6346
J. Phys. Chem. 1991,95,6346-6351 in C 0 2hydrogenations with nickel/zirconia catalysts is interpreted in terms of a nondissociative mechanism, with formate as the pivotal surface intermediate (Figure 7, left side). In addition, the C 0 2 / H 2 reaction to formate supports our findings with other zirconia-based catalyst^.^^-^^ Recent adsorption experimentsMhave indicated that at least two formate species can be distinguished, which are adsorbed on different sites on the zirconia matrix. When asserting the intermediacy of formate on the route to methane, it is important to discuss the presence or absence of other reaction pathways. The experiments in which a catalyst covered with surface formate species is exposed to hydrogen (Figure 5) are informative in this respect. Note that, in an analogous experiment over Pd/Zr02,32we did not observe intermediate formation of gaseous C 0 2 . In the present case, however, C02(g) is detected (Figure 5); therefore it cannot be excluded that gaseous carbon dioxide, released from surface formate, reacts to methane on a further reaction pathway (Figure 7, curved arrow connecting C 0 2 and CHI). However, we are favoring an analogous interpretation as with the other catalysts mentioned above: formate is suggested to represent the surface intermediate in carbon dioxide hydrogenation to methane. The occurrence of C02(g) (as observed in Figure 5) is due to desorption starting from formate, by the reverse reaction of the adsorption process. For further surface reactions of carbon dioxide and hydrogen, the role of lattice anion vacancies must be taken into account. These vacancies are also involved in the interpretation of current results on palladium/zirconia methanation catalysts, where a similar behavior has been observed. These findings are analyzed in terms of a reaction scheme in which the methanation of formate involving anion vacancies is discussed, as reported in more detail at another place.35
C O (b = 2060 cm-I), which represents the only detected surface species in CO/H2 reactions, prior to hydrogenation of the resulting carbonaceous species. Singly bound CO is observed to be produced in two ways, i.e., from gaseous C O and from doubly bound C O (b = 1900 cm-l), which appears to be equilibrated with surface formate. For comparison, we note that for the respective adsorbates on Ni( 111) surfaces, experimental vibrational frequencies of 2020-2060 and 1900-1 960 cm-' have been reported in ref 3 1. In a recent theoretical study,39 ranges of 2040-2062 and 1842-1856 cm-' have been calculated for singly and doubly bound CO on Ni( 100) surfaces. For the products arising from CO hydrogenations over supported nickel catalysts, hydrocarbons with methane as the main component are generally reported. On our coprecipitated Ni/Zr02 catalyst, mainly higher hydrocarbons are produced when starting from CO/H2 (Figure 7,right-hand side), and methane appears to be a side product. Note, however, that methane is the predominantly formed product if CO, is present in the reaction mixture. In contrast, over the Ni,Zrw-derived catalyst the preferential production of methane is also observed upon exposure to CO/H2; with this system, large amounts of C02 are formed by the WGS reaction. From the correlation between C02 formation and methane production, carbon dioxide may be assigned as the likely precursor of the methane product, although this assignment cannot be made unambiguously from the present results. In the following paragraphs, the origins of the exclusive production of methane when starting from C02 and hydrogen will be discussed. A similar behavior observed by Barrault et al." in C 0 2 / H 2 reactions over Ni/La203 and Ni/Ce203 catalysts was explained by assuming a specific environment for carbon species arising from C 0 2 dissociation, different from those produced during CO hydrogenation. From out findings, the CH4 selectivity
Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft and the Schweizerische Bundesamt fIir Energiewirtschaft is gratefully acknowledged. One of us (C.S.) is indebted to the Fonds der Chemischen Industrie for providing a graduate research fellowship.
(39) Maruca, R.; Kusuma, T.; Hick, V.; Companion, A. Surf.Sci. 1990, 236,2 10.
X-ray Diffraction, Electron Paramagnetic Resonance, and Electron Spin Echo Moduiatlon Studles of the PbO-PbCi,-CuCi, Ternary Glass System P. Raghumthan* and S. C. Sivasubramanian Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India (Received: August 27, 1990; In Final Form: January 10, 1991) X-ray diffraction, EPR, and ESEM studies are reported for a novel ternary glass 43PMF56PbCl2-1CuClI. The X-ray PDF data lead to a structural model in which octahedral building units of Cu04C12are predominant. EPR spectra of the glass at X and Q bands have been fitted, by line shape simulation, to the distributed spin Hamiltonian parameters gr(mean) = 2.34, uII= 0.03, g,(mean) = 2.06, uL = 0.008, p = 0.95, Aw(mean)= 131.1 X 10-4 cm-l, and A,(mean) = 5.8 X 10" cm-I. These data suggest a distribution of bonding geometries for Cu2+in the glass structure, with the above-mentioned elongated octahedral CuO4CI2dominating. ESEM results suggest that Cuz+may be surrounded by four Pb2+in the second coordination shell at a distance of 3.8 A.
Introduction Lead oxide, PbO, is known to facilitate glass formation, and glasses formed by mixtures of PbO and the more ionic halides PbC12 and PbF2 have lately aroused much structural From the technological viewpoint, not only are lead glasses exploited as radiation shields, but the halide glasses are becoming (1) Rao, B. G.; Rao, K. J. Phys. Chem. Glasses 1984, 25, 1 1 . (2) Rao, K. J.; Wong,J.; Rao, B. G. Phys. Chem. Glasses 1984. 25, 57. (3) Rao, K. J.; Rao, 8. G.; Elliott, S. R. J . Mater. Scl. 1985, 20, 1678. (4) Reo, B.G.; Rao, K. J. Chem. Phys. 1986, 102, 121. ( 5 ) Rao, K. J.; Rao, B. 0.;Wong, J . J . Chem. Soc., Faraday Trans. I 1988.89. 1779.
0022-365419 1 /2095-6346S02.50/0
increasingly important because of their potential use in infrared optical components and ultra-low-loss optical fibers.6 Also, inorganic glasses containing paramagnetic ions often display further interesting optical and electronic properties. The challenge of structure-property correlations in such glasses necessitates their structural characterization to the finest possible detail. In recent years, much interest has been shown in understanding the structures of inorganic glasses in terms of general models based on a network of close-packed ionic spheres. For example, studies (6) Almeida, R.M. HalIde Glasses f w Infrared and Flberoptics; Martinus Nijhoff Publishers: Boston, 1987.
(33 1991 American Chemical Society
X-ray, EPR, and ESEM Studies of Pb0-PbC12-CuC12 Glass of the ion-pair distribution functions (PDF) of the X-ray diffraction intensities could lead to quantitative information regarding interionic distances up to the first few shells in such close-packed Further, the presence of small amounts of a paramagnetic ion such as Cu2+ would provide an effective electron paramagnetic resonance (EPR) spectroscopic probe for studying the short-range structural detail in the glass network. For the PbO-PbC1,-CuC12 ternary glass under study, it would be of additional interest to observe modulations in the electron spin echo envelope (ESEM)l0 due to superhyperfine interactions arising from the t07Pb nucleus ( I = 21% natural abundance) and thereby probe details of short-range order up to second-neighbor coordination." Inferences regarding comparable ion-pair distances from these two independent experiments, namely, X-ray diffraction and ESEM, should provide a strong proof of the structural model assumed for this glass. Indeed, such comparative studies have been rather sparse.
Experimental Section A. Preparationand Chemical Analyses. Reagent grade Pb304, PbCl,, and CuC12+2H20were homogeneously mixed in an agate mortar in appropriate molar quantities with CCl, as a mixing medium and melted in a quartz crucible. The batch was initially heated slowly above 500 OC until Pb304decomposition to PbO was completed and was then heated vigorously so that a brown liquid was obtained around 650-750 O C . The melt was held for about 2 min and then quenched between polished brass disks. The glass thus prepared was found to be X-ray amorphous and homogeneous without any phase separation, and shall henceforth be referred to as G1. The copper content of this glass was estimated by atomic absorption, while the lead was estimated as PbCrO, by standard gravimetric procedures.', The overall chemical composition of the glass, calculated by assuming that (i) all the Cu2+was present as CuC1, and (ii) silica, if at all present due to traces of PbO reacting with the quartz crucible, was negligible, corresponded to PbO (43 mol %), PbC1, (56mol %), and CuC1, (1 mol %). For demonstrating some interesting points of contrast between the EPR of the glass, G1, and other phase-separated specimens (vide infra), we have also prepared two improperly quenched high-PbO-content "glass + polycrystal" mixed phases. The compositions of these specimens were PbO (60mol %), PbC12 (38 mol %), CuC12 (2 mol %) (hereinafter labeled X1) and PbO (70 mol %), PbC1, (28 mol %), CuClz (2 mol %) (labeled X2). These were subjected to X-ray diffraction and, by comparison with reported data from JCPDS powder diffraction files," the following polycrystalline phases were identified to be mixed in with a glass phase: in XI the major phase-separated components were mendipite, 2PbO*PbC1,, and murdochite, a copper-based oxide of formula cubPbO& In X2, in addition to murdochite and mendipite phases, another oxyhalide of stoichiometry 3PbOgPbC1, was also identified. B. X-ray Analysis and EPR (and H E M ) Spectroscopy. The X-ray diffraction data were recorded with a Seifert (Model Isodebyeflex 2002) automatic powder diffractometer with stepscanning mechanism. Cu K a radiation, which after passing through a Ni filter had a wavelength (weighted over K a l and Ka2) of 1.5418 A, was used in all the measurements. The range of 28 values scanned was 5-1 20°, with a scanning speed of 0.6O min-' . (7) Sundar, H.G. K.; Govinda Rao, B.; Rao, K. J. Phys. Chem. Glasses
1902. 23. 90.
#)Lieheri, G.; Musinu, A.; Paschina, G.; Piccaluga, G.; Pinna, G. J . Chem. Phys. 1986,85.500. (9) Muainu, A.; Paschina, G.; Piccaluga,G. J. Chem. Phys. 1987,86,5141. ( I 0) Kevan, L.; Schwartz, R. N. Time Domaln Electron Spin Resonance; John Wiley & Sons, Inc.: New York, 1979; Chapter 8. (1 1) Raghunathan, P.; Kevan, L. Can. J . Chem. 1988, 66, 1984. (12) Vogel, A. 1. A Textbook ofQuantitatlue Inorganfc Analysls, 4th ed.; Longmans: London, 1978. (13) McClune, W. F.; Mrose, M. E.; Post, B.; Weissmann, S.; McMurdie, H. F. Eds. Powder Diffractlon Flles; JCPDS International Centre for Diffraction Data: Swarthmore, PA, 1985; Vols. 1-35. ~
1rhe Journal of Physical Chemistry, Vol. 95, NO. 16, 1991 6347
The X-band (9 GHz) and Q-band (35 GHz) EPR spectra have been measured at various temperatures with Varian spectrometers E- 109 and E- 112, respectively. All the measured g values were calibrated with respect to the resonance line of DPPH (g = 2.0037); the corresponding field positions are denoted by arrows in all the EPR spectra. Two- and three-pulse ESEM spectra were recorded a t 4.2 K on the University of Houston pulsed X-band EPR spectrometer interfaced to a Nicolet 1280 computer with a 293B pulse programmer; this spectrometer typically produces 1-kW microwave pulses of -60 ns width. Each recorded ESEM was an average of 2000 scans in the time domain. The magnetic field setting for maximum echo intensity corresponded to the X-band g, resonance position observed in the Cu2+ EPR spectrum. The two-pulse experimentlo was performed with the pulse-sequence "a/2-r?r ?-echo" and the echo maximum, V(T),was recorded as a function of time 7 between the pulses. The three-pulse stimulated echo experimentlo was performed by using the sequence "?7/2-~-?r/ 2-T-?r/2-~-echo" along with the usual ( x , x , x ) (-x,-x,x) (-x,-x,-x) - (x,x,-x) phase-cycling procedureI4 to suppress the unwanted 2-pulse echoes at times T = 7 and T = 27 as well as to correct for baseline drifts. The stimulated echo intensity maximum, V(7), was recorded as a function of the time T between the second and third pulses.
+
Results and Discussion A. X-ray Pair Distribution Function (PDF) of G1. The glass G1 was X-ray amorphous. We shall assume that b is the average electronic bulk density of this glass comprising the scattering units j (atomic numbers Zj), with the unit composition index u,, During our scan of 28 values (or, equivalently, a scan of k = 4~ sin 8/A), the oscillatory variation of the diffracted X-ray intensity as a function of the interionic pair distance, r, may be written as the well known PDF1s 2?r2rD(r)CujZi = 2?r2rbEujZj + x * " k i ( k ) sin (kr) dk I
I
(1)
where i ( k ) , the interference function, defined in terms of the corrected and normalized experimental X-ray intensity, I(k), the form factor,I6f(k), and the incoherent scattering intensity,17P ( k ) , has the form
with g(k) = CjuA(k)/Eju. j(0). Experimentally, I ( k ) is o tained by correcting the diffracted intensity for instrumental background, polarization, and absorp tion, as detailed elsewhere,'* and normalized according to Norman's method.Ig Sundar et al.' have shown that errors in the normalization procedure or those due to the inaccuracies in estimating intensities in high-k regions may be effectively minimized by setting up the interference function as
d
W ) = [ N k )- s ( k ) - (C:uh2(k) + ~ u i l , " ( k ) ) l / g Z ( k ) (3) J
+
+
where s ( k ) = a bk ck2. We could readily evaluate the coefficients, A, a, b, and c of the above interference function, eq 3, by setting the PDF, eq 1, equal to zero for pair-wise distances r I 1.4 A. Any negative-going ripples in the region near r I1.4 A could be further reduced by the procedure of Yarnell et a1.,20 (14) Fauth, J. M.; Schwciger, A.; Braunschweiler, L.; Fomr, J.; Ernst, R.
R.J . Magn. Reson. 1986,66, 74. (IS) Warren, B. E. X-Ray Diffraction. Addison-Wesley: Reading, MA,
.---.
1 QhQ
(16) International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, 1974; Vol. IV. (17) Smith Jr, V. H.; Thakker, A. J.; Chapman, D. C. Acta Crystallogr. 1975, A31, 319. (1 8) Sivasubramanian,S.C. Ph.D. Thesis. Indian Institute of Technology, Kanpur, India, 1989. (19) Norman, N . Acta Crystallogr. 1957, 10, 370. (20) Yarnell, J. L.; Katz, M. J.; Wenzel, R. G.; Koenig, S. H. Phys. Rev. 1973, A7, 21 30.
6348 The Journal of Physical Chemistry, Vol. 95, No. 16, 1991
Raghunathan and Sivasubramanian II
33166
!
,-- - - -- -- - -
o0t
I
I1 Z
I
4 Distance
I
6
(1)
I
8
Figure 1. X-ray pair distribution function of glass GI.
whereby the PDF is back-Fourier-transformed to i ( k ) for the condition of PDF = 0 for r I 1.4 A. The new i(k) thus obtained has k values beyond k, (in our experiments k, = 7.0 A-*) and can be retransformed to the corresponding PDF, which is now ripple free at low r values. This smoothing procedure was iterated on our normalized experimental I ( k ) data until convergence was achieved. The PDF obtained in the above manner for G1 is shown in Figure 1. Peaks in this PDF show that the pair distances in G1 correspond to spacings of 2.0, 3.0, 3.8, 4.8, 5.8, and 6.9 A. In previous X-ray and EXAFS studies of binary lead oxyhalide glasses (where no Cu2+was present), Rao et al.,’S2 have assigned the following pair distances: 2.4 A (Pb-0); 2.8 A ( P H I ) ; 4.0 A (predominantly Pb-Pb, with a small number of CI-CI, 0 4 , and 0-0 pairs). Values 5.6 and 6.7 A and higher correspond to Pb-Pb distances in distorted ClPb4 tetrahedra. Since these authors’3 find that at higher (230%)concentrations of PbCI2 the Pb-0 and Pb-CI peaks merge close to -3.0 A, we assign our 3.0-A peak (Figure 1) to both Pb-0 and Pb-CI pairs. Again, our PDF peaks at 5.8 and 6.9 A may be identified with Pb-Pb distances in distorted ClPb4 tetrahedra. The peak at 3.8 A will predominantly correspond to metal-metal distances (Pb-Cu and Pb-Pb) between neighboring octahedral centers. However, the two new pair distances, 2.0 and 4.8 A, found for G1 should involve Cu2+ and may be rationalized as follows. Cu2+ is a well-known Jahn-Teller ion when it is bonded in environments of square-planar or tetragonally distorted octahedral coordinations. Two kinds of distances thus become possible for the same pair in a compound; e.g., in CuC12, Cu2+is surrounded by four CI-at 2.3 A and by another two CI- at 2.95 A, while standard Cu-0 distances in a square-planar arrangement are found to be 2.0 A.2t In light of these typical data, the peak at 2.0 A for G 1 (Figure 1 ) suggests a large number of Cu-0 airs from square-planar units while Cu-Cl distances of 2.95 attributed to distorted octahedral CuO4CI2units could be contributing partly to the peak at 3.0A. The pair distance of 4.8 A found in Figure 1 is tentatively assigned to Cu-Cu or Cu-Pb distances between distorted CICu,Pbex tetrahedra. The model proposed earlier for the binary lead oxychloride glasslV2envisages (i) octahedrally coordinated Pb2+ with at least two oxygens in the octahedron PbO2CI4,(ii) 02-in fourfold coordination of the type OPb4, and (iii) CI- having varied coordination depending upon the concentration of PbCI2. We propose an adaptation of this overall structure wherein the introduction of Cu2+causes either a contracted Cu-0 pair distance of 2.0 A or an elongation of Cu-0 beyond the normal value of 2.4 A to cause a pair distance that would merge with Pb-CI and Cu-CI
Maqnetic field
1
(21) Cotton, F. A,; Wilkinson, G.Advanced Inorganic Chemistry, 3rd ed.; John Wiley & Sons: New York, 1972; pp 912-917.
-
Figure 2. X-band EPR spectrum of glass GI: (a) spectrum at room temperature; (b) spectrum at 77 K;(c) computer simulation.
II
I/
II
I1
Magnetic field
Figure 3. Q-band EPR spectrum of glass G I : (a) spectrum at room temperature; (b) computer simulation.
distances. We note that, although Cu” can exist in three different octahedral environments, namely CuO4CI2,Cu03C13, and Cu02C14,only the first of these would dominate in our model since we have Cu-0 (2.0 A) and Cu-CI (3.0 A) pair distances corresponding only to this geometry. B. EPR Line Shapes of C1, Xl, and X2. In Figures 2 and 3 we depict the X-and Q-band EPR spectra, respectively, of the glass G1. The line shapes recorded at both 77 K and room temperature are identical. The experimental spectra of Figures 2 and 3 remained identical even at liquid helium temperature (recordings not shown), thus ruling out any Cu” motion (mobility
X-ray, EPR, and ESEM Studies of PbO-PbC12-CuC12Glass '/e
The Journal of Physical Chemistry, Vol. 95, No. 16, 1991 6349
of total density
2.09
2.07
91 2.05
0
2.3
8.4
2.3
13.8
2.3
8.4
2.3
0
'
2.03
2.23
2.30
2.45
2.38
911
Figure 4. Fractional probability densities for various (g,, gL) combinations in glass GI.
of V4+ has been observed, for instance, in a more loosely packed Second, the experimental spectra of both Figures 2 and 3 remained the same for crushed as well as uncrushed specimens, indicating that the glass was homogeneous and that there was no microcrystal formation. To ascertain that the low resolution seen at Q-band is not an artifactual effect, we have repeated cur recordings at different signal modulation amplitudes (e.g., the inset in Figure 3) and also over an expanded magnetic field region with slower spectral scans. The spectral features under all these conditions remained the same. In a truly "glass" structure, the environment around the participating atoms will vary somewhat from site to site. In the process the molecular bonding parameters, and hence the EPR spin-Hamiltonian parameters, get statistically d i ~ t r i b u t e d . ~A~ quantitative estimate of this statistical distribution, then, would vastly aid our assignment of a structural model for the glass. Distribution of spin-Hamiltonian parameters is manifested in the experimental EPR spectrum as an inhomogeneous line broadening that shows a line width variation with respect to the applied magnetic field.23-26 E l ~ e w h e r e we , ~ ~have ~ ~ ~reported a fast EPR line shape simulation algorithm to assess the statistical distribution in the spinHamiltonian parameters. Briefly, our method considers the overall line shape of the distributed system as the probability density function of the resonant magnetic field value, which is treated as a random variable. For the Cu2+ sites in GI, our line shape simulation assumes the joint probability density function, p(gll, g,), to be a bivariate normal density function,28with uIl2,uL2as the variances and p as the correlation coefficient between guand g,. The g distribution will, of course, in general lead to hyperfine parameter d i ~ t r i b u t i o nand , ~ ~ our simulation procedure also includes this. Finally, the resonance magnetic field values so calculated are convoluted with a Gaussian broadening function to include a 10-G residual linewidth as has been done by Froncisz and H ~ d e . ~ ~ For the glass GI, the best-fitting simulated spectra are shown as dashed lines in Figures 2 and 3. The following points, which typify EPR spectral absorption in a variety of glass structures studied by us, are worth emphasizing: (i) Both the X-and Qband spectra have been simulated by using the same set of statistical parameters, namely mean gil= 2.34, ull= 0.03, mean g, = 2.06, u L = 0.008, and p = +0.95. (ii) The hyperfine interaction components (from Cu2+) have the values IA,I(mean) = 131.1 X IO-'cm-' and IAJmean) = 5.80 X lo" cm- with a distribution given in terms of the standard deviations u(Al1)= 8.7 X IP cm-I, and u(A,) = 1.9 X l o " cm-'. The All and gllvalues are related by a correlation coefficient -1 and similarly A, and g, values are related by the correlation coefficient + I . (iii) The parallel ~
~~
2) Raghunathan, P.; Das, B. B. Chem. Phys. Lett. 1989, 160, 627. 3) Raghunathan. P. In Electron Magnetfc Resonance of the Solid State; , J. A., Ed.; Canadian Society for Chemistry: Ottawa, 1987. 4) Froncisz, W.; Hyde, J. S.J . Chem. Phys. 1980, 73, 3123. 5) More, C.; Bertrand, P.; Gayda, J. P. J . Magn. Reson. 1987,73, 13. 6) Giugliarelli, G.;Cannistraro, S.Cheni. Phys. 1985, 98, 1IS. 7) Raghunathan, P.; Sivasubramanian, S. C. Proc. h d . Acad. Sci. m. Sci.) 1986, 96, 565. 8) Papoulis, A. Probability, Random Variables and Stochastic Processes; McGraw-Hill, Inc.: New York, 1965.
Magnetic Field
-
F v 5. Room-temperature X-band EPR spectrum of the mixed phase specimen X 1. hyperfine features, which are well resolved in the lower frequency (X-band) spectrum, are undetectably coalesced in the higher frequency (Q-band) spectrum due to the distribution in the gi component. The observation of poorer resolution at the higher microwave frequency is a hallmark of glassy- or amorphous-state EPR spectra. Although it would be more informative to obtain S-band EPR spectra of the glass, we are not presently equipped for it. Our analysis of the gdistribution statistics is presented in Figure 4, which shows the fractional probability densities for various (gi, g,) combinations in glass G1. More than 73% of this density falls in the range 2.3 IgliI 2.38 and 2.05 I g, I 2.07 with the maximum at the mean values. These values show that Cu2+ lies in an octahedral environment with an elongated C4 axis, the electronic ground state being 2Bl (Id$?)). The positive p value suggests that an increase of gllfrom its mean value has a high probability of correspondence with an increased g, value. These g values evidently correspond to structural units such as Cu04C12, Cu03C13,and Cu02C14. Referring again to Figure 4, up to 8% of the total probability density lies in the region 2.38 I g! I 2.45 and 2.07 I g, I 2.09. These values typify a crossover of the Cu2+ coordination from square-planar (that is, very much elongated octahedral) to pseudotetrahedral. Elsewhere in the literature, for example, the typical g values gll= 2.43 and g, = 2.08 have been identified with pseudotetrahedrally coordinated copper.% Similarly, the region (2.23 I H g I2.30), (2.03 I g, I 2.05) has been identified with a pseudotetrahedral (point group DU)environment of copper;30 in our glass, this corresponds to Cu2+ in CuC14 environment. Although the above discussion envisages several environments for Cu2+in G1, the most preferred units are likely to be distorted octahedra of the type Cu04C12. The reason why Cu2+ should prefer oxygen coordination over chlorine coordination (in a typical inorganic structure) ma be adduced on the basis of simple radius-ratio (r+/r-)rules.& If we examine the ionic radii of the relevant packin units,32namely Pb2+ 1.17 A, Cu2+ 0.72 A, 02- 1.32 and C1- 1.81 A, we readily see that, when coordinated to oxygen, Pb2+ can have octahedral or cubic symmetry with as many as eight oxide ions surrounding it. Even for larger anions, such as CI-, Pb2+ will be able to sustain the octahedral environment. At a Cu2+site, however, the oxide coordi-
- w,
-
-
-
(29) Benchini, A.; Gatteschi, D.; Zanchini, C. J . Am. Chem. Soc. 1980,
102, 5234.
(30) Morton, J. R.; h t o n , K. F.; LePage, Y. J. Magn. Reson. 1986,66,
116.
(31) Rao, C. N. R.; Gopalakrishnan, J. New Dfrectiom in Solid State Chemistry; Cambridge University Press: Cambridge, U.K., 1986; p 17. (32) See ref 21, p 52.
Raghunathan and Sivasubramanian
6350 The Journal of Physical Chemistry, Vol. 95, No. 16, 1991
(b)
12660G
/1600 G
Magnetic Field
-
1600 G
Figure 6. Room-temperatureQ-band EPR spectrum of the mixed phase specimen X 1.
c--c-L
400 G
Magnetic Field
-
Figure 8. Room temperature Q-band EPR spectrum of the mixed phase
specimen X2.
200 G
0.8 l'O:
t-
i
0
Magnetic Field
-
Figure 7. Room-temperatureX-band EPR spectrum of the mixed phase specimen X2.
nation leads to an octahedral environment, whereas a concentration of CI-would force a tetrahedral geometry. To sustain amorphicity, propagation of the glass network by octahedral units is essential in our model and thus Cu2+ would prefer greater oxygen coordination. As an interesting contrast to the aforementioned statistical distribution of the EPR data for GI, we briefly demonstrated our EPR results for the two phase-separated mixtures, XI and X2. Figure 5 shows the room-temperature X-band spectrum of a crushed specimen of XI. The spectrum shows well-resolved All features superposed on a broad "glassy" spectrum. The roomtemperature Q-band spectrum of the same specimen, Figure 6, recorded with spectrometer settings that were identical with those used for G1, displays highly resolved hyperfine lines as far as the polycrystalline moiety is concerned. In fact, two or three sets of hyperfine values in the parallel region are discernible. However, the dominating features correspond to the murdochite, Cu6PbOt, structure,'* as evidenced in the X-ray diffraction experiment. Similar X- and Q-band room-temperature EPR spectra for X2 are shown in Figures 7 and 8, respectively, where sharper features corresponding to a polycrystalline phase once again appear along with underlying broad features characteristic of a glassy phase. The sharply resolved parallel hyperfine features in Q-band, and
1.0
0
2.0 T A U OJSeC)
Figure 9. Two-pulse X-band electron spin echo envelope spectrum of
glass GI. 1.o
o.{"
o0t
I
1.0
I
I
2 .o
3.0 T
I
6.O
1
5.0
(v=)
Figure 10. Three-pulse X-band electron spin echo envelope spectrum of glass G 1 .
the resolution of even the perpendicular region (Figure 8b), lead us to infer the presence of only one copper-containing polycrystalline phase.I8
J. Phys. Chem. 1991, 95,6351-6360
C. Two-Pulseand Tbree-Pulse X-Band ESEM Spectra of G1. The two-pulse ESEM spectrum of the glass G1 at 4.2 K is presented in Figure 9, while the three-pulsed ESEM of G1 at 4.2 K appears in Figure IO. Analyses of these modulated echo patterns are based on the appropriate theoretical expressionsIO for the two-pulse ( V ( 7 ) )and three-pulse (V( r ) ) modulated echo intensities for an electron spin (S = with axial g interacting For the local geometry of weakly with a nuclear spin ( I = close-packed ionic spheres envisaged in our model, ESEM spectral simulations based on spherical averaging" and used in the recent literaturell*Mwould be ideally suited. Specifically, our simulation procedure compared the experimental ESEM spectra with the calculated ones to give "best-fit" values for the Cu2+ electron~ O - ~ distance, O ~ P ~ r, the number of 207Pbnuclei, N , involved in this superhyperfine interaction, and the estimated value of the isotropic part of this electron-nucleus interaction, ab To account for the overall magnetization decay by magnetic spin-spin interaction, the calculated ESEM spectra were multiplied by a decay function of the form exp(C, + CIT C2P C37'), which was fitted by a nonlinear least-squares technique to the experimental decay curve." The modulation observed for the two-phase echo decay curve (Figure 9) is faint, indicating that the superhyperfine couplings between a Cu2+electron and close-lying Pb2+ nuclei in the first coordination shell ( 1 3 A) are very weak, if they are present. More interesting results are observed in the three-pulse study of the glass system (Figure 10). Here the decay curve is seen to be modulated by the presence of 207Pbnuclei in the vicinity of Cu2+. The presence of modulation only in the three-pulse echo suggests that Pb2+ is at slightly farther (>3 A) positions, conceivably in the second coordination shell, as would be expected from our X-ray diffraction results. The two dashed lines shown in Figure 10 are the trial-and-error computer simulations, which closely match the experimental echo decay, both computed assuming aim= 0.3 MHz. Figure 10b is the echo decay curve simulated on the basis of three Pb nuclei ( N = 3) surrounding Cuz+at a distance of 3.6 A in our spherical
+
+
(33)Kevan, L.;Bowman, M. K.; Naryana, P. A.; Boeckman, R. K.; Yudanov, V. F.; Tsvetskov, Yu. D. J . Chem. Phys. 1975, 63, 409. (34)Kevan, L. Acc. Chem. Res. 1987, 20, 1.
6351
averaging model. One observes, however, that the simulated echo modulations are too strong, particularly in the early-time ( T = 1 ps) region of the ESEM envelope. Further, trial-and-error procedures were tested to improve the overall fitting, such as the inclusion of 35Cland 37Clnuclear quadrupole and hyperfine interaction terms from distant chlorines, but these led to a suppression of the overall 3-pulse decay. On the other hand, the fitting in the early regions of the decay improved distinctly if we assumed superhyperfine interactions from four Pb nuclei ( N = 4) at a distance of 3.8 A. This is an interesting result, considering that we have assigned an X-ray PDF value of 3.8 to Pb-Pb or P W u scattering distances. The ESEM results therefore further confirm our earlier picture of the second coordination shell in G1. Such types of distance assessment where a direct chemical bonding is not involved are rather difficult to make by other studies. (For example, EXAFS studies2 and neutron diffracti01-1'~primarily yield information regarding the first coordination shell.)
Conclusion By combining the results of our X-ray diffraction, EPR and ESEM studies we have derived useful structural information from a novel 43Pb0-56PbCl2-CuCIZ ternary glass (labeled GI). The X-ray PDF data enable us to propose a general structural model for the glass in which octahedral building units of Cu04C12are predominant. EPR spectra of G1 at X- and Q-band microwave frequencies have been fitted, by line shape analysis procedures, to appropriately distributed spin-Hamiltonian parameters, suggesting that the Cu2+bonding characteristics in the glass structure are distributed. Among the distributed geometries, Cu2+ in the elongated octahedral unit, Cu04C12,appears to make the major contribution, even though other Cu2+ geometries such as pseudotetrahedral may be present to a minor degree. Our ESEM results suggest that Cu2+is surrounded by four Pb2+in the second shell a t a distance of 3.8 A.
Acknowledgment. P.R. is thankful to Professor Larry Kevan for making available pulsed-EPR facilities at the University of Houston. (35)Wright, A. C.;Grimley, D. I.; Sinclair, R. N.; Rao, K. J. J . Phys. (Paris) 1985, CB, 305.
A Simulation Study of Flexible Zwitterionic Monolayers. Interlayer Interaction and Headgroup Conformatlon M. K. Granfeldt and S. J. Miklavic* Physical Chemistry 2, Chemical Center, Box 124, 221 00 Lund, Sweden (Received: January 3, 1991) A simple model for two opposing monolayers of flexible zwitterionic amphiphiles is studied by means of Monte Carlo simulation. The model allows for molecule fluctuations with a component normal to the average hydrocarbon/water interface arising from the partial diffusion of the hydrocarbon chains out of the hydrophobic core. For a range of model parameters we investigate two properties of the system: the headgroup conformation (orientation) and the separation dependence of the interlayer force. From a comparison with spectroscopic data we find that the average "near-parallel" orientation of the zwitterion dipole is as much a consequence of the perpendicular fluctuations as of inter- and intralayer electrostatic interactions. The out-of-plane degree of freedom extends the range of a repulsive entropic contribution to the total interlayer force. The total force, which can be repulsive up to separations of 30 A, is found to have an exponential dependence on surface separation. At larger separations attractive correlation contributions dominate, providing a limit to the 'swelling" of the layers in excess water. The model produces trends qualitatively consistent with force measurements in systems of various phospholipids.
Introduction For some years now very large but short-ranged repulsive forces have been found to exist between charged and electroneutral and their origin has been the subject of intense specuTo whom correspondence should be addressed.
lation and debate ever since. These repulsive forces, over and above ordinary DLVO tYPe forces,3 have an exponential dependence on ( I ) Rand, R. P.; Parsegian, V. A. Eiochim. Eiophys. Acta 1989,988,351. ( 2 ) Israelachvili, J. N. Intermolecular and Surface Forces; Academic
Press: New York, 1985.
0022-3654/91/2095-6351$02.50/0 0 1991 American Chemical Society
