J. Phys. Chem. 1995,99, 7993-8001
7993
Nature of the Thiyl Peroxyl Radical: ESR and ab Initio MO Evidence for Intermolecular Stabilization of the Charge Transfer State, RSSOO'Yurii Razskazovskii, Anny-Odile Colson, and Michael D. Sevilla" Department of Chemistry, Oakland University, Rochester, Michigan 48309 Received: January 3, 1995; In Final Form: March 3, 1995@
The formation, chemistry, and nature of the thiyl peroxyl radicals, RSOO', are investigated by ESR and UV-vis spectroscopy in a variety of organic and aqueous matrices. Experimental evidence suggests that the unusual properties of thiyl peroxyl radicals result from the specific nucleophilic interaction of solvent which stabilizes the charge transfer state, RS+OO'-. Anisotropic oxygen- 17 coupling constants derived from ESR spectra of 0-17 labeled species which are proportional to the unpaired spin at the oxygen are used to estimate the spin density distribution in RSOO' radicals. The couplings and spin density distribution are found to vary with the nature of the thiol and the matrix. For example, in freon the thiyl peroxyl I7O couplings for the terminal and inner oxygens (1701, I7O2) differ for primary (79, 62 G), secondary (84, 57 G), and tertiary alkyl thiols (96,5 1 G), whereas in aqueous systems or methanol all thiols yield RSOO' radicals of approximately the same couplings (80, 62 G). All RSOO' species in polar media have a visible absorption (Amax = ca. 540 nm) and are found to undergo photoisomerization to R S O i and subsequent oxygen addition to form RS0200'. About 5-10% of the spin density is found on the sulfur atom. Results found in neutral or acid aqueous glasses show no pH dependence of either the I7O hyperfine couplings or visible absorption maximum. The degree of charge transfer and the varying oxygen couplings are suggested to be a function of the ability of the medium to act as an electron pair donor. For tertiary thiols solvent access is sterically hindered in freons, which results in 0-17 couplings and spin distribution much like that found for a usual carbon-centered peroxyl radical. The solvent-stabilized charge transfer state, RS+OO'-, is found to be far more thermally stable than the uncomplexed state, which is found to react likely by thermal isomerization to R S O i at 100 K. Ab initio MO calculations are found to mimic the charge transfer state by association of negative ions such as OH- or F- with the sulfur atom.
Introduction The thiyl peroxyl radical, RSOO', has been the subject of considerable interest due to its likely involvement in the autoxidation of and its implication in the oxygen enhancement effect of radiation in living systems.4-I6 It is formed r e v e r ~ i b l y ~by - ~oxygen ~ ' ~ addition to radiation-produced thiyl radicals, reaction 1. RS'
+ 0, =RSOO'
(1)
The thiyl peroxyl radical shows properties which are unique for peroxyl radicals. It shows a visible absorption at 540-560 nm1,4,6,12,17 in contrast with most peroxyl radicals investigated to date, which show no visible absorption. The only other known example of a peroxyl radical, showing visible absorption is the phenyl peroxyl radical which absorbs at 520 Weak bonding to sulfur has been predicted by quantum chemical calculations (9 kcal m01-l)'~ and found experimentally either in liquid (7 kcal mol-')I2 or in a gaseous (11 kcal phase. This results in a reversible equilibrium with oxygen and the thiyl radical (reaction 1). Furthermore, electron spin resonance investigations using 170-labeled oxygen show that the spin density in RSOO' is more equally shared by the two oxygens (0.42, 0.52)8than in any other peroxyl radical in which the spin density on the terminal oxygen varies from 0.6 to 0.7.9 No satisfactory explanation for the unusual behavior of the RSOO' species has been provided to date. Sevilla et aL2I and Chatgilialoglu et aLZ2have performed. ab initio calculations of the spin density and electronic transition energies of CH3SOO' @
Abstract published in Advance ACS Abstracts, April 15, 1995.
0022-3654l95/2099-7993$09.00/0
as a model compound. Surprisingly, these calculations predict no visible absorption,22and the spin density is found to be that of a normal peroxyl radical (with a large spin density on the terminal oxygen).21 Chatgilialoglu22 concluded from their calculations (incorrectly) that RSOO' cannot explain the color found for these species and that some other sulfoxyl species such as RSO' must explain the visible absorption. In previous ESR work, Sevilla et aL9 pointed out that the oxygen spin densities in RSOO' are similar to those found in superoxide ion, 0 2 * - , when complexed to metals or surfaces.23 This suggested the charge transfer state RS+OO'- may be a large contributor to the electronic structure of RSOO'. In this work we present experimental evidence that the unusual properties of the thiyl peroxyl radical result from specific nucleophilic solvation of sulfur, which stabilizes the charge transfer state, RS+OO'-. We find that theoretical MO calculations can mimic this state by association of a negative ion, X-, with the sulfur atom. Our experimental results show that in the poorly solvating medium, freon, the spin density distribution of the tert-butyl-SO0 species is quite different from that found in an aqueous or polar solvent and approaches the spin density distribution in a typical peroxyl radical.
Experimental Section Alkanethiols (Aldrich), thioglycerol (TgSH, W. R. Grace&Co.), and cysteamine (Sigma) were used as received. Neat 1,1,2trichlorotrifluoroethane (TCTFE), butyronitrile, 2-methyltetrahydrofuran, methanol, and ethanol were used to obtain glassy solutions on freezing; other solvents employed for the same purpose were TCTFE/isooctane (1 :1.5 v/v), isooctane/hexane (5:l v/v), 8 M NaC104 (H20), and 6 M H2S04 (H20). Aprotic 0 1995 American Chemical Society
7994 J. Phys. Chem., Vol. 99, No. 20, 1995
Razskazovskii et al.
TABLE 1: Oxygen-17 Hyperfine Couplings of Thiyl Peroxyl Radicals at 77 K thiol
radical
Primary n-C3H7SH
matrix TCTFE methanol+4%HzO TCTFE TCTFElisooctane TCTFE ethanolg 6 M HzSOdH208 8 M NaClOdHzOg Si02 surface
n-CdHgSH n-C 12H25SH CyaSH' CysSHd TgSH' secondary i-C3H7SH tertiary t-CdHgSH
t-CsHllSH
1,,Jnm
550 535
Ait(2)IG"
eb
79 81 79 79 78 82 81 81 79
62 62 62 61 58 61 62 64t 62
0.92 0.93 0.91 0.9 1 0.88 0.93 0.93 0.94 0.92
Ail( 1)/@
TCTFE methanol+4%H20
540h 55 1
84 81
57 62
0.92 0.93
TCTFE TCTFE/isooctane 2-MTHF butyronitrile methanol+4%H~0 TCTFE
535 54 1 540 550 546
96 96,82
51
0.95
80
62 58 61 51
0.92 0.89 0.91 0.95
79 79 96
Numeration begins from the terminal oxygen atom. Splittings in the 96-78 G range are accurate to 1 G. Those in the range 64-58 G that overlap with other lines in the spectrum are accurate to 2 G. Spin density localized in the peroxyl group calculated as (All(1) A11(2))/154. Cya = H3NtCH2CH2-. Cys = HOOCCH(NHz)CHz-. e Tg = HOCH2CH(OH)CH*-. f Reference 8. 8 Radicals were generated by radiolysis. * Approximate position of absorption maximum derived by subtraction of the spectra taken before and after the action of visible light.
+
solvents were dried and stored over molecular sieves. Oxygen17 (37% I7O) was obtained from Icon. Solutes were dissolved in an appropriate solvent or solvent mixture and equilibrated with 0 2 at ca. 0.5 atm except for TCTFE solutions, for which less oxygen-saturating pressure must be used to obtain a good glass. Thioglycerol-loaded silica (10 mg of TgSH per gram) was obtained by mixing equal volumes of silica gel and a TgSW acetone solution followed by removal of acetone in vacuum. Finally, samples were heat treated at ca. 450 K in vacuum to remove volatile products. It is believed that such treatment removes a considerable portion of physically adsorbed water and TgSH molecules and fixes the remaining thiol on the surface due to the formation of R-0-Si bonds.24 UV irradiation of samples with a low-pressure mercury lamp emitting mostly at 253.7 nm was employed to generate free radicals in nonaqueous glasses and on the silica surface. Due to the transparency of any matrix used at this wavelength, only dissolved thiols are believed to be responsible for primary free radical formation via photolytic S-H bond cleavage.25
RSH
+ hv -.RS' + H'
Hydrogen atoms react in organic media forming carbon-based matrix radicals. Thiyl radicals are expected to be trapped in the matrix (or on the surface in silica gel), although they are not always easily detectable because of extremely broad ESR signals usually hidden under the spectra of matrix-derived radicals. Further annealing produced thiyl peroxyl and matrix peroxyl radicals simultaneously. Silica-supported samples were irradiated under vacuum at 77 K followed by annealing at 140 K to remove carbon-based radicals also formed in this system. The sample was subsequently exposed to oxygen at 90 K. ESR spectra were recorded after removal of adsorbed oxygen at 120 K under vacuum. y-Irradiation used to generate free radicals in ethanol, 8 M NaC104, and HzS04 glasses mainly produces matrix-derived radicals, namely, CH3CH(')OH, O'-, and Sod'-, respectively. Thiyl peroxyl radicals were produced on annealing in thermal reactions involving these species. In all systems investigated in this work thiyl peroxyl radicals have been unambiguously identified by use of their high sensitivity to visible and near-IR light.6,13,17 In this work spin densities have been estimated from oxygen17 hyperfine coupling^.^ In earlier work we found that the
oxygen- 17 hyperfine coupling tensor is axially symmetric with principal values All= (a
+ 2B)$
A, = (a - B)en where en is the spin density in the pz orbital of oxygen. Parameters in these expressions were estimated to be a = -40 G and B = -57 G for a number of carbon-based peroxyls (note that aen = For unit spin density in the oxygen pz orbital the expected magnitude of All is 154 G, and thus experimental spin densities can be calculated as 44154. A1 are much smaller, and corresponding features are usually unresolved in the spectra. A Varian Century ESR spectrometer with an E-4531 dual cavity was employed. Peroxyl radical spectra were recorded at 0.2-2 mW microwave power and calibrated with reference to peroxylamine disulfonate (Fremy's salt, g = 2.0056, U N = 13.09 G). Optical spectra were taken in ESR quartz tubes at 77 K with a Shimadzu UV-190 spectrophotometer.
Results Oxygen- 17 hyperfine coupling constants measured for thiyl peroxyl radicals in this work are presented in Table 1. In all cases the assignments of line components in the spectra to RSOO' were made on the basis of the photoisomerization of RSOO' to RS02' (reaction 2 ) under the action of visible and/or near IR light. The sulfonyl free radical, RS02', shows a singlet ESR spectra at g = 2.0055 and a I7O coupling of 56-58 G for isotopically labeled RS02*.63'33'7 RSOO'
+ hv
-
RSO,'
(2)
Such photochemical behavior is highly specific to thiyl peroxyl radicals and has never been reported for any carbon-centered peroxyl radical investigated to date. Details of the systems investigated in the present study are described below. RSOO' in 6 M Sulfuric Acid Glass. The thiyl peroxyl radical was observed in a number of neutral aqueous matrices (glasses) in our previous work.6,8 In this work a highly acidic medium was tested to elucidate the influence of pH on spin density distribution and reactivity of the RSOO' species.
J. Phys. Chem., Vol. 99, No. 20, 1995 7995
Nature of the Thiyl Peroxyl Radical
Id
which displays anisotropic I7O coupling of 56 G, as found previously in neutral solutions.8 This species adds 0 2 to form CyaS0200' (Figure 1C) on annealing to temperatures which allow for oxygen mobility. On further annealing to higher temperatures which allow for bimolecular reaction, we find the C y a S 0 2 0 0 intermediate reacts with thiol, resulting in a sequence of reactions (shown in reaction 5 ) yielding the CyaSO' radical (Figure 1D). Each of these reaction steps was observed in neutral mediass Further, the hyperfine couplings of the intermediates are not significantly different from those found in neutral aqueous glasses such as 8 M NaC104. Tables 1 and 2 show a comparison of the values in the two matrices. The chemistry of the thiyl peroxyl radical has been summarized previously as8
CvaSOO.
I
t /"
hv
0 2
RS*
RSOO*
RS
\
I
I
Figure 1. ESR spectra taken after y-irradiation of 5 mM cysteamine hydrochloride in 6 M H2S04 at 77 K under I7O2.(A) The spectrum of RSOO' radicals recorded after annealing at 150 K. The hyperfine structure originates from tw0~~0-labeled species, RSOI'O' and RSI70 0 . (B) The spectrum of RSOz' radicals with 56 G I7O anisotropic coupling obtained by the action of visible light on RSOO' species. ( C ) The mobilization of oxygen on warming, resulting in the formation of RS0200' radicals. (D) The spectrum showing further warming converting RSOO' to RSO' (reaction 5 ) , displaying the set of 58 G (I7O) and 14 G (1H) couplings. The three markers are 13.09 G , apart centered at g = 2.0056. Wings of the spectra are amplified approximately 10 times.
Samples of 5 mM cysteamine (CyaSH) in 6 M H2S04 glasses were prepared with natural and 0-17-labeled oxygen and irradiated to 0.2 Mrad at 77 K. The initial radicals formed in this glass are 'so4- and 'H. Annealing to 150 K results in the reaction of 'so4- with thiol to form the thiyl radical and subsequently the thiyl peroxyl radical having 0-17 couplings close to those previously found in neutral aqueous media.I0
-
+ CyaSH CyaS' + 0,
HS0,-
+ CyaS'
CyaSOO'
(3)
(4)
Other peroxyl radical intermediates which were not colored initially formed and subsequently converted into thiyl peroxyl species, developing their associated violet coloration. In Figure 1 we show results after formation and subsequent photoisomerization of 0-17-labeled CyaSOO'. Figure 1A shows the ESR spectrum of CyaSOO' found after annealing to 150 K. I7O hyperfine structures for the species CyaSI70O' (61 G) and CyaS0I7O' (82 G) are denoted by the stick diagram. Visible irradiation at 77 K converts CyaSOO' to CyaSO2' (Figure lB),
11
RS -00.
II
0
1
RSH
RSH
RSO*
CyaSO,OO*
'SO4-
0.
02
+
RSOH
RS*
+
RS0200H (5)
where R = Cya, NH3+CH2CH2, The results obtained in acidic media are in accord with this reaction scheme and suggest that the chemistry of thiyl peroxyl radicals does not depend on pH. Further, the fact that the hyperfine couplings of the CyaSOO' radical in acidic media are identical to those found in the neutral glasses suggests protolytic equilibria are not involved in the interaction of the CyaSOO' species with solvent in aqueous matrices. RSOO' in TCTFE Matrix. Employing TCTFE as a lowtemperature matrix for investigation of RSOO' radicals is complicated by the tendency of all primary thiols to form a separate phase on cooling. Dodecanethiol freezes out from TCTFE at any concentration tested, and hyperfine couplings presented in Table 1 for this system correspond to the TCTFE matrix only formally. n-Propanethiol and n-butanethiol freeze out from TCTFE at concentrations above 0.2 and 0.05 M. More dilute solutions used to generate thiyl peroxyl radicals by photolysis look uniform. However, microscopic homogeneity is by no means assured, and the possibility that thiyl peroxyl radicals formed in these systems have thiol molecules in their vicinity cannot be neglected. The ESR spectra observed immediately after UV irradiation of thiol solutions in TCTFE at 77 K are dominated by a broad nearly featureless spectrum of matrix radicals likely formed through chlorine abstraction from TCTFE by hydrogen atoms. UV irradiation also results in a yellow coloration of the samples (absorption maximum about 400 nm). The visible absorption is known to be associated with a broad ESR r e ~ o n a n c e , ~ ~ , ~ ~ which might be expected for trapped RS'. In the present case this signal is obscured by matrix radicals. However, assignment of this absorption to RS' contradicts pulse radiolysis data which suggest a visible absorption maximum at ca. 330 nm.27-31The 400 nm absorption found is more consistent with the formation of a weakly bound complex between RS' and RSH, similar to a protonated RSSP- radical.32 No proper ESR identification of this complex has been reported, although RSS' has been mistakenly identified as this species.33 We believe that the very broad and unresolved structure in this species which is very similar to RS' has prevented its identification. The spectra of degassed samples remain nearly unchanged on storage at 77 K, indicating that reactions of matrix radicals with dissolved thiols at this temperature are insignificant. At the same time, oxygen addition to trapped radicals proceeds spontaneously at
7996 J. Phys. Chem., Vol. 99, No. 20, 1995
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TABLE 2: Hyperfine Couplings and g Values of Sulfoxyl Radicals in Aqueous Media radical matrix Aii(l)/G Aii(2YG gl g2 2.0055 (average) CyaS02'a 6 M H2S04 56 56 2.038 2.007 CyaS0200' 6 M H2S04 105 46 2.021 2.009 CyaSO' 6 M H2S04 58 2.0055 (average) CysS02.b 8 M NaC104 58 58 2.038 2.008 cysso2oo' 8 M NaC104 106 45 2.021 2.0094 cysso' 8 M NaC104 58 Cya = H3N+CH2CH2-. Cys = OOCCH(NH3+)CH2-. (I
m
CH,CH,CH,SOO.
m
(CH,),CHSOO*
r,
Figure 2. ESR spectra of the "0-labeled matrix and thiyl peroxyl radicals in the TCTFE matrix formed by 254 nm photolysis of (A) n-PrSH (0.1 M), (C) i-PrSH (0.2 M), and (E) t-BUSH(0.2 M) in TCTFE followed by annealing at 90 K. Features assigned to RSO"0' and RS"00' species are marked by long and short bars. Spectra B, D, and F show the disappearance of RSOO' species under visible illumination with concomitant formation of RS02' radicals (single line centered at g = 2.0055). Wings of the spectra are amplified 15-20 times relatively to
original spectra.
77 K and completes on annealing at 90 K . This results in the formation of several types of peroxyl radicals stable at this temperature which are distinguishable owing to their different anisotropic 0-17 hyperfine coupling constants. Figure 2 shows these spectra obtained for primary (A), secondary (C), and tertiary (E) thiols. Spectra B, D, and F display the changes induced by visible irradiation. Two species were found to be insensitive to visible light with sets of All(l), All(2) oxygen-17 couplings of 105,45 G and of 94,60 G for the two oxygens of each peroxyl radical. These couplings are assigned to the freon (ClF2CCClFOO') and hydrocarbon peroxyl radicals (ROO'), respectively, and have been reported in previous work? A third type of peroxyl radical is found to be highly sensitive to visible and near-IR light and shows couplings which differ for primary (79,62 G), secondary (84,57 G), and tertiary thiols (96,51 G) (Table 1). These couplings assigned to the thiol peroxyl radical (RSOO') are denoted by stick diagrams in Figure 2. The change
g3
2.002 2.003 2.002 2.0025
ref this work this work this work 8 8 8
in coupling with the type of alkyl group is likely a result of steric hindrance that prevents the solvent-assisted stabilization of the intramolecular charge transfer state RS+OO'- . This mechanism is discussed further below. The formation of thiyl peroxyl radicals at 77 K clearly indicates that thiyl radicals are trapped in this matrix after UV irradiation, as expected. The formation of peroxyl radicals is accompanied by the growth of optical absorption around 540 nm. This absorption disappears simultaneously with the ESR spectrum of the thiyl peroxyl radical when the samples are subjected to visible light. This absorption is, therefore, assigned to RSOO'. Unfortunately, the absorption intensity observed in TCTFE was always too low to allow for accurate location of the maximum. Under the same experimental conditions the 540 nm absorption for RSOO' species was found to vary with the character of the alkyl group. The most intense absorption was observed for primary and secondary thiols. We attempted to check whether this difference originates from variations in absorptivity of different RSOO' species. To do this, we used the intensity of the RS02' signal (formed by visible irradiation of RS0O'containing samples) as a measure of RSOO' concentration in the sample. The RSOO' ESR signal itself was less reliable because of large variations in line shapes of different species and strong overlap of their spectra with signals originating from other peroxyls. Our results suggest that the 540 nm absorption of t-BuSOO' radicals absorbs 3-4 times less strongly than i-PrSOO radicals. RSOO' in IsooctaneIHexane (51 v/v) Matrix. Concentrations of t-BuSOO' radicals generated in this matrix by UV irradiation of t-BUSH solution (0.05-1.0 M) at 77 K with subsequent annealing at 90 K were too low for reliable determination of 0-17 coupling constants and optical absorption measurements, athough their formation was detected by small changes in ESR spectra induced by visible light. This is quite surprising because broad low-field resonance attributable to RS' (RSH complex) was clearly observed in this matrix after irradiation, especially at relatively high concentrations of thiol, disappearing in the course of the reaction with oxygen. The dominant peroxyl radicals formed in this system on annealing were hydrocarbon peroxyl radicals with a characteristic set of 0-17 couplings of 94 and 59 G. RSOO' in TCTFEAsooctane (1:l.S v/v) Matrix. This solvent mixture forms high-quality glass suitable for optical absorption measurements. The formation of peroxyl radicals in UV-irradiated 0.3 M solutions of n-BUSH and t-BUSH proceeds spontaneously at 77 K and results in the appearance of a well-defined new absorption band in the visible region around 540 nm attributable to thiyl peroxyl radicals. These spectra are shown on Figure 3A,C. Spectra B and D illustrate the disappearance of the features attributed to RSOO' species under visible illumination. ESR spectra at m~ = f 5 / 2 region clearly indicate that at least three types of peroxyl radicals are formed. A photosensitive species with an All(1) = 79 G and All(2) = 61 G set of couplings for n-BUSH,marked with arrows in Figure 3A, is identified as the n-butyl thiyl peroxyl radical, while two others are indentified as the usual freon and hydrocarbon peroxyl radicals with typical All( 1) couplings of
J. Phys. Chem., Vol. 99, No. 20, 1995 7997
Nature of the Thiyl Peroxyl Radical >,I
1
CH,CH,CH,SOO300 so0 700 wonm
I
I
(CH,),CHSOO*
'I '
uj
V
A
A
300
mo
700 m n m
e,
(CH,),CSOO*
-
Figure 4. ESR and optical absorption spectra of thiyl peroxyl radicals in methanol glass at 77 K. The samples containing I7O2 and 0.2 M thiols were W-irradiated at 77 K and annealed at 103 K to form the Figure 3. ESR and optical absorption spectra of (A) n-BUSH(0.2 M) and (C) t-BUSH (0.2 M) solutions in the TCTFEhsooctane matrix, Wphotolysed and stored at 77 K to allow peroxyl radical formation. Spectra B and D illustrate the disappearance of features assigned to
I70-labeled thiyl peroxyl radicals (marked by arrows) under visible illumination. The 170features unchanged under illumination belong to freon and hydrocarbon-derived peroxyl radicals. Wings of the spectra are amplified approximately 15 times. 105 and 95 G, respectively. The coupling constant with the terminal oxygen of the t-BuSOO' radical measured in this matrix is equal to 82 G (Figure 3C). However, light-induced changes in the spectrum also indicate the presence of species with an All(1) = 96 G coupling, typical for t-BuSOO' radicals in TCTFE and obscured by the intense signal of hydrocarbon-derived peroxyls formed in this matrix. RSOO' in Alcohol Matrices. The main species detectable by ESR in UV-irradiated solutions of n-PrSH, i-PrSH, and t-BUSH (0.3 M) in methanol are HOCH,' and HCO radicals. Both HOCHpOO' and RSOO' radicals were obtained on annealing at 103 K, the latter possessing the set of 0-17 couplings equal to 79 and 61 G independent of thiol used. These spectra are presented in Figure 4, where features assigned to RSOO radicals are denoted by stick diagrams. Insets in Figure 4 display the optical absorption spectra recorded in these systems. Absorption maxima of RSOO' species in methanol glass are 5-6 nm shifted to the red comparative to their position in TCTFEhsooctane glass (Table 1). y-Irradiation of 0.3 M cysteamine in ethanol at 77 K to 0.3 Mrad produces mainly CH3CHc)OH radicals that convert into their peroxyl radicals when annealed at 95 K (reaction 6). Annealing to 120 K produces coloration typical for the RSOO' radical (formed via
I70-labeledmatrix and thiyl peroxyl radicals (marked by vertical bars). Signals on the wings are amplified 10 times. reactions 7 and 8) with its associated ESR spectrum analysis, which shows 82 and 61 G oxygen-17 couplings. CH,CHOH' CH,CH(OH)OO'
+ 0, - CH,CH(OH)OO'
+ CyaSH -
CyaS' CyaS'
+ CH,CH(OH)OOH
+ 0, - CyaSOO'
(6)
(7)
(8)
t-BuSOO' in Butyronitrile and 2-MTHF Matrices. A butyronitrile matrix was used as a polar non-hydrogen-bonding medium. The matrix and thiyl peroxyl radicals were obtained by UV irradiation of a t-BUSH(0.3 M) solution at 77 K followed by annealing at 105 K. 0-17 couplings and optical absorption characteristics were found to be the same as for the t-BuSOO' radical in a methanol glass. Nearly identical results (except for the position of the absorption maximum) were obtained for this radical in the aprotic and low-polarity 2-MTHF glass (Table 1). RSOO' Species on Silica Surface. To investigate the extent to which the properties of RSOO' radicals are altered by the absence of bulk solvent, we produced RSOO' free radicals on silica surface containing chemisorbed thiol. Thioglycerol was used as a precursor to generate RS' free radicals by photolysis. Hydroxyl groups are known to interact with Si-OH surface groups on heating, forming ether bonds; as a consequence,
7998 J. Phys. Chem., Vol. 99, No. 20, 1995
R*
A I
C
RS*+RSS*+R* 77K
J
f l l
Figure 5. ESR spectra of thioglycerol-loaded silica at 77 K. (A) The spectrum after UV irradiation showing characteristic features of the RSS' radical at g = 2.06 and 2.026 and the carbon-based radical signal. The underlying broad resonance is due to RS' radicals. (B) The sample after annealing at 140 K to remove carbon-based radicals. (C) The spectrum recorded after "0-enriched oxygen admission at 90 K after the removal of exes oxygen in vacuum. Vertical bars indicate the features of "0-labeled thiyl peroxyl radicals. (D) The spectrum recorded after visible irradiation of the RS0O'-containing sample. The signal dominating in the central portion of the spectrum belongs to RSO' radicals. Signals on the wings are amplified 20 times.
thioglycerol should be covalently attached to the surface. The samples containing 10 mg of thioglycerol per gram of Si02 were preheated at 450 K and 0.02 Torr pressure to remove bulk water and at least a part of the physically adsorbed water from the surface and then photolyzed at 77 K. Photolysis results in the appearance of a broad resonance attributable to RS' radicals along with signals typical for RSS' 34 and carbon-based free radicals (Figure 5A). The latter were removed on annealing at 140 K, while the RS' and RSS' remained (Figure 5B). Subsequent oxygen admission at 90 K immediately results in a violet coloration and the corresponding light-sensitive ESR features of the RSOO' species with 0-17 couplings of 79 and 61 G (Figure 5C). The spectrum was recorded at 77 K after removal of excess oxygen at 120 K to improve the resolution in ESR spectra. The changes induced by subsequent action of light clearly indicate the photochemical loss of RSOO', as expected. Varying results are found depending on the wavelength of the light employed. With IR light, the only product found is the R S 0 2 0 0 radical, recognizable due to its characteristic ''0 couplings. This species is likely formed by addition of residual oxygen (probably adsorbed) to RSOi (which is generated photochemically from RSOO). However, on exposure to visible light, RSO' is found in addition to RS0200' (Figure 5D). This indicates that the absence of solvent affects
Razskazovskii et al. the photochemical product. In this case, photochemical cleavage of the RSOO' peroxyl bond with oxygen atom loss or transfer is suggested.
RSOO'
+ hv - RSO' + '0'
(9)
We believe the change in reaction mechanism with wavelength can be explained by the energetics of the reactions involved. In contrast to rearrangement to RSO2', which is exothermic, reaction 9 is endothermic (the peroxyl bond energy is about 2 eV), and an IR photons' energy (about 1- 1.5 eV) is not sufficient to rupture the 0-0 bond. While reaction 9 is observed on silica with visible light (2-3 eV), it is not observed with visible or UV light in solid matrices employed in this work. The matrix likely plays a dual role, reducing the escape of oxygen atoms by the cage effect and by supressing homolytic bond cleavage by the deactivation of reactive excited state(s). Thermal Stability of RSOO' Radicals in Different Matrices. The procedure used in the present work to generate RSOO' free radicals in various matrices shows that the RSOO' species do not thermally isomerize even on warming to 170 K in polar matrices such as 8 M NaC104 glasses. The main product of RSOO' on annealing in polar matrices is the RSO' radical (reaction 5).21 In contrast to these results we find that the t-BuSOO' species in the TCTFE matrix is highly reactive toward isomerization. The optical absorption and ESR features associated with RSOO' species in 0-17 labeled samples are the first to disappear on annealing to 100 K with concomitant growth of features with 105,45 G couplings, likely attributable to the sulfonyl peroxyl radical RS0200'. This strongly suggests that the conversion of RSOO to RS02' occurs at these low temperatures. We attribute this to the lack of solvent stabilization of the RSOO' species. Both RS020O' and the freon peroxyl radicals are expected to be highly reactive hydrogen abstractors as well as good one-electron oxidant^,^ and their stability at the temperature where RSOO' species isomerize further indicates a very high RSOO' reactivity in this matrix. Similar behavior was observed for n-BuSOO' radicals in TCTFE/isooctane glass, where they disappear at 95 K along with TCTFE peroxyls, leaving hydrocarbon peroxyl radicals. Ab Initio Molecular Orbital Calculations. (A) Anhydrous Structures. Several ab initio calculations of CH3SOO' have been performed previously.21.22None of these calculations suggest the near equal spin densities on the peroxyl oxygens or the visible absorption found for RSOO' species. Our extensive investigation of RSOO' radicals in different matrices reported in this work clearly shows that environment considerably affects the spin density distribution. Electron pair donation by the matrix molecules and parent thiol molecules are most probably involved in the interaction with RSOO' radicals. As a consequence, ab initio calculations were performed in this to test the effect of various nucleophilic species on spin density distribution in CH3SO0, which is employed as a model compound. All calculations were performed at the UHF/6-31G* level, and structures were fully optimized except as noted in Table 3.35 Calculations made to ascertain the interaction between CH3SOO' and one water molecule interacting through its oxygen with the sulfur atom revealed little or'no effect of the water molecule on the spin densities and energy levels of the CH3SOO' moiety. The interaction of CH3SOO' with stronger nucleophilic agents such as hydroxide (11), fluoride (111), or methylthiolate ions (IV) was then investigated. A strong bonding association of the oxygen atom of OH- and fluoride to the sulfur atom was found, forming the species CH3S(0H)OO' (structure 11) and CH3S(F)OO'- (structure 111). Methylthiolate anion interacts more weakly, forming structure IV with a relatively long S-S "bond". Nevertheless, it
J. Phys. Chem., Vol. 99,No. 20, 1995 7999
Nature of the Thiyl Peroxyl Radical
TABLE 3: Geometry and Spin Densities Calculated for Structures Shown in Figure 6 at the UHF/6-31G* Level selected optimized spin structure fixed parameters parameters density I full optimization r ( S 0 ) = 1.71 010.89 r(O0) = 1.30 020.13 S -0.019 I1 full optimization r ( S 0 ) = 2.359 010.67 r(O0) = 1.287 0 2 0.35 S -0.01 1 r(S-OH) = 1.74 I11 full optimization r ( S 0 ) = 2.035 0 1 0.75 r ( O 0 ) = 1.286 020.27 r(SF) = 1.77 S -0.016 IV full optimization r ( S 0 ) = 1.814 010.83 r(O0) = 1.287 020.20 r(SS) = 2.844 S -0.015 V internal parameters of H 2 0 and CH3 group; linear configuration of the hvdroeen 010.73 . - bond r ( S 0 ) = 2.141 r ( 0 0 ) = 1.285 0; 0.30 r(S-OH) = 1.819 S -0.015 VI linear configuration of two hydrogen bonds r ( S 0 ) = 1.962 01 0.78 r(O0) = 1.284 0 2 0.25 S -0.017 r(S-OH) = 1.93 VI1 same a s V r ( S 0 ) = 2.426 01 0.58 r(O0) = 1.282 0 2 0.45 r(S-OH) = 1.74 s -0.010 noticeably affects the spin density distribution in the peroxyl moiety (Table 3). On bonding of the negative ion to the sulfur, the negative charge partially transfers to the peroxyl group so the combination appears as an 0 2 ' - complexed with a CH3SX moiety. The loss of charge from X is roughly equal to the gain on the peroxyl oxygens. Note also that the S - 0 0 bond is lengthened substantially by the S-X interaction. ( B ) Effect of Water of Hydration. In order to assess the possible effect of the solvent on the CH3S(OH)OO'- structure, several calculations were performed with water molecules hydrogen bonded to the hydroxide moiety of the structure as well as the peroxyl group. In Figure 6 we show the optimized 6-31G* geometries of the structures considered (V, VI, and VII) and summarize the results of the calculations in Table 3. These calculations show that the bonding of OH- and 0 2 are appreciably affected by waters of solvation. Hydration of the hydroxyl group weakens its association with the CH3SOO' portion of the structure so that the S - 0 0 bond shortens from 2.36 8, without water to 2.14 8, for one water of hydration and 1.96 8, for two waters. The corresponding S-OH bond lengthens from 1.74 8, with no water to 1.93 8, with two waters hydrogen bonded to the hydroxyl group. Calculations with one water hydrogen bonded to each functional group (OH and 00) resulted in a geometry nearly identical to that found for CH3S(0H)OO' without water. The spin density distributions are also affected by the change in structure induced by the waters. Hydration of the hydroxide group reduces its bonding to the sulfur atom and produces a spin density more like that in CH3S O 0 alone. With one water on each group, we find peroxyl oxygen spin densities (0.58, 0.45) very similar to those found experimentally? These calculations show that water of solvation can greatly affect the bonding of 0 2 or OH- and clearly demonstrates the "push-pull"-like bonding between the two oxygen-containing groups attached to the sulfur atom. However, when each group is equally solvated, results similar to those found for the RS(0H)OO'- species alone are found.
Discussion The Innuence of Solvent on Oxygen-17 Couplings. Oxygen17 couplings of RSOO' species in various environments measured in this work clearly show the considerable influence of the environment on unpaired spin density distributions in these species. While carbon-based peroxyl radicals (ROO') show a small variance in I7O hyperfine couplings with the
charge density -0.05
-0.36 0.43
-0.35 -0.52 0.44 -0.23 -0.53
0.51 -0.14 -0.43 0.43 -0.28 -0.5 1 0.47 -0.22
-0.49 0.47 -0.37
-0.45 0.45
electron-withdrawing character of the substituent (R), there is no significant effect of the environment on these couplings. The situation observed in the case of thiyl peroxyl radicals is quite different. The sum of All(0-17) coupling constants is always less than 150 G, which is indicative of some spin delocalization onto sulfur. Previous results for carbon peroxyl radicals suggest for unit spin density that the sum of the oxygen couplings is 154 G.9 In Table 1 this value is used to estimate the total oxygen spin density, and 0.90-0.95 of the spin density is found to reside on the peroxyl group, while the remainer is likely localized on the sulfur atom. In terms of valence-bond structures it means that, in addition to the unusual carbon peroxyl radical valence structures (A, B), structure C should be appended and should have a 5-10% weighting factor. In view of our results which show near equal spin density on oxygens in polar solvents we propose the fourth structure (D) which represents a radicalion pair consisting of the sulfenium cation-nucleohphile complex and superoxide radical anion. Here, full electron transfer from thiyl radical to oxygen molecule has occurred, which in contrast to structure C suggests zero spin density on the sulfur atom. .+
e..
R-S-04
n-
RS-O-O
.+
n
R-S-04
n-
.-
"+ R S (00) e.
X
A
B
C
D
The properties of charge transfer states such as C and D including the degree of intramolecular charge separation are commonly affected by the medium through its static dielectric polarization. In the case of thiyl peroxyl radicals the influence of solvent is quite evident from the data presented in Table 1. However, we find the correlation between the oxygen-17 couplings in RSOO' with the dielectric properties of various solvents to be quite poor. The most important property of the solvent influencing spin density distribution seems to be not polarity but the ability to solvate electron deficient centers through donation of a lone pair. In all solvents except TCTFE and hydrocarbons and, further, even in the absence of bulk solvent (as it occurs on silica surface) RSOO' displays nearly the same oxygen-17 coupling. The same couplings are found regardless of the nature of the alkyl group (R) or the solvent polarity. The other striking feature which can't be explained by the solvent polarization model is the influence of alkyl group
Razskazovskii et al.
8000 J. Phys. Chem., Vol. 99, No. 20, I995 0
-
'
8 8 8 8
' 0-
I11
I1
I
0-
0-
II
VI1
IN I I 1
\
IV
0
V
\
\
VI
Figure 6. UHF 6-31G* optimized structures investigated in this study: (I) CH3SOO'; (11) CH3S(OH-)OO'; (111) CH3S(F-)OO'; (IV) CH3S(SCH3-)OO'; (V) CH3S(OH-)OO.H20; (VI, VII) CH~S(OH-)OOO~H;O.
size on spin density distribution found for thiyl peroxyl radicals in the poorly solvating TCTFE matrix. While the tertiary alkyl group shows a large terminal coupling similar to a normal carbon-based peroxyl radical, the secondary and primary alkyl substituted RSOO' radicals show increasingly more uniform spin distributions. The length of the alkyl group seems less significant than branching, as tert-butyl- and tert-amylsubstituted radicals show the same couplings. Thus, this effect should be attributed to steric hindrance created by bulky alkyl groups. It also emphasizes that the accessibility of solvent molecules to the sulfur atom is an important factor influencing spin density distribution in thiyl peroxyl radicals. The sulfur atom in the electronic ground state of RSOO' is predicted by MO theory to have a partial positive charge. The compounds containing electropositive divalent sulfur attached to an electronegative group such as sulphenyl halides, RSX, are known to be very sensitive to nucleophilic substitution at the sulfur atom. Attack by solvent molecules through donation of a lone electron pair to the sulfur would result in a charge transfer to the peroxyl group in RSOO' with 02'- acting as a leaving group (valence structure D) . Even for TgSOO' on silica, where no solvent interaction is possible, the presence of surface hydroxyl groups and adsorbed water likely allow for interaction with the sulfur atom. Although theoretical calculations do not predict a substantial interaction of water with RSOO', they do predict that stronger nucleophiles such as hydroxide or fluoride ions will assist the charge transfer to the peroxyl group (Table 3). Binding of these negative ions is predicted to result in substantial transfer of its charge to the
peroxyl group, mainly to the terminal oxygen, which is nearly uncharged in the RSOO' radical itself. The changes in theoretical spin density distribution attributed to the binding of negative ions resemble the experimental changes induced by electron pair donating solvents. Thus, we consider the theoretical calculations as only indicative of the sensitivity of the electronic structure of the RSOO' radical to the nucleophilicity of the medium. Optical Absorption Spectra. Our results clearly indicate that optical absorption with maxima at 540-550 nm is a common feature of RSOO' species in a variety of matrices. This feature is unusual for peroxyl radicals, implying that the nature of this electronic transition is closely connected with the presence of the sulfur atom. A new electronic transition in the near-UV region involving the SOMO and as-o* empty orbital is actually predicted for the CH3SOO' radical by MS-Xa calculations.22 However, it seems probable that excitation from the sulfur lone pair to the SOMO localized mostly on the terminal oxygen might also be responsible for the appearance of a new low lying excited state. In any case, the electronic excitation implies a considerable intramolecular charge redistribution, and the corresponding transition energy should be sensitive to medium polarity. The experimentally observed red shift from 539-540 to 546550 nm on going from nonpolar TCTFE to methanol (6 = 64 at 160 K) corresponds to 0.9 kcal mol-'. This energy difference may seem to be small; however, we note a simple treatment of the electrostatic interactions of solvent and solute based on the Onsager relations36assuming a 2-5 D dipole moment change on excitation and a 3-4 8, cavity radius yields values that are
J. Phys. Chem., Vol. 99, No. 20, 1995 8001
Nature of the Thiyl Peroxyl Radical of the same magnitude as the experimental ones. It should be emphasized that consideration presented above doesn't imply any solvent influence on the electronic structure of the chromophore and takes into account electrostatic interactions with the environment only. Specific interactions such as donor-acceptor or hydrogen bonding can alter the energy levels of the isolated system, thus influencing its electronic spectrum "in vacuum". Thermal Stability. A striking difference is found in the reactivity of RSOO' species in TCTFE vs polar media in the dark. RSOO' radicals are the most unstable peroxyl species in TCTFE, and they are the least reactive of the peroxyl radicals in polar media. Further, the reaction mechanisms also differ. In alcohol and aqueous matrices the main reaction pathway is known to be a bimolecular reaction with thiol leading to sulfinyl radicals,21 RSO', which only occurs on warming to allow for molecular diffusion. The disappearance of RSOO' radicals in TCTFE occurs by thermal isomerization to RS02' radicals at 100 K. This species adds oxygen to form R S 0 2 0 0 , as oxygen is mobile at 100 K in TCTFE. According to quantum chemical calculations,8122the CH3S02' radical is substantially more stable than CH3SOO' by 35-50 kcal mol-', so isomerization is expected to be highly favorable and is found to occur at room temperature in aqueous solutions.'2 The stability of RSOO' in alcohol and aqueous media can be explained by nucleophilic stabilization of the charge transfer state by electron pair donation to sulfur. The activation energy for unimolecular isomerization of RSOO' to R S O i would be higher in this complex state than for the uncomplexed radical and would lead to a longer lifetime, as found.
Summary The properties of the thiyl peroxyl radical are suggested in this work to be highly responsive to the solvent. The stabilization of charge transfer from the sulfur atom to the peroxyl group occurs through specific nucleophilic complexation with the sulfur atom in RSOO'. This is the most important interaction influencing the spin density distribution in thiyl peroxyl radicals. For most solvents it dominates all other factors such as steric hindrance arising from alkyl group size. Only in the case of weak solvent interactions as found for TCTFE or hydrocarbon solvents do steric factors come into play. Our theoretical calculations mimic this situation in the presence of nucleophiles such as hydroxide or fluoride anions. In other work the variation in the spin density on the terminal oxygen in peroxyl radicals was shown to correlate with variation in one-electron-oxidation rates in liquid solution^.^ Our results, which indicate large changes in spin distribution with solvent, allow us to predict large variations in reactivity for RSOO' in different environments. For example, in a hydrocarbon or TCTFE environment tert-butyl-S00' should be considerably more reactive toward hydrogen abstraction than in an aqueous environment. Our work has already shown that in such matrices the rate of isomerization of RSOO' to RSO2' is found to be greatly increased.
Acknowledgment. We thank the National Cancer Institute of the National Institutes of Health (Grant ROlCA45424) and the Office of Health and Environmental Research of the Department of Energy (Grant DEFG028ER60455) for support of this work. We thank the DOE National Energy Research Supercomputer Center at Lawrence Livermore National Labora-
tory for generous grants of computer time. A.O.C. was supported as a Predoctoral Fellow by the Oakland University Research Excellence Program in Biotechnology.
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