75
J . Phys. Chem. 1991, 95, 75-79
Matrix Infrared Spectra of the Phosphorus Sulfides PS, P,S, and PS, Zofia Mielke,+ G. Dana Brabson, and Lester Andrews* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: April 9, 1990)
The reaction of P4 and s8 in an argon discharge has produced a number of new phosphorus sulfides for matrix infrared spectroscopy. Contrasting product yields with PH3, H2S, P4SIo,and P4S3in the discharge and OCS in the matrix, photolysis and annealing behavior, and shifts with S-34 and S-321s-34 isotopic mixtures have characterized the PS,P2S,and PSI molecules. Isotopic spectra show that P2S is a linear molecule like P 2 0 and N 2 0 and that PS2 is bent like POz and NO2. Secondary reactions have provided evidence for P2S4, PSY, and P4S. Although sulfur atoms are less reactive than oxygen atoms, many of the same phosphorus species are produced.
Introduction The chemistry of phosphorus sulfides is centered largely on solid compounds. The PS diatomic molecule has, however, been characterized by emission in the 230-275-nm region from a helium discharge seeded with P4S3and the vibrational fundamental determined (733.2 cm-').l Six solid compounds have been wellcharacterized: P4S3.P4S4,P4S5,P4S7,P4S9,and P4SIo. All of these have structures based on the P4 tetrahedron with various numbers of bridging and terminal sulfur atoms.2d Of these, P4S3is the most stable even in water, which hydrolyzes the larger, more reactive sulfides. The former evaporates as PIS3, but the latter decomposes to a significant degree on evaporation.6-8 There is no information available on small volatile phosphorus sulfides other than the diatomic molecule. Recent matrix infrared work on phosphorus oxides in this laboratory has characterized the small phosphorus oxide species PO, PO2, P20, P203,P204, P2O5. and P 4 0 using matrix reactions of 0 atoms or O3molecules with P2, PH3, and P4.9-11 It was desired to prepare and characterize the analogous small phosphorus sulfide molecular species by matrix infrared spectroscopy. Unfortunately, sulfur atoms in the ground state are less reactive than oxygen atoms; the thiophosphoryl species H3PS and P4S could not be prepared from matrix photolysis of carbonyl sulfide with phosphine and with P4. although SPF3 was formed in high yield with PF3.'2.13 It was therefore decided to discharge mixtures of P4 and Sa in order to synthesize small phosphorus sulfide species. The following article describes new phosphorus-sulfur species produced by discharge reactions and trapped in solid argon. Experimental Section A CTI cryogenics Model 22 refrigerator suspended in a stainless steel vacuum chamber affixed with ports for infrared and photolysis windows and for gas deposition lines was used to cool a Csl sample window to 12 f 1 K. The coaxial quartz tube shown in Figure 1 was employed to discharge flowing gas mixtures using a Burdick MW200 diathermy and Evenson-Broida cavity. The 12-mm-0.d. tube entered the vacuum chamber through a 1/2-in. "Ultra torr" fitting; the microwave cavity was placed around the tube in contact with the "Ultra torr" fitting. Elemental sulfur in the finger was heated to 95-120 "C with resistance wire to entrain sufficient Sa vapor in the 6-mm-0.d. center discharge tube to give a strong sky-blue S2emission from the discharged Ar/Sa stream flowing at 2-3 mmol/h. Phosphorus vapor from white phosphorus at 6-10 OC was introduced through a Teflon needle valve9 into the surrounding tube, mixed with the argon/sulfur discharge plume, and collected on the cold sample window. The diathermy was operated at 80% power in order to extend the discharge from the inner tube into the last 1 cm of the outer tube. Similar experiments were done with phosphine replacing the P4 reagent and with 85% 34S-enriched sulfur (Oak Ridge National Laboratory). Typical reagent concentrations were on the order of 0.2% in the argon stream and were varied by a factor of 2 in each direction over the course of these experiments. 'On leave from University of Wroclaw, Wroclaw, Poland.
0022-365419112095-0075$02.50/0
TABLE I: New Infrared Absorptions (in cm-') Produced by Reaction of Phosphorus and Sulfur Atoms 3 2 s '2s134s 3 4 s ident 891.4 783.9 748.9 744.1 729.3 636.7 607.9 518.1 473.9
891.4, 887.8 748.9, 745.2, 741.1 744.1, 740.0, 735.8 729.3. 718.7 636.7. 633.2, 629.2 607.9, 597.7 518.1. 510.2 473.9, 470.5
887.8 780.0 741.1 735.8 718.7 629.2 597.7 5 10.2
P2S ? P2S4 PS2 PS PS2?
cyclic P,S P4SJ
FTIR spectra were recorded on a Nicolet 7199 at 1-cm-' resolution or a Nicolet 60SXB at 0.5-cm-' resolution (KBr beam splitter) during and after sample deposition, after photolysis through glass long-wavelength pass and water filters, and after sample annealing to 32 f 2 K. Wavenumber accuracy is f0.1 cm-'.
Results FTIR spectra of matrix samples prepared by codepositing discharged mixtures of phosphorus and sulfur compounds will be described. Sulfur. Several experiments were performed with s8 vapor seeded into an argon discharge. Strong infrared bands at 679.8, 675.9, and 661.4 cm-' and a weaker band at 642.4 cm-l are due to small sulfur molecular species,I4but no absorption was detected at 476 cm-I for s8. Infrared spectra of s8and smaller molecular sulfur spccies will be discussed in the following paper.15 Weak impurity absorptions were observed at 672.6 cm-' for S20,736.9 cm-' for C2H2,739.1 and 917.9 cm-' for SiS and SiS2,1138.8 cm-' for SO. 1226.0 cm-I for SiO, 1265.4 and 1289.9 cm-' for SSiO, 1355.0 and 1351 .O cm-' for SO2,and 2049.7 cm-' for OCS, (1) Narasungam, N. A.; Subramanian, T. K. B. J. Mol. Specrrosc. 1969, 29, 294.
(2) Leung, Y. C.; Wasser, J.; Van Houten, S.; Vos, A.; Wiegers, G . A.; Wiebenga, E. H. Acta Crystallogr. 1957, 10, 514. (3) Griffin, A. M.; Minshall, P. C.; Sheldrick, G. M. J . Chem. Soc., Chem. Commun. 1976, 809. (4) Meisel, M.; Grunze, H. Z . Anorg. Allg. Chem. 1970, 373, 265. (5) Vos, A,; Olthof, R.; van Balhuis, F.; Batterweg, R. Acra Crystallogr. 1965, 19, 864. (6) Hoffman, H.; Becke-Goehring, M. Phosphorus Sulfides. In Topics in Phosphorus Chemistry; Griffith, E. J., Grayson, M., Eds.; Wiley: New York, 1976; Vol. 8, pp 193-271. (7) Gardner, M. J. Chem. Soc., Dalton Trans. 1973, 691. (8) Andrews, L.; Reynolds, G.;McCluskey, M.; Mielke, Z. Inorg. Chem., in press. (9) Andrews, L.; Withnall, R. J . Am. Chem. SOC.1988, 110, 5605. (IO) Withnall, R.; Andrews, L. J . Phys. Chem. 1988, 92, 4610. (1 1 ) Mielke, Z.; McCluskey, M.; Andrews, L. Chem. Phys. Lett. 1990, 165, 146. (12) Hawkins, M.; Downs, A. J. J. Phys. Chem. 1985.89, 3326. (13) Mielke, Z.; Andrews, L. Unpublished results, 1989. (14) Meyer, B.; Stroyer-Hansen, T. J . Phys. Chem. 1972, 76, 3968. ( I 5) Brabson, G.D.; Mielke, Z.; Andrews, L. J . Phys. Chem., following paper in this issue.
0 1991 American Chemical Society
Mielke et al.
The Journal of Physical Chemistry, Vol, 95, No. 1 , 1991
76
I
p4
/'I
!I
Ar
L -. S
Figure 1. Coaxial quartz discharge tube used for reacting P, and S8 in a microwave-powered argon discharge. The entire sulfur inlet tee was
heated to prevent condensation of sulfur.
0
*1
I1
0930 BAS ai0 ARVENiJMBER
PS
750 735 71C WAVENUMBER
525
5OC 475 WAVEYUMBER
450
Figure 3. Infrared spectra of common phosphorus-sulfur discharge products trapped in solid argon at 12 K: (a) argon discharge seeded with normal isotopic S8 and P4;(b) argon discharge seeded with 85% S-34enriched S8 and P,; (c) argon discharge seeded with 50/50 S-32/S-34 isotopic S8 and P4. 0
1 1
PS
1
C
I
'%io
840 B ~ O WflVENUMBER
780
740 7$0 WRVENUMBER
7ho
S ~ O 630
SIC
WRVENUMBER
Figure 2. Infrared spectra of common phosphorus-sulfur discharge products trapped in solid argon at 12 K (a) argon discharge seeded with S8 and P,; (b) argon discharge seeded with P4Sl0; (c) argon discharge seeded with P4 and codeposited with Ar/OCS = 75/1 sample.
and a strong band was observed at 1157.2 cm-l for S20.16-'9 Annealing these samples produced a marked growth in a sharp 674.5-cm-I absorption, which has been identified as S3 (ezu structure),I5 and a strong, deep blue luminescence due to S2 produced by recombination of sulfur atoms.*O Finally, the argon/sulfur discharge effluent was codeposited with an Ar/PF3 sample to serve as a trap and detector for S atoms. Strong SPF, absorptions observed in this sample were comparable to those produced by photolysis of Ar/PF,/OCS mixture^.'^,'^ Phosphorus. A detailed investigation was done with P4 vapor flowing into the argon discharge without sulfur. Visible spectra of these samples have provided evidence for the P, radical and P4+ molecular cation.21 Infrared studies revealed P4 at 465 cm-I and weak bands at 1218.0, 1226.0, 1240.8, and 1270.3 cm-l due to PO, SiO, P40, and P20,respectively,e11 without new product features. Sulfur and Phosphorus. The major portion of this work reacted sulfur and phosphorus vapors in a discharged argon stream. Infrared spectra revealed the new product bands listed in Table I and shown in Figures 2a and 3a. These include the strong sharp band at 891.4 cm-l (labeled P,S), weaker bands at 748.9 and 744.1 cm-l, the sharp 729.3-cm-l band (labeled PS), the 640.3-, 636.7-cm-' doublet, and sharp bands at 518.1 and 473.9 cm-l. Subsequent experiments reproduced these bands and gave increased SiO, SiS, and SiSzband absorbances. An experiment done with an alumina inner tube gave the above new phosphorus sulfide bands without silicon impurities. The yield of pure sulfur species was reduced (about 50%) by the addition of the P4 reagent relative to pure sulfur experiment^.'^ A sample of P4Slowas evaporated into the discharged argon stream, and the spectrum, illustrated in Figure 2b, is similar to the spectrum in Figure 2a with several exceptions: (a) the 891.4-cm-' band was markedly reduced, (b) the 748.9-, 744.1-, (16) Tang, S.-Y.; Brown,C. W. Inorg. Chem. 1975, 14, 2856. (17) Anderson, J. S.; Ogden, J. S. J . Chem. f h y s . 1969, 51, 4189. (18) Schnockel, H. Angew. Chem., In?. Ed. Engl. 1980, 19, 323. (19) Schnockel, H.: Koppe, R. J . Am. Chem. SOC.1989, 111. 4583. (20) Lee, Y.-P.: Pimentel, G.C. J . Chem. f h y s . 1979, 70, 692. (21) Andrews, L.: Mielke, Z. J . f h y s . Chem. 1990, 94, 2348.
0
O.760
WAVENUMBER L
D
Figure 4. The 720-760-cm-' region in the infrared spectrum of phosphorus-sulfur discharge products: (a) after deposition at 12 K for 8 h; (b) after matrix photolysis with 420-1000-nm radiation for 30 min; (c) after annealing to 32 f 2 K for 5 min.
and 640.3-cm-l bands were enhanced relative to the 729.3-cm-l band, (c) the 518.1- and 473.9-cm-' bands were absent. In addition, sulfur species absorptions in the 600-cm-l region were also observed. A similar argon discharge seeded with P4S3vapor gave comparable matrix absorptions at 748.9,744.1, 729.3 and 636.7 cm-l, and in contrast to the P4Sloexperiment, the 891.4- and 518.1-cm-l absorption intensities were nearly the same as the 744.1- and 729.3-cm-l band intensities. Hence, the intensities of the 891.4- and 518.1-cm-l bands relative to the PS band at 729.3 cm-' increase in experiments with discharged P4Sl0,P4S3,and P4 + SBas the PIS ratio increases. Photolysis and Annealing. Several discharged phosphorus and sulfur samples were subjected to high-pressure mercury arc photolysis. Irradiation by red light (A > 590 nm) increased 891.4and 7 4 4 . 1 - ~ m -bands ~ slightly with no effect on the other absorptions. Visible radiation (A > 420 nm) increased the 891.4-cm-' band by IO%, the 748.9-cm-' band by 50%, the 744.1-cm-' band by 200%, and the 518.1-cm-l band by IO%, decreased the 640.3and 636.7-cm-I bands by 15%, and left the other bands unchanged. Figure 4 shows the marked growth at 744.1 cm-I. Photolysis at X > 290 nm and X > 220 nm continued increasing the 891.4-, 744.1 -, and 5 18.1-cm-I bands and decreasing the 640.3- and 636.7-cm-' bands without significant change in the other absorptions. The most pronounced changes on photolysis were
Matrix Infrared Spectra of PS, P2S, and Ps2 ID
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 77 TABLE 11: Observed and Calculated Frequencies (cm-') for PzS, PS2,and PS," p2s ps2 PS232S
obs
calc
obs
calc
obs
calc
891.4
891.4
887.8
887.9
744.1 740.0 735.8
744.1 740.0 736.1
636.7 633.2 629.2
636.7 633.3 629.6
32.34s
34S
"PIS: LPPS = 1 8 0 O . FPS = 4.84, Fpp= 5.21, Fps.pp= 0.17 mdyn/A. Thc sccond strctching vibration was assumed to be in the range 525 f 20 cm-'. PS2: LSPS = 128O, Fps - Fps,ps = 3.91 mdyn/A. The second strctching frequency was varied in the 560 f: 20 cm-I range. PS,: LSPS = I 20°, Fps - Fps,ps= 3.0 mdyn/A. The second stretching vibration was varied in the range 510 f 15 cm-'.
0
was the P2S band at 891.4 cm-l with weaker bands at 744.1, 729.1, (PS), 636, and 473.9 cm-I. A sulfur species absorption was also observed at 675.5 cm-l. Unfortunately, the strong OCS band masked any 5 18.1-cm-' product absorption. Discussion
observed for the 748.9- and 744.1 -cm-' bands. Sample annealing to 32 f 2 K decreased the 891.4- and 518.1-cm-l bands by 25% and markedly increased the 674.5-cm-] S3band.I5 As shown in Figure 4, the PS absorption was substantially decreased, the 744.1-an-' band was virtually destroyed, the 748.9-cm-' band was increased by 30%, and the 640.3- and 636.7-cm-I bands were decreased. Isotopic Sulfur and Phosphorus. Important experiments were done with 50% and 85% 34S-enriched samples discharged with phosphorus, and infrared spectra are compared in Figure 3. The strong 891.4-cm-I band shifted 3.6 cm-l to 887.8 cm-l (P234S). The sharp 729.3-cm-I band shifted to 718.7 cm-' (P34S). The sharp 744.1-cm-I band shifted to 735.8 cm-' with an intermediate component at 740.0 cm-I; marked growth on X > 420 nm photolysis makes identification of the sharp isotopic triplet for this band straightforward as is shown in Figure 5. The 748.9-cm-I band also exhibited an isotopic triplet at 748.9, 745.2, and 741.1 cm-l. The 636.7-cm-' band gave an isotopic l/2/l triplet at 636.7, 633.2, and 629.2 cm-I with very weak S4absorptions at 642.6 and 624.2 cm-I. The 518.1-cm-' band yielded a 3?3component at 510.2 cm-l and the 473.9-cm-l band exhibited a mixed isotopic component at 470.5 cm-I, but the pure 34Scomponent was masked by P4 and shoulder at 465 and 468 cm-', which are shown in Figure 3. Other Compounds. Analogous experiments were done with PH3 and sulfur, and all product bands listed in Table I, except 518.3 and 473.9 cm-I, were observed with somewhat different relative intensities. The 891.4-cm-I band was reduced relative to the 729.3-cm-l band. An increase in sulfur concentration at constant PH3 concentration left the relative yields of P2Sand PS unchanged but markedly enhanced the 636.7-, 744.1-, and 748.9-cm-I band intensities relative to PS. In addition, a number of new absorptions were observed for H,P,S, species, which will be presented in a later publication. Isotopic sulfur experiments performed with PH3 gave the same isotopic spectra reported above for P, and sulfur discharges. An experiment with Ar/H2S = 75/1 sample in the center discharge tube and P, in the outer tube gave a strong 891.4-cm-I product absorption, almost as strong as in Figure IC, other absorptions comparable to the OCS experiment described below, and weak sulfur species bands at 680 and 676 cm-l. In addition, a strong 5 18.1-cm-I band was also observed. As before, X > 420 nm photolysis markedly increased the 744.1 -cm-l band (3-fold) and substantially increased the 891.4- and 518.1-cm-I bands (2fold). In a different experiment, P, vapor was mixed with discharged argon and codeposited with a separate Ar/OCS = 75/1 sample. The strongest product band in the infrared spectrum (Figure 2c)
The new phosphorus sulfide species produced here will be identified by comparison of yields with different chemical precursors and sulfur isotopic spectra. PS. The sharp new 729.3-cm-' absorption was observed in all discharge experiments with both phosphorus and sulfur present. This absorption was not affected by photolysis, but it decreased on annealing the sample (Figure 4). The S-32/S-34 isotopic ratio 729.3/718.7 = 1.0148 is in excellent agreement with the harmonic ratio for a PS diatomic molecule (1.01478). The 729.3-cm-' matrix absorption is just below the 733.2-cm-] vibrational fundamental for PS deduced from the gas-phase emission spectrum.] A red matrix shift of 3.9 cm-' is appropriate for PS. The analogous PO molecule absorbs at 1218.3 cm-' in solid argon,1° which is 2.0 cm-' below the gas-phase fundamental2* The isotopic shift and agreement with the gas-phase fundamental confirm assignment of the 729.3-cm-' band to PS in solid argon. The PS diatomic molecule serves as a reference point for the stoichiometric identification of other products in these experiments. P2S. The 891.4-cm-' band yield was higher relative to PS in P4 S8 experiments than in PH3 S8 experiments where high PH3 concentration was required. The 891.4-cm-' band intensity relative to PS increased with increasing P / S ratio in the series of experiments where P4Slo,P4S3, and P4 S8 mixtures were discharged. The yield of this new species was highest relative to PS when P4 discharge fragments reacted with S atoms from OCS dissociation on the matrix surface by vacuum-ultraviolet radiation from the discharge. Hence, the 8 9 1 . 4 - ~ m -absorber ~ clearly contains more phosphorus than PS. Evidence from ozone reactions suggests that the reactive phosphorus species in the discharge is primarily P2," which provides the P2S identification for the 891.4-cm-' band. The sharp sulfur isotopic doublet (Figure 3c) confirms that one sulfur atom is present in this species. Vibrational analysis provides further evidence for the P2S identification. The P 2 0 molecule has been characterized as linear like N 2 0 from ab initio c a l c ~ l a t i o n and s ~ ~ matrix infrared spectra." Since the masses of P and S are nearly equal, a linear P2S molecule exhibits "antisymmetric-like" and "symmetric-like" stretching modes. Hence, the 89 1.4-cm-' band involves the terminal P-S vibration out-of-phase with the P-P vibration. The best model for this vibration is the antisymmetric stretching mode of linear centrosymmetric SiS,, which has recently been characterizedI9 and the assignment confirmed by observation of a sharp comtriplet using mixed isotopic sulfur discharges with a Si34S2 ponent at 909.8 cm-I in the present work. The shift from 917.9 (3.9 cm-I) compares cm-I for Si32S2to 914.0 cm-I for 32SSi34S favorably with the shift from 891.4 cm-' for P232Sto 887.8 cm-I for P234S(3.6 cm-I), which shows similar mechanics for these two
+
+
+
(22) Verma, R. D.; Dixit, M. N . Can. J . Phys. 1968, 46, 2079. (23) Lohr, L. L. J . Phys. Chem. 1990, 94, 1807.
78 The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 molecules and supports the linear P2S identification. Simple force constant calculations for the linear P2S species also confirm this assignment. As can be seen in Table 11, the observed P234S fundamental is in very good agreement with the calculated value. The "symmetric-like" stretching mode was assumed to be in the range 525 f 20 cm-l, which is close to the value predicted for SiS2 (514 f IO cm-').I9 HONDO 7.0 electronic structure calculations were performed for linear and triangular P2S using the DZP basis set. Optimized geometries for the linear model gave R(P-P) = 1.86 8,and R(P-S) = 1.90 8, and for the triangular structure provided R(P-P) = 1.96 A and R(P-S) = 2.17 8,. The triangular structure was lower in total energy by 12 kcal/mol at this level. Calculated infrared spectra were as follows: linear, 971 cm-I (strong), 564 cm-' (medium), 152 cm-' (weak): triangular, 773 cm-' (weak), 528 cm-I (medium), 325 cm-I (medium). Such calculations typically predict fundamentals in excess of experimental values by 10-15%. Using 0.9 as a scale factor predicts 874 cm-' for the strongest band of linear P2S and 475 cm-' for the strongest absorption of triangular P,S. Clearly the former is in reasonable agreement with the 89 1.4-cm-' matrix observation and supports the present identification of linear PIS. Finally, ab initio calculations predict the triangular P2S structure to be slightly more stable than the linear form although similar calculations found that linear P 2 0 is more stable than the triangular form.23 This reversal in the structurestability relationship between the oxygen and sulfur compounds occurs because the more diffuse orbitals of the larger S atom can overlap with phosphorus orbitals and form two bonds with less ring strain than the smaller 0 atom. It is possible that triangular P2S is also produced in these experiments; unfortunately, the absorptions are weaker and the region 460-480 cm-' predicted for absorption is masked by P, and other bands, and the triangular P2S molecule, which may be formed, could not be observed here. PS2. The 744.1-cm-I band was favored relative to PS with an increase in sulfur concentration. Two unique characteristics are its most pronounced growth on 420-nm photolysis and demise on annealing (Figure 4). The 744.1-cm-I band becomes a sharp 1 /2/ 1 triplet with a 50/50 S-32/S-34 sample, which indicates a vibration involving two equivalent sulfur atoms (Figure 5 ) . In PH3 experiments with high sulfur concentration, the 744.1-~m-~ and PS absorptions are comparable and both stronger than P,S; under these conditions, P atom products dominate. This evidence suggests one P atom for the 744.1-c1n-~absorber and completes the identification of PS,. The PS2 molecule is expected to be bent like its isoelectronic analogues NO2 and PO,. Since the antisymmetric stretching mode u j in a C, molecule is alone in its b, symmetry class, the terminal sulfur isotopic shift can be used to predict an upper limit for the bond angle. The upper limit of 127 f 4" calculated here for PS, is the only experimental geometric parameter available for this new molecule: MNDO calculations predict a higher angle (1 43").,, I n the case of POz, a 139 f 5" upper limit was predicted, which may be compared to the 135.3" microwave v a l ~ e . ' Isotopic ~~~~ u3 fundamentals for PO, and PS, suggest that the latter has a slightly smaller ( 1 22 f 4") valence angle. In the case of SO,, a 122" upper limit was predicted as compared to the directly measured 119.3" value.26 In support of this trend is the comparison between SO, and S,: the latter valence angle is smaller by about 4" based on multiple sulfur isotopic u3 f~ndamentals.'~ Finally, calculated isotopic fundamentals and force constants for PS, are presented in Table 11. PS;. The 636.7-cm-l band and 640.3-cm-' site splitting were strongest in the P4Sl0discharge experiments and were observed from P4 and PH3 S8reactions in the discharge. These bands diminished considerably after 420- and 290-nm photolysis and after annealing. In the 34S P4 experiment the two bands were
+
+
(24) Bews, J. R.; Glidewell, C. J . Mol. Srrucr. 1982, 86, 217. (25) Kawaguchi, K.; Saito, S.; Hirota, E.; Okashi, N. J. Chem. Phys. 1985,
82, 4893. (26) Allavena, M.; Rysnik, R.; White, D.; Calder, V.; Mann, A. E. J. Chem. Phys. 1969, 50, 3399.
Mielke et al. displaced to 629.2 and 632.6 cm-l. In the 32S/34S experiment with P4, the stronger component dominated and a sharp 1 /2/ 1 triplet was observed at 636.7,633.2, and 629.2 cm-' with very weak sulfur absorptions at 642.6 and 624.2 cm-'. Annealing increased weak sulfur species bands at 642.6 and 624.2 cm-l and the central component also at 633.2 cm-', while the 636.7- and 629.2-cm-I bands decreased. Clearly, the initial 636.7-, 633.2-, 629.2-cm-l triplet is due to a new phosphorus sulfide species with two equivalent S atoms. The 640.3-, 636.7-cm-l doublet tracked with the 744.1-cm-l band in P4Sl0,P,, and PH3 experiments, which provides evidence for a single P atom in this species. The 32/34 ratio 1.01 19 indicates an antisymmetric P-SI stretching mode, but the absorption appears in the region between terminal P-S and bridged P-S-P stretching modes. The apex angle upper limit predicted from the 32/34 isotopic ratio, 11 8 f 4", is less than the 127 f 4" value predicted above for PS,, the same relationship In these cases found for PO; and P0210J'and 03-and 03.27-2s the extra electron provides repulsions that decrease the valence angle, and since the anion electron is largely antibonding the u3 fundamental is reduced. All of the above evidence points to assignment of the 640.3-, 636.7-cm-I doublet to matrix sites of the PSc molecular anion. Here vacuum-ultraviolet radiation from the argon discharge provides photoelectrons for capture by PS2. The analogous PO2- anion and 03-have been observed in P4 discharge experiments with ozone." Other Species. The 748.9-cm-I band is even more dependent on sulfur concentration than PS,; the former increases at the expense of the latter on annealing (Figure 4). The band also becomes a 1/2/ 1 triplet like PS2 with mixed sulfur isotopes, which denotes the antisymmetric P-S2 vibration of a -PS2 group. The S-32/S-34 isotopic shift predicts an S-P-S angle of 138 f 6", slightly higher than the value for PS,. Following the phosphorus oxide species, the 748.9-cm-l band is probably due to sulfo-bridged SPS-PS, analogous to the most stable form of P204.23A new absorption at 750.6 cm-' produced by thermal decomposition of P4Slois probably due to the P2S5molecule analogous to the P20s The S-32/S-34 isotopic shift is a most important diagnostic for product bands in these experiments. A group of weak bands was observed near 800 cm-' with the strongest feature at 783.9 cm-I. This band decreased on photolysis and sharpened and increased on annealing. Sulfur-34 shifted the band to 780.0 cm-', and a sharp doublet was observed with mixed isotopes. Clearly, this vibration involves one sulfur atom, but the lack of a full PS diatomic shift (1 1.4 cm-I) indicates the major participation of other atoms, possibly oxygen. The presence of S 2 0 in these samples arising from reaction of sulfur and silica in the discharge raises the possibility of ternary P,S,O, species contributing to the 783.9-cm-I absorption. Another weak band, which appeared at 607.9 cm-l, cannot be identified. It is important to note that the major product species, which exhibit large sulfur isotopic shifts, can confidently be assigned to new phosphorus-sulfur species. The 518.1-cm-' band exhibits common behavior with P2S, namely, an increase on 420-nm photolysis and a decrease on annealing, but the 891.4-cm-l band increases more on photolysis with increasing energy radiation than does the 518.1-cm-I band. The 51 8.1-cm-' band, however, is more dependent on phosphorus concentration and is not observed in PH3 experiments. Mixed sulfur isotopes produced a sharp doublet indicating a single S atom vibration. This region of the spectrum is characteristic of antisymmetric P-S-P vibrations. The S-32/S-34 isotopic ratio 1.0155 is indicative of such a mode for a P-S-P subgroup having a 96 f 4" angle, treating the subgroup as an isolated triatomic molecule. This subgroup forms angles of 103 f l " and l 10 f l O respectively in P4S3 and P4Slo.6 The 518.1-cm-I absorption is probably due to cyclic planar P4S species mode by reaction of P2S and P,. The analogous cyclic planar P 4 0 molecule produced from P,O and P, is the most stable structural isomer, based on theoretical calculation^.^^^^^ We have no evidence for terminal or ~
~
~~
~~~~
~
(27) Andrews, L.; Spiker, R. C., Jr. J. Phys. Chem. 1972, 76, 3208. (28) Spiker, R. C., Jr.; Andrews. L. J. Chem. Phys. 1973, 59, 1851.
79
J . Phys. Chem. 1991, 95, 79-86 tetrahedral bridged P4S species. The 473.9-cm-' band occurs only in P4 Ss discharge experiments. The 470.5-cm-' band in the 50% 34Sexperiment is twice as intense as the 473.9-cm-I band, suggesting a triplet absorption with the pure % component under P4. Since P4S3absorbs strongly in the region, the 473.9-cm-' band is tentatively assigned to P4S2 with two bridged sulfur atoms. Semiempirical SCF-MO calculations suggest the structure with adjacent P-S-P edges is more stable than that with opposite in such a structure the sulfur motions couple and reveal a mixed isotopic component. Unfortunately, a weak band was also observed at 473.7 cm-' in sulfur discharge experiments,I5and although the present 473.9-cm-I band is much stronger, a sulfur bearing impurity cannot be ruled out. Mechanisms. The simple phosphorus sulfides identified here, namely, PS, P2S, and PS2, were produced in the microwave discharge of P4 and S8 in argon and trapped in a solid argon matrix. Photolysis increased the yields of several products. Photoexcitation of P2 adjacent to sulfur atoms should initiate reaction to form more P2S. Visible photolysis of discharged P4 samples increased P3 owing to diffusion and reaction of P atoms,21 and in the present experiments the P S2 reaction is likely to occur with P atoms dissociated from larger phosphorus clusters. On the other hand, the S3 and S4 speciesI5 can be expected to photodissociate to give S atoms and S2 molecules for further reaction with P atoms in the matrix produced in the discharge of P4, In addition, photodetachment of PS2-contributes to the photochemical yield of PS,.
+
+
(29) McCluskey, M.; Andrews, L. J . Phys. Chem., in press. (30) Lohr, L. L. J . Phys. Chem. 1990, 94, 4832. (31) Glidewell, C. fnorg. Chim. Acta 1984, 81, 187.
Secondary reactions during matrix condensation led to the formation of other products: PS2 PSZ SPSPSZ
+
P2S + P2
-
cyclic PIS
Also notable is the reaction of P2 from the discharge with S atoms produced by photodissociation of OCS on the matrix surface P, + s P2S
-
Annealing the samples to 32 f 2 K allowed for the diffusion and further reaction of trapped sulfur atoms as attested by the observed blue chemiluminescence. Other S atom reactions on annealing markedly reduced PS, PS,, and P2S absorptions and increased SPSPS2.
ConcIusi ons Microwave discharge of P, and S8 mixtures in excess argon produced three simple primary reaction products (PS, P2S, and PS,) that were isolated in solid argon for infrared spectroscopic study. These species were also prepared from other phosphorus and sulfur compounds. The use of enriched S-34 provided isotopic shifts to characterize the normal mode and multiplets to define the sulfur stoichiometry. Photolysis and annealing behavior helped identify the primary and several secondary reaction products including PS,, P4S,and SPSPS2. Sulfur atoms are clearly less reactive than oxygen atoms, but many of the same phosphorus species are produced. Acknowledgment. We gratefully acknowledge financial support from NSF Grant C H E 88-20764 and ab initio calculations performed by M. McCluskey.
Infrared Spectra and Structures of Isotopically Enriched S3 and S4 in Solid Argon G. Dana Brabson, Zofia Mielke,+ and Lester Andrews* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: June 12, 1990)
Sulfur vapor was seeded into a microwave-poweredargon discharge and condensed at 12 K. Infrared spectra revealed sharp strong bands at 680.0, 676.2, 661.6, and 642.4 cm-I, which were also observed by thermal dissociation of Ss flowing in a quartz tube heated to 400-900 O C . Increasing the sulfur concentration in the discharge favored the latter two relative to the former two absorptions. Enriched 34Ssamples gave isotopic shifts for pure sulfur fundamentals and isotopic multiplets that identify these species. Sample annealing produced a strong S2chemiluminescence and a sharp new 674.5-cm-l band. This feature revealed a sharp 1 / 2 / l / l / 2 / l sextet with 50/50 32S/34S, which confirms the observation of C, thiozone; similar multiplets were found for the 680.0- and 676.2-cm-' bands. Calculations from four pairs of symmetrical isotopic u j values gave I I6 f 2' for the valence angle. The 66 1.6- and 642.4-cm-I absorptions behaved differently in the experiments and revealed isotopic multiplets appropriate for S;, these absorptions are assigned to two different open-chain S4 isomers. Weaker bands at 720.5 and 710.1 cm-I, which appeared on annealing, are assigned to ( S J 2 dimer species.
Introduction Sulfur chemistry is important because of its relevance to acid rain and pollution problems and the increasing use of sulfur in organic and inorganic synthesis. For three decades, the photolysis of small molecules such as hydrogen sulfide, H2S, and carbonyl sulfide, OCS, has served as a convenient technique for the generation of sulfur atoms, which then participate in reactions with a variety of organic and inorganic substrates.'g2 The stable and transient products of these reactions have been trapped and studied in frozen inert gas Because of their inherent limitations, electric discharges have been used much less commonly as a source of sulfur atoms.9 Nevertheless, diffusion flames employing discharged sulfur as one reagent have been employed in a series of phosphorus-sulfur University of Wroclaw, Wroclaw, Poland.
0022-3654/9 1/2095-0079$02.50/0
experiments in order to isolate reaction products in solid argon for study by FTIR spectroscopy.I0 Since pure sulfur molecular species are formed in the discharge and/or by reactions in the matrix, IR spectra of the products of discharged sulfur were explored in some detail so that bands due to pure sulfur species ( 1 ) Meyer, B. Chem. Rev. 1976, 76, 361. (2) Gunning, H. E. In Elemental Sulfur; Meyer, B., Ed.; Interscience: New York, 1965, p 265. (3) Long, S . R.; Pimentel, G. C. J . Chem. Phys. 1977, 66, 2219. (4) Smardzewski, R. R. J . Chem. Phys. 1978, 68, 2878. (5) Wight, C. A,; Andrews, L. J . Mol. Spectrosc. 1978, 72, 342. (6) Lee, Y.-P.; Pimentel, G . C. J . Chem. Phys. 1979, 70, 692. (7) Hawkins, M.; Downs, A. J. J . Phys. Chem. 1984, 88, 3042. (8) Hawkins, M.: Downs. A. J. J . Phys. Chem. 1985, 89, 3326. (9) Meyer, B. J . Chem. Phys. 1962, 37, 1577. (10) Mielke, Z . ; Brabson, G.D.; Andrews, L. J . Phys. Chem., preceding paper in this issue.
0 199 1 American Chemical Society
