J. Phys. Chem. 1992, 96,79-84 acetonemethanol interaction. As xA decreases, there is less probability of an acetoneacetone interaction and the temperature dependence of the half-width disappears; indeed, from Table 11, the infinite dilution value of the half-width is completely temperature independent. Each of the four broadening mechanisms mentioned in the Introduction may have been present in pure a c e t ~ n e . ' ~Faster ~*~~ dephasing at elevated temperatures, as the acetone molecules reorient faster, would explain the broadening. However, we would expect the hydrogen bonds present in the methanol solutions to respond similarly, but no temperature trend was seen. Resonance energy transfer would broaden the line if efficient coupling exists but would also be decreased at higher temperatures as the molecules become less well aligned, resulting in line narrowing. The best explanation, following the line shift arguments above, is based on a repulsive mechanism, whereby energy is exchanged between adjacent acetone molecules during a collision. The higher kinetic energy present at higher temperature enhances the collisions, broadening the line. Collisions are less probable as the acetone becomes more dilute, and less mobile, in the presence of methanol, so the temperature trend fades. The increase in line width observed at lower mole fractions could be accounted for through acetonemethanol hydrogen bonds, which would couple the C-C mode to the bath modes. However, this interaction would be temperature dependent, causing a broadening at lower temperatures.1° Rapid collisional deactivation by the methanol would also be present; this would have the opposite temperature trend to the attractive interaction. The actual situation may represent a balanced trade-off between these two interactions, with the narrowing due to decreased hydrogen bonding being balanced by the broadening due to increased collisions. The temperature independence of the infinite dilution width (Table 11) implies these mechanisms were exactly balanced. We are planning further experiments, using non-hydrogen bonding molecules, in the near future, which should provide more insight into this phenomenon.
79
The temperature and mole fraction dependence of the percent Lorentzian data (Figures 6 and 7) strongly indicates that the addition of methanol caused the environment to become increasingly static, based on the discussion around eqs 1 and 2.' Methanolacetone or methanol-methanol hydrogen bonds slowed local motion, causing the peak to become more Gaussian. The increased Lorentzian character of the xA = 0.4 and 0.2 signals as the temperature was elevated (Figure 7, bottom subplots) indicates these hydrogen bonds were broken by thermal motion, which allowed faster relative motion of the acetone and its environment.
Conclusion The shift in frequency of the acetone C-C stretch in pure acetone and as diluted by methanol was consistent with a collisional interaction. The half-widths showed a complicated dependence on temperature and mole fraction, which indicated that the energy relaxation pathway changed during dilution. Based on the frequency shift argument, a collisional path, present in the pure state, was replaced by a combined collisional deactivation and energy transfer through the carbonyl in the presence of methanol. These acted differently as a function of temperature, with the former increasing and the latter decreasing as the temperature was raised. The infinite dilution line width showed no temperature dependence, indicating these two mechanisms needed to be exactly balanced. The Lorentzian character of the vibration decreased strongly as xAdecreased, consistent with slower motion in the environment. This was due to the presence of both acetonemethanol and methanol-methanol hydrogen bonds. As expected, these were temperature dependent, with the line being more Lorentzian at higher temperatures due to the breakdown of the strong interactions. Acknowledgment. This work was funded through grants received from The University of Connecticut Research Foundation. Registry No. Acetone, 67-64-1.
Matrix Reactions of Sulfur Atoms and Fluorlne. Infrared Spectra of SF, SFp, and SF:, in Solid Argon Paniz Hassanzadeh and Lester Andrews* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: July 8, 1991)
Sulfur vapor diluted with argon was passed through a microwave discharge and deposited with argon-fluorine samples onto a CsI window at 12 K. Variation of fluorine and sulfur concentrations over a wide range and sulfur-34 isotopic substitution provided a basis for identification of the SF,SF2, and SF, transient species. The diatomic SF radical showed matrix site absorptions at 818.8 and 822.0 cm-l for %19F and 809.9 and 813.1 cm-I for %I9F. A similar site splitting was also observed for both symmetricand antisymmetric stretching vibrations of SF2at 832.5, 829.5 and 805.0, 804.6, 802.1 cm-', respectively. The SF, radical revealed symmetric equatorial and antisymmdc axial stretching vibrations at 843.8 and 68 1.8 cm-l, respectively. Sample annealing increased weak absorptions due to the higher fluoride species in the series including SF, and SF6. Ab initio quantum chemical calculations verified these assignments.
Introduction Sulfur reacts with excess fluorine to form the stable SF6 molecule; however, this reaction must involve a number of small intermediate species. Electronic absorption, electron resonance, and microwave and infrared spectra of the gaseous SF radical have been reported.I4 Sulfur difluoride has been studied by microwave (1)Di Lonardo, G.;Trombetti, A. Truns. Faraday SOC.1970,66,2694. (2) Byfleet, C. R.;Carrington, A.; Russell, D. K. Mol. Phys. 1971,20,271. Carrington, A,; Currie, G. N.; Miller, T. A. J. Chem. Phys. 1%9,50,2726. (3) Amano. T.; Hirota, E. J. Mol. Specfrosc. 1973,45, 417. Endo, Y.; Saito, S.; Hirota, E. J . Mol. Spectrosc. 1982,92,443.
spectroscopy in the gas phase5 and by infrared spectroscopy in solid argon6 and the gas phase.' This reactive molecule has C, symmetry with a bond length of 158.9 pm and a bond angle of 98.3°.5
The sulfur trifluoride radical has been observed by ESR and finally characterized as a species with one S-F bond different from (4)Endo, Y.; Nagai, K.; Yamada, C.; Hirota, E. J . Mol. Spectrosc. 1983, 97,213. (5) Johnson, D. R.; Powell, F. X. Science 1969,164,950. (6)Hass, A.; Willner, H. Spectrochim. Acta 1978,M A , 541. (7)Deroche, J. C.; Burger, H.; Schulz, P.; Willner, H. J . Mol. Specfrosc. 1981, 89,269.
0022-365419212096-79303.00/0 Q 1992 American Chemical Society
Hassanzadeh and Andrews
80 The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992
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Figure 1. Schematic diagram of the quartz tube with the microwave discharge sulfur atom source, the argon/fluorine deposition line, and the sample window.
the other two S-F bonds and a plane of symmetry.*v9 The SF3 radical is the prototype sulfonyl radical, which is believed to have a nonplaner T-shaped structure, based on detailed ESR observations for the (CF30),S radical.I0 Matrix-isolation experiments involving the vacuum-ultraviolet photolysis of SF,, SF5X, and SF4 diluted in argon produced a weak 682-cm-I infrared band, which was tentatively assigned to the SF3radical.11q12 In related studies, argon containing 1% SF6was passed through a hollow cathode discharge and condensed at 12 K the FTIR spectrum contained sharp bands due to SF5,SF4, and SF2and sharp weak new bands at 844 and 682 cm-I with constant relative intensities. I The SF4molecule, which serves as a precursor and model for the SF3 radical, has been extensively studied, and its infrared spectrum and structure are known, although assignment of the 700-cm-I fundamental has been debated.I4-ls Finally, vacuumultraviolet photolysis of SF6and SF5X during condensation with excess argon and to a lesser extent the fluorine atom/SF4 reaction produced new absorptions a t 812 and 552 cm-I for the SF5radical," which are related to the spectrum of the stable SF6 molecule. l9 In the present work, the sulfur-fluorine intermediate species are built from the constituent atoms. Sulfur and fluorine concentration variation and sulfur-34 substitution are employed to identify the species and characterize the vibrational motions.
Experimental Section The vacuum system and chamber for matrix-isolation studies have been described previously.2b22 A closed-cycle refrigerator (Air Products, Displex Model DE-202) and an indicator/controller were used to cool and monitor the temperature of the CsI window. The thermocouple reading was normally 12 K at the end of the second stage of the refrigerator head. Infrared spectra were collected at 0.12-cm-' resolution in the range of 400-4000 cm-' on a Nicolet 7 199 FTIR spectrometer; wavenumber accuracy is better than f0.1 cm-]. Natural sulfur (Electronic Space Products, Inc., recrystallized) and enriched sulfur-34 material with 7776, 85%, and 98% 34S (Cambridge Isotope Laboratories, Oak Ridge National Lab and (8) Fessenden, R. W.; Schuler, R. H. J. Chem. Phys. 1966, 45, 1845. (9) Colussi, A. J.; Morton, J. R.; Preston, K. F. J . Chem. Phys. 1974,61, 1247. (10) Morton, J. R.; Preston, K. F. J . Phys. Chem. 1973, 77, 2645. ( 1 1) Smardzewski, R. R.; Fox, W. B. J . Chem. Phys. 1977, 67, 2309. (12) Smardzewski, R. R.; Fox, W. B. J . Fluorine Chem. 1976, 7, 353. (13) Brabson, G.D.,unpublished results. (14) Dodd, R. E.; Woodward, L. A.; Robert, H. L. Trans. Faraday SOC. 1956,52, 1052. (15) Tolles, W. M.;Gwinn, W. D.J . Chem. Phys. 1962, 36, 1119. (16) Muetterties, E. L.; Phillips, W. D. J . Am. Chem. Soc. 1959, 71. 1084. (17) Redington, R. L.; Berney, C. V. J . Chem. Phys. 1965, 43, 2020. (18) Frey, R. A.; Redington, R. L.; Khidir-Aljibury, . A. L. J . Chem. Phys. 1971, 54, 344. (19) Swanson, B. I.; Jones, L. H. J. Chem. Phys. 1981, 74, 3205. (20) Kelsall, B. J.; Andrews, L. J. Phys. Chem. 1981, 85, 1288. (21) Brabson, G. D.;Mielke, Z.; Andrews, L. J . Phys. Chem. 1991, 95, 19. (22) Andrews, L.; Lascola, R. J . Am. Chem. SOC.1987, 109, 6243.
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Wavenumber Figure 2. Infrared spectra in the 940-640-cm-' region for discharged sulfur vapor/argon stream codeposited with fluorine/argon mixture on a CsI window at 12 i 1 K. (1) FJAr = 1/1200 and 70 OC sulfur reservoir, (b) F2/Ar = 1/100 and 50 OC sulfur reservoir, (c) sample b after annealing to 30 2 K, and (d) 1-mm orifice discharge tube, FJAr = 1/100 and 100 OC sulfur reservoir.
*
EG&G Mound Applied Technologies) were used as received. A 6 nm 0.d. quartz tube shown in Figure 1 contained the microwave powered sulfur doped argon discharges2' The outer coaxial 12 mm 0.d. tube extended within the vacuum chamber to reduce the conduction of heat to the viton O-ring. This design also prevented air cooling of the inner discharge tube, and consequently a higher amount of sulfur could be sent into discharge without being condensed on the walls. Typically, with 40-60 W of microwave power applied to an Evenson-Broida cavity and a flow rate of 2-3 mmol/h, the discharge could be extended a few millimeters out of the end of the discharge tube. The sulfur reservoir was maintained from 25 O C to 115 O C in order to vary the sulfur concentration. Fluorine (Matheson) was handled in a passivated stainless steel manifold used exclusively for fluorine service.22 The usual impurities in commercial fluorine (HF, CF4, OCF2, and SiF4)were minimal and did not affect the present experiments. Fluorine/ argon samples were codeposited with the discharged argon/sulfur samples, and infrared spectra were recorded.
Results The following experiments were carried out to produce and identify monosulfur fluoride intermediate species in argon matrices. ArgowSulfur Discharge plus Argon-Fluorine. Different mixtures of fluorine-argon (1/ 100, 1/400, 1/800, 1/ 1200) were codeposited with the sulfur-doped argon discharge. The sulfur reservoir temperature was also varied from room temperature (pink argon discharge) to 115 O C (intense blue discharge). When a 1/400 mixture of fluorine/argon was condensed with the room temperature sulfur seeded discharge effluent gas stream only a weak band a t 818.8 cm-I was detected. When the sulfur reservoir temperature was increased to 50 O C , new peaks were observed at 832.5, 829.5, 822.0, 818.8, 805.0, and 802.1 cm-I. Asimilar result was obtained with a 1/800 fluorine/argon mixture. Codeposition of a 1/1200 mixture of fluorine/argon with a 70 O C sulfur reservoir discharge gas stream produced two main peaks at 822.0 and 818.8 cm-I with relatively weaker peaks at 805.0, 804.6, and 802.1 cm-I; the observation of weak S3bands2I at 680.0 and 676.2 cm-' showed that sulfur was in excess (Figure 2a). Annealing the matrix at 25 K caused the intensity of the peak
The Journal of Physical Chemistry, Vol. 96,No. 1 , 1992 81
Matrix Reactions of Sulfur Atoms and Fluorine
BI
rii
C
880
890
870
860
850
830
840
820
810
800
780
790
Wavenumber Figure 3. Infrared spectra in the 890-780-cm-' region for discharged sulfur vapor/argon codeposited with fluorine/argon mixture on a CsI window at 12 j: 1 K; sulfur reservoir at 100 O C and F2/Ar = 1/100 in all experiments: (a) natural isotopic sulfur, (b) sample a after annealing to 30 2 K, (c) 85% "S-enriched sulfur, (d) sample c after annealing to 30 f 2 K, and (e) 50% 34S-enrichedsulfur.
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at 818.8 cm-I to decrease and the peak at 822.0 cm-I to increase without other changes. Upon further annealing at 30 K, the peak at 818.8 cm-I vanished in favor of the 822.0-cm-I peak. As the concentration of fluorine was increased new peaks (843.8, 682.3, and 681.8 cm-I), SF, bands (705.6,856.4,859.7, and 884.5 cm-I), and SF6 absorptions (936.2-938.5 cm-I) appeared; these bands were all enhanced upon annealing.17-19Traces of the oxygenated species OSF, and OF, were also present in all of these experiment^.'^,^^ In addition, a weak HF2- band was found at 1376 cm-1.24 An experiment with 1/100 fluorine/argon and a 50 O C sulfur vapor discharge gave 832.5-,829.5-,822.0-, 805.0-, and 804.6-cm-1peaks along with increased 843.8- and 681.8-cm-I bands, and increased SF4, SF6, and S2F2absorption (752.5 and 708.3 cm-') intensities;25the absence of molecular S3 bands showed that fluorine was in excess (Figure 2b). In this fluorine-rich experiment, annealing to 30 f 2 K decreased all bands in favor of SF4 and SF6 (Figure 2c). Infrared spectra obtained from the reaction of a 1/100 mixture of fluorine in argon with a purplish-blue (100 " C )sulfur discharge effluent are shown in Figures 3a, 3b and 4a, 4b before and after annealing. The observed peaks are also collected in Table I. In spite of the higher sulfur concentration, the higher fluorine con(23) Andrews, L. J . Chem. Phys. 1972, 57, 51. (24) McDonald, S. M.; Andrews, L. J . Chem. Phys. 1974, 70,3134. (25) Hass, A,; Willner, H. Spectrochim. Acta 1979, 35A, 953.
TABLE I: Product Absorptions (cm-I) Observed from tbe Matrix Reaction between Atomic Sulfur and Fluorine "2s 34S shift ( A v ) assignment 921.4 17.2 938.6 SF6 (asym S-F str, v3) 920.8 17.1 937.9 SF6 (asym S-F str, v,) 920.3 17.1 937.4 SF6 (asym S-F str, v 3 ) 873.3 11.2 884.5 SF5 (sym eq S-F str, v J 849.2 10.5 859.1 SF4 (asym eq S-F str, 4,) 846.0 10.4 856.4 SF4 (asym eq S-F str, v6) 834.3 9.5 SF3 (sym eq S-F str) 843.8 823.5 9.0 832.5 SF2 (sym S-F str, Y , ) 820.5 9.0 829.5 SF2 (sym S-F str, v i ) 813.1 8.9 S F (S-F str) 822.0 809.9 8.9 818.8 S F (S-F str) 796.8 16.3 SF5 (asym S-F str) 813.1 795.5 9.5 805.0 SF2 (asym S-F str, v 3 ) 795.1 9.5 804.6 SF2 (asym S-F str, v,) 792.6 9.5 802.1 SF2 (asym S-F str, Y , ) 791.8 9.4 801.2 OSF2 (sym S-F str, v2) 791.3 9.4 800.7 OSF2 (sym S-F str, v 2 ) 789.3 9.9 799.2 OSF2 (sym S-F str, u2) 788.6 10.0 798.6 OSF2 (sym S-F str, v2) 787.2 8.3 795.5 SF< (asym S-F str) 729.1 9.1 738.2 OSF2 (asym S-F str, v s ) 725.9 9.2 OSF2 (asym S-F str, v 5 ) 735.1 SF4 (asym ax S-F str, v8) 695.8 12.9 708.7 695.1 13.0 SF4 (asym ax S-F str, va) 708.1 693.5 12.8 706.3 SF4 (asym ax S-F str, v 8 ) SF4 (asym ax S-F str, v 8 ) 692.8 12.8 705.6 691.3 13.0 704.3 SF4 (asym ax S-F str, v g ) 670.9 11.4 682.3 SF3 (asym ax S-F str) 670.4 11.4 68 1.8 SF3 (asym ax S-F str)
centration allowed for reaction of most of the sulfur leaving only weak S3absorption. Upon annealing, the peaks at 829.5, 818.8, and 802.1 cm-l decreased, and the peaks at 832.5, 822.0, 805.0, and 804.6 cm-I increased. In addition, a relatively higher growth was observed for S2F2and SF4 bands, the 843.8-, 682.3-, and 681.8-cm-l peaks (marked with arrows), and the 937.9-cm-I SF6 band. New peaks at 813.1 and 795.5 cm-I were also produced, which have been identified as SF5and SF
