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Nuclear Magnetic Resonance of Hydrogen Polysulfides in Molten Sulfur by J. B. Hyne, E. Muller, Fundamental Sulfur Research Group, Alberta Sulphur Research Ltd., University of Calgary, Alberta, Canada
and T. K. Wiewiorowski Freeport Sulphur Company, Belle Chasse, Louisiana (Received M a y 9, 1966)
I n a recent paper, Wiewiorowski and Touro' report that the infrared spectrum of hydrogen sulfide in molten sulfur exhibits two bands in the S-H stretching region. The band a t 2570 cm-I is assigned to hydrogen monosulfide and that a t 2498 em-: to hydrogen polysulfides. The presence of hydrogen polysulfide in the system is interpreted in terms of an equilibrium between HzS, and HzSspecies in the molten sulfur. Schmidbaur and co-workers2 have reported that mixtures of hydrogen polysulfides show distinct nmr peaks for the terminal hydrogen atoms of hydrogen polysulfides up to sulfur atom chain lengths of five; for HzSa species and higher sulfanes a single unresolvable nmr peak is found. This behavior is somewhat similar to that previously reported by Grant and Van Wazers for dimethyl polysulfides. In this work the nmr technique has been applied as an analytical tool in the study of the nature of the molecular species present in solutions of HZS in molten sulfur. We report confirmatory evidence for the presence of hydrogen polysulfides in H2S-molten sulfur solutions as suggested by Wiewiorowski and Touro on the basis of the observation of two distinct infrared S-H bands.
Experimental Section and Results
The Nmr Spectrum of Hydrogen Polysulfide Mixtures in CS,. Hydrogen polysulfide mixtures can be readily made by a number of methods, the distribution of specific members of the homologous series being dependent on both the method used and the precise conditions of reagent concentration and temperature employed in each method. The manner in which these various factors affect the sulfane distribution in the mixture will be the subject of a subsequent communication. For the purposes of this study, however, any method yielding a representative mixture of sulfanes provides a sample permitting the establishment of the
nmr spectral responses of each component of the mixture. The nmr spectrum shown in Figure l a is that of a 5% solution in CSZ of a polysulfide mixture generated by treatment of an aqueous solution of sodium sulfide saturated with sulfur (formally Na&L7)with 6 N hydrochloric acid a t -10". The polysulfide mixture separates as a viscous oil and can be easily removed, redissolved, and dried in CSZ. The multiplicity of the nmr spectrum is immediately apparent and the assignment of the nmr peaks to the various sulfanes can be made as was reported by Schmidbaur, et aL2 In this particular sample no HzS or HzSz was present, but the positions of these nmr responses are known from other samples and are indicated in Figure l a and Table I.
Table I : Hydrogen Chemical Shifts of Polysulfides --Chemical In liquid Species
HzS HnSz
HZ85 HzS, a
(Z
2 6)
Shift In
change,
sulfur'
CSa'
ppm
8.74 6.93
9.10 7.16 5.80 5.78 5.65 5.56
0.36 0.23
...
Hi& HZ84
shift, ppm-
5.59 5.46 5.37
... 0.19 0.19 0.19
TMS external = 10.
The Nmr Spectrum of Hydrogen Suljde in Molten Sulfur. I n order to apply the nmr technique to establish the presence of hydrogen polysulfides in the hydrogen sulfide-molten sulfur system, a standard nmr tube containing molten sulfur was maintained a t 180" for 24 hr while gaseous hydrogen sulfide was bubbled through the sulfur. The sample tube was then removed from the thermostat and rapidly transferred to the probe of a Varian A60 nmr spectrometer which had been preheated and tuned at 130". The spectrum obtained is shown in Figure lb. While the concentration of sulfane species was admittedly low, two nmr peaks are clearly present, at 8.74 and 5.37 ppm (1) T.K. Wiewiorowski and F. J. Touro, J . Phys. Chem., 70, 234 (1966). (2) H. Schmidbaur, M. Schmidt, and W. Siebert, Chem. Ber., 97, 3374 (1964). (3) D. Grant and J. R. Van Wazer, J. A m . Chem. Soc., 86, 3012 ( 1964).
Volume 70, Number 11 November 1966
3734
NOTES
CSZ. The nearest corresponding peak in CS2 is that for HZSZat 7.16 ppm. It would therefore appear likely that HzSZ,while absent from the original polysulfide mixture, is generated in situ when the mixture is dissolved in molten sulfur. Since the evolution of HzS was amply apparent and its concentration in the molten sulfur necessarily low, the spectrum was not run to the high-field value of HzS. The sample was maintained at 130" for 100 min and the nmr spectrum obtained is shown in Figure Id. It is apparent that the HzSz,HZS4, and HzS6peaks have decreased markedly in favor of the enhancement of the HzS, (z 6) peak.
4 '
>
Discussion
Figure 1. Nmr spectra of hydrogen polysulfides: (a) hydrogen polysulfides from NatSl.7 in CSZ; (b) hydrogen polysulfides formed by €I& S(l) in molten sulfur; (e) hydrogen polysulfides from NatS4.T added to molten sulfur, after 3 min a t 130'; (d) same as (e) but after 100 min at 130". Note: the signal for each single species is indicated by the number of the sulfur atoms.
+
relative to TMS (external). These peaks should be compared with the 9.10- and 5.56-ppm peaks for HzS and HzS, (z 6) species, respectively, in CSz (see Figure la). The apparent downfield shift of both peaks in molten sulfur compared with CSz is to be expected as a result of the solvent change and the fact that the reference TPIS is external in both cases. The Nmr Spectrum of Hydrogen PolysuljZe Mixture in Molten Sulfur. Having a hydrogen polysulfide mixture of established composition (Figure la) available, it was clearly important to determine the nmr spectrum of this known mixture in molten sulfur. Upon addition of the polysulfide mixture to molten sulfur at 130" in the nmr tube, there was an immediate evolution of H2S gas. The nmr spectrum taken after 3 min had elapsed from dissolution is shown in Figure IC. Three peaks, corresponding to HzS4 (5.59 ppm), H2S5(5.46 ppm), and H2S, (z 6) (5.37 ppm) are apparent. The remnants of an HzS3 peak may be obscured in the high-field shoulder of the H2S4 peak. I n addition, however, a peak a t 6.93 ppm has appeared. Since the H2S4, HzSs, and HzS, peaks are all 0.19 ppm downfield from their observed positions in 5y0 solution in CS2, it is reasonable to assume that the 6.93-ppm peak is also 0.19 ppm downfield from the corresponding peak in CS2, Le., a peak a t 7.12 ppm in
>
>
T h e Journal of Physical Chemistry
The combination of the three nmr experiments described above permits assignment of the various polysulfide peaks in the nmr spectrum in molten sulfur. These assignments are shown in Table I and are compared with the assignment in CS,. The downfield chemical shift of the proton resonance with increasing sulfur chain lengths is an expected consequence of the accompanying increase in the effectiveness of electron withdrawal by sulfur. This pronounced difference in the diamagnetic shielding of the protons in HzS and HzS, is also evidenced by the differences in the infrared absorption coefficients of the S-H group in HZS and H2S,. As previously reported,' the hydrogen polysulfide is characterized by a higher absorption coefficient, and consequently the S-H band in this molecule can be considered as being more polar than in hydrogen monosulfide. Also shown in Table I are the differences in the shifts for the two solvents. It is apparent that the solvent change (CS2to molten sulfur) has a larger effect on the shorter chain sulfanes than on the higher members of the homologous series. If the effect of the solvent change was simply one of bulk susceptibility, constancy in the shift changes might be expected. In the shorter chain sulfanes, however, the change from CS, to molten sulfur environments might be expected to bring about a more dramatic change in nmr response since the protons in the longer chain sulfanes are already in a high sulfur environment within the molecule itself. Despite the lack of constancy in the shift changes in the lower members of the series, however, there can be little doubt that the assignments in liquid sulfur, based on that in CSz, are correct. It would be most surprising if the order of the assignments in such a simple system changed with solvent change. The following points, therefore, appear to be indicated by the results. 1. The dissolution of HzS in molten sulfur leads to
NOTES
the formation of hydrogen polysulfides. Furthermore, it appears that the polysulfides so obtained are mainly, if not exclusively, of the higher members of the homologous series, H2S6and above. A thermodynamic treatment of the chemical equilibria involved indicates that the number-average chain length of hydrogen polysulfides in the system is about 27 sulfur atoms a t 127" and increases with rising temperature. 3. Shorter chain H2S, species (z = 2-5) may be formed as transient intermediates in the formation of H2S, species on dissolution of HZS in molten sulfur. 3. I t has been demonstrated that molten sulfur ran serve as a useful solvent in nmr spectroscopy as well as infrared spectroscopy. Consequently, nmr should facilitate the direct study of numerous organic and inorganir reactions which can take place in liquid sulfur and thus provide a better insight into the chemistry of molten bulfur. (4) T . K. Wiewiorowski, 11. F. >fatson, and C. T. Hodges, Anal C'hem., 37, 1080 (1965).
Dissociation of Palladium Oxide] by Wayne E. Bell, R. E:. Inyard, and XI. Tagami General Atomic Diiisicn of General Dynamics Corporation, J o h n J a y Hopkins Laboratory for Pure and Applied Science, S a n Diego, Califorma (Re& ed .May 11, 1966)
Wohler2 and Schenck and K ~ r z e n ,using ~ static methods, measured the dissociation pressure of PdO(s) in the range 680 to 875". Data of the two investigations are in agreement arid yield the value of about -26 kcaljmole (at 298°K) for the heat of formation of PdO(s). This value does not agree with the -20.4 kcaljmole obtained by Rohler and Jockum4 using a calorinietrir method. Wohler and also Schenck and Kurzen noted a dependence of dissociation pressure on oxygen content of the solid oxide. Wohler attributed this behavior to appreciable solubility of P d in PdO, whereas Schenck and Iiurzen attributed the behavior to impurities in the palladium metal. The present investigation was undertaken (1) to determine the composition range of PdO(s) a t temperatures around 800", (2) to measure the dissociation pressure of the compound using both static and dynamic methods. and ( 3 ) to derive thermodynamic values from the pressure data.
Experimental Section The coniposition (O,/Pd ratio) of PdO was measured
3735
as a function of oxygen pressure using apparatus consisting of a small, closed-end quartz reaction tube mounted in a furnace and connected to a mercury manometer. Means were provided for evacuating the system and pressurizing it with oxygen. I n conducting the experiments, a sample of PdO(s) was placed in the reaction tube, which was sealed in place. The system was evacuated, oxygen was added to a desired pressure, the system was sealed off, and the reaction tube was heated to temperature. Incremental quantities of oxygen were then removed through a vacuum-type valve. Sufficient time (more than 6 hr) was allowed between increments for the pressure to stabilize. The oxygen content of the oxide sample was determined during the course of the experiment by using a material balance calculation. The data used in the calculation were (1) the initial weight and oxygen content of the sample, (2) the quantity of oxygen in the reaction system (exclusive of that in the oxide sample) initially and a t the end of each increment, and (3) the amount of oxygen removed from the system in each increment. The quantity of oxygen in the reaction system was determined from pressure-volume measurements using appropriate temperature corrections. The quantity of oxygen removed in each increment was measured by collecting the oxygen over mercury in an evacuated bulb of known volume and measuring the pressure in the bulb. Dissociation pressures were measured using a static method and a transpiration method essentially as described earliere5 I n the static method, the oxide sample was contained in a dead-end quartz reaction tube and oxygen pressures were read on a mercury manometer. A small sulfuric acid manometer showed when the pressure in the mercury manometer and the pressure in the reaction tube were equal. In the transpiration method, helium carrier gas flowed over an oxide sample contained in a 7-mm i d . quartz tube. Diffusion barriers were located on each side of the sample. The effluent helium-oxygen mixture was analyzed by use of a gas chromatograph. Dissociation pressures were independent of flow rate a t the flow rates used (around 1.5 ml (STP)/min). The metal used was palladium sponge (99.995% purity, Johnson-Matthey). The solid oxide (PdO) used (1) This work was supported in part by the U. S. Atomic Energy Commission under Contract AT(04-3)-164. (2) L. Wohler, 2. Elektrochem., 11, 836 (1905). (3) R. Schenck and F. Kursen, 2. Anorg. Allgem. Chem., 220, 97 (1934).
(4) L. Wohler and N. Jockum, 2. Phusik. C h m . , A167, 169 (1933). (5) W. E. Bell, M.C. Garrison, and U. AIerten, J . Phys. Chem., 64, 145 (1960).
Volume 70, Number 11
Soaember 1906
