Effect of solvent viscosity on the rates of

EFFECT

OF

SOLVENT

RECOMBINATION, DISMUTATION

VISCOSITY

ON

THE

RATES

DISPROPORTIONATION, REACTIONS

L. L. Koroli, I. and V. A. Kuz'min

OF V.

OF

AND

AROMATIC

RADICALS

Khudyakov,

UDC

541.12.038.2-142:541.127:541.515

S o l v e n t v i s c o s i t y e x e r t s a d e c i s i v e e f f e c t on t h e r a t e of f a s t b i m o l e c u l a r r e a c t i o n s of radica][s [1]. p r e s e n t p a p e r is d e v o t e d to an i n v e s t i g a t i o n of the e f f e c t of v i s c o s i t y on t h e r a t e s of r e c o m b i n a t i o n R" + R ' - ~ R - -

The

R

(1)

disproportionation

(2)

RH'+ R H ' - + R + RH 2 RH" ~- R'" --~ R + R ' H

(3)

dismutation R-" ~ - R - - ' _ ~ R + R

2-

(4)

and electron transfer

R-' + R " - + R ~ R ' -

(5)

with p a r t i c i p a t i o n of a r o m a t i c k e t y l , s e m i q u i n o n e , and n i t r o x y [ r a d i c a l s . EXPERIMENTAL We used a pulsed-photolysis unit [2], 1,4-benzoquinone (BQ) and 1,4-naphthoquinone (NQ) recrystallized from hexane, benzophenone (BP) recrystallized from l-propanol, sodium 9,10-anthraquinone-2-sulfonate (NaSO3AQ) recrystallized from alcohol, bidistilled water, cp glycerol, and bidistilled l-propanol. The viscosity (~?, cP) of the solvent was varied by varying the composition of the water-glycerol mixture for constant content of 0.5% l-propanol. For the solutions being investigated, ~ was measured with the Heppler viscometer. All the solutions were deoxygenated with a vacuum unit and were thermostated. For measurement of the activation energies, the experiments were carried out in the temperature range _> 50~ The activation energy for viscous flow of water V = 4.0 ~: 0.5 kcal/mole [3]; for aqueous solutions containing 50% glycerol, we found V = 6.5 -~ 0.i kcal/mole. A pH of 11-12.5 was created by dissolving the alkali in water at a specific concentration; when the reactions were investigated in a water-glycerol mixture, the alkali was dissolved in this mixture at the same concentration. The kinetics of the disappearance of the BQH", NQ-", BQ-", -O3SAQ-", BPH" and BP-" radicals obtained during photolysis of the starting quinones [4-6] and BP through a UFS-5 light filter (k 280-380 nm) [7, 8] was studied according to the variation of the absorbance in the region of absorption maximums of the radicals [4, 7, 8]. We investigated the kinetics of the reactions of BPH" and BP-" with the stable nitroxyl radical ( > N-O') 0 Ph S

5 b y t h e m e t h o d d e s c r i b e d in [9] ([ > N - O ' ] T = 273~

~ 5 . 1 0 - 6 - 2 910 .4 M).

Institute of Chemical Physics, Academy Akademii Nauk SSSR, Seriya Khimichesk~aya, May 25, 1981.

AII t h e v a l u e s of t h e c o n s t a n t s a r e given f o r

of Sciences of the USSR, Moscow. No. 3, pp. 527-531, March, 1982.

0568-5230/82/3103-0471507.50

Translated from Izvestiya Original article submitted

9 1982 P l e n u m P u b l i s h i n g C o r p o r a t i o n

471

QY

t~2k

o~"2d~

~0

o / o

/

8,o

7,o .,/

2,gLgf

~o Fig.

/

/

/ I

I

I,o

gg

,

1

. . . .

2,o ~ r

Fig. 2

Fig. i. Relation of log k to log (T/q) for the reactions: I) reaction (I) with participation of BPH', k = 2kl; 2) reaction (3) with participation of BPH" and nitroxyl radical (pH 6.0); 3) reaction (5) with participation of BP-" and nitroxyl radical (pH 12.0). The broken line denotes the theoretical dependence k = (i/4)kdi f. Fig. 2. Relation of log 2k to log (T/q) for reaction (2) with participation of BQH" (pH 2.0). The broken line denotes the theoretical dependence 2k = (i/4)kdi f. ~g zk / /

~g ~k

g,o

8,o

/

8s~

/

\2

6/0 [

o

/

I

I

I

f,o

Fig. 3 Fig. 4 Fig. 3. Relation of log 2k to log (T/~?) for reaction (4) with participation of BP-': i) pH ii.0; 2) pH 12.0. The broken line denotes the theoretical dependence 2k = (i/4)kdi f X ; P = i0 A. Fig. 4. Relation of log 2kto log (T/q) for reaction (4) with participation of BQ-". The broken line denotes the theoretical dependence 2k = (i/4)kdi f • p = i0 A. DISCUSSION

OF

RESULTS

Recombination [Reaction (i)]. The obtained value of 2k i = (1.6 • 0.3)-109 liters/(mole-sec) for BPH" (water) is in agreement with that found in [7]. The recombinationof BPH" in the viscosity range of i-i00 cP is limited by diffusion, and the obtained values of 2k -~ (1/4)kdi f = (1/4). 8RT/(3000~? ) (Fig. 1) [i]. The various reasons for the deviations of the experimental values from the theoretical ones are discussed in [i]. The anisotropy of the reactivity of this radical is averaged completely during the encounter time [i, I0, ii]. Disproportionation [Reactions (2) and (3)]. The rate of disproportionation of the BQH" radical decreases with increasing viscosity (Fig. 2), but in Solvents with low viscosity the disproportionation of BQH" occurs in the region interfacing the kinetic and diffusion regions [I]. From the relation of (2k) -I to ~, we obtained for the disproportionation of BQH" in a cell [i] 2kehem = (i.0 • 0.2).109 liters /(mole. sec). This value is in agreement with the value of 2kchem obtained for BQH" in [12] in the toluene-2-propanol binary system by a similar method. The obtained data favor the notions that 2kchem does not depend on the nature of the solvent and that the reaction in the cell can be considered on the basis of gas-kinetic collision theory [3].

472

~g zk

y

//

/

//

7,o

,.2r /

8,0

7,0

//

5,o

//o,~

// I

~,0

I

T 2,0 Lg~-

53

' 0

I

i~O

LO

T l,g~-]

Fig. 5 Fig. 6 Fig. 5. Relation of log 2k to log (T/~/) for reaction (4) with participation of NQ-'. The broken line denotes the theoretical dependence 2k = (i/4)kdi f • p = i0/~. Fig. 6. Relation of log 2k to log (T/~) for reaction (4) with participation of -O3SAQ-". The broken line denotes the theoretical dependence 2k = (i/4)kdi f X; P = i0 A. As is evident from the data of Fig. i, the numerical values of 2k 3 for the reaction of BPH' with ~ N-O" are less than the theoretical ones, but in the viscosity range of i-I0 cP (the content of glycerol in water is 0.55%; log (T/7/) = 2.47-1.47) the reaction is limited by the molecular mobility. Our obtained values of AH3 ~ in water and in an aqueous solution containing 50% glycerol are 3.2 • 0.6 and 6.5 + 0.5 kcal/mole, respectively. The obtained values of AH3 ~ are close or in agreement with the corresponding values of V for the given solvents (see Experimental), which confirms the presence of diffusion control [i]. The geometric steric factor for reaction (3) fg < 0.25 was evaluated from the data of Fig. 1 [i, I0, II]. The > N-O" radical is sterically hindered, and reaction (3) occurs, but not during each encounter of the radicals in the solution [i, i0, II]. Dismutation [Reaction (4)]. During pulsed photolysis of BP in an aqueous solution at pH ~ ii.0-12.5 the BP-" radical is formed because for protonation of ]3]?-" pK = 9.2 =~0.1 [7]. The d i s a p p e a r a n c e of BP-" is d i s mutation* and o c c u r s in an aqueous solution in the kinetic region (Fig. 3). The obtained value of AH4 ~ = 9.9 0.5 k c a l / m o l e for reaction (4) with participation of BP-" (water, pH 12.5) significantly exceeds the value of V for water. The measured values of 2k4 = (5.7 =L0.3)- 105 and 2k4 = (6.3 • 0.3). 106 l i t e r s / ( m o l e 9 (water, pH 12.0 and 11.0, respectively) in the limits of e r r o r s of the experiments a g r e e with the c o r r e s p o n d i n g values given in [15], Addition of glycerol (up to 80%) to the aqueous solutions causes a sharp acceleration of the d i s a p p e a r a n c e of BP-" (see Fig. 3). Such a c c e l e r a t i o n of the kinetic e l e c t r o n - t r a n s f e r reactions with participation of a r o m a t i c radical anions with i n c r e a s i n g glycerol content in a w a t e r - g l y c e r o l mixture was observed in [9] and can be explained with the Marcus theory. Indeed, with i n c r e a s i n g glycerol content, the value of l / c o p t 1 / e s t a t d e c r e a s e s ; in a c c o r d a n c e with the Marcus theory, in this c a s e the velocity of electron t r a n s f e r inc r e a s e s . The maximum values of 2t~ a r e achieved with contents of 50% (at pH 11.0) and 80% glycerol (at pH 12.0) in the solution (see Fig. 3). The maximum values of 2k~ a r e close to (pHll.0) o r e x c e e d (pill2.0) thevalue (1/4)kdi f = (1/4)(8RT/30007?)x(see Fig. 3), where • is the Coulomb m e m b e r [4] , equal to 0.45 for a reaction radius p = 10 A and estat = 60 for the given mixture. A further i n c r e a s e of the glycerol content (viscosity increase) leads to r e t a r d a t i o n of the rate of the given diffusion reaction (see Fig. 3). For dismutation of BP-" in h i g h - v i s c o s i t y media the obtained values of 2k4 a r e significantly higher than (l/4)kdif• and 2k~ d e c r e a s e s with i n c r e a s i n g v i s c o s i t y m o r e slowly than (1/4)kdifX. The r e a s o n for such a dependence apparently consists in the i n c r e a s e of p with d e c r e a s i n g mobility of the medium [16, 17].

The rate of reactions (4) with participation of BQ-, NQ-, -O3SAQ'- radicals d e c r e a s e s with i n c r e a s i n g (Figs. 4-6), and the reactions a r e limited by m o l e c u l a r mobility. The values of 2k~ for the reactions of BQ'and -O3SAQ'-" in nonhighly viscous media a r e significantly less than the c o r r e s p o n d i n g values of (1/4)kdif• (see Figs. 5 and 6). It is v e r y probable that reaction (4) with participation of BQ ~ and -OsSAQ: is limited by i m plantation of a p a r t n e r into the f i r s t coordination s p h e r e [6]. In a medium with high viscosity, 2k 4 for this * It is considered that the disappearance of radical anions in bimolecular reactions occurs by electron transfer [7, 13, 14]. At the same time, it is apparently possible to have a contribution of a disappearance reaction according to a second-order law including a step of the formation of a neutral semiquinone or ketyl radical [4].

4 73

reaction decreases with increasing viscosity more slowly than (i/4)kdifX same reason as for reaction (4) with participation of BP-.

(see Figs. 4-6), probably for the

Electron-Transfer Reaction (5). Just as for reaction (3), electron transfer between BP: and > N-O" is limited by molecular mobility and is characterized by the same value of the effective steric factor fef (Fig. i). The electron is apparently transferred to the ) N-O" fragment shielded by methyl groups. C ONC i.

Recombination

of the diphenyloxymethyl

LU SIONS

radical in a water-glycerol

2. Dis mutation of radical anions of 1,4-naphthoquinone ted by molecular mobility in a water-glycerol mixture.

mixture is limited by diffusion.

and Na 9,10-anthraquinone-2-sulfonate

is limi-

3. Dismutation of the benzophenone radical anion in a water-glycerol mixture occurs in the kinetic region. In addition, the reaction rate increases significantly with increasing glycerol content in the mixture in accordance with the Marcus theory. 4. Disproportionation between the diphenyloxymethyl and nitroxyl radicals and electron transfer between the benzophenone radical anion and the nitroxyl radical are limited by the molecular mobility, but the reaction occurs approximately in one out of 15 encounters of the radicals in solution because of spin prohibition and strong anisotropy of the reactivity of the nitroxyl radical. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14. 15. 16.

17.

474

CITED

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