S.B. MILLIKEN,K . MORGAN, ASD R . H. JOHNSEN
3238
Electron Spin Resonance Studies on the Photolysis of Radicals Produced in Ethyl Mercaptan and Ethyl Disulfide by X-Irradiation’
by S. B. Milliken, K. Morgan, and R. H. Johnsen Department of Chemistry, Florida State University, Tallahassee, Florida
52906 (Received March 21, 1967)
The esr spectra and photochemical properties of the radicals generated in ethyl mercaptan and ethyl disulfide by X-irradiation a t 77°K have been studied. The initial products are postulated to be ion pairs with the charges localized on the sulfur atoms. Subsequent photolysis in the case of ethyl mercaptan results in the formation of the RS radical whose presence was previously postulated in liquid ethyl mercaptan.
RSSR +RSSR+ + RSSR-
Introduction This study is concerned with the esr spectrum of irradiated ethyl mercaptan at 77°K before and after photolysis with ultraviolet light. Evidence from the radiation chemistry of various organic systems appears to establish that ionizing radiation expels electrons in condensed media which are thermalized and migrate through the system by diffusion-like processes.2 The electrons then may produce avariety of reactions by dissociative or attachment processes a t room temperature, and a t liquid nitrogen temperatures can be identified by virtue of their paramagnetic properties. After being expelled, they leave behind paramagnetic ions, or neutral radicals resulting from reactions of the parent ion and another molecule. The fate of the electron seems variously t o depend on the extent of hydrogen bonding, the accessability of empty orbitals, and the presence of solutes which are able to react with electrons. Typical reactions are summarized below. (1) Systems without Hydrogen Bonding3
(3)
3-Methylpentane1 which has been studied extensively by means of optica13a and e ~ spectroscopy, r ~ ~ is representative of organic glasses without hydrogen bonding. The yield of trapped electrons is low in these systems, and unless electron scavengers are present, most of the electrons return promptly to the parent ion. Neutralization occurs and a hydrogen atom and a neutral radical are produced. When the electron is trapped or scavenged, the counterion is most often the paramagnetic parent positive ion as shown in reaction la. Dissociative attachment to an electron scavenger (e.g., benzyl chloride) produces a radical and a nonparamagnetic negative ion as suggested by reaction lb. When hydrogen bonding is present as in the aliphatic alcohols, the electron is trapped in high yields. The parent positive ion undergoes reaction to produce a nonparamagnetic positive ion and a neutral free radical. Myron and Freeman’ have suggested that the ion-mole-
t4
RH +R H + e-
+ RX --+R . + X-
+ e-
(if solute is present)
(1%)
(lb)
(2) Systems with Hydrogen Bonding5 ROH +ROH+
+ e-
+ nROH +(ROH),ROH+ + ROH +R’CHOH + ROHz+ e-
(8) Systems with Low-Lying Empty Orbitalse The Journal of Physical Chemistry
(1) This research was supported in part by the U. S.Atomic Energy Commission under Contract AT-(40-1)-2001. This is AEC Document ORO-2001-6. (2) M. R. Ronayne, J. P. Guarino, and W. H. Hamill, Radiation Res., 17,379 (1962). (3) (a) J. B. Gallivan and W. H. Hamill, J . Chem. Phys., 44, 1279 (1966); (b) K.Tsuji and F. Williams, J. A m . Chem. Soc., 89, 152G (1967).
(24 (2b) (2c)
B. Gallivan and W. H. Hamill, Trans. Faraday Soc., 61, 1 (1965). (5) (a) R. S.Alger, T. H. Anderson, and L. A. Webb, J . Chem. Phys., 30, 695 (1959); (b) F. J. Dainton, G. A. Salmon, and J. Tepley, Proc. Roy. SOC.(London), A286, 27 (1965). (6) D.C.Wallace, J. E. Hesse, and F. K. Truby, J . Chem. Phys., 42, 3845 (1965). (4) J.
ESRSTUDIESOF ETHYL MERCAPTAN
cule reaction is assisted by the hydrogen bonding; if so, the RCH-OH radical observed may well be preceded by an alkoxy radical. I n any case, the neutral radical has been identified by the n 1 lines characteristic of an electron localized on the hydrocarbon portion of the molecule indicating the detachment of a hydrogen from a Slight further splitting by the hydroxyl hydrogen and the production of vicinal glycols by radiation suggests the removal of the hydrogen CY to the oxygen. Evidence for the ROH2+ ion is circumstantial but convincing, resting upon actual observation of the species in high-pressure mass spectrometry19the resistance of aldehydes to scavenging: and thermochemical calculations.1° When only first-row elements are present, there are no low-lying vacant orbitals. The ejected electron can thus be expected to be delocalized over several molecules.ll This should be true for both systems with and without hydrogen bonding, the only difference being that the trap appears deeper for the hydrogen-bonded ~ystem.~t~ Thus the electron is released from its trap by light of 16,000 A in 3-;11P; 12,000 A in methyl tetrahydrofuran,12and 7000 A in ethanol. When atoms are present which have vacant orbitals available which are relatively low in energy, the expelled electron becomes associated with a single molecule. The example shown here is an alkyl disulfide studied by Truby.6 The spin-orbital interaction is apt not to be quenched, so that there is an anisotropy observed in the spectrum. The electron is usually localized on an atom containing the available orbitals. In this laboratory some effort has been devoted to the study of alcohol systems with particular attention being paid to the effects of visible and ultraviolet photolysis of the radiation-produced radicals.13-15 A study of the mercaptans seemed like a natural extension of this line of endeavor. Moreover, the above categorization of the chemical behavior of the electron would predict that the electron would behave more nearly like that in disulfides than in the alcohols, and the product yields should reflect that fact. I n a separate study,16 the yields of the chemical products were investigated.
+
Experimental Section The ethyl mercaptan and ethyl disulfide used were the same as in the chemical study, and purification and handling techniques were identical. Samples were sealed in Thermosil tubes 3 mm in diameter for use in the esr spectrometer. Samples for optical studies were placed in ampoules with flattened sides made of the same type of quartz. The esr spectrometery was done
3239
in a Varian 4502 spectrometer. Optical spectra were taken on a Cary spectrometer with a special lowtemperature sample holder. Irradiations were done using the Florida State 3-Mev Van de Graaff accelerator. Bleaching experiments were carried out with an A-H6 mercury arc lamp. The filters used have been described previou~ly.'~Doses were of the order of 10lgev/g.
Discussion The top spectrum of Figure 1 shows ethyl mercaptan immediately after irradiation; the bottom spectrum is the result of subsequent photolysis. The original Xirradiated sample is colored yellow-orange. This color is bleached by near-ultraviolet light but not visible light, and the sample becomes pale yellow or clear. The sample was photolyzed over a range of selected wavelengths from 6000 to 2500 A, and it was found that no change in the radical spectrum occurs above approxi-
A
I. T-Irradiated 2. t-lrradlated
Ethyl Ethyl
Mercaptan Mercaptan,
uv phOtOly8i8, 3300C k 3900
i
Figure 1. Esr spectra of X-irradiated and photolyzed ethyl mercaptan.
(7) J. J. J. Myron and G. R. Freeman, Can. J . Chem., 43, 1484 (1965). (8) B. Smaller and M. Matheson, J. Chem. Phys., 28, 1169 (1958). (9) K.R.Ryan, L. W. Sieck, and J. H. Futrell, ibid., 41, 111 (1964). (10)J. Tebly, A. Habersbergerovan, and K. Vacek, Collection Czech. Chem. Commun., 30, 793 (1965). (11) D.R. Smith and J. J. Pjeroni, Can. J . Chem., 43, 876 (1965). (12) M.R. Ronayne, J. P. Guarino, and W. H. Hamill, J . Am. Chem. SOC.,85, 384 (1963). (13) R.H.Johnsen, J . Phys. Chem., 63, 2041 (1959). (14) R.H.Johnsen and D. A. Becker, ibid., 67, 831 (1963). (15) S. B. Milliken and R. H. Johnsen, ibid.. 71, 2116 (1967). (16) J. J. J. Myron and R. H. Johnsen, ibid., 70, 2951 (1966).
Volume 71,Number 10 September 1967
S. B. MILLIKEN, K. MORGAN, AND R. H. JOHNSEN
3240
mately 3500 A. The arrow indicates g = 2.003, the value for the free electron. The g for the derivative crossover is equal to 2.012. Neither its value nor the concentration of the radical species changes measurably during photolysis. The concentration reflects a yield of 2-3 spins/100 ev. However, this value of the yield is most approximate due to the difficulties of integrating the anisotropic spectra. Both the g value and the estimated yield are based on the known values for these measurements in methanol. The optical spectrum is shown in Figure 2. XIrradiation produces the broad band centered around 4500 A. This band disappears with ultraviolet photolysis using light in the 3500-A region. It is noteworthy that light of wavelengths longer than 3500 A, andin particular in the wavelength region around 4500 A, does not affect the optical absorption of the sample. Both optical and esr absorptions disappear rapidly upon warming of the samples. Because of the lack of structure, the photosensitive species is hard to identify. The bottom esr spectrum, on the other hand, resembles the spectrum of the alkyl sulfide obtained by Kurita and Gordy17 upon the Xirradiation of cystine dihydrochloride modified as one might expect if its free electron were interacting with two protons instead of one. They irradiated their samples in both powdered and crystalline form. The
--\
A f k r inodiotion
--------- A f t u photolysir
I
3000
Before inodiotion
4000 < A< 3000
5000
4000
6000
d Figure 2. Absorption spectra of ethyl mercaptan before and after X-irradiation and following ultraviolet photolysis.
The Journal of Physical Chemistry
spectrum of the irradiated crystal revealed three principal axes of the g tensor and a hyperfine doublet characteristic of splitting by a single hydrogen atom. A calculated spectrum consistent with the observed spectra and the nature of this hyperfine splitting suggested to them that the spin was centered on a sulfur atom which was not free to rotate and thus interacted with only one proton on the adjacent carbon atom. They found evidence for this hyperfine splitting in the spectrum of the amorphous solid; it is also apparent here in the glassy mercaptan. This similarity fairly well characterizes the radical produced by ultraviolet photolysis as the ethyl sulfide radical. Characterization of the photosensitive precursor is more difficult. The mercaptan exhibits a limited amount of hydrogen bonding as well as low-lying vacant orbitals on the sulfur so that trapping of the electron can be expected either by reaction 2b or 3. The lack of resolvable hyperfine structure rules out a reaction analogous to (2c). A likely possibility is that the upper spectrum in Figure 1 is the result of the superposition of two signals, one from the positive parent and one from the negative ion formed by attachment of the electron to ethyl mercaptan. The spectra of irradiated ethyl disulfide and ethyl mercaptan are superposed in Figure 3. The similarity is quite marked except for a small shift in g (crossover) toward higher field. The yields, when corrected for the electron fraction, are about the same for both systems. The g value for the derivative crossover is 2.017, in agreement with the values reported for ethyl disulfide in the literature.6 IYotice that g-(electron) < g(mercaptan crossover) < g (disulfide). In Figure 4, the effects of photolysis on the esr spectrum of irradiated ethyl disulfide are shown. The wavelengths used are the same as those used for ethyl mercaptan. The solid line represents the spectrum immediately after radiolysis, the dotted curve is what remains after several hours of photolysis, after which there is no further change. The sample is initially dark green in color and is bleached by the action of the ultraviolet light. The arrow again indicates g for the free electron. The presence of the sulfide radical is indicated by the broad line on the low-field side of the spectrum. It is neither increased nor decreased by ultraviolet light. The dashed line spectrum is most probably a mixture of alkyl radicals not containing sulfur. It is unique for alkyl disulfides and is identical with those obtained by Truby,6 who photolyzed the entire homologous series up through ten carbons. As sug(17) Y . Kurita and W. Gordy, J. Chem. Phys., 34, 282 (1961).
ESRSTUDIJZS OF ETHYL MERCAPTAN
Figure 3. Comparison of X-irradiated ethyl mercaptan a n ethyl disulfide spectra: -, X-irradiated ethyl mercaptan; - - - -, X-irradiated ethyl disulfide.
3241
and positive ions could account for the line sha*peobserved. His calculation was based on a positive ion with three principal axes in the g tensor, and the electron removed from an orbital shared by the two sulfur atoms. 'The negative ion seemed to be due to an impaired electron occupying a d orbital on a sulfur atom, and had two principal axes. At leaEt some of this circumstantial evidence rests upon properties of disulfides suficiently general to organic siilfur compounds to be true also for mercaptans. Certainly, the initial spectrum and the production of R 8 . radicals are analogous to phenomena observed in ihe disulfide case. Therefore, it seems reasonable that the initial spectrum in irradiated ethyl mercaptan results from an ion pair, and the sequence of events that follow is as shown below.
+ e-
CzH6SH--...+CzH5SH+ e-
+ CzHjSH -+
CzH5SH-
+
C ~ H ~ S H-% - C ~ H ~ S H e,-
+ CzHjSH+-+ CzHjS + H - * €1,* + CzHsSH +CzHsS. + Hz
e, -
Figure 4. Esr spectra of X-irradiated and photolyzed ethyl X-irradiated ethyl disulfide; - - - -, disulfide: -, X-irradiated ethyl disulfide, ultraviolet photolysis a t 3600 A.
gested before, the initial spectrum is probably due to the presence of an ion pair. In contrast to the mercaptans, the only sulfur-containing radical seen after photolysis is that produced directly by the X-irradiation of the disulfide. The evidence for ion pairs was largely circumstantial. (1) When the solid was warmed slightly, there was no gradual decrease in concentration of the radicals i~ might be expected in the case of neutral radicals beginning to diffuse and react. (2) The addition of electron scavengers produced spectra characteristic of the reaction of the scavenger with the electron. (3) A cbange in frequencies from 9 to 35 Gcycles indicated anisotropy. In explaining the k-band spectrum, it was necessary to consider a distribution of orientations of i,he responsible radicals since it is a glass and not a crystal. In doing so, Truby found that an equal mixture of paramagnetic negative
*
The electron is expelled from the molecule by the ionizing radiation as usual. Mercaptan is its own electron scavenger; the electron attaches to another mercaptan molecule, probably in a d orbital on the sulfur atom. Thai: positive hole is also probably located on the sulfur atom in the parent molecule. Since the mercaptan is less hydrogen bonded than the alcohols, one does not expect the formation of a radical by abstraction of' the (Y hydrogen atom by the positive ion as in reaction 2c. The SH group, furthermore, is rigidly held, leading to the observed anisotropy. Any hyperfine splitting is probably due to the hydrogen atom which is attached to the sulfur. The electron trap is deeper than in alcohol (of the order of 3-4 ev), as indicated by the shorter wavelength necessary to bleach the mercaptan. The effect of photolysis then, is simply to rlelease the electron from its trap, permitting it to return to its parent ion and neutralize it. The resulting energy breaks a sulfur-hydrogen bond. In the case of the disulfide, the fragments are bulky and are caged; in the mercaptan, however, the smaller hydrogen ato,m escapes, and a significant concentration of RS.radicals is produced. The ultimate fate of the hydrogen atom is not known. However, since the radical concentration apparently does not diminish during photolysis, the hydrogen atom may be hot enough to absrxact. Acknowledgment. The assistance of J l r . D. Pritchett Volume 71, Number 10 September 1967
RONALD A. MUNSON AND MICHAEL E. LAZARUS
3242
in carrying out the radiolyses is gratefully acknowledged as well as the assistance received from the Florida
State University Institute of Molecular Biophysics in the form of access to the esr spectrometer.
Densities and Apparent Molal Volumes of Molten Phosphoric Acid Solutions
by Ronald A. Munmi and Michael E. Lazarus General Electric Research ccnd Development Center, Schenectady, New York (Recewed March IS, 1967)
The densities of a number of electrolyte solutions in phosphoric acid at 80" and a few a t 150" are reported. Most of the 1-1 electrolytes investigated have apparent molal volumes which are quite close to the molar volume of phosphoric acid. This indicates that these electrolytes enter inko the solvent structure with very little over-all electrostriction of the phosphoric acid. Apparent molal ionic volumes of a number of ions have been calculated on the basis of the assumption of equality of volumes of the lyonium and Lhe lyate ions. It is suggested that this assumption is of value in comparing electrostrictive influences in different solvents.
The densities of ionic solutions may be used to obtain information concerning ion-solvent interactions. The apparent volume changes observed may result from general influences of the ion on the solvent structure or from the displacement of solvent equilibria. In phosphoric acid, two self-dissociative equilibria are of importance, both of which produce extensive ionization. loa
+ HzPO42HaP04 _r H3of + H3P207.-
2H3P04
H4P04'
(1) (2)
Experimental Section Chemical Preparation. Phosphoric acid was prepared from analytical reagent 85% phosphoric scid by water removal under v a ~ u u m . ~It was adjusbed to 100.0% by the crystalline melting point technique used previously.' Sulfuric acid (100.0%) was prepared from analytical reagent fuming sulfuric acid and distilled water. A solution 19.0% by weight perchloric acid and 80.8% phosphoric acid was prepared a t 120° by stirring analytical reagent phosphorus pentoxide with analytical reagent 70% perchloric acid. Lithium perchlorate was prepared by neutralizing lithium carbonate The Journal of Physkcal Chemistry
with analytical reagent perchloric acid. To remove the last trace of water, it was necessary to heat it to 300' just prior to use. It analyzed 92.9'G (expected 93.5%) perchlorate. Lithium dihydrogen phosphate (99.0%) was also prepared from lithium carbonate. Mg(H2P04)2,which was prepared firom magnesium oxide and phosphoric acid, contained 11.2y0 (expected 11.1%) magnesium. Reagent potassium bisulfate (100.1%) and potassium dihydrogen phosphate (99.4%)) which were analyzed by acid-base titration, as well as the other salts, were stored in a vacuum desiccator at 100'. Solutions were made up by weight and all open manipulations were performed in a drybox furnished with dry Nzand Pz05. Procedure. The dilatometer (18 cc) was constructed from a small erlenmeyer flrtsk into which was fitted by means of a ground-glass joint a graduated glass tube with a terminal constriction. The volume of the dilatometer was calibrated with doubly distilled water. Temperature in the oven remained constant to k0.2O. (1) R. A. Munson, J. Phys. Chem., 68, 3374 (1964). (2) R. A. Munson, ibid., 69, 1761 (1965). (3) Inorg. Sun., 1, 101 (1960).
