April, 1962
X-RAYDIFFRACTION OF ARSEKIC TRISULFIDE-IODINE GLASSES
cyclopentyl radicals) from studies of the Hgsensitized decomposition of these cycloalkanes in the gas phase. If, however, one accepts the value 0.9 would be of 1.8 for s-alkyl radicals, a value of expected for klb/k,a. On this basis the value of 1.7 found here seemdl unusually high. The results of this study illustrate the use of ethylene as a scavenger of both hydrogen atoms and radicals in the radiolysis of hydrocarbon liquids.
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733
The technique provides a means of measuring the activation energy for addition of radicals to ethylene in liquid hydrocarbons. Since ethylene, present in the concentration range 0.05 to 1.0 mole %, is effective in scavenging radicals it is suggested that in general radical addition to unsaturated hydrocarbons can be an important secondary reaction at low dose rates in the radiolysis of hydrocarbons if reaction is carried .to 0.1% conversion.
X-RAY DIFFRACTION STUDY OF ARSENIC TRISULFIDE-IODINE GLASSES BYT. E. HOPKINS, R. A. PASTERNAK, E. S. GOULD,AND J. R. HERNDON Department of Chemistry, Stanford Research Institute, Menlo Park, California Received November 9, 1961
The structures of arsenic trisulfide glass and of an arsenic trisulfide-iodine glass (46% by weight iodine) have been studied by X-ray scattering techniques. The data for the former indicate a short order arrangement of the atoms similar to that in the (crystalline) orpiment structure. The data for the iodine-containing glass 8u gest that when iodine is added t o arsenic sulfide, the layers are broken up into a structure (probably a twisted chain) having &+-I and S-S bonds. The fragmentation of layers also is indicated by the marked decrease in viscosity and softening point.
Experimental
height analyzer. A copper tube operating a t 35 kv. and 20 mamp. was used. Since no crystal monochromator was available, matched filters were employed,@the pair of filters being mil nickel, and cobalt oxide-in-plastic.6 The intensities were measured with each of the filters a t 0.01 intervals in s ( = 2 / h sin e), from s = 0.1 to 1.25, by the constant count technique (12800 cts). By taking the difference between the intensities measured with the nickel and the cobalt oxide filters, scattering intensities essentially due only to Cu Ka radiation were obtained. They were corrected for polarization and were brought to absolute scale by the method of Norman.? The scattering factors for As, S, .and I and the method of calculating the incoherent scattering intensities were taken from Compton.* Figure 1shows these corrected intensities, and the calculated total independent scattering curves. These data were converted by the formalism of WarrenSJo to differentialatomic distribution functions and to the atomic distribution functions. In these calculations increments of 0.01 8.-l in sin e/& were taken, and the distributions evaluated at 0.05 8. intervals. The radial distribution curves are shown in Fig. 2 and 3. Electronic distribution functions also were calculated but these did not differ in significant details from the atomic functions, except that peaks were broader.11
For the X-ray scattering experiments a large flat piece of arsenic sulfide glass as obtained from the American Optical Company was used. The samples of iodinecontaining glasses were prepared by fusing appropriate mixtures of powdered arsenic sulfide and elemental iodine; the melts were heated and agitated for 5 min. or more, then poured into a revercied cylindrical sample holder standing on a glass plate and allowed to cool to room temperature. Removal of the plate resulted in a smooth flat surface. The scattering intensities were measured with a Norelco diffractometer equipped with scintillation counter and pulse
As,SS-glass.-The’ differential radial distribution function of As& shqws only two well-resolved peaks, at 2.3 and 3.5 A.; a third broad maximum at about 5.5 8.rises barely above the average density. The two main maxima in the distribution curves obviously represent As-S (2.3 A.) and combined 1,3 As-As plus 1,3 S-S interactions. From
Arsenic trisulfide glass is of interest a t present as a material of high refractive index, transparent in the infrared, which, at the same time, can be processed by techniques suitable for glasses. It recently has been reported2 that the softening point and the viscosity of arsenic trisulfide glass are greatly lowered by the addition of elemental iodine. Indeed, if sufficient iodine is added, the resulting mixtures, which are also glasses, soften below room temperature. So striking are these effects, particularly in the composition region 42-52 weight % iodine,a that we felt a comparative Xray study of the structure of the iodine-containing glasses and arsenic trisulfide glass itself would be of interest. We are here reporting briefly the results of this investigation. After the conclusion of this work, a structural investigation of glasses in the AszSz-PbS system including pure arsenic trisulfide was reported by Petz and co-workers.
(1) The development o f arsenic trisulfide glass is described in detail by R. Frerichs, J . Opt. 8oc. Am., 48, 1153 (1953). See also J. Matsuda, J. Chsm. SOC.J a p m , 88, 208 (1959). (2) 5. 6. Flasehen, A. D. Pearson, and W. R. Northover, J . A p p l . Phys., 81, 219 (1960). (3) Paralleling these effects, the penetrability of arsenia trisulfideiodine glasses has been found to increase by about five powers of ten if the iodine content is rairied from 42 to 52% at 3 7 O . In these testa, the distance penetrated by a needle (tip area about 0.00015 cm.2) under loads varying from 50 to 500 g. after a 5-see. period was measured. (See ASTM Standards, 1958, Part 4, p. 1003.) (4) J. I. Petz, R. F. Kruh, and G. C. Amstuta, J. Chem. Phys., 84, 526 (1961).
Discussion
( 5 ) P. A. Ross, J . Opt. Soc. Am., 16, 433 (1928). (6) Obtained by courtesy of Dr. Milburn, General Eleotric Company, San Francisco. (7) N . Norman, Acta Ct-gst.. 10, 370 (1957). (8) A. H. Compton and 8. K. Allison, “X-Rays in Theory and Experiment,” D. Van Nostrand Co., New York, N. Y., 1935. (9) B. E. Warren, H. Krutter, and 0. Morningstar, J . Am. Chem. Soc., 19, 202 (1936). (10) See also H. P. Klug and L. E. Alexander, “X-Ray Diffraction Procedures,” John Wiley and Sons, Inc., New York, N. Y., 1954, p. 605. (11) The calculations were carried out with a Burroughs 220 eomputer. Copies of the program, in the Burroughs modification of ALGOL. are available from-T. E. H.
T. E. HOPKINS, R. A. PABTERNAK, E, 8. GOULD,AND J. R. HERNDON
734
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Vol. 66
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these two distances the average S-As-S and Ass-As bond angle is calculated to be 99". Fixrther-
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Fig. 3.-Differential radial distribution curves. Bars represent calculated interatomic interaction parameters.
more, from the area of the 2.3 A. peak in the radial distribution curve (Fig. 3), as obtained by integration, the number of As-S int,eractions was estimated to be 15. Assuming coordination numbers of 3 and 2 for As and S, respectively, twelve interactions would have been expected for the As& unit. Considering the limited precision of this study and of the technique in general, the agreement is acceptable. A quantitative interpretation of the area of the second maximum is not justified. On one hand, the definition of the peak is poor; on the other hand, van der Waals interactions between related structural elements almost certainly contribute to this peak also. An appreciable number of As-As, 4s-S) and S-S packing distances in the 3.5 to 4.0 A. range are found in orpiment (crystalline As&!&),l 2 the density of which is only 10% higher than that of the glass. A short-range structural similarity between the glass and orpiinent might be expected. I n crystalline As2S3, the average bond length is 2.3 h., the bond angles are approximately looo, and the coordination numbers of As and S are 3 and 2, respectively. The alternating As and S atoms are arranged in non-planar 12-membered rings, forming two-dimensional sheets. An important feature of the structure is a vector of 4.22 A. between atoms in the ring representing one of the repeat distances of the crystal lattice. The poor resolution of the differential radial distribution shows that order in the glass barely extends beyond the structural unit As&. Thus, if one assumes that the ring structure of orpiment is present also in the glass, (12) N. illorimoto, Mineral. J . (Sapporo), 1, 160 (1954), (in English). A number of the observed arsenic-to-arsenic and arsenic-tosulfur packing distances reported in orpiment are shorter than those corresponding t o the sum of the van der Waals radii of sulfur (1.9 b.1 and arsenic (about 2.0 I.). However the value for arsenic, which lies close t o the radius estimated for the As-3 ion, is muoh more appropriate for an arsenic atom bound t o hydrogen or carbon than for one bound t o oxygen or sulfur. The van der Waals radius for arsenic involved in an As-0 or As-S bond (and therefore bearing a partial positive oharge) doubtless should be smaller, but i t is difficult t o say how much smaller.
April, 1962
X-RAYDIFFRACTION OF ARSEXIC TRISULFIDE-IODINE GLASSES
the rings must adopt a wide range of conformations. Such disorder is ina accord with the absence of a maximum of 4.2%A. in the distribution and with the very small density variation a t r values above 5A. Asveak maximum at 4.2 A. is present in the data reported by Peta and c o - ~ o r k e r s ~apparently, ; their sample retained an additional vestige of orpiment-like order that was not present in our material. The Iodine-containing Glass.-Elemental iodine added to arsenic trisulfide during preparation of the glass is consumed chemically. Very thin layers of the resulting glass do not exhibit the violet color characteristic of molecular iodine, and the radial distribution curve of this glass shows no maximum or shoulder a t 2.66 A., the internuclear distance in molecular iodine. Moreove?, as the glass becomes richer in iodine, the 2.3 A. maximum shifts to 2.45 B., indicating that some As-S bonds have broken and that As-I bonds have been formed. Our data do not exclude the formation of S-I bonds, but because of the low stability of such bonds13 we assume that they are not present in significant number. If the tricovalency of arsenic is maintained, each As-I bond formed releases a sulfur atom having only one bond, almost certainly a thiyl radical. Such monocovalent sulfur atoms would not be expected to survive as such under conditions of glass preparation, where atomic mobility is high; they instead would undergo coupling to form disulfide linkages. Each two As-I bonds formed thus should result in the formation of one S-S bond, but the 5-S interaction, between the two lightest atoms in the ternary systems, mould not be expected to appear as a resolved peak in the radial distribution curves. The introduction of monocovalent iodine atoms into an array of dicovalent sulfurs and tricovalent arsenics necessariky reduces the average number of bonds per atom in the system and thus lowers the average molecular complexity”. The observed decreases in viscosity and softening point reflect this trend; in the glass having 46% iodine by weight (softening point below 60’), very nearly all of the sheets comprising the iodine-free glass have been broken. We have interpreted the radial distribution curves of the iodine-containing glass on the basis of a chain structure, although our data do not preclude the formation of rings containing four or more arsenic atoms. However, such rings would contain ten or more members, and the general difficulty in closing such macro rings in concentrated non-crystalline systems makes the presence of such rings in substantial numbers unlikely. An iodine content of 46 weight % corresponds t o AS~S,.~/&,or approximately to a 1:l ratio. Consumption of d l added iodine would yield roughly equal numbers of disulfide and monosulfide linkages. Discussion of the structure may be simplified further here by assuming that no arsenic atom is bound to more than one iodine (although a few -As12 groups may be present, necessarily a t (13) Compounds having the covalent S-I bonds are exceedingly rare. Moreover, sulfur appears t o form no binary compounds with iodine under ordinary conditions. (See, for example, A. Jakovin and P. A. Archangelski, %. anom. u. allgem. Chem., 226, 350 (1936).)
736
the ends of chains). Bond lengths 2hosen are the “standard” valyes (S-5, 2.08 A.; As-S, 2.28 A,; As-I, 2.56 A.), and the 8-As-S and Ass-As bond angles are taken as loo’, in analogy with the angles in orpiment,ll (some of which lie near 96’, with the others near 103’). Each As-I bond is assumed to make a maximum angle of 120’ with the adjoining 5-As-S plane, the iodine atom being equidistant from the two sulfurs. This allows a ‘(coordination position’’ for the one pair of unshared electrons on each arsenic atom.14 Neither of the two extreme conformations, shown below, for structures having planar chains is satisfactory. The “all-trans” conformation, I, should result in a pattern having maxima at approxiI I
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mately 2.3, 3.4, 5.3, and 6.9 &., corresponding, respectively, to 1,2-, 1,3-, 1,4-, and 1,5-interatomjc interactions in the chain. The 2.3,and 6.9 A. maxima are observed, and the 3.4 A. maximum conceivably could be included in the large “hill” between 3.3 and 4.8 A.,obut there is a minimum observed very near 5.3 A. I n the (‘cis conformation”, 11, which is actually an alternation of cisand trans-arrangements, the base of each trapezoid in the pattern, corresponding to half of the nonbonded 1,4-interactions in the chain, is only 3.1 A. This is far less than the sum of the corresponding van der Waals radii; moreover, the radiFl distribution curve shows a minimum near 3.1 A. Similar objections apply t o other, less regular, planar conformations having both cis and trans structural units, and also to small ring systems, which consist principally of (slightly distorted) cis units.15 A twisted chain is much more likely. Because of the very approximate nature of our study, an accurate estimate of the extent of “twist” is not possible, but our data would fit a twist of about 90’ a t each atom in the chain, i.e., a structure in which the arsenic and sulfur atoms lie on a series of dihedral angles of about 90°, the vertices of which are the bonds constituting the chain. As shown in Fig. 4, this results in a chain in which each bond is nearly parallel to the bond thrice-removed; these (14) See, for example, R. J. Gillespie and R. S. Nyholm, Quart. Revs. (London), 11, 368 (1957). (15) I n this discussion, molecular chains are shown with alternating monosulfide and disulfide linkages. This exaggerates the regularity of the structure, since the two types of linkage would be expected t o be distributed a t random within the chain. I n this way, however, all possible intrachain interactions of less than 7.5 8..except a 1,5-As-As interaction, are considered. This interaction is later incorporated into the semiquantitative treatment with a statistical weight of I,/%.
736
T. E. HOPKINS, R, A. PASTERNAK, E. S.GOULD,AND J. R. HERNDON
VOl. 66
u 0 1.0 2.0 3.0 Fig. $.-Twisted
chain structure proposed for arsenic sulfide-iodine glass.
bonds would be truly parallel if the interbond angles, as well as the dihedral angles, were 90°. I n the bottom section of Fig. 3, the differential radial distribution function for the iodine-rich glass is compared with the calculated interatomic interaction parameters (bars). The weight of the interaction between each pair of atoms is assumed to be proportional to the product of the atomic numbers; interatomic distances are estimated by measurement o[ scale models. (Calculated values only up to 7.0 A. are included, since limitations in our data and the long range random character of glass make such comparisons much less meaningful at high interatomic distances.) The first real corresponds to bonded distances, peak, near 2.45 8., about 2/3 of the weight due to the As-I bonds (2.6 A.) and about 1/3 to the As-S bonds (2.3 A.). Many interactions contribute to the seconfl (very broad) peak with a maximum near 3.8 A. Important contributions arise from the 1,3-iodinesulfur interactions (3.5 A.), from arsenic atoms separated by a single sulfur (3.4 A.), from arsenic atoms separated by a disulfide linkage (4.4 A,), and from iodine atoms separated by an -As-S-SIodine-iodine van der Waals’ As- chain (3.9 ,!%.).I6 contacts between chains (about 4.4 A.) contribute to this peak also; the latter interaction is indicated with dashed lines, for without knowledge of the mode of packing of chains, the number of such contacts cannot be reasonably predicted. The shape of (16) This value is significantly less than 4.4 b., twice the van der Waals radius of iodine. However the iodineiodine vector in this case makes an angle of only about 90’ with the As-I bonds, and nonbonded contact in this direction may be considerably less than the sum of the van der Waals radii. See, for example, L. Pauling, “The Nature of the Chemical Bond,” 3rd Edition, Cornell University Press, Ithaca, N. Y., 1960, p. 264.
the curve, however, suggests between one and two (certainly no more than two) interchain iodineiodine contacts per As2S312unit, corresponding to a very loose packing of chains. The third peak, having a maximum near 5.0 8., represents not on& the totality of l15-interactions in the chain (5.7 A.), but qlso the interactions of “trans” iodine atoms (6.1 A,) separated by -AsS-As- linkages, and a number of additional non-bonded arsenic-iodine intera$ions (5.5 and 5.9 A.). The small peak near 6.8 A., if it is not an artifact, may represent interaction between an arsenic and an iodine separated by a chain of the (6.8 A.). Thus, within the type -S-As-S-S-Aslimits studied, there seems to be no serious inconsistency between our radial distribution curve and the “twisted chain” model proposed for this glass. However, a structure containing large rings, although chemically unlikely, would have much the same short-range order as the chain and should show much the same radial distribution curve. A detailed analysis of the differential radial distribution function for the low-iodine glass (25% by weight iodine) wab not undertaken. The curve is intermediate in character between that of the iodine-free glass and that of the iodine-rich glass, suggesting that the low-iodine glass has structural elements present in both of the ,other glasses. The relatively low peaks on the low iodine curve indicate that this glass is less ordered than either of the other two glasses, and that a structural analysis would be more formidable. Acknowledgment.-It is a pleasure to acknowledge the assistance of Dr. Maurice Huggins for a number of valuable suggestions during the course of this work.
