2850
Communications to the Editor
The Journal of Physical Chemistry, Vol. 82, No. 26, 1978
cm-') have decreased relative to silacyclobutane (927,906, 877, 653, and 532 cm-l), as expected, due to the replacement of weaker Si-C for C-C bonds. As mentioned previously, the complete gas-phase spectra are very rich with combination bands resulting from the effect of the low-frequency ring-puckering vibration. The analysis of this data is very complex and will likely require the use of a multidimensional potential energy function involving the ring-puckering, ring-angle bending, and in-phase SiH2rocking motions. When completed, however, this should add further evidence to the effect that these motions are highly coupled and very sensitive to the molecular conformation.
Acknowledgment. This work was supported by the National Science Foundation under Grant CHE 76-15743.
References and Notes (1) R. M. Irwin, J. M. Cooke, and J. Laane, J . Am. Chem. Soc., 99, 3273 (1977). (2) R. M. Irwin and J. Laane, J . Mol. Spectrosc., 70, 307 (1978). (3) R. M. Irwin, Ph.D. Thesis, Texas A&M University, 1978. (4) J. D. Lewis, T. H. Chao, and J. Laane, J . Chem. Phys., 62, 1932 (1975). (5) J. Blanke, T. H. Chao, and J. Laane, J . Mol. Spectrosc., 38, 483 (1971). (6) V. Schettino, M. P. Marzocchi, and S. Califano, J . Chem. Phys., 51, 5264 (1969); V. Schettino and M. P. Marzocchi, ibid., 57, 4225 (1972). (7) J. Laane, Spectrochim. Acta, PariA, 26, 517 (1970).
COMMUNICATIONS TO THE EDITOR Mossbauer Spectra and Lattice Parameters of Some New Sulfide Phases of Iron
I
I
I
I
i
Publication costs assisted by Argonne National Laboratory
Sir: In the course of investigation on the chemistry of iron sulfide cathodes for molten salt secondary batteries, certain new sulfide phases were found to be present in charged and discharged e l e ~ t r o d e s . l - ~One phase discovered (K6LiFe24S26C1) appears to be similar to the mineral Djerfisherite found in meteorite^.^ This and two other phases, of composition Li2FeS2and 3FeS.2Li2S.Cu2S,have now been prepared in out-of-cell reactions. Their physicochemical characteristics are of importance for the understanding of correlations between molecular structure and properties and the technological development of better electrode materials. We report here on the Mossbauer spectra of these compounds a t room temperature (Figure 1-3) and 77 K, the derived parameters (Table I), and the lattice constants obtained by X-ray diffraction (Table 11). The phase of composition Li2FeS2 was prepared by weighing the stoichiometric amounts of Li2S and FeS, melting the mixture in a graphite crucible at about 1200 "C, cooling to about 850 "C, and finally annealing a t the same temperature for about 3 h. Metallographic examination showed essentially a single phase. Anal. Calcd for Li2FeS2: S, 47.9; Li, 10.4; Fe, 41.7. Found: S, 47.1; Li, 10.5; Fe, 40.3. The phase of composition KfiLiFe2&,&1 (referred to as J phase) was prepared by mixing equimolar amounts of Li2FeS2and LizS and allowing these to react with an iron sheet in molten LiC1-KC1 at 410 "C. Excess LiC1-KC1 and Li2S were removed by hydrolysis (the J phase being insoluble in water) and the product dried under vacuum. Chemical analysis was consistent with the formula given above. Anal. Calcd for K6LiFe24S26C1:Fe, 54.1; s,33.6; K, 9.5; C1, 2.6; Li, 0.28. Found: Fe, 54.6; S, 33.3; K, 9.4; C1, 2.5; Li, 0.29. X-ray diffraction examination showed only a single phase present. The phase containing Cu2S was prepared from the stoichiometric mixture at about 950 "C 0022-3654/78/2082-2850$0 1.OO/O
014
-200
I -IO0
I
I
1
0
IO0
200
I 300
VELOCITY. rnmlrec
Figure 1. Mossbauer spectrum of KGLiFe,,S,6CI I
I
I
i
at 298 K. I
I
0
~
002
Y z
_z
004
0 06
I
1
008 -300
I
I
I
I
-200
-100
0
IO0
I 200
300
I
II
400
500
VELOCITY, rnmlrec
Figure 2. Mossbauer spectrum of FeS.Li,S
at 298 K
and cooling rapidly to room temperature. The sample was further annealed at 800 "C for about 2 h then slowly cooled down to room temperature. Metallographic examination indicated that it was also essentially a single phase. Anal. Calcd for 3FeS.2Li2S.Cu2S: Fe, 32.5; S, 37.4; Cu, 24.7; Li, 5.3, Found: Fe, 31.6; S, 35.4; Cu, 24.4; Li, 5.2. Mossbauer spectra were obtained with a constant-acceleration, pulse height analysis spectrometer built in this laboratory.5 A 10-mCi jsCo-in-Pd source was used. Absorbers were prepared by mixing the finely ground 0 1978 American Chemical Society
Communications to the Editor
The Journal of Physical Chemistry, Vol. 82, No. 26, 1978 2851
TABLE I: Mossbauer Parameters of Sulfide Phases of Irona composition K,LiFe,,S,,Cl Li,S.FeS
T,K
e
W, mm/s
6 , mm/s
A , mm/s
298 77 298
0.082 0.101 0.05 0.03 0.042 0.04 0.03 0.026 0.04 0.026
0.41(I 0.04) 0.45( ?: 0.02) 0.4b 0.4 0.4 0.4 0.4 0.4 0.4 0.4
0.46(f 0.02) 0.61( i 0.01) 0.50( f 0.02) 0.49(i0.01) 0.61(? 0.02) 0.61( * 0.02) 0.55(1 0.03) 0.55(t0.02) 0.66( ?: 0.03) 0.66(+0.03)
0.525(*0.005) 0.560(10.005) 0.62(+0.02) 1.46( i 0.02) 1.06(*0.03) 1.68(*0.02) 0.68( f 0.02) 1.33(k0.02) 0.91(?: 0.03) 1.51(+0.03)
77 3FeS.2Li2S.Cu,S
298
77 a e
is the fractional absorption; W, the line width at half-maximum intensity; 6 , the isomer shift with respect to iron; Line widths constrained if no error limits are given.
A , quadrupole splitting.
TABLE 11: Structure and Lattice Parameters of Sulfide Phases of Iron composition
lattice type cubic pseudo-hexagonal hexagonal
K,LiFe,S,,Cl Li,S.FeS 3FeS.2Li2S.Cu2S
a, A
c, A
10.358(* 0.005) 3.9 3.833( i 0.002)
6.28 6.339(i0.003)
Y, deg
90 90
120 120
1 i
0
-1
0 10
4
0 40
departure of the local symmetry about the iron from a regular octahedron is in line with the X-ray findings of tetrahedral coordination of sulfur around the iron with three different sulfur sites in the J phase.4 Ligand-field theory predicts a high-spin (S = 2) configuration for Fez+ in a tetrahedral environment,s which would suggest a paramagnetic compound with a magnetic hyperfine pattern. However, no magnetic interaction was detected down to liquid nitrogen temperature. Perhaps the effective magnetic field is averaged to zero by a fast electronic spin relaxation. The iron-sulfur bond may also be strongly covalent similar to what has been found for tetragonal FeS where iron(I1) is in a tetrahedral en~ironment.~ The rather low value of 6 is a manifestation of this predominantly covalent character. The Mossbauer spectrum of the phase Li2FeSeappears to consist of two broad lines that are best fitted with two overlapping quadrupole doublets of equal 6 (Table I) but unequal intensity and splittings than tend to equalize a t 77 K. This spectrum would be consistent with two nonequivalent iron positions in the noncubic, pseudohexagonal crystal structure (Table 11). Unfortunately, the limited information obtainable from the powder pattern has not yet afforded a determination of atomic positions and bond configuration. The phase 3FeS.2Li2S-Cu2S,resulting from an apparent substitution of Cues for LizS in Li2FeSz,is hexagonal with increased c and decreased a, as compared to LizFeS2. The Mossbauer spectra are similar, with two quadrupole doublets and slightly increased isomer shifts. The presence of two nonequivalent iron sites is again suggested. No magnetic hyperfine interaction was observed.
1
0 60 -300
-200
-100
0
IO0
200
300
400
500
VELOCITY, mmlrec
Figure 3. Mossbauer spectrum of 3FeS.2LizS.CuzS at 298 K.
specimen with boron nitride powder and forming into disks 2 cm in diameter and 40-60 mg/cm2 thick. Because of the sensitivity of LizFeS2 and the Cu substituted phase to moisture, sample preparation was done inside a glove box under helium. Spectra were fitted using a least-squares computer program developed a t Argonne based on the variable metric minimization method of Davidson.6 Velocity calibration was made with a rolled iron foil enriched in 51Fe and having an effective thickness of 1.9 mg 57Fe/ cm2. Isomer shifts are referred to the centroid of this iron spectrum. Crystallographic data were obtained with filtered Fe K a radiation in a Norelco powder camera of 114.6 mm diameter. Unit-cell parameters were derived by extrapolation of ahkl vs. cos2 8, after correction for film shrinkage. The J-phase spectra are nicely fitted with a single quadrupole doublet having a line width of 0.35-0.40 mm/s. The isomer shift (6) of 0.46 mm/s at room temperature is in the range of values normally found for diamagnetic iron compounds, iron(I1) with intermediate ( S = 1) spin and iron(II1) with high (S = 5/2) spin.7 Valency +2 for the iron would be consistent with the production of this compound in our electrochemical cells a t a potential that corresponds t o the oxidation of iron to the divalent state.3 An asymmetry in the gradient of the electric field at the iron position is indicated by the quadrupole splitting (A). The
Acknowledgment. We express our appreciation to Dr. Frank J. Lynch of the Electronics Division for the use of his spectrometer and help in setting it up, to Dr. A. Martin for preparing the iron sulfide phases, to Dr. Stanley L. Ruby for much valuable advice, and to Mr. R. Teitelbaum for assistance in the computer analysis of the spectra. We are grateful to Drs. F. A. Cafasso and S. Siege1 for their continued interest and support of this work. This research was sponsored by the Division of Basic Energy Sciences of the Department of Energy. References and Notes Nelson and D. S. Webster, "High Energy Battery Program at Argonne National Laboratory", ANL-8064, April 1974. (2) A. E. Martin et al., Paper 54 presented at the Electrochemical Society Meeting, New York, N.Y., Oct 13-17, 1974.
(1) P. A.
Communications to the Editor
The Journal of Physical Chemistry, Vol. 82, No. 26, 1978 C. A. Melendres et al., J . flectrochem. Soc., 124, 1060 (1977). B. Tani, Am. Mineral., 62, 819 (1977). F. J. Lynch and J. B. Baumgardner, Physics Division Report ANL-6391, Argonne National Laboratory (1961). W. C. Davidson, Argonne National Laboratory Report, ANL 5990. T. C. Gibb, "Principles of Mossbauer Spectroscopy", Chapman and Hall. London. 1976. D 76. B. N. Figgis, "Introdu&tionto Ligand Fields", Interscience, New York, 1966, p 157. (9) .E. F. Bertaut, P. Buriet, and J. Chappert, Solid State Commun., 3, 335 (1965).
TABLE I: R a t e C o n s t a n t Measurements for R e a c t i o n s of OH with M e t h a n o l a n d E t h a n o l a t 298 K' CH,OH, mtorr
C. A. Melendres" B. Tani
Chemical Engineering Division Argonne National Laboratory Argonne, Illinois 60439
flash energy, flashes/ J filling
s-'
1012kbi,c m 3 m o l e c u l e - ' s-'
kl',
0.0 2.0 4.0 6.0 6.0 6.0 8.0 10.0
88 88 88 88 88 245 88 88
50 50 50 50 100 50 50 50
40 109 167 251 214 238 288 373
(1.00 i 0.10)
0.0 0.5 1.0 1.5 1.5 1.5 2.0 2.5
88 88 88 88 245 88 88 88
50 50 50 50 50 100 50 50
49 102 142 184 216 195 214 263
(2.62
Received August 8, 7978
Kinetic Rate Constants for the Reaction of OH with Methanol, Ethanol, and Tetrahydrofuran at 298 K Publication costs assisted by the Georgia Institute of Technology
Sir: In the past few years, in an effort to understand the chemistry of the lower atmosphere, OH radical reactions with paraffinic, olefinic, and aromatic hydrocarbons have been extensively studied; however, very little data are available on oxygenated hydrocarbons. Methanol, ethanol, and tetrahydrofuran (THF) are widely used as industrial solvents and enter the troposphere through evaporation. The input of alcohols in urban air could increase further if they are used in internal combustion engines. We report here the results from a kinetics study of the reactions of OH with methanol, ethanol, and tetrahydrofuran. OH + CH30H products (1) OH + C2H50H products (2) OH + CzHsO products (3) The rate coefficient for reaction 1, kl, has been previously measured by Overend and Paraskevopoulosl ( k , = 1.06 x cm3 molecule-I s-I), Campbell et a1.2 (0.94 X cm3 molecule-' s-I), and Osif et ala3(0.9 X cm3 molecule-l s-l). The first two investigators have also measured hz as 3.7 X and 3.4 X cm3 molecule-' s-l, respectively. The only one reported value for the rate coefficient of reaction 3 is that of Winer et ale4who obtained a value of 1.4 X cm3 molecule-' s-l using an environmental chamber. The technique of flash photolysis-resonance fluorescence, employed in this investigation, has been thoroughly documented in the literature and will not be further described Only those details that are necessary for an understanding of the present study have been expanded. OH radicals were produced by photolyzing HzO with vacuum UV light transmitted through a quartz window.
--
+
hv
Hzo
X>160nm*
OH(211)+ H
After a delay of 100 ps following the photolysis, the OH radical decay was measured via resonance fluorescence. All experiments involving reactions 1-3 were carried out under pseudo-first-order conditions with OH being the minor species, Le., [CH30H]/[OHl > 300, [CzH50Hl/[OHl> 75, [THF]/[OH] > 30. Since many flashes were typically required to develop a smooth decay curve, a given gas reaction mixture was replaced approximately every 60 flashes. In this case the percentage of decomposition was 99.9999%. Methanol, ethanol, and tetrahydrofuran were analyzed samples from Baker Chemical Co. All three of these reagents were degassed before use. The results from the OH-CH30H and OH-C2H50H studies are shown in Table I. The bimolecular rate constants k l and k z were computed from the measured pseudo-first-order rate constants using a linear leastsquares analysis. The quoted errors for both hl and k z are 2a values which denote the precision of our measurements.
kl = (1.00 f 0.10) X
cm3 molecule-' s-'
k2 = (2.62 f 0.36) X 10-l' cm3 molecule-' s-l In previous kinetic investigations of OH reactions in our laboratory, under conditions similar to the present measurements, it has been demonstrated that radicalradical reactions such as H OH and OH + OH were of negligible importance. Both modeling calculations as well as variations in the OH concentration have also shown that contributions from reactions such as OH + CH3 and OH CzH5were of negligible importance (see Table I). Our value of kl is in excellent agreement with the direct measurements of Overend and Paraskevopo~los~ (1.06 X cm3 molecule-l s-') and the indirect derivations of Campbell et al.5 (0.94 X cm3 molecule-' s-'). In sharp contrast, the value of kl reported by Osif et a1.,60.9 X cm3 molecule-' s-',~ is in major disagreement with the hl values reported here. Our reported value of k z is noticeably lower (-30%) than that obtained by both Overend and Paraskevopo~los~ (3.7 x cm3 molecule-1 s-') and Campbell et al.5 (3.4 x cm3 molecule-' SKI). One probable source of the discrepancy between our work and that of Overend and Paraskevopoulos (the only direct measurement reported in the literature) is that the latter authors may have overestimated k z due to a significant reaction of OH with the photolysis products from ethanol. In the study by
+
+
0 1978 American
Chemical Society
