Polymer Science Overview - ACS Publications - American Chemical

8 Herman F. Mark and the Structure of Crystals

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 17, 2016 | http://pubs.acs.org Publication Date: December 10, 1981 | doi: 10.1021/bk-1981-0175.ch008

LINUS PAULING Linus Pauling Institute of Science and Medicine, 2700 Sand Hill Road, Menlo Park, CA 94025

Herman Mark is famous for his tremendous contributions to the field of polymer science. He is not so well known for his early work in the field of crystal structure. I think that it was his experience in the crystal-structure field that gave him the background of knowledge that permitted him to make his important contributions to the understanding of polymers. He was, in fact, one of the leading investigators in the field of the use of x-ray diffraction for the determination of the structure of crystals in the years 1923 to 1928, and it was through this work that he developed the feeling for atoms and their interaction with one another that permitted him, later on, to make an effective attack on the problem of the structure and properties of macromolecules. The first volume of the Strukturbericht, written by P. P. Ewald and C. Hermann, covered the period 1913 to 1928. Herman Mark ties for third in the number of page references on crystal structures reviewed in this compendium. First place is held by the mineralogist Viktor M. Goldschmidt, with 89 references, many of them, however, to his great multivolume work Geochemische Verteilungesetze der Elemente. Second p l a c e , with 53 r e f e r e n c e s , i s held by Wheeler P. Davey, who was d i l i g e n t i n applying the powder technique that had been invented by H u l l (independently of Debye) to a l a r g e number of elements and simple compounds. The t i e f o r t h i r d p l a c e , 50 references, i s between Mark and R. W. G. Wyckoff. In h i s x-ray work Mark showed a greater range of i n t e r e s t s than the other i n v e s t i g a t o r s i n the f i e l d . Mark and h i s c o l l a b o r a t o r s s t u d i e d elements, both m e t a l l i c and nonmetallic, minerals, i n o r g a n i c compounds, simple organic compounds, condensed gases, and macromolecular substances, and i n a d d i t i o n they s t u d i e d the physics of x-rays and of the d i f f r a c t i o n phenomenon. Most of Mark's work done during t h i s p e r i o d was done a t the K a i s e r Wilhelm I n s t i t u t e of the Chemistry of F i b e r s or the K a i s e r Wilhelm I n s t i t u t e f o r P h y s i c a l Chemistry and E l e c t r o c h e m i s t r y , Berlin-Dahlen. H i s c o l l a b o r a t o r s during t h i s s i x - y e a r period

0097-6156/81/0175-0093$5.00/0 © 1981 American Chemical Society

In Polymer Science Overview; Stahl, G. Allan; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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i n c l u d e d W. Basche, R. B r i l l , W. Ehrenberg, K. W. G o n e l l , C. G o t t f r i e d , 0. H a s s e l , E. A. Hauser, J . Hengstenberg, H. Hoffmann, H. Kallmann, J . R. Katz, H. Mehner, Κ. H. Meyer, W. N o e t h l i n g , E. Pohland, M. P o l a n y i , P. Rosbaud, J . Steinbach, G. von Susich, Leo S z i l a r d , S. T o l k s d o r f , K. Weissenberg, and E. Wiegner. At the time that Mark began x-ray work the c r y s t a l s t r u c ture had not yet been determined f o r any organic compound. At that time the x-ray techniques could be a p p l i e d with the g r e a t e s t prospect of success i n determining the complete s t r u c ture to c r y s t a l s with high symmetry, e s p e c i a l l y cubic c r y s t a l s . It i s not s u r p r i s i n g that Roscoe G. D i c k i n s o n , who had begun c r y s t a l s t r u c t u r e work i n 1917 i n the C a l i f o r n i a I n s t i t u t e of Technology and who was hoping to be the f i r s t to determine the s t r u c t u r e of an organic c r y s t a l , should have s e l e c t e d hexamethylenetetramine, C5H12N4, one of the few organic compounds that forms c u b i c c r y s t a l s , f o r h i s i n v e s t i g a t i o n , and that Mark h i m s e l f should have s e l e c t e d the same substance. In January 1923 Dickinson and a student of h i s , A l b e r t Raymond, p u b l i s h e d t h e i r determination of the s t r u c t u r e , and e i g h t months l a t e r Mark and G o n e l l p u b l i s h e d t h e i r determination, with e s s e n t i a l l y the same results. Dickinson soon went i n t o another f i e l d of research, but Mark continued f o r some years to i n v e s t i g a t e c r y s t a l s of organic compounds. In most cases he found i t impossible to make a complete determination of the p o s i t i o n s of the atoms, but he u s u a l l y succeeded i n drawing some conclusions about the s t r u c t u r e of the organic molecules from the x-ray r e s u l t s . In 1923, with Eisenberg, he p u b l i s h e d an account of a p r e l i m i n a r y study of urea, followed by s t u d i e s of other organic compounds, u s u a l l y made i n c o l l a b o r a t i o n with von Susich, Mehner, H a s s e l , Hengstenberg, or N o e t h l i n g . The c r y s t a l s i n v e s t i g a t e d i n c l u d e carbon t e t r a i o d i d e , tetramethylmethane, tetranitromethane, p e n t a e r y t h r i t o l , o x a l i c a c i d , metaldehyde, b i p h e n y l , s t i l b e n e , t r i p h e n y l c a r b o n y l , triphenylmethylbromide, f l u o r e n e , phenanthrene, and ( i n 1929), D-glucose, D-fructose, and D - c e l l o b i o s e . In 1925 he reported on h i s work on condensed gases, C02, B2H , NH3, and CS2, c a r r i e d out together with Pohland. His s t r u c t u r e f o r carbon d i o x i d e i s the c o r r e c t one, except that the value of the parameter determining the carbon-oxygen d i s t a n c e i s i n c o r r e c t . He and Pohland reported t h i s bond length to be 1.6 X , r a t h e r than 1.16S. The boron-boron bond length found i n t h e i r study of diborane, 1.8 to 1.9 % , i s c l o s e to the c o r r e c t v a l u e , 1.77&. His f i r s t study of an element was c a r r i e d out i n 1923, with Weissenberg,Gonell, and Wiegner. They r e i n v e s t i g a t e d orthorhombic s u l f e r , which had been reported by W. H. Bragg to have a u n i t c e l l c o n t a i n i n g s i x t e e n s u l f e r atoms. Mark and h i s c o l l a b o r a t o r s found that each of the edges of the u n i t had to be m u l t i p l i e d by 2, to give a c e l l c o n t a i n i n g 128 atoms. They were not able to evaluate the parameters determining the p o s i t i o n s of the atoms. Mark and Hassel i n 1924 reported on t h e i r r e i n v e s t i g a t i o n of g r a p h i t e , which they found 6



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to have the s t r u c t u r e assigned t o i t by H u l l i n 1917, rather than an a l t e r n a t i v e s t r u c t u r e suggested by Debye. They evaluated the parameter as 0 + 0 . 1 0 . J . D. Bernai i n England was i n v e s t i g a t i n g graphite at the same time, and i n the same year he reported s i m i l a r r e s u l t s : the H u l l s t r u c t u r e , with the parameter equal to 0 + 0.06. The s t r u c t u r e of white t i n had been subjected to i n v e s t i g a t i o n f i r s t by B i j l and Kolkmeijer i n 1918, who reported an incorrect structure. In 1923 Mark and P o l a n y i c a r r i e d out a second i n v e s t i g a t i o n , and found the c o r r e c t s t r u c t u r e f o r t h i s t e t r a g o n a l c r y s t a l , a s t r u c t u r e i n v o l v i n g no v a r i a b l e parameters. In 1924 Mark and Hassel reported the r e s u l t s of t h e i r r e i n v e s t i g a t i o n of the s t r u c t u r e of bismuth, whose s t r u c t u r e had been determined i n 1921 by R. W. James, i n Manchester, who assigned the value 0.232 + 0.004 t o the v a r i a b l e parameter. Hassel and Mark v e r i f i e d the James s t r u c t u r e , with the parameter equal t o 0.236 + 0.003. The p r e s e n t l y accepted value f o r the parameter i s 0.2339. Mark a l s o extended h i s i n t e r e s t s i n 1923 t o i n o r g a n i c c r y s t a l s , beginning with calomel, mercurous c h l o r i d e . Calomel had c o n s t i t u t e d a puzzle f o r i n o r g a n i c chemists i n the 19th century, i n that the p o s i t i o n of mercury i n the p e r i o d i c t a b l e i s such as t o lead s t r o n g l y to the conclusion that mercury i s b i v a l e n t , as i n mercuric c h l o r i d e , HgCl2. Calomel has the composition that would permit the formula HgCl t o be w r i t t e n f o r i t , suggesting univalence. Chemists discovered, however, that calomel and s i m i l a r compounds of mercury (mercurous mercury) i o n i z e i n s o l u t i o n t o produce the diatomic c a t i o n H g 2 . Mark and h i s c o l l a b o r a t o r Weissenberg i n 1923 determined the s t r u c t u r e of the c r y s t a l , and found i t t o contain l i n e a r molecules Cl-Hg-Hg-Cl, w i t h the distances i n d i c a t i n g that the molecule contains a mercury-mercury covalent bond as w e l l as mercuryc h l o r i n e covalent bonds. This was an important c o n t r i b u t i o n t o s t r u c t u r a l i n o r g a n i c chemistry. Other i n o r g a n i c c r y s t a l s s t u d i e d by Mark and h i s c o l l a b o r a t o r s , sometimes l e a d i n g t o complete s t r u c t u r e determinations, i n c l u d e strontium c h l o r i d e , z i n c hydroxide, t i n t e t r a i o d i d e , potassium c h l o r a t e , potassium permanganage, and ammonium ferrocyanide. Minerals i n v e s t i g a t e d by them i n c l u d e CaS04 ( a n h y d r i t e ) , BaS04 ( b a r i t e ) , PbSO^, F e T i 0 5 (pseudobrookite), and three forms of A12SÎ05 (cyanite, a n d a l u s i t e , and sillimanite). Mark s x-ray work on f i b r o u s macromolecular substances began i n 1925, w i t h h i s p u b l i c a t i o n , together with Katz, of a paper on c e l l u l o s e . He continued the work on c e l l u l o s e with Meyer and Susich (1929). In 1926 he and Hauser published a report of t h e i r studies of rubber. He had developed e x c e l l e n t ideas about the nature of rubber and the explanation of i t s e x t e n s i b i l i t y and e l a s t i c i t y . I remember that when I v i s i t e d him i n Ludwigshafen i n the summer of 1930 both he and I took ++

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p l e a s u r e i n a demonstration that he showed me. He took a l a r g e p i e c e of unvulcanized rubber, and s t r e t c h e d i t to twice i t s l e n g t h . When i t was r e l e a s e d , i t c o n t r a c t e d to i t s o r i g i n a l l e n g t h . He then s t r e t c h e d the rubber and h e l d i t under a c o l d water f a u c e t , so that i t was cooled i n the stream of water. On being r e l e a s e d , i t remained i n the s t r e t c h e d form, which had crystallized. He d i s c u s s e d the p a r t played by the increased entropy of the c o n t r a c t e d form i n the e x t e n s i b i l i t y of rubber, much to my e d i f i c a t i o n . During t h i s p e r i o d he had already developed an i n t e r e s t i n the f i b r o u s p r o t e i n s . R. 0. Herzog and W. Jancke had made moderately good x-ray f i b e r diagrams of s i l k f i b r o i n i n 1920, and B r i l l i n 1923 had assigned i n d i c e s to about twenty spots i n the f i b e r diagram i n terms of an orthogonal u n i t with i d e n t i t y d i s t a n c e s of 7.0Â along the f i b e r a x i s and 9.3 and 10.4S p e r p e n d i c u l a r to i t . Meyer and Mark i n 1932 then d i s c u s s e d the s t r u c t u r e more d e f i n i t e l y , on the b a s i s of B r i l l ' s x-ray data. They suggested, as B r i l l had e a r l i e r , that the polypeptide chains extend along the f i b e r a x i s , and c o n t a i n g l y c i n e residues a l t e r n a t i n g with amino-acid residues with l a r g e r s i d e c h a i n s , p r i n c i p a l l y a l a n i n e and s e r i n e . Four polypeptide chains, extending along the f i b e r a x i s , would pass through the u n i t c e l l , with one g l y c i n e r e s i d u e and one a l a n i n e r e s i d u e of each chain i n the c e l l . They a l s o suggested that the peptide chains are s t r o n g l y a t t r a c t e d to one another by f o r c e s between the CO groups and the NH groups of adjacent chains. None of t h e i r conclusions could be drawn from the a n a l y s i s of the x-ray diagrams alone; i n s t e a d , they were the r e s u l t p r i m a r i l y of c o n s i d e r a t i o n s of the length of chemical bonds, the bond angles, and the nature of i n t e r m o l e c u l a r f o r c e s . Although the nature of the hydrogen bond was p r e t t y w e l l understood by 1932, twelve years a f t e r the f i r s t important paper on the hydrogen bond, by W. M. Latimer and W. H. Rodebush of the U n i v e r s i t y of C a l i f o r n i a i n Berkeley, had been p u b l i s h e d , Mark, i n common with most other European s c i e n t i s t s , s t i l l had l i t t l e knowledge about t h i s important s t r u c t u r a l f e a t u r e , and a c c o r d i n g l y he and Meyer could not make t h e i r suggestions about the i n t e r a c t i o n of the CO and NH groups p r e c i s e . In f a c t , even a f t e r the l a t e r refinement of experimental techniques, i n c l u d i n g the p r e p a r a t i o n of doubly-oriented f i b e r s of s i l k f i b r o i n by the s t r e t c h i n g and r o l l i n g of silkworn gut and the d i s c o v e r y of the Patterson diagram and other i n c r e a s i n g l y powerful methods of i n t e r p r e t i n g x-ray data, i t was s t i l l not p o s s i b l e to d e r i v e the s t r u c t u r e of s i l k f i b r o i n except through the d e t a i l e d a p p l i c a t i o n of p r i n c i p l e s of s t r u c t u r a l chemistry. By 1948 enough d e t a i l e d x-ray analyses of the s t r u c t u r e of c r y s t a l s of amino a c i d s , simple p e p t i d e s , and other simple substances r e l a t e d to polypeptide chains, a l l c a r r i e d out by Robert B. Corey and hj^s a s s o c i a t e s i n the C a l i f o r n i a I n s t i t u t e of Technology, had provided p r e c i s e information from



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which the i n t e r a t o m i c distances and bond angles i n polypeptide chains could be p r e d i c t e d with high r e l i a b i l i t y , about 0.01Â i n bond lengths and 1° i n bond angles. In p a r t i c u l a r , these observations v e r i f i e d the p r e d i c t e d p l a n a r i t y of the peptide group and the importance of the formation of the maximum p o s s i b l e number of N-H...0=C hydrogen bonds. The a p p l i c a t i o n of these s t r u c t u r a l p r i n c i p l e s and the use of accurate values f o r i n t e r a t o m i c distances and bond angles permitted the exact d e s c r i p t i o n of s e v e r a l p o s s i b l e c o n f i g u r a t i o n s of the p o l y peptide chain, the alpha h e l i x and the two pleated sheets. In p a r t i c u l a r , i t was found that acceptable sheet s t r u c t u r e s of polypeptide chains could not be formed by f u l l y extended p o l y peptide chains; i n s t e a d , the chains need t o be contracted somewhat, and staggered i n the d i r e c t i o n perpendicular to the f i b e r axis and the l a t e r a l hydrogen bonds. The p r e d i c t e d length of the two-residue u n i t of a completely extended polypeptide chain i s 7 . 2 3 Â , that f o r the a n t i p a r a l l e l - c h a i n pleated sheet i s 7.00 X, and that f o r the p a r a l l e l - c h a i n p l e a t e d sheet i s 6 . 6 ? . The c l o s e agreement of the p r e d i c t e d value 7.00 S and the experimental value 6.97 Κ f o r the f i b e r - a x i s u n i t length i n s i l k f i b r o i n i s strong i n d i c a t i o n that the s t r u c t u r e i s based upon a n t i p a r a l l e l - c h a i n p l e a t e d sheets. Moreover, the length of the a. axis i s c a l c u l a t e d t o be 9.5 % f o r the a n t i p a r a l l e l chain pleated sheet, i n good agreement with the observed value 9.4Â. In 1955 R. E. Marsh, R. B. Corey, and L. P a u l i n g showed that the a n t i p a r a l l e l - c h a i n pleated-sheet s t r u c t u r e accounts s a t i s f a c t o r i l y f o r the i n t e n s i t i e s of the x-ray d i f f r a c t i o n maxima o f s i l k f i b r o i n . The nature of the s t r u c t u r e of s i l k f i b r o i n suggested by Meyer and Mark i n 1932 was thus completely s u b s t a n t i a t e d , and somewhat r e f i n e d , twenty-three years l a t e r . In the meantime, as i s w e l l known, Herman Mark had continued to apply h i s understanding of the b a s i c s t r u c t u r e o f f i b r o u s macromolecules i n many important ways. Work on the s t r u c t u r e of c r y s t a l s and f i b e r s was not the only way i n which Mark made use of x-rays. With s e v e r a l c o l l a b o r a t o r s , he reported the r e s u l t s o f a number of s i g n i f i c a n t i n v e s t i g a t i o n s of the physics of x-rays i n 1926 and 1927. With Ehrenberg he reported s t u d i e s of the index of r e f r a c t i o n of x-rays, and with Leo S z i l a r d s t u d i e s v e r i f y i n g the l i n e a r p o l a r i z a t i o n o f x-rays s c a t t e r e d from e l e c t r o n s at 90*. An i n v e s t i g a t i o n of the width of x-ray l i n e s was c a r r i e d out by Mark and Ehrenberg, and Mark and Kallmann reported work on the p r o p e r t i e s of Compton-scattered x-radiâtion and on the theory of the d i s p e r s i o n and s c a t t e r i n g of x-rays. I cannot conclude t h i s account without mentioning another c o n t r i b u t i o n t o science by Herman Mark, not i n v o l v i n g x-rays. I b e l i e v e that t h i s c o n t r i b u t i o n , the d i s c o v e r y of the technique of determining the s t r u c t u r e of gas molecules by the d i f f r a c t i o n of e l e c t r o n s , c o n s t i t u t e s Mark's most important c o n t r i b u t i o n t o



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s t r u c t u r a l chemistry, one which, moreover, was of great s i g n i f i cance i n my own development. In 1930, when I v i s i t e d Herman Mark i n Ludwigshafen, I learned that he and h i s young a s s o c i a t e R. W i e r l , had c o n s t r u c t e d an apparatus f o r s c a t t e r i n g a beam of e l e c t r o n s from gas molecules and had determined the i n t e r atomic d i s t a n c e s i n carbon t e t r a c h l o r i d e and a number of other molecules by a n a l y s i s of the d i f f r a c t i o n p a t t e r n (Die Naturwissenschaften 18, 205, 778, 1930, and l a t e r papers by W i e r l i n Phs. Z e i t . and Ann. Phys. i n 1931 and 1932). The equations d e s c r i b i n g the d i f f r a c t i o n p a t t e r n produced by a wave (x-rays or e l e c t r o n s ) s c a t t e r e d by a molecule had been derived independently by P. Ehrenfest and P. Debye i n 1915. The e l e c t r o n d i f f r a c t i o n p a t t e r n from a molecule such as carbon t e t r a c h l o r i d e showed a s e r i e s of c o n c e n t r i c r i n g s , w i t h d i f f e r e n t i n t e n s i t i e s , and with the r a d i i of the r i n g s and t h e i r i n t e n s i t i e s depending upon the i n t e r a t o m i c d i s t a n c e s and the s c a t t e r i n g power of the atoms i n the molecule. I was overwhelmed by my immediate r e a l i z a t i o n of the s i g n i f i c a n c e of t h i s d i s c o v e r y . For s e v e r a l years I , i n common with other x-ray c r y s t a l l o g r a p h e r s , had been d i s a p p o i n t e d by the repeated f a i l u r e s to determine the s t r u c t u r e s of c r y s t a l s by a p p l i c a t i o n of the known procedures. What was expected to be a simple s t r u c t u r e determination o f t e n turned out to be i m p o s s i b l y complex. For example, i n 1922 the c r y s t a l K2Ni2(S04)^ which I had made and examined was found to have a s t r u c t u r e determined by nineteen parameters, l o c a t i n g the atoms i n the cubic c e l l , and i t was not p o s s i b l e even i n 1930 to determine more than perhaps h a l f a dozen parameters from the x-ray i n t e n s i t i e s . In f a c t , d e s p i t e the i n t e r e s t of many x-ray c r y s t a l l o g r a p h e r s , such as J . D. Bernai i n London, i n amino a c i d s and simple p e p t i d e s because of t h e i r s i g n i f i c a n c e f o r the p r o t e i n problem, even as l a t e as 1937 no one had succeeded i n l o c a t i n g the atoms i n any amino-acid c r y s t a l or simple p e p t i d e . The simplest amino a c i d , g l y c i n e , would probably r e q u i r e i n i t s s t r u c t u r e determination the e v a l u a t i o n of ten parameters, two f o r each of the f i v e heavy atoms, assuming, on the most o p t i m i s t i c assumption, that the molecule has a plane of symmetry, and the methods had not yet been d i s c o v e r e d f o r s o l v i n g such a problem. Whereas the i n v e s t i g a t i o n of any c r y s t a l by x-ray d i f f r a c t i o n was a gamble, i n that a simple molecule might i n t e r a c t w i t h i t s neighbors i n the c r y s t a l i n such a way as to make the s t r u c t u r e complex, no such c o m p l i c a t i n g e f f e c t was p o s s i b l e i n a gas. For example, D i c k i n s o n i n 1923 had found that the u n i t of s t r u c t u r e of t i n t e t r a i o d i d e i s a cube c o n t a i n i n g e i g h t molecules, with the atomic p o s i t i o n s determined by f i v e parameters, which he succeeded i n e v a l u a t i n g . But the Snl4 molecule i s t e t r a h e d r a l , w i t h i t s s t r u c t u r e determined by a s i n g l e parameter, so that one could p r e d i c t w i t h confidence that the i n v e s t i g a t i o n of the vapor by the e l e c t r o n d i f f r a c t i o n method would s u r e l y permit the v e r i f i c a t i o n of the t e t r a h e d r a l s t r u c t u r e and the



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determination of the v a l u e of the one parameter, the t i n - i o d i n e bond l e n g t h , without t r o u b l e . As t h e impact of the s i g n i f i c a n c e of t h i s d i s c o v e r y b u r s t upon me I could not c o n t a i n my enthusiasm, which I expressed t o Mark - my f e e l i n g that i t should be p o s s i b l e i n a r a t h e r short time, perhaps ten years, to o b t a i n a great amount of information about bond lengths and bond angles i n many d i f f e r e n t molecules. I asked Mark i f he and W i e r l were planning to continue with such a program, and he s a i d that they were not. He added that i f I were i n t e r e s t e d i n b u i l d i n g an e l e c t r o n d i f f r a c t i o n apparatus he would be glad t o h e l p , and i n f a c t he gave me the plans of t h e i r apparatus. On my r e t u r n to Pasadena i n September I t a l k e d with a new graduate student i n the C a l i f o r n i a I n s t i t u t e of Technology, Lawrence Brockway, about t h i s p r o j e c t , and he agreed t o undertake the c o n s t r u c t i o n of the apparatus (with the help and advice of my colleague P r o f e s s o r Richard M. Badger). During the f o l l o w i n g twenty-five years the s t r u c t u r e s of molecules o f 225 d i f f e r e n t substances were determined by the e l e c t r o n - d i f f r a c t i o n method i n the C a l i f o r n i a I n s t i t u t e of Technology, through the e f f o r t s of 56 graduate students and p o s t - d o c t o r a l f e l l o w s . These s t u d i e s l e d to the discovery of s e v e r a l v a l u a b l e p r i n c i p l e s of s t r u c t u r a l chemistry. I continue t o have a f e e l i n g o f g r a t i t u d e t o Herman Mark f o r h i s d i s c o v e r y of t h i s important technique and f o r h i s g e n e r o s i t y to me i n connection with i t . Herman Mark i s thought of by most chemists as a pioneer i n polymer s c i e n c e . I t h i n k of him, with a f f e c t i o n and admiration, as a pioneer i n modern s t r u c t u r a l chemistry and an important e a r l y c o n t r i b u t o r to i t s development. RECEIVED February 5, 1981.


Polymer Science Overview - ACS Publications - American Chemical

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