The visible spectrum of S3 and S4

JOURNAL

OF MOLECULIR

SPECTROSCOPY

42, 335-343

(1972)

The Visible Spectrum of Ss and Sq B. MEYER, Department

of

T. STROYER-HANSEN,

Chemistry,

University

of

AND

Washington,

T. V. OOMMEN Seattle,

Washington

98196

A vibrational analysis of the long-known, but unanalyzed and inconclusively assigned absorption around 400 nm in sulfur vapor at 450°C and 20 Torr, with pure isotopes a% and 3% yields To.0 = 23,465 cm-l. All 34 vibrational band heads fit the progression of Y’ = 420 cm-1 and Y” = 590 cm-l. At high dispersion, doublet band heads with unresolved rotational structure can be recognized. Another long-known absorption system around 530 nm reveals neither vibrational nor rotational structure. Photolysis experiments with a 1F3 M solution of chlorosulfanes in an isopentane-methylcyclohexane glass at 77°K and in a krypton matrix at 20°K show that S&L% yields a new species characterized by a progression of bands with Y’ = 400 cm-l, and To,0 = 23,400 f 100 cm-1 in the glass, while photolysis of S&l2 at 77°K yields a species with continuum absorption at 530 nm. Annealing experiments with & in a krypton matrix at 20°K also yield the 530 nm absorption. The correlation of the photolysis reactions and spectra indicates that the 400 nm system is due to S3 while the 530 nm is due to Sq . We discovered that the characteristic absorption of S3 and S* is also found in hot liquid sulfur, in trapped liquid sulfur, and in trapped sulfur vapor. INTRODUCTION

At low temperature, sulfur vapor contains mainly Ss which absorbs below 300 nm. At high temperature and low pressure it contains mainly Ss which absorbs between 240 and 700 nm, due to the Schumann-Runge transition kg-B + temperature and pressure, 3~,-X, with an origin at 31,835 cm-‘. At intermediate two other absorption systems have been reported; one with vibrational structure around 400 nm, and a continuum around 530 nm. Graham (1) was the first to report the absorption at 400 nm. This spectrum was later confirmed by Dobbie and Fox (2) and by Rosen (3). D’Or (4) d e t ermined that this system was very strong in saturated vapor at 560°C. By comparing the intensity with vaporpressure measurement by Preuner and BrockmGller (5)) d’Or concluded that the spectrum could not be due to Sz or Se , as had earlier been assumed, but was probably due to S3 or S4 . This conclusion was later confirmed by studies by Barrow (6), Wieland (7)) and others (8)) but no analysis or conclusive assignment of the spectrum was made. The continuum at 530 nm was studied by Braune and Steinbacher (9) who compared the absorption strength of this system with the vapor-pressure data 335 Copyright 0 1972by AcademicPress, Inc.

336

MEYER,

STROYER-HANSEN,

AND

OOMMEN

and concluded that this system was due to Sq . The authors did not study the 400 nm system. Mass spectroscopic work (10, 11) confirmed that Ss and Sq are present in the vapor. However, the concentration of the two species was found to be so similar that assignment of the vapor composition to the spectra was not feasible. This paper presents results from four types of experiments; (a) gas-phase experiments using ““S and pure “S in medium and high dispersion, (b) photolysis of chlorosulfanes in frozen glasses, (c) photolysis of chlorosulfanes in rare gas matrices, and (d) synthesis of Sd from SZ in matrices. The combination of evidence allows determination of the origin, vibrational analysis, and assignment of the 400 nm system to Ss and assignment of the 530 nm system to Sq . EXPERIMENTAL

High purity sulfur (99.9%) was donated by the Freeport Sulphur Co., and 99% pure 34S was from Oak Ridge National Laboratory. Rare gases, isopentane and methyl-cyclohexane were spectroscopic grade or high purity. The chlorosulfanes SsCIZ and S&l, were prepared from SC12 or &Cl, and H&S according to the procedure described by Feher (12). S&l2 and &Cl2 were 99 % pure, and, due to reaction paths of this synthesis, were not contaminated with each other. Gas-phase experiments were performed in quartz cells with 3, 35, and 1000 mm path length. The cells were equipped with heated windows, and the optical path and the sidearm were fitted with independent heaters. Thus, the sample cell temperature could be changed independently of the vapor pressure. Low-temperature samples were made by freezing a 1O-3 M solution of S3C12 and S&l2 in (iPC) solution in a quartz Dewar with a a 1:2 isopentane-methylcyclohexane built-in optical cell of 20 mm diameter and 40 mm length. In one experiment, a matrix sample of Sa was prepared by condensing a molecular beam of krypton saturated with vapor of S&12 at -2O”C, with following photolysis of the matrix. Matrix samples of Sq were prepared by annealing S2 trapped in krypton at 30°K for 10 or more minutes. Sz preparation and other matrix techniques have been reported earlier in detail (IS). Isotope and low-temperature spectra were recorded on a Cary 14-H spe.ctrograph and a Jarell-Ash f6.3 spectrometer with gratings blazed for 300 and 500 nm, yielding a dispersion of 0.5 or 1.0 nm per mm, respectively. High dispersion spectra were recorded on a 21 ft concave spectrograph. The light source was a tungsten-iodine lamp. Photolysis of the chlorosulfanes was achieved with light from a General Electric AH-6 2kw high-pressure mercury lamp, filtered through water. RESULTS

AND

DISCUSSION

Gas Xpectrum The gas-phase absorption between 360 nm and 500 nm of pure 34S and 3’S vapor is shown in Fig. 1. It consists of a long progression of red degraded bands

VISIBLE

SPECTRUM

337

OF Sz AND Sa

with a Frank-Condon maximum at about 395 nm. The isotope shifts of 34Sand 32S,Fig. 2, indicate that the origin is at 23,465 f 15 cm-‘. This allows tentative vibrational assignment as indicated in Table I. The band spacing is not always even. This is partly because band heads are not well resolved, partly due to the doublet structure of the heads, and partly due to the fact that the rotational structure, at least around the origin, is unresolved, because the Doppler width is larger than the line spacing. All strong bands fit into a progression with V’ = 420 cm-’ and yN = 590 cm-‘. Comparison with ground-state frequencies of the three vibrational modes of the isoelectronic triatomic molecules 03, S02, and E&O(l/i), make us believe that this frequency is one of the stretching vibrations, probably the symmetric stretching vibration, of the molecule. No other vibrational modes were observed.

Top=

400

23 465

450

cm-’

nm

500

FIG. 1. The absorption spectrum of %a and a4Ss.

5c

i

c

-5c

20 000

FIG. 2. Isotope shift of %

23 000 cm -’

Z6C

, 32S3as a function of the frequency of absorption bands.

335

MEYER,

STROYER-HANSEN,

AND

OOMMEN

,_

The 530 nm system, which causes sulfur vapor to appear deep red, is, even in high dispersion, a continuum. Idmtity of species. An extensive study of the influence of temperature and pressure variation on the absorption system at 400 nm and 530 nm, between pressures from 0.1 Torr to 2 atm and a temperature of 300°C to 85O”C, confirmed the excellent data of Wieland (1) and earlier workers, and indicat.es that S3and S4are equally likely absorbers. The vapor composition in this temperature and pressure range has been studied by direct pressure observation (5) and by mass spectroscopy, and discussed at length by other authors (10, 11) . Table II summarizes the optical density of the 400 nm absorption, and the relative abundance of the absorbing species to the total sulfur pressure as a function of trmperature and vapor pressure. Table III contains the same data for the 530 nm bands. For SI , the highest mole fraction is observed between 350°C and 2 Torr and 450°C and 50 Torr. Below 45O”C, the highest mole fraction is formed at the

VISIBLE

0.00

SPECTRUM

339

OF Ss AND Sa

0.01

0.01

0.01

0.03

0.01

0.01

0.05

0.09

0.14

0.20

0

0.03

0.08

0.25

0.60

1.20

R

0.06

0.08

0.13

0.17

0.20

D

0.00

300 R 0

0.02

0.03

I

700

lowest measured vapor pressure, i.e., Sg and large molecules are prevalent. Above 5OO”C, the mole fraction goes through a maximum with increasing pressure for increasing temperature, i.e., Sa is limited by dissociation of S3 to Sz . For ~‘34, the same trend is found, and the maximum mole fraction is formed at 400°C and 10 Torr. The optical density of Sh is always much weaker than that of Ss . A comparison of our absorption data with the vapor-pressure data of Preuner (5) gives a molar extinction coefficient of E = 1 X 10’ for ~‘34 and E = lo6 l/mole.cm for Sz. The color of the vapor changes in the above studied range from light green to dark red. There is no direct relationship between the color and the relative

340

MEYER,

STROYER-HANSEN,

AND OOMMEN

abundance of Ss and Sd , because the total vapor pressure changes far quicker than the relative abundance. As mentioned above, the assignment of the band by earlier authors to Sa or SJ is quite convincing, but Table II and Table III show that a choice between the two is difficult, especially since both spectra are superimposed on a weak background of SS absorption. However, it seems that the intensity of the 530 nm system is more sensitive to pressure and temperature change than that at 400 nm. If true, this would allow the conclusion that they belong to different species.

VISIBLE

SPECTRUM

OF S3 AND

SA

341

However, since the structure and spectra of Sa and Sk are net a priori known and the transition energy and vibrational frequency observed are rcason(I@, able for both molecules, the gas-phase spectra arc obviously not suitable for conclusive assignment of the species. Fortunately, we need no longer depend on vapor data, because we have been able to prepare solid samples in which S3 or Sq can be selected as the only possible elemental sulfur species (16). Photolysis of Sdfanes It was observed in an earlier work (17) that five-minute photolysis of SzClz in rare-gas matrices at 20°K by the light of a high-pressure mercury arc yields more than 90 % Sz and no SCl. The kinetics and the infrared absorption indicate that some S&I, remains. The yield of Sz is greatest in xenon, intermediate in krypton and smallest in argon at 20°K. The Sz spectrum obtained this way is identical to that obtained by depositing sulfur vapor from a Knudsen cell (18). Our recent work on photolysis of E&Cl2 and S&l2 (16) showed, likewise, that photolysis with 250400 nm radiation from a medium- or high-pressure mercury arc selectively breaks S-Cl bands. Figure 3a shows the spectrum of S&l2 in krypton before and after photolysis. The vibrational bands form one progression with at least 16 bands with Y’ = 420 cm-’ and we 2, = 6 cm-‘. The origin is too weak in matrices to be observed. The spectrum of a low3 molar solution of S&l2 in a 1:2 isopentane-methylcyclohexane (iPC) glass at 77”k’, shown in Fig. 3b, before and aftrr photolysis for 5 min, shows the same system with a progression of bands with vibrational spacing around 400 cm-‘, but the vibrational

400

500

600

nm

FIG. 3. (a) Sk?12 before and after photolysis; Kr matrix at 20°K. (b) S&l2 before and after photolysis; iPC glass at 77°K. (c) SaCl~ before and after photolysis; iPC glass at 77°K.

342

MEYER,

STROYElt-HANSl5N,

FIG. 4. New unidentified

absorption

AND OOM3ZISN

system

in sulfur.

is not well resolved. The similarity of the absorption syxtcm and especially the vibrational frequency in the gas phase to that in a matrix or in iPC indicates that all three systems contain the same species. The photolysis of S&l2 in iPC yields a very strong continuous absorption at 530 nm, shown in Fig. 3c. We have not studied S&12 in rare-gas matrices because S&l2 partly decomposes during vaporization. The correlation of the photolysis products with that of the gas phase is less informative for the 530 nm system than that of the 400 nm system, since no vibrational structure is observed, but the lack of structure, by itself, and the similarity of the gas-phasr and the lowtemperature spectra are indication enough for us to conclude that the absorpticn is due to Sq. Further confirmation of the assignment to S, was obtaincad b> annealing S, in krypton matrices, which yield only the 530 nm absorption and no bands at 400 nm. Extended Hiickel calculat,ions on sulfur rings and chains, as w-cl1 as sulfancbs (15) have given good agreement between the known spectra of species and tht spectra here assigned to SI and S* . The calculations predict that longer sulfur chains should absorb at larger wavelength. Unfortunately, we have not succeeded in vaporizing large ehlorosulfancs or dissolving them in glasses. Hen-ever, annealing Sz in krypton at 20°K always reveals 8 weak but clearly discernible absorption bands around 625 nm, with a spacing of 300 cnl-’ (Fig. 4). Weak lines aroung 750 nm with a spacing of 450 cm-’ are found in the vapor. It is not certain whether these bands form two systems, and whether they belong to SS and Ss chains, or whether they could be due to forbidden transitions of S, and Sd , or even due to intrinsic impurities. Studies of hot liquid sulfur, trapped liquid sulfur, and trapped sulfur vapor (16) also revealed the absorption systems at 400 nm and 530 nm. Thcsc rxperimcnts explained (8) that the color of hot sulfur is due to Sa and SA , and not, duta to polymeric sulfur which is yellow. structure

ACKNOWLEDGMENT The authors thank Professor Wieland for the use of unpublished Science Foundation and the Air Pollution Control Organization

data, and the National (EPA) for support.. We

VISIBLE SPECTRUM OF SB AND Sa thank Prof. N. Gregory for use of the Cary 14-H instrument which was purchased with a departmental NSF grant. High dispersion spectra were recorded in the spectroscopic laboratory of Professor S. P. Davies, University of California, Berkeley, Physics Department. One of the authors (T. S.-H.) is grateful to Statens Naturvidenskabelige Forskningsrbd, Denmark, for a grant in partial support of his work. RECEIVED:

November

22, 1971 REFERENCES

1. GRAHAM, Proc. Roy. Sot., Ser. A 84, 311 (1910). 2. J. J. DOBBIE AND J. J. Fox, Proc. Roy. Sot., Ser. A 96,484 (1919). S. L. ROSEN, 2. Phys. 48,545 (1928). 4. L. D’OR, Comp. Rend. Acad. Sci. France 201,1026 (1935). 6. G. PREUNER AND I. BROCKM~LLER, 2. Phys. Chem. 81,129 (1913). 6. R. F. BARROW, private communication. 7. K. WIELAND AND B. HUMPERT, unpublished work, 1965. 8. T. V. OOMMEN, Ph.D. thesis, University of Washington, Seattle, 1970. 9. H. BRAUNE AND E. STEINBACHER, 2. Naturjorsch. 7a, 486 (1952). 10. J. BERKOWITZ AND J. R. MARQU.4RT, J. Chem. Phys. 39,275 (1963). 11. D. DETRY, J. DROWBRT, P. GOLDFINGER, H. KELLER, .~NDH. RICHERT, 2. Phys. Chem. 66, 317 (1967). 12. F. FEH~R, K. NANSED, .IND H. WEBER, 2. Anorg. Allg. Chem. 290,303 (1957). American Elsevier Publishers, 13. B. MEYER, “Low Temperature Spectroscopy,” York, 1971. 14. L. F. PHILLIPS, J. J. SMITH, AND B. MEYER, J. Mol. Spectrosc. 29, 230 (1969). 15. K. SPITZER AND B. MEYER, in preparation. 16. B. MEYER, T.V. OOMMEN, AND D. JENSEN, J. Phys. Chem. 76,912 (1971). 17. A. MORELLE, Ph.D. thesis, University of Washington, Seattle, 1971. 18. L. BREWER, G. D. BRABSON, AND B. MEYER, J. Chem. Phys. 42, 1385 (1965).

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