J . Pkys. Chem. 1986,90, 4533-4536 Li2S04additions. Infrared spectra showed that this is the result of a direct increase of the number of BO4 units, Le., N4increases. Both infrared and Raman results for the x = 0.56series showed that S042-anions are completely dispersed in the boron-oxygen network, which is affected by their presence. This is manifested by an increase of BO4 units and N B O s for the y = 0.15 glass and by an increase of NBO’s mainly for t h e y = 0.50 glass. The presence of Li2S04in the melt seems to favor the formation of BO4 tetrahedra, in the x = 0.20 series, and the formation of N B O s , in the x = 0.56 series. Even though this may lead to the thought that the effect of SO:- is different for the two series, the actual trend is the same. To understand this point, we may recall that the x = 0.20 binary glass contains mostly boroxol rings and tetraborate groups. So, S042- anions cause the distruction of boroxol rings in favor of the more polar tetraborate groups. The
4533
x = 0.56binary glass contains mostly diborate groups and groups having NBOs. By analogy, S042-anions favor the formation of the latter, i.e., the more polar groups, at the expense of diborate groups. The trend appears to be the same; that is, the presence of S042- favors the formation of more polar borate groups than the ones found in the binary glasses.
Acknowledgment. Continuous and enthusiastic support of the work on FIC glasses by Professor C. A. Nicolaides, Director of the Institute of Theoretical and Physical Chemistry, is very gratefully acknowledged. G.D.C. expresses his thanks to Professor W. M. Risen, Jr., of Brown University and Professor C. A. Nicolaides for making his collaboration to this work possible. Registry
No. Li2S04,10377-48-7;Li20, 12057-24-8.
Matrix Isolation Infrared Spectra of the 1:l Molecular Complexes of Sulfur Trioxide with Selected Oxygen-Containing Bases Craig S. Sass and Bruce S. Auk* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: February 3, 1986)
The 1:l molecular complexes between SO3 and a series of oxygen-containing bases have been isolated and characterized in nitrogen matrices. The spectra of the complexes all showed a red shift and splitting of the antisymmetric stretching mode of the SO3 subunit in the complex, typified by product bands at 1362 and 1378 cm-’ in the complex with dimethyl ether. A number of perturbed vibrational modes of the base subunit were observed as well, and generally arose from vibrations involving motion of the oxygen atom of the base. A comparison of the shift of the perturbed base modes in these complexes with previously studied complexes indicated that SO, is a very strong Lewis acid, although only small shifts were observed for the SO3 subunit, and the symmetric stretching mode was not activated to a detectable degree.
Introduction The development of new techniques in recent years has increased interest in the study of molecular complexes, involving both Lewis and Bronsted acids.l” Complexes involving strong Lewis acids such as BF3have been known for years,” and much of the current attention has been focused on relatively weakly bound complexes. Sulfur trioxide, SO,, is both a strong Lewis acid and a potent oxidizing agent and is known to form a few stable room temperature complexes with strong Lewis bases such as (CHJ3N.’+ More often, however, reaction with SO3is sufficiently rapidly that possible intermediate complexes are not observed. The reaction of SO3with H 2 0 has been of particular interest as it is one step in the formation of acid rain. Castelman has shown’O that this reaction proceeds throough a 1:l complex in the gas phase, prior to conversion to H2SO4. This complex has also been observed in cryogenic Workers have reported the formation of a highly reactive species during the reaction of formaldehyde with SO3,which they postulated might be due to a 1:l complex.I3 (1) Bowden, K. H.; Leopold, K. R.; Chance, K. V.; Klemperer, W. J. Chem. Phys. 1980,73, 137. (2) Lcopold, K.L.; Bowen, K. R.; Klemperer, W. J . Chem. Phys. 1980,
Matrix isolation1e17has been a very effective approach to the study of highly reactive chemical species, including Lewis acid-base complexes. Recently, the study of a series of 1:l complexes of SO3with amine bases isolated in nitrogen matrices was reported,I8 marking the first observation of a number of these species in a nonperturbing environment. With the continuing interest in the chemistry of SO3 and the current lack of knowledge about the interaction of SO3 with weak bases, a study was undertaken to characterize the complexes of sulfur trioxide with oxygen bases in inert matrices.
Experimental Section All of the experiments in this study were conducted on a conventional matrix isolation system which has been described previo~sly,’~ including modifications for the handling of SO3.I8 Briefly, solid SO3was placed in an all glass/Teflon vacuum system, and the vapor mixed with N2 to a ratio of approximately 1000/1. This sample was then deposited onto the 14 K cold window through a Teflon needle valve. The oxygen bases employed in this study were dimethyl ether (Matheson), dimethyl-d, ether (ICON), methanol (M. J. Daley),
74 . . , 7211 . - - -.
(3) Jensen, W. G.The Lewis Acid Base Concepts, an Overview; WileyInterscience: New York, 1980. (4) Swanson, B.; Shriver, D. F. Inorg. Chem. 1979, 9, 1406. (5) Amster, R. L.; Taylor, R. C. Spectrochim. Acta 1964, 20, 1487. (6) Grundes, J.; Christian, S. D. J . Am. Chem. SOC.1968, 90, 2239. (7) Watari, F.2.Anorg. Allg. Chem. 1964, 322, 322. (8) Kanda, F. A.; King, A. J. J . Am. Chem. SOC.1931, 73, 2315. (9) Sass, R. L. Acta Crystallogr. 1960, 13, 320. (10)Holland, P. M.;Castelman, A. W. Chem. Phys. Lett. 1978, 56, 511. (11) Tso, T.;Lee.,E. K. C. J . Phys. Chem. 1984,88, 2776. (12) Bondybey, V. E.;Nglish, J. H. J. Mol. Spectrosc. 1985, 107, 221.
(1 3) Nooi, J. R.;Martens, R. J.; Kemper, H. C. Rec. Trav. Chim. P a y s Bas 1972, 91, 367. (14) Craddock, S.; Hinchcliffe, A. J. Matrix Isolation; Cambridge University: New York, 1975. (15) Andrews, L. Annu. Rev. Phys. Chem. 1971, 22, 109. (16) Ault, B. S. Acc. Chem. Res. 1982, 15, 103. (17) Barnes, A. J. In Molecular Interactions; Ratajczak, H., OrvilleThomas, W. J., Eds.;Wiley: New York, 1980. (18) Sass, C. S.; Ault, B. S . J. Phys. Chem. 1986, 90, 1547. (19) Auk, B. S . J. Am. Chem. SOC.1978, 100, 2426.
0022-3654/86/2090-4533$01.50/00 1986 American Chemical Society
4534
Sass and Ault
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986
&Jmv2&-+a'*
I&
'
:dc
'.
g\E?sI ' r m
,
:
929
-
L
A
e43
k
-
60C
'
'
523
c
1
440
'I
Figure 1. Infrared spectra of the product arising from the twin jet codeposition of SO3 and dimethyl ether into nitrogen matrices over selected spectral regions compared to blank experiments of the parent species in N,.
acetone (Daley), furan (Fisher), ethylene oxide (Matheson), tetrahydrofuran (Fisher), and formaldehyde (MCB). Samples of formaldehyde in N 2 were prepared by passing a stream of N2 over solid paraformaldehyde and entraining the monomeric species. All of the remaining bases were purified by repeated freeze-thaw cycles prior to dilution with N,. In a typical experiment, the N2/S03 and N2/base mixtures were sprayed simultaneously onto the 14 K cold window at 2 mmol/h for 20-24 h. Infrared spectra were recorded on an IBM 98 Fourier transform infrared spectrometer at 1-cm-] resolution. Typically, 2000 interferograms were averaged for the final spectra. Several such matrix samples were then annealed to 35 K and recooled to 14 K, and further spectra recorded.
Results Prior to the investigation of the reaction products arising from the codeposition of SO, with the oxygen bases, "blank" experiments were run on each of the reactants alone in nitrogen at several different dilutions. All spectra were in good agreement with literature ~ p e c t r a ; ~the * ~ N2/S03 ~ samples did show some indication of H 2 S 0 4impurity,25which could be minimized but not completely removed. SO, + (CH,),O. Samples of N2/SO3 were codeposited with samples of N2/(CH3),0in three experiments, with total dilutions of up to 2000/ 1/ 1. In these experiments, six new product bands were observed which were not present in a blank experiment of either reactant alone. These were sharp, intense absorptions at 1362 and 1378 cm-I, along with bands of moderate intensity at 549, 872, 875, and 1025 cm-I. When the concentration of the dimethyl ether sample was changed, the intensities of these bands were altered accordingly, and all six bands appeared to maintain a constant intensity ratio. When dimethyl-& ether was employed, the product bands at 549, 1362, and 1378 cm-' were unaffected. However, the product doublet near 875 cm-I and the singlet at 1025 cm-I disappeared, and a sharp new product absorption was noted at 780 cm-I. Infrared spectra from the SO,/(CH,), system are shown in Figure 1 over selected spectral regions. SO, + CH,OH. The codeposition of these two reactants into nitrogen matrices gave rise to a set of four product absorptions, at 600, 1017, 1369, and 1387 cm-'. The former was somewhat broad, while the three latter bands were all quite sharp and distinct. As the concentration of either reactant was increased, the intensity of these product absorptions grew proportionately. (20) Lasseques, J. C.; Grenie, Y . ;Ford, M. T. C . R. Seances Acad. Sci., Ser. B. 1970, 271, 42 1 . (21) Meyer, R.; Serrallach, A,; Gunthard, H. J . Mol. Spectrosc. 1974, 52, 94.
(22) Khoshkoo, H.; Hemple, S. J.; Nixon, E. R. Spectrochim. Acta, Part A 1974, 30A, 8 6 3 . (23) Potts, W. J. Spectrochim. Acta, Part A 1965, ZIA, 5 1 1 . (24) Loisel, J.; Pinan-Lucarre, J. P.; Lorenzelli, V. J . Mol. Struct. 1973, 17,'341. ( 2 5 ) Chackalackal, S . M.; Stafford, F. E. J . Am. Chem. Soc. 1966, 88, 723.
1
)
"
1800
8
I
1720
I
*
1640'
4
:
I
I
'
560
1440 1360 ENERGY (cm')
,
,
480
,
Figure 2. Infrared spectra of the product of the codeposition of sulfur trioxide with formaldehyde in nitrogen matrices compared to blank spectra of each reactant over selected spectral regions.
z
o_tQ
[L
0,
m
1440
1360
4
1280 1200 ENERGY (cm-3
I
560
l
l
480
1
Figure 3. Infrared spectra of the product arising from the codeposition of SO, with ethylene oxide in N, matrices, over spectral regions of interest, compared to blank spectra of the reactants.
+
SO3 (CH,),CO. The codeposition of acetone with sulfur trioxide gave rise to just three product bands, at 516, 549, and 1640 cm-I. The last band was quite close to a weak parent absorption of acetone, but was nonetheless easily resolved. Acetone, unfortunately, has a pair of strong absorptions near 1370 cm-l, which prevented observation of any product bands in the S-0 stretching region. As in the above systems, the three product bands maintained a constant intensity ratio over a range of concentrations in four different experiments. SO, CH20. The means of preparation of the formaldehyde samples precluded precise knowledge of the sample concentration. However, comparison to literature spectra suggests a concentration range of 500/1 to 1000/1. In these codeposition experiments, distinct new product absorptions were observed at 520, 543, 1362, 1372, and 1687 cm-I, the latter three being particularly sharp and intense. In addition, a likely new product absorption was noted at 1475 cm-l. However, impurity H2S04absorbs2squite near this position as well, and this observation must be regarded as tentative. Spectra arising from the codeposition of this pair of reactants are shown in Figure 2 . SO, + Cyclic Ethers. The codeposition of ethylene oxide with sulfur trioxide yielded distinct product absorptions at 5 14, 571, 841, 1271, 1359, and 1383 cm-I, as can be seen in Figure 3. As in the above systems, these last two bands were the most intense and sharp of the set, although all were quite distinct and reproducible from experiment to experiment. One matrix sample containing SO3and ethylene oxide was annealed to approximately 35 K and recooled to 14 K. The spectra recorded after this process were substantially unchanged from the spectra obtained before
+
Infrared Spectra of SO, Complexes annealing, The codeposition of SO, with furan (C4H40)gave rise to just three product bands, at 556, 587 and 1370 cm-'. It should be noted, however, that for these larger cyclic ethers a substantial amount of the spectrum was obscured by the increasing number of parent absorptions. Of particular note is the fact that parent furan has an intense absorption near 1360 cm-I which limited observations in this region. These three product absorptions did maintain a constant intensity ratio in several experiments at concentrations of 500/1 and 1000/1. One sample of furan and sulfur trioxide was annealed as well, and again no distinct changes were observed as a result of this process. Finally, SO, was codeposited with THF (C4H80)in several experiments, and distinct product absorptions were noted at 526, 549, 984, 1356, and 1377 cm-l. Discussion
The codeposition of SO, with the above series of oxygen-containing bases gave rise to a set of product absorptions for each system. These are typified by the set at 549, 872,875, 1025, 1362, and 1378 cm-I in the dimethyl ether experiments. The fact that, for each system, all of the product absorptions maintained a constant intensity ratio suggests the formation of a single reaction product. All of the product absorptions fell within 50 cm-' of absorptions of one of the parent species, indicating that the parent molecules, although perturbed during the formation of the product species, have maintained their basic structural integrity. On the other hand, oxidation products, in general, should have at least some spectral features not near a parent mode. These observations, in agreement with earlier s t ~ d i e s , ~ ~indicate - ~ O formation of a molecular complex or adduct rather than a rearrangement or elimination product. Under the conditions of high dilution employed, the formation of a 1:l complex is most likely, and the product bands observed here for each system are so assigned. This marks the first time that a complex of SO, with an oxygen-containing base (other than HzO) has been observed and spectroscopically characterized. The short-lived existence of such species during room temperature reactions of SO, is also suggested, although these data cannot confirm the previously postulated formation of the S0,-CH20 complex by Nooi et al.I3 The annealing behavior of several of these complexes suggests that they have at least limited stability and that the barrier to rearrangement is not extremely low. As with most molecular complexes, assignment of the product bands is relatively straightforward due to their proximity to the parent absorptions. For example, in the dimethyl ether experiments, the product bands could readily be divided into two groups, those in regions characteristic of the SO, parent at 549, 1362, and 1378 cm-I and those in regions characteristic of the (CH3)20 parent, near 875 and 1025 cm-I. In addition, the first three bands showed no shift upon deuterium substitution, while the latter two did. Accordingly, the first set can be assigned to vibrations of the SO, subunit in the 1:l complex, specifically to one of the deformation modes (549 cm-I) and the two split components of the antisymmetric stretch. This mode is split upon complexation as a consequence of the lowering of symmetry from D j h in free SO, to C, or lower in the complex. The vibrations of the (CH3)2O subunit, near 875 and 1025 cm-I, are readily assigned to the symmetric and antisymmetric C-0-C stretching modes, respectively. Previous studies have shown that these two modes are particularly prone to perturbation upon complex formation.26-2s The slight splitting of the lower band is most likely due to site effects in the nitrogen matrix. With this analogy in place, assignment of the product bands for the remaining systems is quite straightforward. For most systems, a doublet was observed between 1350 and 1400 cm-I in the S-0 antisymmetric stretching region. These are assigned, as above, to the two components of the antisymmetric stretch. For (26) (27) (28) (29) (30)
Auk, B. S. J . Am. Chem. Soc. 1983, 105, 5742. Walther, A.; Ault, B. S. Inorg. Chem. 1984, 23, 3892. Hunt, R. L.; Ault, B. S. Spectrosc. In?. J. 1982, 1, 31. Sass, C. S.; Auk, B. S. J . Phys. Chem. 1985,89, 1002. Sass, C. S.; Ault, B. S. J . Phys. Chem. 1984, 88, 432.
The Journal of Physical Chemistry, Vol. 90,No. 19, 1986 4535 TABLE I: Band Positions' and Assignments for the Infrared Absorptions of the 1:l Molecular Complexes of SO3 with Selected Oxygen Bases
base (CHp)20 CH3OH
"3a.b
b
(CH3)2C0
CH2O C2H40
C4H40 C4H8O
"Zb
4
"be
516 520 514 556 526 488
549 600 549 543 571 587 549 530
874, 1025 1017 1640 1687 841, 1271
1362, 1378 1369, 1387 1362, 1372 1359, 1383 1370 1356, 1377 1397
984
"Band positions in cm-I. bVibrations of the SO, subunit. CVibrationsof the base subunit; for (CH3)20these are the symmetric and antisymmetric C-0-C stretching modes; for CH30Hthis is the C - 0 stretch; for CHzO and (CH3)$0 this is the C=O stretch; for the cyclic ethers, these are ring deformation modes. TABLE 11: Magnitude of the Shift and Splitting of the Antisymmetric Stretching Mode of the SO3 Subunit in Molecular Complexes base AV3 splitting PA," kcal/mol CH2O 30 10 171.7 C2H40
CH3OH (CH3)20
26 19 27
24 18 16
31 436 6Sb
21
C4H4O (CH3)2C0 C4H80
NH3 (CHdJ
0 0
187.9 188.3 192.1 192.2 196.7 198.8 204.0 225.1
'Proton affinity of the base, from ref 31. From ref 18. the furan system, with a strong parent absorption at 1360 cm-', only a single product band was observed in this region, and for the acetone complex, with a strong parent absorption at 1370 cm-l, no product bands were observed in this region. In many systems, one or more product bands were observed in the SO, deformation region between 500 and 600 cm-' and are assigned as deformation modes of the coordinated SO, subunit. These assignments are collected in Table I. For all of the complexes in this study, one or more perturbed modes of the base subunit were observed, such as the 1687- and 1640-cm-' bands in the C H 2 0 and (CH3)2C0complexes with SO,. Table I lists assignments for all of these product absorptions. It is significant that all of the perturbed base modes observed here were vibrations which involved motion of the oxygen atom of the base, such as the C=O stretch for the formaldehyde and acetone complexes. This indicates, as might be anticipated, that the site of coordination is at the oxygen atom, in agreement with numerous studies of this type. Certainly, no evidence was obtained for a hydrogen bonding interaction, which is another possible mode of coordination. Given the low symmetry of the bases under investigation (less than C, in all cases), coordination through the oxygen to the sulfur of SO, must lead to a splitting of the degenerate modes of the SO, subunit, as was observed. The shifting of the antisymmetric S-0 stretching mode to lower energy is also characteristic of Lewis acids, where electron density enters the lowest unoccupied molecular orbital (LUMO), which is typically antibonding in nature. This leads to a weakening of the bonding, and a lowering of the stretching force constant. The magnitude of these shifts is surprisingly small, 15-30 cm-I, considering the high Lewis acidity of SO3. At the same time, the lack of activation of the symmetry stretching mode of the SO, subunit in the complex suggests very little distortion from planarity. This indicates that the LUMO is not strongly antibonding, and the SO, molecule is not readilv* .oerturbed in the formation of a molecular comdex (even in the amine comolexes. the svmmetric stretching mod; was-only very weakly activated). 'The magnitude of the shift of the antisymmetric stretch might well be expected to correlate with the ability of the base to serve as an electron donor, Le., its intrinsic basicity. Such correlation has been applied
4536 The Journal of Physical Chemistry, Vol. 90, No. 19, 1986
p r e v i ~ u s l yand ~ ~ ,has ~ ~ demonstrated at least a qualitative trend. Here, with a splitting of this mode, the center of gravity of the doublet is the quantity most readily compared. Table I1 makes this comparison, listing the average of the split doublet against the proton affinity of the base. Previous results for the bases NH3 and (CH3)3are included for further comparison. As can be seen, the trend is not perfect, but there is a general increase in shift with increasing basicity. The magnitude of the splitting of the two components of the antisymmetric stretching mode is of interest as well. The splitting is likely to be a function of both the degree of asymmetry of the base, and the strength of interaction (in the limit of a very weak interaction, the subunits are sufficiently far apart that the asymmetry of the base is no longer important, while for strongly bound complexes this should be quite important). While the data provide no precise means of determining the strength of interaction for these complexes, the small and roughly comparable shift of the antisymmetric stretch suggests that the interaction strengths, to a first approximation, are comparable. In this case, the asymmetry of the base is the dominant feature, a suggestion which is qualitatively borne out by the data shown in Table 11. C H 2 0 is the simplest and least bulky of the bases and showed the smallest splitting, 10 cm-I. Intermediate values for the splitting were noted for methanol and dimethyl ether, while the cyclic ethers gave rise to the largest splittings. Another trend which might be examined is the shift of the symmetric C 4 - C stretching mode of dimethyl ether as a function of the degree of interaction with a Lewis acid. It has been proposed that this mode is a good indicator of the degree of acidity of the Lewis acid under investigation.26-28 The shift here, 50 cm-I, to 875 cm-I, is larger than any shift seen to date. For comparison, a shift of 40 cm-' was noted2*for the (CH3)2O complex with BBr3, and 33 cm-I for the analogous complex27with GeF,. This suggests (31) Lias, S . G.; Liebman, J. 1984, 13, 695.
F.;Levin, R. D.J . Phys. Chem. Ref.Data
Sass and Ault that SO3is the strongest acid in the series of acids studied by this technique, an observation which is in agreement with the very large shift of the symmetric deformation mode of NH, in its complex18 with SO3. It was not a particular goal of this study to characterize the 1:l complex of SO3with H 2 0 ,in as much as this species has been identified in neon and argon matrices.11.12These workers report a blue shift of the SO3antisymmetric stretching mode from the parent in neon and a red shift in argon. In the present work, this complex was readily identified in nitrogen matrices, due to the presence of impurity water in every experiment, at 1382 cm-l indicating a red shift of 15 cm-'. Matrix shifts are known to increase to the red with increasing polarizability of the matrix mate1ia1.l"'~ Neon is the least interacting of the matrix materials, and it is not impossible that small blue shifts might occur for neon, while red shifts occur for the heavier matrix materials. There is some precedence for shifts of this nature from previous matrix Conclusions The matrix isolation technique combined with twin jet deposition has led to the formation, for the first time, of 1:l complexes of SO3 with a series of oxygen bases. These appear less tightly bound than their nitrogen analogues, but were stable with respect to annealing. The spectroscopic features observed were all consistent with coordination through the oxygen of the base to the sulfur of SO3,leading to a lowering in symmetry of the SO3 subunit in the complex and a splitting of the degenerate modes of the parent.
Acknowledgment. We gratefully acknowledge support of this research by the National Science Foundation through Grant CHE 8400450. Registry NO. S03*(CH,)20, 10371 1-42-8;SO,*CH,OH, 103730-91-2; SO,.(CH,)2CO, 25313-57-9; SO,*CH,O, 10371 1-43-9; SO3C2H40, 103711-44-0; SO,*CpHdO, 10371 1-45-1; SO,*THF, 51771-42-7; N,, 7727-37-9.
