r..~ ? .;, , , im

E L S E V I E R

6 December 1996

Chemical Physics Letters 263 (1996) 263-270

CHEMICAL PHYSICS LETTERS

Hydroxyl radical induced reactions with 1-bromo-2-fluorobenzene in aqueous solutions: formation of

radical cations

H a r i Mohan, J a i P . M i t t a l l

Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India

Received 3 June 1996; in final form 30 September 1996

Abstract

Hydroxyl radicals are observed to react with 1-bromo-2-fluorobenzene to form C6H4BrF-OH and (C6H4BrF) +" in neutral and acidic solutions respectively. The nature and reactivity of the solute radical cation, (C6H4BrF) +', formed under different acidic conditions are discussed.

I . Introduct ion

Hydroxyl radicals ( O H ) are known to react with benzene and substituted derivatives by addition reac- tions to form a hydroxycyclohexadienyl radical which absorbs in the 310-330 nm region [1-7]. In the presence of electron donating substituents (CH 3, OCH3), the additional absorption bands observed at lower pH are assigned to a solute radical cation [8-11]. Strong one-electron oxidants such as SO~-', Ti 2+, Ag 2+ have also been shown to form the solute radical cations [10-15]. The oxidation potential for methoxy substituted benzenes have been determined to be in the range 1.3-1.6 V [10]. In the presence of electron withdrawing substituents (halogens), our re- cent studies have shown that the O H radicals can also react with C6H5X (X = I, Br, C1, F) by an

I AIso affiliated as Honorary Professor with the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India.

electron transfer reaction mechanism in highly acidic solutions [16-20]. S O ; has also been observed to react with C 6 H s X and the oxidation potential is determined to be in the range 2.2-2.4 V [16-20]. The transient optical absorption bands observed in the reaction of "OH radicals with halogenated ben- zenes in the visible region are assigned to the solute radical cation (C6HsX) +'. Although the visible ab- sorption bands observed with these halogenated derivatives are at different positions (I = 630 nm, B r = 550 nm, C1--475 nm, F = 3 8 5 , 400 nm), whether the cationic centre is at the halogen or over the benzene ring remains unanswered. The halogen atom is highly electronegative and the unpaired elec- tron would resonate with the aromatic ring forming a

radical cation. If it is considered to be over the benzene ring, the transition will be strongly influ- enced by the nature of the halogen to give transient optical absorption bands whose 1 vary from 630 nm (X = I) to 385 nm (X = F). In the present work, pulse radiolysis investigations on OH radical in-

0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH S0009-2614(96)01168-2

264 H. Mohan. J.P. Mittal/ Chemical Physics Letters 263 (1996) 263-270

duced reactions with 1-bromo-2-fluorobenzene (BFB) have been carried out to understand the nature and effect of halogen on the formation of the radical cation in a dihalogenated benzene and the possible site of radical cation in the solute molecule.

2. Experimental

l-Bromo-2-fluorobenzene (BFB, purity = 99%) obtained from M / S Aldrich Chemicals was used without further purification. The solutions were pre- pared in deionized 'nanopure' water and freshly prepared solutions were used in each experiment. High energy electron pulses (7 MeV, 50 ns, dose = 15 J kg- J per pulse) used in the present investiga- tions were generated from a linear electron accelera- tor [21-23]. The reaction of 'OH radicals, in neutral aqueous solutions was carried out in an N20 satu- rated solution to convert e~q to O H radicals with G(OH) = 5.6 (G denotes the number of species per 100 eV or micromolar concentration per 10 J of absorbed energy). In acidic solutions, the reaction of "OH radicals was studied in N2/O 2 saturated solu- tions. All other experimental details have been re- ported previously [16-20]. The ESR spectra at 77 K were recorded using a Bruker ESR 300 spectrometer operating at the X-band frequency (u = 9-10 GHz).

3. Results and discussion

3.1. Reaction of O H radicals at neutral pH

Fig. l a shows the transient optical absorption spectrum from the pulse radiolysis of an N20 satu- rated neutral aqueous solution of BFB (1.8 X 10 -3

mol dm-3), which exhibits a band with Amax = 315 nm. The band was not observed in the presence of t-butyl alcohol (0.3 mol dm-3), an efficient OH radical scavenger. This suggests that the band is due to the reaction of O H radicals with the solute. The absorbance of the band remained independent of solute concentration (0.45-3.6)x 10 -3 mol dm -3 and suggests that the entire yield of O H radicals has reacted with the solute. Taking the concentration of the O H radicals to be equal to the concentration of the transient species (315 nm), the extinction coeffi-

,<

0.03

0.0~

0.01

250 350 650

~,/nm

Fig. 1. Transient optical absorption spectrum obtained on the pulse radiolysis of BFB (1 .8× 10 -3 mol d m - 3 ) . (a) N 2 0 satu- rated, pH = 6; (b) N 2 saturated, pH = 1; (c) N2 saturated, pH = 1, t-butyl alcohol = 0.2 mol d m - 3 ; (d) aerated, pH = 1.

cient is thus determined to be equal to 3 X 103 dm 3 mol-~ cm -~. It was observed to decay by second order kinetics with 2k = 2.6 X 109 dm 3 mol- ~ s- 1. The rate constant for the reaction of OH radicals with BFB, determined by formation kinetic studies was equal to 4.1 X 10 9 dm 3 mol - j s -1. The tran- sient band is assigned to an OH-adduct (2, Scheme 1), in analogy with reported studies on a number of substituted benzene derivatives [1-20].

The rate constant for the reaction of 'OH radicals with BFB was also determined by competition kinet- ics using KSCN as the standard solute (ksc N + oH = 1.1X 10 l° dm 3 mol -~ s-2). In N20 saturated solutions of KSCN (5.2 X 10 - 4 mol dm -3) and for different concentrations of BFB (0-1.8 x 10 -3 mol dm-3), the absorbance at 472 nm decreased from 0.073 po 0.032. The bimolecular rate constant was determined to be 4.2 x 10 9 dm 3 mol- ~ s- ~, close to the value determined by formation kinetics studies. These studies suggest that the overall reaction of OH radicals with BFB is essentially by addition to the ring, forming the transient band with Area x = 315 nm. As a result of the competition for OH radicals, the absorbance at 315 nm increased from 0.0021 to 0.0166. This increase at 315 nm should be due to the transient species formed on reaction of O H radicals with BFB. Taking the extinction coefficient at 472

H. Mohan, J.P. Mittal / Chemical Physics Letters 263 (1996) 263-270 265

nm due to (SCN) 2 as 7580 dm 3 mol-1 cm-I the extinction coefficient at 315 nm was determined to be equal to 2.7 × 103 dm 3 mol -~ cm -~, close to the value determined by direct estimation (3 × 103 dm 3 mol- ~ cm- 1) [24].

The OH-adduct of halogenated organic com- pounds is known to undergo acid-catalyzed dehydra- tion to form solute radical cations. Therefore, the formation of solute radical cations of BFB has to be investigated in acidic solutions. In acidic solutions (pH < 3), e~q would not be converted to 'OH radicals on reaction with N20 (N20 + e~-q---~ N 2 + O H + OH-) instead react with H + to form H atoms (e~q+H + ~ H + H 2 0 ) with G ( H ) = 3.2 and G(OH) = 2.8 (at pH = 1). Fig. lb shows the tran- sient optical absorption spectrum from the pulse radiolysis of an N 2 saturated acidic (pH = 1) aque- ous solution of BFB (1.8 × 10 -3 dm 3 mol- 1 s- ~). This would be due to the reaction of H and O H radicals with BFB. The decay showed mixed kinet- ics. The formation rate constant was determined to be 2 × 10 9 dm 3 tool -t s -~. In order to study the reaction of 'OH radicals alone with BFB, in acidic solutions, H atoms should either be scavenged or the contribution of its reaction with BFB should be known separately. The pulse radiolysis of an N 2 saturated acidic (pH = 1) aqueous solution of BFB (1.8 X 10 -3 mol dm -3) containing t-butyl alcohol (0.2 mol dm-3), an efficient O H radical and weak

H atom scavenger, showed little absorption in the 290-350 nm region (Fig. lc). This absorption spec- trum should be due to the transient species formed on reaction of H atoms with BFB and t-butyl alco- hol radicals formed on H atom abstraction by O H radicals. This shows that the contribution of the H atom reaction with BFB is only in the 290-350 nm region and is small. The H atoms could be scav- enged by 02 ( H ' + 02 ~ HO 2) and HO 2 radicals are known to absorb at A < 280 nm [25]. Therefore, the transient optical absorption spectrum from the pulse radiolysis of an aerated aqueous solution of BFB (1.8 X 10 -3 mol dm -3, pH = 1) would mainly be due to the reaction of "OH radicals with BFB (Fig. l d). It also showed an absorption band with Area , = 315 nm, similar to that observed in an N20 saturated solution (Fig. l a). The lower absorbance at 315 nm would be due to the lower G('OH) at pH = 1 in an aerated solution as compared to the G ( O H ) in an N20 saturated solution at pH = 6. In the presence of t-butyl alcohol, little absorption was observed, which also supports the conclusion that the transient spec- trum (Fig. ld) is due to the reaction of O H radicals with BFB.

From these studies it appeared that O H radicals are unable to undergo acid-catalyzed dehydration of the OH-adduct at pH = 1. Therefore, the formation of a solute radical cation has to be investigated in highly acidic solutions.

(1) ( 2 )

B r B r 4- ~ F [ ~ F -p H~--H20

"P "OH ~ -'~ OH --H , '4- H20

(5) C6) +°

O r 8r HCI 0/, _ ~F -.-. "-,. ~F

>4mol dn~ 3

Br Br

<4 tool dr6 3

Scheme 1.

0) (4)

266 H. Mohan, J.P. Mittal / Chemical Physics Letters 263 (1996) 263-270

0.0/. 20 310 nm

~o / / 520 nm

0.03 ~ tc

<a 400 n m

0.02 00 5 3 10

0 i 250 350 450 550 650

A/ nm

Fig. 2. Transient optical absorption spectrum obtained on the pulse radiolysis of aerated BFB (1 .8X10 -3 mol dm-3) . (a) HC104 = 3.9 mol dm-3; (b) HCIO 4 = 7.8 mol dm -3 and (c) 7.8 mol dm-3 HC104 in the absence of BFB. The inset shows the variation of absorbance as a function of HCIO 4 concentration.

3.2. Reaction o f O H radicals in acidic solutions

(a) (HCIO 4 < 4 mol dm3): When the concentra- tion of HCIO 4 was > 1 mol dm -3, the pulse radioly- sis of an aerated aqueous solution of BFB (1.8 x 10 -3 mol dm -3) showed increasing absorption in the region 350-450 nm. Fig. 2a represents the tran- sient optical absorption spectrum from the pulse radiolysis of an aerated acidic (HCIO 4 = 3.9 mol dm -3) aqueous solution of BFB (1.8 x 10 -3 mol dm-3). Absorption bands with /~max = 310, 380 and 395 nm were observed. These bands could not be seen in the presence of t-butyl alcohol (0.2 mol dm-3). In highly acidic solutions, the entire radia- tion energy would not be absorbed by water alone. Radiolysis of HCIO 4 does not produce O H radicals [26,27]; G( 'OH) would decrease with increasing concentration of HCIO 4. In 3.9 mol dm -3 HCIO4, based on electron density variation between H 2 0 and HC104, G ( O H ) is expected to be --2.0. The CIO 4 and CIO 3 radicals formed on pulse radiolysis of HC1Q showed weak absorption in the region 330-340 nm [26-28]. Radiolysis of HCIO 4 ( > 10 mol dm -3) showed a transient absorption band with

/~max = 440 nm, which was assigned to HCIO~" [26,27]. Therefore, the absorption bands (Fig. 2a) are due to the reaction of O H radicals with BFB and not due to the radiolysis products of HCIO 4.

The absorption bands at 380 and 395 nm (Fig. 2a) were blue shifted as compared to those observed with fluorobenzene (385, 400 nm) [19]. The concen- tration of HC104 required for the formation of these bands was 1.9 mol dm -3, slightly higher than re- quired for fluorobenzene (1.5 mol dm- 3) [19] . Ex- cept for these differences, the nature of the absorp- tion spectrum in the 350-450 nm region was similar to that of (C6HsF) +. The blue shift and higher concentration of acid required for the formation of these bands could be due to the presence of Br, a highly electron affinic group, in the solute molecule [29].

The 395 nm band decayed by first order kinetics with k = 3.1 X 10 3 S- ], similar to that of (C6HsF) + [19]. The bimolecular rate constant for the reaction of O H radicals with BFB, as determined by forma- tion kinetics, was 4.4 x 108 dm s mol - l s -~. The nature of the absorption spectrum at A < 340 nm was different from that of (C6HsF) + and it decayed by mixed kinetics, suggesting the presence of more than one species. Taking the extinction coefficient of (C6H5F) + at 400 nm to be equal to 3.57 X 103 dm 3 mol -~ cm -~ [19], - -55% of O H radicals are ob- served to react with C 6 H a B r F to form the solute radical cation. The remaining fraction of O H radi- cals may still react with BFB to form the OH-adduct. This may explain the reason for the different nature and decay of the transient spectrum (A < 340 nm), observed on pulse radiolysis of C6H4BrF (Fig. 2a) compared to that of C6HsF [19].

(b) (HC104 > 4 mol dm3): When the concentra- tion of HC104 was more than 4 mol dm -3, the absorbance at 400 nm showed a slight decrease (Fig. 2). Simultaneously, a new absorption band appeared in the region 450-570 nm. Fig. 2b shows the tran- sient optical absorption spectrum from the pulse radiolysis of an aerated acidic (HC104 = 7.9 mol dm -3) aqueous solution of BFB (1.8 x 10 -3 mol dm-3). The absorption bands with Ama x 520 and 300 nm were observed. The absorption band with ~max =

520 nm is blue shifted as compared to that observed for C6HsBr ('~max = 550 rim) [16]. The concentra- tion of HCIO 4 (5 mol dm -3) required to observe this

H. Mohan, J.P. Mittal / Chemical Physics Letters 263 (1996) 263-270 267

band was also more than that required for the forma- tion of the radical cation of C6HsBr (3 mol dm) [16]. The requirement of a higher concentration of HC104 and blue shift may again be due to the presence of a highly electron affinic group (F) in the molecule [29].

The decay of the transient band at 520 nm (HC104 = 7.8 mol dm 3) was of first order (k = 2.9 X 105 s - l ) and the band at 300 nm showed mixed kinetics suggesting that the entire fraction of OH-adduct is not converted to the solute radical cation even at this high concentration (7.9 mol dm -3) of HC104. The inset of Fig. 2 shows the variation of the absorbance as a function of [HCIO4]. The yield and lifetime of the transient species (A = 520 nm) was observed to increase with HCIO 4 concentration approaching satu- ration when [HC104] was 9.8 mol dm -3.

3.3. Assignment of transient bands

The spectral changes observed on the pulse radi- olysis of aerated acidic solutions of BFB (Fig. 2) are due to the reaction of OH radicals with BFB in the presence of H ÷ as (1) the bands were not observed in the presence of t-butyl alcohol (0.2 mol dm- 3 ), an efficient OH radical scavenger. (2) The bands were also not observed at neutral pH. (3) Thepulse radioi- ysis of an aerated aqueous solution of an equimolar mixture of C6HsBr and C6HsF (1 X 10 -3 mol dm -3) in 3.9 and 7.8 mol dm -3 HC104 showed characteristic absorption bands, similar to those ob- served with individual C6HsF and C6HsBr without any blue shift. Even the concentration of HC104 required for the formation of the absorption bands in an equimolar mixture of fluorobenzene and bromo- benzene was equal to that required for the formation of a solute radical cation from individual solutes. This suggests that the transient species formed on the pulse radiolysis of BFB has a halogen atom, in addition to that present in C6HsF and C6HsBr. (4) The spectral changes were also not due to the radiol- ysis products of HC104 as pulse radiolysis of 7.9 mol dm -3 HC104 did not show such absorption bands (Fig. 2c). (5) The bands were also not due to the reaction of C104//C103 radicals with BFB as (a) the visible absorption bands observed in HCIO 4 with BFB (Fig. 2) were also observed in H2SO 4. The nature of the spectrum, decay and formation kinetics

of the transient bands on the pulse radiolysis of BFB in 4 and 8 mol dm -3 H2SO 4 were similar to those produced in 4 and 8 mol dm-3 HCIO 4 respectively. (b) Pulse radiolysis of a neutral aqueous solution of NaC104 (7.8 mol dm -3) containing BFB (1.8 × 10 -3 mol dm-3) did not show any absorption band in the 400-650 nm region. (6) Pulse radiolysis of an N 2 saturated solution of benzene (1.5 x 10- 3 mol dm- 3) even in 7.8 mol dm -3 HCIO 4 did not show any absorption at A > 350 nm suggesting that the spec- trum (Fig. 2) is due to a transient species involving the participation of a halogen atom. (7) Preliminary studies on the reaction of OH radicals with 1-bromo- 3-fluorobenzene and 1-bromo-4-fluorobenzene have also been carried out and similar results were ob- tained in low and high acid conditions. These studies suggest that the initial reaction of "OH radicals is at the halogen atom.

The bands are therefore assigned to the H+-cata - lyzed dehydration of the OH-adduct (Scheme 1). Since the nature of the spectrum, decay and forma- tion kinetics of the transient species were different in the two acidic conditions, the transient species formed under these conditions should also be of different nature. It is proposed that the removal of the electron may be taking place from the halogen atom and the halogen centered radical cation may then undergo resonance stabilization with the aromatic ring (Scheme 1). The requirement of higher acid concen- tration and shift in the position of the absorption bands as compared to those of (C6HsF) +" and (C6HsBr) ÷ could be explained as being due to the presence of another halogen atom.

The phenoxyl radicals, formed on halogen ion elimination from the OH-adduct absorb in the region of 390 nm [2,30]. The possibility of the formation of such a r a d i c a l (C 6 H 4 B r F + O H C 6 H 4 B r O ' / C 6 H 4 F O + H ÷ + F - / B r - ) was also ruled out as it should be formed even in neutral solutions. Our pulse radiolysis studies showed the absence of any transient absorption at A > 350 nm (Fig. 1).

ESR investigations were also carried out to find the difference in structure of the transient species formed on the pulse radiolysis of BFB in different acidic conditions. The ESR spectrum of ~-irradiated BFB (1 .8× 10 -3 mol dm -3, ~/ dose = 225 J kg -1) in 3.9 and 7.8 mol dm -3 HC104 were identical and

2 6 8 H. Mohan, J.P. Mittal / Chemical Physics Letters 263 (1996) 2 6 3 - 2 7 0

(D 03

b

0 0

t,o

0

a

(.9 ¢'2.

tD

Fig. 3. The ESR spectrum of ~/-irradiated (~,-dose = 255 J kg- ~ ) sample at 77 K (a) empty quartz tube and (b) 7.8 tool dm -3

HCIO 4.

matched the ESR spectrum of ~,-irradiated 7.8 mol dm-3 HCIO4 (Fig. 3). Due to the presence of a large ESR signal from ~/-irradiated HC10, any small dif- ference in the ESR signal of ~/-irradiated BFB in different acidic conditions could not be seen and the possible site of the radical cation could not be deter- mined by ESR.

4. Redox studies

The decay of C12, formed on the pulse radiolysis of an aerated aqueous solution of C1- (2 × 10 -2 mol dm -3, p H = I, h = 345 nm), remained unaf- fected on addition of BFB (1 X 10 -3 mol dm -3) suggesting that electron transfer from BFB to CI~-' is not taking place. Therefore, the redox potential for the (C6H4BrF) / (C6H4BrF) + couple is more than that of the C 1 2 / 2 C1- couple (2.09 V) [31].

The decay of SO 4", formed on the pulse radioly- sis of an N 2 saturated aqueous solution of $2082- (2 x 10 -2 mol dm -3, pH = 7, t-butyl alcohol = 0.2 mol dm-3, A = 460 nm), became faster on the addi- tion of a low concentration of BFB showing electron transfer from BFB to SO4". The bimolecular rate constant for this reaction was determined to be 1 × 108 dm 3 tool-J s - j . Time-resolved studies showed

the formation of a transient band with Area x = 310 nm. Since the position of this band did not match with that of the solute radical cation (Fig. 2), it could not be assigned t o (C6H4BrF) +'. These studies were carried out at pH = 7 and the solute radical cation is unstable at this pH. The reaction of SO 4" with aromatic compounds is known to yield OH-adduct, which absorbs at --~ 310 nm [12]. Therefore, the transient band formed on reaction of SO 4 with BFB is assigned to the OH-adduct, formed on hydrolysis of the solute radical cation [12].

The SO 4 is a strong one-electron oxidant with a redox potential value of 2.43 V [32]. From these studies, it may be concluded that the redox potential value for the (C6H4BrF) / (C6H4BrF) + couple may be between 2.09 and 2.43 V and it would be a strong one-electron oxidant.

Since two different types of transient species were formed under two different acidic conditions, the redox studies were carried out under these conditions to investigate the difference in the redox behavior of these two transient species (Scheme 1). The transient absorption band (~ 'max = 520 nm) formed on the pulse radiolysis of an aerated aqueous solution of BFB (4X 10 -3 mol dm -3, HCIO 4 = 7.8 mol dm -3) was observed to decay faster on the addition of low concentrations of Br - (0.4-2.0) X 10 -4 mol dm -3. The bimolecular rate constant for the reaction of (C6H4BrF) + with Br - was determined to be 4.3 × 109 dm 3 mol-~ s -1. Time-resolved studies showed the formation of a transient band with Area x = 360 nm. Since the Ama x of this transient species matched with that of Br 2 , it could be due to Br~-. When the concentration of HC104 was 3.9 mol dm -3, the decay of the 400 nm band (Fig. 2), was not affected on the addition of Br- . This suggests that the redox potential of this transient species is different from that formed when HC104 was 7.8 mol din-3. There- fore, the nature of the transient species formed under these two acidic conditions would also be different and confirm the earlier optical absorption results.

Similar studies have been carried out with other solutes (Table 1), and in each case the transient species formed when HC104 = 7.8 mol dm -3, was able to oxidize the solute with a high rate constant value (Table 1). However, for the transient species formed when HC104 was 3.9 mol dm -3, the rate constant value was low (Table 1). These studies also

H. Mohan, J.P. Mittal / Chemical Physics Letters 263 (1996) 263-270

Table 1 Bimolecular rate constant value for the reaction of (C6H4BrF) + with different solutes

269

Reaction HCIO 4 (mol dm- 3 ) Bimolecular rate constant (dm 3 mol- ~ s- * ) Possible oxidized species

(C6H4BrF)++ Br- 3.9 < 2 × 107 -- (C6H4BrF)+ '+ Br - 7.8 4.3 X 109 Br 2 (C6H4BrF)+'+ SCN- 3.9 < 2 × 107 -- (C6H4BrF)++ SCN- 7.8 2.5 × 109 (SCN)~ (C6H4BrF)++ CH3SSCH 3 3.9 < 2 × 107 - (C6HaBrF)+ + CH3SSCH 3 7.8 3.1 X 109 (CH3S)~ (C6H4BrF)++(CH3)2S 3.9 < 2 × 107 -- (C6H4BrF)++(CH3)2 S 7.8 3.2 × 109 [(CH3)25]~"

suggest that the nature o f the transient species fo rmed

under these condi t ions are different. The redox be-

hav ior o f these transient species fo rmed under low

(3.9) and high (7.8) acidic condi t ions were s imilar to

those o f C 6 H s F +" and C 6 H s B r + respect ive ly [ 1 6 - 20]. These studies also support the conc lus ion that

the ac id-ca ta lyzed dehydrat ion of the OH-adduc t o f

BFB takes place f rom the ha logen a tom and fo rms

the ha logen centered radical cation, which may

then resonance stabil ize with the aromat ic r ing

(Scheme 1).

5. Conclusions

In the neutral aqueous solut ion o f l -b romo-2 - f luo -

robenzene, the hydroxyl radicals react by addi t ion to

the benzene ring and form a hydroxycyc lohexad ieny l

radical. In acidic solutions, the OH-adduc t undergoes

acid-cata lyzed dehydrat ion to form the solute radical

cation. T w o different types of transient opt ical ab-

sorption spectra observed in low and high acidic

solutions are due to a ha logen centered radical cat ion,

in resonance with the benzene ring. The react iv i ty of

these two transient species was also di f ferent and

matched those o f ( C 6 H s F ) +" and ( C 6 H s B r ) +' re- spect ively.

Acknowledgements

Sincere thanks are due to Professor K.-D. Asmus ,

Director , Radiat ion Laboratory , Univers i ty o f Not re

Dame , U S A , for helpful discussions. The authors are

also thankful to Dr. M.D. Shastri , B A R C , for his

he lp in the E S R exper iments .

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