432
J . Phys. Chem. 1984, 88, 432-440
plexes, based on the displacement of their strongly absorbing H-F fundamentals from the diatomic value.
We gratefully for this research by the National Science Foundation, a Sesquicentennial Associateship from the University of Virginia, a Visiting Fellowship from the Science and Engineering Research
Council (U.K.), and a Fulbright Senior Research Fellowship, the assistance of B. J. Kelsall and R. T. Arlinghaus with several experiments, a Beckman spectrum of H F species in solid argon from B, S, Ault to with the present observations, and word processing of the manuscript by Southampton University. Registry No. HF, 7664-39-3; DF, 14333-26-7; Ar, 7440-37-1.
Matrix Isolation Infrared Spectroscopic Study of Sulfur Dioxide-Amine Complexes Craig S . Sass and Bruce S . Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: April 8, 1983)
The reaction products of sulfur dioxide and a variety of methyl-substituted amines were studied in argon and nitrogen matrices, after formation through both single and twin jet deposition. In each case, a 1:l complex was observed and characterized by a shift in the infrared absorptions of the SO2subunit in the complex. The antisymmetric stretching mode shifted to lower energies upon complexation, from 1350 cm-' for free SO2, to 1338 cm-' in the SO2.NH3complex, to approximately 1270 cm-' for the S02.(CH3),N complex. A similar trend was observed for the symmetric stretching mode of the SO2subunit. Band assignments were confirmed by the use of '*O labeled SO2,and normal coordinate calculations were used to fit the observed frequencies to a set of force constants. The primary S-0 stretching force constant F, was found to decrease linearly with increasing base strength of the amine, indicating the SO2 is serving as a a* acceptor. The infrared spectra were consistent with the theoretically calculated geometry, but an insufficient number of product bands were observed to fully confirm the calculation. In addition, the 1:2 complex SOz.2NH3 was observed spectroscopically for the first time.
Introduction Molecular complexes between Lewis acids and bases have been known for years,'J and complexes involving strong Lewis acids such as BF3 are well characterized.,~~Sulfur dioxide, SO2,is a much weaker Lewis acid, but nonetheless is known to form weakly bound charge-transfer complexes with nitrogen-containing While some thermodynamic data have been obtained for these complexes, little is known about their structure, with the exception of the S02.N(CH3),complex, for which an X-ray crystal structure has been determined.g Several groups have attempted to obtain low-temperature infrared spectra of these adducts, with varying degrees of success. Histasune and co-workers10 reported the solid-state spectrum of a sample formed through a sequential condensation of NH, and SOz, followed by annealing. Reahal" and Nord12 have each reported argon matrix spectra of the SO2.", complex, but substantial points of disagreement exist in all of these studies and no firm conclusions were reached. One group attempted', to obtain gas-phase spectra of the SO2/", system, but only detected thionyl imide, NHSO, and the bisulfate salts. Concurrent with these experimental efforts, a number of theoretical studies have been carried out, on the S02.NH3 complex
by Schaefer and co-workers,14and on the methyl-substituted amine complexes by Kollman et a l l s These groups-reach quite similar conclusions, namely, that the plane of the SO, molecule was approximately perpendicular to the S-N bond, which was itself colinear with the C, axis of the amine. This calculated structure agrees well with the crystal structure determined9 for the trimethylamine (TMA) complex, SO,-TMA. The calculated binding energies ranged from 11 to 15 kcal/mol, which agrees well with the experimental value for SOz.TMA of 10 kcal/mol. In addition, a 1:2 adduct is known for the S 0 2 / N H 3system, and has been postulated for several other amines. While a considerable effort has been directed toward the study of amine complexes of SO,, in part because of their importance in atmospheric chemistry, substantial gaps remain in our understanding of these adducts. The matrix isolation technique has proven to be an excellent technique for the study of relatively weakly bound complexes, and has been applied to both protic acid-base systems16-18as well as to Lewis acid-base systems, particularly those involving NH, c o o r d i n a t i ~ n . ' ~With ~ ~ ~ the considerable interest in these molecular complexes, and the need for a thorough investigation of the SO,.amine complexes, a study was undertaken to isolate and spectroscopically characterize these complexes in both argon and nitrogen matrices.
(1) Jensen, W. G. "The Lewis Acid-Base Concepts, an Overview"; Wi-
Experimental Section All of these experiments in this study were conducted on a conventional matrix isolation system which has been described
ley-Interscience: New York, 1980. (2) Niedenzu, K. Ado. Chem. Ser. 1964, No. 42. (3) Swanson, B.; Shriver, D. F. Inorg. Chem. 1970, 9, 1406. (4) Amster, R. L.; Taylor, R. C. Spectrochim. Acta 1964, 20, 1487. (5) Hull, A. E. J . Am. Chem. SOC.1931, 53, 2598. (6) Grundnes, J.; Christian, S. D. J . Am. Chem. SOC.1968, 90, 2239. (7) Landreth, R.; de Pina, R. G.; Heicklen, J. J . Phys. Chem. 1974, 78, 1378. (8) Scott, W. D.; Lamb, D. J . A m . Chem. SOC.1970, 92, 3943. Chem. (9) van der Helm, D.; Childs, J. D.; Christian, S. D. J . Chem. SOC., Comm. 1969, 887. (10) Hisatsune, I. C.; Heicklen, J. Can. J . Chem. 1975, 53, 2646. (1 1) Reahal, A. S. Thesis, University of Salford, 1981. (12) Nord, L. J . Mol. Struc. 1982, 96, 27. (13) Hata, T.; Kinumaki, S. Nature (London) 1964, 203, 1378.
0022-3654/84/2088-0432$01.50/0
(14) Lucchese, R. R.; Haber, K.; Schaefer, H. F. J . Am. Chem. SOC.1976 -98. - ,7617. --
(15) Douglas, J. E.; Kollman, P. A. J . Am. Chem. S O ~1978, . 100,5226. (16) Auk, B. S.; Pimentel, G. C . J . Phys. Chem. 1973, 77, 1649. (17) Ault, B. S.; Pimentel, G. C. J . Phys. Chem. 1973, 77, 57. (18) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Am. Chem. SOC.1982, 104, 6180. (19) (a) Auk, B. S. Inorg. Chem. 1981, 20, 2817. (b) McNair, A. M.; Auk, B. S . Ibid. 1982, 21, 1762. (20) (a) Ritzhaupt, G.; Devlin, J. P. J . Phys. Chem. 1977, 81, 521. (b) Nelander, B.; Nord, L. Ibid. 1982, 86, 4375.
0 1984 American Chemical Society
IR Study of SOz.Amine Complexes previously.21 Two types of experiments were conducted on this system, depending on the desired product. To enhance production of the 1:1 complex, twin-jet deposition experiments were conducted, by preparing the two reactants in separate vacuum lines. After dilution with argon to ratios between 100/1 and 1000/1, the two samples were sprayed onto the 14 K cold window simultaneously. In other experiments, the amine and SO, were mixed in a single vessel, and argon added so that equilibration among the possible reaction products could occur. This mixture was then deposited onto the cold window from a single line, hence a single-jet experiment. SOz(Matheson), NH3 (Matheson), and (CH3),N (Matheson) were used after purification by one or two freeze-thaw cycles at 77 K. CH,NH2 and (CH3),NH were found to contain considerable amounts of NH3 impurity, and were distilled from a dry ice/chloroform bath prior to sample preparation. All of these amines, with the exception of TMA, adsorbed strongly onto the stainless steel vacuum line, necessitating conditioning prior to an experiment. This adsorption also made the final dilution ratios in argon or nitrogen somewhat inaccurate; the values quoted are the best estimates available. ND3 (99% D, Merck) was also used after purification by freeze-thaw cycles, and the line deuterated by repeated exposure to DzO. Sl8OZ,(99% I8O, Cambridge Isotope) was found to contain considerably less enrichment than indicated by the manufacturer; an enrichment of about 65% was and estimated by the relative band intensities of SI6O2,S16J802, S180zin several experiments. Argon or nitrogen was used as the matrix gas in these experiments and was used without further purification. Samples were deposited for 20-24 h at roughly 2 mmol/h from each line when a twin-jet experiment was conducted, or from one line with a single-jet experiment was run. Final spectra were recorded, both survey and high resolution, on either a Beckman IR12 or a PE983 infrared spectrophotometer. In several experiments, the sample was irradiated by the full light of a medium-pressure mercury arc lamp, sometimes during deposition and sometimes after the matrix sample has been deposited. Normal coordinate calculations were performed at the University of Cincinnati Computer Center with a program from the Research Council of Canada.
Results Prior to investigating the reaction products of SO, with ammonia and methyl-substituted amines, blank spectra were run on each of the reactants at several different dilutions in argon and nitrogen. All of the spectra obtained were in good agreement with literature spectra.11J2-25 SO, + NH,. When a sample of A r / S 0 2 = 1000 was codeposited with a sample of Ar/NH3 = 1000 from separate vacuum lines onto the 15 K cold window, a number of new infrared absorptions were observed which could not be assigned to either parent species. These new absorptions appeared in three spectral regions, a multiplet at 1331, 1327, 1322, 1320, and 1315 cm-I, a singlet at 1014 cm-I, and a singlet at 525 cm-I; all of these bands were of weak-to-medium intensity. When similar samples were studied, but higher concentrations employed, all bands showed an increase in intensity, but maintained the same relative intensity ratios. In addition, the multiplet around 1320 cm-I broadened slightly, so that all five components were not resolvable. Previous studies have shown that nitrogen mat rice^'^,^^ can provide sharper spectra of amine complexes, with less multiplet structure, so a duplicate set of experiments was conducted in N,. When a sample of N2/S02 = 1000 was codeposited with a sample of N2/NH3 = 1000 four new product bands were observed, all singlets, at 1338, 1320, 1038 and 530 cm-I, as well as a distinct (21) Auk, B. S. J. Am. Chem. Sor. 1978, 100, 2426. (22) Milligan, D. E.; Hexter, R. M.; Dressler, K. J . Chem. Phys. 1961, 34, 1009. (23) Purnell, C. J.; Barnes, A. J.; Suzuki, S.; Ball, D. F.; Orville-Thomas, W. J. Chem. Phys. 1976, 12, 77. (24) Hamada, K.; Morishita, H. 2. Phys. Chem. 1975, 97, 295. (25) Glass, W. K.; Pullin, A. D. E, Trans. Faraday Sor. 1961, 57, 546. (26) Lorenz, T. J.; Auk, B. S. Inorg. Chem. 1982, 21, 1758.
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 433 shoulder at 1149 cm-I on the parent SOz band at 1152 cm-I. Additional experiments were conducted in which the SO2/", ratio was varied, and the overall concentrations increased. Under these conditions, the product bands at 530, 1038, and 1338 cm-' maintained a constant intensity ratio, but the product band at 1320 cm-I grew more rapidly than the other bands when NH, was in excess. The ratio of optical density of the 1320-cm-I band to the optical density of the 1338-cm-l band was 0.26 when the total concentration was Nz/NH3/SOz = 2000:1:1, and increased to 0.55 at a total concentration of 1OOO:lO:l. Finally, at low SOz and high NH, concentrations, the shoulder at 1149 cm-' resolved into a distinct product band. Additional experiments were conducted in which the SO2 and NH3 reactants were premixed in a single vacuum line and allowed to come to equilibrium prior to deposition. In this single-jet deposition, the same set of four product bands was detected in good yield; however, the intensity of the 1320 cm-I was increased relative to the other three bands compared to experiments employing the same concentrations and twin-jet deposition. For example, with an overall concentration of 2000:1:1, the ratio of optical densities OD (1320 cm-')/OD (1338 cm-I) was 0.26 for the twin-jet reaction, and 0.35 for the single-jet deposition experiment. Attempts were made to increase the yield of product by ultraviolet irradiation, since SOz has an accessible electronic state and this technique has been used to form aerosolsz7involving SO2. Irradiation was carried out in some experiments during sample deposition, and in other experiments after the matrix was in place. In all experiments, there was a slight increase in yield of product, but no new product bands were observed. Infrared spectra of the reaction products of SO, with NH, are shown in Figure 1. SO2 ND3. These two reactants were deposited in a single-jet experiment to investigate the effect of deuterium substitution on each of the product bands. ND3 exchanges rapidly with hydrogen-containing impurities in the vacuum system, and, although the line was thoroughly deuterated prior to sample preparation, the final level of isotopic substitution with approximately 85 atom % D. Consequently, the samples contained measureable amounts of NDzH and NDHz, but very little NH,. When these two reactants were codeposited at a toal dilution of 1000:2:1 Nz/ S02/ND3,the product bands at 530, 1320, and 1338 cm-I were observed, unshifted from their position in the NH, experiments. However, the product band observed at 1038 cm-' in the NH3 experiments was not observed, but rather two new bands were observed at 807 and 884 cm-I, as is shown in Figure 2. Sl8OZ+ NH,. S1802, with an isotopic enrichment of 65 atom % '*O, was premixed with NH, in several experiments at a concentration of Nz/S180,/NH3 = 1000:1:5, and deposited in single-jet experiments. In these experiments, none of the product bands observed previously were noted, although in most cases two new product bands were observed shifted to lower energy. Product bands were observed at 1319, 1300,1295, 1274, 1125, 1102, and 507 cm-', while the region near 1038 cm-' was obscured by parent N H 3 absorptions at this relatively high NH, level. Spectra obtained from this reaction are shown in Figure 1. SO2 CH3NH2. These two reactants were codeposited in numerous experiments over a wide range of concentrations in twin-jet experiments. The majority of the experiments were conducted at dilutions of NZ/SOz = 1000 and N2/CH3NHz = 1000. Under these conditions, a number of product bands were observed, including a doublet at 1312 and 1305 cm-', a broad band centered at 1261 cm-', and a weak, sharp band at 538 cm-'. At higher concentrations, for example N z / S 0 2 = 500 and N2/ CH3NH2= 500, all of these bands grew in intensity, but the band at 1261 cm-I showed a relatively greater increase than did the remaining bands. A duplicate set of experiments was run with argon as the matrix gas, and very similar results were obtained. At a concentration
+
+
(27) Yu, S-Q.; Spicer, L. D. "Book of Abstracts", 183rd National Meeting of the American Chemical Society, Las Vegas, NV, April 1982; American
Chemical Society: Washington, DC, 1982; PHYS 5 8 .
434
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
Sass and Ault
7i i J
1
75
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1275 v ‘
I
I
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I
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I
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I
”
I
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ENERGY (CM-’)
Figure 1. Infrared spectra, over selected spectral regions, of the reaction products of SO, with NH3 in argon and nitrogen matrices. Also shown are the spectra of the reaction products of isotopically labeled SO, with NH3; all spectra shown are from single-jet deposition experiments.
of A r / S 0 2 = 1000 and Ar/CH3NH2 = 1000, a doublet was observed at 1312 and 1305 cm-’, a broad band at 1259 cm-’, and a singlet at 533 cm-’. These bands also showed the same concentration dependence as their nitrogen matrix counterparts. When these two reactants were premixed with nitrogen in a single vacuum line and deposited in a single-jet experiment, very different results were obtained. Only one of the above product bands, at 1261 cm-’, was observed, and was very intense. In addition, three sharp new bands were observed, at 1134, 11 18,
and 578 cm-’. This experiment was repeated at a number of different concentrations, and in every case only this second set of bands, tripified by the 1261-cm-’ absorption, was observed. These spectra are displayed in Figure 3. S 1 8 4 + CH3NH2.These two reactants were codeposited from separate vacuum lines in several experiments at dilutions of N2/SI8O2= 1000 and N2/CH3NH2= 200. Two isotopic counterparts of each of the bands observed previously in the twin-jet experiments were detected shifted down in energy from the band
IR Study of S02.Amine Complexes
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 435
Figure 2. Infrared spectra of the reaction products of SOz with NH3 and its deuterated counterparts in nitrogen matrices, over the spectral regions of interest.
position observed with normal isotopic SO2,at 506, 516 cm-I and 1265, 1290 cm-'. In addition, a new isotopic doublet was observed at 1091 and 1 1 11 cm-'. A single-jet experiment involving these two reactants was carried out as well, and the band observed at 1261 cm-I with SOzshifted to 1250 cm-l, while the 578-cm-' band had an I8Ocounterpart at 570 cm-'. The bands at 1134 and 1118 cm-I were not observed, probably due to low intensity. Spectra of these reaction products are also shown in Figure 3. SO2 (CH,),NH. These two reactants were investigated in a number of experiments, using both single- and twin-jet deposition. When the two reactants were codeposited from separate vacuum lines at dilutions of 1000/1 each in nitrogen, product bands were observed at 527, 1121, 1275, and 1279 ern-'. At higher concentrations, all of these bands grew in intensity, and maintained a constant intensity ratio to one another. When single-jet deposition was employed, the same set of product bands was observed, and not new bands were detected. SI8O2 (CH3),NH. When Sl8O2and (CH3)2NH were codeposited in a single-jet experiment a t a dilution of 1000:1:5 N,/S1s02/(CH,)2NH, isotopic counterparts of each of the above product bands were observed, at 505, 1077, and 1237 cm-', as is shown in Figure 4. SO, (CH3),N. These two reactants were codeposited in twin-jet experiments at dilutions of 1000/1 each in nitrogen, and two product bands were observed, at 550 and 1120 cm-', while the parent absorption of (CH3),N at 1271 cm-' was significantly broadened. At higher sample concentrations, these two product bands grew, and at the same rate, although the overall yield was not as large as in experiments described above. When the reactants
+
+
+
were codeposited from a single vacuum line, the same set of bands was observed, with the same relative intensities. S1802 (CH3),N. One single-jet experiment was performed with these two reactants, at a dilution of N2/S180,/(CH3)3N= 1000:1:5. The two product bands described above each shifted to lower energy and split into a distinct doublet, at 530, 540 cm-I and 1072, 1094 cm-I. In addition, two new bands were observed, at 1230 and 1254 crn-', suggesting that the pure l60counterpart of these bands was hidden under the parent (CH,),N band at 1271 cm-'. Spectra of the reaction products of (CH&N with SO, and SI8O2are shown in Figure 5 .
+
Discussion SO2 NH,. The matrix reaction of these two species gave rise to a number of new infrared apbsorptions which could not be assigned to either precursor. While similar results were obtained in both argon and nitrogen matrices, multiplet structure was often observed in argon while sharp singlets were evident in nitrogen. Consequently, the nitrogen matrix results will be discussed in detail. While the same four product bands were detected in all experiments, extensive concentration studies indicated that more than one product species was formed. The absorptions at 1338, 1038, and 530 cm-' maintained a constant intensity ratio to one another, and predominated at high sample dilution. On the other hand, the absorption at 1320 cm-l was relatively more prominent either when high NH3 concentrations were used in a twin-jet reaction or when single-jet reactions were studied. These results indicate that the 1320-cm- absorber can be assigned to a different species than the one responsible for the first set of bands.
+
436 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
Sass and Ault
T
-
NZ/MMA
N 2 / S ~ M A SINGLE
J I70
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ENERGY (CM-')
Figure 3. Infrared spectra of the reaction products of SO2 with CH3NH2in both single- and twin-jet deposition experiments,as well as the corresponding products when 180-labeledSO, was employed. Bands marked with an asterisk are due to the reaction products of SO, with impurity NH3 in the CH,NH,
sample.
All of the product bands fell in spectral regions quite close to absorptions of one of the parent molecules; for example, the 1338and 1320-cm-' absorptions lie quite close to the 1350-cm-I antisymmetric stretching mode of the parent SO,. This indicates that the parent moelcules, although perturbed in the reaction product, have maintained their basic structural integrity, and that the product is a molecular adduct rather than a rearrangement or elimination product. This conclusion is in good agreement with previous work6-Sin the field which reported 1:l and 1:2 adducts of NH, with SO,. The spectra obtained here are also in agreement with those obtained by Nord,12 which he subsequently assigned to the 1:l adduct S02.NH3. Consequently, the set of bands at 530, 1038, and 1338 cm-I is assigned to the 1:l adduct between
SO, and NH,. The band at 1320 cm-', which showed a distinctly different concentration behavior and was favored at high ammonia concentration, is assigned to the 1:2 adduct, S02.2NH3, which has been proposed by several workers.8 This assignment is in disagreement with Nord, who assigned to 1320-cm-' absorption to the 34Scounterpart of the 1338-cm-I absorption. While the 34Scounterpart of the 1338-cm-' band should lie near 1320 cm-', the intensity variation of this band with respect to the 1338 cm-I cannot allow this assignment. In addition, 34Soccurs with a natural abundance of 4.22%, compared to 95% for ,,S. This ratio, 95.0/4.22 = 22.5, is not close to the ratio of intensities, which ranged from 1.80 to 3.85. Consequently, assignment to the 1:2 adduct is preferred.
IR Study of SO,.Amine Complexes
I
I
I
1400
1320
The Journal of Physical Chemistry, VoL 88, No. 3, 1984 437
I
I
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Figure 4. Infrared spectra of the reaction products of the codeposition of SO2 and leO-labeled SO2 with dimethylamine in nitrogen matrices.
I
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I
I
135
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I
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I
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I
I
I
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(CM-')
Figure 5. Infrared spectra of the products formed through the codeposition of SO2 and SI8O2and (CH3)3Nin nitrogen matrices.
Band assignments for the observed absorptions are straightassigned to the bending mode of the SO2 unit in the adduct. The forward, due to proximity to parent absorptions and due to isotopic shoulder at 1149 cm-' on the symmetric stretching mode of the behavior. The 1338-cm-' absorption of the 1:l complex showed counterpart at 1125 cm-' and a pure parent SO2 showed a 16,180 a 16*180 absorption at 1319 cm-l and the corresponding 18,180 '@Ocounterpart at 1102 cm-', and is assigned to the symmetric absorption at 1295 cm-'. The intensity pattern was identical with stretching mode of the SO, subunit in the complex. This differs that observed for the labeled SO2 precursor, indicating that the by a few wavenumbers from the band observed by Nord, who two oxygen atoms in the complexes SO2 are equivalent. This reported this mode at 1155 cm-'. result, plus the proximity of the 1338-cm-' band to the parent The product band at 1038 cm-' shifted upon deuteration of the antisymmetric stretching mode, suggests assignment of the ammonia, which indicates that in a vibration of the NH, subunit 1338-cm-' absorption of the antisymmetric stretching of the SO2 in the complex. The band observed at 807 cm-' can be assigned subunit in the 1:l complex. Similar behavior was observed for to the 1:l complex S02.ND, and the intermediate band at 884 the band at 530 cm-', and, in view of the fact that it lies within cm-' to the partially deuterated adduct S02.NHD2. Each of these a few wavenumbers of the bending mode of parent SO2, it is bands lies 50-70 cm-' to higher energies of the symmetric de-
Sass and Ault
438 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 formation mode of the parent. This vibrational mode of NH, has been shown to be very sensitive to coordination and to the strength this mode of the Lewis acid in the c ~ m p l e x . ~In~ -particular, ~~ shifts to higher energy upon coordination, and the magnitude of the shift depends on the strength of interaction, shifting up as highla as 1352 cm-' with BCl, and 1309 cm-' with BF,. SOz, by comparison, is a relatively weak Lewis acid, and a much smaller shift is anticipated, which is consistent with the observed band at 1038 cm-I. Consequently, this product band is assigned to the symmetric deformation mode of the NH3 subunit in the SOZ-NH3 complex. Additional vibrational modes of the N H 3 subunit might be anticipated in the complex; however, previous studies have shown that they are quite insensitive to complexation, and show only small shifts, even with strong Lewis acids. In the present system, these modes are probably shifted very little, and obscured by the intense absorptions of the uncomplexed parent. The 1320-cm-' absorption was the only band observed which could definitively be assigned to the 1:2 complex. In additon, a I6J8O counterpart was observed at 1300 cm-' and a I 8 , l 8 0 counterpart at 1274 cm-'. The location and isotopic behavior of this band suggest its assignment to the antisymmetric stretching complex. It is notemode of the SO, subunit in the SO2.", worthy that the shift of this mode from the parent SO, position is roughly twice as great as in the 1:l complex; this point will be discussed in more detail below. Since, in general, the yield of 1:2 complex was a factor of 2 to 4 less than the 1:l complex, and the symmetric stretching and bending modes were only weakly observed for the 1:l complex, it is not surprising that these modes were not detected for the 1:2 complex. It is difficult to draw firm conclusions about the structure of the 1:1 complex in view of the limited number of product bands observed, and the relatively small shifts from the parent band positions. Nonetheless, one can note that the vibrations of the SO2 subunit undergo no measurable shift upon deuteration of the NH3 subunit. This indicates that the vibrations of the two subunits in the complex are very weakly coupled at best. This could be due to relatively weak interaction between the two subunits, but with an estimated strength of interaction on the other of 10 kcal/mol, one might expect at lest some vibrational coupling. Another mechanism by which very weak coupling could occur is geometric; if the two subunits are at roughly 90' to one another, then coupling could only occur through anharmonic terms, which should be very small. This is the geometry p r e d i ~ t e d ' ~by . ' ~ab initio calculations for the NH3-SOZcomplex, and found by crystallography for the (CH3),N.S0, c ~ m p l e x So, . ~ while no firm conclusions can be reached, the spectra are consistent with the theoretically calculated structure, in which the C,axis of the NH3 subunit is roughly 90° to the Cz axis of the SO, subunit. Comparison should be made at this point to the earlier spectra of Hisatsune and Heicklen,'O after the condensation of alternating layers of SO, and NH, at low temperatures. They assign a variety of bands to the 1:l and 1:2 adducts SO,.NH, and S0,.2NH3. However, thier observed bands bare no resemblance to the current spectra, nor to the spectrum reported by Nordl, for SOyNH,. These workers were unable to make firm band assignments due to the numerous species present concurrently in their low-temperature solids, making comparison to the current spectra difficult. Moreover, these authors note that their spectrum of the "SO,. NH," species has a strong resemblance to ammonium sulfite, NH4SO3. In view of the complexity of their system, it is likely that their spectra are not of the 1:l and 1:2 adducts of SOz with NH,, but of further reaction products such as "$0, and larger aggregate species. SO2 CH,NH,. The spectra obtained in the single- and twin-jet reactions of these two precursors indicate that rather different chemistry occurred. In the twin-jet deposition, at high sample dilution, two sets of product bands were observed and the
+
(28) Hunt, R. L ; Ault, B. S. Spectrosc. Int. J . 1982, 1, 31. (29) Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds", Wiley-Interscience: New York, 1978; 3rd ed. (30) Ennan, A. A.; Kats, B. M.; Ostapchuk, L. V. Koord. Khim. 1976, 2, 393.
set consisting of bands at 1312, 1305, and 538 cm-' was more prominent. The second set, which was characterized by a broad band at 1261 cm-', was favored as the concentration of the amine was increased. In addition, when this system was investigated in a single-jet experiment, only the second set of bands was observed, and the band at 1261 cm-' was very intense. In addition, new weaker bands associated with the 1261-cm-' band were detected at 1134, l 118, and 578 cm-'. The use of l 8 0 labeled SO2helped to identify the two absorbing species. The species responsible for the 1312, 1305-cm-' doublet counterpart and a pure l 8 0 counterpart for each showed a 16,180 of the product bands, with the same isotopic intensity ratio observed for parent SO,, indicating two equivalent oxygen atoms in the product species. These product bands are also close to both and are assigned the parent SO, and to the 1:l adduct SO2-",, to the 1:l adduct SOZCH3NHz,marking the first observation of this complex. The I8O SO, single-jet reaction with CH,NH2 yielded only a single new product band, at 1250 cm-', in addition to the previously counterpart at 1261 cm-I. Moreover, the intensities observed l60 of the 1261- and 1250-cm-' bands did not match that observed for both the parent SO2 and the 1:l adduct. Rather, the intensity data, as well as the observation of only two bands, are consistent with a single oxygen atom in the absorbing species. This suggests that a distinct chemical reaction occurred when the two reactants were allowed to equilibrate at room temperature in the gas phase prior to deposition. The anhydrous gas-phase reaction', of SO, and CH3NH2has been reported to form CH3NS0 and H 2 0 ,and the position of the second set of bands agrees with the gas-phase spectrum of CH3NS0.25Consequently, this set of product bands is assigned to the dehydration product CH3NS0. Band assignments for the 1:l complex are straightforward, by comparison to both parent SO2 and the 1:l complex S02-NH3. The 1305, 1312-cm-I doublet is assigned to the antisymetric stretching mode of the SOz subunit in the complex, which has shifted down roughly 30 cm-I from its position in the NH3 complex. In a similar fashion, the 538-cm-I band is assigned to the bending mode, while no band was observed in the symmetric stretching region. However, when labeled S1802was employed, new bands were observed at 11 11 and 1091 cm-I, which can be and 18*'80 assigned to the symmetric stretching mode of the 16,180 counterparts of the 1:l complex. The pure I6O analogue must be hidden by parent absorptions of SO2 and CH3NH2 which obscure the 1130-1 150-cm-I region. SO, (CH,),NH. The reaction of sulfur dioxide with dimethylamine gave rise to a single set of infrared absorptions, at 527, 1121, 1275 and 1279 cm-' in nitrogen matrices. These bands maintained constant relative intensity over a considerable range of concentrations, and no new bands were observed. In addition, identical spectra were obtained whether single- or twin-jet deposition was employed. The use of partially I8O labeled SO2 revealed l 8 0 counterparts of each of these product bands at 505, 1077, and 1237 cm-'; the regions where the intermediate 16,180 counterparts might be expected were obscured in each case by parent absorption. These observations are similar to those made for the 1:l complexes of SO, with N H 3 and CH3NH2,with a somewhat greater shift from the position of the uncomplexed reactants. Consequently, the product bands are assigned to the 1:l complex S0z.(CH3)zNH. Band assignments are straightforward, using the ammonia and monomethylamine complexes as references. The 1275-, 1279-cm-' doublet is assigned to the antisymmetric stretch of the SO, subunit in the complex, while the 1121-cm-' band is assigned to the symmetric stretch. Finally, the 527-cm-I band is assigned to the bending mode of the SO, unit. Dimethylamine does not have any vibrational modes which are particularly sensitive to complexation, and no product bands were observed which could be assigned to complexed dimethylamine. SO2 (CH,),N. In a manner similar to dimethylamine, a single set of product bands was observed from either the single- or twin-jet deposition of these two reactants. When normal isotopic SOzwas employed, only two bands were observed, at 550 and 1120
+
+
IR Study of SO,.Amine Complexes
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 439
TABLE I: Product Band Positions for SO;Amine Complexes
____species
1JIa
1'2
_____
SO;", S160'aO~NH, SI .NH, SO;ND, SO;NHD, SO;2NH3 S'60'h0.2NH3 S1'O0,.2NH, SO, CH,NH S'60'XOCH3NH, S'XO,~CH,NH, SO, ,(CH,),NH S'80,~(CH,),NH SO,.(CH,),N S160'R0.(CH,),N S1kO,.(CH,),N
1149' 1125 1102
,
1111 1091 1121 1077 1120 1094 1072
530 507 530 530
538 516 506 527 505 550 540 530
1'3
1338 1319 1300 1338 1338 1320 1295 1274 1309d 1290 1265 127Id 1237
!,B
TABLE. 11: Variation of SO, Force Constants with Base Strength
__ for Amine Complexes
PA
1038
Fr
Fs
F,,
9.90a 9.78 9.47
1.65
0.04 0.18 0.31 0.35 0.35
species 807 884
1254 1230
a Vibrational designations are for the SO, subunit. ijB represents a vibrational mode of the base; for NH, complexes this mode is the symmetric deformation. Band positions in cm-', nitrogen matrix positions. Average of site split bands.
'
cm-I. However, when l8O labeled SO, was used, three sets of doublets were observed, the first two of which were to lower energies of the two original product bands. The other doublet was located at 1230 and 1254 cm-I, suggesting that a I6O counterpart might be located near 1270 cm-I. Parent (CH,),N absorbs strongly at 1271 cm-', and undoubtedly is obscuring the l 6 0 counterpart of this latter doublet. The location of these product bands is suggestive of a complex, analogous to those observed with the other methylamines, and is assigned to the 1:l adduct S02.(CH3)3N. Band assignments for these product absorptions are readily made, by analogy to the complexes discussed above. The doublet at 1230 and 1254 cm-', whose I6O counterpart is apparently hidden near 1270 cm-', is assigned to the isotopic bands of the antisymmetric stretching mode of the SO, subunit in the complex, while the 1120-cm-' band is assigned to the symmetric stretch, and the 550-cm-' band to the bending mode. Again, no vibrational modes of the base subunit in the complex were detected, presumably due to lack of shift from the vibrational modes of parent (CH3),NH. Table I summarizes band positions and assignments for all OP the complexes observed here.
Base Strength Effects While, in general, the vibrational modes of the SOz subunit in the 1:l complexes observed here were shifted by relatively small amounts from free SO2, trends could be observed through the methylamine family. The antisymmetric and symmetric stretching modes shifted in a smooth fashion to lower energy upon complexation to stronger bases; for example, v3 was observed at 1338 cm-l for the SO2.", complex, and near 1270 cm-' for the S02.(CH3),N complex. The base strength of the methylamines, as measured by gas-phase proton affinities, is known to increase with increased number of methyl groups,31suggesting a correlation between base strength and magnitude of shift for the SO, antisymmetric stretch. In addition, the shift of the antisymmetric stretching mode for the 1:2 adduct S0,.2NH3 was just about double the shift observed for the 1:1 complex SO2-NH3.This trend can be compared to the related specie^^,-,^ SO, and SO,F, where the coordinating "base" is an electron in the former species and a fluoride ion, in the latter. For these species, v3 shifted down to about 1 0 3 5 and 1 1 7 5 cm-I, respectively, suggesting a lowering of the S-0 stretching force constant. These observations, collectively, indicate that SOzis serving as a P* acceptor,' interacting (31) Aue, D. H.; Webb, H. M.; Bowers, M. T. J . Am. Chem. SOC.1976, 98, 311. (32) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1971, 55, 1003. (33) Garber, K.; Ault, B. S. Inorg. Chem. 1983, 22, 2509. (34) Robinson, E. A,; Lavery, D. S.; Weller, S. Spectrochim. Acta, Part A 1969, 25, 151.
SO, SO,,NH, SO;CH,NH, SO;(CH,),NH SO,.(CH,),N SO;2NH3
1.70 1.76 1.70 1.84
9.15 9.12 9.63
(base), Fro kcal/mol ___0.20 0.23 207 0.26 218 0.30 225 0.33 229
a mdyn/A.
-.-k
c
Q J C 39.5 E 9.7
v
9.3 LLL
9.1
-
I
2 05
I
I
215
I
1
I
225
PROTON AFFINITY(KCAL MOLE-") Figure 6. Plot of the primary S-0 stretching force constant F, for the SO2 subunit in 1:l SO,.amine complexes, as a function of the proton affinity of the amine in the complex.
with the lone electron pair of the nitrogen, and lowering the S-0 force constant. As can be seen in Table I, a similar trend is observed for the symmetric stretching mode. The bending mode, however, showed somewhat different behavior. One would still anticipate P* acceptor behavior, and a shift to lower energies. However, one might also envision steric interference for this vibration with the methyl groups of the coordinated amine, which would hinder the angle-bending motion, and hence increase the bending frequency. For the NH3, CH3"2, and (CH3),NH complexes, the bending mode of the SO, unit remained relatively constant, suggesting a balance between these two effects. The bending mode of the (CH3)3Ncomplex, however, shifted considerably to higher energies, which is likely due to the fact that, in this complex, a methyl group must be located in the interior of the 0-S-0 angle, increasing the steric hindrance considerably. To quantify these effects, normal coordinate calculations were carried on the 1:l complexes. However, since only the three vibrational modes of the SO, subunit were observed consistently, the calculation treated the complex as a perturbed SO, species, without explicitly including the base. The force constants were refined to all of the isotopic data, and a satisfactory fit was obtained. The force constants obtained are tabulated in Table 11; as anticipated, the primary S-0 stretching force constant F, decreased linearly with base strength of the amine, as is depicted in Figure 6. F,, the bending force constant, remained constant for the NH3, CH3NHz,and (CH3)z N H complexes, but increased for the (CH3)3Ncomplex.
Equlibrium Considerations Of the adducts reported here, only the 1 : l complex (CH3),N 6 0 2 has been observed in the gas phase. While equilibrium constants have been obtained for the 1:l and 1:2 adducts of SO2 with NH,, these have assumed dissociation of the complex upon vaporization. In the present study, the 1:l complex with all four amines was observed from both single- and twin-jet deposition, except for CH3NH2,where reaction to form CH,NSO + H,O was observed in the single-jet mode. These results a small amount of complex may be stable in the gas phase, in addition to that formed during the condensation process. In particular, the ob-
J . Phys. Chem. 1984, 88, 440-445
440
servation of the 1:2 complex in the single-jet, but not the twin-jet, experiment gives strong evidence of gas-phase 1:1 complexation. While a 1:2 complex was detected with NH3, the corresponding 1:2 complexes with the methylamines were not observed, even under conditions which strongly favor the formation of larger aggregates. Consequently, one may conclude that these adducts are not stable, except for S02.2NH3,even at cryogenic temperatures.
Conclusions The twin-jet deposition of SO2 with N H 3 or any of the methylamines gave rise to a 1:l adduct isolated in argon and nitrogen matrices. The use of single-jet deposition enhanced the yield of complex for most systems, but, in the case of CH3NH2,a dehydration reaction was instead detected. Only for the SO2/”, system was the 1:2 adduct observed, and this was obtained in best
yield with considerable excess of NH3. These adducts were characterized by vibrations of the perturbed SO2 subunit in the complex; in general the S-0 stretching modes shited to lower energy, while the bending mode shifted to slightly higher energies. These shifts were rationalized in terms of a* acceptor properties of SO,, and increasing donor ability of the methylamines with increasing number of ethyl groups.
Acknowledgment. The authors gratefully acknowledge support of this research by the National Science Foundation, Grant CHE8100119. B.S.A. also thanks the Dreyfus Foundation for a Teacher-Scholar Grant. Registry No. SOz.NH3, 29307-29-7; SO2.2NH3,29307-30-0; SOz.CH3NH2, 67592-19-2; SO,*(CH,)zNH, 21 326-49-8; SOy(CH3)3N, 7664-41-7; CH,NHZ, 74-89-5; (C17634-55-8; SOZ, 7446-09-5; ”3, Hg)zNH, 124-40-3; (CH,),N, 75-50-3.
Matrix Isolation Infrared Spectra of Dioxygen Adducts of Iron( II)Porphyrins and Related Compounds T. Watanabe, T. Ama, and K. Nakamoto* Todd Wehr Chemistry Building, Marquette University, Milwaukee, Wisconsin 53233 (Received: May 2, 1983;
In Final Form: June 16, 1983)
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The O2stretching bands of the following dioxygen adducts were located in Ar matrices at 15 K: Fe(TPP)02 (1 195 and 1106 cm-I), Fe(OEP)02(1 190 and 1104 cm-I), Fe(Pc)02 (1207 cm-’), and Fe(salen)O, (1106 cm-I). The first two compounds form two isomeric dioxygen adducts whose 0, stretching frequencies are 1190 (isomer I) and 1105 cm-I (isomer 11). Isotope scrambling experiments (I6O2+ 160’80 + “0,) were carried out to elucidate the mode of coordination of dioxygen stretching band under the experimental conditions employed. in these adducts. Neither isomer showed splitting of the 160180 Normal coordinate calculations predict that splitting should occur if dioxygen coordinates to the metal in the end-on fashion with a FeOO angle larger than 130’. We propose that isomer I possesses end-on geometry with the FeOO angle smaller than 130’ whereas isomer I1 assumes symmetric side-on geometry. These structures are consistent with the observed relative intensities of the O2stretching bands, the proposed electronic ground states, and general spectra-structure relationships in a series of “base-free” dioxygen adducts of iron(I1) chelates.
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Recently, the matrix cocondensation technique has been used extensively to synthesize a number of novel and unstable compounds.’ In this technique, the metal atom’ or metal halide vapor2 produced at high temperature is reacted with a ligand such as CO and O2diluted in inert gas and the reaction products are frozen immediately on a cold window (- 15 K) for spectroscopic measurements. Vibrational spectra thus obtained provide valuable information about the structure and bonding of unstable or transient cocondensation products. We have already employed this technique for IR investigations of Co(TPP)02 (TPP, tetraphenylporphyrinato a n i ~ n ) ,Co(acacen)02 ~ (acacen, N,N’ethylenebis(acety1acetone iminato) anion): Co(OEP)02 (OEP, octaethylporphyrinato a n i ~ n ) and , ~ Co(J-en)O, (J-en, N,N’ethylenebis(2,2-diacetylethylideneaminato) anion).6 In contrast to these Co(1I) chelates, TPP complexes of Mn(I1) and Fe(I1) are air sensitive. In these cases, we have prepared Mn(TPP)027 and Fe(TPP)OZs by preheating air-stable Mn(TPP)(py) (py, pyridine) and Fe(TPP)(pip), (pip, piperidine), respectively, in the Knudsen cell of our matrix isolation system (1) Moskovits, M.;Ozin, G. A. “Cryochemistry”;Wiley: New York, 1976. (2) Tevault, D.; Nakamoto, K. Inorg. Chem. 1976, 15, 1282. Tevault, D.; Strommen, D. P.; Nakamoto, K. J . Am. Chem. SOC.1977, 99, 2997. (3) Kozuka, M.; Nakamoto, K. J . Am. Chem. SOC.1981, 103, 2162. (4) Urban, M. W.; Nonaka, Y.; Nakamoto, K. Inorg. Chem. 1982, 21, 1046. ( 5 ) Urban, M. W.; Nakamoto, K.; Kincaid, J. Inorg. Chim. Acta 1982,61, 77. (6) Nakamoto, K.; Nonaka, Y.; Ishiguro, T.; Urban, M. W.; Suzuki, M.; Kozuka, M.; Nishida, Y.; Kida, S . J . Am. Chem. SOC.1982, 104, 3386. (7) Urban, M. W.; Nakamoto, K.; Basolo, F. Inorg. Chem. 1982, 21, 3406. (8) Nakamoto, K.; Watanabe, T.; Ama, T.; Urban, M. W. J . Am. Chem. SOC.1982, 104, 3744.
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and by reacting the resultant Mn(TPP) and Fe(TPP), respectively, with 0, diluted in argon. The IR spectrum of Fe(TPP)02 thus obtaineds was of particular interest since it was the first example of a dioxygen adduct of a “base-free” and “unprotected” iron(I1) p ~ r p h y r i n . ~Furthermore, we have found that Fe(TPP)O, exists in two isomeric forms which exhibit the u ( 0 2 ) ( u , stretching) at 1195 and 1106 cm-’ in Ar matrices. In order to understand the nature of these isomers, we have extended our study to the Fe(I1) chelates of OEP, Pc (phthalocyanato anion), and salen (N,N’ethylenebis(salicy1ideneaminato) anion). In particular, we have to elucidate the mode of studied their reactions with 1601s0 coordination of dioxygen to the iron(I1) center in the absence of axial base ligands.
Experimental Section Compounds. All the Fe(I1) chelates studied were prepared by the literature methods: Fe(TPP)(pip),,’O Fe(OEP)(py),,” Fe(Pc)(py),,I2 and Fe(salen)(py).13 The gases, Ar (99.9995%), I6O2 (99.99%), and 180(99.88%), 2 were purchased from Matheson and 160180, and was Monsanto Research. A mixture of 1602, prepared by electrical discharge of an equimolar mixture of 1602 and ’*02 and its mixing ratio determined by Raman spectroscopy. (9) Formation of Fe(TPP)OZwas also suggested by the recent ‘H NMR study (see Latos-Grazynski, L.; Cheng, T.-J.; La Mar, G. N.; Balch, A. L. J . Am. Chem. SOC.1982, 104, 5992). (10) Epstein, L. M.; Straub, D. K.; Maricondi, C. Inorg. Chem. 1967, 6 , 1720. (1 1) Bonnett, R.; Dimsdale, M. J. J . Chem. Soc., Perkin Trans. 1 1972, 2540. (12) Lever, A. B. P.Adu. Inorg. Chem. Radiochem. 1965, 7, 21. (13) Niswander, R. H.; Martell, A. E. Inorg. Chem. 1978, 17, 2341.
0022-3654/84/2088-0440$01.50/00 1984 American Chemical Society
