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JACOB SHAMIR AND HERBERT H. HYMAN
it has been found that in polycrystalline and glassy matrices the Fez+-Fe3+ electron exchange reaction occurs a t a rate which shows a continuous change with temperature through the melt,ing point and therefore in the solid must take place even when the ions are separated by about 100 A.17 Although such a process must remain conjectural in the system under study, it is clear t,hat the reactions of nitrate and nitrite ions during radiolysis are more complex than was t'hought,. I n addition to the formation of NO2 by oxidation by
HzO+ and OH, it appears that electron attachment followed by photolysis can achieve the same result and that these processes may well involve migration of charge over considerable distances. Acknowledgment. We are grateful to the Science Research Council and the University of Leeds for financial assistance. (17) D. Baulch, F. S. Dainton, D. Ledward, and H. Sugier, Trans. Faraday soc., in press.
Raman Spectra of Some Organic Solutes in Anhydrous Hydrogen Fluoride'
by Jacob Shamir and Herbert H. Hyman Chemistry Division, Argonne h'ational Laboratory, Argonne, Illinois
(Received March 21, 1966)
The Raman spectra of ethanol, diethyl ether, tetrahydrofuran, and dioxane have been recorded at various concentrations in solution in anhydrous hydrogen fluoride. The influence of an acidic environment is noted and the effect of the dielectric constant on the formation of spectroscopically isolable protonated species is demonstrated. Good correlation is shown between the production of such new species and significant ionic concentrations as measured by electrical conductivity. The absence of a line Raman spectrum attributable to HF2- is shown for solutions containing sodium fluoride.
We have had a continuing interest in the behavior of anhydrous hydrogen fluoride as a proton-transfer ionizing solvent. Optical methods of studying such solutions have been handicapped by the reaction of hydrogen fluoride solutions with silica-containing materials so that there are all too few examples of such observations. However, the availability of quite transparent grades of polychlorotrifluoroethylene and synthetic sapphire makes possible the application, for example, of Raman spectra studies to solutions of simple organic solutes in anhydrous hydrogen fluoride. Such studies are particularly appropriate because hydrogen fluoride itself has no line Raman spectrum, and that of the organic solute may be easily observed. The Raman spectrum technique is also valuable because of its relationship to the symmetry of the whole molecule so that it is often possible to see distinct changes in such a The Journal of Physical Chemistry
spectrum associated with the transfer of a single proton or fluoride ion. The complementary technique, infrared absorption, is instrumentally much more widely available and has been used for similar studies. It suffers, however, from the substantial absorption found for pure hydrogen fluoride, and it is often hard to interpret unequivocally the rather subtle effects associated with simple proton transfer. This particular study involved ethanol, diethyl ether, dioxane, and tetrahydrofuran chosen largely in the attempt to differentiate hydrogen bonding in samples of low dielectric constant from proton transfer and formation of a new molecule ion in systems of high di~~
(1) Based on work performed under the auspices of the U. 5. Atomic
Energy Commission. (2) J. S. Kirby-Smith and E. A. Jones, J . O p t . SOC.Am., 39, 780 (1949).
RAMAN SPECTRA OF SOMEORGANIC SOLUTES IN ANHYDROUS HYDROGEN FLUORIDE
electric constant. Since the process of proton transfer in H F leaves an excess of fluoride ions, a solution of sodium fluoride was studied as a control.
Experimental Section I n general, techniques for the vacuum line manipulation and optical studies with anhydrous hydrogen fluoride have been reported elsewhere, and material reporting the latest refinements developed at Argonne is in preparation. For the research described in this paper, a rather simple Raman cell mas used, fabricated from some 0.75-in. 0.d. polychlorotrifluoroethylene extruded tubing 0.030 in. thick. Split-ring metal compression type of fittings permitted the attachment of a valve fabricated from the same plastic to one end and a sapphire window to the other end of the tubing so that no metal was in contact with the solution. This tube was then used with the 19-mm standard optics for the Cary 81 Raman spectrophotometer. The hydrogen fluoride was distilled as described el~ewhere.~The organic materials were reagent grade chemicals. The diethyl ether and dioxane were dried with lithium aluminum hydride and were distilled, and the absolute alcohol was dried with calcium oxide and was similarly purified. Solutions were prepared in a vacuum line largely fabricated from polychlorotrifluoroethylene, much as described in references cited above. Solutions were made up by weight and transferred directly into the Raman cell connected to the vacuum line through a ' 1 arrangement.
Observations The observations of the Raman spectrum of anhydrous H F have often been obscured by the presence of a broad fluorescent band. When the 4358-A Hg line is employed, as in this work, the maximum is found at a Raman displacement of about 3000 em-'. A number of additives, particularly such reactive halogen fluorides as ErF5 and the noble gas compound XeF4, have been observed to repress this fluorescence. Until very recently, we were not able to establish to what extent this behavior was inherent in hydrogen fluoride and to what extent it mas a function of fluorescent impurity in the reagent. Recently, samples of pure anhydrous hydrogen fluoride have been obtained with very much lower intensities observed for this fluorescent peak, and we now believe that pure anhydrous hydrogen fluoride is essentially free of fluorescence radiation. Most of the batches of hydrogen fluoride used in this series of measurements did have an amreciable fluorescence background, and intensities in general were measured above this background. The observation that there is no identifiable band A
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spectrum in anhydrous hydrogen fluoride is equivalent to suggesting the absence of a reasonable concentration of any single identifiable species. Presumably, the individual hydrogen fluoride fragments are associated and are in reasonably mobile equilibrium. For many organic solutes added to hydrogen fluoride, the most important chemical process is the proton transfer leaving an excess of solvated fluoride ion, e.g., C2HjOH (HF)n -+ C2HjOH2+ (HF)&'-. The possible spectrum of such a solvated fluoride ion can be readily studied by adding sodium fluoride. We did, indeed, look at the Raman spectrum of such solutions and found they 1%-ereindistinguishable from that of pure hydrogen fluoride; that is to say, there were no identifiable bands. We then conclude that the presence of fluoride ions in the system is not associated with the production of any identifiable molecule, although additional infrared ba:ids are observed associated with the expected shifts in hydrogen flaoride bonding. 3c Raman spectrs were recorded for mixtures of anhydrous hydrogen Euoride and diethyl ether, ethyl alcohol, dioxane, and tetrahydrofuran. For diethyl ether and ethyl alcohol, the entire range of conceiitra t'ion was successfully explored. For dioxane, good Raman spectra were observed in the organic-rich end, but with high concentrations of hydrogen fluoride the background was high, and most' weak Raman bands could not be observed. Similarly with tetrahydrofuran, solutions containing more than about 50 mole 70hydrogen fluoride could not be adequately studied, in this case because the solutions were colored (from pale yellow to deep red depending on time and concentration). Complete tables of the observed Raman bands are a ~ a i l a b l e . ~A few of the bands noted in diethyl ether and ethanol solutions are cited in Tables I and I1 to illustrate the correlation of Raman spcctrum and proton transfer.
+
+
Discussion The analysis of the Raman spectrum of a 15-atom molecule (diethyl ether) is complex6and the rather subtle effect of an acidic environment on this spectrum is being considered in detail and will be reported elsewhere. (3) (a) J. J. Katz, and H. H. Hyman, Rev. Sei. Instr., 24, 1066 (1953); (b) R. H. Mayberry. J. J. Katz, and S. Gordon, ibid., 25, 1133 (1954); (c) R. 11. Adams and J. J. Katz, J . Mol. Spectry., 1, 306 (1957): (d) 13. H. Hyman, L. A. Quarterman, hI. Kilpatrick, and J. ,J. Katz, J . Phys. Chem., 65, 123 (1961): (e) H. H. Hyman and J. J. Katz. "Non-Aqueous Solvent Systems," T. C . Waddington, Ed., Academic Press Inc., London, 1965, p 47 (4) L. A. Quarterman, H. H. Hyman, and J. J. Katz, J . P h w Chem.. 61, 912 (1957). (5) Copies may be obtained from H. H. Hyman, Argonne National Laboratory, 9700 South Cass Ave, ilrgonne, Ill. 60439. (6) I-.AIashiko, S i p p o n ~ a zasshi, ~ so, ~593 (1959). k ~
Volume YO, A'timber 10 October 1066
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JACOB SHAMIR AND HERBERT H. HYMAN
M , so that this observation is not surprising. This does not imply the absence of a significant interaction between diethyl ether and hydrogen fluoride. However it will take the form
Table I: Changes in Relative Intensity of Raman Bands Associated with Proton Transfer in Anhydrous Hydrogen Fluoride Solutions of Diethyl Ether Relative intensity" of diethyl ether, mole 7034 20
----Concn
a
7
A"
ion
380 390 442 790 1000 1155
14
5
N.O.b
N.O.
K.0.
N.O.
113
47
N.O. S.O.
N.O. N.O.
28 12 12 16
36 9 26 24
59
19
N.O.
N.O.
I a t AV = 1460 taken as 100.
N.O.
=
5
rather than (CzHS)zOH+
not observed.
Table I1 : Changes in Relative Intensity of Raman Bands Associated with Proton Transfer in Anhydrous Hydrogen Fluoride Solutions of Ethanol
a
Al'
1 on
350 395 450
I a t AV
=
Relative intensity" Concn of ethanol, mole 66
30
N.O.b N.O.
2.5 5
10 13
14
N.O.
N.O.
890 taken as 100.
N.O.
= not observed.
This discussion will be confined to the effects of proton transfer. We note that for diethyl ether, a solution containing 34 mole yc ether shows no Raman bands attributable to a spectroscopically identifiable ion. The electrical conductivity observations' indicate a concentration of protonated ether ions well under 0.01
The Journal of Physical Chemistry
+ F(HF),=l-.
The 20 mole % ether, with about 20 times this ionic concentration, has a readily identifiable ionic Raman spectrum. The bands whose intensities are substantially reduced are bands which have been attributed to bending or stretching vibrations involving the C-0-C bonds. Presumably, the new bands derive from similar vibrations of the protonated molecules. In the solvent of higher dielectric constant, ethanol, even a 66 mole % solution and an effective ionic concentration of perhaps 0.05 '11 seems to provide an adequate concentration of identifiable ionic species. As mentioned above, neither dioxane nor tetrahydrofuran is a satisfactory solute for Raman studies in the high H F concentration region. Colored solutions are found with the latter and a high background with the former. In each case it is presumed that some reaction is involved in addition to proton transfer. It is worth noting however, that in the concentration regions studied, dioxane, as expected, shows only the slight modification of intensity and frequency shift associated with hydrogen bonding to the oxygen, while tetrahydrofuran is like alcohol and some additional lines are observed, presumably associated with proton transfer and ionization. (7) L. A. Quarterman, H. H. Hyman, and J. J. Katz, J . Phys. Chem., 65, 90 (1961).
