, 116. 37 K. Izaki, M. Matsuhashi, and J. L. Strominger, J. Biol. Chem., 1968, 243, 3180. 151 Structural Studies on Bio-active Molecules 0---I It was shown3 * -40 that cell-wall peptide precursors terminating in -D-Ala-D-Ala accumulate when bacterial cell growth is inhibited by vancomycin and ristocetin. -43In particular, Perkins and co-~orkers~~ showed that cell-wall precursor peptides terminating in -D-AIa-D-Ala form strong 1:l complexes with both van- comycin and ristocetin.Thus, if the hatched area in (25) represents vancomycin or ristocetin, it may reasonably be proposed that they exert their antibiotic action by binding, as indicted in (25), to cell wall precursors terminating in D-Ala-D-Ala. It is the molecular basis for this interaction that we should now be able to study. Since lH n.m.r. parameters (e.g. chemical shift, NOE) are sensitive to intermolecular association, the interaction between the antibiotics and a simple cell-wall analogue, Ac-D-Ala-D-Ala, might usefully be proved by the n.m.r. method. The first studies which utilized this concept were carried out by Brown et a1.44*45However, at the time of this work, the structures of the antibiotics were not known, and the NOEDS technology not yet available.Yet the concept represented an important advance in the area, and the useful discovery was made that the methyl resonance of the C-terminal alanine residue was shifted appreciably upfield upon complex formation with vancomycin. This indicated that in the complex, this methyl group lay over the face of an aromatic ring of vancomycin. Once the structures of the antibiotics were known, it was possible to prove the molecular basis for the antibiotic/cell-wall analogue interaction by using two powerful methods. First, the temperature dependence of amide NH-resonances. These are ca. 6-10 x p.p.m./"C to high field in DMSO solution, when such an NH is exposed fully to the DMSO solvent.This temperature dependence arises 38 P. E. Reynolds, Symp. Soc. Gen. Microhiol., 1966, 16, 47. 39 D. C. Jordan, Biochem. Biophys. Res. Commun.. 1961, 52, 403. 40 C. H. Wallas and J. L. Strominger, J. Bid. Chern., 1963, 238, 2264. 41 A. N. Chatterjee and H. R. Perkins, Biochem. Biophys. Res. Commun., 1966, 24 489. 42 M. Nieto and H. R. Perkins, Biochem. J., 1971, 123, 773. 43 M. Nieto and H. R. Perkins, Biochem. J., 1971, 123, 789. 44 J. P. Brown, J. Feeney, and A. S. V. Burgen, Mol. Pharm.. 1975, 11, 119. 45 J. P. Brown, L. Terenius, J. Feeney, and A. S. V. Burgen, Mol. Pharm.. 1975, 1 , 126. 152 Williams due to the breaking of :S=O-..H-N: hydrogen bonds with increasing tem- perature. In contrast, if an amide NH is involved in an intermolecular hydrogen bond, it is not exposed to solvent, and a much smaller temperature dependence [say (0-2) x p.p.m./"C] results.Using this criterion, and the downfield shifts of amide NH resonances upon intermolecular hydrogen bond formation, it was possible to determine those amide NH-groups of antibiotic and cell-wall analogue which were involved in complex f~rmation.~~*~' Such criteria led, in conjunction with other evidence such as model building (CPK models), to proposals for the interactions shown in Figures 10 and 1 1.46*48 In both these figures, exploded views of the complexes are shown; hydrogen bonds, formed between carbonyl groups of one component and amide NH-groups of the second component, are indicated by broken lines which join the two groups.These are (from left to right in both figures, with the cell-wall analogue group given first), (i) acetyl carbonyl to the C-terminal (seventh residue) NH of the antibiotic, (ii) NH of the C-terminal D-alanine to the carbonyl of the trioxygenated phenylglycine unit (fourth residue) of the antibiotic, (iii) carboxyl oxygen of the C-terminal D-alanine to the NH-group of the fourth residue of the antibiotic, (iv) carboxyl oxygen of the C-terminal ~-alanine to the NH-group of the third residue of the antibiotic and (v) carboxyl oxygen of the C-terminal D-alanine to the NH-group of the second residue of the antibiotic. The residue numbers of the antibiotic (numbered 1 to 7 from the N-terminus) are most readily identified by looking at Figures 10 and 11 in conjunction with Figures 8 and 7, respectively.Note that the proposed interactions allow a hydrophobic interaction of each of the alanine methyl groups with an aromatic ring of the antibiotic: the methyl group of the C-terminal alanine with the trioxygenated aromatic ring of residue 4; and the methyl group of the N-terminal alanine with the biphenyl moiety. The second criterion (intermolecular NOE) can be used to check the validity of the above models. When the two components of either complex are brought together, it is seen that certain protons of the cell-wall analogue are in close proximity to protons of the antibiotics. These proximities are confirmed by irra- diation, in turn, of these protons, and observation of the required NOEs.This use of intermolecular NOEs allows, in principle, the direct mapping of the receptor binding site of a drug. Unfortunately, the method is limited to small receptors, because the receptor has to give an analysable proton spectrum with reasonably sharp lines. We have, however, been able to apply the technique to determine the binding site, in solution, of a DNA tetranucleoside triphosphate for the antibiotic actinomycin D,49 and the method appears to have great potential for future work. 4* J. Kalrnan and D. H. Williams, J. Am. Chem. Soc., 1980, 102, 906. 47 D. H. Williams and D. W. Butcher, J. Am. Chem. Soc., 1981, 103, 5697. 48 D.H. Williams, M. P. Williamson, D. W. Butcher, and S. J. Harnmond, J.Am. Chem. Soc., 1983,105, 1332. 49 D. G. Reid, S. A, Salisbury, and D. H. Williams, Biochemistry, 1983, 22, 1377. 153 Structural Studies on Bio-active Molecules Figure 10 Proposed hydrogen-bonding interactions between the cell-wall peptide analogue Ac-D-Ala-D-Ala (above) and ristocetin A (below,for simplicity, only the glucose unit of the tetrasacchar ide is shown ) 6 Conclusion During the last 20 years, mass spectrometry and nuclear magnetic resonance have evolved to become extremely powerful methods for the elucidation of complex organic structures. In particular, the recent advent of FAB mass spectrometry and 154 Williams Figure 11 Proposed hydrogen-bonding interactions between the cell-wall peptide analogue Ac-D-Ala-D-Ala (above) and vancomycin nuclear Overhauser effect difference spectroscopy is of importance; and it appears that two-dimensional n.m.r.spectroscopyzz is another important advance in extracting structural information from the spectra of complex molecules. It is concluded that if the above techniques are combined with the powerful chromatographic methods of GC and HPLC, and with chemical derivatizations 155 Structural Studies on Bio-active Molecules carried out on a micro-scale, the outlook for the structure elucidation of complex molecules in the future is a very promising one. There is no shortage of challenging and important problems. The challenge is to turn to problems which currently pose great difficulty, e.g. the glycoproteins; and to increase the sensitivity of our methods, so that structures can be solved when only nmol to pmol quantities are available, e.g.in the search for new neuropeptides. Acknowledgements. I wish to thank my colleagues, named in the co-authored references, who have made the described work possible. Additionally, I thank the SERC and the Royal Society for financial support.

ISSN:0306-0012    

DOI:10.1039/CS9841300131

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Crystal Structure Determination: A Critical View By P. G. Jones INSTITUT FUR ANORGANISCHE CHEMIE DER UNIVERSITAT GOTTINGEE, TAMMANNSTR. 4, D 3400 GOTTINGEN, W. GERMANY 1 Introduction Structural information provided by X-ray crystallography has been a cornerstone of chemical research in the last few decades. However, the very importance and widespread use of the technique has created a problem for the chemist, who may not be a specialist crystallographer; how should he critically evaluate published crystal structures? There is an understandable tendency nowadays for crystal structures to be believed implicitly. As an act of faith this is touching, but, as any honest crys- tallographer would admit, many crystallographic papers contain infelicities rang- ing from tendentious statements through over-optimism to downright errors. This article is an attempt to provide the non-crystallographer with a guide to recog- nizing some limitations of published crystal structures, without involving him in detailed theory .The following basic X-ray experiment is assumed: the intensities I and associated standard deviations a(l) are measured with a diffractometer and mathematically reduced to structure factors 14 [and associated a(&)]; these are then used for least-squares refinement of a structural model of the electron density p, leading to refined values of the atomic co-ordinates and thermal vibration parameters (also known as temperature factors). 2 R Values: All Things to All Men? Since crystal structure determinations are essentially quantitative, the question of criteria of accuracy arises.The two usual criteria are (a) R values and (b) estimated standard deviations (e.s.d.’s) of derived parameters such as bond lengths. (These are normally expressed in the form 1.555(6) A, meaning I .555 A with an e.s.d. of 0.006 A).R values are an attempt to express the agreement of observed structure factors F, with F,, those calculated for the refined model. The conventional R value is given by and it is thus clear that the lower the value of R,the better the structure-or is it? Unfortunately, there are several crystallographers’ tricks for artificially reducing R values. To see through these, other factors must be considered; weak reflections, weighting schemes, and data/parameter ratios (‘data’ here means the number.of independent measured intensities-see below). 157 Crysta1 Structure De termination: A Critical View All crystals will show some weak reflections (especially at high diffraction angle 20), characterized by a low ratio of I to a(0. It is general practice to omit from calculations those reflections with I/a(Z)less than a certain threshold value, usually in the range 1.5 to 3 (corresponding to F/cJ(F)3 to 6, since I is proportional to F’). This has three advantages: first, the computing time necessary for refinement is reduced; secondly, the R value is reduced by elimination of those reflections associated with high a; and thirdly, the problem of how to treat reflections with negative I is avoided.The first is often necessary, since few researchers have access to unlimited computer time, but the second is purely cosmetic. Since R can be reduced to an almost arbitrarily low value by use of a suitably high CJ threshold, more valid criteria of accuracy must be sought. One such is the ‘weighted R value’, generally given the symbol R‘ or R,. Weighted R values represent an attempt to take the errors a(F) into account during refinement; reflections with higher a are given less weight. There are several different types of weighting scheme, but one of the commonest is = d(F)LV-+ gF2 whereg is a small constant (typically 0 to 0.001); this takes into account that many common systematic errors depend approximately linearly on Z.The weighted R is then expressed as Since R, values are based on the incorporation of more information (a values) into refinement, and are far less dependent on the CJ threshold, they may be regarded as more ‘realistic’, and R, is thus a better criterion of accuracy than R.Sensible weighting schemes generally lead to lower e.s.d.’s (see below) for refined atom co-ordinates, and it is thus rare to find a structure which has been refined without weights (i.e. with ‘unit weights’); such structures should be viewed with some suspicion. The reference to e.s.d.’s in the last paragraph reminds us that the structural information of an X-ray investigation is contained in refined parameters and their e.s.d.’s, rather than in R values.There is thus a considerable argument in favour of judging the accuracy of a crystal structure in terms of the e.s.d.’s. The omission of too many reflections would tend to increase the e.s.d.’s, since they are to a good approximation inversely proportional to the square root of the number of reflections. A related effect is that higher e.s.d.’s are inevitable if too few reflections are measured and the data/parameter ratio is thus too low-perhaps to save diffractometer time, or to reduce R by omitting higher angle (weaker) reflections. Measurements should normally be made to a diffraction angle 20 of at least 45” for Mo Ka radiation, or 100”for Cu Ka. Structures with 20,,, lower than this, or where no 2U,,, is quoted, may justifiably be regarded with suspicion.* The *They are also far more difficult to solve; much of the effort expended in developing extremely powerful programs and using them to solve such structures could be avoided if proper data were collected in the first place! 158 Jones data/parameter ratio should be at least 8 if at all possible (and may be ca.20 for well-determined structures), yet some structures with a ratio of 5 still appear in the literature. The close relationships between weighting schemes, e.s.d.’s, and weak reflection thresholds have been the subject of several articles in the crystallographic litera- ture. It has been convincingly argued that no weak reflections should be omitted, as this may lead to systematic bias, albeit small, in refined parameters;’ several test structures have shown conclusively that e.s.d.’s are increased by omission of weak reflections, and indeed that reasonable refinements are achieved by using only reflections below a weakness threshold.2 However, the effect on e.s.d.’s has been shown to be small for a range of conventional threshold^,^ and it may be assumed that the practice of omitting weak reflections will continue. Moral: (i) judge accuracy by e.s.d.’s rather than R values (but see next Section for a caveat); (ii) high CT thresholds, low 28,,,, or low data/parameter ratios may be symptoms of a poorly-determined structure.3 Estimated Standard Deviations: Accuracy versus Optimism An ‘accurately determined’ organic crystal structure (at room temperature) may claim bond length e.s.d.’s of 0.002A.This Section will try to explain why the immediate reaction of the reader should be to reach for a pinch of salt. The mathematics and computational aspects of least-squares refinement are well do~umented.~The e.s.d.’s obtained are mathematically sound, but are based solely on information fed to the computer [usually only F and o(F)]and, as such, must be lower bounds of a realistic error estimate. Sources of systematic error are ignored; the most serious is usually inaccuracy of cell constants. It is not always realised that the parameters derived from the measured intensities (via least-squares refinement) are merely fractional co-ordinates of the atoms, based on the unit cell axes.In order to obtain bond lengths (and other molecular dimensions) in absolute units (e.g.A), it is necessary to perform appropriate vector calculations, taking into account the dimensions of the unit cell. The vector r between two atoms with fractional co-ordinates xl,y,, zl, x2,y2,z2in a cell defined by the axis vectors a, b, c is given by r = (x2 -xl) a + (y2-yl) b + (z2-z,)c and the bond length is then r = 4r.r. Any errors in the cell dimensions will thus give rise to additional errors in the bond lengths (over and above those calculated from least-squares refinement). In the case of heavy atom bond lengths, these extra systematic errors may be appreciably greater than the least-squares e.s.d.’s. The conventional method of obtaining cell constants using a four-circle diffractometer is to determine accurate positions of several strong reflections in ’ F.L. Hirshfeld and D. Rabinovich, Aria Crj~.r[al/ogr., 1973. A29. 510. L. Arnberg, S. Hovmoller, and S. Westman, Acra Crysiallogr.. 1979. A35,497. R. E. Stenkamp and L. H. Jensen. Acin Crysiallogr., 1975, B31, 1507. For example. D. W. J. Cruickshank. ‘Least-Squares Refinement of Atomic Parameters’, in ‘Crys- tallographic Computing’, ed. F. R. Ahmed. Munksgaard, Copenhagen. 1970. 159 Crystal Structure Determination: A Critical Vieut terms of the circle angles' 28, o,2, and 4, and to refine the crude cell constants accordingly.6 As with any refinement, e.s.d.'s may be estimated and would typi- cally lie in the range 0.001 to 0.01 A for a lOA axis (with equivalent fractional errors for other axis lengths).Unfortunately, the problem of systematic errors arises once again. Since axis lengths are inversely proportional to sin0 (Bragg's law), a small error in the zero point of the 20 circle will cause systematic errors in the cell constants; for a typical case (Mo Ka radiation, lOA axis, 20, 0.05", 20 of reflections used ca. 20") the calculated axis length would be 10.025A. This additional error is far in excess of the least-squares e.s.d.'s.* Since circle zeros, even on the best-constructed diffractometers, tend to wander, and recalibrating them is tedious, such sources of error are often ignored (until they become so large as to incommode data collection). In our example, this would lead to additional system- atic errors of ca.0.004A in bond lengths.7 An interesting example of the problems of cell constant measurement is provided by the compound 4-nitrophenyl-a-~-glucopyranoside.The structure was first re- ported in space group P2, with a 28.810, b 6.747, c 6.729 A, /? 103.68".*A second, independent determination yielded cell constants a 28.045, b 6.767, c 6.719A, /?90.30".9 The apparent major discrepancies can be removed by an allowed axis transformation of the first cell to a 27.993, b 6.747, c 6.729A, /? 90.17", but the remaining differences, notably ca. IOU in b, are difficult to explain. Moral: (i) quoted cell constant e.s.d.'s may be wildly over-optimistic (as witness the occasional appearance of a fourth or even fifth place of decimals, at which level the thermal expansion over the laboratory temperature range may be significant); (ii) calculated least-squares e.s.d.'s of molecular dimensions should be increased somewhat to allow for errors in cell constants.The latter practice is already implemented in some program systems. 4 Incorrect Space Groups: The Case of the Missing Symmetry It is a sad fact that crystallographers, being human, sometimes fail to assign the correct space group to a structure. In some cases this renders structure solution impossible and the error never reaches the literature; in others, structures are solved, refined, and published in space groups of unnecessarily low symmetry, one or more symmetry elements having gone unrecognized.This may lead to nothing more serious than, say, the refinement of two identical molecules rather than one, *Errors arising from incorrect zeroing of the w and x circles, in contrast to 28, are reflected in increased least-squares e.s.d.'s. tlt should be pointed out that reliablecell constants cun be obtained, r.g. from accurate 20 values free from circle zeroing errors.' These are, however, not straightforwardly obtainable on all makes of diffractometer. 'International Tables for X-Ray Crystallography', Vol. 4, ed. J. A. Ibers and W. C. Hamilton, Kynoch Press. Birmingham, 1974. For a brief review of methods, see Re/: 7.' W. Clegg. Fresenius' Z. And. Cliern., 1982, 312, 22. P. Swaminathan. Actu Crysrallogr., 1982, B38, 184.P. G. Jones, G. M. Sheldrick, A. J. Kirby, and W. B. T. Cruse. Z. Kristallogr.. 1982. 161, 69. 160 Jones and, despite the waste of computer time, the molecular dimensions will still be reliable. Should the omitted symmetry element be a centre of symmetry, the consequences are more serious; refinement will be slow and erratic, and derived parameters unreliable. This is inevitable, and has its mathematical basis in the matrix algebra of refinement. I shall describe here two typical problems of this type and some simple, though not infallible, methods whereby the non-specialist can detect similar cases in the literature. Little familiarity with space groups is necessary; after all, few chemists care to venture into the recondite depths of space group tables," the symmetry operators relating molecules often being chemically unimportant in comparison with the dimensions of a single molecule.Some recent detailed reviews of incor- rectly assigned space groups are highly recommended reading.' '-l4 The first structure is a ternary oxide &Yo3.15 There are four features which raise suspicion; each is in itself quite possible, but the combination of improba- bilities is impressive. (a) The space group is given as PI (synthetic materials sometimes crystallize in non-centrosymmetric space groups, but PI is most un- usual); (b) One cell angle is 89.99'; (c) there are two independent formula units in the asymmetric unit; (d) there are simple mathematical relationships between co-ordinates of pairs of atoms (see Table 1); thus each pair shows Ax 1/2,dy 0.This last is the single strongest indication of 'missing' symmetry. The unit cell a 8.144, b 6.220, c 5.758A, a 117.54,fi 89.99,y I1 1.24" can be trans- formed to a new cell a' 11.030, b'5.758, c'8.144& a'90, B' 114.10, y'90" by the matrix 0 -2 -1/O 0 1/ -1 0 0. l6 The same matrix transforms the reflection indices, whereupon the systematic absences h01,l odd and hkl, h + k odd become apparent (corresponding to monoclinic space groups Cc or C2/c). The atom co-ordinates may be transformed with the transposed inverse matrix (0 -0.5 0/0 -0.5 1 / -100); with an allowed origin shift, the pairs of atoms are then related by the operator x,-y, 0.5 + z. The space group is thus Cc, and the refinement proceeds to R 0.056 (see Table 1).* The second structure, a hydrated oxide H3M5014,15will be discussed more briefly.It was published in the monoclinic space group P2,/rn, with a 5.518, b 16.50, c 5.519& fi 107.0'. In such cases (monoclinic with a and c equal, or triclinic with any two axes equal), a cell with an additional right angle may be constructed; here *The crystallographic reasoning and matrix methods described here are probably unfamiliar to the non-specialist (for a fuller description, see Re/: 17-which is probably the best book on X-ray methods for the beginner). However, several programs are available which use matrix algebra to test for 'missing' symmetry, and which present the results in a form which is easy to interpret.16 See also the 'Morals' below.ID 'International Tables for X-Ray Crystallography', Vol. A, ed. T. Hahn, D. Reidel Publishing Co., Dordrecht, Holland, 1983. 'I R. E. Marsh and V. Schomaker, Inorg. Chern., 1979, 18, 2331. l2 R.E. Marsh and V. Schomaker, Inorg. Chem., 1981, 20, 299. I3 F. H. Herbstein and R. E. Marsh, Acta Crystallogr., 1982, B38, 1051. l4 R. E. Marsh and F. H. Herbstein, Acta Crystallogr., 1983, 839, 280. Is Further details from the author on request. l6 W. Clegg, Acta Crystallogr., 1981, A37, 913. P. Luger, 'Modern X-Ray Analysis on Single Crystals', de Gruyter, Berlin, 1980. 161 Crystal Structure Determination: A Critical View Table I Atom co-ordinates of the ternary oxide X,YO, PI cell Cc cell x Y X J' z XI .600 .659 .769 .671 .288 .399x2 .I02 .658 .I93 x3 .790 323 .396 .588 .169 .709x4 .291 325 .735 x5 .378 .460 .225 .771 .158 .120X6 .881 .451 .540 x7 .698 .268 .948 366 .340 .502X8 .198 ,268 .628 YI .ooo .ooo .oooa .ooo ,849 .oooaY2 .500 .ooo .302 01 359 .669 .986 .165 .003 ,64002 .361 .669 ,986 03 ,546 .91 I .559 .340 .489 ,91804 .047 .909 .659 05 .583 .322 .326 .045 .551 .95306 .082 .320 .304 Fixed zero co-ordinates to define the origin the matrix 1 0 1 1 1 0 -1 / 0 I 0 forms a cell with a 6.565, b 8.872, c 16.50 A, all angles 9Oo.l6A detailed analysis (as above for X,YO,) shows that the structure can be described in the orthorhombic space group Cmcrn.This can be seen from a diagram of the structure (Figure I).Two points should be stressed here. First, in both these cases the overlooked symmetry element was not a centre of symmetry, and the molecular dimensions of the published structures are thus reliable (for cases where this was not so, see below and Refs. 11-14). Secondly, the mere existence of a possible cell with two or three right angles (i.e. with metric symmetry monoclinic or orthorhombic, respectively) does not necessarily mean that the structure belongs to this crystal class. A good example is 6-S,,'* which crystallizes with two independent molecules in a mono- clinic cell with a 'u c. This allows the construction of a metrically orthorhombic cell, but neither the atom co-ordinates nor the intensity data are consistent with such a transformation.Indeed, an attempt to obtain the structure in an ortho- rhombic space group had proved unsuccessful. l9 The problem of deciding if a centre of symmetry is present or not (i.e. dis-R. Steudel, J. Steidel, J. Pickardt, F. Schuster, and R. Reinhardt, Z. Narurfursch., Teil B, 1980,35, 1378. l9 I. Kawada and E. Hellner, Angew. Chem., 1970. 82, 390. 162 Jones 0 00 0 OO@ 0 0 Figure 1 The structure of H,M,O,, projected down the monoclinic b uxis. The orthorhombic symmetry (larger cell) is clearly visible. (Hatched circles M, small circles 0;H atoms not determined) tinguishing between pairs of space groups such as PI, P1; Pnrna, Pna2,) may present some difficulties. Describing a centrosymmetric structure in a non-centrosymmetric space group may well lead to a lower R factor, since the number of parameters is approximately doubled (a modified version of the principle ‘give me enough parameters and I can fit an elephant’); this is therefore no criterion.Statistical tests” can, when carefully used, resolve the ambiguity; unfortunately, careless use may lead to the wrong conclusion and thus to gross errors in published molecular dimensions.2 A partic@arly unfortunate example (potassium tetrox- alate, published in PI instead of P1) has been discussed instructively by Gilmore and Speakman.22 Moral: Overlooked crystallographic symmetry may lead to any (or several) of the following symptoms: synthetic compound in non-centrosymmetric space group; more than one formula unit in the asymmetric unit; accidental exact right angles in the cell; accidentally equal axes; simple mathematical relationships between co-ordinates of pairs of atoms.If the ‘missing’ symmetry is a centre of symmetry, serious systematic errors will be present. 2o W. C. Hamilton, Acta Crysfaflogr.,1965, 18, 502. R. E. Marsh, Arta Crystallogr., 1981, B37, 1985. 22 C. J. Gilmore and J. C. Speakman, Acta Crysfallogr.,1982, B38, 2809. 163 Crystal Structure Determination: A Critical View 5 Absorption Corrections: More Honoured in the Breach.. . The absorption of X-rays by crystals is governed by the equation //Io = exp (-pt) where I/Iois the fractional loss of intensity, t the path length, and p the absorption coefficient. For Mo Kor radiation and organic crystals, p is of the order 0.1 mm-’ and t < 0.8 mm (the maximum X-ray beam diameter usually available), so absorp-tion effects may safely be neglected.The introduction of heavy atoms increases ,u, and in an extreme case such as Au,O,,~~ pt may be as high as 10 (crystal size 0.1 mm, p 110 mm-’) and I/Io 5 x lo-’. It is clear that in such cases severe systematic errors arise because of the different path lengths through the crystal for different diffraction geometries (i.e.different reflections). What is often not appre-ciated, because the necessary integral calculations are complicated, is that serious systematic errors also occur in the case of exactly spherical crystals. The questions thus arise (i) how severe must absorption effects be before a correction is necessary, and (ii) how can a correction be applied.To answer (i) it is helpful to consider the consequences of absorption errors if no correction is applied. It can be shown that absorption is more severe at low 20; the general decrease of intensity with increasing 20 (independent of absorption) is thus to some extent cancelled out. Since this general decrease is caused by thermal motion of the atoms, the apparent thermal motion is artificially decreased by absorption effects. In severe cases this leads to the (physically impossible) phenomenon of negative thermal motion or ‘non positive definite’ atoms, in crystallographers’ jargon. Related effects include the apparent highly anisotropic thermal motion of atoms if the absorption itself is highly anistropic (e.g.for needle- or plate-shaped crystals; see Figure 2), and the appearance of large artefacts in the electron density near heavy atoms.The atomic positions are not severely affected, although the location of light atoms may become difficult in the presence of too many spurious peaks. It is generally agreed that such absorption effects become appreciable when pt is ca. 0.5. Unfortunately, the application of adequate absorption corrections was, until recently, a far from trivial exercise. It involved first the calculation of crystal shape and size by assuming an idealized geometry (e.g.a cylinder) or by indexing the crystal faces and measuring the distances between them, and secondly, calcu- lating path lengths and performing complex integrals for each reflection. If the crystal did not exhibit well-developed faces and did not approximate to a simple shape, the problem was intractable.Nowadays more general and convenient meth- ods are available (e.g. the so-called +-scans, involving measuring a reflection and its equivalents at different diffraction geometries and applying a correction which minimizes the intensity differences; for a brief review of methods see Ref.24). Regrettably, more effort is often expended in inventing excuses for lack of absorp- tion corrections than would have been necessary to perform them in the first place. Here are some examples from recent publications: ’ ‘The expense to information ratio was deemed too high’; ‘with the absorption coefficient as high (my italics) as 23 p.G. Jones, H. Rumpel, E. Schwarzmann, and G. M. Sheldrick, Acra Crystallogr., 1979, B35,1435. 24 N. Walker and D. Stuart, Acra Crysfaffogr.,1983, A39, 158. 1 64 Jones Figure 2 The eflects of neglecting absorption. The ion shown is (HO,PCH,.AsO,H)*-; note the spuriously elongated ellipsoids of the heavy 761 cm-’, no absorption correction was attempted.. . The anisotropic thermal parameters are not given, since they did not stay positive definite’; ‘in view of the very large absorption coefficient (258 cm-’) and the brittleness of the crystals, an absorption correction was out of the question’. Or, freely translated, ‘we knew our data contained severe systematic errors but we didn’t do anything about it’.Moral: Structures with p? 0.5 or greater need an absorption correction. (Since p and the crystal size are generally amongst the published crystal data, the reader can calculate p? for himself). The absence of absorption corrections in such cases reduces the accuracy of the structure and the meaningfulness of the thermal parameters. 6 Thermal Ellipsoids: The Error Dustbin The conventional representation of the thermal motion of an atom by an ellipsoid (inside which the atom has a given chance, usually 50%, of being) is familiar to most chemists. A typical thermal ellipsoid plot of a well-behaved organic crystal structure is given in a related article in Chemistry in Britain.26 Unusually large or anisotropic ellipsoids may genuinely represent unusual thermal motion, e.g.of a long side chain not stabilized by secondary interactions such as hydrogen bonds. 25 L. Falvello. P. G. Jones, 0. Kennard, and G. M. Sheldrick, Actu Crysiallogr., 1977, 633, 3207. 26 P. G. Jones. Chem. Brit., 1981, 17, 222. 165 Crystal Structure Determination: A Critical View, However, if no such obvious cause can be found, the ellipsoids should be regarded with some suspicion; they are a remarkably good way of mopping up systematic errors. One such error, absorption, was discussed in the last Section; if all the ellipsoids in a structure point the same way, the cause may well be absorption (other possibilities include crystal decay, crystal larger than X-ray beam...). An alternative possibility is that the model being refined is in error. One source of error in the model is disorder; one or more atoms are ‘averaged’ over several positions, either by free rotation (eg. of spherical ions such as PF, p)26 or statically (two or more possible positions for the atom(s) involved). In both cases the ‘average’ model may be of a single site smeared out anisotropically. A typical example is furnished by the structures of some MSb,O,, The Mf cations appear to lie on special positions with symmetry mm, but with extremely high temperature factor components in one direction. This model is almost cer- tainly an approximation to a static disorder of M+ over two or more sites, although, as is often the case, such a model could not be refined. Without a full chemical analysis these materials could not have been characterized. The possibility of disorder should be borne in mind if unusual temperature factors are encountered.The converse, that normal temperature factors rule out disorder, is not necessarily true, as was recently shown in spectacular fashion by the structure of the secododecahedrene ( The structure solution appeared, most unexpectedly, to show a molecule of dodecahedrane (2), with normal thermal parameters and a good R value. When the crystal was redissolved, n.m.r. spec- troscopy confirmed the structure as (1). The explanation is that the molecules of (1) in the crystal adopt no fewer than 120 different orientations, the average of which is a good approximation to (2).H H / \ (1) (2)The representation of thermal motion by ellipsoids is only an approximation, and as such has a significant drawback; since the motion of, for instance, a terminal group may be nearer a ‘banana’ than an ellipsoid, the use of the latter as a model causes a systematic shortening of apparent bond lengths. This effect is known as ‘libration’. As an example, consider the structure of carbonyl gold(r) chloride, (OC)AUCI,~~in which the molecules are constrained by symmetry to be exactly 27 D. Bodenstein, W. Clegg, G. Jlger, P. G. Jones, H. Rumpel, E. Schwarzmann, and G. M. Sheldrick, Z. Naturforsch., Teil B, 1983, 38, 172. 0.Ermer, Angew. Chem., 1983, 95. 251.l9 P. G. Jones, Z. Naturforsch., Teil B, 1982, 37, 823. 166 Jones linear. The bond lengths Au-C 1.93, C-0 1.1 1 8, are unexpectedly short; this is almost certainly due to libration, and thus no great significance should be attached to these unreliable results. In general, if structural correlations are to be derived from small differences (ca. 0.01 A) in bond lengths, the systematic errors intro- duced by libration may be highly significant, and in severe cases invalidate the discussion.* In some cases a correction for libration may be applied (and where extreme accuracy is sought, such a correction should always be attempted) by a detailed analysis of the anistropic thermal parameter^.^'. 31 The relative advantages of the two commonest methods have recently been reviewed.32 The structure of (0C)AuCI is regrettably a good example of a case where a libration correction is impossible; the light atom parameters are imprecisely determined in the presence of gold, and, despite the application of an absorption correction, the extremely severe absorption would almost certainly lead to systematic errors in the thermal parameters. (In fact no anisotropic refinement of C and 0 was attempted).Moral: (i) unusual temperature factors may reflect systematic errors in intensity data or errors in the structural model (in particular, disorder); (ii) appreciable thermal motion, especially perpendicular to a bond, causes an apparent shortening of a crystallographically determined bond length.For the most accurate structures, a libration correction should be applied; if this is not done, discussions based on small differences on bond lengths may be invalid. 7 Problems with Light Atoms: Small is Beautiful? Since crystallographic methods provide a picture of the electron density, the positions of atoms with more electrons will be more accurately determined (this has been implicitly assumed above). However, given a reasonable absorption cor- rection, it is usually possible to locate non-hydrogen atoms without difficulty, even in the presence of heavy transition metals (hydrogen atoms are a special case-see below). A more serious problem may be to distinguish similar light atoms (C, N, 0,F), especially if the exact chemical constitution of the compound is not known.A case in point is the SO,F- ion, for which the 0 and F sites are often impossible to distinguish (and may indeed be di~ordered).,~ More serious chemically is the inability to distinguish between alternatives such as M-NCO, M-OCN or M-NSO, M-OSN. Hydrogen atoms present a special, extreme case of the problems of light atom location. The single electron is difficult to find, and the apparent position will be shifted towards the atom X of the X-H bond (because the H electron spends a *One extenuating circumstance is that a series of related compounds may well exhibit similar thermal motion. and thus similar librational effects; a discussion of trends in bond length would thus remain valid. 30 W. R. Busing and H.A. Levy, Acta Crystallogr., 1964, 17, 142. 3' V. Schomaker and K. N. Trueblood, ACIU Crystullogr., 1968, B24, 63. 32 R. Srinivasan and N. R. Jagannathan, Acta Crysfaflogr.,1982, B38, 2093. 33 P. G.Jones and 0.Kennard, Acta Crystallogr., 1978. B34, 335. 167 Crystal Structure Determination: A Critical View finite time away from the H nucleus); accurate neutron diffraction data and spec- troscopic data establish the internuclear length of a C-H bond as ca. l.O8A, whereas X-ray data generally give about 0.96A* 34 The treatment of H atoms in X-ray structure determination may be considered under two headings. A. H Atom Determination in the presence of Heavy Elements.-Small features in the residual electron density are examined when all other atoms have been found and satisfactorily refined.With luck, judgement, and good data, those in chemically sensible positions can be accepted as H atoms, although their refinement will probably be extremely imprecise unless certain constraints are used (e.g. fix the C-H bond at 0.96 A*). It should now go without saying that adequate absorption corrections are necessary to minimise artefacts in the electron density. A far more reliable method of locating H atoms is neutron diffraction, where the H atom contributions to the diffraction are not overwhelmed by those of the heavier elements; this, however, requires very large crystals and access to an atomic reactor. To obviate this problem to some extent, Or~en~~ has developed an indirect method; possible H sites (‘holes’ in the X-ray structure) are analysed in respect of their potential energy, based on interaction curves between H and other atoms.High energy sites are ruled out. Although the energies calculated are very approx- imate, the success rate is impressive. Agreement with known neutron structures is achieved in virtually every case, thus lending confidence in calculated H atom positions when no neutron data are available. The method has also shown many published H atom positions to be false.35 B. H Atom Location in Organic Structures.-Given moderately accurate data and no disorder, H atoms are usually obvious features in the electron density when all other atoms are accounted for. The trend nowadays, in view of the imprecision of refined H atom positions, is to include the H atoms in the refinement using a ‘riding model’ incorporating various constraints (eg.C-H 0.96 A, as mentioned above, H-C-H 109.5’ for sp3 C, fixed H atom temperature factors). It therefore seems unnecessary to identify H atoms in the electron density, since their idealized positions are generated by the program. The consequence of this attitude is clear; if the atoms of the C/N/O framework have been wrongly identified (perhaps on the basis of a preconceived structure), the H atoms will also be wrongly placed. This error should be detected from several symptoms; the temperature factors of the wrongly assigned C/N/O atoms may be unusually high or low (another aspect of the ‘error dustbin’-but who checks temperature factors carefully, especially now that many journals do not publish them); bond lengths and, in particular, non- bonded X e.-H contacts may be anomalous. Nevertheless, it is a safe assumption *This value is incorporated into some program systems in order to include H atoms at geometrically calculated, rather than X-ray determined, sites. 34 M. R. Churchill, fnorg. Chem., 1973, 12, 1213. 35 A. G. Orpen, J. Chem. SUC.,Dalton Trans., 1980, 2509. 168 Jones (a) (bl Figure 3 Problems in light atom assignment :diflerence electron density of the largely planar molecule C,3H, N,O. Formulae: (a), as initially reJined; (b), corrected version. The electron density clearly siows all hydrogen atoms of the rings and excess density at two atoms; the assigned atom types were amended accordingly (Reproduced by permission from Ref.36) that several such incorrect structures have been published; obvious candidates are large natural products, which are often of unknown structure and which may yield poor quality X-ray data. An example of this problem is given in Figure 3. The structure C,,H,,N,O was solved without difficulty, but it was not immediately clear which atom types were which. A careful analysis of the difference electron density after initial refinement allowed the C/N/O assignement to be corrected and all H atoms to be located. Moral: (i) H atom positions in heavy atom structures (e.g. cluster hydrides) may be erroneous if no supporting data, such as potential energy calculations, are available; (ii) H atom positions in organic structures are probably correct if the difference electron-density has been critically inspected; (iii) bonding modes of ions such as NCO-are difficult to determine with X-ray methods alone.36 M. Noltemeyer, G. M. Sheldrick, H.-U. Hoppe, and A. Zeeck, J. Antibiofics, 1982, 35, 549. 169 Crystal Structure Determination: A Critical View 8 Acentric Structures: Through the Looking-glass? Acentric (non-cen trosymmetric) structures provide their own peculiar pit falls. These arise mostly from the breakdown of Friedel’s law (which states that the intensity of a reflection hkl is equal to that of hkf;the breakdown is caused by the phenomenon known as anomalous scattering of X-rays).The differences in in- tensity between hkf and hkl are generally immeasurably small for light atom structures; if heavier atoms are present, however, an accurate analysis of the measured differences can distinguish between a given structural model and its mirror image. For chiral materials, this corresponds to the determination of the absolute configuration (as in the pioneering work of Bijvoet ’7). Clearly it is a great advantage to be able to determine absolute configuration by X-ray methods. The converse, though equally clear, is sometimes ignored; for any acentric structure containing heavier atoms, the refinement must take the anoma- lous scattering into account. This may be done either by (a) refining an alternative model with all co-ordinates changed in sign and accepting the model with the lower R value (valid for most space groups, but the difference in R may be very small-see below) or (b) refining a factor q multiplying the imaginary components of the anomalous scattering contributions of all atoms; a value of + 1 indicates a correct model, -1 that a change of sign of co-ordinates is necessary. The rj method, first suggested by Rogers,37 is valid for all space groups and appears to be the more powerful of the two.Bearing these comments in mind, it is distressing to find many published acentric structures for which no mention of anomalous scattering (or absolute configuration) is made, even with very heavy atoms present (e.g. Au ”). It was long ago demonstrated that the neglect of anomalous scattering can lead to systematic errors in bond lengths.38 A good example of this is shown by the gold(1) complex (Ph,Sb),Au+ (C,F,),AU-,,~ which crystallizes in the acentric space group P3cl.* The refinement was initially unsatisfactory in that the six independent Au-Sb bond lengths showed a considerable scatter (2.52-2.72 A).Changing the sign of the co-ordinates reduced this scatter to 2.59-2.67 A (typical e.s.d.’s 0.01 A) al-though having little effect on the R value. The latter model was accepted on the basis of this reduced scatter (the rj refinement was at that time unavailable). The former model, with its inappropriate treatment of anomalous scattering, would have led to systematic errors of up to 0.17 A in bond lengths.*Experienced crystallographers will have noticed that in P3c.I. as in all acentric space groups with glide planes, there is no ‘absolute configuration’; here the analysis of anomalous scattering determines instead the ‘polar axis direction’. This is one of several oversimplifications in this Section, which do not, however invalidate its general principles. tNote added in proof: A greatly expanded version of Section 8 has been submitted to Acta Crys- tallographica. Further recommended reading: D. Rogers and F. H. Allen, Acra Crystalfogr., 1979, B35, 2823 (critique of published absolute configurations): H. D. Hack, Acta Crystallogr., 1983, A39,876 (a suggested alternative to the q method). ” D. Rogers, Acta Crystallogr., 1981, A37.734. 38 D. W. J. Cruickshank and W. S. McDonald, Acra Crj..sraNogr., 1967, 23, 9 39 P. G. Jones, Z. Noturforsch., Teil 13,1982, 37, 937. 170 Jones Morul: All published acentric structures containing heavy atoms (how heavy is a heavy atom? Not very: the presence of one P atom in an organic structure can allow the determination of its absolute configuration) should also contain some mention of the treatment of anomalous scattering effects. If these effects were ignored, the bond lengths may be associated with systematic errors many times larger than the e.s.d.'s quoted. 9 Miscellany This last section presents a brief selection of errors not easily classifiable under the headings 2-8. The experienced reader will doubtless be able to add to the list. A.Gross Errors in Published Data.-Since crystallographic publications contain lists of co-ordinates, bond lengths, ete., it is not surprising that numerical errors creep in from time to time. This can prove very confusing for the reader who tries to perform calculations with (for example) permuted axes. Fortunately, the Cam- bridge Crystallographic Data Centre checks all structures destined for its files, and erring authors (as the present author knows only too well) are asked to provide corrected values. B. Insufficient Data.-Apart from the problem of unsuitably low 20 cutoff (Section 2). close examination of crystallographic datasets may reveal that not all indepen- dent data are present. In some high symmetry space groups, the crystallographer may not have been familiar with the index range of the unique data (e.g.Laue group 6/m, data collection appropriate to 6/n7n7m1')).In acentric space groups, it is often merely a matter of taste (and machine time) whether Friedel opposites are collected, especially for light atom structures (see Section 8). However, the deter- mination of absolute configuration without Friedel opposites may not be 100% reliable. C. Mis-interpretation of Peaks.-Crystallographic methods enable us to locate peaks in the electron density. The assignment of atom types to these peaks is more subjective, and care should be taken not to confuse atoms of similar atomic number (see Section 7). A striking example of such a problem is the structure of the (alleged) novel chlorine(vi1) compound [CIF,]' [CuF,]-.It has recently been suggested4' that traces of water and the use of silica vessels had led instead to [Cu(H,O),]'+ [SiF,I2-. the cell of which is almost identical to that reported for the 'chlorine( VII)' compound. Acknobt3ledgement.s and E.wuses. It is a pleasure to acknowledge the helpful sug- gestions of many colleagues, in particular Prof. G. M. Sheldrick, during the preparation of this article. Two excellent but rather well-hidden essays acted as inspiration and model for much of the material presented here.41 '"H. G. von Schnering and D. Vu, Angcw. Chmi.. 1983. 95. 421. J1 J. A. Ibers. 'Problem Crystal Structures'. and J. Donohue. 'Incorrect Crystal Structures: Can They Be Avoided?'.in 'Critical Evaluation of Chemical and Physical Structural Information', ed. D. R. Lide jr. and M. A. Paul. Nat. Acad. Sci.. Washington D.C.. 1974 171 Crystal Structure Determination: A Critical View It is appropriate in these concluding paragraphs to explain why a crys-tallographer should seek to sow the seeds of doubt in the minds of those who are, after all, his customers. The answer is very simple: only by making the chemist more familiar with the problems involved, and constructively critical of the results obtained, can consistently high standards of published crystal structures be achieved. I have tried, wherever possible, to give examples of problems and errors taken from my own work. The diligent reader could certainly find some that I have overlooked. Finally, it is hoped that this article will stimulate the chemist to find out more about crystallographic methods, both by reading some of the excellent general texts available”, 42 and, much better, by active collaboration with departmental crystallographers. 42 J. D. Dunitz, ‘X-Ray Analysis and the Structure oforganic Molecules’, Cornell U.P.,New York,1979. 172

ISSN:0306-0012    

DOI:10.1039/CS9841300157

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

LENNARD-JONES LECTURE Recent Experimental and Theoretical Work on MolecularlySimple Liquid Mixtures* By L. A. K. Staveley? THE INORGANIC CHEMISTRY LABORATORY, THE UNIVERSITY OF OXFORD 1 Introduction The thermodynamic properties of liquid mixtures have interested chemists for many years. An enormous amount of experimental information has been amassed, and a great deal of effort has been devoted to developing theories which seek to account for the observed behaviour of such mixtures. These theories have more than academic interest and importance, for the better our understanding of liquid mixtures, the better are the chances of successfully predicting the properties of a particular mixture. Reliable predictions of this kind may have considerable tech- nological value, since they can save the time and money which would be needed to determine the required properties experimentally.The thermodynamic properties of a liquid mixture are generally considered with reference to the corresponding properties of an ideal liquid mixture. The ideal solution, like the ideal gas, is a convenient fiction. In an ideal liquid mixture, the chemical potential piof a component i at a temperature Tmust obey the equation pi= p: + RTlnx,, (1) where xi is the mole fraction of component i, and py is the chemical potential (or Gibbs energy per mole) of the pure liquid i at the temperature T.(Strictly speaking, py is a function of the pressure acting on the system, but most of the experimental work on liquid mixtures has been carried out at low pressures, of the order of one atmosphere, and moderate pressure changes in this region have almost no effect on py).If the mixture of the vapours in equilibrium with the liquid mixture can be regarded as a mixture of ideal gases, it follows from equation (1) that the partial vapour pressure piof component i is given by the equation I Ip. =pox.I, (2) where pq is the vapour pressure of pure liquid i at the same temperature. We are concerned in this review with real, and hence non-ideal, liquid mixtures, and we shall confine ourselves to mixtures of only two components. If x1moles of *Based on the second Lennard-Jones Lecture, delivered at the University of Hull on 28th March, 1983 tAddress: Uplands, Dunstan Road, Old Headington, Oxford OX3 9BY 173 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures a liquid 1 and (1 -x,) moles of a liquid 2 are mixed to give one mole of a solution at a temperature T, then for any extensive thermodynamic function X,the change AX, in this function is AX, = X, -Xl,q -(1 -.x,)x":, (3) where X,, X(: and fiare respectively the values of X for a mole of the mixture, a mole of pure component 1, and a mole of pure component 2.AX, can also be written as AX, = dX,(id) + XE, (4) where XE is the excess function. For the Gibbs energy, it readily follows from equation (I) that dG,(id) = Rnx,ln.u, + (1 --x,)In( 1 -x,] (5) Another consequence of equation (1) is that if two liquids mix to form an ideal solution, they do so with no enthalpy change and with no volume change.So for a real mixture we have AH, = HE,the enthalpy of mixing, and AV, = VE, the volume increase on mixing. SEis given by (HE-GE)/T.In this review, values of the four excess functions GE,HE, I/E, and SEall refer to the formation of one mole of mixture. GE,HE,and VEcan all be determined by experiment. Of interest is not only their composition dependence at a fixed temperature, but also (for a given concentration of the mixture) their temperature dependence (at constant pressure), and their pressure dependence (at a constant temperature). The aim of a theory is to calcu- late these excess functions for particular mixtures. Without wishing to decry the value of theories which seek to do this semi-empirically, for example by assuming some particular equation of state, we shall only be concerned with theories which attempt the prediction of the excess functions of a given system on the most fundamental basis possible, namely in terms of the intermolecular forces which operate between the molecules involved in that system.When such theories began to be advanced-ssentially in the years following the Second World War-almost all the experimental data on the excess functions of binary liquid systems then available referred to mixtures of substances which are liquid at ordinary tem- peratures, involving therefore such compounds as benzene, tetrachloromethane, n-hexane, and so on. While the molecules of such substances may be simple to, say, an organic chemist, they are complex and difficult when it is a matter of finding a quantitative expression for the intermolecular energy for a pair of molecules as a function of their separation and mutual orientation.Meanwhile, however, infor- mation on intermolecular forces between small molecules was increasing in quan- tity and quality, and has continued to do so. So it was clear that these new statistical-mechanical theories of liquid mixtures should first be tested on mixtures of the simplest and smallest molecules, or in other words on mixtures of liquefied gases. This was the primary consideration which prompted the initiation and development of the kind of experimental cryogenic work described in this review.Staveley One of the first of the more fundamental theoretical approaches to the liquid mixture problem was based on the cell model pioneered by Lennard-Jones and Devonshire, * who used it to calculate the properties of molecularly simple liquids such as liquid argon. In more recent years, increasing use has been made of perturbation treatments, which start from a suitable reference system. For this a frequent choice is the 12-6 fluid, that is a liquid for which the potential energy between any two molecules depends only on their separation and is adequately represented by the famous Lennard-Jones 12-6 potential. The popularity of this choice is largely due to its suitability for computer simulation calculations of the properties of such a fluid.Again, one method of handling the problem of aniso- tropy presented by non-spherical molecules such as those of nitrogen and ethane is to regard the intermolecular forces as forces operating from two centres within a molecule which can be represented by isotropic Lennard-Jones potentials. So, one way and another, the influence of Lennard-Jones in the theoretical field with which we are concerned has indeed been far-reaching. 2 Experimental Methods A. The Determination of GE.-GE is found by measuring the vapour pressure of mixtures of known composition. An equation connecting GEwith the mole fraction x of one component is assumed, such as the Redlich-Kister equation (6). G" = ARTx (1 -X) [l + B(~x-1) + C(2x -1)2.. ..] (6) and the experimental results are then used to determine A, B, C... . If the com- position of the vapour in equilibrium with the liquid mixture is also measured, GE can be evaluated without having to assume any particular form of equation connecting GEand x. But in either case, it is essential to allow for the imperfection of the vapour phase. To deal adequately with this for pressures up to a few atmospheres it is not necessary to proceed beyond the second virial coefficient. There is, however, a serious shortage of experimental data on even the second virial coefficients of gas mixtures at low temperatures, and indeed even for some of the pure gases themselves, and work to remedy this deficiency would be valuable. The importance of correcting for the non-ideality of the vapour phase is strik- ingly demonstrated by Figure I, which shows the dependence on composition of the total vapour pressure of liquid mixtures of argon and krypton.2 It will be seen that the deviations from ideality (i.e.from Raoult's law) are small, but also that they are apparently negative at higher argon concentrations.When, however, allowance is made for the non-ideality of the vapour phase (which is tantamount to converting the partial pressures into fugacities), it is then found that the devi- ations from Raoult's law are positive over the whole composition range. B. The Determination of p.-Most of the published values have been obtained by the straightforward method of measuring the amount of each gas needed to fill J.E. Lennard-Jones and A. F. Devonshire, Proc. R. Soc. London, Ser. A, 1937, 163. 53; 1938, 165. 1. R. H. Davies. A. G. Duncan, G. Saville, and L. A. K. Staveley, Trans. Furuduj. Soc., 1967, 63, 855. 175 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures I 1 x(Ar1 Figure 1 Total vapour pressure for the Ar + Kr system. Upper curve, at 115.77K, the triple-point temperature of krypton; lower curve, at 103.94K a pyknometer of known volume at a known low temperature with a mixture, the composition of which is of course given by the amounts of the two components used to form it. The actual mixing therefore takes place in the pyknometer. The determination of the amount of a component condensed into the pyknometer depends on accurate pressure measurements, which give the pressure drop on removing the gaseous component from a reservoir of known volume maintained in a thermostat near room temperature.VE values obtained in this way are those for mixing at the saturation vapour pressure of the system at the chosen tem- perature. Since the liquid level in the stem of the pyknometer has to be seen, the apparatus is made of glass, and generally is not used at vapour pressures above -I .5 MPa. However, this relatively simple technique can be supplemented by studies of the equation of state of pure and mixed liquefied gases to which reference will be made later (Section 2D). The value of such studies is that they enable one to estimate the change with pressure of the excess functions.They can be used to give VE at a chosen temperature and at the saturation vapour pressure of a particular mixture by extrapolating the p, V isotherm for the mixture back to the saturation vapour pressure. It becomes more difficult to perform this extrapo- lation accurately, the higher the temperature, owing to the increasing steepness of the p,V isotherms, while at low temperatures near the triple-point of one of the components the liquid range over which p,V measurements can be made will be limited by the onset of solidification at a comparatively low applied pressure. Recently, Haynes et have described a magnetic method for determining ~1.~7 W. M. Haynes, M. J. Hiza. and N. V. Frederick, Rev. ScI.Instrum. 1976. 47, 1237 W. M. Haynes, Rcv. Sci. Instrum.. 1977. 48,39. 136 Staveley the densities of pure and mixed liquefied gases in which the quantity measured is the magnetic field required to balance the gravitational force on a barium ferrite cylinder immersed in the liquid. By carrying out experiments without any liquid (i.e. with the barium ferrite cylinder in a vacuum), the technique can be made to give absolute densities. This method can be used to -5 MPa, and while it is much more sophisticated than the pyknometric technique it clearly has considerable potential. Another method, developed by Singh and Miller,' depends on measurements of the dielectric constant E (permittivity). The connection between E, the molar vol- ume V, and the Mosotti-Clausius function M (the total polarization) is given for a pure species i by equation (7).Mi= Fi(Ei-l)/(Ei +2). (7) Ot T/ K Figure 2 Excess volume ,for the mkture 0.503 N, +0.497 CH, at the saturation vapour pressure. Circles, refs. 6 and 7; crosses, reJ 8 (Reproduced by permission from J. Chem. Thermodyn.. 1978, 10, 151) S. P. Singh and R. C. Miller, J. Chem. Thermociw., 1972, 4, 85; 1973, 5, 207.'Y.-P. Liu and R. C. Miller. J. Chem. Thermodyn., 1972. 4, 85.'D. R. Massengill and R. C. Miller, J. Chem. Thermoclyn., 1973, 5, 207.'M. Nunes da Ponte, W. B. Streett. and L. A. K. Staveley, J. Chem. Thermocfin., 1978, 10, 151. 177 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures It is assumed that M for a mixture is the mole-fraction average of the pure component values (i.e. that the excess of the function, ME, is zero).On this assumption, VE can be calculated from equation (8). This method readily lends itself to measurements over a range of temperature and pressure. All of the methods mentioned above are capable of a precision of a few parts in lo4. For binary mixtures of condensed gases, is often of the order of one per cent of the molar volume, and for such systems the experimental values of Vf (i.e. VEfor the equimolar mixture) should be reliable to 1 to 2 per cent. Figure 2 is a plot of VE at the saturation vapour pressure against temperature for an approxi- mately equimolar mixture of nitrogen and methane which incorporates results obtained in two laboratories.It will be seen that there is satisfactory agreement between the two sets of measurements. C. The Determination of HE.-This proved to be more difficult than the deter- mination of GEand VE.HEcan be deduced from measurements of GEat more than one temperature, using the relation (?G"/?T),= -Ht/TZ (9) but to obtain reliable values in this way requires exceptionally good GE data, and a direct calorimetric method is to be preferred. A series of calorimeters for mea- suring the enthalpy of mixing of two liquefied gases has been constructed in Oxford.'-l2 Known amounts of the pure components are liquefied into separate cavities. These cavities are separated in the earlier models by a metal diaphragm, and in the later versions by a valve, and together form a closed system.Mixing is accomplished by puncturing the diaphragm or by opening the valve and shaking the whole cryostat. Provision is made for monitoring the temperature of the vessel in which mixing takes place, and for supplying it with a known amount of energy. The mixing is almost always an endothermic process, and the measurement of HE becomes, in effect, a measurement of the energy which has to be supplied to counterbalance the temperature drop on mixing. An idea of the performance of such calorimeters is given by the results for the argon + methane system plotted in Figure 3. The accuracy is believed to be -1 to 2 per cent. More recently, a calorimeter has been constructed by Professor Streett and his collaborators at Cornell University (see ref: 35).This instrument is similar in many respects to the Oxford models, but it is more completely automated and the mixing of the two liquids is effected by a rotating stirrer. Figure 4 embodies what are thought to be all the HEresults for the nitrogen + oxygen system. For certain types ' R. A. H. Pool and L. A. K. Staveley. Trrins. Frrrrirki!. Soc ., 1957. 53. 11x6. 'O R. A. H. Pool. G. Saville, T. M. Herrington, B. D. C. Shields. and L. A. K. Staveley. Trrrrrs. Frirtiduj, Soc.. 1962. 58, 1692. I' K. L. Lewis G. Saville, and L. A. K. Staveley. J. CIr~nr.T/icwm~~~~i.,1975. 7. 3x9. K. L. Lewis and L. A. K. Staveley, J. Chenr. T/~er~?ro(/j.~r..1975. 7. X55. 178 Staveley II I I I Figure 3 Enthalpy of mixingfor the Ar + CH, system at 91.5 K.Full circles, ref. 11; open circles, ref. 13; square, re$ 14; cross, ref: 15 (Reproduced by permission from J. Chem. Thermodyn., 1975, 7, 855) of calorimeter-for example, combustion calorimeters, and those intended for the measurement of the enthalpy of mixing of liquids at or near room temperature- certain substances or systems on which careful experiments have been carried out in several laboratories are now generally recognized to be suitable for standard- izing a new calorimeter or for checking its performance. In the field of liquefied gas mixtures, a standard system with which to check HE measurements would also seem to be desirable, and the nitrogen + oxygen system would appear to be a good candidate for this.The two substances are readily available in a state of high purity, and the HEmeasurements can conveniently be made at or near the normal boiling- point of nitrogen. In the low-temperature calorimeters just described, the vapour phase cannot be wholly eliminated, whereas this can of course be done in those operating at or near room temperature. Corrections therefore have to be made for the transference of material between the vapour and liquid phases. There is no objection to this so long as the corrections can convincingly be made with adequate precision. If the vapour pressures are not too high, these corrections do not present a serious problem in HE determinations. But as the temperature rises and the critical region is ap- proached, the corrections become ever larger and more difficult to make accu- l3 M.Lambert and M. Simon, Physica,1962, 28, 1191.'' V. Mathot, Nuovo Cimenro, 1958, 9 Suppl. I, 356. Is J. Jeener, Rev. Sci. Instrum., 1957, 28, 263. 179 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures II I I 1I"" AL 60-a-'0 0//b x IN2) Figure 4 Enthalpy ofmixingfor the N, + 0,system. Open circles, 80.5 K, ref: 6;jilled circles, 77.7 K, ref: 19; open triangles, 80.3 K, ref: 1 l;jilledsquares,77 K, ref: 17; open squares, 77.5 K, ref. 18; solid triangle, value for 74 K derivedfrom GE,ref: 16 rately, and eventually it is necessary to change to a different type of calorimeter, such as the flow calorimeter.20 D.Equation of State Studies.-In recent years, an increasing amount of work has been carried out on the effect of pressure on the excess functions of liquefied gas mixtures, which has already added considerably to the useful data on these systems which is available for contemplation by theoreticians. It is now known that these functions can present a very different appearance at, say, 100 MPa from that given by the low-pressure values. From the molecular point of view, the effect of applying '' A. G. Duncan and L. A. K. Staveley. Truns. Faraday Soc., 1966, 62, 548. C. M. Knobler, R. J. J. van Heijningen, and J. J. M. Beenakker, Physica,1961, 27, 296. W. Kohler, Ph.D. Dissertation, University of Gottingen, 1964.l9 W. B. Streett, J. A. Zollweg, and K. P. Wallis, unpublished results, privately communicated. *' S. E. Mosedale and C. J. Wormald, J. Chem. Thermodyn., 1977, 9, 483. 180 Staveley 3 pressure is to change the balance between the forces of attraction and repulsion. As the liquid is compressed, the forces of repulsion become increasingly dominant, in that they have to oppose not only the intermolecular forces of attraction but also the externally applied pressure. The first experiments of this kind were made in the Soviet Union by Blagoi and Sorokin,21*22 who studied mixtures of argon + krypton and of krypton + methane to about 50MPa. Since then, similar studies have been made to rather higher pressures at Oxford, and more recently still at the Instituto Superior Tecnico in Lisbon and at the School of Chemical Engineering at Cornell University.The technique used in all these laboratories consists essentially in finding the amount of liquid which fills a metal cell of known volume at a known temperature under a measured applied pressure. (An alternative technique, briefly described in Section 2B, is that depending on permittivity measurements). Experi- ments must of course be done on the pure components as well as on at least one mixture of known composition. Given values of GE and HE at a low (or zero) pressure, their values at higher pressures can at once be obtained from the equation of state results by using them to calculate the changes in the excess functions from the relations given in equations (10) and (11).It will be seen from equation (11) that the calculation of the change of HEwith pressure requires a knowledge of the temperature coefficient of VE, so that the p, V,T studies must be made over a range of temperature. To illustrate the kind of results obtained, we show in Figure 5 the effect of pressure on VEfor an approximately equimolar mixture of liquid nitrogen and methane,8 in Figure 6 the pressure dependence of HE, SE,and GEfor the same solution, and in Figure 7 the pressure dependence of HE and GE for the argon + krypton system.23 Certain features at once emerge from these figures which are common to all the systems so far studied in this way. First, it will be seen that the applied pressure has its greatest effect over about the first 20MPa.Sec- ondly, there are much larger changes in HEand SEthan in G.EAlso, VE becomes numerically very small at high pressures. It will also be noted that for the nitrogen + methane system eventually becomes positive, so that at high pres- sures the molecules behave in the mixture as if they are slightly larger than they are in the pure liquids. One cannot therefore claim that a system has been comprehensively investigated until its behaviour under pressure has been investigated. But it is also necessary that it should be studied over as wide a temperature range as possible. Figure 2 '' Yu. P. Blagoi and V. A. Sorokin, Sb. Nauchn. Tr. Fiz.-Tekh. Inst. Nizk Temp. Akad. Nauk Ukr. SSR, 1969, 5, 5.22 Yu. P. Blagoi, A. E. Butko, S. A. Michailenko, and V. A. Sorokin, Akad. Sci. Ukr. SSR, Phys. Tech. Inst. LOW Temps., 1975, UDK 534.22, 538.34. 23 S. F. Barreiros, J. C. G. Calado, P. Clancy, M. Nunes da Ponte, and W. B. Streett, J. Phys. Chem., 1982, 86, 1722. 181 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures 120.00 K I: ' 1 1 I ' I ' 1 1' I ' 0 20 LO 60 80 100 120 P/ MPa Figure 5 Dependence on pressure of the excess volumeJor the mixture 0.503 N, + 0.497CH,, ref. 8 (Reproduced by permission from J. Chem. Thermodyn., 1978,10,151) I I I I I 1 I 1 -I -250 200 -L I 2 150-A v) I? 100-3 W-50-5 W' -2 0-\\ -50-I I I I 1 I 1 1 1 0 10 20 30 40 50 60 70 80 ' 182 Staveley I I I I I I 20 LO , 60 IAGE B A P/MPa Figure 7 Dependence on pressure of G" and HE,for the mixture 0.485Ar + 0.515 Kr, at 134.3K (A) and 142.7K (B), ref: 23 (Reproduced by permission from J.Phys. Chem., 1982, 86, 1722) illustrates the very marked effect which a rise in temperature can have on p.The influence of temperature on HEcan be still more striking. Figure 8 presents results for HEin the argon + methane system at the saturation vapour pressure, those at the lower two temperatures having been obtained using a calorimeter of the type described above,24 and those at the higher three temperatures with a flow cal- orimeter.20 It will be seen that HE decreases with rising temperature and changes sign well before the critical temperature (151 K) of argon, the more volatile com- ponent, is reached.Figure 9 for the argon + krypton system once again shows that is strongly temperature-dependent at low pressures, but it also demonstrates that at sufficiently high pressures, VE (now small) is almost unaffected by a change in temperature. 3 The Importance of the Enthalpy of Mixing (HE) The effort required to obtain reliable values of HEhas undoubtedly been worth- '' A. J. Kidnay, K. L. Lewis, J. C. G. Calado, and L. A. K. Staveley, J. Chem. Thermodyn., 1975,7,847. 183 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures T/ K Figure 8 Temperature dependence of the enthalpy of mixing ,for an equimolar Ar + CH, mixture at the saturation vapour pressure. Squares, rets.11 and 24; triangles, reJ 20 (Reproduced by permission from J. Chem. Thermodyn., 1977, 9, 483) a -1 c I2 m E w< -2 > -3 115 125 135 145 T/K Figure 9 Showing the effect of temperature on VEfor the mixture 0.485 Ar + 0.5 15 Kr at the constant pressures given in MPa by the$gure at the left-hand end of each curve, plotled from the results of Barreiros et al., ref. 23 184 Staveley while. In the first place, HE(and likewise SE)can be described as a more sensitive function than GE.Some experimental evidence which supports this statement has just been presented in Figures 6 and 7. Later, in comparing theoretical predictions with experiment, we shall see that calculated values of HEare much affected by the assumptions and approximations made in the theory.The reason for this is that if a theory gives an expression from which G” can be calculated, then to estimate HEand SEit is necessary to go further and evaluate (aGE/aT),, which clearly makes a reasonably successful calculation of HEand SEa more exacting undertaking than the prediction of GE. But there is another reason which puts a premium on HEdata, and this relates to the so-called combining rules. The intermolecular potential energy for a pair of molecules is often characterized by the parameters E and o,E being the depth of the potential well, and othe separation at which the energy is zero. Since in a mixture of two components 1 and 2, the three pair interactions 1-1, 2-2, and 1-2 have to be considered, the problem arises of the relation between E,, and o12for the unlike pair and the corresponding parameters for the two like pairs.It has been recog- nized for some time that the original Lorentz-Berthelot combining rules, namely equations (12) and (13). c12= $(oI1+ aZ2)(Lorentz) (12) ~)4= (E~~~ (Berthelot) (13) are not adequate, and so they are commonly modified by introducing parameters ,Jl ,and k ,, such that Equations (14) and (1 5)-or relations effectively equivalent to them-have been widely used in theoretical work. It was soon found that the calculated excess functions are sensitive to the values of j,, and k,, (especially the latter), small though these values may appear to be.We shall later refer to an attempt which has been made to improve on equation (13), and so eliminate the parameter k,,. But in most of the comparisons so far carried out between theoretical and experimental values of the excess functions for particular systems, to derive numerical values from the theory it has been necessary to sacrifice two pieces of experimental information to fixj,, and k,,, usually one value of GE(for a certain concentration and temperature) and one value of VE. If, therefore, the experimental data avail- able for testing the theory are limited to GEand VE, there is then little left to test. There remain only the concentration dependence of GE and VE (which is often almost symmetrical anyway, at least for GE),and their values (if available) at other temperatures.If HEis also known, however, the situation is much improved, and the addition of experimental values of the ‘sensitive’ function HEto those of G“ and VE must therefore be rated much more highly than merely as a fifty per cent increase in the available information. 185 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures 4 Some Observations on Current Molecular Theories of Solutions It would need considerable space to do justice to all the theories relevant to our subject, and indeed to just one type of theory, and our comments here must necessarily be brief. There are two systems of absolutely fundamental importance on account of their extreme molecular simplicity, namely argon + krypton and krypton + xenon.The intermolecular potentials in these systems are a function of the molecular separation only, there being no question of any angular dependence, and they have been very thoroughly studied. Two other very basic systems are argon + methane and krypton + methane, since the methane molecule is compact, highly symmetrical, and has no dipole or quadrupole moment. In testing earlier theories, it was often assumed that small diatomic molecules like those of nitrogen, oxygen and carbon monoxide could be regarded as being spherical, but clearly it is better to avoid this approximation and to recognize the anisotropic character of the field of force around such molecules. In any case, one of the objectives of the experimental work has been to obtain data for systems of molecules which, while simple, have characteristics such that the system would generate information of value in the interpretation of the properties of mixtures of substances which have larger molecules and which are liquid at ordinary temperatures. It was clearly desirable, therefore, to make a systematic study of the effect of polarity in the molecules of one or both components of a mixture. Accordingly, in the last decade a number of systems have been investigated, the molecules in which, while small, have dipoles, quadrupoles, or octopoles. Hydro- gen chloride and hydrogen bromide were chosen to represent dipolar molecules, though we shall see that the apparent simplicity of these molecules is probably deceptive.Dinitrogen oxide (nitrous oxide, N20) and ethene (C2H4) have been used to provide quadrupolar molecules. (Dinitrogen oxide has a dipole moment, but this is so small that its influence is negligible. It was preferred to carbon dioxide, since the relatively high melting-point and triple-point pressure of carbon dioxide create experimental difficulties). Symmetrical diatomic molecules such as those of nitrogen and oxygen also have quadrupole moments, but no dipole moment. Tetrafluoromethane, CF,, has been used as a component whose molecules are octopoles, but not dipoles or quadrupoles. It should be noted that polar diatomic molecules like those of hydrogen chloride and hydrogen bromide have a quadru- pole moment as well as a dipole moment.The types of system which have now been studied experimentally, with examples of the actual systems chosen, include the following: non-polar + dipolar (Xe + HCI; Xe + HBr); dipolar + dipolar (HCl + HBr); non-polar + quadru-polar (Xe + N,O; Ar + N,; Xe + C2H,); non-polar + octopolar (Xe + CF,); dipolar + quadrupolar (HCI + N2O); quadrupolar + quadrupolar) (N20+ C2H4); dipolar + octopolar (HCI + CF,). In the early stages of this ex- perimental programme, Gubbins and Gray and their collaborator^^^ --27 devel-oped a perturbation theory of solutions designed to examine the effect on the excess 25 C.-H. Twu, K. E. Gubbins, and C. G. Gray, Mol. Pi7y.s.. 1975, 29. 713. M. Flytzani-Stephanopoulas. K. E. Gubbins, and C.G. Gray. Mol. Phj.s., 1975, 30, 1649.’’ C.-H. Twu, K. E. Gubbins, and C. G. Gray. J. Chem. Phys.. 1976, 64.5186. 186 Staveley functions of dipoles, quadrupoles, and octopoles in the molecules of one or both components, and also of anisotropy in the dispersion and repulsive forces. Much of the rest of this review will be a consideration of some of the systems just mentioned, in which the experimental results (often obtained in Oxford) will be compared with the results of theoretical calculations carried out by Gubbins and his co-workers. A perturbation theory treats the real system by starting with a simpler system as a reference, and then converting this into the real system, as it were, by intro- ducing a suitable kind of perturbation.28 Here the perturbation takes the form of introducing into the molecules of one or both components a dipole, quadrupole, or octopole, and it also becomes desirable even for diatomic molecules like those of nitrogen and hydrogen chloride to allow for the angular dependence of the dispersion and repulsive (overlap) forces. It is advisable to choose as the reference system one which approaches as closely as possible to the real system to be treated.In recent theoretical studies of the effect of polarity, the fluid chosen as the reference has been one for which an n-6 potential operates between the centres of a pair of molecules. If n = 12, we have the Lennard-Jones 12-6 fluid, the properties of which have been thoroughly stud- ied by computer simulation methods.As shown by Ro~linson,~~ the properties of an n-6 fluid can be determined by relating them to those of the 12-6 fluid. To deal with a reference mixture of these idealized molecules, use is made of the well-tried van der Waals- I conformal solution theory,30 which equates the reference mixture with a pure fluid for which the CJ and E parameters are given by equations (16) and (1 7). 12 Since these equations require expressions for gI2and E,~,it is at this stage that the problem of the combining rules arises, and at which, if equations (14) and (15) are used, as is generally the case, the parameters k, and j, (or quantitities equivalent to them) enter the calculations. As regards the introduction of perturbation terms, we may illustrate this by considering the Xe + HCI system, where the perturbation concerns the two pair potentials involving HCI, namely HCI-HC1 and Xe-HC1.3 The HCI-HCI poten- tial is obtained by adding to the central n-6 expression the terms for dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interaction, plus terms to allow for the anisotropy in the dispersion and overlap forces.So even for such simple 2H J. A. Pople. Proc. R. Soc. London, Scr. A. 1954. 221. 498.'')J. S. Rowlinson, Mol. Ph~s.1964, 8, 107. 3" J. S. Rowlinson and F. L. Swinton. Liquids and Liquid Mixtures. 3rd edn.. Butterworths, London. 1982, chapter 8. 3' J. C. G. Calado, C. G. Gray, K. E. Gubbins, A. M. F. Palavra. V. A. M. Soares, L. A. K. Staveley, and C.-H.Twu, J. Cliem. Soc. Farodq. Trans. I. 1978. 74. 893. 187 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures molecule the expression for the potential energy has already become quite elabo- rate. The Xe-HCl potential is not so complex, consisting of an n-6 expression supplemented by terms for anisotropy and for the attraction resulting from the dipole induced in the zenon atom by the dipole and quadrupole of a hydrogen chloride molecule. It is, of course, important that the intermolecular potential functions adopted for the two pairs of like molecules involved in the treatment of a binary mixture should give the best representation of, or consistency with, the bulk properties of the pure components.It might be thought that we are dealing with substances which are so simple and so well-known that all the necessary information on their bulk properties would be available. But this is by no means always the case. For hydrogen chloride, for example, it would be invaluable to have p, V,T data on the liquid up to, say, 100MPa, from which the configurational energy could be calcu- lated as a function of volume. So here is another field in which the experimentalist can make a valuable contribution. The first thermodynamic function to be calculated is the Helmholtz function A, which is expanded in terms of the perturbing potential-not as a series, which generally converges too slowly, but as the Pade approximation --~___-A = A, +A, [1 -:,,A,] where A, is the value of the Helmholtz function for the reference system.(The term A, vanishes as a result of the choice of the reference potential). Once an expression for A has been obtained, the required excess functions for the mixture are derived from this using standard thermodynamic relations. For example, use of the equa- tion (r7A/2V)T = -p (19) leads at once to an equation of state for the mixture from which V" can be calculated . 5 Comparison of Theory and Experiment for some Selected Systems A. Xenon +Hydrogen Chloride.-The comparison of theory and experiment is usually made by comparing the actual and calculated values of the excess func- tions, but in Figure 10 the predicted and measured behaviour of this very non-ideal system is shown on a different basis, namely as plots of the total vapour pressure against the composition of liquid and vapour phases at three different tem- peratures.The problem of the indeterminate parameter in the cross-interaction potential [in effect, the k12of equation (15)] was resolved by assuming the experi- mental value of the vapour pressure for the equimolar mixture at the highest temperature. For this system, the value of/, in equation (1 4) proves to be virtually zero. The agreement between the calculated curves and the experimental points in Figure 10 is impressive. The asymmetry of the dependence of VE on composition is also found to be reproduced reasonably well. When, however, the comparison 188 Staveley xlHCI), y(HCI1 Figure 10 Total vapour pressure.for the system Xe + HCI at three temperutures. x(HC1) and y(HC1) are the mole,fractions of HCl in the liquidphuse (open circles) and the vapour phase (filled) circles respectively.The curves ure culculuted3 ' is made for HE,the outcome is rather different.32 Two calculated curves are shown in Figure 11, neither of which is in close agreement with experiment. Curve A was calculated using an HCI-HC1 potential which omitted terms to deal with the anisotropy of the dispersion and overlap forces, whereas for the calculated curve B these terms were included. It will be seen that their inclusion makes a consid-erable difference, and brings the calculated HE values nearer to, but still not into agreement with, the experimental results.On the other hand, inclusion of the anisotropy terms makes very little difference to the calculated GE and values. This is another illustration of what we have called the sensitivity of the function HE. In 1957, K~hler~~proposed an alternative to equation (13) (the Berthelot rule), based on London's formula for dispersion energy. Kohler's expression for E, is~ L. Q. Lobo. L. A. K. Staveley, P. Clancy, and K. E. Gubbins, J. Cliem. Soc., Faraday Trans 1. 1980, 76.174. '3 F. Kohler. Monarsh. Cliem.. 1957, 88, 857. (See also F. Kohler. J. Fischer, and E. Wilhelm, J. Mol. Srrwi., 1982, 84, 245). 189 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures 0 0.2 0.4 0.6 Q8 x(HCI) Figure 11 Enthalpy of mixing for the Xe + HCl system.Circles, experimental values. A and B are calculated curves (see text)32 where and a2 are the polarizabilities of molecules 1 and 2. Moser et al.34have applied equation (20) to the Xe + HCl system. Using the Lorentz and Berthelot relations and an HCI-HC1 potential similar to, but not identical with that used for the calculations we have just summarized, the phase diagram shown by the broken lines in Figure 12 was obtained. When Kohler’s equation (20) was substituted for the Berthelot relation, but no other changes made, calculation gave the full lines in Figure 12, which now correctly show that the system has a positive azeotrope. This is a striking demonstration of the importance of the combining rules.It seems likely that increasing use will be made of Kohler’s equation. It has recently been applied in calculations on the system carbon dioxide + ethane. The Lorentz equation (12) was also used, so that no experimental values of the excess functions had to be sacrificed. Excellent agreement between theory and experiment was obtained for HE and VE,though the agreement for GE, perhaps surprisingly, was rather less good.35 When we have spoken of a molecule having a dipole, quadrupole, or octopole, in theoretical treatments these have been taken to be a point dipole or a point multipole. Even with this simplification, the mathematics rapidly becomes compli- cated and sophisticated. Nevertheless, there is evidence which suggests that it may 34 B.Moser, K. Lucds. and K. E. Gubbins, Fluid Phase Equilibria, 1981, 7,153.’’ K. P. Wallis, P. Clancy, J. A. Zollweg. and W. B. Streett, paper submitted to J. Chem. Thernzodyn. 190 Staveley x(Xt). y (Xc) Figure 12 Vapour pressure-composition diagram for the Xe + HCI system. Circles, experi- mental values; broken curves, calculated using equation (13); continuous curves, calculated using equation(Reproduced by permission from Fluid Phase Equilibria, 1981, 7, 153) be desirable to try and improve on this approximation. For example, the way in which two hydrogen chloride molecules associate suggests hydrogen-bonding rather than simple electrostatic attraction. In the solid state, hydrogen chloride and its deuterated analogue each exist in two forms, a low-temperature orientationally ordered orthorhombic form and a high-temperature orientationally disordered cubic form.The structure of the ordered form of deuterium chloride, as determined by neutron diffra~tion,~~ is shown in Figure 13. The molecules form zigzag chains, the angle between adjacent molecules being almost 90". This is not the way in which dipolar molecules would preferentially group themselves on simple electrostatic grounds. The structure does, however, strongly suggest that the real ordering force is attraction between the proton or deuteron and the negative charge offered by a pair of unshared electrons in a p-orbital of the chlorine atom. In the low-temperature form of hydrogen chloride and deuterium chloride, the orientational ordering is of course long-range.While the high-temperature form is orientationally disordered, and the liquid no doubt still more so, neutron scattering studies have indicated that the same kind of ordering persists on a local scale even above the melting-p~int.~' It therefore seems that ideally an intermolecular poten- tial for HCI-HCl interaction which is to be used to treat the properties of the pure liquid and its mixtures should be based on a model that does not simply assign a 36 E. Slindor and R. F. C. Farrow, Nature (London), 1967. 215. 1265. 37 H. Boutin and G. J. Safford, Proc. Symposium on Inelastic Scattering of Neutrons, Bombay, 1964,II. 393. 191 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures \ I\ / / I I 1\ \ y/Figure 13 Structure of orderedphase of solid DCl.36Smaller circles, deuterium atoms; larger circles, chlorine atoms.Planes containing molecules shown as open circles are separated from those composed of molecules shown asJllled circles by half the unit cell dimension in the direction perpendicular to the paper point dipole and a point quadrupole to the molecule, but has charges so distributed within it as to be consistent with the tendency to form a hydrogen bond between a proton and a pair of p-electrons. Perhaps the cause (or part of the cause) of the discrepancy between the experimental and calculated values of HE for the HCl + Xe system is to be found here. These considerations raise the question whether one could choose a better representative of compounds whose molecules have a dipole moment than hydro- gen chloride.Unfortunately, there are very few substances which are liquid at sufficiently low temperatures for their mixtures with, say, liquid xenon to be investigated, and which at the same time have small molecules that are incapable of hydrogen-bonding but nevertheless have a sufficiently large dipole moment. One possibility is CIF, though apart from experimental difficulties which might arise from the chemical reactivity of this compound, some of the information on its physical properties which is needed to apply the kind of theory we are discussing may not be available at present. B. Xenon + Dinitrogen Oxide.-This system was chosen as a model for a mixture of non-polar molecules and molecules with a quadrupole moment.38 The linear 3x J.R.S. Machado, K. E. Gubbins, L. Q. Lobo, and L. A. K. Staveley, J. Chern. Soc., Fnraday Trans. I, 1980, 76, 2496. 192 Staveley molecule of N20has, in fact, a dipole moment, but this is very small (0.166 D, = 0.55 x 10-30Cm), and although terms for the dipole were included in the N20-N20potential in the perturbation treatment, their effect on the calculated excess functions is trivial. In an early perturbation study of the influence of polarity, Chambers and McDonald39 examined this by considering the excess Gibbs energy of an equimolar mixture of a non-polar liquid with one having molecules of dipole moment p.The result they obtained using the Pade approxi- mant is shown graphically in Figure 14. If one compares on this basis two imag- -I /'-E" LOO Figure 14 Showing the calculated e ect on G".for an equimolar mixture of a dipole moment p in the molecules of one component 5 (Reproduced by permission from Mol. Phys., 1975, 29, 1053) inary cases, one having p = 0.5 D and the other p = 1 D, the dipoles of the less polar molecules would only have about one-tenth of the effect produced by the more polar molecules. For molecules with ,u = 0.166 D, the effect would be negli- gible. In the theoretical treatment of the Xe + N20system, the parameters were given values to fit the experimental results for GEand VE for the equimolar mixture.The calculated HE is in excellent agreement with experiment (Figure 15). Initially, anisotropic dispersion and overlap terms were included in the N,O-N20potential, but it was found that virtually no difference was made to the calculated HE values if the anisotropy of the dispersion and overlap forces was ignored. This is in contrast to the position for the Xe + HCI system, where allowance for anisotropy substantially improves the calculated HE values (Figure 11). C. Xenon + Tetrafluoromethane.-For this system, chosen as a model for a mixture 3y M. V. Chambers and I. R. McDonald. Mol. P~Js.,1975, 29. 1053 193 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures . .E 80 c I d 601 ,c r ~ LO1 J C w z w -201c9 I I I I 0.2 0.4 0.6 0.8 x(N,O) Figure 15 Excess ,functions .for the system Xe + N,O.The points and the broken line are esperimental, und the continuous curves ure calculated38 of non-polar molecules and molecules having an octopole but no dipole or quadru- pole, the calculated values of HE are again in excellent agreement with experiment4* (Figure 16). x(Xe) Figure 16 Excess functions .for the system Xe + CF . The points and the broken line are experimental, and the continuous curves are calculated'0 'O L. Q. Lobo, D. W. McClure, L. A. K. Staveley, P. Clancy. K. E. Gubbins, and C. G. Gray, .I.Chern. Soc.. Faraday Trans. 2, 1981, 77, 425. 194 Staveley D. Dinitrogen Oxide +Ethene.-Systems of two quadrupolar molecules can be of two kinds, depending on whether the quadrupoles of the two species have the same sign or opposite signs.The way two quadrupolar molecules tend to associate is different, according to whether the signs of the two quadrupoles are the same or not (Figure 17). An example of a mixture where the molecules of the two com-If - +I 0 Figure 17 Showing the mode of association of two molecules with quadrupoles of the same sign(A), and of opposite sign (B) ponents have quadrupoles of opposite sign is N20 +C,H,. For the linear N,O molecule the charge distribution corresponds to that in the lower of the two quadrupoles in B of Figure 17, while that in the C2H4 molecule is represented by the upper quadrupole in B.The experimental values of GE, HE,and VE are plotted against composition in Figure 1K41 The values of are numerically small, but what is very unusual about this system is the extremely unsymmetrical concentration-dependence of VE. This is not reproduced by the perturbation theory applied to the previous systems we have discussed, and indeed it is doubtful if any current theory of solutions could do this. We shall see that for this system the incorporation of anisotropic overlap and dispersion terms does give improved calculated values of HE. But first a further complication in handling this system must be mentioned, which arises from the fact that the ethene molecule lacks axial symmetry, so that the quadrupole tensor has 4' L.Q. Lobo, L. A. K. Staveley, P. Clancy, K. E. Gubbins, and J. R. S. Machado, J. Chem. Soc., Faradq Trons.2. 1983, 79, 1399. 195 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures Figure 18 Experimental results.for the excess functions for the system N,O + C,H441 300 /’C/ c 0 I’I zE 20(3 / / / / // w z 100 ,I / I I I 1 ( 0.2 0.4 0.6 0.8 x(N20) Figure 19 Enthalpy ofmixing for the system N,O + C2H4. The points are the experimental values, and the three curves are calculated (see te.~t)~’ Staveley two principal components, and not just the one which suffices for a linear molecule like N,O. This can be dealt with either by making an ‘effective axial approxi- mation’, or by introducing the correct non-axial representation of the two indepen- dent components of the quadrupole moment. It is interesting that the ‘effective axial approximation’ gives good results for the properties of pure ethene, but inferior results for the N,O + C,H, mixture.Figure 19 sumarizes the results of three calculations of HE. For curve A, the ‘effective axial approximation’ was assumed, and the anisotropic corrections were omitted from the N,O-N,O poten-tial. In obtaining curve B, these corrections were still omitted, but the non-axial treatment of the quadrupole was adopted, while for curve C this treatment was combined with the inclusion of the anisotropic terms in the N,O-N,O potential. Clearly, curve C gives the best, though not perfect, agreement with the experi- mental results. E.Krypton + Nitrogen Oxide.-The reason for including this system in this review is rather different. Chemists have long been interested in liquid mixtures in which one component tends to associate. An example of such a system, provided by familiar substances, is benzene + acetic acid. Kr + NO may lay claim to be, from the molecular point of view, the simplest system of this kind. Nitrogen oxide, by virtue of the one unpaired electron in its molecule, forms a dimer. Molecules of acetic acid, which associate by hydrogen-bonding, undoubtedly form cyclic dimers, but they can in principle produce larger aggregates in more concentrated solutions or in the pure liquid, whereas with nitrogen oxide the association must presumably stop at the dimer stage. Accordingly, the system Kr + NO is well suited for adoption as a model system for a study of the effects of dimerization by one component.So far, only GE and VE at one temperature have been deter- mined,42 and further work could be profitably undertaken. The system is strongly non-ideal, having a positive azeotrope, and the dependence of VEon composition is somewhat asymmetric. It has been possible to make an analysis of GEto estimate how much of this function is due to the dimerization, and how much to what one might call the general causes of non-ideality. This led to the conclusion that roughly 60 per cent of the observed GEfor the equimolar mixture can be ascribed to the monomer-dimer equilibrium.F. Argon, Krypton, or Xenon + Ethane.-The long liquid range of ethane makes it possible to study the binary mixtures of this substance with each of the rare gases argon, krypton, and xenon. The Ar + C,H, system is strongly non-ideal, with Gt, the value for the equimolar mixture, equal to 374Jmol-’ at 90.2K.I2 For Kr + C,H,, GE is much less, being 80Jmol-’ at 117 K. The system Xe + C,H, was found to show negative departures from Raoult’s Law, with Gy = -29 J mol- at 163 K.43 Furthermore, liquid ethane and xenon were found to mix exothermically. In short, for this system, all the four excess functions GE, 42 J. C. G. Calado and L. A. K. Staveley, Fluid Phnse EquiMvia, 1979, 3, 153. 43 J. C. G. Calado. E. F. S. Gomes de Azevedo, and V.A. M. Soares, Chem. Eng. Commun., 1980,s. 149. 197 Recent Experimental and Theoretical Work on Molecularly Simple Liquid Mixtures HE, SE,and VE are negative. This implies some special kind of attraction or association between a xenon atom and an ethane molecule, though the reason for this is by no means obvious. The ethane molecule has a quadrupole moment, and the xenon atom is the most polarizable of the rare gas atoms, so inevitably quadrupole-induced dipole attraction must enter into the Xe-C,H, potential. But it is doubtful if any of the current theories would or could predict negative values for all four excess functions. Perhaps the spatial charge distribution in the ethane molecule happens to be particularly effective in relation to the size of the xenon atom, in which case a more detailed model will be required to do justice to the system.6 Conclusions It is fair to say that perturbation theory has achieved notable successes in account- ing quantitatively for the observed thermodynamic properties of molecularly sim- ple binary liquid mixtures in terms of the intermolecular forces involved. Where agreement between theory and experiment is less impressive, it would seem that the reason for this must be sought in the inadequacy of the potentials used rather than in any shortcomings of the theory as such. The reader unfamiliar with this field of physical chemistry may have been surprised by the complexity of the inter- molecular potentials already used for pairs of like molecules as simple as those of hydrogen chloride and dinitrogen oxide.Nevertheless, it begins to appear that there are limitations to what can be achieved if the polarity of a molecule is dealt with by assigning it a point dipole, point quadrupole, or higher multipole. It may prove necessary to move to point charge models which represent more realistically the electronic structure of the molecules, though calculations which are already lengthy and complicated will no doubt become still more so. The work surveyed in this review can be regarded as belonging to the first stage of an approach to the problem of the molecular interpretation of the thermo- dynamic properties of liquid mixtures. Proceeding from the simplest systems, the experimental work should deal with mixtures of molecules of increasing size and complexity, and so ultimately lead to an understanding on a molecular basis of the properties of mixtures of important and familiar substances which are liquid at ordinary temperatures.Few would question the desirability of achieving this goal, but it must be admitted that it still seems a long way off.

ISSN:0306-0012    

DOI:10.1039/CS9841300173

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

NYHOLM LECTURE* Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education By Peter J. Fensham FACULTY OF EDUCATION, MONASH UNIVERSITY, CLAYTON, V I C TORI A, AUSTRAL I A 1 Introduction In Britain, Australia, and many other countries, a very great deal of effort was devoted during the 1960s and the 1970s to developing new approaches to the teaching of chemistry in schools. Among the social influences that led to these efforts were the very great changes in chemistry itself, both as a discipline of knowledge and as a professional research and development field, that occurred in the years of post war recovery after 1945. It is timely now to make a realistic appraisal of what has been achieved through these efforts and in doing so to try to define and contribute to some of the challenges that confront us in the 1980s in the field of chemical education.In many countries, there do now exist much improved courses for educating in chemistry those pupils (about 20% of any school age group) from whom the future chemists and other science based professionals will come. With minor ups and downs in the supply of these professionals, the school systems (including those in many developing countries) are preparing this minority group of secondary age pupils with a quite reasonable efficiency. Indeed, in a number of countries as the economic conditions worsened in the 1970s, supply of this technical manpower resource overshot the demands, and problems of unemployment and under- employment of these skilled resources have occurred.On the other hand, the last five to ten years in particular have provided enough evidence that the sorts of chemistry courses we have developed have not been adequate for the purpose of educating the rising masses of secondary school students. They remain inadequately literate in the physical sciences and they do not have a sense of personal satisfaction and useful achievement from their encounters in schools with the subject of chemistry. The hopes expressed earlier in the 1970s by some that these learners might be educated through chemistry have not been sustained whether the criteria are immediate interest, sustained interest, positive learning, sense of achievement, or awareness of the many roles of chemistry in society.*Delivered at a Meeting of the Education Division of The Royal Society of Chemistry on 26 April, 1983, at the Scientific Societies’ Lecture Theatre, Savile Row, London W1 and at the Universities of East Anglia, Hull, Leeds, Queens, Coleraine, and Glasgow 199 Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education Throughout the 1970s concerns about the inadequacy of school science (includ- ing chemistry) education were expressed internationally via a number of slogans -an important method of highlighting issues in education. The Social Relevance of Chemistry Curricula was questioned early in the 1970s. Chemistry (and other science) education in schools was then called on by the late 1970s to contribute to an appreciation of the interplay between Science and Society.Technology was added to this duo and S.S. and T. is now a contemporary slogan for the issue of a more meaningful chemistry education in schools. At a meeting of the chemical education division of I.U.P.A.C. in Maryland in 1981 a plenary session was held entitled Tailoring Chemistry Curricula to Culture -one more way of expressing the sense of inadequacy about the contemporary efforts in chemical education. In the 1980s these concerns have crystallized in a number of places into formal acknowledgement of failure, and new policies have been established. In Britain, a Secondary Science Curriculum Review is under way.2 In Scotland, new policies to develop curricular programmes based on the Munn and Dunning Reports are beginning.In the Asia Region of UNESCO (to which Australia belongs) one of the two top priority educational projects for the next five years, is entitled, Science for All. Throughout the British Commonwealth a new priority area is entitled, Science Education in relation to the World of Work-a somewhat euphemistic title for a concern that stems from the existence of high levels of unemployed young persons among the post school population in most of these countries. A general question being asked in all of these stirrings and new beginnings can be phrased as follows: (i) Can chemistry, as a subject field, contribute to the schooling of the 80 + % of learners in each age group who are most unlikely to study chemistry again after leaving school? In the light of the above evaluations of present chemistry courses, two broad approaches to this question can be considered. The first is to suggest that there may be quite radically different ways to define chemistry for schools so that it does make an effective contribution to this 80%.The second is to suggest that there may be new ways to communicate chemistry, however it is defined, so that the learning of it is much better by all learners than occurs at present. The latter approach would also have a direct relevance to a more subtle evaluation that contemporary research is beginning to provide of the present school courses. This relates to the level of understanding of chemistry, as distinct from the examination efficiency, that the more ‘successful’ 20% acquire as they are prepared for possible study of chemistry beyond school.It appears that criteria of ‘understanding’ are much less well met by a great deal of present chemical education than is commonly believed. This new evidence is coming from countries like Australia, Germany, Sweden, USA, Britain, and New Zealand where these aspects of chemical learning have so far been explored. I.U.P.A.C., ‘Teaching Chemistry in a diverse world;. Proceedings Sixth International Conference on Chemical Education, University of Maryland, 1981. R. West, Sch. Sci. Rev. 1983, 64, 407. 200 Fensham A second question is thus posed to chemical educators by this further type of evaluation of much chemical learning that is occurring in our schools with the 20% of learners who may go on with further chemical study.(ii) How can chemistry be taught for greater understanding? Answers to this second question would, of course, also be a contribution under the second approach suggested above to question (i). Two fields of current research in chemical education contribute directly to the questions above and these will be drawn on in the two parts of this paper that follow. Both fields also involve conceptions, misconceptions, and alternative frameworks about chemical education. 2 Defining Chemical Curricula for Schools Historical and sociological studies of the curricula of schooling began to gain prominence with the work in the 70s of persons like Bourdieu and Passeron3 in France, and Young4 in Britain.With respect to the fields like chemistry, Layton’ illustrated from a study of some 19th century innovations in Britain how the curriculum for schools is shaped and constrained by a variety of competing social forces. Jenkins6 and Waring7 have carried out other analyses within this general approach as have Bailey’ and Fensham’ in Australia. The basic starting point for this type of research is the recognition that chemistry teaching in schools does not take place in a social or political vacuum. School systems and schools themselves are established by societies to fulfil social purposes, and the explicit and implicit curricula of the school are both major instruments for these purposes.Some of these purposes are political, such as sorting out those who will move, from school, into certain positions of power and influence and those who will not. Some are economic, in that they relate to ensuring that appropriate numbers and sorts of adequately trained manpower become available from the schools to maintain and develop the national economy. Chemistry as part of the total curriculum of schooling has and is serving both these purposes. It does sort out learners and it is an important ingredient in the preparation of a number of key technical groups in most economies. It has also been associated with another important social demand, namely, keeping the disci- pline of chemistry going and growing.Layton has referred to this purpose as ‘subject maintenance’ and it is certainly from the ranks of the successful 20% that university chemistry departments draw their own students and in due course their future colleagues. Figure 1 presents diagrammatically this societary view of the role of schools and P. Bourdieu and J.-C. Passeron, ‘Reproduction in Education, Society and Culture’, Sage, London, 1977. M. F. D. Young, ‘Knowledge and Control’, Macmillan, London, 1971. D. Layton, ‘Science for the People’, Allen and Unwin, London, 1973. E. W. Jenkins, ‘From Armstrong to Nuffield’, Murray, London, 1979. ’ M. Waring, ‘Social pressures and curriculum innovation’, Methuen, London, 1979. R. F. Bailey, ‘The decisions and control of Victorian curricula for chemistry, 1935-1975’, M.Ed.thesis, Monash University, Clayton, Victoria, 1978. P. J. Fensham, J. Curric. Sfud., 1980, 12, 189. 20 I Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education Individual Soclal Figure 1 The place of chemistry classes within schooling and the societary demands and constraints that affect their curricula of chemistry curricula in particular. The three social demands already mentioned are indicated in the top half of the diagram. From the 1940s to the early 1960s when the first products of curriculum renewal began to be used in schools, these three were the primary (almost only) tasks for the teaching of chemistry in schools. Only about 20% of an age group participated in the full scope of secondary education in many countries (including Britain and Australia). Chemistry teaching began with this group at or beyond 14 or 15 years of age, the compulsory age for schooling after which the majority of pupils left to enter the world of work.For many, it was the experience of employment that provided a second and more significant learning experience for adulthood than the school system had been able to achieve. Within the minority at school, chemistry was predominantly a field of study for boys who were, in any case, by the final years of the secondary school, a very clear majority of the remaining pupils. From 1960 onwards, the complement of the secondary school began to change rapidly with many more of an age group (and hence a wider cross-section of the society) participating.By the 1980s, in Australia and Britain, more than 60% of the age group were in school at 16+, and 30 to 40% of the 17+ group were in their sixth year of secondary schooling. Nor are the figures for these countries unusual, for many developed as well as some developing countries exceed this level of participation in schooling. A feature of this increase has been the greater involvement of girls. In Australia for example, by the late 1970s girls formed the majority in the final year of secondary schooling. Since 1975 these much changed demands on the school have been further complicated by the high levels of un- employment in many countries, especially among youth between the ages of 15-20.In Britain and Australia there is now more than 20% unemployment in Fensham this age group and many more than this fraction will experience some periods of unemployment after the completion (whether with formal success or not) of schooling. Unemployment among this potential school age group is now as significant a fraction as the percentage who will move on, with success, from schooling into the varieties of higher education. It is thus not surprising that schools and their curricula now face a number of other social demands. Among these are those we can call cultural since all these persons will live in societies and cultures that are inextricably influenced by and dependent upon the chemical and other sciences. Another social purpose stems from the need to have an informed populace who will support, or withhold support from, those developments that are possible because of technologies or products that are chemically based.Then there is always the purpose that relates to assisting individuals in all sorts of ways to share in and respond to the great human en- deavours we associate with chemistry. It is over the demands from these societary sources (see the lower part of Figure 1) that the concerns and admissions of failure of present chemistry curricula have largely arisen. Chemistry curricula can and do change. It is instructive to examine them over 1940s 1960s 1980s (Human 1 Conceptual (Theoretical) Secondary Schooling 95% of 15-16 ages 60% of 16-17 agesafter age 14 35% of 17-18 ages Majority male Majority Female Unemployment of 15 -19 ages very low very low 2031.Figure 2 Changes in the content of school chemistry 194&1980 and some features of the secondary school age population 203 Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education ‘0‘level C. S.E. Figure 3 Content analysis of examination questions for ‘0’level and C.S.E. Chemistry1980-81 the time scales when the societary changes mentioned above have occurred. Con- tent analysis is a method of classifying text books, course statements and exam- ination papers so that the knowledge to be learnt in a subject field is classified into broad categories. It is important in applying this technique to distinguish between the rhetoric of a curriculum and those aspects of it that, in various ways, become perceived by teachers and learners alike as being the ‘knowledge or learning of worth’. An analysis of the chemistry courses in Australian schools reveals a significant change in the knowledge of worth from the 1940s to the new curricula of the later 1960s.Conceptual or theoretical aspects of chemistry moved from about one third of the content to about two thirds, displacing much descriptive or factual content (including almost all the details of industrial applications of chemistry and the considerable number of references to historical persons in the development of the subject’s coverage of substances and explanatory ideas). Some further curricular reforms in the late 1970s did little to change the theoretical/descriptive proportion but did re-introduce some applications of chemistry.These analyses of content are shown diagrammatically in Figure 2 along with some data about the changing secondary school population. A similar analysis has been made using the ‘0’level and C.S.E. examination papers of the English examination boards for 1980 and 1981 as the source of ‘knowledge of worth’. Some of the alternative papers which, as yet, are taken only by a small proportion of chemistry pupils in Britain are excluded. The results of this analysis (see Figure 3) indicate a not very dissimilar type of knowledge content in these two kinds of chemistry courses even though the C.S.E.curricula were intended to extend meaningful learning of chemistry from the 20th percentile to about the 50th percentile of the age range. Few questions occur on the C.S.E. examinations that could not be asked on the 204 Fensham ‘0’level papers. The proportions of theoretical (C)and descriptive (D) knowledge are not very different although descriptive knowledge of chemical applications is more evident on the C.S.E. papers. There is no obvious block of questions on C.S.E. papers that relate chemical knowledge to social, economic, or political aspects of society. Nor do the common types of household or public chemicals emerge as the substances to be learnt about. There are, of course, a number of questions of both theoretical and descriptive chemistry on the ‘0’level papers that do not occur on the C.S.E.papers, but they are extensions of the same sorts of knowledge rather than being representative of other sorts of learning such as historical origins of the subject, technological applications, or explorations of the manner of chemical reasoning from data to theory. Another finding of these content analyses is the way the knowledge content is arranged for learning. By far the commonest arrangement in existing courses is as a sequence based on the development of a theoretical concept from its simplest or definitional form to more and more differentiated aspects of the same concept. This type of sequence is shown in Figure 4. A typical example of this arrangement is the introduction of acids and bases, quickly followed by their abstract definitions, which leads into their differentiation into weak and strong by quantitative aspects of these types of substances, and then followed by their interactions, etc.Again, chemical equilibrium is introduced and pursued through a variety of widely (and bewilderingly to the learner) different sorts of chemical systems-gases, solution, precipitation, complex formation, redox, etc. These familiar and obvious sequences will be contrasted later in the paper with other arrangements. It will suffice at this point to note that they are characterized by increasing complexity, and by a sequence of learning that means each step is important if subsequent steps are to be learnt, and which has no obvious termination except as the particular course content for a given level determines.Furthermore, particular chemicals or reactions are merely exemplary for the conceptual learning and not the actual focus of the learning. For example, LEARNING SEQUENCE FOR CONCEPTUAL KNOWLEDGE ~ ~ ~~ ~ ~ TEACHER CONTRIBUTION ~~ ~ ~~~-APPROPRIATE VERBAL IMPUTS NO CONCEPT A-CONCEPT A-CONCEPT A CONCEPT A DEFINED DEVELOPED-QUANTIFIED’~~~.? ? ? t i I I I I I I OPTIONAL ASSOCIATED EXPERIENCES OR APPLICATIONS Figure 4 The teachingllearning sequence for conceptual knowledge 205 Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education the reaction of hydrogen with iodine is commonly referred to in the sequence on equilibrium but the reaction with chlorine (an obvious analogue for many learners) is never discussed at this point.In many courses there are recommendations that practical work should precede or follow these conceptual steps if possible and in some of the more recent courses applications of a conceptual step are quite often given in the texts or course outlines. However, there is a sense in which these are optional extras since the focus for the learning remains on the conceptual knowledge and does not move to the realities of the practical work or to the applications as chemical events in their own right . It will, I believe, be evident that content for learning that is chosen and organized in the ways that emerge from these analyses have the following characteristics: (i) It is predominantly related to the theoretical knowledge of chemistry.(ii) It involves a sequential demand for conceptual learning without an obvious focus or terminal that has chemical reality. (iii) It expects the learner to be motivated to the theoretical knowledge of the discipline of chemistry or by the fact that chemistry is a required subject for his/vocational goals. (iv) It is preparatory for still more complex aspects in a later year of school, or in some course (such as university studies) that could follow schooling. Such characteristics require a persistence of interest and intellectual achievement in learners that many are not likely to maintain. Courses of this type are, however, likely to fulfil the three upper social functions in Figure 1 that involve selection, preparation and a high status for academic or theoretical knowledge.These sorts of analyses also indicate that during the 1970s, in both Britain and Australia, attempts were made to extend effective chemical learning to a much wider cross-section of secondary learners by using essentially similar, but reduced chemical content for learning. If these efforts are now part of the acknowledged failure that has been evidenced above, it is useful to summarize, as above, their characteristics since these will need to be avoided in any attempts to define alter- native chemical education for the 80%, which is likely increasingly to include, as part of the population of secondary schooling (through legislation or financial incentives), those groups who at present make up the politically embarrassing youth unemployed.If the success of present courses has been improvement of the education of a minority in chemistry, their failure is the education of the majority through essen-tially similar sorts of chemistry. Using these characteristics of present chemistry courses it is possible to list another set of characteristics that could be an alternative basis for learning from chemistry. One set of alternative characteristics for the content of the chemistry to be learnt could be: (i) It should have as its foci aspects of chemistry other than the theoretical concepts. (ii) It should have goals for learning that are obvious to (and in some cases determined by) the everyday lives of learners.206 F‘ensham (iii) It should draw its motivation from the interests of the learners, that is, from chemistry’s prospects of enhancing their mastery of the personal and social processes of living. (iv) It should include knowledge content that contributes to the terminal learning goals and is not simply preparatory to some future learning in the subject. (v) It should be capable of being learned in some sense by the majority of learners. It will not, however, be sufficient to consider chemistry as a corpus of human endeavour and to devise programmes for chemical education that are consistent with these alternative characteristics.This is but one of the tasks that is needed, although it is the one appropriate to address in this paper. The other task is the establishment of conditions of organisation and support within school and school systems, and of reward and understanding in the commu- nity so that such alternative contributions of chemical education become viable, attractive and effective. Pupils and their parents will need to perceive them as of personal worth, and employers and other community groups as having social worth. Many pupils among the 80%see little personal worth in present chemistry and their parents see only the extrinsic worth that achievement in chemistry keeps more doors open for employment possibilities. Present chemistry courses are perceived by employers as having vocational worth in some cases and more gener- ally as of moral worth since they involve hard and demanding attitudes of study.These socio-political tasks in relation to particular subject areas in the school curriculum will be a matter for each country and the appropriate conditions in one will not be transferable. It is, however, likely in all countries that the support of professional chemists, and in particular of powerful academic chemists in univer- sities, will be necessary if these changes or alternative forms of chemical education are to gain acceptance and not be seen as of little worth in comparison with traditional courses. In the case of C.S.E.and ‘0’level chemistry in England, to run these courses in parallel seems inevitably to restrict and downgrade the former, as well as not providing the ‘0’ level group with many aspects of chemistry from which they might also have gained benefit earlier in the secondary years.Garforth” has succinctly stated one of the problems associated with the recog- nition of radically new conceptions of a field of learning drawn from the richness of chemistry. ‘It may well be that there is a corpus of knowledge without which no syllabus could be called chemistry. Equally it may be that by our schooling, subsequent training and teaching we cannot see anything diferent adequately $lling the space called chemistry at the school level’. The potential of chemistry as a science and as a field of human endeavour for education in schools is far greater than is at present being exploited.This is not difficult to show. If chemists of all types are asked to list the features of their subject-the objects, people, events, facts, and ideas which they recognize in their lives as chemists-it is not difficult to obtain a list that can only be described as lo F. Garforth, ‘Chemistry to 16+ Examination’, Educ. Sci., 1983, No.102, 29. 207 Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education a very rich diverse conglomerate on which the word CHEMISTRY confers a common identity. It can only be represented by a multi-faceted collage of raw materials, processes and products, historical and contemporary persons, knowl- edge and numbers, commercial and research procedures, etc., etc.Once this multi-faceted corpus, CHEMISTRY, is drawn up, it is possible to view it from a variety of frames of reference, which in a sense lead to transects of the corpus. One such transect might list the processes and procedures chemists use for their purpose. Another would emphasize social or industrial or personal products. Again, there is a transect that represents the historical development of the subject and the contributions of its historical persons. There are the transects of descriptive properties of substances and the conceptual knowledge used to describe, gener- alize, quantify, and explain. These transects will constantly intersect each other and their junctions will focus on a substance, its properties, its uses, its descriptive origins and the persons of its past and present, etc.The content of school chemistry has, as has been shown above, at different times recognized some different transects of this corpus of CHEMISTRY, but there has rarely been much of its richness evident in the knowledge of worth. The particular strengths for education of the intersecting transects are in most contemporary courses even reduced from what they were in the 1940s. The chemical education of most recent developments for schools has been in a sense a journey along the surface formed by two dimensional transects (descriptive properties and theoretical concepts). It has been drawn and defined from within the corpus. Its limited two-dimensionality is reinforced by the representations of this sort of chemistry that make up the pages of a text-book, the sheets of examination papers or the words and symbols on blackboards so heavily used in chemistry teaching.When the education of the 80% is being considered, it is helpful to try to stay outside the corpus, viewing it from many angles, and selecting from it rather than becoming immersed in just some of its aspects. Nelson"*'2 has made a valuable contribution to this type of consideration of chemistry by urging that the transects of pure and applied chemistry can and should be traversed together. If this is done, he argues, chemical education will also regain the contribution of the intuitive approaches that are so valuable to chemists but which have been overshadowed by rationality in present courses.The 80% need to be educated about chemistry rather than in it. This does not mean at all that they should be taken on a quick and superficial tour of the surface of the corpus. Rather, it means that they are more likely to gain useful insights and knowledge about chemistry if they study in depth a few well chosen examples of its intersecting transects. In this sense the depth (compared with the breadth of conceptual coverage) of the chemical education of the 80% could be greater than for the 20% with their concentration on the theoretical. Later studies and experi- " P. G. Nelson, 'What is Chemistry, that I may teach it?' Department of Chemistry, The University, Hull, 1981. P.G. Nelson, Educ. Chem., 1983, 20, 122. 208 Fensham ence in employment, of course, add a number of other aspects of chemistry to those of the 20% who do eventually become practising chemists. How can such an approach to CHEMISTRY be used to define a chemical education that has the characteristics that may be appropriate for the 80%?Three possibilities will now be discussed to illustrate that there do appear to be good prospects for these alternative frameworks for a content of chemical education. In considering their merits the set of alternative characteristics is one sort of criterion to use. Another way of setting up criteria of worth is to list consistent learning outcomes that we know are not well achieved at present and desirable curricular experiences that also seem to be rather generally lacking.Table 1 lists some of these sorts of outcomes and experiences. Table 1 Some outcomes and experiences for more effective chemical education GOALS OF CHEMISTRY FOR ALL ~-~~~~~ ~ ~ EVERY STUDENT SHOULD BE ABLE ~ ~-__ 1. TO EXPLAIN A CHEMICALLY-BASED APPLICATION 2. TO EXPLAIN HOW THE SUBSTANCES OF EVERYDAY LIFE CAN BE REGARDED AS CHEMICALS 3. TO STATE (WITH RELEVANT DETAILS) THE SORTS OF PEOPLE WHO FIND EMPLOYMENT IN THE FIELD OF CHEMISTRY !YERY STUDENT SHOULD HAVE 4. PRACTICE IN THE APPLICATION OF CHEMISTRY TO REAL (DOMES- TIC, LEISURE, COMMUNITY, ETC) PROBLEMS 5. MEANINGFUL EXPERIENCES OF EACH OF THE MAJOR ACTIVITIES OF CHEMISTS 6.EXPERIENCE, WITH JOY AND EXCITEMENT, OF PHENOMENA THAT ATTRACT PEOPLE TO CHEMISTRY 7. SOME EXPERIENCE OF THE POWER OF CHEMICAL KNOWLEDGE. A. Learning what Chemists do.-Chemists work with a great variety of chemicals and reacting systems. However, it seems that, beyond these specific details, chem- istry is a relatively simple science in terms of the number of different activities that chemists do. In Table 2 there is a list of six of these essentially chemical ‘doings’. It probably needs a few additions but groups of chemists considering it have not been able to double or treble it. Each of these six has countless examples in research chemistry, in applied chemistry, and in the chemistry of living at home and in society. If we are prepared to put a focus on these activities it should not be difficult to enable every learner to have several meaningful examples of each type of doing.Such a content for chemical education could meet the alternative characteristics (i) to (iv) on pages 206 and 207 but what about (v)? 209 Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education Table 2 Essential activities or ‘doings ’that are characteristically chemical CHEMISTS’ NAMES PRACTICAL EVERYDA Y EXAMPLES PURIFICATION -SEPARATE -CLEANING CLOTHES SYNTHESIS -MAKE -FIBRE GLASS PATCHING ANALYSIS -IDENTIFY -DISTINGUISHING TURPENTINE FROM KEROSINE STRUCTURAL STUDIES, /DETERGENTS FIND ALTERNATIVE PROPERTY SEARCHING / \CHOOSING MATERIALS /INHIBIT -STEMMING CORROSION REACTION CONTROL FACILITATE -DEVELOPING A FILM Some idea about how many may be able to learn some of these practical skills can be gained from considering their teaching/learning sequence in Figure 5.The learning of practical skills has quite different stages for learning and instruction from those for conceptual learning in Figure 4.These differences are encouraging to the possibility of successful learning [alternative characteristic (v) on p.2071 at least to the level of improved skill. Indeed, it is known that much more complicated but desirable practical skills, such as driving a car, are learnt to this level by very large proportions of the population. TEA CHER CONTRIBUTIONS ~ ~ ~~~______~~ DEMO NSTRA TE, ENCOURAGE PRO VIDE 11 1 AND REQ WIRE PRACTICE, KNOWLEDGE COPY CORRECT ERRORS, BASES AND NOTE CONTROL FOR OF VARIANTS SKILL r I INO + PRIMITIVE + IMPROVED -* MASTERED SKILL SKILL SKILL SKILL Figure 5 TeachinglLearning sequence for practical skill acquisition 210 Fensham B.Learning about Chemicals as the Substances of Everyday Life.-Gillespie,‘3 Watts and Bayliss,14 Cole, Watts, and Bucat,” and numerous others have all criticized modern courses for their neglect of chemicals as the primary content of chemical education. Some of the teachers of the Alchem course in Alberta have tried to discipline themselves to introduce chemical topics to their beginning learners only if they can physically show them the chemicals involved.If it is the 80% with whom we are concerned, an emphasis on chemicals will need also to shift its focus, at least for a good deal of the time, from chemist’s chemicals to the chemicals of everyday life. In endeavouring to convince a class of 16 year olds of the power of chemical formulae for describing the composition of everyday substances, the author was confronted with more than one hundred different substances the pupils found from the contents data on the packets and bottles in their own kitchens and bathrooms. His reference books (quite reasonable compared with most teachers) provided the formulae for only fifteen of the substances. A British Pharmacopoeia and Mercks Index enabled formulae, properties, and uses of more than 90 to be reported back to the class.Most school teachers, at least in Australia, do not seem to have been introduced to these invaluable resources for everyday chemicals. Consideration of the source of available chemicals provides an easy and poten- tially motivating content for a chemical education which is based on the chemistry of chemicals and which takes their social, economic and political realities seriously. At any time in history, the available chemicals in a country or region are either natural raw materials, manufactured locally, or imported. At another time, what falls into each category will have changed if new chemical skills are acquired and the other determining social conditions have also changed. C. Learning about Chemical Applications.-Fehr’6, in the context of mathematics, pointed to a truism that would have profound effects if it was applied to chemical education: The chemistry of an application is not the same as chemistry with applications.The simplicity of this statement is beguiling, but its disturbing impact on our present practice is evident when the learning of an application (or a chemical technology) (see Figure 6) is compared with the learning of conceptual knowledge in Figure 4. Learning a chemical application begins with experience of the application and this initial focus (or terminal) remains very visible as the various aspects of the application (around the circle in Figure 6) are considered in the learning experi- ences. l3 R.J. Gillespie, Chem. Aust., 1980, 47, 499. l4 D. W. Watts and N. S. Bayliss, ‘Chemistry for Australian Schools’, Australian Academy of Science Report No. 23, 1979. l5 A. R. H. Cole, D. W. Watts, and R. B. Bucat, ‘Chemical Properties and Reactions’, Cole, Watts, and Bucat, Perth, 1981. l6 H. F. Fehr, Educ. Stud. Math., 1968, 1, 347. 21 1 Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education TEACHER CON TRlBUTlONS -EXPERIENCE OF THE ASPECTS OF-(APPLICATION ] Figure 6 A model for learning about a chemical issue, application, or technology In the case of conceptual learning the applications are simply references from the primary learning focus which is progression along the conceptual sequence. When an application from the corpus of chemistry is taken, the knowledge on which it is based immediately will be seen to belong to a number of these concep- tual sequences. In other words, the focus on an application quite radically re-orders the association of chemical knowledge and each bit draws its meaning from the focus of the learning, the application itself.Consider as an example the lead accumulator, which is one of the applications of chemistry that must be in the short list for having made the greatest change in the culture of many societies, because it has made individuals (both normal and impaired) mobile on land, sea, and in the air. If the chemistry on which this example is based is studied it will include the redox potentials of the electrode systems but only as one piece of chemistry along with topics like the solubility equilibrium of insoluble salts, the reversibility of the physical form of a solid as it goes in and out of solution, and the evolution of gas in systems involving metals and acids.Indeed when the question of alternatives for this application is consid- ered these other aspects to its chemistry turn out to be very critical since there are any numbers of redox pairs that could produce a similar e.m.f. When this same application is mentioned as part of conceptually oriented courses (as in Figure 4)it is only ever referred to when the redox sequence is being studied and thus many other aspects of its chemistry and social impact are over- looked. 212 Fensham 3 Teaching Chemical Education for Better Understanding It is now appropriate to address the second key question since a growing body of recent research is throwing light on it.Almost the whole of the efforts of curriculum reform in the last twenty years has assumed that chemistry is learnt as a result of two sorts of classroom interactions. These are shown as (a) and (b) in Figure 7. The first assumes that most learners in school have blank minds as far as chemical knowledge is concerned. These blank minds encounter teachers who have chemical knowledge in their minds (S,) which will be transferred to some more or less degree if the conditions (classroom climate, textbooks, practical experiences, intrinsic and extrinsic motivation, etc.)are right.Alternatively, it has been known that some learners do have primitive ideas (howl- ers or misconceptions) but as in (b) in Figure 7, it has again been assumed that if the conditions are right these will be easily displaced when they encounter the powerful ideas and chemical knowledge of the teacher. The recent research to which reference has been made is increasingly providing evidence that for many key topics in science education these assumptions are wrong. Studies in New Zealand (Osborne and Wittrock’ 7), Germany (Minssen and Nentwig’* and Duit * 9), England (Sutton”), Sweden (Anderson2 ’) and Australia (Fensham, West, and Garrard22 and Mitchell23), point to at least two other interactions (c) and (d) in Figure 7. Much of these data are from the ‘successful 20%’. In (c) of Figure 7 significant groups of learners have well formed ideas about chemical phenomena (SCJ that are not displaced by the teachers efforts.Rather, these learners accommodate the teacher’s knowledge for school purposes like examinations while retaining their original ideas because they seem more useful for dealing with the phenomena of the real world. Then there are learners, as in (d) of Figure 7 who reject the teacher’s knowledge and simply retain their own, becoming ‘failures’ or dropping out of chemical studies. Table 3a lists the terms that are being used to describe the learners’ ideas and those they meet through the formal teaching of science in school. In summary, this research indicates that many learners bring to the classroom from their outside and previous experience intuitive knowledge that has been described by the researchers with a variety of terms.In our own research24 the term ‘Children’s science’ (SCJ has been used (see Figure 7) because the ideas seem to involve powerful logic but limited experiences, and because a number of the ideas seem to be like those that were orthodoxy in the sciences at earlier periods in their development. For example, many learners have a view of burning that seems to be like that of the phlogistinists. ” R. J. Osborne and M. Wittrock, Sci.Educ., 1983, 67, 489. M. Minssen and P. Nentwig, J. Chem. Educ., 1983, 60, 476. l9 R. Duit, J. Sci. Educ., 1981, 3, 291. 2o C. R. Sutton, Eur. J. Sci. Educ., 1980, 2, 107.” B. Anderson and L. Renstrom, ‘Oxidation of Steel Wool’, EKNA Report No. 7, Goteborgs Univer- sity, 1982. 22 P. J. Fensham, L. H. T. West, and J. E. Garrard, Res. Sci. Educ., 1981, 11, 121. ” I. J. Mitchell, unpublished results, Faculty of Education, Monash University, Melbourne, 1983. 24 J. K. Gilbert, R. J. Osborne, and P. .I.Fensham, Sci. Educ., 1982, 66,623. 213 Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education (a) pupil tubulu rum, (b) teacher dominance (c) pupil two worlds, (d) pupil dominance teaching learning0 -Learner Teacher Learner (a1 teaching -learning 0 Learner Teacher Learner (b) teaching m @learning Learner Teacher Learner (C) teaching 8+ * learning Learner Teacher Learner (d 1 Figure 7 Interactions between the science frameworks of teachers and their learners In school these views, now more commonly regarded as conceptions or alterna- tive frameworks, meet a variety of views of chemistry from teachers. Sometimes these teacher views are like the present public orthodoxy of chemical science or a good textbook.Sometimes they are a teacher’s version of this topic or even a view that enables complex topics to be examined-examination science. For example, many learners at school get the impression that the three types of cubic lattices represent a common form of crystal structure, whereas they are in fact rarities among the crystal forms of real solids. They are teachable and examinable.As Table 3b also indicates, this sort of research also seems to be suggesting (West, Pines, and Sutton2’) that if the discrepancy between the learners’ and the teacher’s views is not large (or if the learners do not have relevant views), then evolutionary learning by interactions (a) and (b) of Figure 7 can occur. If, on the other hand, the discrepancies are large, interactions (c) and (d) of Figure 7 are 25 L. H. T. West, A. L. Pines, and C. R. Sutton, paper submitted to Rev. Educ. Res., 1983. 214 Fensham Table 3 (a) Some of the terms used to describe learners’ ideas and the sorts of science knowledge they encounter in school (b) The consequence of discrepancy between these different conceptions for learning OUTSIDE SCHOOL INSIDE SCHOOL EXPERIENCES SITUATIONS (intuitive) (non-intuitive) Naive theories Scientific theories Cultural theories Formal science Preconscious theories Teachers’ science Gut knowledge Disciplinary knowledge Personal knowledge Objective knowledge Children’s science Examination science Real world theories Alternative frameworks Children’s conceptions DISCREPANCY LEARNING Small Evolutionary Large Revolutionary probable and only by quite radical or revolutionary experiences will the learners’ views be much affected.Some of the chemical topics for which there is evidence that significant groups of learners (15% to more than 50%) hold views at variance with what formal chemistry expects are listed in Table 4. Several of these strongly held views have made us aware that many learners do not understand the concepts of conservation of matter or of atoms naturally, both of which much teaching of chemistry has taken as almost axiomatic. The ideas about burning and about the destruction of matter as a source of the energy of reactions, and about the non-conservation of atoms now help us to understand why stoich- eiometry so often turns out to be a great problem area for learning.These newly found complexities in the communication of these basic conceptual ideas in chemistry have enabled us to appreciate why the ‘understanding’ of many of the 20% falls short of our hopes. They also are strong indicators of why at least a number of the 80% never get to grips with chemistry and leave school without benefit from this subject.The alternative approaches discussed in Part 2 for the 80% are much more directly experiential and may have great advantage also to the learning of the 20%. A Case Example.-A recent study of learners’ understanding about the reaction of dilute hydrochloric acid and magnesium ribbon will illustrate some of these findings. This reaction was chosen because it seemed likely to be a simple and familiar one. Each learner was asked to observe the reaction as it occurred in test 215 Conceptions, Misconceptions, and Alternative Frameworks in Chemical Education Table 4 Examples of phenomena in Chemistry and learners’ views of them (Revolutionary learning) PHENOMENON EXPLANATION Burning Substances lose mass (‘Phlogistonist’) Disappearance in reaction Matter eliminated as energy (‘Einsteinian’) Involvement of gases Gases are normally air (Air =Oxygen =Gas) Sub-division of compounds Essential nature of substances (a non-atomic view) Existence of phases Substances have a natural phase.Heat can melt or heat can boil but no sense of three phases as reversible and simply a function of the conditions Nature of matter Fusion of experiential reactions with objective properties tubes held by them until the strip of ribbon had finished reacting with the excess acid. They were then individually interviewed concerning their observations and explanations for the reaction. Table 5 lists some common responses from the samples of learners who were drawn from the classrooms at different levels in several schools.The youngest group of learners had not encountered chemical symbolism or the use of equations to describe reactions, while the older learners were familiar with both. The first striking result was that the older groups reported so much less of the easily observable. There seems to be a tendency for more chemical learning to lead to less observation and to more mere repetition of the words used by teachers to describe this reaction. This may be symptomatic of a tendency for conceptual chemical education to divorce itself from the realities of chemical systems. A second finding was the varied and uncertain answers given by the youngest group to ‘what happened to the metal?’ and the confident convergent answer from the older students.However, this difference was more apparent than real when the older ones were asked ‘What do you mean by ‘dissolves’?’ This question elicited varying ideas. Some learners thought the magnesium would now be ions clustered at the bottom of the tube because ‘they are heavier than water’. Others thought they dissolved enroute to turning into the energy of reaction. One other result was the distinctly different answers to the question ‘Where does the gas come from?’ The answer, ‘the acid’, was re-inforced by the teachers in most cases and seems to be an example of teacher or examination science. Obser- vationally, ‘the metal’ or ‘the metal-acid interface’ are clearly more correct and in fact the product hydrogen has two components, one of which comes from the acid and one from the metal.Questions like ‘Where does the energy come from?’ or ‘What has happened to the acid?’ produce answers which indicate a complexity in this system that was Fensham Table 5 Some responses to the reaction of magnesium metal with acid RESULTS OF INTERVIEWS Year 9 14+ Year 11 16 + Year 12 17+ What do you observe? droplets up tube tube gets warm metal disappears bubbles of gas . ..(sometimes)Mg dissolves H, bubbles ...(sometimes)Mg dissolves H, evolved What happens to the metal? dissolves dissolves dissolves not sure disappears Where does gas come from? metal acid acid unexpected to the researcher.Among the total findings there are data from partic- ular schools that suggest evolutionary learning and other data are indicative of the evolutionary gap. Ausube126, twenty years ago, when the great phase of curriculum development was getting underway, urged that the way to improve the understanding of learning was to ‘find out what the learner already knows and teach him accordingly’. So far the research that has been described has been much more concerned with the first clause of Ausubel’s dictum than with the second. There has been some research on what it may mean to ‘teach accordingly’, but insufficient as yet to report in this general review. This type of research has, as indicated above, shown that the apparently simple task of asking learners questions (which many teachers may have thought they were doing) is one that can involve quite different types of teacher-learner inter-actions. In research, these interactions are entertaining and fruitful-full of ha-has and ah-ahs! Their transfer to the general repertoire of teachers in classrooms could be just as rewarding, but given the strength of present teaching patterns it is not likely to be easy. It is, however, a worthy challenge if the two questions of this paper are to find answers in the rest of this century. 26 D. P. Ausubel, ‘Educational Psychology-a cognitive view’, Holt, Rinehart, and Winston, New York, 1968. 217

ISSN:0306-0012    

DOI:10.1039/CS9841300199

出版商:RSC    

年代:1984

数据来源: RSC