4432
J. Phys. Chem. 1982, 86, 4432-4435
tribution function for realgar vapor “resembles the radial distribution function for orpiment to a certain extent” is now understandable, since As$, molecules are the dominant species in both vapors. It was also found that fused (amorphous) and precipitated (crystalline) As2S3 gave identical vapor-phase electron diffraction patterns. Our new value Doo(AsS)= 89.8 f 1.5 kcal/mol is lower than the incorrect values reported by Faure et al.’ (114 kcal/mol) and Pashinkin et a1.2 (96-102 kcal/mol) for reasons noted above. A linear Birge-Sponer extrapolation of the ground-state vibrational levels5 of ASS yields D(LBX) = 116 kcal/mol, but this will significantly overes-
timate Doo for a relatively covalent molecule such as ASS. Correction for the degree of ionic-covalent character20 yields Doo(AsS)/D(LBX) = 0.74 and Doo(AsS) 86 kcal/mol, in good agreement with our experimental value. A correlation of Doo values in the series PO (142), As0 (114), BiO(80) with PS (104) and BiS (73) also indicates 90 kcal/mol to be a reasonable value for DOJAsS).
Acknowledgment. This research was supported by the National Science Foundation under Grant DAR 79-12296. (20) D. L. Hildenbrand, J. Electrochem. Soc., 126, 1396 (1979).
Pulse Radiolytic and Product Analysis Studies of the Reaction of Hydroxyl Radicals with Cinnamic Acid. The Relative Extent of Addition to the Ring and Side Chain‘ K. Bobrowskl’2 and N. V. Raghavan Radiation of Laboratory, University of Notre Dame. Notre Dame, Indiana 46556 (Received: July 21, 1981; In Final Form: July 16, 1982)
Using pulse radiolysis with optical detection and high-pressure liquid chromatography (HPLC), we show that reaction of OH radicals with cinnamic acid (CA) in aqueous solutions leads to addition to both the ring and the olefinic group. The relative extent of the above two pathways was estimated as 3:7, respectively. Benzyland hydroxycyclohexadienyl-typeradicals were observed with absorption maxima at 320 (310) and 370 (365) nm depending on the pH of the solution. In the pH region 4.9-5.7 the absorption at 305-315 nm decays during the first 5 ks after the pulse. The dependence of the rate constants and absorption spectra on pH suggests that this decay is due to an equilibration process between acid-base forms of benzyl-type radicals formed through OH addition to the olefinic group.
,R
Introduction The OH radical reacts with benzene and most of its derivatives predominantly by addition to the ringa3l4 It has been recently shown from substituent effects that hydroxyl radical behaves like a typical electrophile.6-10 Even though extensive work has been done on the pattern of addition of OH to benzene derivatives, very few quantitative studies have been carried out if the substituent on the benzene ring possesses a double bond (for example, styrene or cinnamic acid). In these systems hydroxyl radical can add either to the benzene ring or to the side chain
(1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2267 of the Notre Dame Radiation Laboratory. (2) On leave of absence from the Institute of Nuclear Research, 03-195 Warsaw, Poland. (3) Neta, P.; Dorfman, L. M. Adu. Chem. Ser. 1968, No. 81, 222. (4) Jefcoate, C. R. E.; Norman, R. 0. C. J. Chem. SOC.E 1968, 48. (5) Raghavan, N. V.; Steenken, S. J. Am. Chem. SOC.1980,102,3495. (6) Selvarajan, N.; Raghavan, N. V. J . Phys. Chem. 1980, 84, 2548. (7) Volkert, 0.; Bors, W.; Schulte-Frohlinde, D. Z. Naturforsch. E 1961, 22, 480. (8) Samuni, A.; Neta, P. J . Phys. Chem. 1973, 77, 1629. (9) Neta, P. Radiat. Res. 1973, 56, 201. (10) Steenken, S.; O’Neill, P. J . Phys. Chem. 1978, 82, 372.
The first attempt to obtain the relative extent of the above two pathways was Swallow’s’’ work concerning pulse radiolysis of styrene. From the position of the absorption maxima and the known extinction coefficients, he estimated that about 80% of the OH radicals add to the ring and about 20% of the remaining OH radicals add to the 0 position of the vinyl group in styrene. In contrast Mehnert and c o - ~ o r k e r s ~later ~ - ~ *concluded that addition (11) Swallow, A. J. Adu. Chem. Ser. 1968, No. 81, 499. (12) Brede, 0.; Helmstreit, W.; Mehnert, R. J. Prakt. Chem. 1974,316, 402. (13) Brede, 0.;BOs, J.; Helmstreit, W.; Mehnert, R. Z. Chem. 1977,17, 447. (14) Brede, 0.; BOs, J.; Mehnert, R. Radiochem. Radioanal. Lett. 1979, 39, 259.
0 1982 American Chemical Society 0022-3654/82/2086-4432$01.25/0
The Journal of Physical Chemistry, Vol. 86, No. 22, 7982 4433
Reaction of Hydroxyl Radicals with Cinnamic Acid
to the side chain was favored. Accurate determination of the relative extent of the two pathways from the Mehnert data is difficult. However, the quantity can be estimated quantitatively if the initial OH adducts are converted into stable products. In the case of styrene such product analysis studies were difficult since reference compounds are not readily available. All three isomeric hydroxycinnamic acids (OHCA) are known, so that we have set out to investigate the radiolysis of aqueous solutions of cinnamic acid by both pulse radiolysis and product analysis methods. The latter were carried out by high-performance liquid chromatography. The method most commonly used for the conversion of hydroxycyclohexadienyl radicals into stable products involves oxidation by oxidants such as K,[Fe(CH),] or IrCl,2- to yield p h e n o l ~ . ~ J ~AJcombi~ nation of these techniques helps to obtain a quantitative estimation of the relative extent of the above pathways. We find that in this case addition to the side chain is favored, in contrast to the above-mentioned results in styrene.
Experimental Section The solutions were irradiated by 5-11s electron pulses from an ARC0 LP-7 linear accelerator. The dose per pulse was such that it produced only 2-4 pM of radicals and minimized their second-order decay on the microsecond time scale. Dose effects were measured with pulses of up to 50-11s duration. Optical detection and signal averaging were carried out by the computer-controlled pulse radiolysis apparatus described pre~ious1y.l~The absorbance measurements were compared to (SCN),-- with a reference value of 470001a taken for the product of yield and extinction coefficient at 480 nm in N20-saturatedsolutions. Kinetic analysis was carried out by fitting a least-meansquare exponential buildup or decay to the observed data using a Hewlett-Packard 9830 calculator having a peripheral 9862 X-Y p10tter.l~ The cinnamic acid (CA) and hydroxy derivatives of cinnamic acid (OHCA) were obtained from Aldrich and were of the highest purity available. They were used as received. Styrene was obtained from Eastman. All other inorganic reagents were from Matheson, Baker, Mallinckrodt, or Fisher. For the pulse radiolysis experiments solutions were prepared by using reagent-gradewater from a Millipore Milli-Q system. Fresh solutions were prepared before each irradiation. They were deoxygenated by bubbling with N2 or N20. In the latter case the N 2 0 was used in order to remove oxygen and to convert ea; into OH by the reaction N 2 0 ea; + H20 N2 OH OH-. The pH of the solutions was adjusted with NaOH, HC104, Na2HP04,and NaH2P04. Optical absorption spectra of the solution before and after the irradiation were recorded on a Cary 219 spectrophotometer. The absorption spectra were corrected for the depletion of the solute (S) in the wavelength where the solute absorbs light (X I320 nm). For product analysis studies, 0.5 mM solutions of cinnamic acid containing 0.3 mM oxidant were saturated with N20 and ,OCo y irradiated by using dose rates of 3.12 X 1017 eV g-l min-' as determined by Fricke dosimetry. Samples (150 pL) of the irradiated solutions were analyzed
+
-
+
+
(15)Steenken, S.; Raghaven, N. V. J. Phys. Chem. 1979,83,3101. Bhatia, K.; Madhavan, V.; Schuler, R. H. J. Phys. (16)Klein, G.W.; Chem. 1975,17,1767. (17)Patterson, L. K.; Lilie, J. Int. J.Radiat. Phys. Chem. 1974,6,129. Patterson, L. K.; Janata, E. J. Phys. Chem. 1980, (la)Schuler, R. H.; 84, 2088.
(19)Schuler, R. H.; Buzzard, G. Znt. J. Radiat. Phys. Chem. 1976,8, 563.
I
300
350
400
X-nm
Flgure 1. Absorption spectra (correctedfor the bleaching of the parent compound) of the intermediates obtained after pulse radiolysis of aqueous solutions of cinnamic acid saturated with N,O at different pHs (immediately after the pulse), 40 ps after the pulse for pH 14: (A)pH 4.0, (0)pH 6.0, (m) pH 14.
with the high-performance liquid chromatography apparatus previously describeda20 The isomeric hydroxycinnamic acids were detected optically at 267 nm. Details of the separation and detection have been described elsewhere.21 For quantitative estimation of the products the possibility of the thermal reactions, if any, between isomeric hydroxycinnamic acid and various oxidants IrCh2and Fe(CN)t- was checked. A facile thermal reaction was observed between IrCls2- and isomeric hydroxycinnamic acids. On the contrary, no thermal reaction was observed if Fe(CN)63-was used as the reactant instead of IrCl:-.
Results and Discussion The radiolysis of water may be represented by reaction 1.
H 2 0 k- ea;,
OH, H, H30+,H202,H2
(1)
In N20-saturatedsolutions ea; is quantitatively converted into OH radicals: eaq- + N20 + H 2 0
-
OH-
+ N2 + OH
(2)
such that yield of OH per 100 eV of absorbed radiation G(OH) is equal to 6.0. Since G(H) = 0.55, in irradiated N20-saturated aqueous solutions the OH radical constitutes >90% of all the radical produced. Pulse Radiolysis Studies. The transient absorption spectra obtained under various experimental conditions in the pulse radiolysis of cinnamic acid are given in Figure 1. Under these conditions the spectrum represents the products of the reaction of OH radicals with the solute with a contribution of about 10% from the H radicals. As the H adducts have a sharp spectrum with the absorption maximum at X = 320 nm (see Figure 2), a significant contribution to the observed spectrum is to be expected. In almost neutral N20-saturated aqueous solutions (pH 6.0) three absorption maxima were observed immediately after the pulse at 310, 320, and 365 nm. After the correction for the H adduct, the spectrum is composed only of two peaks, one at 310 nm and the second at 365 nm with relative absorbances 7600 and 5150, respectively. The rate of the formation of the transient followed first-order kinetics. From the fitting of the exponential increase of the (20)Raghavan, N.V. J. Chromatogr. 1979,168, 523. (21)Raghavan, N.V.;Bobrowski, K.; Selvarajan, N., paper in preparation.
4434
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982
, 9000-
8000-
700C60005000-
\,
I
1 1
'
\
I I
I
?
\ i;
40003000
Bobrowski and Raghavan
l0,OOO~
b
I
Flgure 2. Absorption spectrum (corrected for the bleaching of the parent compound) of hydrogen addltion to CA observed at pH 1, saturated with N,, also containing 0.5 M tert-butyl alcohol to remove OH radicals.
Scheme I
A- nm Flgure 3. Absorption spectra of the Intermediatesobtained after pulse radiolysis of aqueous solutions of CA at pH 6.0 ( 0 )immediately after the pulse, (A)after the completion of the fast decay process, and (m) at pH 14, 40 ps after the pulse.
-
310- and 365-nm absorptions after the pulse and the concentration of cinnamic acid (in the range 6 X 10-"5 X M)8 a rate constant k3a,b = 8.1 X lo9 M-' s-' was calculatedn for the addition reaction 3 (see Scheme I). On the other hand, the transient absorption spectrum taken immediately after the pulse in acid solution below pH 4.0 was distinctly different from those observed at higher pHs. This spectrum exhibits two intense bands with the absorption maxima located a t 320 and 370 nm. After a correlation similar to that in the case of neutral solutions relative absorbances 6800 and 4350 were found, respectively. The absorption obtained a t 320 (pH 4) or 310 (pH 6) nm is consistent with that expected for benzyl-type r a d i ~ a l s . ' ' J ~ J ~Similar , ~ ~ peaks were observed in the pulse radiolysis of styrene and l-meth~lstyrene.'~,~ These peaks are therefore attributed mainly to adducts of OH radicals a t the /3 position in the vinyl group, to give radicals resembling benzyl (forms I1 and I11 in Scheme I). The absorption obtained at 370 (pH 4) or 365 (pH 6) nm is partly caused by a hydroxycyclohexadienyl-typeradical formed by addition of OH to the benzene ring of the cinnamic acid molecule. Taking into account product analysis data (see below text) the observed spectra represent -70% of
benzyl-type radicals and 30% of hydroxycyclohexadienyl-type radicals. This is in excellent agreement with values estimated by Mehnert and c o - w o r k e r ~ . ~ ~ - ' ~ The reaction of H atoms with cinnamic acid was studied after removal of most of the OH radicals through reaction with tert-butyl alcohol at pH 1. The experimental conditions for obtaining the absorption spectrum of the transient as seen in Figure 2 were 5 x M cinnamic acid, 5 X lo-' M tert-butyl alcohol, and lo-' M HC104, nitrogen-purged aqueous solution. The observed absorption absorption spectrum was corrected for the depletion of the solute (S) in the wavelength range where the solute absorbs the light and for the contribution to the absorption of t-BuOH radicals. The spectrum observed immediately after the pulse has a very sharp absorption band with the maximum at X = 320 nm and a second very broad absorption band with the maximum around 370 nm. From the rate-constants data it is estimated that the fraction of OH radicals scavenged by the alcohol is more than 98%. Using Mehnert'~'~ result that H atoms react predominantly (185%) by addition to the olefinic group, the observed spectrum represents 85% of benzyl-type radicals and 15% of cyclohexadienyl radicals. Assuming that absorption by the latter radical at X = 320 nm is negligible in comparison to the first one, a value 8500 M-' cm-' for 6320 of PhCHCH,COOH was estimated. The spectrum of the OH adduct to the olefinic group of cinnamic acid is expected to resemble that ofthe H adduct,24so the values 9700 M-l cm-' for 6320 of PhCHCH(0H)COOH and 3300 M-' cm-' for €370 of Ph(OH)CH=CHCOOH were calculated. Our values for the extinction coefficients appear reasonable and are very close to the values given for such types of radicals,11-1423 In a very narrow region of pH (4.9-5.7), the absorption spectrum in an aqueous solution of CA undergoes a small change during the first 5 p s after the pulse (Figure 3). The spectrum after decay coincides fairly accurately with the spectrum obtained in strongly alkaline solution. In the region 305-315 nm the observed absorption decays fast in a first-order reaction independent of dose and solute concentration but slightly dependent upon H+ concentration. The decay of the absorption is much more pro-
(22) When one uses the calculated ratio of both ring and olefinic group addition, the rate constants for OH addition to the ring and OH addition to the olefinic group are equal to 2.4 X I O 9 and 5.7 X IO9 M-I s-l, respectively. (23) Schneider, C. Ado. Chem. Ser. 1967, No. 911, 219.
(24) Because the absorption at X = 370 (365) nm represents only 30% of hydroxycyclohexadienyl-type radicals, such an assumption is justifiable, in view of the fact that for many aromatic compounds the extinction coefficients of such adducts are in the range 3000-5000 M-' cm-'.
.n
HC=CH-C
HC=CH-C~-
O 'H
tOH
"E OH
OH
I
H&CH
-cBO
+OH HC=CH-C
/o
@OH
The Journal of Physical Chemistty, Vol. 86, No. 22, 1982 4435
Reaction of Hydroxyl Radicals with Cinnamic Acid
:'
. .(... ..'....:,..-.' , ....(.
.. .
pH 565 '-..._._-''.
