J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 711-716 71 1 Pulse Radiolytic One-electron Reduction of 2-Hydroxy- and 2,6=Dihydroxy=9,1O=anthraquinones Haridas Pal,* Tulsi Mukherjee and Jai P. Mittal Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085,India The semiquinone free radicals produced by one-electron reduction of 2-hydroxy-9,lO-anthraquinone (2HAQ) and 2,6-dihydroxy-9,10-anthraquinone (26DHAQ) in aqueous formate solution, water-isopropyl alcohol-acetone mixed solvent and isopropyl alcohol have been studied using the pulse radiolysis technique. The absorption characteristics, kinetic parameters of formation and decay, acid-base behaviour and redox characteristics of the semiquinones have been investigated and compared with the corresponding characteristics of a few intramole- cularly hydrogen-bonded anthrasemiquinone derivatives.The non-hydrogen-bonded semiquinones show two pK, values (4.7 and 10.7 for 2HAQ and 5.4 and 8.7 for 26DHAQ, respectively) within the pH range 1-14, whereas other intramolecularly hydrogen-bonded semiquinones show only one pK, . The one-electron reduction potential (€,) values for 2HAQ (-440 mV) and 26DHAQ (-400 mV) are more negative than those of the intramolecularly hydrogen-bonded systems. Pulse radiolysis is an important technique for studying the electron-accepting properties of the quinones by character- ising their semiquinone radicals formed by one-electron reduction.'-24 A difference in substitution in simple quinones may cause different characteristic changes in their proper- tie^.^'-^' In the present paper we report on the character- istics of the semiquinones of 2HAQ and 26DHAQ (where no intramolecular hydrogen bonding exists) in various solvents and compare the results with those of other similar systems with intramolecular hydrogen bonding. Since both 2HAQ and 26DHAQ are not sufficiently soluble in water below pH x 8, a water-isopropyl alcohol-acetone mixed-solvent system (30.2 : 5 :1 molar ratio) was mostly used in the present st~dy.'~,'~,~~ At pH > 8, however, the reduced semiquinone radicals were also produced and investigated in aqueous solutions containing 10-mol dm-3 sodium formate as an additive. The reduced semiquinone radicals were also studied in pure isopropyl alcohol solution for comparison purposes.In the above aqueous-organic mixed-solvent system, since the bulk of the medium remains essentially aqueous, reporting of the pH of the solutions is j~stified.~~,~',~ Experimental 26DHAQ (Aldrich) was purified by repeated crystallisation from methanol. 2HAQ was prepared from 2-amino-9,lO- anthraquinone (TCI, Japan) by diazotization followed by hydrolysis in hot acidic water. 2HAQ thus prepared was purified by crystallisation from a water-methanol mixture. The crystals showed a melting point of 306"C, the reported value. Isopropyl alcohol and acetone were of spectroscopic grade from Spectrochem India. Other chemicals used were of the purest grade available from Fluka, Merck or BDH and were used without further purification.Triply distilled water was used for making all aqueous solutions and in the aqueous-organic mixed solvent. Solutions were made alka- line to 1 mol dmP3 NaOH for studies at pH x 14. Details of the pulse radiolysis and other experimental arrangements are as described earlier. 15-'7 The absorbed dose was measured by thiocyanate dosimetry (5 x mol dm-air-saturated KSCN sol~tion~~.~~)following the absorbance of the (SCN);- radicals at 500 nm, produced under isodosage conditions. Results and Discussion Absorption Spectra and pK, Values of 2HAQ and 26DHAQ In the pH range (1-14) of our investigation, 2HAQ can exist either in neutral or in monoanionic form and 26DHAQ can exist either in neutral, monoanionic or dianionic form, depending on the pH of the solutions, as the phenolic OH groups participate in the protolytic equilibria.For the conve- nience of presentation, the neutral, monoanionic and dia- nionic forms are generalised as QH2, QH- and Q2-, respectively. Spectroscopic data and the acid dissociation constants of 2HAQ and 26DHAQ are listed in Table 1, along with those of unsubstituted anthaquinone (AQ)34 and other anthraquinone derivatives, namely 1,4-dihydroxy-9,10-anthraquinone (14DHQ),24 1,5-dihydroxy-9,10-anthra-quinone (15DHAQ)I6 and 1,8-dihydroxy-9,10-anthraquinone (18DHAQ).16 From Table 1 it is seen that the pK,, and pK,, values for the present systems are much lower than those of 14DHAQ, lSDHAQ and 18DHAQ, as the last three quin- ones have strong intramolecularly hydrogen-bonded struc- tures, resulting in deprotonation at higher pH.Considering the position and intensity of the absorption peaks of all the hydroxyquinones listed in Table 1, it is seen that for 2HAQ and 26DHAQ the charge-transfer (CT) of the longest-wavelength absorption band is much weaker (lower absorption maxima and low molar absorption coefficient for the CT band) than with Table 1 Absorption characteristics of 2HAQ, 26DHAQ and other related quinones in aqueous 5 mol dm-3 isopropyl alcohol-1 mol dm -acetone A/nm (&/lo2m2mol-') quinone neutral monoanion dianion pK,, pK,, ' AQb2HAQ 26DHAQ 330 (5.01) 375 (1.77) 332 (2.50) 350 (8.45) 400 (1.80) -481 (2.60) 348 (4.12) 333 (18.7) 422 (8.90) 480 (2.78) --344 (27.0) 420 (15.4) 500 (1.56) -7.4 7.1 -8.7 - 14DHAQ' 15DHAQd 18DHAQd 465 (8.32) 477 (8.43) 420 (9.60) 430 (12.2) 547 (9.74) 581 (8.63) 480 (12.2) 495 (11.0) 562 (10.4) 600 (10.8) 475 (13.3) 495 (12.4) 9.9 10.6 9.7 12.7 12.5 12.1 (I Error limits in pK, values are k0.l; ref.34; 'ref. 24; ref. 16. those for 14DHAQ, l5DHAQ and 18DHAQ, of which 14DHAQ has the strongest CT character indicated by the longest-wavelength absorption maximum for the CT band. This is due to the quasi-aromatic structure of 14DHAQ resulting from intramolecular hydrogen bonding.,' Difference (Semiquinone -Quinone) Absorption Spectra and the Acid Dissociation Constants of the Semiquinone Radicals On delivery of an electron pulse to a nitrogen-bubbled solu- tion of a quinone (ca.lop4mol drn-,) in an aqueous-organic mixed solvent, semiquinone radicals are formed as follows : H,O4 H', e,;, OH', other products (1) OH'(H') + CH,CHOHCH, -, H,O(H,) + CH,tOHCH, (11) e,; + CH,COCH, +(CH,COCH,)'-(111) (CH,COCH,)'-+ H20+CH,tOHCH, + OH-(IV) CH,eOHCH, + QH,(QH-or Q2-) + QH;(QH;-or QH*'-) + CH,COCH, (V) In N,O-saturated aqueous solution of quinones mol drn-,) containing lo-' mol dmP3 sodium formate, the semi- quinone radicals are formed as follows : Hz eai + N,O -N, + OH' + OH-(VI)OH'(H') + HCO, ___* H20 + C0;-(VW C0;-+ QH-(Q2-) -QH;-(QHo2-) + CO, (VIII) In reactions (V) and (VIII) the product semiquinone radicals can be either of neutral (QH;), monoanionic (QHi-) or dia- nionic (QHo2-) form depending on the pH of the solutions.In pure isopropyl alcohol some solvated electrons are gen- erated, G(es-) = 100 nmol J-l, which may reduce the quin- ones .directly.? The main reducing species, however, are the CH,COHCH, radicals [reaction (IX)] formed either from the higher excited states, or, by a neutralisation reaction involving es- to give H' followed by reaction (11)and also by ion-molecule reactions. CH,eOHCH, + QH, +QH; + CH,COCH, (IX) The semiquinone-quinone difference absorption spectra obtained in aqueous-organic mixed solvent [after completion of reaction (V)] for 2HAQ and 26DHAQ at different pH are shown in Fig. 1 and 2, respectively. A careful investigation of these spectra clearly suggests that for both the systems the semiquinone exists in three different forms of protonation at pH 1.5, 7 and 13.The pK, values associated with the different forms of the semiquinones of 2HAQ and 26DHAQ have been estimated following the changes in AA with pH at suitable wavelengths where there is no ground-state absorption, and fitting the experimental points according to eqn. (1). t The G values for the free radicals is given by the number of molecules formed per 100 eV of energy absorbed, or, in SI units, by the number of mol formed upon absorption of 1 J of energy per kg. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 32 , 2 0 300 400 500 600 700 800 A/nm Fig. 1 Semiquinone -quinone difference absorption spectra of the reduced semiquinone radical of 2HAQ in aqueous-organic mixed solvent at pH 1.5 (O),7.0 (*) and 13 (0).Dose, ca.8.7 Gy. where AA is the observed absorbance at any pH and A,, A, and A, are the absorbances if the semiquinones exist exclu- sively in forms 1, 2 and 3, respectively. pK,, and pK,, are the first and second acid dissociation constants of the radicals, respectively. Fig. 3 shows the AA us. pH plot for the semi- quinones of 2HAQ (at 615 nm) and 26DHAQ (at 600 nm) along with the respective computer-fitted curves. The pK,, and pK,, values thus obtained for the semiquinones of 2HAQ and 26DHAQ are listed in Table 2. From comparing the semiquinone-quinone difference absorption spectra in aqueous-organic mixed solvent at pH 1.5 with those in pure isopropyl alcohol (Fig.4) it is suggested that the neutral semi- quinone radical (QH;) exists in the acidic pH region (pH z 1.5) for both of the q~inones.'~~'~ The pK,, and pK,, values, as listed in Table 2, are therefore associated with the following equilibria for the semiquinones of both 2HAQ and 26DHAQ. pK.1 QH; QH;-+ H+ (X) PK~Z QH;-QHo2-+ Hf (XI)Note from Table 2 that the pK,, values for the semiquinones of 2HAQ and 26DHAQ are a little higher than those of the semiquinones of 14DHAQ, l5DHAQ and 18DHAQ. Because 50 R 30 -nv) 44.-C 3 uj 10 -% WI 2 -10 -Y 2 -30 -300 400 500 600 700 800 A/n m Fig. 2 Semiquinone -quinone difference absorption spectra of the reduced semiquinone radical of 26DHAQ in aqueous-organic mixed solvent at pH 1.5 (O),7.0 (*) and 13 (0).Dose, ca.8.7 Gy. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I 11 h .-u)4-C ujam8 m 0 FY I2 I I I I 2 5 8 11 14 PH Fig. 3 Variation of AA with pH for the semiquinone radicals of 2HAQ (0)at 615 nm and 26DHAQ (*) at 600nm. Dose, ca. 16 Gy. of the intramolecular hydrogen bonding, the semiquinone monoanions of 14DHAQ, 15DHAQ and 18DHAQ become more stable than the corresponding monoanions of the semi- quinones of 2HAQ and 26DHAQ. This stability effect causes Table 2 Absorption characteristics and pK, values of the semi- quinone radicals of 2HAQ, 26DHAQ and other related quinones in aqueous 5 rnol dmP3 isopropyl alcohol-1 rnol dmP3 acetone quinone neutral monoanion dianion pK,, pK,, a AQb 389 (8.9) 385 (6.7) 4.4 -490 (5.2) 2HAQ 380 (6.6) 400 (6.6) 405 (6.5) 4.7 10.7 450 (4.4) 460 (5.1) 26DHAQ 390 (1 1.0) 410 (9.0) 420 (14.7) 5.4 8.7 450 (4.5) 500 (5.7) 14DHAQ' 410 (1 1.6) 388 (5.8) 3.3 >14 475 (13.7) 15DHAQd 410 (12.4) 390 (8.0) 3.65 >14 440 (13.0) 18DHAQdse 400 (15.8) 380 (8.5) 3.95 >14 450 (14.7) a Error limits in the pK, values are kO.1; ref.34; ref. 24; ref. 16; in ref. 16 the A,,, and E values were wrongly quoted in Table 2 for 18DHAQ semiquinone. I + Ii 300 500 700 A/n rn Fig. 4 Semiquinone -quinone difference absorption spectra of the reduced semiquinone radicals of 2HAQ (0)and 26DHAQ (0)in pure isopropyl alcohol.Dose, ca. 10Gy. differences in the pK,, values for the two sets of semi- quinones. The lower pK,, values (Table 2) for the semiquinones of 2HAQ and 26DHAQ are also due to the absence of intra- molecular hydrogen bonding. The monoanionic forms of the semiquinones of 14DHAQ, l5DHAQ and 18DHAQ have stronger intramolecularly hydrogen-bonded structures than the corresponding neutral forms. This causes the second stage of deprotonation [reaction (XI)] to be very difficult for the semiquinones of 14DHAQ, l5DHAQ and 18DHAQ, making the pK,, values of these semiquinones exceptionally high compared with those of the semiquinones of 2HAQ and 26DHAQ, as represented in Scheme 1. Note that for hydroxy-substituted anthrasemiquinones, having no intramolecular hydrogen bonding, the pK,, values are a little higher than that of unsubstituted anthra-semiquinone (Table 2).Apparently, the electrondonating substituent (hydroxy group) increases the effective electron density of the semiquinonoid moiety, making the semi-quinonoid hydroxy group less acidic compared with that in unsubstituted anthrasemiquinone. This hypothesis is further supported by the observation that substitution of the hydroxy group by an amino group, having a stronger electrondonating effect, causes an increase in the pK,, value of the semiq~inones.'~ In the case of the semiquinones of 14DHAQ, 15DHAQ and MDHAQ, the effect of intramolecu-lar hydrogen bonding predominates over the electron-donating effect of the substituents, resulting in a lower pK,, than the pK, of the unsubstituted anthrasemiquinone.The semiquinone -quinone difference absorption spectra for both 2HAQ and 26DHAQ at pH x 13 were also obtained in N,O-saturated aqueous solutions of the quinones containing lo-' mol dm- formate. The absorption spectra (not shown in the figures) were qualitatively similar to the corresponding spectra in aqueous-organic mixed solvent at pH x 13. Therefore, the same radical species are formed by the reaction of either CH,COHCH,, e,; or C0;- with the quinones. Corrected Absorption Spectra of the Semiquinone Radicals The corrected absorption spectra of the different ionic forms of the semiquinones were obtained by correcting the corre- sponding difference (semiquinone -quinone) absorption spectra.' 6*1 Different GR values were chosen for different experimental conditions. Thus for mixed aqueous-organic ~olvent,'~.'~,~~G, is taken to be 6.2 molecules (100 ev)-' (or 6.42 x mol J-'); for pure isopropyl alcoh01,~~~"~~~ GR is taken to be 5.1 molecules (100 eV)-' (or 5.28 x mol J-'); and for aqueous formate,'5-'6*24*36~37 GR is taken to be 6.5 molecules (100 eV)-' (or 6.73 x lop7mol J-').The corrected absorption spectra for the neutral (pH 1.5), monoanionic (pH 7) and dianionic (pH x 13) forms of the semiquinones of 2HAQ and 26HAQ in aqueous-organic mixed solvent are shown in Fig. 5 and 6, respectively. The corrected spectra obtained in pure isopropyl alcohol and in aqueous formate solution in pH x 13 (not shown in the figure) are similar to those obtained in mixed aqueous- organic solvent at pH 1.5 and 13, respectively, for both of the quinones.The spectroscopic parameters of the semiquinones of 2HAQ and 26DHAQ are listed in Table 2 along with those of the semiquinones of AQ, 14DHAQ, l5DHAQ and 18DHAQ for comparison. From the table it is seen that the neutral forms of the semiquinones of both AQ and its hydroxy derivatives absorb at the same wavelength region, i.e. ca. 375-410 nm. Therefore, hydroxy substitution has very little effect on the position of the absorption peak for the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2HAQ 0' 0' 0'm0-OH 0-0-neutral monoanion dianion 26DHAQ 0' 0' ?* OH 0-0-neutral monoanion dianion 14DHAQ 0 .O..H.0 -H.o..H'o O.."'O neutral monoanion dianion Scheme 1 neutral semiquinone radicals except that it causes a large solvent or in pure isopropyl alcohol were used to estimate the increment in the molar absorption coefficients. The effect is reaction rates of CH,eOHCH, radicals with the quinones. considerably high for semiquinone radicals where intramole- The rate constants were obtained by monitoring the growth cular hydrogen bonding is possible. The absorption charac- of the semiquinone absorbance at wavelengths >600 nm teristics of the monoanionic form of the semiquinone radicals where the ground-state quinones do not have any absorption. of hydroxy anthraquinones are also qualitatively similar to In isopropyl alcohol the reaction of es- with the quinones is those of the unsubstituted anthrasemiquinone monoanion, very fast and does not interfere with the reaction of except that the absorption peaks are much stronger for the CH,eOHCH, with the quinones.Rate constants for the former systems, especially when the derivatives have intra- reaction of C0;-with 2HAQ and 26DHAQ were obtained molecularly hydrogen-bonded structures. A quantitative by monitoring the growth of the semiquinone absorbances at correlation between the strength of hydrogen bonding and >600 nm, using N,O-saturated aqueous solutions of the the corresponding change in the absorption characteristics of quinones (5 x to 4 x mol dm-3) containing lo-' the anthrasemiquinones is beyond the scope of this report.mol dm-3 sodium formate. The rate constants for the reac- tions between ea; and the quinones were obtained by observ- Kinetic Parameters of tbe Semiquioooe Radicals ing the rate of disappearance of absorbance for ea; at 700 nm in aqueous solutions containing different concentrations Nitrogen-bubbled solutions of the quinones (5 x to 4 x mol dm-3) either in aqueous-organic mixed 16 IIf n z 3 2i:_0 300 400 500 600 700 800 300 400 500 600 700 800 A/nm Ifnm Fig. 5 Absorption spectra of the reduced semiquinone radicals of Fig. 6 Absorption spectra of the reduced semiquinone radicals of 2HAQ in aqueous-organic mixed solvent, obtained from the data 26DHAQ in aqueous-organic mixed solvent, obtained from the data given in Fig.1 after correction for the parent depletion; pH 1.5 (O), given in Fig. 2 after correction for the parent depletion; pH 1.5 (O), 7.0 (*) and 13 (0) 7.0 (*) and 13 (0) Pure isopropyl alcohol; ,, J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Kinetic parameters of the semiquinone radicals of 2HAQ and 26DHAQ in aqueous 5 mol dm-, isopropyl alcohol-1 mol dm-' acetone rate constant reaction PH /dm3 rno1-ls-l ~~~ 2HAQ + CH,tOHCH, 26DHAQ + CH,COHCH, 2HAQ + CH,tOHCH, a a --1.5 7 (1.3 f0.1) x 109 (0.7 f 0.1) x 109 (1.3 f0.1) x 109 (1.3 rt 0.1) x 109 26DHAQ + CH,tOHCH, 2HAQ + ea; 26DHAQ + ea; 2HAQ + C0;-26DHAQ + C0;-2 (2HAQ semiquinone) 2 (26DHAQ semiquinone) 13 1.5 7 13 llb 1lb llb 1lb 1.5 1.5 (0.9 f0.1) x 109 (2.3 rt 0.1) x 109 (1.2 f0.1) x 109 (0.7 f0.1) x 109 (1.0 0.1) x 109 (1.0 f0.1) x 109 (0.4 f0.2) x 109 (0.8 f0.1) x 10" (1.4 f0.1) x 10" (0.5 f0.3) x lo9 aqueous lo-' rnol dm-3 sodium formate solution.0.05 h v).z 0.03 3 ujn v a O,O1 (5 x to 4 x mol dm-3) of the quinones and 5 x lo-' mol dm-3 tert-butyl alcohol at appropriate pH. Corrections were applied for the much slower decay of eag absorbance in solutions containing no quinones. The second- order rate constants (Table 3) were obtained for all the above reactions from the linear dependence of the pseudo-first-order rate constants with quinone concentration. The semiquinone absorbances are seen to decay in the acidic pH region (pH z 1.5) following bimolecular second- order kinetics, obviously due to the disproportionation of the semiquinone into the parent quinone and the corresponding hydroquinone [reaction (XII)], as is observed for all other anthrasemiquinone radicals.7*24 2(semiquinone) +quinone + hydroquinone (XII) The second-order rate constants for the semiquinone decay are listed in Table 3. At higher pH (~6) the decay of the semiquinones is too slow to extract meaningful kinetic parameters. The semiquinone to quinone and hydroquinone equilibrium, as observed for other substituted anthra-semiquinones,' 5-' 7,24 was not observed for the present systems over the entire pH range (1-14) covered in this study. This could be because either the attainment of the equili- bration is very slow for the present systems or there is no equilibration at all.One-electron Reduction Potential (E') The one-electron reduction potentials (El) of 2HAQ and 26DHAQ were measured in water-isopropyl alcohol-acetone mixed solvent at pH 7 and 11, following the electron-transfer equilibria between the semiquinone and a suitable redox ref- erence,15-1 7.2 2-24 1,l'-dimethyl-4,4'-bipyridylium dichloride -0.01 i(MBP2+; E' = -330 mV3' us. NHE at 25°C). We have -0.01 0 100 200 300 400 500 tirnejps Fig. 7 Oscilloscopic traces showing attainment of electron-transfer equilibrium in aqueous-organic mixed solvent at pH 11. Wavelength, 610 nm; dose, ca. 6.3 Gy. (a) [MBP2+] = mol drn-,. A (b) [2HAQ] = 1.9 x lop4 mol dm-,. B (b) [26DHAQ] = 1.4 x mol dm-,.A (c) [2HAQ] = 1.56 x mol dm-, and [MBP2+] = 7.4 x mol drn-,. B (c) [26DHAQ] = 1.22 x lo4 mol dm-, and [MBP2+] = 5.5 x mol dm-,. Table 4 One-electron reduction potentials of 2HAQ and 26DHAQ in aqueous 5 mol dm-, isopropyl alcohol-1 mol dm-, acetone E l/mVa quinone PH 7 pH 11 AQb -445 -445 2HAQ -440 -440 26DHAQ -400 -400 14DHAQ -249 -304 1 5DHAQd -306 -340 1 8DHAQd -325 -405 a f20 mV; ref. 34; ref. 24; ref. 16. recently e~tablished~~ that, owing to a change in ion solva- tion, the E' value of the MBP2+/MBP'+ couple changes from -450 mV vs. NHE in aqueous formate solution39 to -330 mV us. NHE in aqueous 5 mol dmP3 isopropyl alcohol-1 mol dm-3 acetone. Most of the measurements for redox calculations were made at 610 nm (the weaker absorp- tion peak of the MBP*+ radical38) where the interference from the parent quinone and semiquinone absorption is nil and minimal, respectively.Owing to the very high molar absorption coefficient of MBP'+ at 395 nm,38*39 the redox measurements were also made at this wavelength, in addition to the 610 nm measurements. Single pulses of low dose (ca. 6 Gy) were given to N2-bubbled solutions of the quinones (1 x to 2 x mol dmd3) and MBP2+ redox reference (1 x lop5 to 2 x lop4 mol dmp3) for all redox studies. Within a few hundred ms, the parents and radicals came into an equilibrium, semiquinone + MBP2+squinone + MBP" (XIII) Fig. 7 shows some typical redox equilibration traces for 2HAQ-MBP2+ and 26DHAQ-MBP2+ pairs at pH 11.0.The E values in mV us. NHE at 25 "Cwere calculated using eqn. (3) E'(quinone/semiquinone) = E~(MBP~+/MBP*+)-59 log K (3) where K is the equilibrium constant for reaction (XIII) and is expressed as [quinone],, [MBP' +Ieq (4)K= [semiquinone],,[MBP2 + leq The El values thus obtained for 2HAQ and 26DHAQ at pH 7 and 11 are listed in Table 4 along with the E' values of 716 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 unsubstituted anthraquinone and other anthraquinone deriv- atives for comparison. The latter values have been cor-re~ted,~~wherever necessary, for E'(MBP2+/MBP'+) = -330 mV us. NHE at 25 "C. It is seen from Table 4 that the E' values of 2HAQ and 26DHAQ are similar to that of the 5 6 7 8 A. J. Swallow, A.B. Ross and W. P. Helman, Radiat. Phys. Chem., 1981,17, 127. R. L. Willson, Chem. Commun., 1971,1249. K. B. Pate1 and R. L. Willson, J. Chem. SOC., Faraday Trans. I, 1973,69, 814. P. S. Rao and E. Hayon, J. Phys. Chem., 1973,77,2274. unsubstituted anthraquinone, but are more negative than those of the other hydroxy anthraquinones for which intra- molecular hydrogen bonding is possible. Thus the semi- quinones of AQ, 2HAQ and 26DHAQ are stronger reducing agents than the other intramolecularly hydrogen-bonded hydroxy-anthrasemiquinones.This observation is in accord- 9 10 11 12 E. Hayon, T. Ibata, N. N. Lichtin and M. Simic, J. Phys. Chem., 1972,76,2072. D. Meisel and P. Neta, J. Am. Chem. SOC., 1975,97,5198. P. Wardman and E. D. Clarke, J. Chem. SOC., Faraday Trans., 1976,72, 1377.K. P. Clark and H. I. J. Stonehill, J. Chem. SOC., Faraday Trans., 1977,73, 722. ance with expectation, because the intramolecular hydrogen bonding will introduce a +6 charge on the carbonyl oxygen, making the quinonoid moiety more electron afinic than that of simple AQ and its derivatives where there is no intramole- cular hydrogen bonding. 13 14 15 B. E. Hulme, E. J. Land and G. 0. Phillips, Chem. Commun., 1969,518. B. E. Hulme, E. J. Land and G. 0. Phillips, J. Chem. SOC., Faraday Trans., 1972,68,1992. H. Pal, D. K. Palit, T. Mukherjee and J. P. Mittal, Radiat. Phys. Chem., 1991,37,227. 16 H. Pal, D. K. Palit, T. Mukherjee and J. P. Mittal, J. Chem. SOC., Conclusion 17 Faraday Trans., 1991,87,1109. H. Pal, D. K. Palit, T. Mukherjee and J.P. Mittal, Radiat. Phys. The characteristics of the semiquinone radicals formed by one-electron reduction of 2HAQ and 26DHAQ have been found to be quite dissimilar to those of 14DHAQ, l5DHAQ and 18DHAQ. The dissimilarities have been interpreted in terms of the presence and absence of intramolecular hydro- 18 19 20 Chem., 1992,40,529. E. McAlpine, R. S. Sinclair, T. G. Truscott and E. J. Land, J. Chem. SOC., Faraday Trans. I, 1978,74,597. E. J. Land, E. McAlpine, R. S. Sinclair and T. G. Truscott, J. Chem. SOC., Faraday Trans. I, 1976,72,2091. E. J. Land, T. Mukherjee, A. J. Swallow and J. M. Bruce, J. gen bonding in these systems. Where the semiquinones of 14DHAQ, 1 5DHAQ and 18DHAQ disproportionate quickly, establishing an equilibrium among the semiquinone, the parent quinone and the hydroquinone over a pH range of ca.6 to 12, the semiquinones of 2HAQ and 26DHAQ do not show the attainment of such equilibrium, at least within the longest time period of our observation (5 ms). Regarding the redox characteristics, the semiquinones of 2HAQ and 26DHAQ are very similar to that of AQ and are stronger reducing agents than the other hydroxy-substituted anthra- semiquinones where intramolecular hydrogen bonds are 21 22 23 24 25 26 Chem. SOC., Faraday Trans. 1, 1983,79,391. N. J. F. Dodd and T. Mukherjee, Biochem. Pharmacol., 1984,33, 379. T. Mukherjee, B. Cercek, N. J. F. Dodd and A. J. Swallow, Radiat. Phys. Chem., 1987,30,271. T. Mukherjee, E. J. Land, A. J. Swallow, P. M. Guyan and J. M. Bruce, J. Chem.SOC., Faraday Trans. I, 1988,842855. T. Mukherjee, A. J. Swallow, P. M. Guyan and J. M. Bruce, J. Chem. SOC., Faraday Trans., 1990,86,1483. D. K. Palit, H. Pal, T. Mukherjee and J. P. Mittal, J. Photochem. Photobiol., A: Chem., 1990,52,375. D. K. Palit, H. Pal, T. Mukherjee and J. P. Mittal, J. Chem. SOC., Faraday Trans., 1990,86,3861. present. 27 H. Pal, D. K. Palit, T. Mukherjee and J. P. Mittal, J. Photochem. The acid dissociation constants of the semiquinones of 2HAQ and 26DHAQ are very different from those of the semiquinones of other hydroxyanthraquinones having intra- molecular hydrogen bonding. It is also seen that for the present hydroxyanthrasemiquinones, where no intramolecu- lar hydrogen bonding is possible, the molar absorption coeffi- 28 29 30 31 Photobiol., A: Chem., 1991,62, 183.S. R. Flom and P. F. Barbara, J. Phys. Chem., 1985,89,4489. G. Smulevich, J. Chem. Phys., 1985,83, 14. C. C. Westcott, pH Measurements, Academic Press, New York, 1978. R. G. Bates, M. Paabo and R. A. Robinson, J. Phys. Chem., 1963,67, 1833. cients for their absorption peaks are much lower than those for hydroxyanthrasemiquinones with intramolecularly hydrogen-bonded structures. 32 33 J. W. T. Spinks and R. J. Woods, An Introduction to Radiation Chemistry, Wiley, New York, 2nd edn., 1976. G. E. Adams, J. W. Boag, J. Currant and B. D. Michael, in Pulse Radiolysis, ed. M. Ebert, J. P. Keene, A. J. 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SOC., Faraday Trans. I, 1978,74,665. Busi, Reidel, Dordrecht, Holland, 1982, p. 289. 4 A. J. Swallow, in Functions of Quinones in Energy Conserving Systems, ed. B. L. Trumpower, Academic Press, London, 1982. Paper 3/04979E; Received 17th August, 1993