J . Chem. Soc., Faraday Trans. I, 1989, 85(1), 11-21 Raman and Infrared Spectroscopy of the A1C13-SOC1, System Pamela A. Mosier-Boss, Roger D. Boss, Cedric J. Gabriel and Stanislaw Szpak Naval Ocean Systems Center, San Diego, CA 92152-5000, U.S.A. Jerry J . Smith Naval Weapons Center, China Lake, CA 93555-6001, U.S.A. Robert J . Nowak Ofice of Naval Research, Arlington, VA 2221 7-5000, U.S.A. The structural aspects of the AlC1,-SOCI, system have been examined by vibrational spectroscopy. The SOCI, molecule exhibits amphoteric charac- ter, i.e. it can act simultaneously as a donor through the oxygen atom and as an acceptor through the sulphur atom. The liquid state contains loosely bound, open-chain dimers/oligomers, (Cl,SO), with n 2 2. The dissolution of AI,Cl, in SOCI, occurs dissociatively with the formation of C1,SO -+ AlCl, adducts.At higher AICI, concentrations, an increase in solution electrical conductivity is attributed to the reaction : 2 C1,SO -+ AlCl, e [Cl,Al( +- OSCl,),]' + AlCl, Hecht,' followed by Spandau and Brunneck, proposed the existence of 1 : 1 and 2: 1 AlC1,-SOC1, adducts. In the early 1960s, Long and Bailey3 examined the structural features of these adducts and concluded that complexation occurred through the oxygen atom of SOCl,. A few years later, Auborn and co-workers4 employed this system in the construction of Li galvanic cells of highest practical energy densities. Their work prompted further inquiry into the nature, structure and reactivity of the various SOCl, complexes associated with the Li-SOCl, cell Here, we discuss the structure of neat SOCl, and is reactivity through interaction with a set of selected miscible liquids.We include an analysis of the structural changes and species produced by the addition of Al,Cl, to SOCl,.* Experimental Chemicals Thionyl chloride was refluxed under an argon or helium atmosphere to remove dissolved HCl and SO, with the end point indicated by pH paper located in the gas exit stream. The SOCl, was then distilled and the middle fraction was collected and stored under argon. Aluminium chloride, benzene, carbon tetrachloride and hexanes, all of spectroscopic grade (Fluka), and toluene and methylene chloride (Aldrich, gold label) were used as received. Solutions All solutions were prepared in a glove bag by diluting known volumes of SOCl, with CCl,, CH,Cl,, C,H,, or C,H,CH3.The AlC1,-SOC1, solutions were prepared from known amounts of AI,Cl, and filtered through a glass-fibre filter prior to use. 1112 Raman and I.R. Spectroscopy of AlC1,-SOCl, Instrumentation Infrared spectra were obtained on a Nicolet 5DXB FT-IR spectrometer with a resolution of 2cm-'. Standard demountable cells with NaCl windows were used throughout. The pathlength was maintained at 2.5 x m; spectra were subtractively normalized to compensate for solvent absorption. The Raman spectra were recorded on a system comprising a Lexel model 85 Ar-ion laser emitting at 488 nm, which was chopped at 20 Hz, a sample chamber with coupling and collecting optics, a Spex model 1400-1 I scanning double monochromator with photon-counting preamplifier and line driver, a photon-counting integratorg and a digital synchronous detector.The samples were contained in a Helma model 162 F stoppered quartz cell. The cell chamber was flushed with dry nitrogen to prevent condensation on the cell windows when operating below room temperature. Tem- peratures were controlled to within kO.1 "C. Most of the spectra were recorded at a laser power of 50 mW. Resolution of Spectral Bands The recorded infrared absorption and Raman-scat tering bands were computer-analysed and decomposed into their component Voigt profiles using a previously described procedure.1° In a dynamic system, comprising a number of species, N , with overlapping spectral lines, the experimentally observed spectral band intensity, I( v), is a superposition of at least N lines, and can be approximated by eqn (1) N I(v) = C,(v-a)+ c CiL( ...) i- 1 where a, C, and Ci are adjustable parameters and f( ...) is defined by eqn (2) where and v,, Av, and AvG are, respectively, the centre frequency, and the full widths at half maximum of the Lorentzian and Gaussian distributions that characterize the line.The linear term Co(v-a) has been added to account for a sloping baseline. a = Av, d ( l n 2)/Av,, cu = 2(v- vo) d(In 2)/Av,, Molecular-orbital Calculations Calculations were performed using AMPAC, a general purpose, semi-empirical molecular- orbital package developed at the University of Texas (Austin, TX). These calculations yield information on electronic and core-core repulsion energies, heats of formation and vibrational frequencies. Results and Discussion The molecular structure of a liquid arises from interactions between neighbouring molecules.This premise allows us to follow structural changes as the molecules adapt themselves to the changing environment, e.g. to changes in solution composition or temperature. For example, SOCl,, being an amphoteric molecule, may form dimers/ oligomers. In the more complex AlC1,-SOCI, system, such interactions can lead to the formation of molecular adducts and ionic species.8P. A . Mosier-Boss et al. 1.0 13 V , ( A ' ) 6,(0SCI 1 - - I ICl1 I 200 400 600 800 1000 1200 1400 wavenumber/cm-' Fig. 1. Raman spectrum of neat SOCI,: vibrational modes and assignments. Method of Analysis The SOCI, molecule is of the Z X Y , type and a member of the C, symmetry point group." It has six normal modes of vibration, all Raman and infrared active.The Raman spectrum of neat SOCI, and the assignments are shown in fig. 1. The structural characteristics of SOCI, in the liquid state as well as the AlCl,-SOCl, system are ascertained by examining the position and lineshape of v,(A'), the symmetric S-0 stretching vibration at 1231 cm-l and the v2(A') and v,(A') symmetric and asymmetric S-Cl stretching vibrations at 492 and 455 cm-', respectively ; these vibrational modes are sensitive to changes in the charge distribution arising from molecular interactions. The S=O bond of SOCI, has partial double bond character which results from the superposition of pn - dn back-bonding from 0 to S upon the S + 0 bond.', According to the valence-shell electron-pair repulsion (VSEPR) model, bonding through the oxygen atom should lessen the pn --+ dn back-bonding and, hence, lower the S=O bond- order and stretching frequency.Conversely, bonding through the sulphur atom increases the pn -, dn back-bonding, thus raising the S-0 stretching frequency. Withdrawal of electron density from the S=O bond will decrease the repulsion between the lone pair of electrons on the sulphur and the chlorine atoms of SOCI,, which, in turn, strengthens the S-Cl bonds and shifts the symmetric and asymmetric S-Cl stretching vibrations to higher frequency. Moreover, coordination through the sulphur atom will reduce the repulsion between the sulphur lone pair of electrons and the chlorines, also shifting the symmetric and asymmetric S-Cl stretching vibrations to higher frequencies.Examples of this kind of behaviour have been reported for the metallic complexes of (CH,),SO, in which complexation with Zn2+, A13+, Ni2+, Co2+, Fe2+ and Fe3+ ions occurs through the oxygen atom while complexation with Pd2+ and Pt2+ occurs through the sulphur atom.', Thionyl Chloride in the Liquid State In the liquid state, the S-0 stretching frequency is at 1231 cm-'; whereas, in the gaseous state it occurs at 1251 cm-'.14 This rather small shift to lower frequency upon14 Raman and I.R. Spectroscopy of AlCl,-SOCl, 1 .O A 0 B 1150 1175 1200 1225 1250 1275 1300 w avenumber im- ' Fig. 2. Decomposition of S-0 stretching spectral band of neat SOCl, into Voigt profiles: A, 23.5 "C; B, -20.0 "C.condensation of SOC1, implies that the intermolecular interactions are weak. At 23.5 "C the S-0 stretch of neat SOC1, at 1231 cm-l, shown in fig. 2, is actually a composite band which has been resolved into two Voigt profiles with peaks at 1242.6 and 1230.5 cm-l, profiles (a) and (b) respectively. However, at -20 "C the S-0 stretch of neat SOCl, can be resolved into three Voigt profiles with peaks at 1243.5, 1230.8 and 1221.5 cm-', profiles (a), (b) and (c), respectively, in fig. 2. This band structure suggests that pure SOCl, is a weakly associated liquid, which, in view of the rather low latent heat of vaporization and the numerical value of the slope of the fluidity as a function of specific The existence of the association implies that the SOCl, molecule is amphoteric, i.e.it acts simultaneously as a donor through the oxygen atom, and as an acceptor through the sulphur. Thionyl chloride may self-associate in two ways ; via sulphur-to-oxygen bonding in either a cyclic-dimer form, or an open chain form, which may contain more than two members, and via sulphur-sulphur association. In longer chains, a cooperative effect may increase the donor character of the S=O bond. Dilution experiments were performed to determine the structure of the associated indicates the presence of small, interacting molecular clusters.P . A . Mosier-Boss et al. 15 species of SOC1,. Infrared spectroscopy was used to examine the changes in the lineshape and position of the S-0 stretching vibrational band upon dilution in three types of solvents : (a) poor acceptors (inert solvents), e.g.C6H14 and eel,; (b) n-interacting solvents, e.g. C,H,CH,; and (c) a polar liquid, e.g. CH,Cl, (with p = 1.6 Dt).', On the basis of the VSEPR model, as well as more general donor-acceptor considerations," the following characteristics of the S-0 stretch of the monomer and associated species of SOCl, are expected. For a monomer, the S-0 stretching vibrational peak should be fairly narrow and its peak position should be solvent dependent. The cyclic dimer, should also exhibit a fairly narrow peak but its position should be relatively solvent independent. Furthermore, because of the decrease in repulsion between the lone pair of electrons on the sulphur atom and the negative charge on the chlorine atoms, the separation between the S-Cl symmetric and asymmetric stretches, v, and v,, respectively, should be less than that for the monomer.For an open chain dimer/ oligomer, the peak position of the S-0 stretching frequency is expected to be solvent dependent and the peak should be broader than that of the monomer. The open-chain dimer probably has two different S=O bonds. The extent to which these differ will depend on the strength of the interaction within the dimer. If the strength of the interaction were on the order of an ionic or covalent bond then both these S=O vibrational bands would be observed. However, as the strength of the interaction decreases, the separation between these two bands would also decrease eventually resulting in one broad band. This would be the expected result for dipole4ipole and dipole-induced-dipole interactions.Finally, the separation between v, and v, is expected to be greater for the open dimer than for the monomer due to increased repulsion between the chlorines and lone pair of electrons on the sulphur atom. Results of dilution experiments are as follows. In all solvents, within the SOC1, concentration range studied, the S-0 Voigt profile at 1221.5 cm-', v(c) in fig. 2, is not observed. For this reason and the fact that it is observed only in neat SOCl, at low temperatures, this band is attributed to higher aggregates, i.e. to trimers, tetramers etc. Furthermore, it is seen that the peak position of the other two Voigt profiles is solvent dependent. In CCI, and hexanes, which are considered to be 'inert' solvents, the high- frequency peak, v(a), is narrower than the low-frequency peak, v(b).In these solvents, the association of SOC1, is expected to be the dominant reaction. Therefore the high frequency peak, v(a) is assigned to the monomer and the low frequency peak, v(b), to dimer. The ratio of the areas of these peaks is consistent with these assignments. Since v(b) is broader than v(a), we conclude that the dimer is one of the open type as shown in fig. 3(a). Further evidence for an open structure can be found in the separation between the symmetric and asymmetric S-C1 stretches, v, and v,. In the gaseous state this separation is 37 cm-l, whereas in the liquid state, it is 47 cm-'.14 In methylene chloride, both v(a) and v(b) of the S-0 stretching bands are very broad.With a dipole moment of 1.60 D, CH,C1, is a weakly polar solvent; thus one expects dipole4ipole interactions to occur between CH,C1, and SOCl,. Such interactions have been observed for (CH,),SO and CHCl,.15*16 The behaviour of the S-0 stretching composite band of SOCl, solutions in C,H,CH, is similar to that observed in CCl, and C6H14; i.e. v(a) is fairly narrow and v(b) is broader. From this we conclude that any interactions between SOC1, and C,H,CH, are very weak and that self-association of SOC1, is the dominant reaction. The results of the MO calculations provide additional insights into the possible structures of neat SOCI, in the liquid state. In particular, calculations have been performed for dimeric SOCI, using the structures S-0-S-0 in cyclic form and in open chain, as well as the 0-S-S-0 type of association.The calculations showed that the oxygen-sulphur associated dimers are favoured, fig. 3. The sulphur-sulphur t 1 D = 3.33564 x C m.16 Raman and I.R. Spectroscopy of AlC1,-SOCl, C l C l 0 ' C l Fig. 3. MO-optimized structures: (a) open dimer; (6) cyclic dimer. associated species gives an S-S bond length exceeding 6 (6 x lo-'" m) thereby suggesting little or no association uia this bonding arrangement. Furthermore, the MO calculations indicate that ordered dimeric structures, as well as longer oligomeric structures, can exist in the liquid state with essentially the same stability as the molecular SOCl, itself, and, yet, are not of significantly different stability so as to dominate the structure of the liquid SOCl,.The dilution experiments supported by molecular-orbital calculations indicate the presence of dimers with the open-chain spatial arrangement illustrated in fig. 3 (a) rather than the cyclic form, fig. 3(b). In fact, calculations of cyclic dimer structures invariably led to optimized geometries in which the cyclic dimer opened to form an open-chain dimer. These MO calculations yield structures whereby the lone pairs of the sulphurs are directed away from one another. This implies that the repulsion between the lone pair of electrons on the sulphur atom is principally responsible for molecular structure of SOC1, dimers. Similar arguments for the formation of the cyclic dimers results in a highly improbable structure; the repulsion between the lone pair of electrons with the simultaneous requirement for the charge transfer for 0, -, S, and 0, + S,, creates a distorted arrangement.In summary, SOCl, in the liquid state consists of weakly associated species having an open structure. The presence of cyclic dimers, which could be possible, is excluded on a basis of MO calculations.P. A . Mosier-Boss et al. 17 0 100 200 300 400 500 600 1000 1050 1100 1150 1200 1250 Raman shift/m-' Fig. 4. Evolution of Raman Spectra as a function of AICI, concentration (mol drn-,). The AlC1,-SOCI, System With the addition of Al,Cl,, a covalent compound containing halogen bridges, a new set of Raman bands appears and indicates the formation of adduct(s). In the course of 1 : 1 adduct formation, the Al-Cl-A1 bonds of the acceptor molecular must first be broken.The improvement in coordination makes the formation of the 1 : 1 adduct energetically favourable. The progressive changes in the Raman spectra are shown in fig. 4. Upon the addition of Al,Cl, the Raman spectra become more complex. In addition to bands at 1108, 523, 383, 217, 167 and 114 cm-l, a band at ca. 1055 cm-l emerges, and gains in intensity with increasing Al,Cl, concentration. It is noteworthy that the appearance of this band is accompanied by an increase in the solution conductivity.*18 Raman and I.R. Spectroscopy of AlC1,-SOCl, I I 1 I I I I 200 400 600 800 1000 1200 1400 w avenumber/cm-' Fig. 5. Polarized (a) and depolarized (b) Raman Spectra of the equimolar AlC1,-SOCl, solution.The polarized and depolarized Raman spectra of the equimolar A1Cl3-SOC1, solution are shown in fig. 5. The most significant changes are the disappearance of the band at 123 1 cm-' the S-0 stretching vibration of neat SOCl,, and an emergence of a new band at ca. 1108 cm-'. Such a change in the S-0 stretching frequency accompanied by the shift observed in the symmetric and asymmetric S-Cl stretches to higher frequencies (i.e. from 492 to 523 and from 455 to 500cm-', respectively) is consistent with complexation of AlCl, through the oxygen atom of SOC1,. The observed vibrational modes of the AlC1,-SOC1, complex(es) and their assignments are shown in table 1. These assignments correspond with those made for the AlCl, complexes with tetrahydrofuran, C,H,O l9 and nitromethane, CH,N0,.20 Species and Equilibria An examination of the 1000-1 300 cm-' spectral region, fig.4, indicates the formation of, at least, two distinct species. The first species, with the S-0 stretching vibrational frequency v,(A') at 1108 cm-', has been attributed to the 1 : 1 adduct, C1,SO -+ AlC1,. The identification of the species exhibiting a broad band at 1055 cm-' is less certain. For example, the broadness of this peak as well as the shift of v,(A') to lower frequency indicates further electron withdrawal from the oxygen atom perhaps from the formation of a 1 : 2 complex, Cl,SO(AlCI,), by the reaction (1) Such a conclusion is consistent with the existence of 1 : 2 complex in the solid state., Spectroscopically, this equilibrium requires that the ratio of the area of the band at 1055 cm-l to the area of the 1108 cm-' band should increase with increasing AlCl, concentration.However, it is observed that this ratio is nearly constant at 0.3 0.1 for all concentrations of AlC1, in which the two bands can be resolved. Fig. 6 shows the decomposition of the S-0 stretch of the AlC1, adducts. Furthermore, an increase in the solution electrical conductivity which coincides with the appearance of this band offers an alternative interpretation, namely that of formation of ionic species. Molecular adducts are neutral molecules. Formation of ions from the C1,SO -+ AlCl, 2 C1,SO + AlC1, + C1,SO -+ (AlCl,), + SOC1,.P. A . Mosier-Boss et al. 19 Table 1. Vibrational assignments and calculated frequencies for the Raman spectrum of the 1 : 1 AlC1,-SOCl, complex and other observed vibrations for the 1 : 1 solution vibrational mode vobs.d/cm-' Vcalcd/cm-' observed calculated frequency frequency 0-Al-Cl bending of complex AlCl, rocking of complex AlCl, bending of complex AICl, bending of complex Cl-Al-Cl bending of AlCl; deformation of complex SCl, bending of free SOCl, SCI, bending of complex Al-0-S bending of complex SOCI, torsion of free SOCl, SOCl, torsion of complex SOCl, deformation of free SOCl, SOCl, deformation of complex, Al-CI A1-0 stretching of complex A1-0 stretchingb A1-0 stretchingb S-Cl asymmetric stretching of free SOCl, AI-CI asymmetric stretching of complex S-Cl asymmetric stretching of free SOCl, S-Cl asymmetric stretching of complex S-Cl symmetric stretching of complex AlCl, degenerate stretching of complexc S-0 stretchingb S-0 stretching of complex S-0 stretching of free SOCl, symmetric stretching of AlCl, and AICl, < 110 114 - 167 181 (w) - 194" (P) 217 274 282 318 355 344 (P> 383 419 (w) 428 (w) 455" 492" (PI 500 523 560 (w) 1055 1 108 (P) 1231 (P) 35,101 116 121 157 184 167 192 283 - - 340 335 503 - - 566 395,566 615 608 645 588 1330 1374 (w) weak.(p) polarized. a not observed in the equimolar solution. band due to either Cl,(SO) --+ (AlCl,), or Cl,Al( + OSCl,);. compare with 525 cm-' for the 1 : 1 AlCl, : tetra- hydrofuran complex, 532 cm-' for the 1 : 1 AlCI,: nitromethane complex, and both bands are weak in the Raman. adduct can occur either via the halide ion transfer or the internal exchange route, eqn (11) and (111), respectively.C1,SO -, AlCl, SOCl+ + AlC1; (11) (111) 2 C1,SO -, AlC1, + Cl,Al( t OSC1,); + AlCl,. Of these ions, the presence of the AlC1, ions is documented by a weak band at 181 cm-', and attributed to the C1-Al-C1 bending mode, fig. 5. The stronger bands associated with a AlCl,, complex ion, i.e. the bands at 122 and 350 cm-', are obscured by vibrational modes of the free and complexed SOC1,. The AlC1, ion also has a band at 495 cm-l which is strong in the i.r.,, An attempt was made to see this band by adding LiCl to 4.0 mol dm-, AlCl, in SOCl,. Such a solution would contain AlCl; as well as Li(SOC1,)~. Owing to the overlapping S-Cl vibrational bands of neat SOC1, as well as SOC1, complexed by AlC1, and Li+, it was not possible to discern the 495 cm-l band of AlCl,.There is no spectroscopic evidence for the positively charged SOCl+ species, as required by eqn (11) and suggested by others.'-, On the other hand, the evidence for the20 Raman and I.R. Spectroscopy of AlC1,-SOCI, 0.2 - 950 1000 1050 1100 1150 1200 Fig. 6. Decomposition of the S-0 stretching spectral band associated with the aluminium complex of 4.0 mol dm-I AlC1, in SOCl, at -20.0 "C into Voigt profiles. presence of the [CI,Al(+ OSCl,),]' ions, eqn (111), is as follows. The S=O bond in the onium ion must be weakened due to the reduced pn -+ dn back-bonding, thus lowering the vibrational frequency us. that of a neutral adduct. The appearance of a new band at 1055 cm-l supports its presence. Furthermore, because the [CI,AI( + OSCI,),]' would have a positive charge on the A1 atom, its A1-0 bond would be stronger than that for a neutral complex.This, in turn, would shift the Al-0 stretching vibrations to higher frequencies. Indeed, in the neutral adduct the A1-0 stretching mode occurs at 383.3 cm-l while in the conductive, equimolar AlC1,-SOC1, solution, new weak bands attributed to the ionic complex, were found at 419 and 428 cm-'. In addition, eqn (111) requires that the concentration of the AICl, ion must be equal to that of the Cl,AI(t OSCI,),+ ion. The equilibrium constant expression may be rearranged to give eqn (IV) : (IV) which requires that the areas of the bands at 1055 and 1108 cm-' be in constant proportion, i.e. independent of the AICl, concentration, as observed. The results of the MO calculations of relative stabilities of the mono- and di-solvated onium complex favour the latter as the dominant species responsible for the increase in solution conductivity. The suggested structure is a tetrahedral arrangement about the Al with SOCl, coordination through the oxygen, i.e.consistent with eqn (IV). On the basis of the argument presented, the 1055cm-' band is assigned to the [CI,AI( + OSCI,),]' ion. [CI,Al( + OSCl,),]' = K[Cl,SO + AICl,] Conclusions (a) Neat SOCl, is an associated liquid which forms open chain dimers/oligomers (CI,SO), with n = 2,3,4 . . . ; (b) aluminium chloride dissolves dissociatively in SOCI, i.e. as AlCl, not Al,Cl,; (c) complexation of SOCl, with AIC1, occurs through the oxygen to form the adduct, CI,SO -+ AICl,; ( d ) the C1,SO -+ AICl, adduct dissociates to yield the ionic species [CI,Al(+ OSCl,),]' and AICl,.This work was in part supported by the Office of Naval Research.P. A. hfosier- Boss et al. 21 References I H. Hecht. Z. Anorg. Client.. 1947, 254. 44. 2 H. Spandau and E. Brunneck. Z. Atiorg. Cltem., 1952. 270, 201 ; 2. 3 D. A. Long and R. T. Bailey. Trans. F~irutlq. Soc., 1963. 59. 594. 4 J. J. Auborn. K. W. French. S. I. Lieberman. V. K. Shah and A. Heller. J. Elecrrocheni. Soc.. 1973, 5 K. C. Tsaur and R. Pollard. J. Electroc*iwnt. Soc-., 1984. 131. 975; 984; 1986. 133. 2296. 6 M. J. Madou, J. J. Smith and S. Szpak. J. El~~c~trocherii. 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