Radical intermediates in the photochemical decomposition of p-toluenesulphonate (a kinetic spin trapping study)

229

J. Photochem. Photobioi. A: Chem., 71 (1993) 229-235

Radical intermediates in the photochemical decomposition p-toluenesulphonate (a kinetic spin trapping study) V. Brezov%,

A. Stagko

and

of

S. BiskupX

Faculty of Chemical Technology, Slovak Technical University, Radlimtiho

9, CM1237

Brotislava (Slovak Republic)

(Received October 12, 1992.;accepted December 10, 1992)

Abstract On irradiation of aqueous sodium p-toluenesulphonate solutions, the NaSOs’ radical is formed, whose lifetime is approximately 5 X1W3 s, and which is characterized by a singlet electron paramagnetic resonance (EPR) spectrum with g = 20035. In the presence of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) the main product found is the NaSO,‘-DMPO adduct with splitting constants aN= 1.45 mT and (I~= 1.635 mT. Its formation and decay can be described by formal first-order kinetics and a corresponding reaction scheme is suggested. HO.-DMPO and carbon-centred R,C’-DMPO adducts are observed as minor byproducts.

1. Introduction Alkylbenzenesulphonates are important components of surfactants and frequently cause the pollution of natural aquatic environments. An effective method for their removal from water is photochemical degradation by solar irradiation [l]. The reaction mechanisms for the photolysis of benzenesulphonic and alkylbenzenesulphonic acids in aqueous solution have been proposed [2, 31. The formation of hydroxyl radicals as reactive intermediates as well as carbon- and sulphurcentred radicals was described using the spin trapping technique [4]. This study reports the direct observation of free NaSO; radicals and further radical intermediates, produced by irradiation of aqueous sodium ptoluenesulphonate solution, and describes their kinetics of formation using the spin trapping technique.

2. Experimental

details

2.1. Materials 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), purchased from Aldrich (USA), was freshly distilled at 75 “C! and 0.5 Torr. The colourless liquid was stored in a freezer at -25 “C under argon. The spin trap 3,5-dibromo-4-nitrosobenzenesulphonic acid (DBNBS) was produced by Sigma. p-Toluenesulphonic acid and all the chemicals used were

1010-6030/93/$6.00

of analytical grade purity from Lachema Brno (Czechoslovakia). Potassium ferrioxalate was prepared in our laboratory according to ref. 5. The sodium p-toluenesulphonate (PTS) was obtained by dissolvingp-toluenesulphonic acid in a saturated solution of sodium hydrogen carbonate at 22 “C. The solution was neutralized to pH 7. 2.2. Apparatus A Bruker 2OOD electron paramagnetic resonance (EPR) spectrometer (Germany), equipped with a field frequency lock and coupled to an Aspect 2000 computer, was used to obtain the EPR spectra. The aqueous samples placed in a flat cell were irradiated directly in a TM 102 cylindrical cavity. A 500 W high-pressure mercury lamp (Narva, Germany) with an originally constructed power supply was employed as the irradiation source. Its irradiation flux ($0) was measured directly in the EPR cavity using ferrioxalate actinometry [5] and was found to be (&=4.4x 10m9 mol s-l. The cavity was continuously flushed with argon at 20 “C to eliminate overheating by irradiation. The EPR spectra obtained were analysed and simulated using standard Bruker programs. 2.3. Pmcedures The aqueous solutions, containing various concentrations of PTS as specified below, were saturated with argon and then pumped into the quartz flat cell. This was placed in the spectrometer cavity and exposed to various irradiation periods; si-

Q 1993 - Elsevier Sequoia. All rights reserved

230

V: Brezowi et al. I Photochemicd

decomposition of p-toluenesulphonate

multaneously, the EPR spectra or the intensity of one selected individual line were recorded. Representative spectrometer settings were as follows: centre field, 347.5 mT; sweep width, 7 mT; scan time 20 or 50 s; microwave frequency, 9.72 GHz; microwave power, 10 mW; spectrometer gain, 5 X lo4 to 5 x 105. If the intensity of an individual line was recorded (Figs. 3 and 5, see Section 3), the modulation was increased to 0.25 mT and the magnetic field was set on the maximal intensity of the fist line in the low field. Its position was characterized in an initial experiment using a marker. In further measurements, this position was locked using the field frequency lock and the spectrometer was put in a time-base mode. Photochemical reactions were carried out in a quartz photochemical immersion well {Applied Photophysics, UK} at 25 “C. A 125 W mediumpressure mercury lamp (Applied Photophysics, UK) was used as irradiation source. The sulphur dioxide evolved during the irradiation of aqueous PTS solution was identified as BaSO, as described in ref. 2.

3. Results

and discussion

The results of three types of EPR experiments are presented according to the composition of the irradiated system: (1) aqueous solutions of PTS; (2) aqueous solutions with constant initial concentration of PTS ([PTS]O= 1 M) and with variable initial concentrations of DMPO; (3) aqueous solutions with constant initial concentration of DMPO ([DMPO]‘=O.Ol M) and with variable concentrations of PTS. The UV absorption spectrum of DMPO in aqueous solution exhibits only one strong absorption peak at 227 nm. Consequently, the irradiation of the high-pressure mercury lamp is mostly absorbed by PTS. 3.1. PTS solutions On irradiation of PTS aqueous solution ([PTS]O= 1 M) a singlet spectrum with g=2.0035 was observed (Fig. l(a)). This intermediate was identified as the NaSO,’ radical in the experiments with the DMPO trap as stated below. The stationary concentration of the generated radical (approximately 5X lo-” M) remains unchanged for longer than 30 min. During interruption of irradiation the concentration of the radical decreases rapidly to zero and then increases again on irradiation as shown in Fig. l(b), where the changes in the maximum line intensity are

Fig. 1. (a) EPR spectra observed after various irradiation periods of 1 M aqueous PTS solution. (b) Change in the maximum line intensity of the spectrum in (a) during interruption of irradiation ( T , irradiation start; 1, irradiation stop).

recorded. From the slope of the line after the irradiation stops and from the time resolution available in our experimental arrangement, it may be concluded that the half-life of the radical formed is shorter than 0.1 s. If we consider the irradiation flux ($= 4.4~ lo-’ mol s-l), molar absorptivity of PTS (Ezao= 3700 mol-’ dm2) [6], path length (1=0.003 dm), calculated rate of photoabsorption of PTS in the cell (@=9X10-’ mol dme3 s-l), quantum yield (4 =O.l) [2, 31 and the estimated stationary NaSO,’ concentration ([NaS03JST= 5 X lo-’ mol dme3), the approximate lifetime for the NaSO; radical is calculated to be 5 x 10m3 s. 3.2. 1 M PTS, 0.2-0.001 M DMPO solutions When the experiments described above were carried out in the presence of DMPO ([DMPO]O= 0.05 M), a spectrum with g= 2.0057 was obtained (inset in Fig. 2). The splitting constants extracted from the simulation, uN = 1.45 mT and aH= 1.635 mT, confirm the formation of the NaSO,’ adduct of DMPO. Similar values for this adduct are reported in refs. 7 and 8.

K &ezov& et al. / Photochemical decomposition

Fig. 2. EPR spectra of N&O;-DMPO obtained in aqueous solutions after various irradiation times ([PTS]‘= 1 M; [DMPO]‘=O.Ol M). Insets: experimental and simulated spectra of NaSOs*-DMPO with splitting constants nH and IQ+

Spectra recorded after various irradiation periods are presented in Fig. 2. The adduct concentration increases with time and its approximate value after 960 s of irradiation is lo-’ M. More accurate monitoring of this time dependence was obtained by following the changes in the maximum intensity of the first line, I!-,, with irradiation time. Before this, experiments similar to those described in Fig. 2 were carried out to investigate the changes in the spectral linewidths and to look for the formation of furtlier products in addition to NaSO,‘. The linewidth remained approximately constant and additional products to NaSO,’ were found only under extreme substrate concentrations as described below. The dependence of the intensity of the first line, It_I, on the irradiation time at various DMPO concentrations in 1 M PTS aqueous solution is shown in Fig. 3. I,-, continuously increases in systems with high initial DMPO concentrations ([DMPO]’ = 0.05-0.2 M). At very low initial DMPO concentrations ([DMPO]‘= 0.0025 and 0.001 M) a stationary concentration of N&O,‘-DMPO adduct was established after a few seconds (Fig. 3(b)).

of p-toluenesulphonote

231

Fig. 3. Time dependence of the line intensity, I,_, of the spectral line I- 1 in 1 M PTS at various initial DMPO concentrations [DMPO]‘: (a) [DMP0]0=0.054L2 M; (b) [DMPO]0=0.0014X01 M. Insets show characteristic EPR spectra for these concentration ratios.

After a time period of 8 min the irradiation was stopped and the decay of the NaSO,‘-DMPO adduct was followed for a further 8 min. The decay of the NaSO,‘-DMPO adduct is significantly dependent on the initial DMPO concentration. For [DMPO]O= 0.01-0.001 M the decrease in adduct concentration is rapid (Fig. 3(b)). This dependence will be discussed in Section 3.4. The insets in Fig. 3 show the spectra whose line intensities I,_, were measured. The analysis of these spectra is given in Fig. 4. The spectrum in the inset of Fig. 3(b), according to its simulation in Fig. 4(a), contains a carbon-centred radical ‘CR, in a minor concentration. The main contribution originates from the NaSO,‘-DMPO adduct and the intensity of the first line 1,_ 1 is a good measure of NaSO,’ formation in Fig. 3(b). A more specific identification of the carboncentred radical adduct R3C-DMPO can be obtained, In the literature [8] it has been reported that DMPO adducts in aqueous solutions with uH > 2.4 mT originate from phenyl-centred radicals and those with lower uH values originate from aliphatic carbon-centred radicals. In the primay step of PTS photolysis the p-CH,C,H,’ radical is expected. However, the value a,=2.38 rnT is indicative of an aliphatic carbon-centred radical. In additional experiments employing DBNBS as

232

K Brezovd et al. I Photochemical decomposition of p-toluenesulphonate

15% if [DMPO]O= 0.075 M. At [DMPO]O<0.05 M, the fraction of ‘OH adduct is considerably lower than 10% and Zl-l is a good measure of NaSO, adduct concentration. This is valid for the measurements depicted in Fig. 3(b) which were considered in the kinetic evaluations.

Fig. 4. Characteristic experimental and simulated EPR spectra of radical products observed at various ratios between DMPO and PTS ([DMPOlO/[PTS]“) in irradiated aqueous solutions: (a) [DMPO]“/[pTs]o 0.1; simulation parameters (mT): @I) oN=1.56, 0,=2.38; (b2) aN=1.51, aH=l.51; (b3) a,=1.45, +,=I.635

trap an adduct with parameters 2xa,=0.07 mT, 2xa,=1.125 mT, a,=1.425 mT and g-2.0067 was found. These parameters are characteristic of the RCH,‘-DBNBS adduct, where R is an EPR silent substituent. In our system, RCH,’ could be represented by GH,CH;, which is formed by the isomerization of the primary photolyticp-CH,C,H, radical as described in ref. 3, or by p-NaSO,C,H,CH,; which is formed from PTS by He abstraction due to the radicals produced during photolysis. A more complex spectrum is shown in Fig. 3(a). Its analysis is given in Fig. 4(b). In addition to NaSO,’ and the carbon-centred ‘CR3, ‘OH radical added to DMPO is observed. Unfortunately, the lines of the DMPO-OH’ adduct interfere almost exactly with those of NaSO,‘-DMPO. Consequently, Z,_1 cannot be used to characterize NaS03’-DMPO adduct formation. A precise separation of Z,- i into ‘OH and NaSOS’ contributions is difficult. An approximate analysis of the spectrum in Fig. 4(b), at [DMPO]‘=0.2 M, shows that this spectrum consists of about 60% NaSO,’ and 40% ‘OH adduct. At lower initial DMPO concentrations the proportion of the ‘OH adduct decreases to approximately 30% if [DMPO]‘= 0.1 M and to

3.3 0.01-1.0 M PTS, 0.01 M DMPO solutions The dependence of the intensity of the first line (I,_,) of the EPR spectrum (insets in Fig. 5) on the irradiation time at various initial PTS concentrations is shown in Fig. 5. Over the whole concentration range of PTS ([PTS]O=O.Ol-1 M) the line intensity Z,-, increases with the initial PTS concentration. The spectrum from the inset in Fig. 5(a) is simulated in Fig. 4(a). The contribution of other adducts (e.g. DMPO-OH) is negligible here (less than 5%) and these experiments were used for kinetic analysis. In Fig. 5(b), with [PTS]“=O.l-O.O1 M, the adduct concentration is considerably lower and the gain of the spectrometer is increased by a factor of five in comparison with Fig. 5(a). The relative contribution of the ‘OH adduct is again evident and it increases at low initial PTS concentration ([PTS]“=O.O1 M) to about 15%. Again, an exact separation into NaSO; and ‘OH contributions is not possible, and these experiments were not considered in the i

Fig. 5. Time dependence of the line intensity, I,_,, of the spectral line I- 1 in 0.01 M DMPO at various initial PTS concentrations [PTSP: (a) [PTSP=O.t-1 M; (b) [PTS]D=O.O1~.l M. Insets show characteristic EPR spectra for these _concentration ratios.

‘v. Brezovb et al. / Photochemical decomposition of p-toluenesulphonate

kinetic evaluations. Relatively high ‘DMPO-OH adduct concentrations are observed if the ratio [DMPO]“/[PTS]o>O.l (Figs. 3(a) and 5(b)). This can be explained by the considerably longer lifetime of NaSO,’ (5 x lo-” s) when compared with hydroxyl radicals (much less than 10e5 s). 3.4. Kinetic evaluations The kinetics of radical formation and decay, followed by EPR, were fitted by the Marquardt minimization procedure of the least-squares method to exponential functions and their parameters were evaluated. The increase in It-, during irradiation was described by the saturation function JY1 =C,* exp( -k*hf)+1l*-ST

(I)

where F-l is the intensity of the first line in the spectrum of the NaS03’-DMPO adduct, k” is the rate constant of NaSO,‘-DMPO adduct formation during irradiation and I;“_“: is the stationary intensity of JL1. The fitting procedure for the ideal saturation curve leads to results in which I;rsF = - c* The de&y of the NaSO,‘-DMPO adduct after switching off the irradiation was described by the formal first-order kinetic equation Z,_,=C,

exp-kt)+C,

(2)

where II-, is the intensity of the first line in the spectrum of the NaS03’-DMPO adduct, C1 and C, are the components of the intensity of the first line in the spectrum of the NaSO,‘-DMPO adduct immediately after switching off the irradiation and k is the rate constant of the NaSO;-DMPO adduct decay after switching off the irradiation. The parameters of the fitting procedure for the relevant systems are summarized in Tables 1 and 2. The EPR spin trapping technique tinfirms the formation of carbon-, hydroxyland NaSOiDMPO adducts in the irradiated aqueous PTS solution. The irradiation of PTS ([PTS]“= 1 M) was also carried out in a photochemical immersion reactor at a temperature of 25 “C in an argon atmosphere. In accordance with ref. 2, the evolution of sulphur dioxide was observed in the irradiated systems. The formation of hydroxyl radicals via the decomposition of SO,* radicals during the photolysis of p-toluenesulphonic acid in acidic media was proposed by Ogata et al. [2] and is also confirmed in our experiments as shown in Fig. 4(b). Based on the results obtained the following reaction scheme is proposed

233

TABLE 1. Parameters’ of the fitting procedure for systems with constant initial concentration of PTS ([PTS]‘=l M) and with variable initial concentrations of DMPO k’ (s-‘)

[DMPO]O (mm01 dm-“)

c:

10.0 7.5 5.0 2.5 1.0

- 2205 f 0.23 - 1559 f 0.32 -836*0.01 -305fl.11 -189f1.67

2366 f: 0.23 1749f0.24 1014 f 0.01 355.9fO.l 194.4*0.11

o.567x10-z*o.73 0.692x lo-‘f0.94 0.830x lo-‘f 0.02 0.564x10-‘zt1.65 0.143 f 2.46

[DMPO]’

C,

C,

k (s-l)

761 k 0.51 407 * 0.74 177 * 0.64 9.7k2.16 -2.0k10.7

0.351 x lo-**0.53 0.439 x lo-‘*0.55 0.750x lo-“*oso 0.337x10-‘*0&l 0.448x10-‘i-l.22

(mmol

dmm3) 10.0 7.5 5.0 2.5 1.0 ‘Standard

1.540f0.21 1336kO.19 846 f 0.18 368 f 0.39 216*0.81 deviations

in per cent. Top, formation:

bottom, decay.

TABLE 2. Parameters” of the fitting procedure for systems with constant initial concentration of DMPO ([DMPO]“=O.Ol M) and with variable initial concentrations of PTS

VV

k* (s-l)

c:

(mol

dme3) 1.0 0.75 0.5 0.375 0.25 0.10

- 168OrtO.22 - 1454 f0.36 - 1154f0.62 -985kO.35 -852~kO.33 -414*0.59

1788 f 0.25 1597f0.38 1314f0.64 1031* 0.37 877 kO.35 427kO.93

0.322x10-*kO.52 0.311x10-‘+0.82 0.331 x lo-2f1.49 0.245X10-2~0.67 0.232x10-*~0.60 0.193x10-**1.48

[p=l”

G

G

k (s-l)

(mol dm-‘) 1.0 0.75 0.5 0.375 0.25 0.10 ‘Standard

1113kO.26 878 f 0.48 64X*0.64 370* 1.09 309* 1.47 92G.36 deviations

344 kO.89 409 * 1.07 426 k-O.96 361 k1.18 289&1&l 173*2.97

in per cent. Top, formation;

PTS+hv+

K+‘SO,Na

R’+PTs2-

products

DMPO + K -%

+‘SO,Na

‘DMPO-R

‘SO,Na + DMPO A ‘S03Na 2

0.201 x 10-Z* 0.43 0.190x10-2*0.73 0.167x10-‘kO.98 0.168x10~**1.68 0.161~10~*+2.23 0.152x10-‘k7.97

SO* +‘OH

‘DMPO-S03Na

bottom, decay.

234

K Brewvd et al. / Photochemical decomposition of p-toluenesulphonate

DMPO +‘OH 2

b)

‘DMPO-OH

‘DMPO-S03Na

+ ‘R -!--+ diamagnetic

‘DMPO-SO,Na

+ SO, --%

diamagnetic

products products

where C$ is the rate of photoabsorption by PTS and 4 is the corresponding quantum yield. The concentration of radical adducts is very low, and so in the kinetic analysis some simplifications may be proposed: (1) rDMPOSO,Na] GZ[DMPO]‘; (2) [DMPOSO,Na] -C [PTS]‘; (3) k5
-_ . I-\ J

m

i

,,‘.

9,’

.

,_4’

l

;,’

IO.02

[‘RI =

(3)

44 + W”M-RI k,[DMPO] + ks

= k7[‘DMPOS03Na]ST[‘R]

+k8[‘DMPO-S03Na]ST[S02]

(5)

the stationary concentration of the and NaSO,‘-DMPO adduct, [NaSO,‘-DMPOIST, is expressed in the following form [NaSO;-DMPOIST=

2 [DMPO]+ 7

n

I

=

0

“O t

1.,5<-1

,,,’

.

(4)

Equation (4) may be simplified assuming k,[DMPO] z+ ks. Under steady state conditions, the rates of NaSO,‘-DMPO adduct formation and decomposition are equal. During irradiation, the reactive short-lived ‘R and NaSO,’ radicals are generated in the system and it can be assumed that the NaSO,‘-DMPO adduct is decomposed predominantly by carbon radical addition, which results in the formation of diamagnetic products [9]. The decomposition of the NaSO,‘-DMPO adduct by reaction with SO, during irradiation may be neglected [Z]. For steady state conditions of NaSO,‘-DMPO adduct formation and decay, it may be written kJSO,Na][DMPO]

i

,,,@

44 k,[PTS] + k,[DMPO]

[5 O,Na] =

_

<

,’

F

7

[PTS]

(6)

The stationary concentration of the NaSOiDMPO adduct is actually represented by the values of I;“-“T extracted from EPR experiments. Figures 6(a) and 6(b) indicate a linear depenof dence of I:_“: on the initial concentrations DMPO and PTS in agreement with eqn. (6). After switching off the irradiation, the decomposition of the NaSO,‘-DMPO adduct may be

[PTS]O.

M

Fig. 6. (a) Linear dependence of the steady state line intensity, @T, of the spectral line I- 1 on the initial DMPO concentration (Rz=0.99).@) Lineardependenceof thesteadystatelineintensity, r;l_“:, of the spectral line l-1 on the initial PTS concentration (R’=O.97). (c) The dependence of the NaSO;-DMPO adduct decay rate constant (&and l//c) on the initial DMPO concentration. (d) The dependence of the NaSOj-DMPO adduct decay rate constant k on the initial PTS concentration.

successfully described by formal first-order kinetics and the rate constants of decay have been calculated (Tables 1 and 2). The concentration of photogenerated reactive NaSO,’ radicals rapidly decreases to zero (Fig. 1). However, the decomposition of the NaSO,‘-DMPO adduct is a more complex reaction. The rate constants obtained for NaSO,‘-DMPO adduct decay (k) exhibit a dependence on the initial PTS and DMPO concentrations (Figs. 6(c) and 6(d)). A slow formation of sulphur dioxide from NaSOa radicals was confirmed in the system, but we were unable to determine SO, quantitatively in the EPR cell, due to its very low concentration. NaSO,‘-DMPO adduct formation and NaSOJ’ decomposition are in competition. Consequently, the concentration of sulphur dioxide in the system may be approximated by the following equation

PO1 =k where

PSI

(7)

,DMPOl

k, is the experimenta!

constant.

K Brezovd et al. I Photochemical decomposition of p-toluenesulphonate

The formal first-order kinetic equation for the decay of the NaSO,‘-DMPO adduct may be written in the form - d[‘DMPz*03Na1

=kJDMPO-SO,Na][SO,]

(8) The first-order rate constant k (calculated from the experimental data) may be obtained as k =k,k,

Pm [DMPO]

(91

In agreement with eqn. (9), a linear dependence was found for k vs. [PTS]’ in Fig. 6(d) and for l/k vs. [DMPO]’ in Fig. 6(c).

235

References 1 R. Frank and W. Klbpffer, Ecotoxicol. Environ. Safety, 17 (1989) 323. Y. Ogata, K Takagi and S. Yamada, Bull. Chem Sot. 3p?~, 2 50 (1977) 2205. 3 M. Nakamura and Y. Ogata, Bull. Chem. Sac. Jpn., 50 (1977) 2396. J. C. Evans, S. K. Jackson, C. C. Rowlands and M. D. Barratt, Tetrahedron, 41 (1985) 5195. 5 J. F. Rabek, Eqwimental Methoa3 in Photochemistry and Photophysics, Wiley, New York, 1985, p. 945. 6 J. M. Arends, H. Cerfontain, I. S. Herschberg, A. J. Prinsen and C. M. Wanders, Anal. Chem., 36 (1964) 1803. 7 Y. Kirini, T. Ohkuma and T. Kwan, Chem. Pharm BuR, 29 (1981) 29. 8 S. W. Li Anson and C. F. Chignell, Res. Gem. Intermed., 14 (1990) 235. 9 E. G. Janzen, P. H. Krygsman, D. A. Lindsay and D. L. Haire, J. Am. Chem. Sot., 112 (1990) 8279. 4