Gerald D. Jacobs
A Physical Chemistry Experiment
Northern Michigan University Marquette, Michigan 49855
on Clathrates
O v e r the past several years we have been changing the emphasis in the physical chemistry lahoratory course from one of many individual, sometimes unrelated, experiments, to one of fewer but more comprehensive determinations. Esch experiment requires the mastery of several new techniques. I n this way we feel the student begins to understand how the chemist accumulates information from as many experimental methods as are required to solve a given problem. The experiment discussed in this paper has proven to he of great interest to the students, and introduces them to several new experimental techniques. At the same time it focuses attention on chemical bonding through a whole class of compounds which are just now beginning to he understood. The experiment requires thepreparation of an inorganic complex, the growth of some clathrate single crystals, the use of X-ray diffraction techniques in determining crystal structures, and.finally the determination of the density of a small crystal by the flotation method. The experiment may be expanded in several ways to provide additional work for the interested student. Theoretical
Clathrates constitute the class of inclusion compounds in which the guest molecule is completely surrounded by the structure of the host. This complete spatial enclosure sets clathrates aside from other types of inclusion compounds, such as those formed with urea, where the guest molecules are trapped in channels open at the
Figure 1.
Surroundings of one benzene molesvle in the clathrate Ni(CNh-
NHa. C6Hs.
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ends (1). The name clathrate was first suggested by H. 14. Powell following his X-ray structure determination of the hydroquinone compound. The name derives from the Latin 'Lclathratus" meaning enclosed or protected by the cross bars of a grating. The clathrate compound prepared in this experiment was first reported in 1897 by Hofmann and IGispert (2). They stated that a precipitate of composition [Ni(CN)1NHa. C6H6]was formed when benzene was added to a solution of nickel cyanide in aqueous ammonia containing acetic acid. Since that time many organic compounds have been substituted for the henzene as the guest component. Included in the list are aniline, furan, phenol, pyridine, pyrrole, and thiophene. An article giving many of the physical and chemical properties, as well as some commercial uses of this clathrate, was published earlier in THIS JOURNAL by Bhatnagar (3). Figure 1 illustrates part of the structure of the clathrate to show the surroundings of one benzene molecule. This cage and its enclosed molecule of henzene are often referred to as the unit cell (3). From the experimental data, one determines that there are two "formula units" per unit cell; thus Figure 1 is not the best representation of the unit cell. A more convenient model of the unit cell for demonstrating the two "formula units" is shown in Figure 2. For detailed information on internuclear distances, angles, etc., the reader is referred to the original article on the crystal structure determination by Rayuer and Powell (4). In Figure 2 one should note that the benzene molecules project through the sides of theunit cell so that one-half
Journol of Chemicol Education
Figure 2.
Another choice of unit sell for the clolhrote.
of each ring is considered to belong to the unit cell pictured. There are, thus, two benzene molecules associated with each unit cell. Experimental
The clathrate crystals are prepared by a layering technique, and then an X-ray diffraction pattern taken of a powdered sample. Several, small, single crystals are set aside for the density determination, from which the number of formula units per unit cell will he calculated. Preparation of the Sample. The monoammine nickel (11) cyanide complex is prepared by adding 9.0 g (0.05 moles) of Ni(CN)2.4Hn0 to approximately 300 ml of concentrated ammonium hydroxide. I t is important to note that this reaction should he carried out in a well-ventilated hood, and only under careful supenrision and proper safety precautions. It should be emphasized to the student that cyanides and their derivativesare deadly poisons! (5). The resulting mixture is stirred for one to two hours, and then filtered through a sintered glass filter. The deep blue solution is the monoammine nickel (11) cyanide complex. The clathrate is formed by careful layering of benzene on top of the complex solution. Actually, we have the students pour equal amounts of the complex solution inlo small Erlenmeyer flasks, and then layer pure benzene on some and 1:1 mixtures of benzene:p-xylene on others. The p-xylene is too large to enter the crystal, and serves only as a dilutmt. The clathrate crystd.lr, hegin to grow in a. few hours a t the interface of the layers. The student may observe the difference in growth rate between the flasks containing pure benzene and those which contain the bensene:p-xylene mixture. Interested students might also try growing crystals which contain some of the other guest molecules mentioned earlier in this paper. Once the crystals reach a critical size, they will fall to t,he bottom of the flask. The clathrate is insoluble in water. Approximately one week should he allowed for the clrtthration process to produce a good crop of crystals for use in the remaining parts of the experiment. Using a. Bllchner funnel, the crystals are now harvested and air dried. The orystdls usually vary in size from a few tenths of a millimeter an an edge to several millimeters. Some of the smaller, more perfect crystals should he removed a t this point, snd set aside for the density determination. Enough of the crystals are ground to a fine powder (ahout 325 mesh) so that a suitable X-ray powder diffraction pattern may be obtained. We normally use collodian as a hinder, and extrude the sample from a 0.5 mm diameter capillary. The X-ray pattern is then taken, and the resulting film prepared for measurement. The determination of the density of the crystals poses a very interesting problem to the ~tudent,i.e., finding the density of a. very smell solid. We have done this quite successfully for this elsthrate, using the method of flotation. A cylinder is placed in a eonstmt temperature bath (20°C) and partially filled with etbyl iodide (density 1.933). Several of the small crystals are t,hen floated on the surface of the ethyl iodide. With thorough stirring after each addition, chloroform (density 1.489) is added dropwise, unt,il a solution is ohtained in which the crystals remain suspended. Assuming the density of the crystals now matches that of the solution, we need only to measure the density of the solution. This may be done in several ways. We have obtained goad results using the Westphal balance or the method of refractive indices (6).
Calculations
The d values for each of the measured lines on the X-ray film are calculated, using the standard Bragg equation (7). The clathrate crystal formed between benzene and monoammine nickel (11) cyanide is of the tetragonal class (a = b # c, (Y = @ = y = 90'). The easiest way to determine the dimensions of the unit cell is to use graphical methods. Two charts for indexing tetragonal
crystals are in common use: Bunn Chart and Bjurstrom Chart. The Bunn Chart is available commercially (8). The log of the measured d values are plotted on a scale compatible with the Bunn Chart, and when a suitable match is found, the log (c/a) value can be read as the ordinate on the chart. I n addition, the Miller indices (hkl) may be assigned from the chart. The value of a (a = b) for the unit cell can be calculated by substitution of a d h value, ~ and corresponding values of (hkO) into the equation for a tetragonal system (9).
Similarly, the value of c for the unit cell is found by substitution of dwl and corresponding (001) values, into the preceding equation. The values for a and c may also be determined by the Bjurstrom method. I n this method a "fan diagram" is constructed by plotting the values of l/dZ as ordinate values and joining these points, by straight lines, to a common point on the abscissa. We commonly use a scale of 1 cm = 0.05 A-%. This fan diagram is then superimposed on the BjurStrom Chart, until a match is found. The Bjurstrom Chart may he easily constructed on the basis of tan 0 = c / a and examples are given in many reference books (10). A match occurs when the lines of the fan diagram lntersect the lines of the Bjurstrom Chart in a vertical line. The c / a ratio can then be read from the chart, and the individual values of a and c determined the same as in the Bunn method. Once the dimensions of theunit cell are known, as well as the density of the crystal, it is a straightforward calculation to determine the number of formula units per unit cell. Additional experimental work on this system has been carried out by some students to include single crystal X-ray diffraction studies, utilizing the Weissenberg camera. A comparison of experimental values found by a representative group of students with the accepted values taken from the literature is shown in the table.
Com~arisonof Exoerimental Parameters
Literature Values (4)
4
a c
f","$Xunits/unit cell
7.24 8.28 A 1.58 g/ce 2
Student Values (average) 7.26 A 8.31 A 1.58 g/cc 2
Literature Cited HAGAN, Sraran MARTINETTE,"CI&thmte Inclusion Compounds," Reinhold Publishing Corp.. New York, 1962, p. 7. (2) HOFMANN, K. A.. A N D KPIBPERT. Ti,, 2. Anoiy. Chern.. 15, 204 (1897). (3) BXATNAOAR, V. M., J. CXEI.EDUO., 40, 646 (1963). (4) R * r m n , J. H.. A N D POWELL. H. M.. J. Chem. Soc.. 319 (1952). ( 5 ) KAUFPMAN. G. B.. FAUST.G. E., AND TUN.P.. I. CXEM.EDUC..45, 141 (1968): (6) MIDOLEY. H.G..Ada Cryat.. 4, 565 (1951). (7) Az*na~a.L. V., *No B u ~ n 0 . n . M. J., "The Powder Method in X-ray Crystallography," MeGraw-Hill Book Co.. New York. 1958, p. 56. (8) Polycrystal Book Servioe, P.O. Box 11567,Pittsburgh, Pa.,15238. (9) See reference (7). p. 62. (10) See reference (71,p. 66.
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Volume 47, Number 5, M a y 1970
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