Pulse radiolysis of pyridine and methylpyridines in aqueous solutions

Radiat. Phys, Chem. Vol. 41~ No. 6, pp. 825 834, 1993 Printed in Great Britain. All rights reserved

0146-5724/93 $6.00 + 0.00 Copyright { 1993 Pergamon Press Ltd

PULSE RADIOLYSIS OF P Y R I D I N E A N D M E T H Y L P Y R I D I N E S IN A Q U E O U S SOLUTIONS S. SOLAR,I N. GETOFF,I* K. SEHESTED2 and J. HOLCMAN2 ~lnstitut t'~r Theoretische Chemie und Strahlenchemie der Universitfit Wien, Wfihringerslr. 38, A-1090 Vienna, Austria and 2Section of Chemical Reactivity, Environmental Science and Technology Department. Riso National Laboratory, DK 4000 Roskilde, Denmark

(Receired 2 May 1992,"ac.'epted 31 August 1992) Abstract The radicals formed from pyridine, 3-methylpyridine, 3,5-dimethylpyridine, 2,6-dimethylpyridine and 2,4,6-trimethylpyridine by attack of tt, Gq, OH and O'- in aqueous solutions were investigated by pulse radiolysis in the pH-range I 13.8. The UV vis. absorption spectra as well as the formation and decay kinetics for the protonated and unprotonated forms of the methylpyridine radicals studied are presented. The pK~cvalues for the OH-adducts were determined.

INTRODUCTION In previous papers (Solar et al., 1988, 1991) we have reported spectroscopic and kinetic characteristics of transients produced by attack of OHradicals, H-atoms and eaq o n a number of pyridinecarboxylic acids in acid and alkaline aqueous solutions. It has been established that in acid solutions the position of the carboxylic group in the pyridine ring affects to some extent the reactivity of the substrate towards OH-radicals. The H-attack is more influenced by the position of the carboxylic groups both in acid and alkaline solutions. An interesting feature was the simultaneous formation of H-adducts and pyridinyl radicals in acid solution as a result of the H-attack on the pyridine carboxylic acids. In the case of 3,5-pyridine dicarboxylic acid (3,5-PDCA) even an intramolecular electron transfer, transforming the H-adduct into pyridinyl radical, was observed (Solar et al., 1988). This transformation was suggested to occur from the H-adduct of 3,5-PDCA at ipsoposition. The ratios of the measured H-adducts to pyridinyl radicals of each investigated compound were determined. The highest yields of H-adducts compared to those of pyridinyl radicals were observed when o- and p-positions to the N-atom in the ring were blocked by - - C O O H group. The H-attack distribution in respect to the pyridinyl radical formation in acid solution seems not to follow a simple pattern. In neutral and basic solutions, however, the reaction of H-atoms with the pyridine carboxylic acids leads exclusively to formation of H-adducts and *Address all correspondence to: Professor Nikola Getoff, Institut f/Jr Theoretische Chemie und Strahlenchemie, W/ihringerstr. 38, A-1090 Wien, Austria. tG-value = number of transformed molecules per 100eV absorbed energy. For conversion in SI-units multiply G by 0.10364 to obtain the yield in # m o l x J-~. 825

that of eaq to pyridinyl radicals, respectively (Solar et al., 1991). All these findings as well as an incompletely elucidated reaction pattern, notably those of H-attack on pyridine derivatives in acid solution, made it worthwhile to perform comparative studies on methyl-pyridines. As model substrates for these investigations were chosen: 3-methylpyridine (3-MP), 3,5-dimethylpyridine (3,5-1utidine; 3,5-DMP), 2,6dimethylpyridine (2,6-1utidine; 2,6-DMP), 2,4,6trimethylpyridine (sym. collidin; 2,4,6-TMP) and pyridine was included for the sake of comparison. EXPERIMENTAL

Pulse radio@~'is equipment The 10 MeV Linac at Riso (Haimson Research Corp., HRC-712) providing pulses of 0.2 1 #s duration and a detection system consisting of 450 W xenon-lamp, quartz cell (light path: 5.1 cm), a Perkin Elmer double quartz prism m o n o c h r o m a t o r and photomultiplier IP28 (previously described: Sehested et al., 1975) equipped with a LeCroy (model 9600) storage oscilloscope and an IBM PC/AT3 computer on line for data processing were used. Hexacyanoferrate(II)-dosimeter [G(OH + eaq) = 5.9?; ~420= 1000din 3 mol ~ cm J; Schuler et al., 1981] was used for determination of the absorbed dose. The measured transient absorptions at a given range were normalized to a dose of 10 Gy (1 krad) and presented as O.D./cm as a function of the wavelength (2 in nm). The experimental uncertainty of the measured rate constants amounts + 10% and of the extinction coefficients _+ 10%.

Preparation o f solutions and procedures The methylpyridines applied p.a. or purum degree (Merck or Fluka) were distilled under reduced pressure in the presence of argon before use and kept

S. SOLAR et al.

826

0,015

OD

o.8 ©

0.020

o Oh

0.6

o~

~ ,,

0.4

0.015-

H3 C - ~ C H 3 1,5 -

I t

0D cm

O I

;I

o~

I I

0.010. t

!

I ! ! ! !

I,

',B

I

/J

~

1,0-

i

D

I

I

i

oI

" L

I "~

A

II /,

a

0.010-

0

?

0.005-

I

II

0.2

l

I

g

o

0.s-

l/II

I

200

220

240

I

260 nm

I

I 0

',o.,%.o__ o_o.O°ooOo. i

~

T~ -

•

300

400

500

nm

Fig. 1. Transient absorption spectra obtained by OH-attack on pyridine (Py). (A) 5 x 10 3mol dm 3 Py, 1.25x10 3moldm 302 , p H 1 (HCIO4).(B) I x l 0 ~mol dm 3 Py, 2.8x I0 2mol dm 3 N20, p H - 9 . 8 (NaOH). Insert: absorption spectra of 2 x 10 4 py at pH 1 (A) and pH 9.8 (B). in amber glass bottles. All other chemicals used were o f p.a. quality. The solutions were freshly prepared in triply distilled water and saturated with either argon or N20, which converts e L into OH (N20 + eaq--*OH + N> + OH , k = 0.91 x 10mdm ~ mol ~s '; Janata and Schuler 1982). Some solutions were saturated with oxygen (1.25 x 10 3tool dm O2) or air ( 0 . 2 5 x 10 3mol d m s 02) in order to scavenge selectively H and e,q (H+O2--*HO2, k = 2.1 x 10 m d m 3 m o l i s i; e~q Jr- 02---*0"2 , k = 1 . 9 x 10 mdm 3 mol ~ s ~; Buxton et al., 1988). For measurement o f the pK,-values of the OHadducts 1 m M phosphate buffer was applied.

0~,

~ ~0 " - 0 ~

300

400

00

0.0151.0 I

! 1

I

,B

, o

0.010-

t e I

Reactivity of O H methylpyridines

radicals

with

,5

-\ ,

t 0.015

A~ A

1.0

5)o 6' I Ia

O.OLO-

O

[/,q~~B

1~

J /^ /,'

v/',

lit

°.5

I

I

: °,I

0.005-

0 200

l

',."L j ~-

300

1

I

, ~

0

and

H,CCH,A /1

H3C " l ~

0.5

I I

0.005-

pyridine

Spectroscopic and kinetic characteristics of the transients resulting from the attack o f OH, H and eaq on pyridine in aqueous solutions at pH 1 and 7 have

I

?

nrn

RESULTS AND DISCUSSION

,Qo

I ! !

500

Fig. 3. Transient absorption spectra obtained by OH-attack on 3,5-dimethylpyridine (3,5-DMP). (A) 10 ~mol dm 3,5-DMP, 1.25 x 10 Xmol dm ~ 02, p H I (HClO4). (B) I0 ~moldm 33,5-DMP, 2.8 x 10 2moldm 3N20, pH 10 (NaOH). Insert: absorption spectra of 2 x 10 4mol dm 3,5-DMP at pH I (A) and pH 9.5 (B).

0?m oo cm

O~

400

250 nm

~-o-O-o.

d

~

'

T,

W

o

1,.

,

,

200 ~ 2 s o

l

-'-°5

~o~

500 nm

Fig. 2. Transient absorption spectra observed by the attack of OH-radicals on 3-methylpyridine (3-MP). (A) 5 x 10 3moldm 3 3-MP, 1.25 x 10 - 3 m o l d m 3 O2' pH 2 (HCIO4). (B) 10 3 to 10-2 mol dm -3 3-MP, 2.8 x 10 -2 mol dm 3 N20, pH 9.5-10.5 (NaOH). Insert: absorption spectra of 2 x 10 4moldm -3 3-MPat pH l (A) and pH 9.5 (B).

"-~

r,m

0

3;0

4;0

5;0

nm

Fig. 4. Transient absorption spectra obtained by the OHattack on 2,6-dimethyl-pyridine (2,6-DMP). (A) 10 3tool dm 3 2,6-DMP, 1.25× 10 3moldm-3 02 , pH 1 (HCIO4). (B) I0 3moldm 32,6-DMP, 2.8 × I0 2moldm 3N20, pH 10 (NaOH). Insert: Absorption spectra of 2 x 10 4tool dm 3 2,6-DMP at pH 1 (A) and pH 9.5 (B).

Pulse radiolysis of pyridine and methylpyridines

x

x

J

x

x

x

x

x

~

~

~

827

x

x

x

x

x

x

o

~

~

~ ~

~

x

._=

~

.i

j~-

~

~/

~ ~

~ ~

~

-

~ ~

I ,-r" 0

x

x

x

x

J

x

x

x

x

x

x

x

~ x

~2 x

x

×

,,-q r , i ~

x ~

x

x

×

._;

06

r-i

i .r_,

J

kO

828

S. SOLAR

1,5-

CH3

o.o2o-

o.._QD cm

'~

~.0 ",,

; B

I

0.015-

/

I

i I

! 0.010-

I

,, '

o

l

J

z

' II

I

o

,'

I

I

200

250

nm

0.005-

0

i

i

i

300

400

500

i

nm

600

Fig. 5. Transient absorption spectra obtained by OH-attack

on 2,4,6-trimethyl-pyridine (2,4,6-TMP). (A) 10 ~tool dm 2,4,6-TMP, 1.25 x 10 ~mo] dm ~ O 2, pH 1 (HC104). (B) 10 3moldm ~ 2,4,6-TMP, 2.8 x 10 : m o l d m ~ N_~O, pH 10 (NaOH). insert: Absorption spectra of 2 x 10 4tool dm ~ 2,4,6-TMP at pH 1 (A) and pH 9.5 (B).

been reported (Cercek and Ebert, 1967). Kinetic data for the formation of the pyridine species were also given by Simi6 and Ebert (1971). The transient absorption spectra however, were only measured up to 380 nm. Hence, for a comparison with the transients of methylpyridines investigated in this work some pulse radiolysis experiments with pyridine were also performed. The observed transient absorption spectra resulting from the OH-attack on pyridine at pH 1 and 9.8 are presented in Fig. 1. In acid solution the spectrum shows a maximum at 315 nm and a very weak absorption in the range from 400 to 550 rim. A strong absorption maximum at 322 nm and two weak bands (475 and 510 rim) were observed in alkaline solution. The evaluated spectroscopic and kinetic data of the OH-adducts of pyridine and of the investigated methylpyridines are compiled in Table 1. The absorption spectra of the starting compounds in acid and alkaline solutions are given as an insert on each figure. Our rate constants for the formation and decay of the pyridine OH-adducts in acid and basic solutions are somewhat higher in comparison with those previously reported by Cercek and Ebert (1967). The absorption spectra of the OH-adducts resulting from 3-MP in acid and alkaline solutions are presented in Fig. 2. Here, again the protonated form of the substrate leads to species having one small, but pronounced maximum in the UV-range (335 nm), whereas the OH-adduct of the unprotonated molecule shows two absorption maxima: a very strong one (320 rim) and a broad absorption band in the

el a/.

visible range (460 nm). The spectroscopic and kinetic data are given in "Fable 1. The rate constant for the transient formation in alkaline solution is in fair agreement with a previously published value (Shevchuk el a/., 1969). In the case of 3,5-DMP the absorption spectra of the OH-adducts formed in acid and alkaline solutions are similar to those of pyridine and 3-MP shown above (Figs I and 2). The only difl'erences are in respect to 2 ..... and ~-values (see Table 1 and Fig. 3). The transient absorption spectra of the OHadducts resultin from 2,6-DMP at pH 2 and 10 are presented in Fig. 4, together with the absorption spectra of the substrate given as insert. It is interesting to note that in this case two strong absorption maxima were registered also in the acid media. The transient absorption maxima in alkaline solution shifted somewhat to longer wavelengths in comparison to the previous ones. The evaluated data are compiled in Table 1. The OH-adducts from 2,4,6-TMP possess two even much more pronounced absorption maxinla in acid and alkaline solutions (Fig. 5). Their shift to longer wavelengths is increased due to the occupation of three methyl groups on the pyridine ring. Summing up it can be proposed that the observed spectra resuhing from OH-attack on pyridine and methylpyridincs are those of OH-adducts to the pyridine ring. This is corroborated by the fact that observation of acid catalyzed water elimination from the OH-adduct to lhe pyridine ring by pulse radiolysis requires substitution with two methoxy-groups (Steenken, 1987). ,As the ability to eliminate water from OH-adducts of methylated benzenes decreases with increasing ionization potential (Sehested and Holcman, 1978) and as the methoxyl substitution in benzene ring lowers the ionization potential ,-a 2 3 times more than methyl substitution the acid catalyzed water elimination is not considered probable even for trimethylpyridines. Indeed no indication ['or water elimination was observed. This assignment is additionally corroborated by preliminary experiments where 2,4,6-TMP was oxidized by the SO'4 radical anion in acid (pH 4) and in basic (pH 9.5) solutions both with and without t-butanol used as an OH radical scavenger. The obtained transient spectra are very similar to that observed by the OH-attack on the same substrate at the corresponding pH. Based on this fact it is assumed that in the presence of SO'4 species first the radical cation of 2,4,6-TMP is formed, which, however, reacts fast with water resulting in OH-adduet. In alkaline media (pH 9 10) the OH reactivity with the same substrates in their unprotonated form is essentially higher which is consistent with the electrophilic nature of the OH-radical. For methylated benzenes the rate constant for the direct H-atom abstraction from methyl group by OH-radical 4-4.7 x 10~dm ~ tool ~ s ~ per methyl group was reported (Sehested et al., 1975). If this rate

Pulse radiolysis of pyridine and methylpyridines

829

Table 2. pK-valuesof the pyridineand methylpyridineOH-adducts Substrate

Measured at 2 (nm)

pK~-value solute

pK-value OH-adduct

320 320 330 320 340

5.20 5.85 6.75 6.15; 6.34 7.40

4.55 5.3 6.8 5.7 6.6

Pyridine 3-MP 3,5-DMP 2,6-DMP 2,4,6-TMP

applies to methylpyridines the methyl H-atom abstraction is the dominating reaction in the acid range while much less important at pH 10. However, as the present results do not allow for differentiation between the spectrum of the ring OH-adducts and products of the H-atom abstraction (e.g. substituted methyl radicals) thc spectra produced by OH radical in acid as well as in alkaline solution were arbitrarily ascribed to a single species, OH-ring adducts and G(OH) values were used in calculation of/-max given in Table 1.

pK,,-values of the OH-adducts

e.g.

H3C~

~

H3C~

OH

+ H+

"N" "OH

(1)

Based on the strong spectroscopic differences between acid and alkaline medium discussed above, it was of interest to determine the dissociation constants of the OH-adducts of the investigated methylpyridines [see e.g. reaction (1)]. For this purpose the O.D.-values of the buffered substrate sol-

+ 0.1 + 0.1 + 0.1 _+ 0.1 _+ 0.1

ApK~ 0.65 0.55 -0.05 0.45; 0.64 0.80

log

(k~/k.)

1.48 1.27 0.67 0.96 0.51

utions (10 3 tool dm ~) were measured at suitable wavelengths as a function of pH. From the strong change of the O.D.-values the pK,-values of the OH-adducts were determined and presented in Table 2. Except for 3,5-DMP pK~-values of the OH-adducts are lower than pK[s of the original compounds. As the spectra resulting from the OH-reaction with acid form of the solutes are substantially different from those obtained with the basic forms the procedure applied will yield titration curves even if there were no protolytic equilibria concerning products of the OHattack. However, if this was the case the difference in pK, of the parent compound and that obtained experimentally, ApK~, should equal log (kb/ka) where k~ and k~ are the rate constant of the OH-reaction with protonated and unprotonated solute, respectively. As it can be seen in Table 2, ApK, and log (kb/ka) differ significantly. This indicates that the obtained pK~-values concern genuine protolytic equilibria of OH-adducts, however, a different reactivity pattern of the protonated and unprotonated substrates might influence the accuracy of the obtained pK~-values.

Reactiri O' of H-atoms with pyridine and methylpyridiI?CS

0-015t o I

l'

°° l

0.010

I

I

0.005-

0

o . . ~

360

0.010OD cm

0 I

,,.0%

400 i

560

"~.7"%_'o

600 ~m i

! I

0.005°~ o

i

300

40o

65o

7&

Fig. 6. Transient absorption spectra obtained by H-attack pyridine (I), 3-methyl-pyridine (2), 3-5-dimethylpyridine (3), 2,6-dimethylpyridine(4) and 2,4,6-trimethylpyridine(5) at pH 1. RPC 4 1 / ~ D

The absorption spectrum of H-adduct to pyridine in the UV-range and the rate constant for its formation in acid solution has been previously reported (Cercek and Ebert, 1967). Our pulse radiolytic experiments showed that these species also have an absorption band with ,~max = 515 nm (Fig. 6, spectrum 1). The H-adducts of all investigated methylpyridines possess two absorption maxima: in the UV (320-370 nm) and in the visible range (500-700 nm). The observed absorption maxima and extinction coefficients are found to be strongly dependent on the number and position of the substituted methyl groups. This fact indicates their influence upon the site of H-attack. From the inspection of the rate constants in Table 3 it can be concluded that positions 2 and 6, if not blocked by --CH3 groups, are the most sensitive sites for H-addition on the pyridine ring, while positions 3, 4 and 5 seem to be less reactive. One of the aims of our study was to examine whether in addition to H-adducts also pyridinyl radicals are formed in acid solution. This pattern has been previously observed for pyridine carboxylic acids (Solar et al., 1988, 1991). Based on the obtained data a straightforward evaluation of the formation of

830

S. SOLAR et al. T able 3. Spectroscopic and kinetic characteristics of transients produced by reaction of H-atoms with pyridine and methylpyridines in acid solution (pH = 1) Kinetics (dm ~ tool ~ s ~) ~'ma~

}'max

Substrate

(nm)

Pyridine (pK - 5.2)

H3C~

(dm 3 mol

--

i cm

335

2240

515

500

345

3000

530

1000

H~C-~CH3

3,5-Dimethylpyridine (pK - 6.72: 6.75)

350

4500

Decay

03 ~- H-Add.

2.0 × I(P

5.0 × 107

3.5 X 10~

4.0 × 10:

4.0 x 102

5.5 X 10 8

219

×

10ah

9.0 × 10~ 550

1800

H3

340

18oo

2,6-Dimethylpyridine (pK = 6.15; 6.34)

580

1600

H3C~~CH3

350

2900

2,4,6-Trimethylpyridine (pK = 7.43; 7.40)

610

1050

CL?ICH3

Formation

1.7 × 10~'~

3-Methylpyridine

(pK = 5.85)

I)

2.7 × 1 0 2.3 × 10~¢

2.1 × 10~

3.2 × [0 s

6.8 × 107

2.0 × 10~

2.8 × 10~

I.I × I(f'

C.H3

"Cercek and Ebert (1967); Umeasured at 350 nm; Cat 530 n m

the pyridinyl radicals is not possible. Very likely, their absorption may be expected to be below 300 nm, as found in alkaline solutions (see below). Because of the strong substrate absorption in this range and uncertainties in the corrections needed a decisive resolution of this problem could not be achieved.

One electron reduction of pyridine and methylpyridines Formation of pyridinyl radicals. The rate constant for reaction of eaq with pyridine, k = 1 × 101°dm3mol I s ~ has been previously determined by Cercek and Ebert (1967), whereas Fessenden and Neta (1973) reported ESR-spectra of radicals resulting from the eaq attack on pyridine and 3,5-DMP. However, the absorption spectra of the transients formed by reaction of e~ with pyridine and methylpyridines have not been reported. For the study of the one electron reduction of pyridine and the investigated methylpyridines deoxygenated solutions of 10 -4 mol dm -3 substrate in the presence of 10 -2 mol dm 3 sodium formate at pH 9.5 were used. The formate scavenges O H and H resulting in the CO" 2 radical anion. A subsequent one electron transfer from the C O ' : radical anion, known as a potent one-electron reductant (Schwarz and Dodson, 1989), to the substrate was expected. How-

ever, no such reaction was observed in the time scale of our experiment which is in agreement with earlier findings (AI-Hayaly and Buxton, 1988; Neta, 1972: Land and Swallow, 1968) that the CO'2 radical reacts only with N-protonated pyridine species. Alternatively isopropanol, also yielding strongly reducing radicals by scavenging H atoms and OH radicals (Schwarz and Dodson, 1989), showed negative results. A reduction of the pyridine compounds under investigation was observed only by a direct attack of e,q, e.g.

H3C~ +

e~q---~

H3C~.

The rate constants, k (eaq + S ) , for reaction (2) are given in Table 4. They are showing a weak decreasing tendency with increasing number of methyl groups from k = 7.7 × 109 dm 3 mol ~ s (pyridine) to 4.4 × 109din 3 mol ~ s ~ (2,4,6-TMP). This trend can be rationalized in terms of electron donating properties of the methyl substituent as opposed to the nucleophilic character of e~q. The electron adducts of all studied substrates resulting from reaction (2) are converted into the

Pulse radiolysis of pyridine and methylpyridines

831

Table 4. Spectroscopic and kinetic data of transients formed by one electron reduction of pyridine and methylpyridines, pH 9.5 (airfree) 2m~x (nm)

Substrate

%.~ ' cm

(dm~ mol

k (%q + S) ~) (din~ mol t s

')

255

3050

7.7 x 109 1.0 x 109~

255sh 270

-4500

7.5 x 109

250sh 275

5500

Pyridine

(pK= 5.2) H3C-~ 3-Methylpyridine = 5.85)

(pK

H3C'~CH3 3,5-Dimethylpyridine (pK = 6.72; 6.75)

H3CLN(~)/['CH3

7.0 x 109

250sh

2,6-Dimethylpyridine (pK - 6.15; 6.34)

H3C~CH 3 2,4,6-Trimethylpyridine

--

270

5150

245 270

4350 5850

50

×

,o~

4.4 X 10 9 (2.5 x 10~° at pH - 5)

(pK = 7.43; 7.40) ~Buxton et al. (1988).

c o r r e s p o n d i n g pyridinyl radicals in a q u e o u s solution, e.g.

+

H20---~

+

OH-.

H

(3)

T h e a b s o r p t i o n spectra o f the pyridinyl radicals are s h o w n in Figs 7-10. W i t h exception o f the pyridine transient, h a v i n g one m a x i m u m at 255 n m , the following three s u b s t r a t e s , 3-MP, 3 , 5 - D M P a n d 2,6D M P p o s s e s s a s h o u l d e r a r o u n d 250 n m a n d a s t r o n g m a x i m u m at a b o u t 2 7 0 n m . 2 , 4 , 6 - T M P has t w o a b s o r p t i o n b a n d s , at 245 a n d 270 n m . T h e m o l a r extinction coefficients (E) o f the o b s e r v e d pyridinyl radicals s h o w an increasing t e n d e n c y s t a r t i n g f r o m

0D cm

(A)

(B)

0D

O ~*NN '~ 'cH~ H H

0015-

H3C~CH3

cm

0.015

[C~Ol

e

0010 0 005

0

,°'o' I°, B .

~

o

0.010 0.005

!

!

o~O/ l o

~

H

f,

I

-°"w°P"

I 0

O,, "O'-o ~.O

200

'

250

(~

30

~

'

350nm

Fig. 7. Transient absorption spectra obtained by one electron reduction of pyridine (A) and of 3-MP (B) at pH 9.5. S o l u t i o n s : 10 -4 mol dm -3 substrate, 10 -2 mol dm -3 sodium formate, airfree.

0

200

250

300

350 nm

Fig. 8. Transient absorption spectrum obtained by one electron reduction of 3,5-DMP (10-4mol dm -3) in the presence of 10 2 mol dm 3 sodium formate, pH 9.5, airfree.

832

S. SOLAR et al.

o_o

cm

H~CH~CH3

0.015

P'o I

O.OLO-

I

0 I !

o_Qa

I

em

0.010 I

'o

0.005

0.005-

\

q

!

\

o /

~0~0 --O~D..O~ ~0.. 0

I

200

:

I

0 300

400

350 nm

oo

cm

!

|

?

'o o

0.005 E = 3050 dm 3 mol ~ cm ' for the pyridine transient and reaching E = 5850 dm 3 mol 1 cm ' for 2,4,6T M P species (Table 4).

5

I

k4

" ---~eaq

2.5 x 107dm3mo] is 1

=

(Buxton et al., 1988) e~q + N20--*OH + O H

(4)

-

g

300

attack on pyridine and

In N20 saturated solutions at pH 13.8 the primary species of water radiolysis (e,q, H and OH) are converted into O" with G(O ) = 6 . 5 [reactions (4)-(6); Fielden, 1967]. This G-value has been taken for calculation of the extinction coefficients of the transients resulting from H-abstraction on the methyl groups of the substrates under investigation. The ~-values and the kinetic data of these species are presented in Table 5. H + OH

!

0.010-

Fig. 9. Transient absorption spectrum obtained by one electron reduction of 2,6-DMP (10 4mol dm 3) in the presence of 10-2mol dm 3 sodium formate, pH 9.5, airfree.

Transients produced hy O" methylpyridines

500 nm I

i

300

2£;0

1 io,'

4 ~'~,..,

i

i

400

~ - - -

500

",o

i"

nm

Fig. I I. Transient absorption spectra obtained by reaction of O" radical anions with pyridine (1), 3-MP (2), 3,5-DMP (3), 2,6-DMP (4) and 2,4,6-TMP (5) in strong alkaline solutions (pH 13.8). Solutions: 10 3mol dm ) substrate, 0.5moldm 3 NaOH, 2.8 × 10 2mol dm ~ N~O. OHIO"

+ H +,

pK=ll.84

(Elliot and McCracken, 1989).

(6)

It is known that O" radical anions are reacting preferentially with the side groups of aromatic compounds by H-atom abstraction (Sehested et al., 1975: Neta and Schuler, 1975). Hence, it is expected that in strongly alkaline solutions (pH > 13) reaction (7) can take place, e.g.

+ N2

k5=0.91 x 10]°dm3mol Is l (Janata and Schuler, 1982)

(5)

e.g.

+ O'-

oo

cm

H2,.~O P_ A 3

i°~

,o,,

0.015

H~C ~ " C H ~ H

i

1

+

OH-. (7)

I !

0.010

! |

2

0%

_e,o

% %

0

0.005

'o *o

0

200

I

i

250

300

i

350 nm

Fig. 10. Transient absorption spectrum obtained by one electron reduction of 2,4,6-TMP (10-4mol dm -3) in the presence of 10 -2 mol dm -3 sodium formate, pH 9.5, airfree.

The absorption spectra of the transients resulting from the reaction of O" with pyridine and methylpyridines under investigation at pH 13.8 in the presence of 2.8 x 10 2 mol dm 3 N20 are presented in Fig. 11 and pertinent kinetic data in Table 5. Obviously, the transient spectrum of Py (Fig. 11, spectrum 1; 2 < 2 7 0 n m , shoulder at ~ 3 2 0 n m ) strongly differs from the methylated pyridines which have absorption bands around 300-340 nm and between 380-550 nm. This fact might be explained by the assumption that O" reacts with pyridine by formation of an adduct, whereas the major reaction

Pulse radiolysis of pyridin and methylpyridines

833

Table 5. Spectroscopic and kinetic characteristics of transients produced by reaction of O' radical anions with pyridine and methylpyridines in strong alkaline solution (pH 13.8) Kinetics (dm~ molt s-i) Substrate

~max

(nm)

~max

(dms mol t cm L) Formation

© Pyridine (pK = 5.2)

310sh

Decay

2.0 x 10s

0.2 ×

1.0 x 109

2.3 x lOs

1.3 x

4.2

10 9

~ 800

H~C-~ 3-Methylpyridine (pK = 5.85)

450

460

300

1800

400 450

400 550

310

1600

445

650

315

1550

455 490

700 500

H3C'~CH3 3,5-Dimethylpyridine (pK = 6.72; 6.34)

H~C~'/LCH3 2,6-Dimethylpyridine (pK - 6.15; 6.34)

10 9

x 108

1.1 x 109

2.0 x 108

1.4 x

1.6 x 10~

C.H3 H3C~"N'~CH~ 2,4,6-Trimethylpyridine (pK = 7.43; 7.40)

of the methylated pyridines is the H - a b s t r a c t i o n from the methyl groups. This a s s u m p t i o n is also supported by the build-up rate constants (Table 5), where the reaction o f O" with pyridine is roughly one order of m a g n i t u d e lower t h a n those of the methylated pyridines under investigation. The a b s o r p t i o n spectrum of the above postulated pyridine O" adduct also differentiates from the c o r r e s p o n d i n g OH-adduct. This difference could be rationalized in terms of the dissociation of hydroxyl p r o t o n analogous to t h a t described for pyridine dicarboxylic acids (Taniguchi, 1987) rather than hypothetical a b s t r a c t i o n of a ring-hydrogen atom. O u r rate c o n s t a n t for reaction of O ' - with pyridine k = 2 x 108 d m 3 tool -1 s -1 is substantially higher t h a n the earlier reported limit ~<0.7 x 10Sdm 3 mol -~ s -~ (Neta a n d Schuler, 1975) we can offer no explanation of this discrepancy. F r o m Table 5 it can be seen that the rate constants for O" reaction with methylated pyridines are 5-7 times higher t h a n that for pyridine a n d do not vary essentially with the n u m b e r o f methyl substituents. This b e h a v i o u r indicates prevailing methyl H - a t o m abstraction. Indeed the rate c o n s t a n t for O ' - reactions with methylated benzenes are all within the range of 1 . 8 - 2 . 6 × 109dm3mol Is l (Sehested a n d H o l c m a n , 1978). The spectra obtained from O" -attack on methyl pyridines (Fig. 11) resemble the features of O H - a d d u c t s observed at

10 9

pH 9.8 (Figs 1 5) on the other hand, they resemble spectra derived of methyl substituted benzyl radicals (Sehested and H o l c m a n , 1978) only shifted to the red. We suggest that these spectra in the m a j o r portion belong to the products of methylic H - a t o m abstraction, a l t h o u g h a smaller a m o u n t of the ring O H adduct is most p r o b a b l y present in all of them. Acknowledgements--We thank Miss H. Corfitzen and Mr T. Johansen for skillful technical assistance. Two of us, S.S. and N.G. thank Riso National Laboratory, Danish Education and Science Ministry, and the Federal Ministry for Science and Research in Austria for financial support.

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834

S. SOLAR et al.

Fessenden R. W. and Neta P. (1973) ESR-spectra of radicals produced by reduction of pyridine and pyrazine. Chem. Phys. Lett. 18, 14 17. Fielden E. M. and Hart E. J. (1967) Primary radical yields in pulse-irradiated alkaline aqueous solution. Radiat. Res. 32, 564 580. Janata E. and Schuler R. H. (1982) Rate constant for scavenging e~q in N20-saturated solutions. J. Phys. Chem. 86, 2078 2084. Land E. J. and Swallow A. J. (1968) One electron reactions in biochemical systems as studied by pulse radiolysis, l. Nicotinamide-adenine dinucleotide and related compounds. Biochim. Biophys. Acta 162, 327 337. Neta P. (1972) Electron spin resonance study of radicals produced by one electron reduction of pyridines. Radiat. Res. 52, 471 480. Neta P. and Schuler R. H. (1975) The mode of reaction of O" radicals with aromatic systems. Radiat. Res. 64, 233 236. Schuler R. H., Hartzell A. L. and Behar B. (1981) Track effects in radiation chemistry. Concentration dependence for the scavenging of OH by ferrocyanide in N,Osaturated solutions. J. Phys. Chem. 85, 192. Schwarz H. A. and Dodson R. W. (1989) Reduction potentials of CO'1 and the alcohol radicals. J. Phys. (Twin. 93, 409M. 14.

Sehested K. and Holcman J. (1978) Reactions of the radical cations of methylated benzene derivatives in aqueous solution. J. Phys. Chem. 82, 651~53. Sehested K., Corfitzen H., Christensen H. C. and Hart E. J. (1975) Rates of reactions of O" , OH and H with methylated benzenes in aqueous solutions. Optical spectra of radicals. J. Phys. Chem. 79, 310 315. Shevchuk L. G., Zhikharev V. S. and Vysotskaya N. A. (1969) Kinetics of the reactions of hydroxyl radicals with benzene and pyridine derivatives. Zh. Org. Khim. 5, 1655 1658. Simid M. and Ebert M. (1971) Pulse radiolysis of aqueous solutions of carboxy, carboamids and pyridyl derivatives of pyridine. Int. J. Radiat. Phys. Chem. 3, 259 272. Solar S., Getoff N., Holcman J. and Sehested K. (1991) Pulse radiolysis of pyridine-carboxylic acids in aqueous solutions. Radiat. Phys. Chem. 38, 323 332. Solar S., Solar W., Getoff N., Holcman J. and Sehested K. (I 988) Reactivity of H, OH and e,q with nicotinic acid. A pulse radiolysis study. Radiat. Phys. Chem. 32, 585 592. Steenken S. (1987) Addition~limination paths in electron transfer reactions between radicals and molecules. J. chem. Sot., Faraday Trans. 1 83, 113 124. Taniguchi H. (1987) ESR study of the dissociation of hydroxyl protons in hydroxyl radical adducts to pyridine dicarboxylic acids. J. Phys. Chem. 91, 5213 5217.