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Kinetic and spectral characteristics of transients formed in the pulse radiolysis of phenylthiourea in aqueous solution
Radiar. Phys. Chem. Vol. 43, No. 4, pp. 365-369, Printed in Great Britain. All rights resewed
0969-806X/94 $6.00 + 0.00 Copyright 0 1994 Pergamon Press Ltd
1994
KINETIC AND SPECTRAL CHARACTERISTICS OF TRANSIENTS FORMED IN THE PULSE RADIOLYSIS OF PHENYLTHIOUREA IN AQUEOUS SOLUTION G. R. DEY, D. B. NAIK, K. KISHORR and P. N. MOORTHY? Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-400085, India (Received 22 July 1992; accepted 2 December
1992)
Abstract-The reactions of primary species such as e4, H-atom and OH radicals as well as some one electron reductants and oxidants with phenylthiourea have been studied at various pHs. The kinetic, spectral, acid-base and redox properties of the transients have been determined using the pulse radiolysis technique.
INTRODUCTION Phenylthiourea (PTU) is a well known corrosion inhibitor (Ayres, 1970). It is used in reactor decon-
tamination formulations to minimise corrosive attack on the structural materials used in the nuclear reactors. During the process of decontamination the compound will be subjected to high radiation fields. Hence, it is necessary to study its reactions with the radiolytic species produced in irradiated water. We have studied the reactions of e,, H, OH and reducing and oxidising species such as (CH,),COH, COOH, Br; , T12+, SO,-, Cl; with PTU at different pHs using the pulse radiolysis technique. The detailed spectral, kinetic, acid-base and redox properties of the transient species produced by these reactions are reported in this communication.
EXPERIMENTAL Phenylthiourea (Koch-light) was purified by repeated crystallization from water. All other chemicals used were AnalaR grade reagents. Water from “Barnstead Nanopure System” having conductivity
~0.1 PS cm-’ was used for preparing all the solutions. Gases employed to saturate the solutions, viz. N,O, N,, O2 were of Iolar/Instrument grade from Indian Oxygen Ltd. The pH of the solutions was adjusted by using phosphate and borate buffers or plain H,SO, and NaOH solutions of suitable concentrations for different ranges of pH. Details of pulse radiolysis set up have been reported earlier (Guha et al., 1987). A 7 MeV linear electron accelerator (Ray Technology, England) giving 50 ns single pulses (dose z 15 Gy/pulse) was used for irradiating the solutions. Aerated decimolar KSCN solutions were used for determining the absorbed dose using a value 21520 dm’mol-’ cm-’ per 100 eV for GE at 500 nm tAuthor to whom all correspondence should be addressed.
(Fielden, 1982). Spectral and kinetic data acquisition and analysis were carried out with an IBM-PC-XT computer. RESULTS AND
DISCUSSION
Phenylthiourea was found to react readily with the primary species of water radiolysis viz. e,, H-atom and OH radicals and also with some one electron reductants and oxidants. Reaction
of e&
The reaction of e, with PTU was studied at pHs 4.8 and 9 in N, purged 1O-3 mol dm-3 PTU solutions containing 1 mol dm-3 t-butanol. The transient spectra were identical at both the pHs and the one at pH 9 is shown in Fig. 1. It shows one absorption band with A,, at 345 nm. The rate constant for the reaction of e, with PTU at pH 4.8 and 9, determined both by following the decay of e, absorption at 720 nm and the formation of the transient absorption at 345 nm were equal and is given in Table 1. This value is comparable to that obtained for the e, reaction with thiourea (Ramnani et al., 1991). Increase in concentration of PTU did not have any effect on the yield or spectrum of the transient indicating the absence of any interaction between the radical and the parent molecule. The transient species decayed by second order kinetics. Kinetic salt effect experiments indicated this species to be a singly charged species. As PTU is neutral at this pH, electron addition should give rise to a negatively charged species. The spectral and kinetic properties of the transients formed by the reaction of e@;with PTU are given in Table 1. This electron adduct was found to be a strong reductant as shown by its reactions with various solutes such as thionine, safranine-T, methyl viologen and anthraquinone-2-sulphonate, producing the well known semi-reduced species of these compounds. The 365
G. R. DEY et al.
366
0.01
-
\
l
-•-.
Wavelength
(nm)
Fig. 1. Transient spectrum obtained by e& reaction with F’TU at pH 9 immediately after the electron pulse in N, purged 10m3mol dm-’ FTU solution containing 1 mol dm-’ f-butanol.
rate constants for these reactions as determined by following the formation of the semi-reduced species of these solutes are given in Table 2. Isopropyl ketyl and CO; radicals could not reduce PTU at pHs 4.8 and 9 indicating that the one electron reduction potential of PTU in this pH range is more negative than - 1.9 V vs NHE. The electron adduct of PTU has LX at a longer wavelength viz. 345 nm as compared to 280 nm in the case of thiourea (Ramnani et al., 1991). This may be due to the delocalization of the electron over a bigger molecule (due to the presence of benzene ring in PTU). The electron adduct is essentially a carbon centered radical formed by the addition of e, to the carbon-sulphur double bond. Reaction of H-atom
Reaction of H-atom with PTU was studied at pH -0 and at very high acid concentration viz. 3.6 mol drn-) H,SOI, using N, purged 10e3 mol drnm3 PTU containing 1 moldmm3 f-butanol. Figure 2(a) and (b) shows the transient absorption spectra obtained under these conditions. The spectrum at pH 0 has a maximum at 330 nm whereas that in 3.6 mol drne3 acid shows two maxima at 330 and 410 nm. The 330 nm band can be assigned to H-atom addition to the benzene ring as cyclohexadienyl radicals are known to absorb in this wavelength region (Sauer and Ward, 1967). The absorption at 410nm can be attributed to the addition of H-atom to the carbonsulphur double bond. This was substantiated by studying the reaction of H-atoms with thiourea in 3.6 mol drns3 acid solutions where only the 410 nm absorption band was observed but not the 330 nm Table
1.Spectral
Reducing species
e.4 H
(CH,),COH COOH ‘0.5 mol dm-’
band. The rate constant for the reaction of H-atom with PTU in 3.6moldm-’ acid solutions was much lower than at pH 0, possibly due to protonation of phenylthiourea. Electron density on the PTU molecule would be reduced on protonation and hence addition of H-atom may be more difficult leading to lowering of the rate constant for its reaction. The rate constants for the formation of the transient were different at 330 and 410 nm indicating the formation of two different species. The spectral and kinetic properties of the transient formed are listed in Table 1. Reactions of one electron reducing species
Reducing species such as COOH and (CH,),COH radicals could react with PTU at high acid concentrations only, viz. 3.6 mol dm-’ H,SO, where PTU may be present in the protonated form. The absorption spectra of the transients formed by the reaction of these radicals with PTU at this low pH were identical with A,,,,, at 410 nm and the spectrum obtained by reaction with isopropyl ketyl radical is shown in Fig. 2(c). The yield of the transient increased with increasing acid concentration and became constant beyond 3.6mol drne3 H,SO,. Similar experiments with thiourea also led to the formation of a transient having an absorption spectrum very similar to that observed with PTU. This indicates that the reaction site is the same in both cases i.e. the C==S bond. An absorption band in the 410 nm region was also observed in the case of H-atom reaction with PTU at high acid concentrations [see Fig. 2(b)]. Both the H-adduct and the species formed by the reaction
and kinetic parameters for the transient species formed by the reaction of one electron reducing radicals with PTU PH
I nl”” (nm)
9.0 -0’ cot
345 330 330 410 410 410
H,SO,;
73.6 mol dm-’
(dm’mo;-’ 6300 7200 6600 H,SO,.
cm-‘)
krOrmYLlon ( x 10e9 dm’ mol-’ s-‘)
2k/cl (s_‘)
4.2 12.0 0.5 0.1 0.5 1.1
6.0 x IO’ 4.2 x IO’ 1.2 x IO5 1.4 v IO’ -
Characteristics of transients formed from phenylthiourea Table 2. Rate constants for the reactions of electron adduct of F’TU with various solutes at pH 7 Rate constant (x 10-9dm’mol-‘s-‘)
Solute Methyl viologen Safranine-T Thionine Anthraauinone-2-sulohonate
6.1 2.0 4.5 4.6
System: N, purged 10-j mol dm-’ FTU in 1 mol dm-’ r-butanol + solute. Solute concentration l-3 x IO-‘mol dm-‘. Formation of solute transient followed at its A,,,.
of COOH and (CH,),COH radical with PTU decayed by second order kinetics with comparable 2k/d values indicating the species to be identical. It is possible that COOH and (CH,)$OH radicals react with PTU by electron transfer but the resulting species gets protonated quickly to give a species which is identical to the H-adduct. The transient species produced by the reaction of these radicals with PTU was found to be non-reducing in nature and could not reduce even thionine whose reduction potential is very low viz. 0.05 V vs NHE (Guha et al., 1987).
Reactions of OH radical
Figure 3 shows the transient absorption spectra obtained by the reaction of OH radicals with PTU (5 x 10e4 mol dmm3) at pH 9 and -0 in pulse irradiated N,O saturated and 0, saturated solutions respectively. It was found that the PTU was inert towards HO, radicals. But the transient species formed by OH reaction at pH 0 was found to react with O2 with a rate constant of N 10’ dm3 mol-’ s-‘. However this rate is too slow to affect the initial yield
367
of the transient species. The spectrum shows two absorption bands with A,,,, at 315 and 580 nm at pH 9.0. At pH -0, instead of two absorption bands, there is only one band with L,,, at 410 nm. This indicated the presence of a pK,, of the transient species. The plot of A0.D. at 420 nm vs pH for 0, saturated 10e3 mol drnd3 PTU solutions is shown in the inset of Fig. 3, and from this, the pK, of the transient species was evaluated to be 1.6. Rate constants for the reaction of OH radicals with PTU at pHs 0 and 9.0 were determined by following the build up of the transient absorption at the respective A,,,,, and the values are given in Table 3. The value at pH 9 is comparable to that reported earlier for thiourea (Ramnani et al., 1991). The transient produced by OH reaction with PTU at pH -0 shows an absorption band in the same region as the H-adduct and the species produced by the reaction of COOH or (CH,),COH radicals with PTU in 3.6 mol drn-) acid solutions (viz. the 410 nm band) but it is difficult to say whether the species are identical or not. The 315 nm absorption band at pH 9 can be attributed to hydroxycyclohexadienyl radical formed by addition to the benzene ring (Dorfman et al., 1962). But the 580nm band cannot be due to a C-centered radical such as C,H,NHC(SOH)NH2 radical. The 580 nm band was also formed by the reaction of specific one electron oxidants with PTU. Studies on the kinetic salt effect in both the cases indicated that the species was neutral. It is proposed that an intramolecular 3-electron bond is formed between sulphur and nitrogen after the addition of OH radical to the C=S double bond or H abstraction from -NH,. Such sulphur-nitrogen 3-electron bonds
0.07
0.06
0.05
0.04 d 2
0.03
0.02
0.01
0 240
300
400
500
Wavelength
600
700
(nm)
Fig. 2. Transient spectra obtained by H-atom reaction with F’TU (Nr purged 10m3mol dm-’ solutions containing 1 mot dm-’ t-butanol): (a) at pH 0 (A); (b) in 3.6 mol dm-3 H,SO, solutions (a); and (c) by reaction of isopropyl ketyl radicals with FTU (N2 purged IO-’ mol dm-3 solutions containing 1 mol drnd3 isopropanol) in 3.6 mol dm-’ H,SO, solutions (0).
G.
368
0
0.008
-
‘r n 6
0.006
-
0.004
-
0.002
-
DEY er al.
R.
a
0 ’ 240
300
400
600
500
Wavelength
700
(nm)
Fig. 3. Nonnalised transient spectra obtained by: OH radical reaction with 10-j mol dm-’ PTU solutions, N,O saturated, at pH 9 (0) and 0, saturated at pH 0 (A); and Br; reaction with IO-’ mol dm-’ PTU solutions containing 0.1 mol dme3 KBr, N,O saturated at pH 9 (0) and 0, saturated at pH 0 (A). Inset: plot of A0.D. vs pH for Bri reaction (0) and for OH reaction (0) at 420 nm. [PTU] = IO-’ mol dm-).
have been reported (Asmus, 1990).
earlier in case of methionine
CSHsNHCSNH, + OH-CJHS(OH)NHSNH,
C,,,NHL’NH
and
+ H 20
The 410 nm band obtained at lower pH may be due to the cation radical which is facilitated at 0 pH by the presence of H+ ions. Similar band is observed in the case of oxidation by Br; radicals as shown in the following paragraph. The absence of 330nm band viz. the band due to hydroxycyclohexadienyl radical at lower pHs indicates that at this pH, this pathway is not preferred. Even at pH 9 only about 33% of the OH radicals react with the benzene ring as the O.D. at 580 nm in the case OH radical reaction is only about 2/3 of the O.D. obtained in the case of Br; radical reaction (see Fig. 3). The spectral and kinetic properties of the semi-oxidised species are given in Table 3. Reactions of other oxidizing species
The reactions of various oxidizing radicals with PTU were studied in the pH range O-9, using 5 x 10m4mol drne3 PTU solutions containing appropriate concentrations of the solutes giving the oxidant
radicals and appropriate ambient i.e. N,O, O2 or Nz. It was found that while Br;, SO,, T12+ and Cl, could oxidise PTU, N, could not do so. The transient spectrum produced by the reaction of Br; with PTU at pH 9 is shown in Fig. 3 and it can be seen that the 580 nm band is similar to that obtained in the case of OH radical reaction with PTU. Similar spectra were obtained by the reactions of SO; with PTU at pH 9, T12+ with PTU at pH 3.5 and Cl, with PTU at pH 2. It is possible that the cation radical formed undergoes very fast deprotonation to give a species similar to that obtained in the case of OH radical reaction with PTU. At pH 0, the reaction of Br, with PTU was studied in 0, saturated solutions. The spectrum obtained (Fig. 3) was identical to that obtained by the OH radical reaction with PTU at this pH indicating the formation of similar species i.e. cation radicals. The pK, values of these species are also very close to each other (pK, = 1.6 in the case of species produced by OH radical reaction and 2.0 in the case of species produced by Br; reaction with PTU). The decay of the transient species followed first order kinetics at pH 0 and second order kinetics at pH 9 as in the case of OH radical reaction product. All these results indicate that at pH 0, the species produced by OH and Br, reaction with PTU are identical. Rate constants for the reactions of various oxidants with
Table 3. Spectral and kinetic parameters of the transient species formed by the reaction oxidising radicals with PTU Species
PH
Arn”l (nm)
OH
-0 9.0
Br;
-0 9.0
410 315 580 410 580
;3 Cl;
9.0 3.5 2.0
580 580
*Decay
first order
(dm’mo;-’ 7200 4100 I060 7000 I700 -
cm-‘)
of one electron
klarmallon (xlO~Pdm’mol~‘s ‘) 2.6 3.8 2.5 2.5 4.3 3.2 4.0
2kitl (s ‘) 1.8 4.4 1.0 2.2 1.0
x x x x x
IO** 10s 106 104’ 106
Characteristics of transients formed from phenylthiourea
PTU and the spectral and the kinetic properties of the transient species are given in Table 3. The semi-oxidised species produced from PTU was rather inert and could not oxidise even ascorbate ions. CONCLUSION
Phenylthiourea shows high reactivity towards epi, H-atom, OH and other oxidising radicals. OH radical reaction with PTU leads to the formation of two species by OH addition to the benzene ring and the carbon-sulphur double bond respectively. Only in highly acidic solutions, isopropyl ketyl and COOH radicals could react with PTU and the resulting species were found to be non-reducing in nature. However, the electron adduct formed in the pH range 4.8-9 is a strong one electron reductant. REFERENCES
Asmus K. D. (1990) Sulfur-centered free radicals. Mel/r. Enzym. 186, 168.
369
Ayres J. A. (1970) Corrosion. In Decontamination of Nuclear Reactors and Equipment (Edited by Ayres J. A.) p. 177. Ronald Press, New York. Dorfman L. M., Taub I. A. and Buehler R. E. (1962) Pulse radiolysis studies. I. Transient spectra and reaction rate constants in irradiated aqueous solutions of benxene. J. Chem. Phys. 36, 3051. Fielden E. M. (1982) Chemical dosimetry of pulsed electron and X-ray sources in the I-20 MeV range. In The Study of Fast Processes and TranstM Species by Electron Pulse Radiolysis (Edited by Baxendale J. H. and Busi F.) p. 59. Riedel, Dordrecht. Guha S. N., Moorthy P. N., Kishore K., Naik D. B. and Rao K. N. (1987) One electron reduction of thionine studied by pulse radiolysis. Proc. Indkm Acad. Sci., Chem. Sci. 99, 26 I. Ramnani S. P., Dhanya S. and Bhattacharya P. K. (1991) Pulse radiolysis of thiourea in aqueous solutions. Int. Symp. on Radiochemistry and Radiation Chemistry, Plutonium 50 years, Bombay. Sauer M. C. Jr and Ward B. (1967) The reactions of hydrogen atoms with benzene and toluene studied by pulse radiolysis: Reaction rate constants and transient spectra in gas phase and aqueous solution. J. Phys. Gem. 71, 3971.
0969-806X/94 $6.00 + 0.00 Copyright 0 1994 Pergamon Press Ltd
1994
KINETIC AND SPECTRAL CHARACTERISTICS OF TRANSIENTS FORMED IN THE PULSE RADIOLYSIS OF PHENYLTHIOUREA IN AQUEOUS SOLUTION G. R. DEY, D. B. NAIK, K. KISHORR and P. N. MOORTHY? Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-400085, India (Received 22 July 1992; accepted 2 December
1992)
Abstract-The reactions of primary species such as e4, H-atom and OH radicals as well as some one electron reductants and oxidants with phenylthiourea have been studied at various pHs. The kinetic, spectral, acid-base and redox properties of the transients have been determined using the pulse radiolysis technique.
INTRODUCTION Phenylthiourea (PTU) is a well known corrosion inhibitor (Ayres, 1970). It is used in reactor decon-
tamination formulations to minimise corrosive attack on the structural materials used in the nuclear reactors. During the process of decontamination the compound will be subjected to high radiation fields. Hence, it is necessary to study its reactions with the radiolytic species produced in irradiated water. We have studied the reactions of e,, H, OH and reducing and oxidising species such as (CH,),COH, COOH, Br; , T12+, SO,-, Cl; with PTU at different pHs using the pulse radiolysis technique. The detailed spectral, kinetic, acid-base and redox properties of the transient species produced by these reactions are reported in this communication.
EXPERIMENTAL Phenylthiourea (Koch-light) was purified by repeated crystallization from water. All other chemicals used were AnalaR grade reagents. Water from “Barnstead Nanopure System” having conductivity
~0.1 PS cm-’ was used for preparing all the solutions. Gases employed to saturate the solutions, viz. N,O, N,, O2 were of Iolar/Instrument grade from Indian Oxygen Ltd. The pH of the solutions was adjusted by using phosphate and borate buffers or plain H,SO, and NaOH solutions of suitable concentrations for different ranges of pH. Details of pulse radiolysis set up have been reported earlier (Guha et al., 1987). A 7 MeV linear electron accelerator (Ray Technology, England) giving 50 ns single pulses (dose z 15 Gy/pulse) was used for irradiating the solutions. Aerated decimolar KSCN solutions were used for determining the absorbed dose using a value 21520 dm’mol-’ cm-’ per 100 eV for GE at 500 nm tAuthor to whom all correspondence should be addressed.
(Fielden, 1982). Spectral and kinetic data acquisition and analysis were carried out with an IBM-PC-XT computer. RESULTS AND
DISCUSSION
Phenylthiourea was found to react readily with the primary species of water radiolysis viz. e,, H-atom and OH radicals and also with some one electron reductants and oxidants. Reaction
of e&
The reaction of e, with PTU was studied at pHs 4.8 and 9 in N, purged 1O-3 mol dm-3 PTU solutions containing 1 mol dm-3 t-butanol. The transient spectra were identical at both the pHs and the one at pH 9 is shown in Fig. 1. It shows one absorption band with A,, at 345 nm. The rate constant for the reaction of e, with PTU at pH 4.8 and 9, determined both by following the decay of e, absorption at 720 nm and the formation of the transient absorption at 345 nm were equal and is given in Table 1. This value is comparable to that obtained for the e, reaction with thiourea (Ramnani et al., 1991). Increase in concentration of PTU did not have any effect on the yield or spectrum of the transient indicating the absence of any interaction between the radical and the parent molecule. The transient species decayed by second order kinetics. Kinetic salt effect experiments indicated this species to be a singly charged species. As PTU is neutral at this pH, electron addition should give rise to a negatively charged species. The spectral and kinetic properties of the transients formed by the reaction of e@;with PTU are given in Table 1. This electron adduct was found to be a strong reductant as shown by its reactions with various solutes such as thionine, safranine-T, methyl viologen and anthraquinone-2-sulphonate, producing the well known semi-reduced species of these compounds. The 365
G. R. DEY et al.
366
0.01
-
\
l
-•-.
Wavelength
(nm)
Fig. 1. Transient spectrum obtained by e& reaction with F’TU at pH 9 immediately after the electron pulse in N, purged 10m3mol dm-’ FTU solution containing 1 mol dm-’ f-butanol.
rate constants for these reactions as determined by following the formation of the semi-reduced species of these solutes are given in Table 2. Isopropyl ketyl and CO; radicals could not reduce PTU at pHs 4.8 and 9 indicating that the one electron reduction potential of PTU in this pH range is more negative than - 1.9 V vs NHE. The electron adduct of PTU has LX at a longer wavelength viz. 345 nm as compared to 280 nm in the case of thiourea (Ramnani et al., 1991). This may be due to the delocalization of the electron over a bigger molecule (due to the presence of benzene ring in PTU). The electron adduct is essentially a carbon centered radical formed by the addition of e, to the carbon-sulphur double bond. Reaction of H-atom
Reaction of H-atom with PTU was studied at pH -0 and at very high acid concentration viz. 3.6 mol drn-) H,SOI, using N, purged 10e3 mol drnm3 PTU containing 1 moldmm3 f-butanol. Figure 2(a) and (b) shows the transient absorption spectra obtained under these conditions. The spectrum at pH 0 has a maximum at 330 nm whereas that in 3.6 mol drne3 acid shows two maxima at 330 and 410 nm. The 330 nm band can be assigned to H-atom addition to the benzene ring as cyclohexadienyl radicals are known to absorb in this wavelength region (Sauer and Ward, 1967). The absorption at 410nm can be attributed to the addition of H-atom to the carbonsulphur double bond. This was substantiated by studying the reaction of H-atoms with thiourea in 3.6 mol drns3 acid solutions where only the 410 nm absorption band was observed but not the 330 nm Table
1.Spectral
Reducing species
e.4 H
(CH,),COH COOH ‘0.5 mol dm-’
band. The rate constant for the reaction of H-atom with PTU in 3.6moldm-’ acid solutions was much lower than at pH 0, possibly due to protonation of phenylthiourea. Electron density on the PTU molecule would be reduced on protonation and hence addition of H-atom may be more difficult leading to lowering of the rate constant for its reaction. The rate constants for the formation of the transient were different at 330 and 410 nm indicating the formation of two different species. The spectral and kinetic properties of the transient formed are listed in Table 1. Reactions of one electron reducing species
Reducing species such as COOH and (CH,),COH radicals could react with PTU at high acid concentrations only, viz. 3.6 mol dm-’ H,SO, where PTU may be present in the protonated form. The absorption spectra of the transients formed by the reaction of these radicals with PTU at this low pH were identical with A,,,,, at 410 nm and the spectrum obtained by reaction with isopropyl ketyl radical is shown in Fig. 2(c). The yield of the transient increased with increasing acid concentration and became constant beyond 3.6mol drne3 H,SO,. Similar experiments with thiourea also led to the formation of a transient having an absorption spectrum very similar to that observed with PTU. This indicates that the reaction site is the same in both cases i.e. the C==S bond. An absorption band in the 410 nm region was also observed in the case of H-atom reaction with PTU at high acid concentrations [see Fig. 2(b)]. Both the H-adduct and the species formed by the reaction
and kinetic parameters for the transient species formed by the reaction of one electron reducing radicals with PTU PH
I nl”” (nm)
9.0 -0’ cot
345 330 330 410 410 410
H,SO,;
73.6 mol dm-’
(dm’mo;-’ 6300 7200 6600 H,SO,.
cm-‘)
krOrmYLlon ( x 10e9 dm’ mol-’ s-‘)
2k/cl (s_‘)
4.2 12.0 0.5 0.1 0.5 1.1
6.0 x IO’ 4.2 x IO’ 1.2 x IO5 1.4 v IO’ -
Characteristics of transients formed from phenylthiourea Table 2. Rate constants for the reactions of electron adduct of F’TU with various solutes at pH 7 Rate constant (x 10-9dm’mol-‘s-‘)
Solute Methyl viologen Safranine-T Thionine Anthraauinone-2-sulohonate
6.1 2.0 4.5 4.6
System: N, purged 10-j mol dm-’ FTU in 1 mol dm-’ r-butanol + solute. Solute concentration l-3 x IO-‘mol dm-‘. Formation of solute transient followed at its A,,,.
of COOH and (CH,),COH radical with PTU decayed by second order kinetics with comparable 2k/d values indicating the species to be identical. It is possible that COOH and (CH,)$OH radicals react with PTU by electron transfer but the resulting species gets protonated quickly to give a species which is identical to the H-adduct. The transient species produced by the reaction of these radicals with PTU was found to be non-reducing in nature and could not reduce even thionine whose reduction potential is very low viz. 0.05 V vs NHE (Guha et al., 1987).
Reactions of OH radical
Figure 3 shows the transient absorption spectra obtained by the reaction of OH radicals with PTU (5 x 10e4 mol dmm3) at pH 9 and -0 in pulse irradiated N,O saturated and 0, saturated solutions respectively. It was found that the PTU was inert towards HO, radicals. But the transient species formed by OH reaction at pH 0 was found to react with O2 with a rate constant of N 10’ dm3 mol-’ s-‘. However this rate is too slow to affect the initial yield
367
of the transient species. The spectrum shows two absorption bands with A,,,, at 315 and 580 nm at pH 9.0. At pH -0, instead of two absorption bands, there is only one band with L,,, at 410 nm. This indicated the presence of a pK,, of the transient species. The plot of A0.D. at 420 nm vs pH for 0, saturated 10e3 mol drnd3 PTU solutions is shown in the inset of Fig. 3, and from this, the pK, of the transient species was evaluated to be 1.6. Rate constants for the reaction of OH radicals with PTU at pHs 0 and 9.0 were determined by following the build up of the transient absorption at the respective A,,,,, and the values are given in Table 3. The value at pH 9 is comparable to that reported earlier for thiourea (Ramnani et al., 1991). The transient produced by OH reaction with PTU at pH -0 shows an absorption band in the same region as the H-adduct and the species produced by the reaction of COOH or (CH,),COH radicals with PTU in 3.6 mol drn-) acid solutions (viz. the 410 nm band) but it is difficult to say whether the species are identical or not. The 315 nm absorption band at pH 9 can be attributed to hydroxycyclohexadienyl radical formed by addition to the benzene ring (Dorfman et al., 1962). But the 580nm band cannot be due to a C-centered radical such as C,H,NHC(SOH)NH2 radical. The 580 nm band was also formed by the reaction of specific one electron oxidants with PTU. Studies on the kinetic salt effect in both the cases indicated that the species was neutral. It is proposed that an intramolecular 3-electron bond is formed between sulphur and nitrogen after the addition of OH radical to the C=S double bond or H abstraction from -NH,. Such sulphur-nitrogen 3-electron bonds
0.07
0.06
0.05
0.04 d 2
0.03
0.02
0.01
0 240
300
400
500
Wavelength
600
700
(nm)
Fig. 2. Transient spectra obtained by H-atom reaction with F’TU (Nr purged 10m3mol dm-’ solutions containing 1 mot dm-’ t-butanol): (a) at pH 0 (A); (b) in 3.6 mol dm-3 H,SO, solutions (a); and (c) by reaction of isopropyl ketyl radicals with FTU (N2 purged IO-’ mol dm-3 solutions containing 1 mol drnd3 isopropanol) in 3.6 mol dm-’ H,SO, solutions (0).
G.
368
0
0.008
-
‘r n 6
0.006
-
0.004
-
0.002
-
DEY er al.
R.
a
0 ’ 240
300
400
600
500
Wavelength
700
(nm)
Fig. 3. Nonnalised transient spectra obtained by: OH radical reaction with 10-j mol dm-’ PTU solutions, N,O saturated, at pH 9 (0) and 0, saturated at pH 0 (A); and Br; reaction with IO-’ mol dm-’ PTU solutions containing 0.1 mol dme3 KBr, N,O saturated at pH 9 (0) and 0, saturated at pH 0 (A). Inset: plot of A0.D. vs pH for Bri reaction (0) and for OH reaction (0) at 420 nm. [PTU] = IO-’ mol dm-).
have been reported (Asmus, 1990).
earlier in case of methionine
CSHsNHCSNH, + OH-CJHS(OH)NHSNH,
C,,,NHL’NH
and
+ H 20
The 410 nm band obtained at lower pH may be due to the cation radical which is facilitated at 0 pH by the presence of H+ ions. Similar band is observed in the case of oxidation by Br; radicals as shown in the following paragraph. The absence of 330nm band viz. the band due to hydroxycyclohexadienyl radical at lower pHs indicates that at this pH, this pathway is not preferred. Even at pH 9 only about 33% of the OH radicals react with the benzene ring as the O.D. at 580 nm in the case OH radical reaction is only about 2/3 of the O.D. obtained in the case of Br; radical reaction (see Fig. 3). The spectral and kinetic properties of the semi-oxidised species are given in Table 3. Reactions of other oxidizing species
The reactions of various oxidizing radicals with PTU were studied in the pH range O-9, using 5 x 10m4mol drne3 PTU solutions containing appropriate concentrations of the solutes giving the oxidant
radicals and appropriate ambient i.e. N,O, O2 or Nz. It was found that while Br;, SO,, T12+ and Cl, could oxidise PTU, N, could not do so. The transient spectrum produced by the reaction of Br; with PTU at pH 9 is shown in Fig. 3 and it can be seen that the 580 nm band is similar to that obtained in the case of OH radical reaction with PTU. Similar spectra were obtained by the reactions of SO; with PTU at pH 9, T12+ with PTU at pH 3.5 and Cl, with PTU at pH 2. It is possible that the cation radical formed undergoes very fast deprotonation to give a species similar to that obtained in the case of OH radical reaction with PTU. At pH 0, the reaction of Br, with PTU was studied in 0, saturated solutions. The spectrum obtained (Fig. 3) was identical to that obtained by the OH radical reaction with PTU at this pH indicating the formation of similar species i.e. cation radicals. The pK, values of these species are also very close to each other (pK, = 1.6 in the case of species produced by OH radical reaction and 2.0 in the case of species produced by Br; reaction with PTU). The decay of the transient species followed first order kinetics at pH 0 and second order kinetics at pH 9 as in the case of OH radical reaction product. All these results indicate that at pH 0, the species produced by OH and Br, reaction with PTU are identical. Rate constants for the reactions of various oxidants with
Table 3. Spectral and kinetic parameters of the transient species formed by the reaction oxidising radicals with PTU Species
PH
Arn”l (nm)
OH
-0 9.0
Br;
-0 9.0
410 315 580 410 580
;3 Cl;
9.0 3.5 2.0
580 580
*Decay
first order
(dm’mo;-’ 7200 4100 I060 7000 I700 -
cm-‘)
of one electron
klarmallon (xlO~Pdm’mol~‘s ‘) 2.6 3.8 2.5 2.5 4.3 3.2 4.0
2kitl (s ‘) 1.8 4.4 1.0 2.2 1.0
x x x x x
IO** 10s 106 104’ 106
Characteristics of transients formed from phenylthiourea
PTU and the spectral and the kinetic properties of the transient species are given in Table 3. The semi-oxidised species produced from PTU was rather inert and could not oxidise even ascorbate ions. CONCLUSION
Phenylthiourea shows high reactivity towards epi, H-atom, OH and other oxidising radicals. OH radical reaction with PTU leads to the formation of two species by OH addition to the benzene ring and the carbon-sulphur double bond respectively. Only in highly acidic solutions, isopropyl ketyl and COOH radicals could react with PTU and the resulting species were found to be non-reducing in nature. However, the electron adduct formed in the pH range 4.8-9 is a strong one electron reductant. REFERENCES
Asmus K. D. (1990) Sulfur-centered free radicals. Mel/r. Enzym. 186, 168.
369
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