1359
PMROF SULFUR RADICALS
Paramagnetic Resonance of Sulfur Radicals in Synthetic Sodalites by S. D. McLaughlan and D. J. Marshall Royal Radar Establishment, Malvern, Worcester, England
(Received September 19, 1969)
The paramagnetic resonance spectra of sulfur-doped synthetic sodalites, produced by the hydrothermal pro cess, have been examined after various heat treatments. The blue-colored variety of sodalite, known as ultramarine, has been shown to arise from the presence of SS-radicals while the radicals responsible for green ultramarine have been identified with reasonable certainty. Results suggest that the sulfur is quite mobile in the sodalite and that the effect of heat treatment under vacuum is to make the sulfur aggregate to form radicals like Sa-. Some comments are made on the effects of varying the Al:Si ratio of the framework.
1. Introduction This investigation is part of a general study being made on sulfur-doped sodalites in connection with their photochromic properties. No correlation between photochromic behavior and the sulfur radicals reported here was established. However, some indication of the behavior of sulfur in sodalites was obtained and this may well be important for complete understanding of all the processes involved. The chemical composition of sodalite is 6NaAlSiOl 2NaCl. It has an open aluminosilicate framework with a group of four Na+ ions arranged tetrahedrally around one C1- ion in each cavity of the framew0rk.l The C1- ions can be replaced by OH-, Br-, I-, or sulfur in some form. At high sulfur levels, blue powders are generally produced which are normally referred to as ultramarines (natural species, lazurite). Ultramarine is a generic name used for a wide range of compounds derived from sodalite and can be obtained in various colors, e.g., blue, green, red, and violet. An early review of the constitution and structure of ultramarine is given by Jaeger2 but no firm conclusions as to the source of the color were given. Since that time various paramagnetic resonance studies have been made on natural and synthetic ultramarines3f4and it has been generally agreed that the coloration arises from sulfur radicals. However, the actual radicals responsible have not been identified. Hofmann, et u Z . , ~ have by a pyrolytic process prepared ultramarines which were blue, green, violet, and red. On the basis of chemical analysis and spin concentration measurements, they concluded that the optical absorption bands at 590, 520, and 380 nm were due to S2- and/or Sa-, S20 and SZ2-radicals, respectively. More recently, Hodgson, et u Z . , ~ observed two of the radicals reported here during an investigation of photochromic sodalites. They attributed one to an unspecified sulfur-containing species and the other to a defect on oxygen and neither affected the photochromic properties of the sodalite. Here these radicals are tentatively identified as specific sulfur radicals and the conditions under which they are produced are described.
The sulfur radical responsible for blue ultramarine is identified with reasonable certainty, while green ultramarine is produced when another sulfur radical is present in addition to that responsible for blue ultramarine.
2. Experimental Section The sodalites were synthesized by the hydrothermal process in welded platinum capsules of 0.25-1.5 ml capacity. Grade 1 alumina (Johnson Matthey, Ltd.) was thoroughly mixed with finely crushed “Spectrosil” silica (Thermal Syndicate, Ltd.) in the ratio A1203: 2Si02 and the required quantity was weighed into the capsule together with sulfur in the form of N&S-9H20 or Na2S4,and in some cases sodium chloride or bromide was also added. Sodium hydroxide solution was then added and the capsule sealed. The capsules were heated in commercially available apparatus using Tuttle-type autoclaves. The temperature was maintained at 410-420” and 550-600 bars pressure for 3-6 days. The autoclaves were then allowed to cool, the capsules opened, and the product thoroughly washed and dried at 110”. The sodalites crystallized as euhedral rhombic dodecahedra which could be from 5 to 500 pm in size but were usually in the range 15-25 pm. Each product was identified by X-ray diffraction using Debye-Scherrer cameras with filtered Cu radiation. Unit cell parameters were determined from carefully measured films using the Straumanis technique and the Nelson-Riley extrapolation methoda7 Selected powders were ana(1) L. Pauling, Natl. Acad. Sci., 16, 453 (1930); Z.Krist., 74, 213 (1930). (2) F. M.Jaeger, Trans. Faraday SOC.,25,320(1929). (3) D. M. Gardner and G. K. Fraenkel, J . Amer. Chem. SOC.,77, 6399 (1955). (4) Y.Matsunaga, Can. J . Chem., 38,309(1960). (5) U.Hofmann, E. Herzenstiel, E. Schonemann, and K. H. Schware, Z. Anorg. Allg. Chem., 367,119 (1969). (6) W.G.Hodgson, J. S. Brinen, and E. F. Williams, J . Chem. Phys., 47, 3719 (1967). (7) J. B. Nelson and D. P. Riley, Proc. Phys. SOC.,57, 160 (1945). Volume 74, Number 6 March 19, 1970
s. D. MCLAUGHLAN AND D. J. MARSHALL
1360 lyzed for halide and sulfur content by mass spectrometry before and after heat treatments. The paramagnetic resonance spectra of the powders were examined using a 9-GHz superheterodyne spectrometer, capable of working between room temperature and 1.4"K. The powder was contained in a thin polyethylene bag which covered the bottom of the Hall rectangular cavity. A small block of ruby having the c axis vertical (so that the Cra+ resonance lines were independent of magnetic field orientation) was placed in a corner of the cavity and the Cra+ lines used to compare the spectral intensities of the radicals in the powders.
3. Results Samples containing hydroxyl or halide ions and having high sulfur doping levels were generally blue and were usually examined without treatment. The lightly doped powders (up to 0.5 at. % S) were colorless as grown. If these mere heated to 900" in an evacuated nickel tube they changed color from white to yellow, then green and sometimes blue. The time taken to cover the whole color sequence seemed to depend on the composition of the powder. It was also found that this behavior was generally characteristic of powders doped with NazS4rather than NazS. Colorless or pale blue sodalites doped at higher levels (1.6-2.0 at. % S) with NazS4 became deep blue after a few minutes at 800900" in air. Because the crystal size was so small we were obliged to work with powders, usually about 50 mg for each investigation. Hence the angular dependence of the epr spectra relative to the crystal axes could not be studied. Similarly, any attempt at isotopic enrichment was unlikely to provide any information due to the complexity of overlapping spectra. However, the spectroscopic g values can be readily deduced from the observed spectra.* These can then be compared with those from sulfur radicals in other lattices as they do not seem to be sensitive to their environment. Powders which had a fairly high sulfur content (-2.0 at. %) consisted of mixed blue and colorless crystals. X-ray powder diffraction studies showed the colorless crystals to be hydroxysodalite and the blue, ultramarine. As the sulfur concentration was increased further, only blue crystals were obtained. The ultramarine was found to have the soodalite structure but with a unit cell edge of 9.03-9.06 A. The diffraction pattern, however, contained a first-order line of variable intensity arising from the (100) planes, which is not present in other sodalites. These blue powders showed the most unambiguous sulfur radical spectrum at 1.4" (see Figure 1). This can be interpreted as a rhombic spectrum having g values gl = 2.005, 92 = 2.036 and 93 = 2.046. The line width is estimated to be about 10 G, similar to that observed for the F-center in sodalitee6 Above 77°K the The Journal of Physical Chemistry
,
;92=2*036 I e2.005
SI3= 2.046
9,
Figure 1. Paramagnetic resonance spectrum of the SSradical in sodalite powder (blue ultramarine) at 1.4'K.
radical begins to rotate and a t room temperature an almost symmetrical line at g = 2.027 f 0.001 is observed (in good agreement with g1 92 g3/3 = 2.028). We will refer to this radical as C. For comparison with previous work on ultramarine, in particular that of Gardner and Fraenkel,3 we decided to examine one of the commercially available ultramarines-"Recliitt's blue." From this powder we observed an isotropic line g = 2.029 as reported by Gardner and Fraenkel and this remained isotropic even at 1.4OK Some boroultramarine, produced by fusing borax, boric acid, and sodium sulfide to form a deep blue glass, was also examined and 0.002, in this case a rhombic spectrum gl = 2.008 gz = 2.036 f 0,001, and 93 = 2.046 A 0.002 was observed at room temperature, similar to that observed by Matsunaga.4 We believe that all these resonances arise from the same sulfur radical in different environments. Some lightly doped sodalites having -0.2 at. % S, ie., those used for photochromic purposes, were also examined before and after treatment in an evacuated nickel tube at 900". Before treatment, no sulfur resonances could be observed probably because the spectra were highly anisotropic or because they were nonparamagnetic. Powders doped with KazS were largely unchanged after heat treatment while those having the sulfur added as NazS4 altered radically, the rate of change increasing with the sulfur content. After a short time (-0.5 hr, depending on the composition of the powder) and quenching to room temperature, the powders were found to be still white in color. The epr spectra observed from these powders consisted mainly of a single line g = 2.005 arising from a radical which
+ +
*
(8) F. K. Kneubtlhl,J . Chem. Phys., 55,1074 (1960)
Prim OF SULFUR RADICALS
1361
Table I: Comparison of Sulfur Radicals Previously Observed with Those in Sodalite
Radical
Host
So3-
Na.&OR.2Hz0
02
01
2.004
2.004
S ~ O Z - N ~ Z S Z O ~ . ~ 2.0030 H ~ O f 0.0002
2.0050 f 0.0002
2.0083
A
Sodalite
2,005 i 0.001
2.005 k 0.001
...
SSOB
NagS203.5H~O 2.0035 Sodalite
2.001"
2.0106 2.011 & 0.002
2.0287 2.029 It 0.002
2.015 f 0.001
S3-
NaCl
2.0014 i 0.0003
2.0308 f 0.0003
2.0465
C
Sodalite
2.005 i 0.001
2.036 =k 0,001
2.046 =k 0.001
a
2.004
2.005 & 0.001
.., j=
j=
...
0.0002
.,.
...
0.0003
2.028 =k 0.001
Method of identification and origin of results
%O3- spectrum observed ref 9 Isotopic enrichment, ref 16 This paper and Hodgson, et al., ref 6 Ref 17 This paper and Hodgson, et al., ref 6 Isotopic enrichment ENDOR, ref 18, 19 This paper
hIeasurement made by W. Hodgson, et al.
we will label A. Further heating (total treatment time -1 hr) resulted in the powder turning yellow in color and in the formation of a further radical B in addition to A. This had g values gl = 2.002 (difficult to determine due to overlap with A), gz = 2.011, and g3 = 2.029. Above 77°K this radical began to rotate and a t room temperature its spectrum consisted of an isotropic line at g = 2.015 (again in good agreement with gl g2 93/3 = 2.014). When the heat treatment time was extended still further (-2 hr, but dependent on the actual sulfur concentration) the powder turned green. The epr spectrum then consisted of radicals A, B, and C in roughly equal proportions. Further heating generally resulted in a blue powder which showed only radical C in the epr spectrum. It was found that the epr spectra of radicals A, B, and C were most readily separated out in the green powders at room temperature as here all these radicals give more or less isotropic resonance lines with the minimum of overlap (see Figure 2).
+ +
4.
Orotat
OS
Identification of Sulfur Radicals
Since only powders were available it was not possible to relate the axes of the g tensor to the crystallographic axes of sodalite. However, one can determine whether the crystalline field acting on the radicals is cubic, axial, or rhombic, and the principal values of the g tensor. For sulfur radicals, with the exception of Sz-,the g values tend to be rather insensitive to the environment so that the radicals can be determined with a fair degree of certainty by comparing values obtained from other host lattices. Radical C is a particularly good example. In Table I, the g values obtained for the S3- radical in NaCl are compared with those of radical C and one can see that there is good agreement. Similarly, radical B would appear to be SSO- beyond reasonable doubt. One cannot be so certain in the case of radical A as there ape two possibilities, either so3- or SZOZ-. This difficulty arises because of the similarity of sulfur and oxy-
gen so that these radicals give similar spin resonance spectra. However, the spectrum of SrOz- previously observed was slightly rhombic. This was not observed but may have been concealed by the resonance line width and also by radical B which was generally present at a lower level. We feel that the SO3- radical is more likely. Thus it would appear that initially the sulfur is in some form whose epr spectrum could not be detected, probably monotornic, and this is first converted to so%-,the progression with increasing time of heating being monatomic -+ SO3sulfur (?)
(white powder)
+ sso(yellow powder) +SO3- + SSO- + S3- (green powder) 4803-
----,
s3-
(blue powder)
The visible optical absorption spectrum of these powders was studied. The fact that a colorless powder was obtained when only S03- was present agrees with the results of Chantry, et aLj9who found that the main absorption bands occurred at 270 and 240 nm. The yellow coloration which arises from a band at 390 nm might be associated with the SSO- radical. However, the possibility that it arises from some radical which is either nonparamagnetic or has a highly anisotropic g tensor cannot be ruled out (e.g., S2- or Sz-). The blue powder shows a single absorption band at 610 nm due to the S3- radical. This conclusion is supported by Morton,'O who performed a combined optical esr experiment on blue-green K I : S3- and RbI :Sacrystals. He showed that the coloration is due to the (9) G.W. Chantry, A. Horsfield, J. R. Morton, J. R. Rowlands, and D. H. Whiffen, Mol. Phys., 5,233 (1962). (10) J. R. Morton, Colloque Ampere XV, North-Holland (Amstep dam), 1969,p 299. Volume 74, Number 6 March 19, 1970
s. D. MCLAUGHLAN AKII D. J. MARSHALL
1.362,
A
77OK
1.4OK
MAGNETIC FIELD H
-
Figure 2. Temperature dependence of the radical spectrum observed from “green ultramarine.” The three types of radical present are readily separated a t 300°K due to rotational averaging.
zB1 + 2A1 allowed transition of the SB- radical, and suggested that this might also account for the color of ultramarine. Green powders are produced by the simultaneous presence of the S3- and SSO- radicals. In the samples examined the intensity of the SO3- spectrum could be increased by X-irradiating the powders but not that of SSO- and Sa- spectra. Further evidence for the presence of Sa- in blue ultramarine is given by Holzer, el al.,” who compared the Ramari vibrational spectra of RbI and K I known to contain S3- with that of ultramarine, and observed an identical vibrational frequency of 545 cm-’ from each.
5. Discussion Chloro- or bromosodalites which showed the color change white-yellow-green-blue were analyzed and found to contain slightly less sulfur after treatment. Thus on heating the powders under vacuum, only a small amount of sulfur volatilizes, most atoms eventually aggregating to form Sa- radicals. This bluecolored sodalite is quite stable when heated in air but can be bleached by heating to 900” in a stream of flowing hydrogen and is then photochromic. Thus it would appear that this treatment redisperses the sulfur throughout the lattice in some monatomic form. If the sulfur is replaced by Se or Te the color of the ultramarine changes from blue to blood red-yellow (see ref 2). This lends additional support to connecting the blue coloration with some sulfur radical. The progression of the absorption band to shorter wavelengths is probably due to the greater spin-orbit coupling of the heavier atoms. The absolute sulfur content seems to have little bearing on the final coloration. However, those having a high sulfur content are blue initially and can only be The Journal of Physical Chemistry
bleached by heat treating in hydrogen. The higher the sulfur concentration in the uncolored powders, the quicker they go through the color cycle during heat treatment. Powders grown with the sulfur added in the form of T\Tad% show this color cycle more readily than those doped with NazS-9Hz0. It seems to be immaterial whether the sodalite contains a halide or hydroxide. The structure of Ss is known, the sulfur atom spacings being 2.12 and bond angle 105°.12 Thus it would appear to be difficult to get the S3- radical into the cage with the four sodium ions which are normally tetrahedrally coordinated round the central anion at a distance of -2.8 b. Furtherniore there is no indication of any 23Na(I = 100%) hyperfine interaction on any of these radicals, but this may be due to a very small overlap on to the sodium nuclei. It is possible that the sodium ions are not present and that the S3- radical occupies the whole cage, free to rotate. This can be the case if one assumes that the A1 :Si ratio is not 1:1. Indeed Jaeger points out that ultramarines having an Al: Si ratio of 1 : 1.5 have a deeper color. Barth13 suggests that the composition of the framework is not necessarily [Al~si&]~- but is more generally [(Al, S i ) 1 ~ 0 ~ 4where ] ~ - the number of negative valences n is fixed by the A1 :Si ratio. Hence if one had an Si-rich cage there would be no need to have four Xa+ ions present for charge balance. The cage might then contain only the sulfur radical and this may explain its ability to rotate. The above remarks are also applicable to the SSO- and SOa- radicals. Hoffman14gives the “theoretical” composition of the various colored forms of sodalites. The transition from white to green to blue, he states, is connected with a reduction in the number of sodium atoms present. The more recent work of Barror and Cole16also suggests that the sodium content of sodalites can be varied over wide limits. (See Table I for citation of ref 16-19.) 6. Conclusions Under certain conditions the sulfur atoms seem to be extremely mobile in sodalites and this fact may well have far reaching effects on the photochromic properties. The reasons for the different behavior of NazS. 9Hz0 and NazS4-doped sodalites are not known but it would appear to be advantageous to add the sulfur as NazS-9Hz0to obtain photochromic sodalite as it seems (11) W. Holzer, W.F. Murphy, and H. Bernstein, to be published; see ref 10.
(12) B. E.Warren and J. T. Burwell, J . Chem. Phys., 3, 6 (1935). (13) T.F.W. Barth, 2.Krist., 83,405 (1932). (14) R. Hoffman in “Das Ultramarin,” Braunschweig, 1902,p 87. (15) R. M. Barrer and J. F. Cole, J . Phys. Chem. Solids, 29, 1755 (1968). (16) J. R.Morton, Can. J. Chem., 43,1948 (1965). (17) J. R.Morton, J . Phys. Chern., 71,89 (1967). (18) J. Schneider, B. Dischler, and A . Rauber, Phys. Stat. Sol., 13, 141 (1966). (19) J. Suwalski and H. Seidel, ibid., 13,159 (1966).
THEMOLECULAR STRUCTURE O F Si2Bli'7 to be less prone to aggregate when introduced in this form. The S3- and SSO- radicals are free to rotate above 77°K and do not show any apparent line broadening due to interaction with the four sodium nuclei which ought to be in the cage. These facts, coupled with Jaeger's observations that silicon rich ultramarines have a deeper color, suggest that the A1:Si ratio may have considerable bearing on the types of radicals formed. If the cage can be devoid of sodium atoms, then it might be possible to obtain an F center which would not show the usual 13-line epr spectrum due to the four adjacent sodium nuclei. Indeed the presence of atomic hydrogen, apparently in a sodalite cage not
1363
containing sodium, will be reported in a related publication. Further work is planned using samples of known Al: Si ratio and single crystals as soon as they become available.
Acknozoledgments. We would like to thank H. W. Evans for heat treating the samples and for assisting in the epr investigations. Thanks are also due to P. A. Forrester and 14.J. Taylor for interesting discussions and R. J. Heritage for analyzing the samples. This paper is published with the permission of the Controller Her Majesty's Stationery Office.
The Molecular Structure of Perfluoroborodisilane, Si,BF,, as Determined by Electron Diffraction
by C. H. Chang, R. F. Porter, and S. H. Bauer Department of Chemiatry, Cornell University, Ithaca, New Y O T ~14850 (Received August 16, 1969)
An electron diffraction structure analysis of SizBF7 in the gas phase shows that the connectivity in the molecule is gaSi-SiFz-BFz,confirming conzlusions derived from nmr spsctra. Bond lengths are (Si-B) =.2.008 i 0.017 A, (Si-Si) = 2.361 f 0.012 A, (B-F) = 1.309 & 0,009 A, and (Si-F)ave= 1.575 f 0.002 A. The bond angles are LSiSiB = 125.0 f 2.9", LSiBF = 120.6 =t1.3') LBSiF = 109.1 f 2.4', LSiSiF(center) = 1.0'. The most populous conformations are those in 102.9' f 1.7", and LSiSiF(termina1) = 109.5 which the terminal -SiFa and -BFz are staggered with respect to the centeral -SiFz. By assuming a potential function of the form V = l/zV0(l - cos 30), a barrier height of 2.35 kcal/mol for rotation about the Si-Si bond was estimated, using Karle's method.
Introduction Aside from the high-temperature silicon borides, l t 2 only a few compounds containing silicon-boron bonds have been reported and for the latter no structural data are available. A series of compounds BSi,F2,+3 with n ranging from 2 to 13, mere prepared by Timms, et ~ l . by , ~ low temperature acid-base reactions between S i r z and BF3.4 From nmr, ir, and mass spectral data, they concluded that these compounds should be classified as a homologous series SiF3-(SiF2),-1-BF2. Although SiBF5 was later synthesized by Timmss in the reaction between B F and SiFe, the yield was very poor. The simplest member of this series available in sufficient quantity is Si2BF7. This report is on the molecular structure of SiZBF7 as determined by electron diffraction in the gas phase.
Experimental Section Samples of SizBF7 which contained small amounts of
silicon oxyfluoride, (Si20F6bp = -23.3") as detected from mass spectra, were provided by Thompson and Margrave.e These were purified by distilling off the impurity at - 18". An ampoule containing the liquid sample Si2BF, was connected directly to the nozzle lead tube of the electron diffraction apparatus. The amount of vapor injected into the apparatus was controlled with a Teflon needle valve. Where needed Kel-F grease was used on glass joints and stopcocks in the inlet system. Sectored electron diffraction photographs were taken with the sample at room temperature. The chamber (1) C. F. Cline, J . Electrochem. Soc., 106, 322 (1959). (2) V. I. Matkovich, Acta Cryst., 13, 679 (1960). (3) P. L. Timms, et al., J. Amer. Chem. Soc., 87, 3819 (1965). (4) P. L. Timms, R. A . Kent, T. C. Ehlert, and J. L. IMargrave, ibid., 87, 2824 (1965). (5) P. L. Timms, Chem. Eng. News, 44, (39), 50 (1966). (6) J. C. Thompson and J. L. Margrave, private communication.
Volume 74, Number 6 March 19, 1070
