J. Phys. Chem. 1995, 99, 551-555
551
Zintl-Molecules in Vapors over Alloys of Heavy Main-Group Elements Lutz Poth and Konrad G. Weil* Institut fur Physikalische Chemie, Technische Hochschule Darmstadt, Petersenstrasse 20, 64287 Darmstadt, Germany Received: September 26, 1994; In Final Form: October 26, 1994@
We report on Knudsen effusion mass spectroscopic experiments, aimed at the detection of intermetallic molecules in the vapor over mixtures of antimony or bismuth with thallium or elements of groups 14 and 16 and of binary molecules containing elements of groups 14 and 16. The discussion is concentrated on the following species: PbSbzTe, Pb2Te2, SnSbBiTe, SnBizTe, Sn2Te2, and TlSbTe2. All these molecules are present in the respective vapors with comparatively high abundances. They contain four atoms and 20 valence electrons. Furthermore, the 22-valence-electron species SbzTe;?and Bi2Te2 can be observed. The tetraatomic species are isoelectronic to Sb4 or Sbd2-. Hence it can be assumed that the 20-electron species have tetrahedral structures, while the 22-electron species are planar.
1. Introduction
In preceding publication~l-~we have shown that several ternary molecules could be identified in the equilibrium vapors over mixtures of alkali metals with elements of the groups 13/ 15 and 14/15. They have the general formula MxAxB4-x,M = alkali atom, A = element of group 14, and B = Sb, As, or MaA‘xB4-x, A’ = element of group 13. One can explain them using a suggestion from Zint14 by transfer of one electron from each alkali metal atom to the elements of group 13 or 14, so that the tetramer nucleus AxB4-x contains 20 valence electrons. Table 1 contains these molecules together with the con-esponding anions derived by formal separation of charges within the molecules. All these molecules contain Zintl-anions that are isoelectronic to Sb4. Furthermore, we demonstrated5 the existence of molecules with the general stoichiometry CxB4-xHalxwith C = group 16 element, B = Sb, and Hal = C1, Br in the vapor over mixtures of group 15/16/17 elements. Examples are Sb3TeBr, SbzTezBr2, and SbsTeCl. One can interpret these molecules in a similar way. They contain a tetramer unit (CxB4-xy’+with 20 valence electrons and x Hal-. In the framework of Zintl’s concept, one expects the (AxB4-xy’- and (CxB4-xy’+ to be tetrahedral ions. In all these studies we used Knudsen effusion mass spectroscopy. Within the Knudsen cell the system is in an equilibrium state. Hence, the results mentioned above indicate that molecules that contain formal ionic entities with four atoms and 20 valence electrons are of pronounced thermodynamic stability. In this paper we will inquire whether tetramer neutral species with 20 valence electrons can be found in the vapor over alloys of heavy main-group elements. 2. Experimental Section We used a single-focusing 90’-sector field mass spectrometer
MM 30K from VG Micromass, which is especially modified for Knudsen effusion mass spectrometry. The ions were generated by electron impact with an electron source operating at low electron energies to avoid fragmentation of the molecules. The emission current was 100 PA. Ion intensities were measured with a photomultiplier and monitored with a fast ion counter. Mass discrimination effects were avoided by use of a Daly conversion dynode. @Abstractpublished in Advance ACS Abstracts, December 15, 1994.
0022-3654/95/2099-0551$09.00/0
TABLE 1: Molecules with Zintl-Anions molecules corresponding anions CszInSb3 (InSb3)’CsSnSb3 (SnSb3)Cs2SnzSbz (Sn~Sb2)~CsSnBiSb2 (SnBiSb2)CsSnBizSb (SnBi2Sb)(snAs3)NaSnAs3 NaGeAs3 (GeAs3)We used Knudsen cells machined from stainless steel with an orifice diameter of 0.8 mm. The cell temperature is measured with a Ni/Cr/Ni thermocouple, which is fastened to the bottom of the cell. The evaporating species could be identified by their mass to charge ratio and their isotopic pattern and in all cases are characterized by their appearance energy. By use of a shutter in front of the Knudsen cell we were able to c o n f i i their origin from within the Knudsen cell and to distinguish them from background signals. We prepared all mixtures used in the experiments from the elements by melting the appropriate amounts in a quartz ampule. Antimony, thallium, and lead of 99.999% purity and tellurium, tin, and bismuth of 99.9999% purity were purchased from Johnson Matthey Alfa Products. If not stated otherwise, equal amounts of the elements are mixed together. The abundances of intermetallic molecules are rather insensitive to the composition of the condensed material. 3. Results 3.1. Composition of the Vapor over LeadJAntimonyl Tellurium. Figure 1 shows a mass spectrum that we obtained under the following conditions. The Knudsen cell contained a sample of composition PbSbzTe at a temperature of 960 K. The evaporating vapor is analyzed as described in the Experimental Section. Figure 1 shows that in the vapor over a solid of composition PbSbzTe all those species (Pb, Sb2, Sb4, Te, and Te2) could be identified, which are already known from the vapors over the pure elements. In addition SbzTe2 appears. This molecule is already known from investigations of the gaseous phase over binary antimony/telluriumalloys6 and of the ternary antimony/ telluriumhalogen systems mentioned in the Introduction. Over the ternary Pb/Sb/Te system, however, SbzTe2 appears with much smaller intensity, as it does in the vapor over the binary SbTe alloy. 0 1995 American Chemical Society
Poth and Weil
552 J. Phys. Chem., Vol. 99, No. 2, 1995
I
1, i
578 ll5so PbSb
Ti
dn
1, it
IO?
I 0.9
0.55
350 400 450 500 550 600 650 wn Figure 1. Compressed mass spectrum of the vapor over lead/antimony/ tellurium at a Knudsen cell temperature of 960 K and an electron energy of 9.6 eV. In all compressed mass spectra we use a logarithc scale for the intensity of the different molecule peaks, but linear scales for the isotope intensities within a peak. Assignment: ( 1 ) Te+; ( 2 ) Pb+; (3) Sbz+; (4)Tez+; ( 5 ) PbTe+; (6) PbTe2+;(7) Sb4+; (8) SbZTez+;(9) PbzTe'; (10) PbSbzTe+;(11) PbzTez'.
0.2
105
TABLE 2: Stoichiometries and Appearance Potentials of Binary Lead Tellurides Observed in the Vapor over LeadAntimonyVTellurium PbTe 8.3 f 0.3 eV PbTe2 8.0 f 0.3 eV 7.7 f 0.3 eV Pb2Te Pb2Tel 7.6 f 0.3 eV Furthermore, several binary leadtellurium molecules could be detected. They are summarized, together with their appearance potentials, in Table 2. Leadtellurium molecules in the vapor over the ternary l e a d antimony/tellurium system appear with four different stoichiometries, while binary leadantimony molecules could not be identified at all. Of special interest is a molecule of the stoichiometry PbSb2Te, which is the only ternary molecule in this vapor. The experimental mass spectrum in the mass range 566-592 amu is shown in Figure 2 and compared with a calculated one for PbSb2Te, using the natural isotopic abundances of the elements. An appearance energy of 7.4 & 0.3 eV for PbSb2Te could be derived from the ionization efficiency curve by linear extrapolation. This low ionization energy, which is about the same as those of Sb4 and Pb2Te2, indicates that PbSb*Te+ is formed as a parent ion. The ionization efficiency curve of PbSbzTe+ shows a significant break at 10.2 eV, indicating onset of fragmentation. The fragmentation product seems to be PbSbTe+. This ion can only be observed at electron energies above 10 eV. The appearence energy of PbSbTe+ of 10 eV can be correlated well with the break in the ionization efficiency curve of PbSb2Te'. This observation indicates the following fragmentation reaction. PbSb,Te-
10 eV
PbSbTe'
+ Sb + e-
3.2. The Systems Tin/Antimony/Bismuthellurium and TiniBismuthellurium. At electron energies below 10 eV we identified all species in the vapor over tidantimonyhismuthl tellurium which are known from the vapors over the pure elements. The only exception is tin. No signal that could be assigned to Sn was found in the spectrum. Figure 3 shows a
1 y' i , i , i 1
Figure 2. Measured and calculated mass spectra of PbSbZTe.
c ,
,
i,
102
2
3
,
IO' 0 1
1so
50
250
350
nvn
106,
1
1I
12 I
9 .^
500
I
15
600
700
800
m/n
Figure 3. Compressed mass spectrum of the vapor over tinhismuthl antimony/tellurium at a temperature of 950 K and an electron energy of 10 eV. Assignment: (1) Te+; ( 2 ) Bi+; (3) SnTe'; (4) Tez+; ( 5 ) BiSb+; (6) BiTe+; (7) SnTez+; (8) Biz+; (9) BiSh2+; (10) Sb4+; (11) SbzTez+;(12) BiSb3+; (13) SnBiSbTe'; (14) Bi3+; (15) BiZSb*+;(16) SnBizTe+; (17) Bi?Sb+;(18) Bil+. mass spectrum of the vapor over Sn/Bi/SbRe. We see that the composition of the vapor is dominated by binary Sb/Bi and Snl Te molecules. The molecule Sb2Te2 is detectable but occurs with a lower intensity than over binary antimony/tellurium alloys. Binary molecules of group 14 and group 15 elements cannot be observed. Signals of SnSbzTe should appear in the mass spectrum in a mass range 480 amu < m/n -= 500 amu. Signals of such a molecule could not be observed because this mass range is covered by the intensive signals of Sb4 and Sb2Te2. However, we could observe signals which can be attributed to a molecule of the stoichiometry SnSbBiTe, as will be described in the next paragraph. Signals of mixed binary antimonyhismuth molecules are present with high intensities. The signals with the highest intensities are those of the molecule Sb3Bi. Three additional signals can be observed in the mass spectrum of Sb3Bi when the sensitivity is increased. They are in the mass range 484
J. Phys. Chem., Vol. 99, No. 2, 1995 553
Zintl-Molecules in Vapors
TABLE 3: Stoichiometries and Appearance Potentials of
Binary Bismuth Tellurides Observed in the Vapor over T~ismuWTellurium BiTe 7.8 f 0.3 eV 7.4 f 0.3 eV 7.4 f 0.3 eV not determined ~
I
.L
0.551
570
574
578
582
m/n
Figure 4. Measured (top) and a calculated (bottom) combined mass spectra of the ions Sb3Bi' and SnSbBiTe+. The small figure at the bottom shows a calculated mass spectrum for SnSbBiTe+alone. I 10 10
2
j
3
1
:l--LLu
Figure 6. Measured and calculated mass spectra for SnBizTe.
10 2
10
'
50
150
250
350
d n
Figure 5. Compressed mass spectrum of the vapor over tin/bismuth/ tellurium at a temperature of IO00 K and an electron energy of 10 eV. Assignment: (1) Te+; ( 2 ) Bif; (3) SnTe+; (4) Tez'; ( 5 ) BiTe+; (6) SnTez+;(7) Biz+; (8) BiTeZ+;(9) SnzTeZ'; (IO) BiZTe'; (11) Bi3+; (12) SnBi?Te+;(13) BizTez+; (14) B&+. amu < mtn < 488 amu and cannot be assigned to the isotopic pattern of Sb3Bi. Figure 4 shows these signals. The very intense signals from Sb3Bi extend the ordinate range. The weak signals indicate the presence of the new molecule SnSbBiTe. Comparison of calculated mass spectra of similar molecules and the observed one supports the assignment and confirms the existence of SnSbBiTe. Further signals, indicating the existence of another molecule can be observed in the mass range 658 amu m/n < 670 m u . They are partly overlapped by the intense signals of SbZBiz. A comparison with calculated mass spectra shows that these signals can be assigned to the former unknown molecule SnBizTe. This assignment can be supported by experiments with the ternary mixture of tin, bismuth, and tellurium. In this case the mass spectrum shown in Figure 5 can be registered. Signals of bismuth species and of tin tellurides dominate this spectrum.
In addition we found the bismuth tellurides collected in Table 3. From these we observed only BiTe over the quaternary mixture. In this mass spectrum the isotopic pattern of the molecule SnBizTe can be resolved without partial coverage of signals by SbzBiz. This mass spectrum is shown in Figure 6 together with a calculated mass spectrum for SnBizTe. The smaller signals in the mass range 672 amu < m/n 678 amu can be assigned to the molecule BizTez. The appearance potentials of the new molecules could be determined to 7.5 & 0.8 eV for SnSbBiTe and 7.0 f 0.8 eV for SnBizTe, respectively. The ionization efficiency curves of both molecules show breaks at an electron energy of about 10.4 eV, as does the ionization efficiency curve of PbSbzTe. Due to the small abundance of the mother molecules, we did not find signals of fragmentation products of these molecules. 3.3. The System ThalliudAntimonylTellurium. In the vapor over thallium/antimony/telluriumall those molecules could be detected which are known from the binary antimonyl tellurium system. Further signals of T1, Th, and of several thalliudtellurium molecules could be observed, whereas TlSb and TlSb2, described by Balducci et al.,' who have investigated the vapor over antimonylthallium alloys, could not be identified over the ternary mixture. A compressed mass spectrum of the vapor over this alloy is given in Figure 7, and a review of the identified binary thallium/ tellurium molecules and their ionization energies is given in Table 4. Only one ternary molecule could be identified in the vapor over this mixture. The mass spectrum of this molecule indicates the stoichiometry TlSbTez and is shown together with a calculated mass spectrum for TlSbTez in Figure 8. Its appearance energy, determined from the ionization efficiency curve, is 7.6 & 0.4 eV.
554 J. Phys. Chem., Vol. 99, No. 2, 1995
Poth and Weil
I 105,
1
1
1 o4
Figure 7. Compressed mass spectrum of the vapor over thallium/ antimony/telluriumat a temperature of 1050 K and an electron energy of 10 eV. Assignment: (1) Te+; (2) T1+;(3) Sb2+; (4) Te2+;(5) TlTe+; (6) Tl2’; (7) TlTe2’; (8) Sbd+;(9) SbzTe*+;(10) Tl2Te+;(1 1) TlSbTe*+; ( 12) TlZTe*+,
d n
0 9 ct
1 1
J
Figure 8. Measured and calculated mass spectra for TlSbTez. TABLE 4: Stoichiometries and Appearance Potentials of Binary Thallium Tellurides Observed in the Vapor over Thallium/Antimony/Tellurium TlTe 7.1 i0.3 eV 7.6 f 0.3 eV TlTe2 T12Te 6.5 0.3 eV T12Te2 8.2 i0.3 eV
*
4. Discussion Mass spectrometric investigations of the vapor over alloys of group 15 elements have shown that in the tetrameric molecules one or two atoms can easily be substituted by an element of the same group. Drowarth et a1.* could detect all molecules Sb4, SbsAs, Sb2As2, SbAs3, and As4 over antimony/ arsenic alloys. The same holds true for the molecules Sb4, Sb3Bi, Sb2Bi2, SbBi3, and Bi4, which can be observed in the vapor over antimonyhismuth alloys.g The interpretation of the molecules mentioned in the Introduction is based on the assumption that entities consisting of four atoms and 20 valence electrons are of pronounced stability. This interpretation is supported by facts from structural inorganic chemistry. In Zintl-phases several stable tetraatomic entities are known. They are tetrahedral if they have 20 valence electrons and square planar if they have 22 valence electrons. Examples for the tetrahedra are the homoatomic groups Ge44-,
SQ~-, and Pb44- in the appropriate alkali compounds1° or the heteroatomic g r o ~ p s ~ ~Sn3T15- ’ ~ in NasSnsTl as well as Pb2Sbz2- and Sn2Biz2- in crypt-stabilized alkali compounds. Examples for the square planar entities are Sb42- and Bi42- in the c o m p o ~ n d s ~ ~CallSblo, -~’ YbllSblo, (2,2,2-crypt Kf)2Sb42-, and CallBilo. The bonding characteristics of the Sb4 analogous clusters which form the subject of this study may be adequately described following the analysis of Mingos.18 The Sb4 cluster gives an example for edge-localized bonding, which means that each edge of the cluster polyhedron is associated with a bonding skeletal electron pair. In this situation the “effective atomic number rule” is applicable, according to which the number of cluster valence electrons N is related to the number of edges E of the cluster polyhedron consisting of M = 4 atoms by the formula
This rule specifies six edges for a tetraatomic 20-valenceelectron cluster. As quantum chemical investigation^^^^^^ on a variety of these clusters show, their main bonding features are similar to those found in Sb4. If one electron pair is added to a tetraatomic 20-valenceelectron cluster, this pair will occupy a strongly antibonding molecular orbital,19thus breaking one cluster bond. This effect leads to a pronounced stretching of one of the six cluster edges and generally to a planar structure which is therefore typical for tetraatomic 22-valence-electron systems. All tetraatomic molecules in the vapor over binary alloys of group 14 and 16 elements have in common that species with 20 valence electrons are dominating. In the vapor over tin/ tellurium alloys Drowarth et aL21did detect Sn2Te2 as the unique tetraatomic molecule, and Sokolov et al.22detected the molecules Sn2Te2 and PbSnTe2 in the vapor over leadtidtellurium alloys. Drowarth and co-workers already speculated about a tetrahedral structure for Sn2Te2 and ascribed its high abundance to its electron number, isoelectronic to Sb4. It can be stated that all the newly detected tetraatomic molecules in the vapors over lead/antimony/tellurium and tin/ antimonyhismutldtellurium contain 20 valence electrons. It seems that starting with Sb4, not only antimony atoms can be substituted by an atom of the same group but a whole Sbz group can be substituted by the isoelectronic entities PbTe or SnTe. So the new molecules can be interpreted as intermediates in the sequence from the homoatomic tetramers of group 15 elements and the isoelectronic molecules of the type AzB2 (A = element of group 14, B = element of group 16). Formally, the stoichiometries of these molecules can be interpreted by ionic separation. If an electron is transferred from tellurium to lead, we arrive at the formulation Pb-(Sb2)Te+. An electron transfer from tellurium to lead seems to be energetically unfavorable. The electronegativity difference should lead to an electron transfer in the opposite direction. The pronounced stability, however, of a tetrahedral 20-electron system may overcompensate the electronegativity effect. According to this concept, an element of group 13 can only be incorporated in the antimony tetrahedra if two antimony atoms are substituted by tellurium to compensate the resulting lack of two electrons. So the detection of the molecule TlSbTe2 in the vapor over thallium/antimony/tellurium confirms the aforementioned concept. A summary of all tetraatomic molecules with 16 up to 23 valence electrons which one can formulate in the systems lead antimony/tellurium, tinhismuthhellurium, and thaUiudantimony/ tellurium is given in Figure 9.
Zintl-Molecules in Vapors 16
17
18
J. Phys. Chem., Vol. 99,No. 2, 1995 555 20
19
21
22
23
Na*(SnA%)xpi
'/
, I
(Cs+)z(InSb,)(Cs+)z(InSb,)-
'1'
BiTe,
(Sb3Te)+B r - B Sb, W ,PbSb2Te PbSbJe Pb,Sb, Pb,
Pb,Sb
Pb,SbTe
(Sb,Te2)2+(Br-)2
Pb,Te SnBi, Sn,Bi,
Sn,
Sn,Bi
SnpITe
Sn,Te TISb,
TI,Sb,
I Pb,Te, I
TISb,Te
R SnBi,Te
SnEiTe,
SnTe,
Sn,Te,
I
TISbTe,
1
TISbTe,
Pb2Te2
TITe,
TI,SbTe
TI,Te
Figure 9. A summary of all tetraatomic molecules with 16-23 valence electrons which one can formulate in the systems lead/antimony/ tellurium,tin/bismuth/tellurium, and thallium/antimony/teUurium.Marked are those molecules which could be observed in this study by mass spectroscopy.
Marked are those molecules which could be really detected in this study. With the exception of TlzTe2 all detected molecules are those with 20 or 22 valence electrons. So the number of valence electrons seems to be the decisive criterion for the stability of these molecules. The only tetraatomic molecule with less than 20 electrons is Tl2Te2, which can be observed in the vapor over thallium/ antimony/tellurium in higher abundance than TlSbTe2. Apparently, the 20-electron stability is not the only building principle in this system. A possible reason for this observation could be the greater electronegativity difference between thallium and tellurium. As shown for the PbSb2Te molecule, the Zintlconcept requires a formal electron transfer from the more electronegative element tellurium to the less electronegative thallium. Furthermore, thallium has to accept two electrons to reach the valence electron count of antimony. This step seems to be too unfavorable to be compensated by the stability of the resulting 20-electron system. The influence of the electronegativity difference of the atoms in a tetrahedral 20-electron system has been evaluated by Cave et d.19 by quantum mechanical calculations. They have calculated geometries and energies for the neutral molecules BizSbz, PbzTe2, and Tl212 and found that these systems are destabilized by an increasing electronegativity difference. For T124 no local minimum for a tetrahedral arrangement could be found. This shows that greater electronegativity differences are destabilizing the tetrahedral 20-electron systems. Accordingly, already for Pb2Te2 a distorted tetrahedral structure has to be discussed. When the electronegativity difference surpasses a certain value, square planar structures are preferred. This was demonstratedby infrared studies of matrix-isolated Si202, Sn202, and Pb202.23,24In addition, mass spectrometric investigations of the vapor over PbO show that Pb202 is only one molecule in a series of polymers from PbO to (PbO)6.25 Obviously, only if the tetrahedral structure is energetically preferred can one find a single dominating stoichiometry. According to this consideration, the observation of Tl2Te2 in addition to TlSbTez demonstrates the limit of the applicability of the Zintl-concept. The wide applicability of the Zintl-concept for the interpretation of stoichiometries of different molecules containing heavy main-group elements can be seen in Figure 10. Three different
Figure 10. Wide applicability of the Zintl-concept for the interpretation of different molecules containing heavy main-group elements.
paths lead to the formation of stable 20-electron tetrahedra: electron transfer from alkali atoms, leading to Zintl-anions, electron transfer to halogen atoms under formation of Zintlcations, and internal electron transfer and formation of neutral tetraatomic tetrahedra. In recent years theoreticians also became interested in such molecules. Schmidt et al. calculated structures and energies for N ~ G ~ A GeAs3 s ~ . is ~ a~simple tetrahedron, while the sodium position is close to one of the triangular faces containing the germanium atom. Hagelberg et aL20 performed Hartree-Fock calculations with SbsTeBr and Sb2Te2Br2. In both cases the minimum energy structure contains a distorted tetrahedral antimony/tellurium nucleus. The calculations of both groups c o n f i i the concept of an electron transfer to or from the tetrahedral nucleus.
Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is greatly acknowledged. We thank Dr. F. Hagelberg, SUNY at Albany, for stimulating suggestions. References and Notes (1) Hartman, A.; Weil, K. G. Angew. Chem., Znt. Ed. Engl. 1988,27, 1091. (2) Hartmann, A.; Weil, K. G. Z. Phys. D 1989,12,11. (3) Hartmann, A.; Weil, K. G. High Temp. Sci. 1990,27, 31. (4) S c h ~ e rH.; , Eisenmann, B.; Muller, W. Angew. Chem. 1973,85, 742. (5) Poth, L.; Wed, K. G. Ber. Bunsen-Ges. Phys. Chem. 1992,96,1621. (6) Sullivan, C. L.; &he, M. J.; Carlson, K. D. High. Temp. Sci. 1974, 6,80. (7) Balducci, G.;Piacente, V. J . Chem. Soc., Chem. Commun. 1980, 1980,1287. (8) Drowarth, J.; Smoes, S.; Vanderauwera-Mahieu, A. J. Chem. Thermodyn. 1978,10, 453. (9) Kohl, F. J.; Carlson, K. D. J. Am. Chem. SOC. 1968,90,4814. (10) Muller, W.; Volk, K. Z . Naturforsch. 1977,32b, 709. (11) Blase, W.; Cordier, G. Z . Kristallogr. 1990,193, 319. (12) Critchlow, S. C.; Corbett, J. D. Znorg. Chem. 1985,24,979. (13) Critchlow, S. C.; Corbett, J. D. Znorg. Chem. 1982,21,3286. (14) Deller, K.; Eisenmann, B. Z. Naturforsch. 1976,8 3 1 , 29. (15) Clark, H. L.; Simpson, H. D.; Steinfink, H. Znorg. Chem. 1970,9, 1962. (16) Critchlow, S. C.; Corbett, J. D. Znorg. Chem. 1984,23,770. (17) Cisar, A.; Corbett, J. D. Inorg. Chem. 1977,16,2482. (18) Mingos, D. M. P.; Johnston, R. L. Structure and Bonding, 1987, 68,29. (19) Cave, R. J.; Davidson, E. R.; Sautet, P.; Canadell, E.; Eisenstein, 0.J . Am. Chem. SOC. 1989,Ill, 8105. (20) Hagelberg, F.; Srinivas, S.; Sahoo, N.; Das, T. P.; Weil, K. G. In preparation. (21) Colin, R.; Drowarth, J. Trans. Faraday SOC. 1964,60,673. (22) Sokolov, V. V.; Belousov, V. I.; Shol'ts, V. B.; Sidorov, L. N. Russ. J . Phys. Chem. 1966.40,885. (23) Anderson, J. S.; Ogden, J. S. J. Chem. Phys. 1969,51, 4189. (24) Ogden, J. S.; Ricks, M. J. J. Chem. Phys. 1972,56, 1658. (25) Drowart, J.; Collin, R.; Exsteen, G . Trans. Faraday SOC. 1965.61, 1376. (26) Schmidt, P. C. Private communication. JP9425981
