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AB initio calculations and matrix ftir studies of the sulfur trioxide radical anion
Volume 129. number 2
CHEMICAL PHYSICS LETTERS
22 August 1986
AB INITIO CALCULATIONS AND MATRIX FTIR STUDIES OF THE SULFUR TRIOXIDE RADICAL ANION
David M. STANBURY, Thomas A. HOLME, Zakya H. KAFAFI and John L. MARGRAVE Department of Chemistry, Rice Universily Houston, TX 77251, USA Received 14 April 1986; in final form 28 May 1986
The structures and vibrational frequencies of SO, (Csv) and SO; (C&J have been calculated at the UHF SCF/3-21 + G * level. By cowndensation of Cs atoms and SOs in an AI matrix the FTIR spectrum of C#Os has been measured. The molecule is proposed to have C, symmetry with SO3 binding to Cs in a bidentate fashion.
1. Introduction
The radical anion of sulfur trioxide, SO,, is of interest for a variety of reasons. In terms of structure and bonding it may be expected to be a link between the well-known species SO3 and SO;-. In terms of reactivity it has been implicated as an intermediate in the aqueous redox chemistry of SOi- [ 11. It may be an important species in the environmental chemistry of so2 [2]. There have been several informative studies of SO,. By using flash photolysis and pulse radiolysis Hayon et al. have measured the electronic spectrum of the SOg in aqueous solution; the spectrum has a peak at 255 nm with E = 1.15 X lo3 M-l cm-l [3]. Huie and Neta have reported an estimate of 0.63 V for the reduction potential of the SO,/SOi- couple [4]. Lower limits to the electron affinity of SO, in the gas phase have been reported by Rothe et al. (EA > 1.70 eV) [S] and by Dotan et al. (EA > 2.7 eV) [6]. SO, has been detected by ESR during photolysis of aqueous solution of SO% [7], in various irradiated salts [B] , and from photolysis of SO, adsorbed on MgO surfaces [9]. C3,, symmetry was assumed on the basis of a Walsh diagram, and the hyperfine interaction in the ESR spectra was interpreted by means of an s/p orbital hybridization scheme to yield an O-S-O bond angle of 112”. Hoyever, these methods yield bond-angles accurate only within several degrees, and no information is provided regarding the bond lengths. 0 009.2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Matrix isolation IR studies of SO, to our knowledge have not been reported, but there has been a recent study of the related species, HOSO,, which was generated by photolysis of H20 or H202 in the presence of SO, [lo]. In this FTIR study only the region between 4000 and 700 cm-l was investigated; moreover, the proton in this molecule is rather covalently bound, to one 0 atom. In the current investigation a closer approximation to free SO, is achieved by the reaction of Cs atoms with SO3 in an Ar matrix, and the spectrum has been scanned in the region 4000300 cm-l. In addition, UHF SCF calculations have been carried out in order to obtain an accurate geometry for free SO,.
2. Experimental A sample of SO3 was prepared by trap-to-trap distillation under vacuum of the vapors from a sample of oleum fl 11. The sample was finally trapped at -170°C in a Pyrex flask equipped with a Teflon highvacuum stopcock. During the matrix isolation experiments this flask was immersed in ice-water to reduce the SO3 vapor pressure to ~5 Torr [ 111. Matrix isolation studies were performed on the multisurface matrix isolation/FTIR apparatus as previously described [ 121. Vapors of SO3 were admitted to tile apparatus through a stainless-steel and brass manifold with a stainless-steel metering valve. Cs 181
Volume 129, number 2
CHEMICAL PHYSICS LETTERS
vapors were generated in a stainless-steel crucible within a tantalum furnace by heating a mixture of Li and CsCl in the range of 380 to 450°C [ 131. Concentrations of Cs and SO, in the matrices were estimated by use of a quartz crystal microbalance. Ar (99.9995% pure, Matheson) was cocondensed with these vapors onto a Rh-plated Cu surface maintained at 13 K. Ab initio calculations were performed on an AS-9000 computer with the program GAMESS [ 141.
3. Results 3.1. Ab initio calculations Calculations for molecules of the type reported here require some attention to the choice of method. The unrestricted Hartree-Fock method was used because the molecules are open-shell. The basis set 3-21 + G* was selected because the molecules are hypervalent and bear a negative charge, thus requiring d orbitals for S and diffuse sp(L) shells for S and 0 [ 15,16 1. The exponents were sp(0) = 0.0845, sp(S) = 0.0405, and d(S) = 0.65 [ 171. Ground state geometries for SO, and SO, were obtained by an energy minimization procedure [ 181 under the constraints of C, and C2v symmetry, respectively. These symmetries were imposed because they are predicted in the Walsh diagrams. No attempt was made to introduce configuration interaction into these calculations, since we were primarily interested in geometries which are well predicted at the UHF SCF level. The computed geometry for SO, was RS_O = 1.518AandBo_S_0 = 113.9”. This compares favorably with the calculations of Hirao using correlated wavefunctions (Rs_o = 1.497 A and 80_s_0 = 113.8’) [ 191 and the deductions from its ESR spectrum (8,-,_s_0 = 115’) [20] and IR spectrum (OO_s_O = 110”) [21]. For SO, the computed geometry was R,_, = 1.483 A and 8,_,_, = 113.8’; the ESRderived geometry was reported as 112” [7-g]. These calculations also predicted vibrational frequencies for SO, at 1175 (degenerate pair), 1000,604, and 5 11 cm-l (degenerate pair), and they verified the assumed C, symmetry. It is generally recognised that frequencies calculated by this method are = 10% too high because of our use of uncorrelated wavefunctions [22], and the neglect of anharmonicities. 182
22 August 1986
3.2. Matrix FTIR observations FTIR spectra of SO,/Ar matrices were qualitatively similar to those recently reported by Rondybey and English [23]. Prominent peaks at 1385,527, and 491 cm-l were assigned to SO,, while smaller peaks were seen for H2S04 (895 cm-l), H2S04 *H20 (1475, 1240 cm-l), SO, -Hz0 (1400 cm-l), (SOs)2 (1367 cm-l), (SOS), (1025,487 cm-l), and SO2 (1355, 1351,1152,1147, and 519 cm-l). The H2S04 presumably arises from reaction of SO, with residual H20 in the apparatus, the hydrates from reaction of SO, with H20 in the matrix, and SO2 from reduction of SO3 at the walls of the delivery system. The intensities of the peaks assigned to (SOs)2 and (SO& depended on the M/R ratio of argon to sulfur trioxide. A spectrum of Cs in Ar showed small peaks due to contamination by SO,, but no other significant features. As shown in fig. 1, new features were seen for Ar matrices containing both Cs and SO,. Matrices were prepared with the molar compositions Cs:SO, :Ar = 1.9:4.3:1000,1.9:2.5:1000,2.5:2.5:1000,and 0.5 :2.5 : 1000. Spectra were taken over the range 4000-300 cm-l. The new features were measured at 1191,1093,1090,1041,990,985,965,585, and 504 cm-l. A weak peak was also observed at 474 cm-l,, but it was barely resolved from the noise. Of these new features, the peaks at 1041,990, and 985 can be assigned to SOT on the basis of Milligan and Jacox’s study of SOT [21]. The pair of peaks at 1093 and 1090 cm-1 probably arises from splitting due to different matrix sites, and so we designate the pair as a peakat1091cm-1.Thesixpeaksat1191,1091, 965,585,504, and 474 cm-l had relative intensities that remained constant with changes in the composition of the matrices. We thus assign them to the new species CsSO,, with some reservations regarding the significance of the peak at 474 cm-l. There is a ossibility that Cs,SO, has also been prepared. SO,% aq and crystalline metal sulfites have IR bands at 928-990,928-952,600-639, and 457-499 cm -l [24]. A controversial [24] study of ‘I$ SO, in an Ar matrix (prepared by reaction of Tl20 with SO,) reported additional peaks at 1064 and 1086 cm-l [25]. It is clear, however, that our most prominent peak, at 1191 cm-l, is at too high energy to be due to Cs2S03. Since the relative intensities of
Volume129, number 2 I
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1150 II00 WAVENUMBERS,
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1250 1200
I 1000
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b
0.4 003 8 $
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Fig. 1. PTIR spectra.Lowerspectraare SOa:AR= 4.3: 1000;up~spectraareC!s:SOs:Ar=1.9:4.3:1000. (a),is the mid-infrared,(b) is the far-infrared. this absorption to the other four peaks remained constant with different Cs concentrations, since the Cs concentrations were kept fairly low, and since Cs vapors are largely monatomic, we conclude that the species responsible for the observed peaks is CsSO,.
4. Discussion The D,, molecule, SO,, has S-O bond lengths of 1.43 A and O-S-O bond angles of 120’ [ 111. The structure of SOi- is somewhat dependent on its environment [26], but in simple ionic crystals such as Na2S03 it has C, symmetry with S-O bond lengths of 1SO4 A and O-S-O bond angles of 105.7’. Vibra-
22 August1986
tional spectra of SO!- m ’ aqueous solution are consistent with C,, symmetry in this medium as well [27]. Our calculated geometry for SO, has bond lengths and angles very close to the means of the corresponding parameters for SO3 and SO%-, which is in accord with expectations from simple bonding theory [8]. Tetratomic molecules of this symmetry have six normal modes, four of which form degenerate pairs, so that four vibrational frequencies are expected. Our observation of more than four IR bands for CsS03 in an Ar matrix, however, requires symmetry lower than C,,. The comparable isovalent species, CsC103 apparently has C, symmetry in low temperature matrices [28] ; it has been proposed in this case that Cs is bound to all three oxygen atoms. On the other hand, NaClO, and LiClO, appear to have C, symmetry, with M+ bound symmetrically to two of the oxygen atoms [29]. preference for the bidentate structure by the lighter alkali metals may simply be due to their smaller ionic radii. Since the O-S-O bone angle is larger in SO, (113.8”) than the O-Cl-O bond angle in ClO, (107’) [30] a preference for bidentate binding in CsSO, is not unexpected. For CsSO, with C, symmetry six fundamentals are expected for the SO? moiety; two extra bands arise from splitting of the doubly degenerate u3 and v4 modes of the C3v species. Relative to C3v SO,, in C, symmetry with SO, binding to Cs in a bidentate fashion the unique S-O bond will have enhanced double bond character, while the other two S-O bonds will have diminished double bond character. This leads to a description of the six modes as: y1 = S=O str., v2 = S(-O), sym. str., v3 = O=S-0 sym. bend, u4 = O-S-O sym. bend, v5 = S(-O)2 asym. str., P6 = O=S-Oasym. bend. A tentative assignment of the bands is presented in table 1, along with the experimental frequencies for SO, and SOi- and the UHF SCF calculated frequencies for SO,. Clearly the three high-energy bands correspond to stretching modes vl, v2, and Q while the three at low energies correspond to the deformation modes u3, v4, and v6. Among the stretching modes, v2 should occur at lowest energy; the assignments of vl and v5 were made so as to give the closest agreement with the UHF SCF frequencies. The bending modes were assigned on the expectation that Y6would occur at higher energy than v3, and that u4 would occur at lower energy than vs. 183
22 August 1986
CHEMICAL PHYSICS LETTERS
Volume 129, number 2
Table 1 Infrared vibrational frequencies (cm-‘) of sulfur trioxide and its anions Species
“1
“2
“3
so,
so; W
1065 1000
486 604
1389 1175
so; csso3
1093, 967 “1
504, 620
a)
- d)c)
I( JJ 1191, vs933965, v2
“3
b) This work, UHF SCF results. a) Ref. [23]. c) This work; experimental results; tentative assignment.
Acknowledgement DMS acknowledges support of this research by the National Science Foundation through Grant CHE-8215501 and by the Robert A. Welch Foundation through Grant C-846. We are grateful to Mike Schmidt at North Dakota State University for his advice on using the GAMESS program, and to Bruce Ault for preprints of his work.
[l] W.K. WiImarth, D.M. Stanbury, J.E. Byrd, H.N. PO and C.-P.Chua,Coord.Chem. Rev.51 (1983) 155.
184
529 511 J I (474), “4 469585, “6
d) Ref. [24].
The greater degree of covalence in HOSO is reflected in its IR spectrum. Peaks were found at 1309 (S( =0)2 asym. st.), 1097 (S(=O), sym. St.), and 759 cm-1 (S-OH st .) [lo]. Such a wide range of frequencies is not found for CsS03, consistent with the notion that its three S-O bonds are more equivalent. The reaction of SO, with Cs may be thought of as an acid-base reaction. Adduct formation with amines [31] and oxygen [32] bases has recently been reported by Sass and Ault. In these cases, however, it is argued that bonding occurs through sulfur rather than the oxygens. Studies with isotopically substituted SO3 will be necessary in order to assign the spectrum definitively and to obtain the force field parameters. However, the present study does provide useful preliminary information regarding the structure and vibrations of the sulfur trioxide radical anion. The results should allow an estimate to be made of the Franck-Condon barrier for electron-transfer reactions of SOS- [ 1,331.
References
“4
[ 21 J.G. Calve& ed., SOa, NO and NO2 oxidation mechanisms: atomospheric considerations (Arm hbor Science Ann Arbor, 1983); R.E. Huie and N.C. Peterson, in: Trace atmospheric constituents,ed. S.E. Schwartz (Wiley, New York, 1983) ch. 2; M.R. Hoffman and S.D. Boyce, in: Trace atmospheric constituents, ed. S.E. Schwartz (Wiley, New York, 1983) ch. 3. (31 E. Hayon, A. Treinin and J. WiIf, J. Am. Chem. Sot. 94 (1972) 47. [4] R.E. Huie and P. Neta, J. Phys. Chem. 88 (1984) 5665. [S] E.W. Rothe, S.Y. Tang and G.P. Reck, J. Chem. Phys. 62 (1975) 3829. [6] I. Dotan and F.S. Klein, Intern. J. Mass Spectrom. Ion Phys. 29 (1979) 137. [7] O.P. Chawla, N.L. Arthur and R.W. Fessenden, J. Phys: Chem. 77 (1973) 772. [ 81 P.W. Atkins and M.C.R. Symons, The structure of morganicradicah (Ehevier, Amsterdam, 1967) pp. 172,173. [9] B.Y. Taarit and J.H. Lunsford, J. Phys. Chem. 77 (1973) 1365. [lo] S. Hashimoto, G. Inoue and H. Akimoto, Chem. Phys. Letters 107 (1984) 198. [ 11) G. Nickless, ed., Inorganic sulfura chemistry (Elsevier, Amsterdam, 1968) pp. 388-395. [12] R.H. Hauge, L. Fredin, Z.H. Kafafi and J.L. Margrave, Appl. Spectry. (July 1986), to be published. [13] Z.H. Kafafi, RX Hauge, W.E. BiIlups’and J.L. Margrave, Inorg. Chem. 23 (1984) 177. M. Dapuis, D. Spangler and J.J. Wendlowski, NRCC Software Catalog, University of CaIifornia,~Berkeley, Vol. 1 (1980). W.J. Pietro, MM. Francl, W.J. Hehre, D.J. DeFrees, J.A. Pople and J.S. Binkley, J. Am. Chem. Sot. 104 (1982) 5039. T. Clark, J. Chandrasekhar, G.W. Spitznagel and P. von R. Schleyer, J. Comput. Chem. 4 (1983) 294. [17] G.W. Spitrnagel, Diplomarbeit, Erlangen (1982). [18] H.B. Schlegel, J. Comp. Chem. 3 (1982) 214. [19] K. Hirao, J. Chem. Phys. 83 (1985) 1433. [ 201 A. Reuveni, Z. Luz and B.L. Silver, J. Chem. Phys. 5 3 (1970) 4619.
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CHEMICAL PHYSICS LETTERS
[21] D.E. Milligan and ME. Jacox, J. Chem. Phys. 55 (1971) 1003. [22] J.A. Pople, R. Krishnan, H.B. Schlegeland J.S. Binkley, Intern, J. Quantum. Chem. S13 (1979) 225. (231 V.E. Bondybey and J.H. English, J. Mol. Spectry. 109 (1985) 221. [24] L. Peter and B. Meyer, Inorg. Chem. 24 (1985) 3071. [25] S.J. David and B.S. Ault, Inorg. Chem. 23 (1984) 1211. [26] P. Kierkegaard, L.O. Larsson and B. Nyberg, Acta. Chem. Stand. 26 (1972) 218. [27] J.D. Brown and B.P. Straughan, J. Chem. Sot. Dalton Trans. (1972) 1750.
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[28] I.R. Beat-tie and J.E. Parkinson, J. Chem. Sot. Dalton Trans. (1983) 1185. [29] G. Ritrhaupt, H.H. Richardson and J.P. Devlin, High Temp. Sci. 19 (1985) 163. [ 301 A.F. Well, Structural inorganic chemistry, 5th Ed. (Oxford Univ. Press, Oxford, 1984) p. 405. [31] L.S. Sass and B.S. Ault, J. Phys. Chem. 90 (1986), to be published. [32] L.S. Sass and B.S. Ault, submitted for publication. [ 331 S.B. Rabin and D.M. Stanbury, work in progress.
1185
CHEMICAL PHYSICS LETTERS
22 August 1986
AB INITIO CALCULATIONS AND MATRIX FTIR STUDIES OF THE SULFUR TRIOXIDE RADICAL ANION
David M. STANBURY, Thomas A. HOLME, Zakya H. KAFAFI and John L. MARGRAVE Department of Chemistry, Rice Universily Houston, TX 77251, USA Received 14 April 1986; in final form 28 May 1986
The structures and vibrational frequencies of SO, (Csv) and SO; (C&J have been calculated at the UHF SCF/3-21 + G * level. By cowndensation of Cs atoms and SOs in an AI matrix the FTIR spectrum of C#Os has been measured. The molecule is proposed to have C, symmetry with SO3 binding to Cs in a bidentate fashion.
1. Introduction
The radical anion of sulfur trioxide, SO,, is of interest for a variety of reasons. In terms of structure and bonding it may be expected to be a link between the well-known species SO3 and SO;-. In terms of reactivity it has been implicated as an intermediate in the aqueous redox chemistry of SOi- [ 11. It may be an important species in the environmental chemistry of so2 [2]. There have been several informative studies of SO,. By using flash photolysis and pulse radiolysis Hayon et al. have measured the electronic spectrum of the SOg in aqueous solution; the spectrum has a peak at 255 nm with E = 1.15 X lo3 M-l cm-l [3]. Huie and Neta have reported an estimate of 0.63 V for the reduction potential of the SO,/SOi- couple [4]. Lower limits to the electron affinity of SO, in the gas phase have been reported by Rothe et al. (EA > 1.70 eV) [S] and by Dotan et al. (EA > 2.7 eV) [6]. SO, has been detected by ESR during photolysis of aqueous solution of SO% [7], in various irradiated salts [B] , and from photolysis of SO, adsorbed on MgO surfaces [9]. C3,, symmetry was assumed on the basis of a Walsh diagram, and the hyperfine interaction in the ESR spectra was interpreted by means of an s/p orbital hybridization scheme to yield an O-S-O bond angle of 112”. Hoyever, these methods yield bond-angles accurate only within several degrees, and no information is provided regarding the bond lengths. 0 009.2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Matrix isolation IR studies of SO, to our knowledge have not been reported, but there has been a recent study of the related species, HOSO,, which was generated by photolysis of H20 or H202 in the presence of SO, [lo]. In this FTIR study only the region between 4000 and 700 cm-l was investigated; moreover, the proton in this molecule is rather covalently bound, to one 0 atom. In the current investigation a closer approximation to free SO, is achieved by the reaction of Cs atoms with SO3 in an Ar matrix, and the spectrum has been scanned in the region 4000300 cm-l. In addition, UHF SCF calculations have been carried out in order to obtain an accurate geometry for free SO,.
2. Experimental A sample of SO3 was prepared by trap-to-trap distillation under vacuum of the vapors from a sample of oleum fl 11. The sample was finally trapped at -170°C in a Pyrex flask equipped with a Teflon highvacuum stopcock. During the matrix isolation experiments this flask was immersed in ice-water to reduce the SO3 vapor pressure to ~5 Torr [ 111. Matrix isolation studies were performed on the multisurface matrix isolation/FTIR apparatus as previously described [ 121. Vapors of SO3 were admitted to tile apparatus through a stainless-steel and brass manifold with a stainless-steel metering valve. Cs 181
Volume 129, number 2
CHEMICAL PHYSICS LETTERS
vapors were generated in a stainless-steel crucible within a tantalum furnace by heating a mixture of Li and CsCl in the range of 380 to 450°C [ 131. Concentrations of Cs and SO, in the matrices were estimated by use of a quartz crystal microbalance. Ar (99.9995% pure, Matheson) was cocondensed with these vapors onto a Rh-plated Cu surface maintained at 13 K. Ab initio calculations were performed on an AS-9000 computer with the program GAMESS [ 141.
3. Results 3.1. Ab initio calculations Calculations for molecules of the type reported here require some attention to the choice of method. The unrestricted Hartree-Fock method was used because the molecules are open-shell. The basis set 3-21 + G* was selected because the molecules are hypervalent and bear a negative charge, thus requiring d orbitals for S and diffuse sp(L) shells for S and 0 [ 15,16 1. The exponents were sp(0) = 0.0845, sp(S) = 0.0405, and d(S) = 0.65 [ 171. Ground state geometries for SO, and SO, were obtained by an energy minimization procedure [ 181 under the constraints of C, and C2v symmetry, respectively. These symmetries were imposed because they are predicted in the Walsh diagrams. No attempt was made to introduce configuration interaction into these calculations, since we were primarily interested in geometries which are well predicted at the UHF SCF level. The computed geometry for SO, was RS_O = 1.518AandBo_S_0 = 113.9”. This compares favorably with the calculations of Hirao using correlated wavefunctions (Rs_o = 1.497 A and 80_s_0 = 113.8’) [ 191 and the deductions from its ESR spectrum (8,-,_s_0 = 115’) [20] and IR spectrum (OO_s_O = 110”) [21]. For SO, the computed geometry was R,_, = 1.483 A and 8,_,_, = 113.8’; the ESRderived geometry was reported as 112” [7-g]. These calculations also predicted vibrational frequencies for SO, at 1175 (degenerate pair), 1000,604, and 5 11 cm-l (degenerate pair), and they verified the assumed C, symmetry. It is generally recognised that frequencies calculated by this method are = 10% too high because of our use of uncorrelated wavefunctions [22], and the neglect of anharmonicities. 182
22 August 1986
3.2. Matrix FTIR observations FTIR spectra of SO,/Ar matrices were qualitatively similar to those recently reported by Rondybey and English [23]. Prominent peaks at 1385,527, and 491 cm-l were assigned to SO,, while smaller peaks were seen for H2S04 (895 cm-l), H2S04 *H20 (1475, 1240 cm-l), SO, -Hz0 (1400 cm-l), (SOs)2 (1367 cm-l), (SOS), (1025,487 cm-l), and SO2 (1355, 1351,1152,1147, and 519 cm-l). The H2S04 presumably arises from reaction of SO, with residual H20 in the apparatus, the hydrates from reaction of SO, with H20 in the matrix, and SO2 from reduction of SO3 at the walls of the delivery system. The intensities of the peaks assigned to (SOs)2 and (SO& depended on the M/R ratio of argon to sulfur trioxide. A spectrum of Cs in Ar showed small peaks due to contamination by SO,, but no other significant features. As shown in fig. 1, new features were seen for Ar matrices containing both Cs and SO,. Matrices were prepared with the molar compositions Cs:SO, :Ar = 1.9:4.3:1000,1.9:2.5:1000,2.5:2.5:1000,and 0.5 :2.5 : 1000. Spectra were taken over the range 4000-300 cm-l. The new features were measured at 1191,1093,1090,1041,990,985,965,585, and 504 cm-l. A weak peak was also observed at 474 cm-l,, but it was barely resolved from the noise. Of these new features, the peaks at 1041,990, and 985 can be assigned to SOT on the basis of Milligan and Jacox’s study of SOT [21]. The pair of peaks at 1093 and 1090 cm-1 probably arises from splitting due to different matrix sites, and so we designate the pair as a peakat1091cm-1.Thesixpeaksat1191,1091, 965,585,504, and 474 cm-l had relative intensities that remained constant with changes in the composition of the matrices. We thus assign them to the new species CsSO,, with some reservations regarding the significance of the peak at 474 cm-l. There is a ossibility that Cs,SO, has also been prepared. SO,% aq and crystalline metal sulfites have IR bands at 928-990,928-952,600-639, and 457-499 cm -l [24]. A controversial [24] study of ‘I$ SO, in an Ar matrix (prepared by reaction of Tl20 with SO,) reported additional peaks at 1064 and 1086 cm-l [25]. It is clear, however, that our most prominent peak, at 1191 cm-l, is at too high energy to be due to Cs2S03. Since the relative intensities of
Volume129, number 2 I
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1150 II00 WAVENUMBERS,
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1250 1200
I 1000
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0.4 003 8 $
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-.
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I
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Fig. 1. PTIR spectra.Lowerspectraare SOa:AR= 4.3: 1000;up~spectraareC!s:SOs:Ar=1.9:4.3:1000. (a),is the mid-infrared,(b) is the far-infrared. this absorption to the other four peaks remained constant with different Cs concentrations, since the Cs concentrations were kept fairly low, and since Cs vapors are largely monatomic, we conclude that the species responsible for the observed peaks is CsSO,.
4. Discussion The D,, molecule, SO,, has S-O bond lengths of 1.43 A and O-S-O bond angles of 120’ [ 111. The structure of SOi- is somewhat dependent on its environment [26], but in simple ionic crystals such as Na2S03 it has C, symmetry with S-O bond lengths of 1SO4 A and O-S-O bond angles of 105.7’. Vibra-
22 August1986
tional spectra of SO!- m ’ aqueous solution are consistent with C,, symmetry in this medium as well [27]. Our calculated geometry for SO, has bond lengths and angles very close to the means of the corresponding parameters for SO3 and SO%-, which is in accord with expectations from simple bonding theory [8]. Tetratomic molecules of this symmetry have six normal modes, four of which form degenerate pairs, so that four vibrational frequencies are expected. Our observation of more than four IR bands for CsS03 in an Ar matrix, however, requires symmetry lower than C,,. The comparable isovalent species, CsC103 apparently has C, symmetry in low temperature matrices [28] ; it has been proposed in this case that Cs is bound to all three oxygen atoms. On the other hand, NaClO, and LiClO, appear to have C, symmetry, with M+ bound symmetrically to two of the oxygen atoms [29]. preference for the bidentate structure by the lighter alkali metals may simply be due to their smaller ionic radii. Since the O-S-O bone angle is larger in SO, (113.8”) than the O-Cl-O bond angle in ClO, (107’) [30] a preference for bidentate binding in CsSO, is not unexpected. For CsSO, with C, symmetry six fundamentals are expected for the SO? moiety; two extra bands arise from splitting of the doubly degenerate u3 and v4 modes of the C3v species. Relative to C3v SO,, in C, symmetry with SO, binding to Cs in a bidentate fashion the unique S-O bond will have enhanced double bond character, while the other two S-O bonds will have diminished double bond character. This leads to a description of the six modes as: y1 = S=O str., v2 = S(-O), sym. str., v3 = O=S-0 sym. bend, u4 = O-S-O sym. bend, v5 = S(-O)2 asym. str., P6 = O=S-Oasym. bend. A tentative assignment of the bands is presented in table 1, along with the experimental frequencies for SO, and SOi- and the UHF SCF calculated frequencies for SO,. Clearly the three high-energy bands correspond to stretching modes vl, v2, and Q while the three at low energies correspond to the deformation modes u3, v4, and v6. Among the stretching modes, v2 should occur at lowest energy; the assignments of vl and v5 were made so as to give the closest agreement with the UHF SCF frequencies. The bending modes were assigned on the expectation that Y6would occur at higher energy than v3, and that u4 would occur at lower energy than vs. 183
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CHEMICAL PHYSICS LETTERS
Volume 129, number 2
Table 1 Infrared vibrational frequencies (cm-‘) of sulfur trioxide and its anions Species
“1
“2
“3
so,
so; W
1065 1000
486 604
1389 1175
so; csso3
1093, 967 “1
504, 620
a)
- d)c)
I( JJ 1191, vs933965, v2
“3
b) This work, UHF SCF results. a) Ref. [23]. c) This work; experimental results; tentative assignment.
Acknowledgement DMS acknowledges support of this research by the National Science Foundation through Grant CHE-8215501 and by the Robert A. Welch Foundation through Grant C-846. We are grateful to Mike Schmidt at North Dakota State University for his advice on using the GAMESS program, and to Bruce Ault for preprints of his work.
[l] W.K. WiImarth, D.M. Stanbury, J.E. Byrd, H.N. PO and C.-P.Chua,Coord.Chem. Rev.51 (1983) 155.
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d) Ref. [24].
The greater degree of covalence in HOSO is reflected in its IR spectrum. Peaks were found at 1309 (S( =0)2 asym. st.), 1097 (S(=O), sym. St.), and 759 cm-1 (S-OH st .) [lo]. Such a wide range of frequencies is not found for CsS03, consistent with the notion that its three S-O bonds are more equivalent. The reaction of SO, with Cs may be thought of as an acid-base reaction. Adduct formation with amines [31] and oxygen [32] bases has recently been reported by Sass and Ault. In these cases, however, it is argued that bonding occurs through sulfur rather than the oxygens. Studies with isotopically substituted SO3 will be necessary in order to assign the spectrum definitively and to obtain the force field parameters. However, the present study does provide useful preliminary information regarding the structure and vibrations of the sulfur trioxide radical anion. The results should allow an estimate to be made of the Franck-Condon barrier for electron-transfer reactions of SOS- [ 1,331.
References
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[ 21 J.G. Calve& ed., SOa, NO and NO2 oxidation mechanisms: atomospheric considerations (Arm hbor Science Ann Arbor, 1983); R.E. Huie and N.C. Peterson, in: Trace atmospheric constituents,ed. S.E. Schwartz (Wiley, New York, 1983) ch. 2; M.R. Hoffman and S.D. Boyce, in: Trace atmospheric constituents, ed. S.E. Schwartz (Wiley, New York, 1983) ch. 3. (31 E. Hayon, A. Treinin and J. WiIf, J. Am. Chem. Sot. 94 (1972) 47. [4] R.E. Huie and P. Neta, J. Phys. Chem. 88 (1984) 5665. [S] E.W. Rothe, S.Y. Tang and G.P. Reck, J. Chem. Phys. 62 (1975) 3829. [6] I. Dotan and F.S. Klein, Intern. J. Mass Spectrom. Ion Phys. 29 (1979) 137. [7] O.P. Chawla, N.L. Arthur and R.W. Fessenden, J. Phys: Chem. 77 (1973) 772. [ 81 P.W. Atkins and M.C.R. Symons, The structure of morganicradicah (Ehevier, Amsterdam, 1967) pp. 172,173. [9] B.Y. Taarit and J.H. Lunsford, J. Phys. Chem. 77 (1973) 1365. [lo] S. Hashimoto, G. Inoue and H. Akimoto, Chem. Phys. Letters 107 (1984) 198. [ 11) G. Nickless, ed., Inorganic sulfura chemistry (Elsevier, Amsterdam, 1968) pp. 388-395. [12] R.H. Hauge, L. Fredin, Z.H. Kafafi and J.L. Margrave, Appl. Spectry. (July 1986), to be published. [13] Z.H. Kafafi, RX Hauge, W.E. BiIlups’and J.L. Margrave, Inorg. Chem. 23 (1984) 177. M. Dapuis, D. Spangler and J.J. Wendlowski, NRCC Software Catalog, University of CaIifornia,~Berkeley, Vol. 1 (1980). W.J. Pietro, MM. Francl, W.J. Hehre, D.J. DeFrees, J.A. Pople and J.S. Binkley, J. Am. Chem. Sot. 104 (1982) 5039. T. Clark, J. Chandrasekhar, G.W. Spitznagel and P. von R. Schleyer, J. Comput. Chem. 4 (1983) 294. [17] G.W. Spitrnagel, Diplomarbeit, Erlangen (1982). [18] H.B. Schlegel, J. Comp. Chem. 3 (1982) 214. [19] K. Hirao, J. Chem. Phys. 83 (1985) 1433. [ 201 A. Reuveni, Z. Luz and B.L. Silver, J. Chem. Phys. 5 3 (1970) 4619.
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[21] D.E. Milligan and ME. Jacox, J. Chem. Phys. 55 (1971) 1003. [22] J.A. Pople, R. Krishnan, H.B. Schlegeland J.S. Binkley, Intern, J. Quantum. Chem. S13 (1979) 225. (231 V.E. Bondybey and J.H. English, J. Mol. Spectry. 109 (1985) 221. [24] L. Peter and B. Meyer, Inorg. Chem. 24 (1985) 3071. [25] S.J. David and B.S. Ault, Inorg. Chem. 23 (1984) 1211. [26] P. Kierkegaard, L.O. Larsson and B. Nyberg, Acta. Chem. Stand. 26 (1972) 218. [27] J.D. Brown and B.P. Straughan, J. Chem. Sot. Dalton Trans. (1972) 1750.
22 August 1986
[28] I.R. Beat-tie and J.E. Parkinson, J. Chem. Sot. Dalton Trans. (1983) 1185. [29] G. Ritrhaupt, H.H. Richardson and J.P. Devlin, High Temp. Sci. 19 (1985) 163. [ 301 A.F. Well, Structural inorganic chemistry, 5th Ed. (Oxford Univ. Press, Oxford, 1984) p. 405. [31] L.S. Sass and B.S. Ault, J. Phys. Chem. 90 (1986), to be published. [32] L.S. Sass and B.S. Ault, submitted for publication. [ 331 S.B. Rabin and D.M. Stanbury, work in progress.
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