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Disulfur Monoxide. 111. Its Infrared Spectrum and Thermodynamic Functions
by Uldis Blukis and Rollie J. Myers Inorganic Materials Research Division, Lawrence Radiation Laboratory, and Department of Chemistry, College of Chemistry, University of California, Berkeley, California (Received September 21, 1964)
A study of the infrared spectrum of frozen films of S20has led to a complete infrared assignment including the bending frequency a t 388 f 2 cm.-I. The thermodynamic functions ( P o - H o o ) / Tand (H" - H " o ) / T for gaseous S20are tabulated for 273.15-300O0K, Estimates of the standard enthalpy of formation for gaseous S20are given, and its thermodynamic stability is discussed.
Introduction Now that disulfur monoxide, S20,is a well-established molecular species, its chemical and physical properties can be studied. Current work is being done on its chemical reactions.2 This paper is concerned with its thermodynamic properties and an assignment of its vibration spectrum. A previous determination of the standard enthalpy of formation of SzO has been obtained from ionization potentials,S but, since this is only an approximate minimum value, other possible values will be discussed.
Experimental The samples of SzO, containing about 50% S02, were prepared by the usual discharge method.' For our infrared studies the samples were sprayed on a salt window cooled to 77°K. in a low temperature cell.' To study the decomposition of SzO, several spectra were taken after the window had been allowed to warm to 100 and to 28OoK., but the spectra were always taken after the window was cooled back to 77'K. These spectra were taken several years ago on a modified PerkinElmer Model 12C single-beam instrument utilizing either KBr or CsI optics. The wave length scale in the CsI region was determined from the water bands that appeared in the background of the single-beam instrument.
Infrared Results I n the region 600-1400 cm.-l two bands were observed in the frozen films which could be ascribed to SzO. The frequencies of these bands were identical, within our resolution, to the 679 and 1165 cm.-' found by ,Jones5 in the gas phase. The 1165-cm.-' band is The Journal of Phgsical Chemistry
close to an SO2 absorption,6and the two could never be entirely resolved. The 679-cm. -l absorption is completely free of SO, absorption. When the salt window was warmed to 100°K., the S20 bands a t 1165 and 679 cm.-' decreased greatly. Afrer warming to 28OOK. the 679-cm.-' band could not be observed, but an absorption of about one-third the original intensity remained at 1130 em.-'. These observations are consistent with diffusion of S20in an SO2matrix a t 100°K. followed by decomposition into SO2 and sulfur. Since the band near 1100 cni.-' can be ascribed to an S-0 stretch, it is clear, as observed by Schenk,' that some oxygen remains bonded to the produced sulfur a t temperatures as high as 280°K. Since SzO is a bent triatomic molecule, it should have one bending and two stretching frequencies. The 1165 and 679 cm.-l can be readily assigned to the two stretches, and the bend would nornialIy be found at a lower frequency. A measurement of microwave satellite intensitiesIb gave a value of 370 f 30 cm.-l for the lowest frequency vibration for S20,and the spectroscopic temperature coefficient measurements8 indicate that there is a frequency of about 450 cm.-'. In the (1) (a) D. J. Meschi and R. 3. Myers, J . A m . Chem. Soc., 78, 6220 (1956); (b) J . Mol. Spectry., 3, 405 (1959). (2) P. W. Schenk and R. Steudel, Angew. Chem., 7 5 , 793 (1963); P. W. Schenk and W. Holst, 2. anOTg. allgem. Chem., 319, 337 (1963). (3) R. Hagemann, Compt. rend., 255, 1102 (1962).
(4) E. D. Becker and G. C. Pimentel, J . Chem. Phys., 25, 224 (1956). (5) A. V. Jones, ibid., 18, 1263 (1950). (6) R. N. Wiener and E. R. Nixon, ibid., 25, 175 (1956); P. A . Gigukre and M .Falk, Can. J . Chem., 34, 1833 (1956). (7) P. W. Schenk, Z. anorg. allgem. chem., 248, 297 (1941). (8) E. Kondrat'eva and V. Kondrat'ev, Zh. Fit. Khim., 14, 1528 (1940).
DISULFURMONOXIDE.INFRARED SPECTRUM A N D THERMODYNAMIC FUXCTIONS
range 300-450 cm.-’ a single SzO band was found a t 388 f 2 cm.-l in a frozen film a t 77OK This band had the same warm-up characteristics as did the 679-cm.-‘ absorption. The bending frequency in the gas phase would be expected to differ from 388 cm.-’ by less than 10 cm.-’.
Thermodynamic Functions On the basis of the infrared measurements in this paper and the previous we know, without doubt, all three fundamental vibrational frequencies for SzO. The microwave worklb has also supplied the rotational constants and the symmetry (G). The thermodynamic functions for gaseous SzO on the basis of a rigid-rotor, harmonic-oscillator model were calculated from these data, and they are listed in Table I.
Table I : Thermodynamic Functions for S 2 0 a -(Fo - H ” a ) / T
T
273,15 298.15 400
53.78 54,56 57.25 59.41 61,26 62.87 64.31 65,61 66,80 67.88 68.91 69.85 70.72 71.56 75.09 77.90 80.24
500 600 700 800 900 1000 1100 1200 1300 1400 1500 2000 2500 3000
(HO
- H”o)/T
8.78 8.92 9.45 9.91 10.30 10.64 10.92 11.16 11.37 11.55 11.70 11.84 11.97 12.08 12.48 12.74 12,92
a In cal./(mole OK.) calculated using Z J J 0 = 618.14 X 1O-l” g . 3 hf(S,O) = 80.12 g./mole, P = 1 a t m . , v1 679 cm.-l, v 2 388 em.-’, a n d V I 1165 cm.-l.
Thermodynamic Stability S20,called LLSUlfUr 1iIonoxide” or L L S z 0 2in ” the older literature, has been produced in a variety of nonit is present in S~S“1’1S’’a3S significant amounts in t’hose high temperature oxygensulfur equilibria which are rich in sulfur. F~~a study of the thermodynamic stabilit’y of 820 in these and systen-,sthe standard enthalpy of forlnatiorl AH’, [2S(rh) ’/ZOz(g) -.t SzO(g),a t O O K . ] is needed. In the absence of a direct measurement an estimated AH’, would be useful. An approximate value for A H o , can be based on bond
+
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angles in S p , SO, S20, and SO, are siniilar.1b.10 Furthermore, the bond dissociation energies Do(S-0) and Do(OS-O) are nearly equal. l 1 These observations imply that Do(S-S) and D o ( S - 0 ) niay be used to calculate AHoo for SzO. With these assumptions AH’, = - 34 kcal./mole is obtained. From nieasurenients of ionization potentials, ultraviolet spectruni, and equilibria in the sulfur-oxygen systeni experimental lower limits for AH’, can be established. These limits indicate that SzO is less stable than is indicated by the bond energy calculation. Hagemann3 has measured the appearance potentials of SO+ from both SO, and SZO. His nieasurenients give a value of 2.0 f 0.4 e.v. for the energy required to produce SzO 0 from SO2 S. This value gives AHo, = -17 f 9 kcal./mole. Hageniann implies in his paper that this value is a lower limit for AH’,. In his work on the ultraviolet absorption spectrum of S20 Jones5 noted a well-defined predissociation limit a t 3153.8 A. in a progression which starts from the ground vibrational state. This value corresponds to an energy for photodissociation of 90.5 kcal.,/niole. If Sz 0 were the products of this dissociation. then a o S20would be - 1 kcal ’mole. This minimum M o for S value seems to be somewhat too high. If SO were the products, then a minimum AH’, would be - 23 kcal./mole. These are niininiuni values because of the unknown amount of excess energy carried off by the products. Excess energy in the form of electronic excitation is unlikely because the first excited states of both the S atom and SO are sufficiently high12 to give a value for AHooof about 0 kcal./mole. Photodissociation studies of SOZl3and NO2I4 have shown that the excess rotational and vibrational energy of the products can be small. The most complete study of sulfur-oxygen vapor in equilibrium has been done by Dewing and RichardThey used silver beads in equilibrium with the
+
+
+
+
(9) R. G. W. Norrish and G. A. Oldershaw, Proc. Roy.SOC.(London), A249, 498 (1959); R. G. W. Norrish and A . P. Zeelenberg, ibid., A240, 293 (1957); A. L. Myerson, F. R. Taylor, and P. L. Hanst, J . Chem. Phys., 26, 1309 (1957). p, A , J , P h y s . Chem,, 64, 190 (1960), (11) All the thermodynamic data are taken from Table 1 of this paper and Appendix 7 of G. N. Lewis, M . Randall, K. S. Pitrer, and L. Brewer, “Thermodynamics,” McGraw-Hill Book Co., Inc., New York, N. Y . , 1961. There is some uncertainty about the values of 101 and 123.5 kcal./mole used for the dissociation energies of s2and so, respectively. See R.Colin, P. Goldfinger, and M .Jeunehomme, Nature, 187, 408 (1960); also see W. D. 1leGrath and J. F. McGarrey, J . Chem. P ~ w . 37, , 1574 (1962). (12) The ’4state of SO is not known, but its energy should be similar t o that found in 02. (13) P. Warnek, F. F. Marmo, and J. 0. Sullivan, J . Chem. Phys.. 40. 1132 (1964).
Volume 69, .Vumber 4
A p r i l 1965
ULDISBLUKISAND ROLLIEJ. MYERS
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vapor a t 1000-1500’1
