RADICALS PRODUCED IN IRRADIATED AQUEOUS SOLUTIONS OF THIOLS
2277
Electron Spin Resonance Study of Radicals Produced in Irradiated Aqueous Solutions of Thiols1 by P. Neta and Richard W. Fessenden* Radiation Research Laboratories and Department of Chemistry, Mellon Institute of Science, Carnegie-Mellon Uni’nicersity,Pittsburgh, Pennsylvania 16215 (Received February 1 , 1971) Publication costs assisted by the Carnegie-Mellon University and the U.S.Atomic Energy Commission
Radicals produced by reaction of hydrated electrons and hydroxyl radicals with several organic thiols in aqueous solutions have been studied by electron spin resonance. Irradiation with high-energy electrons was carried out directly in the esr cavity. Spectra of the radicals CHzCOO- and -SCHCOO- (g = 2.0086, CZC“ = 13.4 G) were obtained from alkaline solutions of mercnptoacetate. The variation in intensity of the two spectra when various scavengers were added shows that the two species were formed by reactions of casand OH, respectively. The radical SCH&OO- is also expected to be formed by reaction of OH but could not be observed under our experimental conditions. Similarly, the radicals -0OCCHzCHC00- (g = 2.0033, UC” = 20.4 G, U C H , ~= 23.6 G) and -OOCCH,C(S-)COO- (g = 2.0075, CZ& = 8.7 G) in solutions of mercaptosuccinate were found to result from the reaction of eaq- and OH, respectively. Substituted alkyl radicals produced by hydrogen abstraction from 3-mercaptopropionate, cysteine, thiodiglycolic acid, and dithiodiglycolic acid have also been observed.
Introduction The radiation chemistry of aqueous solutions of thiols has been the subject of several extensive investigations with particular emphasis on cysteine and related compounds (cf. ref 2-6). The radicals produced from these compounds have been studied both by pulse r a d i o l y ~ i sand ~ , ~by esr s p e c t r o s ~ o p y .The ~ ~ ~esr studies, however, did not involve radiolytic production but relied on chemical oxidation using a mixing technique. It seemed, therefore, desirable to carry out an esr investigation of the radicals produced by in situ radiolysis of aqueous solutions of these compounds. This method has been developed and used in this laboratory for the study of the radiolysis of several groups of organic compounds in aqueous solution^,^^^^ and its application was expected t o help elucidate the mechanism of reactions of OH and eas- with thiols. Most of the studies of thiols have dealt solely with radicals of the type RS, which are the main product of hydrogen abstraction. Rate constants for the different paths of abstraction have been measured for the reaction of hydrogen atoms with cysteine“ but no measurements exist for the OH reactions. From the available data, abstraction is expected to take place from C-H a t rates one to t x o orders of magnitude slower than from S-H. Although the radiobiological importance of mercaptoalkyl radicals may be less than that of thiyl radicals we wish to draw attention to their formation and to present esr data on some radicals of this type which have not been observed previously. Experimental Section Mercaptoacetic acid, mercaptosuccinic acid, and thio-
diglycolic acid were Baker Grade reagents, 3-mercaptopropionic acid was obtained from Eastman, dithiodiglycolic acid from Aldrich, and cysteine from Cyclo. The inorganic materials were Baker Analyzed reagents. Water was doubly distilled. The pH was adjusted using potassium hydroxide, perchloric acid, or sodium tetraborate. Solutions were deoxygenated by bubbling with nitrogen or were saturated with nitrous oxide. The irradiation arrangement and other details of the experiment were as previously described.9 Results and Discussion The possible reactions of thiols with the primary (1) Supported in part by the U. S. Atomic Energy Commission. Presented in part a t the 161st National Meeting of the American Chemical Society, Los Angeles, Calif., March 28-April 2, 1971. (2) V. G. Wilkening, M. Lal, M. Arends, and D. A. Armstrong, Can. J. Chem., 45, 1209 (1967). (3) A. Al-Thannon, R. M . Peterson, and C. N. Trumbore, J . P h y s . Chem., 72, 2395 (1968). (4) J. E. Packer and R. V. Winchester, Can. J . Chem., 48, 417 (1970). (5) G. E. Adams, G. S. McNaughton, and B. D. Michael, “Chemistry of Ionization and Excitation,” G. R. A. Johnson and G. Scholes, Ed., Taylor and Francis, London, 1967, p 281; Trans. Faraday Soc., 64, 902 (1968). (6) Mi. Karmann, A. Granzow, G. Meissner, and A. Henglein, Znt. J. Radiat. P h y s . Chem., 1, 395 (1969). (7) W. T.V. Wolf, J. C.’Kertesz, and W. C. Landgraf, J . Magn. Resonance, 1, 618 (1969). (8) W. A. Armstrong and W. G. Humphreys, Can. J . Chem., 45, 2589 (1967). (9) K. Eiben and R. W.Fessenden, J. P h y s . Chem., 72, 3387 (1968); 75, 1186 (1971). (10) P. Neta and R. W. Fessenden, ibid., 74, 2263, 3362 (1970); 75, 738 (1971). (11) M. Anbar and P. Neta, I n t . J . A p p l . Radiat. Isotop,., 18, 493 (1967).
T h e Journal of Physical Chemistry, Vol. 76, N o . 16, 1971
2278
P. NETAAND RICHARD W. FESSENDEN
RzCHSH
RzCHSH
+H
+RzCSH
+ H2
RzCHs + HzO + OH I+ +R&SH + HzO
These reactions may be followed by
+ RzCHSH -+R2CH2+ RzCHS RzCHS + R2CHSH Jr R2CHSSHCHR2 RzcH
2RzCHS +R2CHSSCHR2 Rate constants for these reactions [excepting (6) ] have been measured for cysteine and several other compounds.~~6~11~12 The dissociated form of the thiol, RzCHS- (present at pH > lo), has been found to undergo reactions 1, 5, and 7 at rates lower than those for the acid form.6s11812 The dimeric complex in equilibrium 8 was observed by pulse radiolysis in both the acid (RSSHR)13 and the basic (RSSR)- forms.jr6 The radical recombination, eq 9 written specifically for RzCHS can take place with all the types of radicals present. Because the g factors of sulfur-containing radicals potentially can vary more than those of their oxygen analogs this parameter can be of some use in identifying the radicals found. Values for two main types of radicals are known from studies on single crystals. Thiyl radicals typically have a large g-factor anisotropy (principal values such as 2.003, 2.025, and 2.053 found for HOOCCH(KH2)CH2S in L-cysteine dihydrochloridell) with an isotropic value of 2.027. The two mercaptoalkyl radicals which have been studied in crystals, H02CCHzScHCOzH15 and CHBSCHCH1CH(NHB+)CO2-,l6have moderate anisotropy (2.002 t o 2.011 and 2.0017 to 2.0080, respectively) with an isotropic g factor of about 2.0030. I n the present work no radicals were encountered with g factors above 2.01, and therefore it is likely that none of our spectra belong to thiyl radicals. It should be noted that the radicals reported by Wolf, et al.,' as thiyl radicals do not have g factors as high as 2.027. The simplest compound investigated was mercaptoacetic acid. Some of the spectra obtained with this compound are shown in Figure 1. Two radicals could be observed. The radical CH2COO- formed by reaction with cas-, eq 1, can be identified by its 21.2-G triplet with g = 2.0032 in agreement with previous meas u r e m e n t ~ . ~ , ~The 0 13.4-G doublet with g = 2.0086 can be assigned to the -SCHCOO- radical formed by reaction of OH, eq 6. Increasing the concentration of -SCH2COO- caused a decrease in the signal intensity of the spectrum of CH2COO- apparently as a result of The Journal of Physical Chemistry, Vol. 7'6, - i o . 16,19rI
k i 3 4 G - i
CH2Coo-
I -21
18G-----*
Figure 1. Second-derivative esr spectra from deoxygenated aqueous solutions of 0.01 M mercaptoacetate at pH 12.4 during irradiation with 2.8-MeV electrons. Microwave power levels were 2.5 mW (top), 34 mW (center), and 100 mW (bottom). Magnetic field increases to the right. The stick spectra show the relationship of the lines. The large signal from the silica cell is seen just above the centei of the spectrum and is recorded at a gain 100 times less than the other portions. Approximate positions of gain change indicated by vertical slash marks.
reaction 7. The addition of N 2 0 to the solution caused the disappearance of the spectrum of CH,COO- and an increase in the intensity of the spectrum of -SCH(12) R. Braams, Radiat. Res., 27, 319 (1966). (13) M. Simic and M. Z. Hoffman, J . Amer. Chem. SOC.,9 2 , 6096 (1970). (14) Y . Kurita and W , Gordy, J . Chem. Phys., 34, 282 (1961). (15) Y. Kurita and 117. Gordy, ibid., 34, 1285 (1961). (16) D. G. Cadena, Jr., and J. R. Rowlands, J . Chem. Soc. B , 488 (1968).
RADICALS PRODUCED IN IRRADIATED AQUEOUS SOLUTIONS OF THIOLS COO-. In the presence of another electron scavenger, acetone, the spectrum of CH2COO- was again absent. High concentrations of OH scavengers such as methanol or isopropyl alcohol had little effect on the spectrum of CH2COO-. The fact that CH2COO- was not observed from solutions containing both S20 and CH30H shows that CH,O- cannot react with -SCH2COO- in the same way as ea,-. It is seen in Figure 1 that the two radicals are affected differently by the change in microwave power. Whereas the signals of CH2C00- saturate similarly to those of other alkyl radicals (maximum peak height at -5 mW), those of -ScHCOO- do not saturate up to the maximum power investigated (100 mW). (The line width for -ScHCOO- in Figure 1 is -0.26 G and is noticeably larger than for CH2COO-. The implied decrease in relaxation time is probably a result of the gfactor anisotropy, see below.) This finding helps in distinguishing between the two types of radicals and prompted us to use high power levels for the study of sulfur-containing radicals. The studies of sulfur-cen,~ microwave power tered radicals by Wolf, et C Z ~ . utilized levels to 200 mW. Spectra similar to those in Figure 1 were obtained from solutions of mercaptoacetate in the pH region 1013.5. At pH 8.6 only CH2COO- was observed. In acid solutions (pH 4, 1, 0.3) no spectra could be observed. The absence of CH2COOH in the acid region is probably a result both of a lower production rate and a more rapid recombination. The conversion of ea,-- to H replaces reaction 1 by reactions 2-4 of which reaction 2 is predominant (about 90% of the total). The absence of lines of HSCHCOOH or of HSCHC00- in both the acid and neutral regions can be a result of a low production rate in both pH regions although line broadening caused by proton exchange may also be a contributing factor to low line intensities. The production of -ScHCOO- in the basic region is probably also low but here the double negative charge will have a big effect on the recombination ratel7 and the steady-state concentration will be correspondingly higher. The moderate concentrations of -ScHCOOobserved are consistent with a relatively low production rate. I n all the experiments between pH 0.3 and 13.5 no spectrum could be observed from the SCH,COOH or SCH2COO- radicals. These radicals are expected to undergo reaction 8 in the forward direction (with a rate constant higher than l o 9 sec-l for RS-)6 to form the complex ([RSSR-]/[RS-] [RS] l o 4 LW-1).5 The g-factor anisotropy of such a radical might cause sufficient line broadening that the spectrum would be unobservable by esr spectroscopy at the concentrations existing. On the other hand, a spectrum attributed to the radical SCH2COOH produced by mixing the thiol with Ce4+has been observed by esr' spectroscopy in acid solution.' It is possible that the cerium prevents for-
-
2279
mation of the complex (RSSHR) by forming a stronger complex with SCH~COOH. We attempted to dupli, ~ the same socate the experiment of Wolf, et ~ l . using lution and spectrometer conditions, but utilizing radiation instead of Ce4+ for the production of radicals. No spectrum could be observed even in the presence of Ce3+. We conclude therefore that the steady-state conentration of radicals in our experiment is lower. The results obtained with irradiated aqueous solutions of mercaptosuccinic acid are similar to those with mercaptoacetic acid. Two representative spectra are shown in Figure 2. Lines from more than three radicals are present. The reaction of ea,- produces -00CCHCH2C00- according to reaction 1. This radical has previously been formed by H abstraction from s ~ c c i n a t e ' ~and ~ ' ~by H addition to fumarate20 and the present hyperfine constants (see Table I) are in good agreement with the other values. The reaction of OH radicals with mercaptosuccinate produces the radical -OOCC(S-)CH2COO- according to reaction 6. This radical has a relatively high g factor (2.0076) comparable to that of -ScHCOO- (2.0086). The third radical obtained from mercaptosuccinate (Figure 2) cannot be readily identified. The 20.15-G doublet with g = 2.0048 can possibly be assigned to the radical -0OCCHC (=S)COO - formed by secondary reactions or by reaction with an impurity but no positive identification is suggested. Some additional lines which could not be analyzed are also present in the spectrum. I n acid solutions, as in the case of mercaptoacetic acid, no spectrum could be observed. -4spectrum of only a single radical is observed from irradiated solutions of 3-mercaptopropionic acid (but only at high pH and high microwave power level). It has a 17.1-G doublet of 19.3-G triplets with g = 2.0058. This spectrum is assigned to the radical -SCHCHzCOO- formed by hydrogen abstraction according to reaction 6. The a-proton hyperfine constant (17.1 G) though higher than those of the radicals obtained from mercaptoacetate and mercaptosuccinate is nevertheless too low to be assigned to the radical -SCH2CHCOO-. It is expected that attack by OH radicals would be more favorable at the position adjacent to the sulfhydryl group than at that adjacent to the carboxyl group. The radical 6H2CH2COO- produced by reaction af ea,- with 3-mercaptopropionate was not observed. Cysteine behaved in a fashion similar to that of 3mercaptopropionate in that a spectrum of a single radical was observed only from alkaline solution. This spectrum is a 10.0-G triplet, which is further split by (17) The second-order decay rate constant of the radical. HCQHCOOH is two orders of magnitude higher than t h a t of -0CHCOO[M. Simic, P. Iieta, and E. Hayon, J . Phys. Chem., 73, 4214 (1969) I. (18) H. Fischer, K.-H. Hellwege, and M. Lehnig, B e y . Buaaenges. P h y s . Chem., 72, 1166 (1968). (19) G. P. Laroff and R. W.Fessenden, submitted for publication In J . Chem. P h y s . (20) P. Neta, J . Phys. Chem., in press. The Journal of Physical Chemistry, Val. 76, N o . 16,1971
2280
P. NETAAND RICHARD W. FESSENDEN
k
20.35 G4-
k-223.590
4
-
I
-OOC$CH,CO 0-
k--8.67G
Figure 2. Esr spectra from deoxygenated aqueous solutions of 0.03 iM mercaptosuccinate at pH 13.1 during irradiation. Microwave power levels were 2.5 mW (top) and 34 mW (bottom). Two assigned sets of lines are indicated by the stick figures. The doublet mentioned in the text is composed of the fourth strong line from the low-field end and the line just above the signal from the cell. Other unassigned lines are present.
4 2 . m -I
I------------ 15.17G-
1-
- 4 2 . 2 6 G a 16.476
Figure 3. Esr spectrum from an NzO saturated aqueous solution of 0.003 M thiodiglycolate a t pH 8.5 (buffered with sodium tetraborate) during irradiation. Microwave power was 19 mW. The observed lines were considerably weaker a t a power of 2.5 mW.
one nitrogen (5.4 G) and two unequivalent protons (0.5 and 1.2 G), and has g = 2.0037; it could be asAlthough signed to the radical -SCH&(NH,)COO-. both the CH2 and C H positions are activated (by sulfhydryl and amino groups, respectively), the main OH substituted attack Occurs at the more or tertiary CH position. The two hydrogens of the The Journal of Physical Chemistry, Vol. 76, N o . 16, 1.971
amino groups are not equivalent as was found in radicals obtained from other amino acids.10c,21 The spectrum obtained from irradiated solutions of thiodiglycolic acid is shown in Figure 3. Three different radicals are observed. The main species (21) H. Paul and H. Fischer, Be?. Bunsenges. Phys. Chem., 73, 972 (1969).
2281
RADICALS PRODUCED IN IRRADIATED AQUEOUSSOLUTIONS OF THIOLS Table I : Structure and Hyperfine Constants of Radicals Produced in Irradiated Aqueous Solutions of Thiols Hyperfine constants*
B
Radical observed
Solute irradiated
-SCHzCOO-
CHICOO-SCHCOO -OOCCHCH~COO-
-0OCCHCHzCOO-
I S-
botora
UCHB =
2.00322 2.00861 2.00328
21.18 13.41 = 20.38'
U C E ~=
i
UGHH
U C H ~ H=
-OOCCCHzCOO -
23.59'
aCRnH= 8.67
2.00755
I
S-SCHzCHzCOO -
-SCHCH~COO-
i
2,00575
U C H ~=
17.08
U O H ~= ~ 19.26
-SCHzCCOO -
-SCHzCHCOO -
2 00369 I
I
I
NHz
NHa
-0OCCHzSCHzC00 -
-0OCCHzSSCHzCOO -0OCCHzSSCHzCOOOH(or -0OCCHzSHzOz
+
+
+
CHzCOO-OOCCHSCH~COO-
2.00320 2.00562
(?) CH~SCH~COO-
2.00468
-OOCCHSSCH~COO-0OCCHzS02
aCHnH = 10.01 a N = 5.43 U N H ~ H '= 0.51 aNH2HZ= 1.24 U C H ~ H= 21.17 UCHH = 15.17 zH = 2.99 U O H = ~ ~ 16.47 acHzH= 2.26 acaH = 11 U C H ~= ~ 1.82
I
:.I
-2,0046 2.00840
OH -) a The g factors are measured relative to the peak from the silica cell and are accurate to =!=0.00005. Second-order corrections have been made [R. W. Fessenden, J . Chem. Phys., 37,747 (1962)]. * Hyperfine constants are given in gauss and are accurate to 1 0 . 0 3 G. = 20.41, ' Hyperfine constants reported to be somewhat dependent on ionic strength; values in the presence of 1 ~12NapSol: U C H = ~ ~22.75 G (ref 18).
formed by reaction of OH is -OOCcHSCHzCOO-. It has a 15.17-G doublet of 2.99-G triplets with g = 2.0056. TWOof the six lines are masked by the signal from the silica cell. The observed lines increased in the presence of N20 and were also observed at pH 9.5 and 13.2. The second radical observed is CHzCOOformed by a reaction of eaq- similar to reaction 1.
; :di
-7
IlJ 4
1.82G
Figure 4. Esr spectra from NzO-saturated solutions of 0.01 M dithiodiglycolate at pH 12.8 during irradiation. Microwave power was 2.5 mW. Top spectrum-fresh solution, bottom spectrum-solution bubbled with oxygen for 65 hr and then oxygen removed by NzO. Gain reduction by a factorof 10at position of slash mark on lower trace.
-OOCCHzSCHzCOO-
+ eaq- +
-OOCCH2S-
+ CH2COO-
(10)
Although two lines of this triplet appear in the same position as lines of other radicals, they could be detected by the appropriate change in intensity when SzO was present; the calculated hyperfine constant and g factor agree very well with previous values. A third radical appears to be present in the spectrum in Figure 3. This radical has a 16.47-G triplet of 2.26-G triplets with g = 2.0047. A reasonable assignment is cHzSCH2COO-, but the mechanism of formation is not clear. Representative spectra obtained from irradiated solutions of dithiodiglycolic acid are shall-n in Figure 4. The results in this case are complicated by the fact that this compound undergoes hydrolysis and disproportionation in alkaline The main reactions are (11) and (12). The Journal of Physical Chemistry, Vol. 7 6 , KO.16,1971
2282
P. NETAAND RICHARD W. FESSENDEN
+ OH- + -0OCCH2SOH + -SCHZCOO-
-0OCCHzSSCHzCOO-
(11)
2-00CCHzSOH + -0OCCH2SH
+ -0OCCHZSOZH
(12)
I n mildly alkaline solutions (pH 9-11) the reaction of OH radicals with the solute is expected t o form -0OCeHSSCH2COO-. This radical appears to show a doublet of about ll-G splitting with g = 2.0046 and no splitting ( 12 these weak lines increased in intensity as shown in the top spectrum of Figure 4 and a strong triplet appeared. All of these signals increased with time indicating that the slow reactions 11 and 12 were taking place. The spectrum obtained after a long hydrolysis time is shown at the bottom of Figure 4. The triplet observed has a = 1.82 G and g = 2.0084. Since the signal intensity was very high, it was possible to show that no other lines of this spectrum occur in the region of the signal from the cell. The additional lines in the top spectrum could not be interpreted and are probably formed from intermediates in the oxidation processes. On the basis of reactions 11 and 12 the triplet observed in Figure 4 (bottom) can reasonably be assigned to the radical -0OCCHzS02 formed by the reaction -0OCCHZSOZ-
+ OH
----f
-00CCH2S02
+ OH-
(13)
which is similar to the oxidation reaction of sulfite by OH. It should be noted that this radical has a structure which is isoelectronic with -00CCHz$J02- (a" = 8.5 G19). Aromatic radicals of this type have been observed in solution23and the ring proton splittings are rather small. The g factor reported for C6H5S02is 2.0044. On this basis the parameters assigned here to -OOCCH2S02do not seem unreasonable. This radical is clearly different from the radical suggested by Wolf, et u Z . , ~ to be formed by the reaction of -OOCCH2S with 02. This latter radical shows g = 2.018 (and no proton splittings), consistent mith values for other peroxy radicals, and must have the structure -0OCCHZ-
s-0-0,.
The spectrum of cHZCOO- was not observed from solutions of dithiodiglycolate. The reaction of eaqprobably produces -OOCCH2S and -SCH2C00-. However, if the compound was allowed time to hydrolyze according t o reaction 11 and then irradiated, weak lines of CH2COO- were observed in addition to the bottom spectrum in Figure 4. Similar results to those obtained from very alkaline solutions of (SCHZThe Journal of Physical Chemistry, Vol. 7 6 , N o . 16, 2971
COO-), were also obtained from irradiated solutions IlzOz at pH 13. The first reaction of -SCHgCOOis of course the thermal oxidation of -SCH,COO- to (SCHZCOO-)2 by hydrogen peroxide which is then followed by hydrolysis and disproportionation. No spectra could be observed with cystine, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, and thiophenol. The reasons could be the lack of additional charged groups to lengthen the lifetime or the splitting into too many lines which decreases the intensity. Weak lines were observed from solutions of thioacetamide and thiourea, which appear t o be due to CHae(S-)i\THB and SH&(S-)NH,, but no clear analysis could be made. The structures of the radicals proposed to explain the various spectra and the corresponding spectral parameters are summarized in Table I. As mentioned above, two single crystal studies of mercaptoalkyl radicals have been reported. I n both cases the isotropic value of a,," is about 15 G indicating considerable delocalization onto the sulfur. The isotropic g factors in these two cases are about 2.0050. The parameters found here for -00CCeHSCH2C00- in aqueous solution are very similar. AdanisZ4has also reported radicals of this type [CH&HSCHZCH3and CH,(CH,),eHSCH2(CHZ),CH8] in hydrocarbon solution as well as two of the type RCHSSR'. The values of amHare similar (17 G) but no g factors were reported. He also observed splittings from CH, protons located across a single sulfur (but not from those across a disulfide group). Radicals of the type RzCS- have not been previously reported and their hyperfine constants are best discussed by reference to the oxygen analogs. The two direct comparisons which can be made are -SCHCOO- (aH = 13.4 G) n i t h -0cHCOO- (a" = 14.2 G)g and -OOCC(S-)CH2COO- (aH = 8.7 G) with -OOCc(O-)CH,COO- (a" = 8.4 G).I9 I n each case the hyperfine constants are remarkably similar. This result is a bit surprising in view of the results reported by Adams24 on two thioether radicals. He found that for both radicals the value of a," is 3 G higher than for the corresponding ether radical. The lack of any such difference for the radicals reported here suggests that a, for the ether radical is unusually 1 0 ~ . This effect may be related to a nonplanarity at the radical site.25 With radicals like -0cHCOO- and -SCHCOO- an increased degree of conjugation may keep the radicals planar. The g factors for the mercaptoalkyl radicals reported here vary in a way which is very similar to that found
+
(22) "Organic Sulfur Compounds," N . Kharasch, Ed., Pergamon Press, Elmsford, N . Y . , 1961. (23) M. McMillan and W ,A . Waters, J . Chem. SOC. B , 422 (1966). (24) J. Q. Adams, J . Amer. Chem. Soc., 92, 4535 (1970). (25) A. J. Dobbs, B. C. Gilbert, and R. 0. C. Korman, Chem. Commun., 1353 (1969).
2253
INFRARED SPECTRUM OF MATRIX-ISOLATED URANTUN MOKOXIDE for the oxygen analogs but the deviations from the freeelectron value are larger as a result of the larger spinorbit coupling for sulfur. Sulfur in a /3 position [-SCHzdl(SHz)COO-] shifts the g factor very little. The two radicals of the type RCHSR’ have g factors of about 2.0050 in agreement with the values found for such radicals in the solid (see above). The three radicals of the type R2dlS- do not show the same g factor but show a markedly higher value if the carboxyl group is in the a position [-SCHCOO- and -OOCdl(S-) CH,COO- as compared with -ScHCH,COO-]. The increase with an a-carboxyl group (-0.0025) is similar to but larger than the increase from, e.g., cHzO(g = 2.0037j9 to -0CHCOO- ( g = 2.0044).9 Apparently there is some interaction of the S- and COzgroups. The ratio of observed line widths (3.5’ to 0.25) for
thiyl and mercaptoalkyl radicals seems in accord with the known g-factor anisotropies and the idea that the width is a result of incomplete averaging of this anisotropy. The thiyl radicals have about a five times greater anisotropy and with similar tumbling rates the line width ratio should be the square of this or 25. The observed ratio (14) is in reasonable agreement with this rather crude calculation. I n conclusion, it was observed that thiols react with hydrated electrons to lose SH- and form alkyl radicals according to reaction 1. The reaction with hydroxyl radicals involves hydrogen abstraction from CH groups (reaction 6) in addition t o abstraction from the SH group (reaction 5). Radicals of the types RzCS-, RzcCHzS-, R2CSR’, R26SSR’, and RSOz mere identified in this study and their hyperfine constants and g factors were determined and discussed.
The Infrared Spectrum of Matrix-Isolated Uranium Monoxide1 by S. Abramowitz,* N. Acquista, National Bureau of Standards, Washington, D . C. 20234
and K. R. Thompson Department of Chemistry, University of Florida, Gainesville, Florida
32601
(Receiz;ed February 18, 1971)
Publication costs assisted by the National Bureau of Standards
The infrared spectra of matrix-isolated uranium oxides have been observed. By varying the O/U ratio of the condensed phase from 1.5 to 3.0 and comparing the observed spectra of the matrix-isolated vapors in equilibrium with t h e condensed phases it has been possible to assign a frequency of 776.0 cm-I to UL60. Verification of this assignment has been obtained in similar experiments using oxygen-18 enriched uranium oxides; a frequency of 736.2 c$-’ has been observed for UlSO. This yields a force constant to the harmonic approximation of 5.32 mdyn/A for this species. d stretching mode of UOz has also been assigned for UlaOz, UlSOz, and U160180.
Introduction
Experimental Section
AIass spectrometric studies of the vapors in equilibrium with the uranium-oxygen system coupled with Knudsen effusion techniques have yielded the vapor pressures of U, UO, UO,, and U03.2-6 It has also been shown in these studies that the ratios of the vapor pressures of these species a t a given temperature can vary by several orders of magnitude as the O/U ratio changes between 1.5 < O/U < 3. I n particular these studies have shown that the vapor in equilibrium with substcichiometric UOZ contains UO, UOz, and some U. The ratio of the UO/UOz reaches about 15 a t about 200°K in the three phase system of UOz(s), U(1), and vapor. We have used these extensive data to pattern our matrix isolation studies of the uranium-oxygen system.
The electron beam furnace, cryogenic system consisting of an Air Products cryotip and Perkin-Elmer 99G monochromator and Perkin-Elmer 301 spectro(1) This research was supported in part by the Defense Atomic Support Agency, Washington, D . C., and in part by the National Science Foundation under Grant KO.GP-9316. (2) R. J. Ackermann, P . W. Gilles, and R. J. Thorn, J . Chem. Phys., 25, 1089 (1956). (3) R. J. Ackermann and R. J. Thorn, I n t . A t . Energy Ag., Tech. R e p . Ser., 39 (1965). (4) A. Pattoret, J. Drowart, and S. Smoes, Thermodyn. 2\rucl. Mater., Proc. S y m p . , 1967 (1968). (5) R. J. Ackermann, E. G. Rauh, and N. S. Chandrasekharaiah, J . P h y s . Chem., 73, 762 (1969). (6) R. K. Edwards, M . S. Chandrasekharaiah, and P. M. Danielson, High T e m p . Sei., 1, 98 (1969).
T h e Journal of Physical Chemistry, Vol. 76, N o . 16, 1971
