ANALYTICAL CHEMISTRY
736 ACKNOWLEDGMENT
The authors gratefully acknowledge the continued interest and support of George L. Clark, and the helpful cooperation and technical assistance of the personnel of the Statistical Service Unit of the University of Illinois. One of the authors (C. R. H.) wishes to express special appreciation to Armour and Co. for support by pre- and postdoctoral fellowships a t the university. Several months after this symposium there appeared a paper by Hughes (6) which gives a brief description of a new Caltech punched-card procedure whichemploysmanp of the improvements, including machine selection of cards and intersperse gang-punching, which are presented here. LITERATURE CITED
(P) Beevers, C. A., and Lipson, H., Nature, 1 3 7 , 8 2 5 (1936). (2) Cox, E. G., Gross, L., and Jeffrey, G. A., Ibid., 1 5 9 , 4 3 3 (1947).
(3) Cox, E. G., Gross, L., and Jeffrey, G. d.,Proc. Leeds Phil. SOC., 5, 1 (1947). (4) Cox, E. G., and Jeffrey, G. A,, Acta C ~ y s t .2, , 3 4 1 (1949).
(5) Hodgson, M. L., Clews, C. J. B., and Cochran, W., Ibid., 2, 113 (1949). (6) Hughes, E. W., “Punched Card Methods in Crystal Structure
Calculations,” from “Computing Methods and the Phase Problem in X-Ray Crystal Analysis,” ed. by R. Papinsky, pp. 141-7, State College, Pa., The Pennsylvania State College, 1952. (7) Lipson, H., and Beevers, C. .4.,PTOC. Phys. SOC.(London),48, 772 (1936). ( 8 ) Lonsdale, K., “Simplified Structure Factor and Electron Density Formulae for the 230 Space Groups of Mathematical Crystallography,” London, G. Bell & Sons, 1936. (9) Rimsky, A., Eller, G. v., Rose, -4.-J., and Guilhaumou, J., Bull. soc.franC. mineral., 74, 197 (1951). (10) Robertson, J. hl., Sature, 138, 683 (1936). (11) Robertson, J. h l . , Phil. M a g . , 21, 176 (1936). (12) Shaffer, P. A , , Jr., Schomaker, V., and Pauling, L., J . Chem. Phys., 14, 648 (1946).
RECEIVED for review November 19, 1952. Accepted March 28, 1953.
low Temperature X-Ray Crystallography BENJAMIN POST AND ISIDOR FANKUCHEN, Polytechnic Institute of Brooklyn, Brooklyn 2, A’. Y . The use of low temperature x-ray diffraction techniques extends the range of x-ray crystallographic analysis to substances which are liquids or gases at room temperatures. Within the past few years the crystal structures of a large number of low melting compounds have been determined in this way. These techniques and to investigate phase transformamay also be used for identification purposes - tions in these compounds.
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NTIL very recently a large number of compounds of considerable chemical and physical importance could not be investigated by the methods of single crystal x-ray diffraction merely because they happened to be liquids or gases a t room temperature. I n a f e v cases, Debye-Schemer techniques were used to investigate these compounds a t low temperatures where they were crystalline. As might be expected, hon-ever, only a limited amount of structural information could be obtained in this way. I n only a very few instances were efforts made to utilize singlecrystal techniques a t low temperatures; thus, in 1932, Cox studied the crystal structure of benzene (6) and, in 1936, T‘onnegut and Warren (28) determined the crystal structure of bromine from x-ray studies of single crystals a t - 150” C. The methods used in these investigations did not lend themselves readily to studies of a wide variety of compounds. I n 1947, Kaufman and Fankuchen (11),in the course of a study of the crystal structure of cyclo-octatetraene (melting point -17” C.), greatly simplified the techniques used in low temperature x-ray investigations. Their procedure involved cooling of the liquid sample, previously sealed into a thin-walled glass capillary tube, with a stream of cold dry gas. The progress of single crystal growth and orientation was observed and controlled with the aid of a polarizing microscope mounted on the x-ray camera. The light source was placed behind the x-ray collimator slit. When crystals of satisfactory size were grown and properly oriented, the light source was removed and the camera moved up, cooling stream and all, along the track to the x-ray source. From this point on, the xray investigation does not differ from investigation of materials which are solid a t room temperature. Improved and simplified versions of this apparatus have since been described (1, 20). The simple and inexpensive equipment required can readily be assembled in any reasonably rell-equipped laboratory to provide means of x-ray and microscopic euaminations down to the temperature of liquid air. As a result, low temperature single-crystal studies are now being conducted in more than a score of laboratories throughout the world.
These investigations may be divided into two major categories: determinations of crystal structures, and studies of structural transformations and disorder in crystals. DETERMINATION OF CRYSTAL STRUCTURE
Results of the crystal structure determinations are of considerable interest, particularly to chemists. Thus, a study of diketene (10) showed clearly that the molecule has the 3-buteno-p-lactone configuration, and that alternative structures which have been proposed must be rejected. An investigation of formic acid ( 9 ) showed, among other results, that the molecules do not associate as dimers in the solid, but are linked head-to-tail by hydrogen bonds to form infinite chains. Investigations of nitric acid and its hydrates (14) have elucidated the nature of the hydrogen bonding in the crystals. Many other crystal structure determinations have been completed a t low temperatures, including hydrazine ( 4 ) , sulfur dioyide (18), nitric oxide ( 7 ) , nitrogen pentoxide (8), pentaborane ( 6 ) ,methanol ( I S ) , and nickel carbonyl (12). STRUCTURAL TRANSFORBlATIONS AND DISORDER
Investigations of structural transformations and disorder in l o a melting organic compounds include studies of thiophene ( d ) , cyclopentane (19), tert-butyl chloride and bromide (21), cycloheyane (16), and 1,2-dichloroethane (15). Because this field of study is so new, the list of projects under way is far longer than the list of published low- temperature studies. The range of work in progress is nide and rapidly growing; it includes studies of the crystal structures of interhalogen compounds, various hydrides of boron, a number of simple hydrocarbons, organic and inorganic acids, etc. ( 1 7 ) . The applications of low temperature x-ray crystallography extend bel-ond investigations of crystal structures of substances which are liquids or gases a t room temperature. The techniques described above offer convenient methods of investigating phase transformations in substances which are solid a t room temperature. X-ray studies can also readily be made of rubbers and other polymers a t low temperatures.
V O L U M E 25, NO. 5, M A Y 1 9 5 3
737
Burbank (3) recently pointed out that collecting the x-ray data a t low temperatures offers one of the simplest means of improving precision of determination of bond lengths and bond angles, especially in relatively low melting crystals. The amplitude of thermal vibration a t room temperature, in substances like these, is so great that the location of the true centers of the broadened atomic peaks often is extremely difficult; and, in electron density projections, serious overlapping of adjacent atomic peaks is frequently encountered. ildvantage can be taken of the reduction in thermal motion with reduction in temperature by collecting the x-ray data at, say, the temperature of liquid air. The atomic peaks, a t this temperature, are effectively sharpened; centers are more easily located and overlapping reduced. It is clear that this brief listing by no means exhausts the possibilities inherent in the application of these simple low temperature techniques to x-ray crystallography. LITERATURE CITED
(1) Abrahams, S. S.,Collin, R. L., Lipscomb, W. S . , and Reed, T. B., Rea. Sei. Instr., 21,396 (1950). (2) Abrahams, S. C., and Lipscomb, W. K., Acta Cryst., 5, 93 (1952). (3) Burbank. R. D.. Ibid.. in mess. (4) Collin, R. L., and Lipscomb, W.K.,J . Chem. Phys., 18, 556 (1950); Acta Cryst., 4, 10 (1951). (5) Cox, E. G., S u t u r e , 122, 401 (1928); Proc. Roy. SOC.,A-135, 491 (1932).
(6) Dulmage, W.J. and Lipscomb, W. N., J . Am. Chem. Soc., 37, 3539 (1961); Acta Cryst., 5, 260 (1952). (7) Dulmage, W. J., Meyers, E. A , , and Lipscomb, W. N., J . Chem. Phys., 19,1432 (1951). (8) Grison, E., Eriks, K., and de Vries, J. L., Ibid., 3, 290 (1950). (9) Holteberg, F., Post, B., and Fankuchen, I., J . Chem. Phys., 20, 198 (1952). (10) Kate, L., and Lipscomb, W. N., Acta Cryst., 5, 313 (1952). (11) Kaufman, H. S., Fankuchen, I., and Mark, H., J. Chem. Phys., 15,414 (1947). (12) Ladell, J., Post, B., and Fankuchen, I., Acta Cryst., 5, 795 (1952). (13) Lipscomb, W. N., and Tauer, K. J., Ibid., 5,606 (1952). (14) Luaaati, V., Compt. rend., 229, 1349 (1949); 230, 101 (1951). (15) Milberg, M. E., and Lipscomb, W. X., Acta Cryst., 4, 369 (1951). (16) Oda, T., X-Rays, 5,26 (1948). (17) Post, B., and Fankuchen, I., “Low Temperature Bulletin,” 2nd ed., Division of Applied Physics, Polytechnic Institute of Brooklyn, July 29,1952. (18) Post, B., Schwarta, R. S.and Fankuchen, I., Acta Cryst., 5, 372 (1952). (19) Post, B., Schwarta, R. S., and Fankuchen, I., J . Am. Chem. Soc., 73,5113 (1951). (20) Post, B., Schwarta, R. S.,and Fankuchen, I., Rev. Sci. Instr., 22.218 (1951). (21) Schwarta, R. S., Post, B., and Fankuchen, I., J . Am. Chem. Soc., 73,4490 (1951). (22) Vonnegut, B., and Warren, B. E., Ibid., 58, 2459 (1936). RECEIVEDfor review November 19, 1952. Accepted December 17, 1952. IT-ork carried o u t under contract with Office of Naval Research.
Reactions of MercuryW Compounds with Ammonia X-Ray Difraction Studies WILLIAM N. LLPSCO-MB School of Chemistry, University of Minnesota, Minneapolis 14, Minn. X-ray diffraction studies of the products formed when ammonia reacts with mercury(1) compounds were carried out in order to elucidate the reactions by a method which, together with chemical evidence, would give an unambiguous answer to this problem. The compounds Hg(NH&C12, HgNHzCI, and HgzNCI.Hz0 were identified as products of the reaction of ammonia with mercurous chloride. No other products could be identified, thus specifically refuting earlier evidence for the corresponding mercury(1) analogs and for oxides or hydroxides of either valence state of mercury in this reaction. The bonding in all
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T I S inevitable that any study of the reactions of mercury(1)
compounds with ammonia will disagree with a large fraction of the existing literature. The reaction between calomel and ammonia was known to the alchemists, and attempts to discover the nature of the black precipitate have been made a t all stages of chemical history. As a generalized summary, the questions have been concerned with the presence of amido and ammino compounds of various formulas, as well as the presence of the chloride of Millon’s base and mercury oxide or hydroxide, and the question of mercury(1) versus mercury(I1) oxidation states for nearly all of these compounds. Writing, for example, HgNHICl as Hg2NCI.NH4C1or writing Hg( IXH~)~CIg as Hgz?TC1.3NH4CIhas been at times advocated or refuted; although the inference of a close relation among these compounds is chemically justifiable, this particular relation is incorrect. Finally, the only compounds demonstrable in any reasonable amounts by x-ray diffraction techniques are the three mercury(I1) compounds, Hg(SH7)?CI2,
three of these compounds has been shown to be the same. Complete structure determinations, now available for all three compounds, indicate that mercury forms linear bonds and nitrogen forms tetrahedral bonds. This reaction occurs in the Group I separation of the metals, and is taught in most elementary courses in qualitative analysis. The usual product associated with this reaction in most textbooks is HgNHSCI. However, depending upon the relative concentrations of ammonia and of the ammonium chloride produced by the reaction, Hg(NH&CIs and Hg2NCI.HzO frequently appear.
HgSHSCI, and Hg&Cl.H,O, or its dehydration product, HgZNCI. The close structural similarities of these compounds are described. MERCURY-4MMOYIA COMPOUNDS
Hg(NH,)gClz and Related Compounds. This structure was describrd by 11acGillavry and Bijvoet ( 5 ) as simple cubic iyith 111 ammonia at 000, chlorine a t - - -, and mercury placed statistically 222 so that on the average - Hg is present at the center of each face of 6 the cube (Figure 1). The value of the cube edge, a = 4.06 A , , is so small that the diffraction pattern is necessarily very simple indeed. Wyckoff (9) questioned the correctness of this structure, and suggested that the true unit is undoubtedly larger. However, experiments in this laboratory have confirmed the correctness of the original structure. Very well exposed photo-
