One-electron transfer reactions of some hydroxynaphthoquinones Solvent and substitution effect as studied by pulse radiolysis

~ ~~

Madhab C. Rath, Haridas Pal and Tulsi Mukherjee* Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India

The properties of one-electron reduced species of 1,4-naphthoquinone (NQ), 2-hydroxynaphthoquinone (2HNQ), 5- hydroxynaphthoquinone (5HNQ) and 5,8-dihydroxynaphthoquionone (58DHNQ) in water-isopropanol (Pr'OH) (5 mol dm - ,)- acetone (1 mol dm-3) mixed solvent (MS), Pr'OH and water have been investigated by a pulse radiolysis technique. Semiquinones of 2HNQ produced at acidic pH resemble those produced in pure Pr'OH and have no absorption beyond 500 nm. These were assigned as neutral semiquinones. The semiquinones produced at higher pH ( > 11) were assigned as dianionic semi- quinones. Both the dianionic semiquinones and the one-electron oxidised species of 2 HNQ have absorption beyond 600 nm, but to a different extent since the difference of two electrons between the species leads to a difference in the extent of conjugation of the aromatic ring with the oxygen atom of the OH group. The pKs of the parent molecules and the semiquinones were deter- mined in MS. The one-electron reduction potentials (E') of the quinones were calculated and compared with those of other related quinones. The kinetics of formation and decay of both the one-electron reduced and the one-electron oxidised species have been calculated. The k, values for the reduced and the oxidised species were of the order of lo9 dm3 mol-' s-'. The corresponding k , values were of the order of lo8 dm3 mol-' s - ' for the oxidised species, but the decay of the semiquinones was very slow. The effect of the position of the OH groups in NQ on the semiquinone characteristics has been discussed for 2HNQ, 5HNQ and 58DHNQ in MS and aqueous solutions.

Hydroxyquinones constitute the central moiety in quinone anti-tumour agents as well as in several dye intermediates. The study of semiquinones derived from such quinones has physico-chemical significance.'-' These molecules can undergo controlled one-electron reduction as well as oxida- tion by electron transfer between the quinone and the hydroxy group. Radiation-induced formation by pulse radiolysis, fol- lowed by kinetic spectrophotometry, is a proven method for generating and characterising these semiquinones. We have earlier reported the semiquinone chemistry of 5HNQ,'*' 58DHNQ3-' and many other substituted quinones6-' in aqueous solution. Functional groups at different positions may explain their different characteristics.

In the present work we report the characteristics of the semiquinones of NQ, 2HNQ, 5HNQ and 58DHNQ and compare them with other related quinones in water contain- ing 0.1 mol dm-, Pr'OH or formate, pure Pr'OH and aqueous-organic mixed solvent (water-Pr'OH-acetone in 30.2 : 5 : 1 molar ratio). These quinones are not sufficiently soluble in water, therefore, we have used aqueous-organic mixed solvent (MS) as the main solvent system for the study. In the MS, since the bulk of the medium remains essentially aqueous, reporting the pH of the solutions is justified. The characteristics of the fully reduced (two-electron) form of 2HNQ in the MS are also reported. The study of one-electron oxidation of 2HNQ has been carried out in aqueous solution above pH 5 by OH', N,' and 0'- redicals.

Experimental 2HNQ, 5HNQ and NQ (Aldrich) were purified by repeated crystallisation from methanol. Pr'OH and acetone were of Spectroscopic grade from Spectrochem India. Other chemicals were from Aldrich, Merck, TCI Japan and Fluka and were used without further purification. Nanopure water from a 'Barnstead Nanopure' system, having conductivity less than 0.1 pS, was used for the preparation of aqueous solutions and solutions in MS. The pH of the solutions was adjusted with a

phosphate buffer and the extremes of the pH scale were adjusted by addition of HC10, and NaOH.

The solutions were irradiated with 50 ns pulses from a 7 MeV linear electron accelerator at BARC. Details of the experimental arrangements have been reported elsewhere.6-8 The absorbed dose was measured by thiocyanate dosimetryg (5 x lo-' mol dm-3 air-saturated KSCN) following the absorbance of (SCN),'- radicals at 500 nm, using G = 2.9 and E = 7100 dm3 mol-' cm-'. Iolar grade (Indian Oxygen Ltd.) N, and N 2 0 were used for bubbling. A Gamma-cell type Co-60 y-irradiation source with dose rate ca. 10 Gy min-' was used for the study of fully reduced species of 2HNQ in MS.

Results and Discussion Ground-state Characteristics of NQ, 2HNQ, SHNQ and 58DHNQ

2HNQ, 5HNQ and 58DHNQ can exist in either neutral or anionic form depending on the pH, whereas NQ and AQ exist only in the neutral form in the pH range 1-14. The absorption characteristics of these compounds are given in Table 1. The OH group attached to the aromatic ring forms stronger intra- molecular H bonding with the quinonoid oxygen atom than the OH group attached to the quinonoid ring. This is reflected in their pK, values, which are lower in water than in MS, since MS is less polar than water. The pK, values were calcu- lated by plotting absorbance us. pH; 2HNQ in MS is shown as an example in Fig. 1.

R' = RZ = R3 = H, NQ

wR1 R' = OH, RZ = R3 = H, 2HNQ

R' = R3 = H, R2 = OH, 5HNQ

R' = H, R2 = R3 = OH, 58DHNQ P o

J . Chem. SOC., Faraday Trans., 1996,92(1 l), 1891-1897 1891

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Table 1 Absorption characteristics of NQ, 2HNQ, 5HNQ and 58DHNQ in different solutions

Laxlnm (&/lo3 dm3 mol-' cm-')

quinone solvent neutral monoanion dianion PK( 1 ) PK(2) ref.

NQ 2HNQ 2HNQ 2HNQ 5HNQ 5HNQ 58DHNQ 58DHNQ

MS MS

Pr'OH MS

MS

b

a

a

330 (2.51) 338 (3.15) 458 (3.04) 4.2 338 (3.1) 458 (2.96) 4.1 332 (2.94) 425 (3.6) 525 (3.9) 9.4 425 (3.6) 525 (3.9) 8.85 1

520 (5.2) 580 (6.2) 615 (11.5) 7.85 10.7 3 520 (5.2) 580 (6.2) 615 (1 1.6) 8.8 12.4

~~~ ~ ~

a Aqueous solution containing 0.1 mol dmP3 formate. Aqueous isopropanol(O.1 mol dm-3).

7 . 0 - 1

PH Fig. 1 resents the computer best fit [eqn. (l)].

A us. pH for 2HNQ at 458 nm (0) in MS. Solid line rep-

Spectral characteristics (difference absorption spectra) of the semiquinone radicals

The one-electron reduction of 2HNQ was carried out in MS, Pr'OH and water bubbled with N, gas. The concentration of the quinone was ca. mol dm-3. The quinone was reduced by the acetoneketyl radicals, CH3C'(OH)CH3 in MS on irradiation by the electron pulse. The reactions are given below:

H,O- -+ H', OH', eaq- and other related products (I)

CH3CH(OH)CH3 + H'/OH' -+ CH3C'(OH)CH3 + H,/H,O

CH3COCH3 + eaq- -+ (CH3COCH3)'- (111)

(CH3COCH3)'- + H,O -+ CH3C'(OH)CH3 + OH- (IV)

CH,C'(OH)CH, + Q-(QH) + QH-(QH,') + CH3COCH3

In water (0.1 mol dm-3 Pr'OH) the reduction of quinone takes place by eaq- and CH3C'(OH)CH3. The reduction of quinones in pure Pr'OH solution is carried out by CH,C'(OH)CH, radicals formed from the primary intermedi- ates, especially the higher excited states.* The semiquinones were characterised at four different pHs in MS and water. It is evident from their difference spectra (not shown) that the semiquinones at pH ca. 2 in MS and in Pr'OH are equivalent and assigned to the neutral semiquinone having two OH groups, which account for the two pKs (Scheme 1). In the pH

0 0

0 0

i OH

I 0'

Scheme 1

I 0'

.4~Ilti+

?- mo- I 0'

range 6-8 the semiquinone exists in the monoanionic form, and at higher pH (> 11) in MS the semiquinones are di- anionic. The negative charge on the two oxygen atoms is in conjugation with the benzene ring which accounts for the high absorption in the long-wavelength region beyond 600 nm. However, for the neutral and monoanionic semiquinones there is less absorption beyond 600 nm. The possible struc- tures of the semiquinones derived by one-electron reduction of 2HNQ at different pHs are shown in Scheme 1. The resulting differences between the absorbances of 2 HNQ and its semi- quinone were measured immediately after the completion of reactions (1)-(V). The difference absorption spectra in MS at pH 1.8, 6.7, 10.3, 13.0 and in pure Pr'OH and that in water at different pHs are not shown, but they are qualitatively similar to those in the MS of corresponding pH. The negative value of the absorbance in the wavelength range 420-520 nm indi- cates that the parent molecule itself absorbs more strongly than the semiquinone in this region. A plot of A A (at 390 nm) us. pH gave a pKa(l) value of 5.5 in MS (Fig. 2) and 4.7 in aqueous formate, for the semiquinone corresponding to the equilibrium

where the experimentally observed AA values were fitted to the modified Hendersen equation3

Where, AA, and AA, are the observed differences in absorb- ance at pH values well before and well beyond the pKa. Simi-

1892 J . Chem. SOC., Faraday Trans., 1996, Vol. 92

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PH 2 3 4 5 6 7 8

i - 10 I I 1 I I 1 112.0

8 9 10 I I 12 13 14

PH Fig. 2 410 nm in MS. Solid line represents the computer best fit [eqn. (l)].

AA. (0) and E (A) 0s. pH for the semiquinone of 2HNQ at

larly, a plot of AA (at 410 nm) us. pH gave a pK,(2) value of 10.1 (Fig. 2) which corresponds to the equilibrium

The one-electron reductions of 5HNQ and 58DHNQ in MS were carried out at different pHs. The pK, corresponding to equilibrium (VI) was 4.7 (3.65 in aqueous formate)' for 5HNQ and 2.8 (2.7 in aqueous formate) for 58DHNQ. The pK, corresponding to equilibrium (VII) could not be calcu- lated with accuracy but is greater than 12.5 for both the semi- quinones. The spectral characteristics of these two quinones in MS are similar to those in aqueous formate s~ lu t ion . ' .~

The one-electron reduction of N Q was carried out in MS at different pHs and in pure Pr'OH solution bubbled with N, gas." A pK, value of 4.3 was obtained, corresponding to the equilibrium :

QH'+Q'- + H + (VIII)

the pK,( 1) values of the semiquinones of different quinones in MS are in the order, 2HNQ > 5HNQ > N Q > 58DHNQ and in aqueous solution 2HNQ > N Q > 5HNQ > 58DHNQ. This clearly indicates that the acidic character of ring- substituted OH is greater than that of OH in the quinonoid ring, both in water and MS. Also, 5HNQ semiquinone is a stronger acid than N Q semiquinone in water, but a weaker

one in MS. Experiments for N Q were carried out in 1 mol dmP3 Pr'OH which is more aqueous in nature than MS and values in both 7 mol dm-3 Pr'OH '' and 1 mol dm-3 Pr'OH are given in Table 2. Similarly the pK,(2) values of these semiquinones in MS are in the order 58DHNQ > 5HNQ > 2HNQ, The explanation of this observation is dis- cussed in the next section.

Corrected absorption spectra of semiquinone radicals

The difference absorption spectra of the semiquinone radicals were corrected by applying a correction for the parent deple- tion. At a given wavelength, 1, the molar absorption coeffi- cient ( E ~ , of the semiquinone radical may be calculated from:

where A(SCN)z.- is the absorbance of (SCN),'- at 500 nm observed in an air-saturated solution of KSCN (5 x lo-, mol dm- 3, under the same experimental conditions. The values G(SCN)z.- = 2.9 and E ( ~ ~ ~ ( ~ . - = 7100 dm3 mol-' cm-' were used for the ca lc~la t ion .~ Different GR values were taken for different solutions: 6.2 for MS," 6.0 for water16 and 5.1 for pure Pr'OH s ~ l u t i o n . ~ ~ " and E~ are the molar absorption coefficients of the radicals and the parents, respectively. The corrected absorption spectra of the semiquinones of 2HNQ in MS at pH 1.8, 6.7, 10.3, 13.0 and in Pr'OH are shown in Fig. 3.

The spectroscopic parameters and pK, values of the semi- quinones of NQ, 2HNQ, SHNQ, 58DHNQ and other related quinones in different solvents are listed in Table 2. It is clear that there is a difference in the pK,( 1) values of a semiquinone in water and MS. It is less acidic in MS than in water, indicat- ing that the neutral semiquinone is more stabilised in MS, whereas its ionic form is more stabilised in water. This occurs since MS is less polar than water. The substitution of the OH groups at different positions in NQ results in a difference in their pK, values. The acidic character of the neutral semi- quinones of NQ, 2HNQ, 5HNQ and 58DHNQ in MS follows the order, 2HNQ < 5HNQ < N Q < 58DHNQ. The OH group at the 2-position in the semiquinone of 2HNQ forms a weak intramolecular H bond with the 0 atom at the 1- position, compared with the OH group at the 5-position in the semiquinone of 5HNQ and 5-, 8-positions in the semi- quinone of 58DHNQ. Hence, at the 2-position it behaves

Table 2 Absorption characteristics and pK, values of semiquinone radicals of 2HNQ, 5HNQ and other related quinones in different solvents

AmaJnm (&/lo3 dm3 mol-' cm-')

quinone solvent neutral monoanion dianion PK,(1) PKa(2) ref.

NQ NQ

AQ

2HAQ

2HNQ

2HNQ 2HNQ 5HNQ 5HNQ 58DHNQ

58DHNQ

MS b

C

MS

MS

MS

Pr'OH

MS

MS

a

a

a

370 (8.1) 370 (7.2)

389 (8.9)

380 (6.6)

360 (9.0)

365 (7.0) 370 (5.9) 370 (1 1.6) 370 (12.6) 380 (10.3)

380 (11.8) 760 (2.7)

375 (7.8) 390 (1 2.5)

385 6.7) 490 (5.2) 400 (6.6) 450 (4.4) 375 (8.6)

385 (10.8) 385 (12.2) 380 8.6) 430 (7.4) 380 (10.5) 425 (8.9)

405 (6.5) 460 (5.1) 370 (7.3) 620 (2.0)

4.3 4.1 3.8 4.4

4.7

5.5

4.7 4.7 3.6 2.8

2.7

10.7

10.1

> 12.5 > 12

11

12

13

14

1

3

Aqueous solution containing 0.1 rnol dmP3 formate. Aqueous isopropanol(7 rnol dm-3). Aqueous isopropanpol(1 mol dm-3).

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90

70

7

I

7

5 0 E E

0

U N z

30

10

320 4 0 0 5 0 0 600

Unm Fig. 3 Corrected absorption spectra of the semiquinone of 2HNQ in MS at pH 1.8 (O), 6.7 (A), 10.3 (0, 13.0 (A) and in Pr'OH (0)

more like an electron-donating agent to the aromatic ring. This reduces the acidic character of the neutral semiquinone of 2HNQ compared with that of 5HNQ and NQ. In the semi- quinone of SHNQ, a six-membered ring is formed by intra- molecular H-bonding with the 0 atom at the 4-position, however, the electron density in the aromatic ring system is still increased slightly causing it to be less acidic than the NQ semiquinone. However, in the case of the semiquinone of 58DHNQ the two H-bonded rings on both sides of the aro- matic ring drain away the electron density causing it to be more acidic than those mentioned above. However, in aqueous solution, the replacement of one H atom by an OH group at the 2-position has a reverse effect on the acidic behaviour of the semiquinone compared with that at the 5- position of NQ. This may be due to the higher polarity of water compared to that of MS. Similarly, the acidic behaviour of the monoanionic forms of the semiquinones of 2HNQ, 5HNQ and 58DHNQ follows the order 2HNQ > 5HNQ > 58DHNQ. This can also be explained on the basis of intramolecular H bonding in the radical anion. The stabil- ity of the doubly H-bonded 58DHNQ semiquinone anion (pKa(l) = 2.7, pKa(2) > 13.8)3 is unmatched in the free radical

literature of quinones. A similar effect was found for 1HAQ12 and 2HAQI3 semiquinones.

Kinetics of the growth and decay of semiquinone radicals

Nitrogen-bubbled solutions of 2HNQ, 5HNQ and 58DHNQ (5 x mol dm-') in MS and Pr'OH solu- tions were used for these studies. The rate constants were cal- culated by monitoring the growth and decay of the semiquinones at their peak absorption wavelength. The second-order rate constants were obtained from the slope of the linear plots observed on plotting the pseudo-first-order rate constants vs. the quinone concentrations. The values obtained are listed in Table 3. The formation and decay of the semiquinones in MS at pH 1.8 are faster than at pH 6.7, 10.3 and 13.0 for 2HNQ. Similarly the formation of the semi- quinones of 5HNQ at pH 7.0 in MS and in Pr'OH is faster than in MS at pH 10.4 and 13.0, however, their decay in these cases is very slow. For 58DHNQ the rate constant of forma- tion of the semiquinone at all pHs is comparable, but there was no decay of the semiquinones, even after a few ms. At acidic pH in MS, and in Pr'OH the semiquinones probably disproportionate into the parent quinone and the correspond- ing hydroquinone

2(semiquinone) -+ quinone + hydroquinone (IX)

At higher pH (> 6) the decay of the semiquinones is not com- plete in the timescale of a few hundred ps. This could be due to the establishment of an equilibrium in the reaction (IX). This effect is more pronounced for 58DHNQ and 5HNQ than for 2HNQ.

to 2 x

One-electron reduction potential

The one-electron reduction potential (El) of NQ, 2HNQ, 5HNQ and 58HNQ in MS and water were determined at pH 7. Different redox standards were used for the calculation of El, following the electron-transfer equilibria between the semi- quinone radicals and the redox standards. 1,l'-Dimethyl-4,4- bipyridylium dichloride [MBP' + ; E' = - 330 mV in MS] l 8

was used for both NQ and 2HNQ in MS; duroquinone [DQ; E' = -240 mV in MS l5> l8 and water"] for 2HNQ in MS and in water; 1,4-dihydroxy-9,10-anthraquinone-6-sulfonate [quinizarin-6-sulfonate; E' = -249 mV in water2'] for 2HNQ in water and oxygen (0,) for 5HNQ in both solvents and 58DHNQ in MS. The E' of O2 in water is known to be - 155 rnV.'l We have determined the E' value of 0, in MS at pH 7 to be - 180 mV, taking DQ as the redox standard.

Single pulses of low dose ca. (6-9 Gy) were applied to the N,-bubbled solutions of the quinones (5 x lo-' to 2 x

Table 3 Kinetic parameters of the semiquinones of 2HNQ in MS and Pr'OH

quinone reaction solvent PH kf/10-9 dm3 mol-' s - ' kJlO-' dm3 mol-' s - l

2HNQ Q + CH,C'OHCH, MS MS MS MS

Pr'OH 5HNQ Q + CH3C'OHCH3 MS

MS MS MS

Pr'OH 58DHNQ Q + CH3C'OHCH3 MS

MS MS MS

1.8 6.7 10.3 13.0

2.0 7.0 10.4 13.0

1.3 5.2 7.0 11.0

3.9 f 0.2 1.0 _+ 0.1 1.3 f 0.1 2.3 f 0.2 1.0 f 0.1 3.1 f 0.2 2.9 f 0.2 1.7 0.1 1.4 f 0.1 3.1 f 0.2 1.52 _+ 0.1 1.97 f 0.1 1.48 f 0.1 1.46 f 0.1

2.05 f 0.2 2.05 f 0.2 1.5 f 0.1 1.5 & 0.1 3.6 f 0.2

a

a

a

a

a

a

a

a

a

Very slow decay due to the disproportionation equilibrium.

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mol dm-3) and the redox reference (5 x to 2 x loF4 mol dm-3) for all the redox studies. Within a few tens of ps the electron-transfer equilibrium (X) was established for 2HNQ systems but for SHNQ and 58DHNQ systems it took a few hundreds of ps. In our experimental condition the equi- librium was attained after 400 ps in MS, whereas in aqueous solution it was attained after 230 ps for 5HNQ and 1200 ps for 58DHNQ. Air-saturated solutions of SHNQ in MS and in water were used for the redox study. The concentration of 0, in water is known to be 2.65 x mol dm-3 and in MS it was calculated to be 5.35 x mol dm-3. Oxygen- saturated solutions of 58DHNQ in MS were used for the redox study. The concentration of 0, in the 0,-saturated solution was calculated to be 4.13 x mol dm-3. A typical electron-transfer trace is shown in Fig. 4 for the redox couple 2HNQ and D Q in aqueous solution, where the semi- quinone radicals of D Q were monitored at 445 nm. The corre- sponding absorption of the semiquinone radicals of 2HNQ at this wavelength was accounted for by the following equation

0.6 -

0.5-

3 0.4- uj n 3 0.3-

0.2 -

0.1 -

0.0 -

v)

c c. .-

semiquinone + ref e quinone + ref'- (XI

+([ref'-]Acref.-) (3)

The E' values were calculated using the relation:

E'(quinone/semiquinone) = E'(ref/ref'-) - 59 log K (4)

where K , the equilibrium constant for equilibrium (X), was calculated from the absorbed dose and the equilibrium absorption at the wavelength of observation. The E' values obtained at pH 7 are listed in Table 4 together with those for other related compounds. It is found that the E' values of 2 HNQ and 5HNQ in MS are less than those in water and the

time - Fig. 4 Typical oscilloscopic trace at 445 nm showing the electron- transfer equilibrium with time. A, 1.2 x mol dm-3 duroquinone; (B), 1.2 x mol dm-3 2HNQ in aqueous solution at pH 7, dose 9 Gy.

rnol dm-' duroquinone and 3.2 x

Table 4 One-electron reduction potentials of NQ, ZHNQ, SHNQ, 0, and other related quinones in different solvents at pH 7

E'IrnV' E'ImV" compounds in MS ref. in water ref.

2HNQ - 350 - -310 -

5HNQ - 125 - 95 1 - 200 10

58DHNQ - 115 -110 3 NQ

- 180 - 155 21 0 2 -445 12 AQ

2HAQ -440 13

- - -

- -

- - - -

E' of 2HNQ is less than that of 5HNQ in both solvents. The E' values of NQ, 2HNQ and 5HNQ in MS follow the order 2HNQ < N Q < 5HNQ. The presence of the OH group at the 2-position increases the electron density mainly in the quino- noid ring while that at the 5-position allows a sharing of elec- tron density between the benzene ring and the quinonoid ring. The intramolecular H bonding in 2 HNQ is weaker than that in 5HNQ and hence the reduction potential of 2HNQ is lower than that of NQ while the reduction potential of SHNQ is higher than that of NQ. The E' values of 58DHNQ and 5HNQ are similar as are those of 2HAQ and AQ. This could be due to the small effect of the OH group on the quinonoid ring in 2HAQ. The small difference in the E' of 58DHNQ in water and MS can be explained by the exceptional stability of the semiquinone anion at pH 7 owing to double intramolecu- lar H b ~ n d i n g . ~

Two-electron reduction of 2HNQ in MS

Two-electron reduction of quinones to hydroquinones is best carried out by steady-state y-radi~lysis.~ Two-electron reduction of 2HNQ in MS was carried out by y-radiolysis of N,-bubbled solutions of 2HNQ (8.6 x mol dmV3) at dif- ferent pH for 45 min using Co-60 y-source of dose rate ca. 10 Gy min-'. The absorption spectra of the fully reduced species were recorded (Fig. 5). The absorption maxima are different in different pH ranges e.g. 323 nm in the pH range 4.6 to 8.0, 327 nm in the pH range 8.5 to 11.2 and 342 nm beyond the pH 12.3. This indicates that the hydroquinone exists in at least three different forms over the pH range. Further, it is evident from their spectral features that there is complete reduction of the quinone during this irradiation process. It was observed that the two-electron reduced form of 2HNQ (hydroquinone) returns to the parent quinone form on exposure to air. This process was very slow below pH ca. 5.0 owing to the slow electron transfer between the hydroquinone and oxygen at acidic pH. This was confirmed by bubbling oxygen through the irradiated solution, when the rate of oxidation increased.

The pK, s of the fully reduced hydroquinone of 2HNQ were obtained by plotting the experimentally observed A values us. pH and by fitting with eqn. (l).37'3 The pK, values were 6.4, 9.5 and 11.6, less than the corresponding pK,s of 5HNQ in aqueous formate.' These results may be explained on the basis of the lower stability of the fully reduced hydroquinone of 2HNQ compared with that of 5HNQ.

One-electron oxidation of 2HNQ

Spectral properties. The presence of a hydroxyl group allows both 2HNQ and SHNQ to undergo one-electron oxidation, i.e. loss of an electron, when the quinone is reacted with a

pH 4.6 pH 8.0

. . . . . . . . . pH 10.3 pH 13.5

- - - - - -

-. -. -. -.

-.-. -.-. -.-. -.-__._

- _ _ ..-. . . . _ _ _ _ _ l ~ l ~ I ~ l ~ l - ~

300 350 400 450 500 550 Nnm

Fig. 5 Ground-state absorption spectra of the two-electron reduced species of 2HNQ formed by y-radiolysis in MS at pH 4.6 (-), 8.0 (---) 10.3 (----) and 13.5 (--- -), dose = 450 Gy

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suitable radical oxidant such as OH', 0'- or N,' e t ~ . " - ~ ~ 4 One-electron oxidation studies were carried out at different

rnol dm-3) and 5HNQ (5 x rnol m-3) in the absence ' 3 pHs in N,O-saturated aqueous solutions of 2HNQ (5 x

The and radiation presence chemical of 5 x lo-' reactions mol dm-, are: sodium a i d e (NaN,). 'T - g * 5 - : eaq- + N,O 4 N, + 0'- ( X I 4 % -

0'- + H,O - + O H ' + OH- (XI@ ; 1 - m

OH' + OH- 4 0'- + H,O (XII)

QH + OH'-,Q'+ H,O (XIII) 0

Scheme 2 illustrates the oxidation of 2HNQ by these oxidants. The resultant one-electron oxidised quinone-quinone differ- ence absorption spectra on completion of the above reactions were recorded. The corrected absorption spectra (Fig. 6 and 7) of the semi-oxidised quinones were obtained using eqn. (2), taking G , = 6.0. The spectral characteristics are given in Table 5. It was found for 2HNQ that there is an absorption of the semi-oxidised quinones beyond 600 nm, however, this is less than that of the semiquinone radicals at higher pH (> 10). It is clear from the two schemes that there is a difference of two electrons between the one-electron oxidised quinone (Q')

1 . 1 ' 1 - I , , .

A -.-pH 10.4, OH' -A-PH 13.0, 0'-

\ p q < a N 3 - 4 1 - A,'

\&A* l ' l , l ' , - l -

0 0

@OH

0

pH Na\ 7.0

H+

pH 12.7

0

Scheme 2

320 400 500 600 700 ,Unm

Fig. 6 Corrected absorption spectra of the one-electron oxidised species of 2HNQ in aqueous solution, pH 8.0 (a), 12.7 (A) and NaN, (0.05 mol dm-3), pH 7,O (A)

Table 5 Absorption characteristics of the transients produced by one-electron oxidation of 2HNQ and 5HNQ in aqueous solution

l,,,/nm (&/lo3 dm3 mol- cm- l )

quinone OH' 0.- N3'

2HNQ 340 (3.8) 340 (2.8) 340 (3.4) 440 (2.5) 450 (2.8) 450 (2.75) 420 (2.2) 400 (3.3) 420 (2.8) 5HNQ 540 (3.1) 540 (3.1) 540 (3.1)

and the semiquinone Q*'-) at higher pH (> 10). This extra negative charge on the oxygen atom in the semiquinone is delocalised over the aromatic ring through conjugation and this may be the reason for the weak absorption of the semi- oxidised 2HNQ beyond 600 nm. A similar effect was observed in case of 5HNQ.

Kinetic parameters. The kinetics of formation and decay of the semi-oxidised quinones were studied in aqueous solutions containing, separately 2 x rnol dm-3 2HNQ and 5HNQ saturated with N,O gas. The rate constants were cal- culated by observing the growth and the decay of the species at their absorption maxima. The second-order rate constants of formation (k,) and decay (kd) of the semi-oxidised quinones formed in different conditions are listed in Table 6 ; k, is of the order lo9 dm3 mol- s- ' and k, of the order lo8 dm3 mol- s-'. These are higher than the rate constants for formation and decay of the semiquinones. It was found from the decay traces that an equilibrium could exist at pH 2 12.0.

Conclusion The characteristics of the semiquinone radicals formed by one-electron reduction of 2HNQ have been found to be quite dissimilar to those of NQ, SHNQ and 58DHNQ. These differ- ences have been interpreted in terms of the intramolecular H

Table 6 Kinetic parameters of the transients produced by one- electron oxidation of 2HNQ and 5HNQ in aqueous solution

~~

k,/109 k,J108 quinone oxidant pH dm3 mol-' s-l dm3 mol-' s - '

~ ~~

2HNQ OH' 7.5 11.0 f 0.5 4.7 f 0.2 2.7 _+ 0.2 2HNQ 0'- 12.5 7.4 _+ 0.4

2HNQ N,' 7.0 6.0 & 0.3 0.7 f 0.2 8.0 f 0.4 5HNQ OH' 10.4 2.6 _+ 0.2

SHNQ 0 - 13.0 1.3 f 0.1 3.3 f 0.2 5HNQ N,' 10.4 3.9 f 0.2 4.3 2 0.2

18% J . Chem. Soc., Faraday Trans., 1996, VoL 92

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bonding and the electron-donating ability of the OH group@) attached at different positions on the NQ. The presence of OH group@) at 2-, 5- and 8-positions in the parent NQ molecule increases the electron density in the ring system to a different extent, which is reflected in the pK, and E' values. The E' of 2HNQ and 5HNQ are higher in water than in MS since water is the more polar solvent. The one-electron reduced species in MS at pH 13 and the one-electron oxidised species in aqueous solution for both 2HNQ and 5HNQ have similar elemental stoichiometry, however, they differ in the number of electrons. This leads to a difference in the extent of conjugation of the aromatic ring with the oxygen atom of the OH -group, which is demonstrated by their extent of absorption beyond 600 nm.

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Paper 5/07045G; Received 25th October, 1995

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