Indian Journal of Chemistry Vol. 4 1A, July 2002, pp. 1385-1 390

Synthesis and characterization of a chelating resin containing ONO donor tridentate Schiff base and its coordination compounds with copper(II), nickel(II) , cobalt(II), iron(III), zinc(II), cadmium(II), manganese(II),

molybdenum(VI) , zirconium(IV) and uranium(VI)

A Syamal, D Kumar*, A K Singh, P K Gupta, Jaipal & L K Sharma Department of Chemistry , Regional Engineering College, Kurukshetra 136 11 9, Haryana, Indi a

Received 24 December 200 I .. revised I April 2002

New polystyrene supported coordination compounds have been sy nthesized by the reac tion of metal salt/metal coordination compound with the polys tyrene supported Schi ff base (PS-LH2) (obtained by the reaction of chloromethylated polystyrene, 3-formylsalicy li c ac id and ethanolamine) . The compounds have the fo rmulae : PS-LCu· DMF. PS-LNi ·3DM F, PS- LCo·3DMF, PS-LFeCI·2DMF, PS- LZn·DM F, PS-LCd·DMF, PS-LMn·3DMF, PS- LMo02·DM F, PS­LZr(OHh·2D MF and PS-LU02·DM F (DM F = dimethylformamide). The compounds have been characterized by e lemental analyses, IR , electronic, ES R spectral and magnetic susceptib il ity measurements . The Cu(II ), Ni (II ), Co(l l), Fe(lIl ) and Mn(lI) compounds are paramagnetic whi le the Zn(II ), Cd(II ), Zr(IV), Mo(V I) and U(V I) compounds are di amagneti c. The magnetic and ES R da ta indicate the magnetically dilute nature of the metal centres. The shi fts of the v(C=N)(azomethine), v(C- O)(phenolic) and v(C- O)(alcoholic) st retches have been mon itored to fi nd out the donor sites of PS-LH2' A square planar structure fo r PS- LCu·DM F; a tetrahedral structure fo r PS-LZn·DM F and PS- LCd· DM F; an octahedral structure for PS- LNi·3DMF, PS- LCo·3DMF, PS- LFeCI·2DM F, PS- LM n·3DMF, PS- LMo02·DMF and PS- LUOr DMF have been suggested for the compounds . PS- LZr(OH)2·2DMF is pe ntagonal bipyrami dal.

The reaction of polymer-anchored ligand with metal ions provides an easy route for the sy nthesis of immobili zed transition metal compounds. Several polymer-anchored ligands like porphyrins, polydentate amines, crown ethers, iminodiacetic ac id, acetylacetone, tripeptide and their metal absorbing property have been reported 1.2 . However, some of these studies are limited by leaching of the metal ions fro m the polymer and no concerted effort has been given to study the structural aspects of the metal bound to the chelating resins. A structural study of such metal bound polymers seems useful and in teresting in view of their numerous applications3

.s

such as in organic synthesis, immobilizati on of enzymes, biological systems, application of dyes, water treatment , chromatography, catalys is, as substrate carriers, protecting groups and metal absorbing agents. The Schi ff bases are the most versatile and thoroughly studied ligands in coord ination chemistry. On account of their pronounced coordinating properties, a number of Schiff bases have been anchored on polystyrene

. 6· 10 I h' h h d matn x . ntis paper, we report t e sy nt eses an

E-mail: dkumau·eck @yahoo.com

characteri zation of coordination compounds of the polystyrene supported Schiff base(PS-LH2)(l) with Cu(II), Ni (II), Co(lI), Fe(lII), Zn(II), Cd(II), Mn(II), Mo(VI), Zr(lV) and U(VI).

Materials and Methods Ch loromethy lated polystyrene(PS-CI)( contain ing

1.17 mmol CI per g of resin and 2% cross linked with di vinylbenzene) [Fluka A ' G (Switzerl and»); copper(IJ ) acetate monohydrate, pet. ether (60-80°C) [IDPL] ; nickel(I1) acetate tetrahydrate, zinc(II) acetate dihydrate, dioxouranium(VI) acetate dihydrate, ethanolamine[SD's fine chemicals]; absolute ethanol [Bengal Chemicals and Pharmaceuticals Works Ltd .]; manganese(ll) acetate tetrahydrate [Sarabhai]; cadmium(I1) acetate dihydrate, cobalt(lI) acetate tetrahydrate, iron(III) chloride(anhydrous) and [Zr4(OH)g(H20 )16]Clg[BDH); methanol, acetone, ethy l acetate, triethylamine, DMF, ammoni um molybdate(VI) tetrahydrate[Ranbaxy] were used for the syntheses. 3-Formylsalicy lic ac id, zirconiu m(IV) acetate and bis(acetylacetonato)dioxomolybde­nu m(VI) were sy nthesized by fo llowing the literature procedures7

. The so lvents were dried over molecular sieves.

1386 INDIAN J CHEM, SEC A, JUL Y 2002

The metal contents and the coordinated DMF in the polystyrene supported coordination compounds were analysed as per the reported methods9

. The magnetic susceptibilities were determined by the Gouy method using Hg[Co(NCS)4] as the cali brant. A double ended one side sealed tube with zero diamagnetic susceptibility was used for measuring the magnetic susceptibility. The diamagnetic correction of the metal-ligand system was calculated using the Pascal 's constant II and following a procedure specially designed for polystyrene-anchored compounds9

. The susceptibilities were corrected for temperature independent paramagnetism term usi ng the following values: Cu(ll) (60 x 1O-6cgs units), Ni(II) and Co(Il) (200 x 10-6cgs units), Fe(TII) (zero) and Mn(II) (zero). ESR spectrum was recorded at liquid nitrogen temperature in polycrystalline solids on a Varian V 4502-12 X-band ESR spectrometer with 100 KHz modulation using diphenylpicrylhydrazide as a g­marker and monitoring the frequency with a frequency meter. For the calculation of g values, a second order correction was used with the resonance line: 7A2/4H2•3 for Cu(II). Reflectance spectra were recorded on a Beckman DU spectrophotometer attached with a reflectance arrangement. Infrared spectra were recorded in KBr pellets in the range 400-4000 cm- I on a Nicolet FTIR spectrophotometer using polystyrene as a cali brant.

Synthesis of polystyrene 3 -formylsalicylate( PS-FSal) Chloromethylated polystyrene (PS-Cl) (l.0 g) was

swelled in DMF (20 ml) for 45 mjn . To this , a solution of 3-formylsalicyclic acid (0.78 g, 4.68 mmol) in DMF (20 ml) was added. Ethyl acetate (100 ml) and triethylamine (1.5 g, 15 mmol) were added to above mixture. The mixture was heated under reflux for 8 h while stirring magnetically. This was allowed to cool to room temperature. The pale-yellow coloured res in was suction filtered, washed with DMF, ethyl acetate, ethanol and acetone and was dried in vacuo at room temperature.

Synthesis of polystyrene-anchored Schiff base (PS­LH2) (I)

Polystyrene 3-formylsalicylate (PS-FSal)(l.O g) was swelled in DMF (50 ml) for 45 min. To this suspension , a DMF solution (50 ml) of ethanolamine(0.286 g, 4.68 mmol) and ethyl actate (100 ml) was added while stirring magnetically and the mixture was refluxed for 8 h. The mixture was cooled to room temperature. The yellow coloured

polystyrene-anchored ligand was suction filtered, washed with DMF, ethyl acetate, methanol and acetone and dried in vacuo at room temperature.

General method for the syntheses of PS-LM ·xDM F [where M = Cu(II), Ni(ll), Mn(I/), Zn(ll), Cd(ll), U02(1I) and x = l or 3}

The polystyrene-anchored Schi ff base (PS-LH2) (0 .5 g) was suspended in DMF (20 ml) for 45 min. To this suspension, a DMF solution (30-50 ml) of appropriate metal acetate (1.17 mmol) was added . The mixture was refluxed on a heating mantle for 8 h while stirring magnetically. This was allowed to cool to room temperature. The product was suct ion filtered, washed with DMF, ethanol, methanol and acetone and was dried in vacuo at room temperature.

Synthesis of PS-LMo02'DMF

The polystyrene-anchored Schiff base (0.5 g) was swelled in DMF (20 ml) for 45 min . To this suspension, a DMF solution (40 ml ) of bis (acetylacetonato) dioxomolybdenum(V I)(0.38 g, 1.17 mmol) was added. The mixture was heated under reflux for 7 h while stirring magnetically. The product was allowed to cool to room temperature, filtered and washed with DMF, ethanol and acetone. The compound was dried in vacuo at room temperature.

Synthesis of PS-LFeCI·2DMF

To a swelled suspension of the polystyrene­anchored Schiff base (0.5 g) in DMF (20 ml ) for 45 min, a DMF solution (50 ml) of iron(III) chloride (anhydrous) (1.19 g, l.l7 mmol) was added. The mixture was heated under reflux for 6 h wh ile stirring magnetically and the condenser was fitted with CaCb guard tube. The product was cooled to room temperature, filtered, washed with DMF, ethanol and pet. ether and dried in vacuo at room temperature.

Synthesis of PS-LCo·3DMF

The polystyrene-anchored ligand (0.5 g) was swelled in DMF (20 ml) for 45 min and nitrogen gas was passed through the mixture. To th is suspension, a hot DMF solution (40 ml) of coba!t(II) acetate tetrahydrate (0.291 g, 1.17 mmo!), fl ushed with N2,

was added and the mixture was allowed to cool to room temperature. The brown coloured product formed was suct ion filtered, washed thoroughly with de-aerated (by passing nitrogen) DMF, ethanol and acetone and dried in vacuo at room temperature.

SY AMAL et al.: CHELATING RESIN CONTAINING SCHIFF BASE COMPLEXES 1387

Synthesis of PS-LZr(OHh·2DMF The polystyrene-anchored Schiff base (0.5 g) was

swelled in DMF (20 ml) for 45 min . To thi s suspension, a freshly prepared DMF solution (40 ml ) of zirconium(IY ) acetate( 1.17 mmol) was added . The mixture was heated under reflux for 7 h while stirring magnetically. The product was cooled to room temperature, suction filtered and washed with DMF, ethanol, acetone and pet. ether. The resin was dried in vacuo at room temperature.

Results and Discussion The synthesis of polystyrene-anchored Schiff base

(I) derived from 3-formylsalicylic acid and ethanolamine was carried out in two-steps. In first step, PS-CI and 3-formylsalicy lic ac id reacts together to form polystyrene 3-formylsalicy late (PS-FSal ). In second step, PS-FSal reacts with ethanolamine in I : 4 mol ar ratio to form PS-LH2 (1) (Scheme I).

The reaction of PS-CI and 3-formylsalicylic acid was carried out in 1 : 4 molar ratio and this leads to 100% conversion as is evident from the fact that PS­FSal does not contain any chl orine. The reaction of PS-CI and 3-formylsal icy li c ac id in the ratio 1 : < 4, always resulted in the product containing chlorine. The chloromethylated polystyrene is white in colour while PS-LH2 is yellow in colour. Thi s colour of PS­LH? remains unchanged even after repeated washing with DMF, ethyl acetate, ethanol and acetone. The reaction of PS- LH2 wi th metal sa lt/metal coordinati on compound in 1 : 2 ligand: metal molar ratio in DMF gives polystyrene-anchored compounds. The forma tion of the compounds may be represented by the fo ll ow ing equations:

PS-C1

? 1I0~OIl DMF. ethyl acc tntc

+ ~CI-IO Irict hy laminc

° II

+ HCI

PS - OL OH

~CI-IO PS- FSal

DMF, ethyl acetate PS- FSal + NH2-(CH2)2-0H • I + H20

Scheme I-Preparative method of polystyrene supported Schiff base (I)

DMF PS-LH2 + M(CH3COOh -----7

renu x

PS-LM·DMF + 2CH3COOH

[M = Cu(II) , Zn(II), Cd(II), U02(II )]

PS-LM·3DMF + 2CH3COOH

[M = Ni(IJ), Co(ll), Mn(Il)]

DMF PS-LH2 + Mo02(acach -----7 PS-LMo02·DMF

renux

+ 2acacH

DMF PS-LH2 + FeCI3 -----7 PS- LFeCI·2DMF + 2HCI

renux

DMF 4PS-LH2 + [Zr4(OH)g(H20 )1 6](CH3COO)g -----7

renu x

4PS-LZr(OHh·2DMF + 16H20 + 8CH3COOH

PS-LH2 is yellow in colour, as the reaction of PS­LH? with metal salt/metal coordination compound pro~eeds, the colour of PS-LH2 changes fro m ye llow to cream, yellowish-brown, orange, brown and green . The colour of compounds does not change even after repeated washing with DMF, methanol, ethanol and pet. ether. PS-LH2 and its compounds are insoluble in water and common organic solvents. The percent reaction conversion of the compounds is in 'lhe range 52.3-93.7 (Table I ). The metal binding capac ity of PS- LH2 is 44.3-74.0 X 10-2 mmol of metal per g of resin (Table 1). There is no apparent correlation between percent reaction conversion and size of the metal ions. The coordinated DMF was lost completel y upon heating the compounds in air.

The v(C=O)(carboxylic) stretch In 3-formylsalicylic ac id occurs at 1660 cm- I

. The appearance of a new band at 1730 cm- I due to

v(C=O)(ester) in PS-FSal confirms the covalent bond formation via ester lin kage between 3-fo rmylsali cy li c ac id and PS-CI7. The v(C=N)(azomethine), v(C­

O)(phenolic) and v(C-O)(alcoholic) stretches occur at 1625 , 1520 and 1210 cm- I respectively in the Schi ff base deri ved from sa licylaldehyde and ethanolamine '2. These bands occur at 1630, 15 15 and 1225 cm- I respec ti ve ly in PS-LH2. In the polystyrene­anchored compounds, the v(C=N)(azomethine) undergoes a negative shi ft by 5-30 cm- I

, v(C­O)(pheno lic) undergoes a positi ve shift by ~ 10 cm- I

and v(C- O)(alcoholic) undergoes a negative shift by 5- 30 cm- I

. These shifts in IR frequencies after coord ination with metal ions indicate the ONO donor tridentate behaviour '2 of PS- LH2' The IR data rule out

1388 INDIAN J CHEM, SEC A, JULY 2002

the presence of a dimetallic structure and indicate a monometallic structure as in the case of a dimetallic structure, the v(C-O)(phenolic) stretch is expected to undergo a positive shift 13 by > 10 cm-I

. DMP shows a band at 1680 cm- I due to the v(C=O) stretch8. This band shifts to lower energy by 5-30 cm- I in the compounds indicating oxygen coordination of DMP8. PS-LU02·DMP shows the Vasy (O=U=O) at 905 cm- I and this band occurs in the usual range (870-950 cm- I

) observed for the majority of trans-U02

compounds9.'4

. The force constant (Ju-o) and the U-O bond length in the present dioxouranium(VI) compound are 6.80 mdyn/A and l.74 A respectively. These values are in the expected range (6.58-7.03 mdyn/A and l.60-l.92 A) reported for the majority of U02(II) compounds9

. '5 . PS-LMo02·DMP exhibits the

vsy(O=Mo=O) and vasy(O=Mo=O) stretches at 945 and 905 cm- I and these occur in the usual range(892-964 cm- I

; 840-925 cm- I) reported for the majority of

Mo02(II) compounds 16. The presence of

vsy(O=Mo=O) and vasy(O=Mo=O) bands indicates a cis-Mo02 structure as the compounds with trans­Mo02 structure exhibit only the vasy(O=Mo=O) since the vsy(O=Mo=O) is IR inactive '6. PS­LZr(OHh·2DMP exhibits a band at 1118 cm- I due to the o(Zr- OH)I7.

The room temperature magnetic susceptibilities and magnetic moments of the polystyrene-anchored compounds are presented in Table 2. The polystyrene­anchored copper(II), nickel(II), cobalt(II), iron(III) and manganese(II) compounds exhibit magnetic moments of l.86, 3.05, 4.80, 5.87 and 5.92 B.M.

Table I--Characterization data of polystyrene-anchored compounds"

Compound Colour M

PS- LCu·DMF Dark-brown 4.7 (4.65)

PS-LNi·3DMF Green 3.8 (3.82)

PS- LCo·3DMF Brown 3.1 (3.09)

PS-LFeCI ·2DMF Dark-brown 4. 1 (4.14)

PS- LZn·DMF Cream 2.9 (2.93)

PS- LCd·DMF Cream 6.9 (6.97)

PS- LMn·3DMF Dark-brown 3.3 (3.32)

PS- LMo02·DMF Cream 6.9 (6.93)

PS-LZr(OH)2·2DMF Yellowish-brown 6.5 (6.50)

PS-LU02·DMF Orange 10.6 (10.65)

"Abbreviations: PS-LH2 = I, DMF = dimethylformamide.

Found (Calc.) (%) CI DMF

5.3 (5.34)

13.9 ( 14.2)

115 (11.48)

2.6 (2.63) 10.7 (10.8 1)

3.2 (3.28)

4.4 (4.53)

13.1 (13.23)

5.1 (5.27)

10.3 (10.4)

3.2 (3.26)

Metal binding capacit/ (X,10-2

) (mmol/g of resi n)

74.0

64.7

52.6

73.4

44.3

61.4

60.0

71.9

71.2

44.5

Conversionc

(%)

85 .2

84.6

68.5

93.7

52.3

75 .3

78.6

88.7

92.6

61.3

bCalculated from the observed value of metal ion percentage in the resin using the formula [M% (observed) x 10]/(atomic weight of metal). cPercentage reaction conversion (p) = (observed metal ion percentage x 100)/(calculated metal ion percentage on the basis of 100% reaction conversion of polymer-anchored ligand to polymer-anchored compound).

Table 2-Magnetic moment and electronic spectral data of polystyrene-anchored compounds

Compound Mia X~rr Magnetic Moment' Temperature V ITI:u. D. B' (em" ) 13 13°%

(10-6 egs unit s) (B.M.) (K ) (em" ) (em- ') ( 10";; egs units)

PS- LCu·DMF - 852 1442 1.86 300.5 17700

PS-LNi·3DMF - 943 3895 3.05 297.5 8700 870 581 0.55 45 15400 21300

PS- LCo·3DMF - 1131 965 1 4.80 300.0 8300 935 754 0.776 22.4 12500 17860

PS- LFeCJ.2DMF - 846 1451 8 5.87 297.0 12720 19600 25000

PS- LMn·3 DMF - 1005 14834 5.92 295 .0 18440 22980 25220

:I corr In. ~rr. = 2.83 (XM x n B.M.

SY AMAL et al .: CHELATING RESIN CONTAINING SCHIFF BASE COMPLEXES 1389

respectively. These values are indicative of the magnetically dilute nature of the compounds ' 8. The presence of polymer backbone prevents the M-M interaction in polystyrene-anchored compounds and this leads to a magnetically dilute environment around the metal ions. ESR data (v ide supra) also support the magnetically dilute nature of the compounds. The zinc(II), cadrnium(II), molybdenum(VI), zirconium(IV) and uranium(VI) compounds are diamagnetic as expected for d'0, tf, and f system. Thus, the data suggest a tetrahedral structure for zi nc(II) and cadmjum(II) compounds; an octahedral structure for molybdenum(VI) and uranium(VI) compounds and a pentagonal bipyramidal structure for zirconium(IV) compound.

Nujol mull electronic spectra of the compounds could not be recorded as the compounds do not form a good mull and hence the reflectance spectra of the compounds were recorded . The compounds being insoluble in common solvents, the solution electronic spectra also could not be recorded. The copper(II) compound exhibits a band at 17700 cm- I

characteristic of square planar CuN03 coordination sphere ' 9. The absence of a band at 8000-10000 cm- I

rules out the presence of tetrahedral structure20. The polystyrene-anchored nickel(II) compound exhibits three bands at 8700, 15400 and 21300 cm- I due to the spin allowed tranSItions: 3 A 2g--j3T2g(VI) ,

3 A2g--j3Tl g(F)(V2) and 3 A2g--j3TI g(P)(V3), respec ti ve ly,

in an octahedral sy mmetr/ I. The V2/VI value for the nickel(II) compound is 1.77 and this lies in the usual range (1.6- 1.82) reported for the majority of octahedral nickel(II) co mpounds21

• The e lectronic spectral parameters22 of the present polystyrene­anchored nickel (II) compound are as follows : Dq =

870 cm- I, 8' = 58 1 cm- I, ~ = 0.55 and ~o = 45%. The reduction of Racah parameter from the free ion va lue of 1056 cm- I to 581 cm- I and ~o value of 45 % are indicative of the strong covalen t nature of the compound . The polystyrene-anchored cobalt(II) compound shows three bands at 8300, 12500 and 17860 cm- I due to 4Tl g(F)--j4T2g(VI), 4Tl g(F)--j4A 2g(V2)

and 4T Ig( F)--j 4TI g(P)(V3) transitions , respectively, in an octahedral symmetry21. The e lectronic spectral parameters22 of the present cobalt(U) compound are as fo ll ows: Dq = 935 cm- I

, 8' = 754 cm- I, ~ = 0.776 and ~o = 22.4%. The lower value of 8' in comparison to the free ion value of 97 I cm- I and ~o value of 22.4% suggest the covalent nature of the cobalt( II) compound. The polystyrene-bound iron(lll)

compound exhibits three bands at 12720, 19600 and 25000 cm- I due to 6A Ig--j4Tlg(G), 6AI g--j4T2g(G) and

6Al g--j4AIg(G) tranSitIOns, respectively, in an octahedral symmetr/ I. The polystyrene-anchored manganese(II) compound exhibits three bands at 18440, 22980 and 25220 cm- I due to 6Alg--j4Tlg(G) ,

6Alg--j4T2g(G) and 6A lg--j4AIg(G) transitions, respectively , in an octahedral environmental21 . It is of interest to note that although both Mn(II) and Fe(III) have 6 A Ig ground state, all the bands occur in Fe(lII) compound at lower energy than those of Mn(II) compound. Thi s is due to the lower value of Racah parameters (8 and C) in the iron(III) compound in comparison to those of manganese(II) compound21.

The presence of diamagnetic large pol ymer backbone keeps the metal centres in the polystyrene­anchored compounds considerably separated, which avoid dipolar broadening. As a result, reasonably good ESR spectrum was observed in polycrystalline solids in the absence of a host diamagnetic coordination compound diluent. The ESR spectrum of polystyrene-anchored copper(II) compound exhibits

two g values(gll = 2.25, g.L = 2.08) and this indicates the presence of tetragonal type symmetry about the Cu(lI) ion8

. The ESR parameters8,23 of the present

Cu(lI) compound are as follows: All = 154 x 10-4

cm- I, AJ. = 33 x 10--4 cm- I, G = 3.18, CX~II = 0.75, (a')2

= 0.33, K = 0.51 and Pd = 1.55 x 1O-2cm- ' . The ESR

data indicate that gil> g.L and All > AJ. which are indicative of the presence of the unpaired electron in the d?_/ orbital24 . For ionic environments, gil is

normally ~ 2.3 and is < 2.3 for covalent environments. The gil value in our copper(II ) co mpound indicates that the metal-ligand bonding in the compound is covalent. For tetragonal copper(ll) compounds, if G is less than 4.0, the ligand formin g the copper(II) compound is regarded as a strong fi eld ligand . In our polystyrene-anchored copper(II) compound, the G value of 3. 18 indicates the strong field nature of the polystyrene-anchored ligand. The

a~lI value of 0.75 and (a')2 of 0.33 for our copper(l l)

compound indicates its covalent nature. The smaller the va lue of a 2, the more covalent is the bonding; a 2 =

1 indicates completely ioni c bonding, whil e r:/ = 0.5 indicates complete covalent bonding24. The larger the va lue of the (a')2, the more covalent is the bonding; (a')2 = 0 suggests a complete ionic bonding24. The

values of K and Pel for our copper(U) compound are 0.51 and 1.55 x 10-2 cm- I respectively. The positi ve

1390 lNDlAN J CHEM, SEC A, JULY 2002

value of K suggests23 that All should be greater than A.l

and thi s trend in All and A.l values was also observed by us. The lower value of P" in the copper(ll) compound in comparison to that of the free ion value of 3.5 x 10-2 cm- I indica tes the presence of covalent character between the meta l-li gand bonding. The ESR

spectrum of PS- LCu·DMF does not show any band

- 1500 G due to the ~Ms = 2 transition and this precl udes the presence of M- M interaction. From the chlorine content of the starting chloromethylated po lystyrene, a rough calculation indicates that the metal atoms are placed on pheny l rings(of polystyrene) which are 6 to 7 styrene units apart when the percent conversion is 100% and the styrene units are more than seven when the percent conversion is < 100%. This results in a magnetically dilute environment around the metal ato m as the pathway for the dimer formation with M-M interaction is blocked . However, since the polymer is cross- linked with polymer chains overlapped and twi sted , some of the reac ti ve groups may come c lose to one another resulting in some M-M interaction which was not detectable by ESR stud ies.

Acknowledgement The authors are thankful to the Mini stry of Human

Resource Development (Govt. of Indi a) and the Faculty Research Fund, Regional Engineering College, Kurukshetra (Haryana), for financial support of thi s work under the thru st area programme.

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