J. Phys. Chem. 1995,99, 9918-9923
9918
Aggregation of Thionyl Chloride in Organic Solvents Bum-Soo Kim and Su-Moon Park* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 Received: November 28, 1994; In Final Form: March 28, 1995@
The aggregation chemistry of thionyl chloride has been studied in organic solvents including dimethyl sulfoxide, N,N'-dimethylformamide, and propylene carbonate employing ultraviolet (UV) absorption spectroscopic and cyclic voltammetric experiments. Results indicate that thionyl chloride undergoes a self-association equilibrium at high concentrations in these solvents. Equilibrium constants of dimerization reactions in these solvents and molar absorptivities of dimers at their absorption maxima are reported. At higher concentrations, aggregates of more than three thionyl chloride molecules are observed.
Introduction During the past few decades, batteries based on the lithiumthionyl chloride (Li-SOCl2) chemistry have been receiving a great deal of attention due to their light weights, high energy as well as power densities, and long shelf-lives.' The reduction mechanism of thionyl chloride has been the subject of extensive investigations to understand the discharge mechanism of the Thionyl chloride itself, as well as its nonaqueous solutions, has been employed for studies on its reduction mechanism. Although many investigations on the reduction mechanism were conducted in nonaqueous systems, relatively few studies addressed the solution chemistry of thionyl chloride with solvents or with itself.2'-26 A good understanding of the solution chemistry of thionyl chloride is important not only for a better understanding of its reduction mechanism as an active material for lithium-thionyl chloride batteries but also for its use as a chlorination agent widely used in synthetic organic chemistry. Long et al. carried out Raman spectroscopic studies of in SOC4 and reported formation of a 1:l complex between AlC13 and SOC12 in excess SOC12.2' The structure of the complex was believed to be C12SO AlC13. The possibility of a 1:2 complex was also suggested for a loosely attached second AIC13 molecule. Venkatasetty22 studied the reduction of thionyl chloride in several different nonaqueous solvents such as dimethyl sulfite, N,N-dimethylformamide (DMF), and acetonitrile (ACN) using cyclic voltammetric techniques. He suggested that the donor-acceptor complexes are formed between SOC4 and solvents. David and Hallam employed infrared band shapes and intensity measurements to study the solvent effects on thionyl chloride.23 No interactions between SOC12 and nonpolar solvents such as CC4, C6H6, n-hexane, and CS2 were observed in their study. In polar solvents such as CHBr3 and CH212, they reported a strong charge-transfer interaction between the thionyl group (=S=O) and the solvent, e.g., C12S=O. **HCBr3. Similar results were reported by Nanni et al.,24who observed the formation of a coordination compound of thionyl chloride with pyridine. The formation of oligomers, (SOC12)n,with n > 2 in neat thionyl chloride has been discussed by Mosier-Boss et al. from their Raman spectroscopic studies as well as molecular orbital (MO) calculation^.^^*^^ From the calculations, open chain aggregates, which contain more than two SOCl2 molecules, appear thermodynamically more stable than a closed cyclic
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* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracrs, May 15. 1995. 0022-3654/95/2099-99 18$09.00/0
dimeric form, in which sulfur and oxygen atoms are linked alternately to form a cyclobutane-like tetragon.26The calculated Cl C,I
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heats of formation are -22 and -47 kcal mol-', respectively, for the monomer and open chain dimer. We have been studying the electrochemistry of thionyl ~ h l o r i d and e ~ its ~ ~reduction ~~ products, Le., sulfur28and sulfur dioxide29,for a better understanding of electrochemical reduction of thionyl chloride. As a continuing effort to study the electrochemical and chemical properties of thionyl chloride, we have conducted UV absorption spectroscopic and cyclic voltammetric studies on thionyl chloride solutions and present evidence for formation of aggregates of thionyl chloride in nonaqueous solutions in this communication. Solvents used for the study include dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and propylene carbonate (PC).
Experimental Section Tetra-n-butylammonium perchlorate (TBAP, Southwestern Analytical, electrometric grade) was used after being dried overnight in vacuo at about 90 "C. Thionyl chloride (Aldrich's 99+%) was used after distillation. N,N-Dimethylformamide (DMF, EM Science's OmniSolve, glass distilled), dimethyl sulfoxide (DMSO, EM Science's OmniSolve, glass distilled) and propylene carbonate (PC, Aldrich's anhydrous 99+%) was used after fractional distillation under a helium atmosphere with a reflux ratio of 5 :1. These solvents were dried over activated molecular sieves (Davidson Chemical, Baltimore, MD; type 4A) for at least 3 days prior to distillation. A sealed platinum disk (area 0.332 cm2) was used as a working electrode with two other molybdenum wires as counter and pseudoreference electrodes in nonaqueous solutions containing small amounts of thionyl chloride and 0.1 M TBAP as a supporting electrolyte. A Princeton Applied Research (PAR) Model 173 potentiostat-galvanostat was used along with a PAR 175 universal programmer for recording cyclic voltammograms (CVs). Spectral measurements were made in the solvents without electrolytes using a Hewlett-Packard 8452A photodiode array spectrometer or a Perkin-Elmer Lamda-5 spectrophotometer and a pair of matched 1.00 mm path length quartz cells. 0 1995 American Chemical Society
Aggregation of Thionyl Chloride in Organic Solvents
J. Phys. Chem., Vol. 99, No. 24, 1995 9919 0.00045
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Figure 1. Cyclic voltammograms of (a) 0.30 mM and (b) 10.0 mM thionyl chloride in DMSO with 0.10 M TBAP as a supporting electrolyte. Scan rate was 100 mV/s.
All the samples were prepared in a glovebox under the dry argon atmosphere to avoid contamination by oxygen and water.
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Figure 2. The ip,c vs DMSO.
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when thionyl chloride is reduced at the first CV peak.29 We thus assign it to the reduction of sulfur. The CV peaks showing up beyond this potential are attributed to the reduction of sulfur and/or sulfur d i ~ x i d e . ~ ' -We ~ ~ wish to point out here that the second CV peak observed in 10 mM does not have its equivalent reduction wave in 0.30 mM. Figure 2 shows the scan rate dependency for the first reduction peak current in the 10 mM SOC4 solution. The i, vs VI'* plot, where i, is the peak current and v is the scan rate, displays a negative deviation from linearity at higher scan rates.
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Results Figure 1 shows typical cyclic voltammograms recorded for the reduction of 0.30 mM (a) and 10.0 mM (b) thionyl chloride at a platinum electrode, respectively, in DMSO with TBAP used as a supporting electrolyte. Only one reduction CV peak is observed at -0.75 V at a low SOC12 concentration, while at least three peaks, one at -0.75 V, the second at -1.03 V, and the third at about -1.25 V, are observed in the 10 mM solution. The CV peak at -0.75 V in 0.30 mM is also observed at the same potential in the more concentrated (10 mM) solution. That is, the species reduced at low concentrations is also reduced at higher concentrations at the same potential. Similar CVs are observed in other solvents such as DMF, PC, and acetonitrile, although the respective currents for the first and second peaks are different. Also, their peak potentials were about the same in different solvents within about f 1 5 mV. The second CV peak could be due to the reduction of a product generated following the first electron t r a n ~ f e r , ~I .an ~,' adduct formed between thionyl chloride and the solvent, or dimeridoligomeric species present at higher SOClz concentrations. Bowden and Dey3,4assigned this peak to the reduction of SO.SOC12, an intermediate species formed after the first electron transfer, and Dampier and Cole" attributed it to the reduction of S and SO>. Also, thionyl chloride is known to interact with supporting electrolytes such as AlC13.2'.30-32 Formation of dimeric or oligomeric species was considered by Mosier-Boss et al.25.26 The third CV peak is observed at the same potential as that of sulfur reduction,28which is produced according to the reaction
017
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240
280 280 300 Wavelength. nm
320
340
360
1 380
Figure 3. A series of spectra recorded from (a) 0.30, (b), 0.90, (c) 1.5, (d) 2.1, (e) 4.0, and (f) 6.0 mM SOC12 solutions in DMSO. Spectra recorded at lower concentrations than 0.30 mM are not shown for clarity.
The extent of negative deviations from the linearity was greater when higher concentrations of thionyl chloride were used. On the other hand, the i, YS v1I2 plot for the 0.30 mM solutions followed the expected linearity. These observations indicate that a preceding reaction takes place before the electron transfer; the negative deviations observed at higher concentrations suggest that the rate of the chemical reaction presents a rate-limiting step for the electrochemical reaction when the scan rate is higher.33 Preceding chemical reactions such as complexation of SOCl2 with the solvent (DMSO), dimerization, and/or combination of the two might be presenting a rate-limiting step for the reduction of SOC12. Note also that the CV peak height observed for the 10 mM solution in Figure l a is only about 17 times of that in Figure 1b, although their concentration ratio is about 33. This observation also suggests that a CE (chemicalelectrochemical) mechanism,33in which a chemical equilibrium reaction precedes the electron transfer, must be operating. Finally, the ratio of the second peak current to the first increased monotonically as the scan rate increased, suggesting that the equilibrium reaction is modulated by how fast the potential is scanned. To investigate whether or not the formation of intermolecular charge-transfer (CT) or self-association complexes is involved in the equilibrium reaction and affects the cyclic voltammograms described above, we studied absorption spectra of thionyl chloride solutions in the UV region as a function of concentration. A series of spectra recorded from SOC12 solutions in DMSO at various SOCl2 concentrations are shown in Figure 3.
Kim and Park
9920 J. Phys. Chem., Vol. 99, No. 24, 1995
-1
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0
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Figure 4. A series of spectra recorded from (a) 2.5, (b) 4.5, (c) 7.5, (d) 9.5, (e) 11.5, and (0 14.5 mM SOClz solutions in propylene carbonate. Spectra recorded at lower concentrations than 2.5 mM are not shown for clarity.
We observe a major absorption peak at 239 nm at lower thionyl chloride concentrations (‘0.30 mM) in the spectra shown in Figure 3. Spectra recorded at lower concentrations than 0.30 mM, in which only one absorption maximum is observed at 239 nm, are not shown here. As the concentration of SOC12 increases, the absorption peak at 276 nm increases faster than that at 239 nm. The absorbance at 239 nm, when plotted as a function of the SOCl? concentration (not shown), is linear in a lower concentration range of up to 1.5 mM SOCl2 and deviates negatively from Lambert-Beer’s law as the concentration increases. On the other hand, the absorbance at 276 nm deviates positively from the Lambert-Beer line at higher SOC12 concentrations. This is an indication that thionyl chloride undergoes an equilibrium reaction such as dimerization. The spectral features of SOC12 in DMF (not shown) are very similar to those observed in DMSO. When the concentration of SOCl2 is low, one sharp peak is observed at 248 nm. When the concentration reaches 0.30 mM, one additional peak at 274 nm begins to develop. This peak increases more rapidly than that at 248 nm as the concentration of thionyl chloride increases. When the concentration is high, the peak shown at 274 nm is dominant with the one at 248 nm becoming a shoulder. The changes in absorbance values at 248 and 274 nm as a function of [SOC12] resemble those in DMSO. While the spectral behavior of thionyl chloride is similar in the above two solvents, it is significantly different in propylene carbonate. A series of spectra recorded as a function of [SOC12] in propylene carbonate are shown in Figure 4. When the concentration is lower than about 2.5 mM, only one peak at 196 nm is observed. Then, a peak at 273 nm starts to develop when the concentration becomes higher than about 3 mM. As the concentration increases further, a new absorption peak at 227 nm starts to grow faster than the other two, and the one at 273 nm becomes a shoulder (not shown). When the concentration becomes high, the 227 nm peak becomes dominant. Also, the absorption peak at 196 nm shows a slight red shift at higher concentrations. In order to better resolve the absorption bands, a spectrum obtained at the lowest concentration, where no or very low longer wavelength absorption band was observed, was normalized to the absorbance values observed in the shorter wavelength region of the spectra at higher concentrations and subtracted it from the spectra obtained at higher concentrations. This operation assumes that the peak absorbance obtained at lower concentrations arises from monomer molecules only and is not affected significantly by the presence of small amounts of the dimeric or oligomeric species. A series of difference spectra
-1
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240 280
280
300
320
340 3dO
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Figure 5. Difference spectra obtained from (a) 0.50, (b) 0.90, (c) 1S, (d) 2.1, (e) 4.0, and (06.0 mM SOClz in DMSO with the normalized spectrum obtained from 0.30 mM in DMSO used as a reference.
thus obtained in DMSO solutions are shown in Figure 5. We notice in these spectra that.the absorbance decreases in a spectral region below about 210 nm while they increase in two longer wavelength regions, Le., at about 230 and 273 nm. A slight red shift of the band at 273 nm is observed as the SOC12 concentration is increased. We assign this band to the absorption of the dimer from the observations to be described below. The formation of the CT complex with the solvent is ruled out since the CT band at 273 nm should also appear in the low concentration range due to a large solventlsolute ratio. Moreover, the absorbance at this new maximum at 273 nm should be directly proportional to the added concentration of SOC12, which was not the case in our measurements. No evidence was found for the formation of complexes between SOC4 and acetonitrile, DMF, or pyridine in an earlier report on spectroscopic studies of the bicomponent systems.34 The band should then arise from the electronic transition of dimeric or oligomeric aggregates. The band at 230 nm will be addressed more below when the results in propylene carbonate are described. The most important evidence for the dimerization equilibrium comes from the fact that the increase in absorbance values at the increased thionyl chloride concentration follows the dimerization model. In order to show this quantitatively, we here derive an equation similar to that used for the dimerization of aromatic compounds studied by nuclear magnetic resonance spectrometric method^.'^ For a dimerization reaction of a compound B,
the equilibrium constant Kd has an expression
(3) where [B]o is the added concentration of the monomer and [B2] is the equilibrium concentration of the dimer, B2. Since the absorbance A at 273 nm is assumed to result from the absorption of the dimer, it would follow an expression
where € 6 2 is the molar absorptivity of the dimer and L is the path length of the cell. After obtaining [Bz] from eq 4, substituting it into eq 3 followed by a proper rearrangement of the resulting equation after dropping a quadratic term, we obtain an expression
Aggregation of Thionyl Chloride in Organic Solvents
J. Phys. Chem., Vol. 99, No. 24, 1995 9921 C.00067
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The quadratic term was dropped based on an assumption that its contribution should be very small in most cases. Equation 5 indicates that the [B]&A vs 1/[B]o plot should result in a straight line with a slope of 1/(&B2) and an intercept of 4 / ~ ~ 2 if the dimerization reaction takes place. A plot of data obtained in DMSO according to eq 5 is shown in Figure 6; the plot is a reasonably good straight line with a correlation coefficient of 0.99. At higher thionyl chloride concentrations, the line deviates in a positive direction showing smaller increases in corresponding absorbance values than expected, which could be due to 0.00035 the formation of larger aggregates, i.e., n > 2 (see below) and 100 200 300 400 500 600 700 800 900 10001100 1200 also the breakdown of the assumption made on the quadratic l/[SOCIP] term during the derivation of eq 5. It is also possible that Figure 6. The [SOCI&L/A us 1/[SOC12] plot for the data shown in inaccuracies encountered during absorbance measurements at Figures 3 and 5 . higher concentrations could have resulted in deviations from Beer's law due to high absorbance values. Measurements of ." 1 high absorbance values were inevitable because of high SOC12 1.4concentrations. A dimerization constant, Kd, of 3.1 x lo3 L/mol 1.2and the molar absorptivity, EB2, of 1.3 x lo3 are obtained from the slope and intercept of the plot. This dimerization constant 1is translated into a free energy of formation for the dimer of about -4.8 kcal mol-'. A Kd value of 3.3 x lo3 L/mol and ef 0.86 B 2 of 1.0 x lo3 are obtained in DMF. 0.6a Formation of larger aggregates is also shown to occur when 0.4the spectra are obtained from more concentrated solutions in 0.2DMSO. As the concentration of thionyl chloride increases the absorbance becomes too large to be recorded with a spectro0photometer. We therefore used two 1.00mm path length cells, with one containing a 10.0 mM SOClz solution used as a -o*?&O 200 250 300 350 4 1 0 reference and the other containing solutions of higher concentraWavelength, nm tions as a sample. Spectra thus obtained would represent the Figure 7. Difference spectra obtained from solutions of (a) 12, (bj "hardware"-compensated difference spectra, as opposed to the 14, (cj 16, (d) 18, and (e) 20 mM SOClz in DMSO with a 10 mM "software"-compensated ones shown in Figure 5. A series of SOCl? solution used as a reference. Spectra recorded at higher concentrations than 20 mM are not included here. spectra thus obtained from SOC4 solutions of up to 20 mM are shown in Figure 7. It is seen from the absorbance vs 2.51 I concentration plot in Figure 8 that the band at 262 nm, which I b we believe has the same origin as the one observed at 273 nm in previous experiments, levels off at higher concentrations, and a new band with its peak absorbance in the 306-320 nm region becomes dominant. We believe the difference in wavelength of about 11 nm for the 273 nm band originated from the different ways the two sets of difference spectra were obtained in Figures 5 (by software subtraction after spectral normalization) and 7 (by hardware compensation). The longer wavelength band, first observed at 306, shifts gradually to about 320 nm as the concentration becomes higher. This observation indicates that the equilibrium shifts to the formation of a new species, which 4 absorbs at longer wavelengths in the 306-320 nm region at higher [SOC12]. We believe that this band originates from larger 40 15 20 25 30 35 10 aggregates such as trimers. Concentration, mM We attempted to treat the curve for the 306 nm band shown Figure 8. Absorbances of (a) 260 nm and (b) 320 nm bands plotted in Figure 8, but the complexity of equations resulting from the against [SOC12]. presence of dimers, trimers, and perhaps larger aggregates prevented us from obtaining any meaningful results. When the very low level of transmitted light of about or less than 1%. As absorbance is plotted as a function of log[SOCl2] (not shown), a result, linearity of the measurements would be poor. The we had a slope of about 5 from the linear portion, which includes second could be that larger aggregates may start to form at the data obtained from up to the 30 mM solution. This suggests higher concentrations as noted above, when results shown in that aggregates as large as a pentamer might have been formed. Figure 5 were discussed. In other words, the first originates At higher concentrations, the absorbance was seen to deviate from the unavoidable experimental errors/artifacts due to from linearity perhaps for two reasons. The first is due to high extreme conditions under which experiments were run, while absorbance values for the measurements, which translate into a the second comes from the chemistry effects. I
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9922 J. Phys. Chem., Vol. 99, No. 24, 1995
Kim and Park
dissociation reaction, it would display the electrochemical behavior specific to the adduct formation. The second piece of evidence for the closed form is the lack of spectral shift for this band as pointed out above. The solvent dependency of the dimer would not be different from that of the monomer due to the presence of the =S=O group if the dimer had an open form. The closed form dimer is not expected to show a solvent effect as it should not have a permanent dipole moment. The third piece evidence comes from reduction peak potentials for the monomer and the dimer. The dimeric species would usually require less energy for their reduction than monomers, but the CVs shown in Figure 1 indicate that the monomeric species is easier to reduce than its dimeric counterpart. This results from a larger energy to add an electron to the cyclic form than to the thiocarbonyl group of the open form. The reduction of dimers of the closed form may also result in ring opening, which would take an extra amount of energy. As a result, the reduction potential for the dimer should be more negative than the monomer. While the dimer itself does not appear to have the chargeDiscussion transfer character as shown by the lack of solvent dependencies of its spectra, it must have formed via charge transfer.40 This The spectroscopic results clearly indicate that the absorption is because the formation constant is highly solvent dependent. band of the monomer thionyl chloride molecule results from Kfvalues are 3.3 x lo3 Wmol in DMF, 3.1 x lo3 L/mol The n * transition. The absorption peak assigned to the the n in DMSO, and 11 L/mol in PC, with approximately the same monomeric species is observed at 248, 239, and 198 nm in molar absorptivities in these solvents. In other words, the DMF, DMSO, and PC, which have dielectric constants of 36.7, equilibrium in a highly dissociative solvent, propylene carbonate, 46.6, and 64.4, respectively. In other words, the monomer peak does not lead to effective formation of aggregates. Once they shows a large blue shift as the solvent polarity increases. This are formed, they act more like a molecule than an adduct. is readily expected as the ground-state molecule would have a The results obtained here lead us to conclude that both fairly large dipole, which interacts strongly with polar solvent monomeric and dimeric species undergo the electrochemical molecules. The excited state, however, has the nonpolar reduction at different potentials. Also, spectroscopic observax-electronic character, and the combined result of these effects tions at higher concentrations of thionyl chloride indicate that would lead to a blue shift in the absorption band in a more larger oligomers are formed as well. The new species absorbing polar solvent. Generally, polar solvents would also diminish their vibrational structures as well as molar a b s ~ r p t i v i t i e s . ~ ~ - ~ at ~ 227 nm in propylene carbonate at higher concentrations than 9.5 mM must have been produced by the reaction of the dimer The molar absorptivity estimated for the band was about 7200 absorbing at 273 nm with an extra monomer molecule. This is L-mol*cm-' in DMF and DMSO, while it is about 920 in because the increase in the 273 nm band levels off at higher propylene carbonate, from Lambert-Beer's slopes. The large concentrations while the new band at 227 nm increases. The blue shift shown in propylene carbonate and the change in molar new species could then be formed by an interaction of the dimer absorptivities are consistent with the characteristics of n-x* of closed form with a monomer molecule standing perpendicular transitions expected to be observed for the thiocarbonyl group, to the four-membered ring of the dimer molecule. The oxygen =s=o. end of the third thionyl chloride molecule may act as an electron On the contrary, the reduction potential of thionyl chloride acceptor from two sulfur atoms of the dimer. Part of lone pair is not expected to show as large polarity dependencies as spectral electrons on the S=O group of this molecule may then be tied bands do. This is because the distance between the lowest up and the n-x* transition may require more energy. This empty molecular orbital (LEMO), i.e., the x*-orbital in this case would lead to the increase in transition energy for the trimer. and the vacuum level determines the reduction potential,33and The source of such an interaction may be the charge transfer the n*-orbital is not significantly affected by the solvent polarity. perhaps from larger sulfur atoms of the dimer to the S=O group. This is indeed the case for reduction potentials of many While we have no way of proving the proposed interaction and compounds in different solvents;39 shifts in redox potentials the structure based on our data, it explains the shorter result mostly from chemistries involved in electron transfer wavelength observed for this band. reactions such as preceding or following chemical reactions rather than from the solvent polarities. Our results also explain the electrochemical behavior of The spectral bands assigned to the dimeric species are thionyl chloride when its concentration is high. If the concenremarkably solvent independent. The absorption peaks are tration is low only one peak is observed as shown in Figure la. located at 274, 276, and 273 nm in DMF, DMSO, and PC, Two well-defined peaks were observed at intermediate concenrespectively. This is an indication that both the ground and trations. When the concentration is higher than 100 mM, excited states do not interact significantly with the solvent voltammograms showing slowly increasing currents over a wide dipole. range of potential with a few bumps (not shown) are observed. The presence of oligomers of different sizes as demonstrated Under normal circumstances, the formation of a loosely bound in this study explains the very uncharacteristic voltammogram adduct only shifts the oxidation or reduction potentials upon observed at very high thionyl concentrations. This also explains addition of more complexing agents rather than giving another some inconsistencies in reported observations for the thionyl well-defined reduction current peak as shown in Figure 1. This chloride reduction in the literature, depending on the concentrais the first piece of evidence for the closed form dimer; if it tion of thionyl had an open form adduct that may undergo an effective The results obtained in propylene carbonate are somewhat peculiar in that two different forms of aggregates appear to be obtained (Figure 4). The absorption peak at 273 nm follows the equation for dimerization equilibrium with a Kd of 11 Umol and an .332 of 1.5 x lo3. The new absorption band appearing at 227 nm, however, does not follow the behavior typical of dimerization. Its concentration dependency fits neither the first nor the second order with respect to the [SOClp]. From the A vs log[SOClz] plot, a power dependency of about 3 was obtained and, thus, we believe that the aggregate formed at higher thionyl chloride concentrations may consist of more than three thionyl chloride monomer molecules. We also believe that the band observed at about 230 nm in a set of difference spectra recorded in DMSO and shown in Figure 5 has the same origin as this band at 227 nm in propylene carbonate. The only difference observed in propylene carbonate and in DMSO or DMF was perhaps that no significant amounts of trimers have formed due to relatively lower SOCl:! concentrations used in DMSO and DMF.
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Aggregation of Thionyl Chloride in Organic Solvents In conclusion, it has been shown by our study that thionyl chloride undergoes a dimerization or oligomerization equilibrium at higher concentrations. It appears that oligomers as large as a pentamer may be formed at concentrations higher than 20 mM in solvents such as DMF and DMSO. Oligomers of larger sizes may be present in solutions of even higher concentrations or in neat thionyl chloride. Our results suggest that the CV peak assignments made in the earlier l i t e r a t ~ r e ~ *and ~ %therefore " the related electrochemistry need to be reinterpreted. Cyclic voltammograms of neat SOCl2 containing only a supporting electrolyte such as LiAlC4, which is used as battery electrolytes, this exhibit a broad, heavily convoluted peak;2,5-'0.'2,'5-'7~20,27 may result from the reduction of many different oligomeric species present in the solution. Therefore, irreproducible results reported on thionyl chloride reduction depending on the concentrations used in the literature should be recasted.2-20
Acknowledgment. Grateful acknowledgment is made to Sandia National Laboratories for supporting this research through a contract (78-2526). References and Notes (1) Schlaikjer, C. R. In Lithium Batteries; Gabano, J. P., Ed.; Academic Press: London, 1983; p 304. (2) Schlaikjer, C. R.; Goebel, F.; Marincic, N. J. Electrochem. SOC. 1979. 126, 513. (3) Bowden, W. L.; Dey, A. N. J. Electrochem. SOC.1979, 126, 2035. (4) Bowden, W. L.; Dey, A. N. J . Electrochem. SOC.1980, 127, 1419. (5) Kolomoets, A. M.; Pleshakov, M. S.; Dudnikov, V. I. Soviet Electrochem. 1981, 17, 326. (6) Doddapaneni, N . Proceedings of the 30th Power Sources Synposium; The Electrochemical Society: Pennington, NJ, 1982. (7) Istone, W. K.; Brodd, R. J. J . Electrochem. SOC.1982, 129, 1853. (8) Istone, W. K.; Brodd, R. J. J . Electrochem. SOC.1984, 131, 2467. (9) Madou, M. J.; Szpak, S. J . Electrochem. SOC.1984, 131, 2471. (IO) Hagan. W. P.; Hampson, N. A,; Packer, R. K. Electrochim. Acta 1986, 31, 699. (11) Dampier, F. W.; Cole, T. A. J . Electrochem. SOC.1986, 133, 938. (12) Madou, M. J.; Smith, J. J.; Szpak, S. J . Electrochem. SOC.1987, 134, 2794.
J. Phys. Chem., Vol. 99, No. 24, 1995 9923 (13) Chiu, J. G.; Wang, Y. Y.; Wan, C. C. J . Power Sources 1987, 21, 119. (14) McDonald, R. C. J. Power. Sources 1988, 135, 403. (15) Hills, A. J.; Hampson, N. A. J. Power Sources 1988, 24, 253. (16) Mosier-Boss, P. A.; Szpak, S.; Smith, J. J.; Nowak, R. J. J . Electrochem. SOC. 1989, 136, 2455. (17) Bagotsky, V. S.; Kazarinov, V. E.; Vol'fkovich, Yu. M.; Kanevsky, L. S.; Beketayeva, L. A. J . Power Sources 1989, 26, 427. (18) Hu, H.-Y.; KO, S.-W. J. Power Sources 1989, 26, 419. (19) Schlaikjer, K. R. J . Power Sources 1989, 26, 161. (20) Choi, Y.-K.; Kim, B.4.; Park, S.-M. J . Electrochem. SOC. 1993, 140, 11. (21) Long, D. A.; Bailey, R. T. Trans. Faraday SOC. 1963, 59, 594. (22) Venkatasetty, H. V. J . Electrochem. SOC.1980, 127, 2531. (23) David, J. G.; Hallam, H. E. J . Mol. Struct. 1970, 5, 31. (24) Nanni, P.; Viani, J. Mol. Struct. 1972, 14, 413. (25) Mosier-Boss, P. A,; Boss, R. D.; Gabriel, C. J.; Szpak, S.; Smith, J. J.; Nowak, R. J. J . Chem. Soc., Faraday Trans. 1989, 85, 11. (26) Mosier-Boss, P. A.; Szpak, S.; Smith, J. J.; Nowak, R. J . J. Electrochem. SOC. 1989, 136, 1282. (27) Kim, B.-S.; Park, S.-M. J . Electrochem. SOC. 1995, 142, 34. (28) Kim, B.-S.; Park, S.-M. J . Electrochem. SOC. 1993, 140, 115. (29) Kim, B.-S.; Park, S.-M. J . Electrochem. SOC. 1995, 142, 26. (30) Spandau, H.; Brunneck, E. Z . Anorg. Chem. 1952, 270, 201. (31) Spandau, H.; Brunneck, E. Z . Anorg. Chem. 1955, 278, 197. (32) Kolomoets, A. M.; Pleshakov, M. S.; Dudnikov, V. I. Soviet Electrochem. (English Translated Edition) 1981, 17, 326. (33) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; Chapter 11. See Chapters 6 and 11 for discussions on reaction mechanisms and Chapter 1 for interpretation of redox reactions in terms of energy levels of molecular orbitals. (34) Anthony, J. F. Ph.D. Dissertation, University of Georgia, Athens, Georgia, 1968. (35) Park, S.-M.; Hemdon, W. C. Tetrghedron Lett. 1978, No. 27, 2363. (36) Rao, C. N. R. Ultra-Violet and Visible Spectroscopy; Butterworth & Co.: London; 1975; Chapter 2. (37) Reichardt, C. Solvents and Solvent EfSects in Organic Chemistry; VCH: Weinheim, 1988; Chapter 6. (38) McConnell, H. J . Chem. Phys. 1952, 20, 700. (39) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous Systems; Marcel Dekker: New York, 1970. (40) Foster, R. Organic Charge-Transfer Complexes; Academic Press: London, 1969. JP943 1604
