J. Phys. Chem. 1991, 95, 3676-3681
3676
seems that the simple Hiickel method is not suitable for discussing the difference in the reactivities of the secondary hydrogens. Deuterium Isotope Effect. Our values of k H / k Dfor acetone and cyclopentanone a t room temperature are 4.1 and 3.0, respectively. Yip and Siebrand measured the deuteration effect on the reaction rate constant of acetone with ethanol.'* They reported 3.7 of k H / k Dat room temperature. Our values are close to this value. k H / k Dfor aliphatic carbonyls may be about this value. Formoshinho et al. made a theoretical calculation of the value of k H / k Dfor the reactions of acetone with methanol, 2-propanol, and c-C,. They obtained 3.5, 3.0, 4.0, respectively," which are very close to the experimental values. As for the temperature dependence no one has examined k H / k Dfor this reaction experimentally or theoretically. The isotope effects on the reaction rate constants have been discussed on the basis of the classical transition-state theory. The theory is based on the assumption that thermally activated species determined by a Boltzmann distribution leads to the reaction, and the isotope effect is provided by the difference in the zero-point energies. With the use of the frequencies of the C-H stretching - ~2900 cm-' and UC-D = 2050 vibrations reported by IR ( v ~ = cm-I), the value of k H / k Dis estimated to be 7 and 14 at 27 and -43 OC, respectively, if the C-H bond is completely broken in the transition state. k H / k Dmay become smaller depending on the structure of the transition state. However, the transition-state theory predicts a large temperature dependence on k H / k Dand cannot account for our observation that there is little temperature effect on k H / k D .On the other hand, the tunneling mechanism has been invoked to interpret the reaction rates that could not be
accounted for by the classical theory. It is thus tempting to try to interpret our results in terms of the tunneling mechanism, particularly because our values are very close to the values calculated by Formoshinho et aLI3 on the basis of the tunneling mechanism. However, a reliable calculation on the values of kH/kDat various temperatures must be made before we can discuss the reaction mechanism further. Conclusion We have examined hydrogen abstraction reactions of carbonyls, especially acetone, with alkanes and 2-propanol using timeresolved EPR and identified several intermediate radicals. We have demonstrated that the dependence of the hydrogen abstraction reaction rates on the position of the hydrogen can be studied by the analyses of the CIDEP spectra. For alkanes we have found that all the secondary hydrogens are abstracted with almost equal rate constants. The relative reactivities for the primary, secondary, and tertiary hydrogens are obtained for acetone and benzophenone to be kl/k3 I0.01 and k2/k3N 0.1. This tendency is qualitatively explained by the frontier electron theory based on the simple Hiickel MO, but this treatment fails to explain our results on the reactivities among secondary hydrogens. The deuterium isotope effect in the reaction of acetone and cyclopentanone with 2propanol gives kH/kDN 3-4 over the temperature range examined (room temperature to -43 "C). This result cannot be explained by the classical transition-state theory.
Acknowledgment. We thank Mr. Patrick K. Walsh of the University of Minnesota for his help in improving the manuscript.
Reactlons of HS and S* wlth Molecular Oxygen, H2S, HS-, and S2': Formatlon of SO2', HSSH', HSS2', and HSS' J. Zbu, K. Petit, A. 0. Colson, S.DeBolt, and M. D. Sevilla* Department of Chemistry, Oakland University, Rochester, Michigan 48309-4401 (Received: November 1, 1990)
An electron spin resonance investigation of the reactions of HS' and So- produced from H2S, HS-, and S2- in glassy matrices at low temperatures is presented. co-60 irradiation of 8 M NaCIO,, 12 M LiCl, and alkali-metal hydroxide glasses at 77 K results in the formation of 0'-, C12'-, and e-. Upon annealing to about 150 K 0-or C12+ reacts with the solutes H2S, HS-,or S* to form HS'and So-radicals. In the presence of molecular oxygen HS' and'S each react to form sulfur dioxide anion radical, SO2'-. I'O isotopic studies verify the source of the oxygen in SO2'- is the molecular oxygen dissolved in the matrices. In the absence of molecular oxygen, competing processes are clearly observed; i.e., HS' and So-attack HIS and HS- to form dimer radicals HSSH'- and HSS'". At low pH we find that HS' attacks H2S to form HSS'. Mechanisms for the formation of these species are proposed, and hyperfine couplings and gvalues are reported. Ab initio molecular orbital calculations are performed to aid our understanding of the electronic structure of the various radical species formed and the energetics of their reactions.
Introduction Extensive studies of thiyl radicals and their reactions have been made because of their biological significance.I-I0 It is well-known (1) Volman, D. H.; Wolstenholme, J.; Hadley, S.G. J. Phys. Chem. 1967, 71, 1798. (2) Norman, R. 0. C.; Storey, P. M. J. Chem. Soc. B 1971, 5, 1009. (3) Symons, M.C. R. J . Chem. Soc. Perkin Trans. 2 1974, 1618. (4) Box, H. C.; Budzinski,E. E. J. Chem. Sa..Perktn Trans. 2 1976,553. ( 5 ) Nelson, D. J.; Petersen, R. L.; Symom, M. C. R. J . Chem. Soc., Perkin Trans. 2 1977, 2005. (6) Schonle, G.; Rahman, M. M.; Schindler, R. N. Ber. Bumen-Ges. Phys.
.
CHem. 66. - -. -.... 1981. -. . , 91. . . .
that thiols are able to donate hydrogen atoms to repair free-radical damage on biomolecules and form more stable thiyl radical^.^*-^' Thiyl radicals have been found to form dimer anion radicals, RSSRO-, through reaction with thiolates and to form perthiyl ,~~ radicals, RSS', through reaction with parent t h i ~ l ? , ~Recently, in the presence of molecular oxygen, the formation of sulfoxyl (10) Swarts, S.; Bccker, D.; DeBolt, S.; Sevilla, M. D. J. Phys. Chem. 1989. -. -.7
93. 155. --.
(11) Redpath, J. L. Radiar. Res. 1973, 54, 364. (12) Wolfenden, B. S.;Willson, R. L. J. Chem. Soc.. Perkin Trans. 2 1982, 1105.
(7) &villa, M. D.; Becker, D.; Swarts, S.;Herrington, J. Biochem. Biophys. Res. Commun. 1987, 144 (2), 1037. ( 8 ) Bccker, D.; Swarts, S.; Champagne, M.; Sevilla. M. D. Int. J . Radiat. Biol. 1988, 53, 767. (9) Simic, M . G. Mutation Res. 1988, 202, 377.
0022-3654/91/2095-3676S02.50/0
(13) Bonifacic, M.; Asmus, K. D. J . Phys. Chem. 1984,88,6286. (14) Von Sonntag, C. The Chemical Basis ofRadiation Biology; Taylor and Francis: London, 1987. (15) Petersen, R. L.; Nelson, D. J.; Symons, M. C. R. J . Chem. Soc.,
Perkin Trans. 2 1978, 225.
0 1991 American Chemical Society
Reactions of HS' and S'radicals has been elaborated through ESR studies at low temperatures.'b'8 These results suggest that thiols may not always act as radioprotectors and may manifest prooxidant action wherein they promote rather than inhibit oxidizing damage.18J9 For example, the sulfonyl peroxy radical, R S 0 2 0 0 ' , has been suggested as one of the intermediates responsible for oxidizing damage.18 The reactions of the simplest of the sulfhydryl radicals, HS', are not as well-known as those of the more complex thiyl radicals, RS'. Pulse radiolysis studies of H2S ,in aqueous solutions have shown that the attack of OH' results inthe rapid formation (>lo9 M-l s-l) of HS' or S'.20,21 The mercapto radical formed further reacts with HS- to form the complex HSSW- radical, and it also reacts with oxygen to form peroxyl radical, SO0'-, or sulfur dioxide anion radical, SO2'- (OS0'-).21 (The identity is unclear from this previous work.) In mass spectrometric investigations, the reaction of HS' radical with O2has been reported to be too slow at room temperature to be followed.6 Since the major results in pulse radiolysis studies are based on the analysis of the easily detectable HSSH'- and HSSo2-radicals (A, = 380 nm), the reactions involved are not clear. Up to this time, to the best of our knowledge, neither So- nor HS' radicals have been clearly detected in frozen aqueous matrices by ESR, though So- is well characterized in alkali-metal single crystals,22-u and HSSH' has been suggested to be formed through the reaction of mercapto radical with HS-in glassy mat rice^.^^,^^ In this work through ESR spectroscopy we have investigated the formation of H S and So- and their reactions with molecular oxygen and sulfur solutes (H2S, HS-, S2-) in frozen aqueous matrices.
Experimental Section Samples were prepared from commercially available compounds (Aldrich Chemical Co.) without further purification. Labeled oxygen (50% 1702)was obtained from Icon. From the studies of the formation of the'S and HS' radicals, 8 M NaC104, 12 M LiCl, and 8-10 M alkali-metal hydroxide samples, containing 0.001-0.02 M Na2S, were bubbled with O2 or N2 at room temperature and frozen at 77 K in 4-mm quartz tubes to form glasses. These samples were given approximately 1.8 KGy of Co-60 yirradiation. For the studies of the reactions of S'- and HS' with HS- and H2S the same techniques, except higher concentrations (0.034.2 M Na2S), were employed. The pH was controlled by addition of HCI or NaOH. After irradiation the samples were annealed in the ESR cavity to higher temperatures where various reaction steps were followed. The spectra were recorded on a Varian E-Line spectrometer with dual cavity and low-temperature capability and stored on an on-line computer? The g values and hyperfine splittings were measured against Fremy's salt ( A N = 13.09 G and g = 2.0056). The three calibration marks on the spectra shown are the three Fremy's salt resonances. The spin intensity was determined by using cupric sulfate, CuS04.5H20, as a ~tandard.~'*~*
-
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3677
B
4
gn,l
150W100K
7 170W104K
I
-tt+ Figure 1. First-derivative ESR spectra found after irradiation of a sample of 0.02 M Na2S in 8 M NaOH at 77 K and subsequent annealing. The spectra illustrate the conversion of'0 (A) to So- (B) and the reaction of S- with O2to form SO2- (C). The positions of g,, values measured for 0'-and So- radicals are taken as arrow marked, which are at 75% of height of the peaks (from the baseline on the high field side). Where two temperaturts are indicated in all figures, the first indicates the temperature to which the sample was warmed and the second at which the spectrum was recorded. All spectra are contaminated by small components due to quartz background. The dotted line in (B) shows the g = 2.00 region after subtraction of the background signal. The three markers are 13.09 G apart with the center marker at g = 2.0056.
Results and Discussion Formation of Surfide Anion Radical (S'). Figure lA,B shows the ESR spectra of free radicals induced by y-radiation of a frozen aqueous solution of Na2S (0.02 M) in 8 M NaOH at 77 K after annealing to temperature shown in the figure. At 77 K the major initial species present are e- and O*. Upon photobleaching with visible light the single symmetrical absorption band (g = 2.002) ) of the trapped electron is lost in part by dielectron ( Q ~formation, but 0'-remains in the spectra even after annealing the sample to 150 K (Figure lA), indicating that the glassy matrix is rigid, and both 0'-and Sz- are immobile at this temperature. These results are consistent with that reported by Blandemer et al.,29 except for the determination of g values.30 As the sample is warmed to higher temperatures, the spectrum of 0'-decreases in intensity as a new component at lower field increases in intensity, indicating'0 converts to a new radical with,,g = 2.1 5 and gh = 1.997. This conversion was completed at about 170 K (Figure 1B). The,,g found here is close to that reported for So- in KBr and KCl single crystals22 and within 2.10-2.40 reported for thiyls.S,8 Therefore, we assign this new radical to sulfide anion which is formed by one-electron oxidation of S2- by radical, S*-,
O.-.
Sz-
(16) Scvdla, M. D.; Yan, M.; Becker, D. Biochem. Biophys. Res. Commun. 1988, I55 (I), 405. (17) Sevilla, M. D.; Yan.. M.;. Becker.. D.:. Gillich. S. Free Rad. Res. Cohmk. 1989,.6 (2-3); 99. (18) Sevilla, M. D.; Becker, D.; Yan, M. Inr. J . Radiar. Biol. 1990, 57, (11, 65. (19) Schoneich. C.; A",K. D.; Dillinger, U.; Bruchhausen, F. Biochem. Biophys. Res. Commun. 1989, 161 (I), 113. (20) Karmann, W.; Meissner, G.; Henglein, A. Z . Naturforsch. 1967, ZZB, 273. (21) Mills, G.; Schmidt, K. H.; Matheson, M. S.: Meisel, D. J . Phys. Chem. 1981, 91, 1590. (22) Hausmann, A. Z . Phys. 1966, 192, 313. (23) Vannotti, L. E.; Morton, L. R. Phys. Reu. 1968, 174 (2), 448. (24) Bill, H.; Dohrer, D.; Schwan, L.; Sigmund, E. Solid Stare Commun. 1980, 31, 383. (25) Razskazovsky, Yu. V.; Melnikov, M. Ya. Khim. Vys. Energ. 1989, 23, ( I ) , 47. (26) Razskazovsky, Yu. v.; Melnikov, M. Ya. J. Photochem. 1984,27,239. (27) Zhu, J.; Sevilla. M. D. J . Phys. Chem. 1990, 94, 1447. (28) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance-Elementary Theory and Practical Applications; Chapman and Hall: New York, 1986; pp 157, 239.
I#
-
+ 0'- H+
So-
+ OH-
(1) It should be mentioned that the spectrum shown in Figure 1B is contaminated in the center by the absorption spectra of a quartz background and a small amount of SO2' and dimer radicals. The dotted line in Figure 1b shows the spectrum of'S in the g = 2.00 region and is the result of computer subtraction of these absorptions. A comparison of the S- and 0'-spectra shows both have axial symmetry. The,,g absorption is at lower field and broader than that of'0 as expected due to its larger spin-orbital coupling constant3 (A) and weaker matrix interactions (AE). (29) Blandemer, J. M.; Shields, L.; Symons, M. C. R. J. Chem. Soc. 1964, 4352. (30) The g,, values are measured at a position corresponding to 75%of the height of the peak (measured from the baseline on the high field side). Anisotropic spectral simulations show that this position is the proper position for the measurement of g,, and that the position is almost unaffected by line broadening. Previous values reported for 0- are in error due to improper measurement. For the detailed analysis for gvalua please scf ref 28, pp 157, 239.
Zhu et al.
3678 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
" C
150W102K
so; '
S-. z
I
V
itt-
C
Figure 2. Spectra obtained after the irradiation of 0.002 M Na2S in 12 M LiCl at 77 K and subsequent annealing to temperaturesshown in the figure. The results show the formation of'S by a second reaction route, Le., one-electron oxidation of HS-(S2-)by CI; (A) to form'S (B). In the presence of 02, 'S reacts with O2to form SO2' (C).
TABLE I: Anisotropic g Values for 0'- and S- in Various Frozen Aqueous Matrices 0'S' gmiu
matrix (4%2) LiOH (saturated) 2.052 8 M NaOH 2.059 IO M KOH 2.064 12 M LiCl 8 M NaCIO,
(*&I) 2.000
2.000 2.000
2.054
2.002
(ftG2)
(fO.OO1)
2.153 2.164 2.173 2.098 2.137
1.997 1.996 1.996 1.991 1.997
Similar results are observed in 8 M NaC104,as well as in mixtures of NaOH and NaCI04. Since C10, scavenges an electron by the reaction C104- e- C103- + 0'(2)
+
-
it is clear that the electron plays no important role in the Soformation in the matrices at high pH. 'S was produced by a second route. Figure 2A,B shows spectra found after irradiation of a 2 mM N a 8 in 12 M LiCl sample (pH 8) at 77 K and subsequent annealing. The initial spectrum taken at 112 K after photobleaching with visible light at 77 K shows that the major species is Clz*. The S* radical beings to appear at about 140 K as the Clz'- radical decays (Figure 2B), implying that the formation of S- occurs through one-electron oxidation of the major species present at pH 8, HS-, by C12'- (reaction 3). Since H S is not observed as an intermediate, the HS'formed must undergo deprotonation to form So- (reaction 4). In fact, the pK, of the HS' radical is likely to be substantially lower than 7;20J as a consequence, So- will dominate at pH's near or above 7. HS-
+ - +
+ Cl2'HS'
Figure 3. ESR spectra of y-irradiated 0.2 M NazS in N2-saturated 12 M LiCl (A and B) and in 02-saturated12 M LiCL (C) at pH 1 at 77 K. (A) The spectrum present is due to HS' and HSS'. The poorly resolved broad component shown on the lower field side is due to H S (B) Warming to 157 K shows the decay of HS'and the increase of HSS' (g, = 2.071, gu = 2.025, g, = 2.002; cf. with (A)). (C) The spectrum shows that the SO2' radical is the major oxidative product in the reaction of HS' with molecular oxygen. The components marked by arrows are due to unidentified species.
HS'
So-
H+
2C1-
(3) (4)
Table I lists g values determined for S- and 0'-in various frozen aqueous matrices. Values of,,g for'S and'0 in aqueous matrices of alkali-metal hydroxides increase as the alkali-metal ion increases in size. Since smaller g shifts imply a greater splitting of the degenerate p orbitals in'S and 0,' the smaller alkali-metal ions likely interact more strongly with these anion radicals. For the matrix anions a similar argument would have the CI- ion as most interacting and the HO- at least; however, this is unlikely. The effect of the matrix anion is likely to be felt indirectly through interactions with the matrix cation or in the binding of water and not directly on So- or 0.-. Formation of the Mercapto Radical, H S . The mercapto radical, HS', is only observed at low pH's. At pH < 2 in N2saturated LiCl frozen solution the events differ from those at higher pH. The well-resolved S- spectral components in the
so;
' '
166K
'Jr ~
Figure 4. Spectrum found after irradiation of 0.04 M Na# in deuterated 8 M NaCIO, and annealing to 166 K. This spectrum shows the full g anisotropy for the SO;-.
foregoing discussion are replaced by a poorly resolved broad component (2.25 > gvalue > 2.00); see Figure 3A. On the basis of the similarities of the spectra of HS' with RS',S**and the product H S S (Figure 3B) with RSS',8 we assign this species to the HS' radical. The HSS' radical is formed through the reaction of HS' with H2S and is discussed in more detail later. It is known that the hydrated electron reacts readily with H2S but not appreciably with HS-." Since H2S dominates at pH 90%).
reducing agent and reduce O2to 02-(reaction 9) at near diffusion controlled rates.33b In agreement we find evidence for the production of 02' during the decay of SO2* in oxygenated systems. so,*- 02 so2 0 2 . (9)
+
-
+
At pH 1.0 reaction of HS' with Ozis observed in 02-saturated 12 M Lick see Figure 3C. The product is also SO2'. This finding suggests that even HSOO' (the protonated form of SOO'-) is unstable toward rearrangement to SO2'- as in reaction 10. HS'
+0 2
-
[H+- -SOW-]
-
H++ S02'-
( 10)
Reactions of S-and H S in Anoxic Matrices: Formation of HSSW- and H S S 2 . Experiments were carried out to elucidate reactions of S'- with sulfur anions in the absence of oxygen. Higher concentrations of sulfide were employed in these studies to promote reactions with the parent. Figure 6A displays the spectra of y-irradiated NazS (0.2 M) in N,-saturated 12 M LiCl adjusted to neutral pH. For comparison, the spectrum of a diluted N a g (0.0015 M) sample prepared by the same technique is shown in Figure 6C. After warming to about 140 K, HSSH'- appears concomitant with the formation of So- in about equal intensity. HSSH'- shows a characteristic spectrum with an 8-G coupling due to two equivalent hydr0gens.2~J~These results indicate that the reaction of So- with HIS occurs as depicted in reaction 11.
+ HZS
HSSH'(1 1) Continued warming the sample to 160 K results in the decay of a loss of HSSHO-, and the appearance of a the remaining S*-, spectrum (Figure 6B) showing one hydrogen hyperfine coupling, a" = SG, at g ~ " The . low field component of this new species (gmx= 2.021) is at a lower field than that of HSSH'- (gma,= 2.014). The spectrum in Figure 6B is assigned to HSSZ-. This species is produced by the deprotonation of HSSH' (reaction 12) So-
(34) Lin, M. J.; Lunsford, J. H. J . Phys. Chem. 1976, 80, 2015.
3680 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 TABLE IIk
GAUSSIAN 82 ab Initio
Zhu et al.
MO calculation for 50;
Cwphga vo nasi8 SeP
basis setb ST03G
I7O
xx
YY
22
a total
12.7 -8.0 4.7 10.2 -8.5 1.7 -18.0 16.1 -1.9 14.3 -22.4 -8.1 13.6 -23.8 -10.2 14.5 -23.8 -9.3
13.4 -8.0 5.4 11.4 -8.5 2.9 -15.1 16.1 1.0 15.2 -22.4 -7.3 14.5 -23.8 -9.3 15.4 -23.8 -8.4
-26.0 -8.0 -34.0 -21.6 -8.5 -30.1 33.1 16.1 49.2 -29.3 -22.4 -51.7 28.0 -23.8 -51.8 -29.9 -23.8 -53.7
total
C3.0
C3.0
-30.8
A
a total
ST03G'
I7O
A
a
total
))S
A
a
total 631G*
170
A
a total 631+~*
170
A
a
total D95*
expt
170
A
$14Figure 8. ESR spectrum of y-irradiated 0.02 M Na2S in N2-saturated deuterated 12 M LiCl (pH = 1) shows the spectral components for DSS' radical (g, = 2.071, gu = 2.025, and g, = 2.002). The spectra marked by arrows are due to unidentified species.
170
#The principal elements of the total hyperfine coupling tensor are the sum of the anisotropic ( A ) and isotropic (a) couplings. bS02' was geometry optimized at each basis set.
concentrations of parent sulfide anion. It should be mentioned that the HSS'" radical formed in NaC104 is further oxidized by oxygen to form SO2' radical, even though the solution was thoroughly purged with N2. This is due to the oxygen formation during y-irradiation of the perchlorate glass. Since the oxygen concentration is low compared to that of sulfide ion, the dimer species is observed first, followed by the oxidation of the dimer to SO2'- (reaction 15). It is known that HSS2- (= So- + HS')
B
Figwe 7. ESR spectra found after y-irradiation of 0.2 M Na$ (adjusted to neutral pH) in a N2-saturated12 M LiCl (D20) after annealing to the temperatures shown in the figure. The major radical species are identified as DSSD- (A) and DSS'*- (B).
and possibly through the reaction of'S with HS- at these higher temperatures (reaction 13). The assignment for HSS'I- is further confirmed by the results taken from a deuterated sample which eliminate the hydrogen couplings and more clearly show the g values (Figure 7 and Table 111).
+ -
HSSH'-
HSSo2-
+ H+
(12)
HSHSS2(13) The pK, for HIS is 7.0; as a consequence, HS- is the major species at pH's between 7 and 12.35 This implies the formation of HSSH' may occur through another reaction path as suggested from the pulse radiolysis,20J1i.e., by the reaction of H S with HS': S-
-
HS' + HSHSSH'(14) If SO, the HS' produced by oneelectron oxidation of HS- by C12' (in LiCI, reaction 3) or 0'-(in NaC10,) must react with HSbefore deprotonation. This is only likely in solutions of high (35) McQuarrie, D. A.; Rock, P. A. General Chemistry; W. H. Freeman and Company: New York. 1984; p 760. (36) Steudel, R.; Albertsen, J.; Zink, K. Eer. Bunsen-Ges. Phys. Chem. 1989, 93, 502.
+ 202
-
2S02'-
+ H+
( 1 5)
the formation of H W 2 - radical from'S and HS- is reversible.M Thus, the formation of SO2'- may simply be from the direct addition of O2 to the So- or HS' in equilibrium with HSSo2-. Formation of H S S . At pH
