Indian Journal of ChemistryVol. 46A. April 2007. pp. 582-588

Spectral and thermal characterization of newly synthesized arylazo benzimidazolemercury(II) complexes

Sawsan S HaggagFaculty of Science, Chemistry Department, Alexandria University, P.O. Box 426, Alexandria 2J 321, Egypt

Email: smsh2006@hotmail.com

Mercury(ll) complexes of benzimidazole and its derivatives, (viz., benzimidazole (Benz.) (I); lJ-l-2-(naphthyl a-azo)benzimidazole (a-NaBenz.) (II); and I H-2-(arylazo)benzimidazole (III» of the type HaaBZR, where R=I-I (a), -C1-I3 (b),-NO! (c) and-OH (d) have been synthesized and characterized by elemental analysis, infrared. mass and 11-1NM R spectra.

The stoichiometric ratios for the Hg(lI) complexes have been identified as (1:1), (1:2) or (2: I) with the contribution ofcoordinated water and chloride in complex formation. The probable structures for the mercury(JI) complexes are alsoproposed. The results from mass spectral studies are in agreement with the TGA of the fragmentation and mass losses.

Mercury compounds can easily be absorbed byinhalation and through the skin causing dangerous healthproblems. The toxicity of mercury salts and inorganicmercury compounds or species is known as the majorreason for renal failure, sudden life-threatening,profound circulatory collapse with tachycardia,hypotension and peripheral vasoconstriction, vomiting,and bloody diarrhoeal. The brain is mainly the criticalorgan for chronic inorganic mercury poisoning2

.

Regardless of the form of mercury causing toxicity,chelation therapy should be started when mercurypoisoning is suspected in critically ill patients3

. Thesynthesis and study of mercury(JI) chelates is considereda major discipline in the impact processes of chelationtherapy. Investigations on usingferrocineliminophosphine as a novel chelating ligand forsynthesis and structural characterization of mercury(II)complexes~ are reported. Chelating triazine-3-thione wasused as a chelating ligand to synthesize mercury(II) andmethyl mercury complexes5

. A study was made to reportthe synthesis, structure and analytical appl ications ofmercury(II) complexes with thiazole derivative6

.

Mercury(II) complexes of the vitamin B 1 anatgonistoxythiamine were synthensized, characterized andreported? Hg(II)-complexes of the Schiff base derivedfrom 4-aminophenyl benzimidazole and 2,2'-dehydropyrollidene-N-aldehyde were synthesized andcharacterized by elemental analysis, conductance, IH_NMR and IR measurements8

. cis-3,7-Dimethyl-2,6-octadienthiosemicarbazone and thiosemicarbazide

ligands derived from reaction between benzofuran-2-carboxyhydrazide and phenylisothiocyanate (BCPT)/p-chloro-phenylisothiocyanate (BCCIPT) weresynthesized and their Hg(II) complexes identified andtested for their antimicrobial activity against bacterialspecies9

•IO

. The complexes of organomercury(II)chloride [RHgCl] with piperidine dithiocarbamate and2-aminopyridine dithiocarbamate were synthesizedcharacterized and their bacterial strains and fungalstrians activities were also evaluated II. Silica gel-functionalized-pyrimidine derivative was synthesizedand the metal complexation and interaction propertieswith mercury(II) and other transition metal ions wererecently reported 12. Mercury(II)-azido bridged polymericcomplexes was synthesized on the basis of directreaction of mercury(II) acetate with the l-alkyl-2-arylazo imidazoles and sodium azide in methanolsolution 13.

Benzimidazole (benz) (I) and related ligands are ofa considerable interest as strong chelating compoundstowards transition metal ions 14. Metal complexes ofthese ligands were extensively studied as models ofsome important biological moleculesl5

. We reporthere the synthesis of mercury(II) complexes withbenzimidazole arylazo derivatives (I, II, III) andcharacterization of the reaction complex products.

Materials and MethodsAll chemicals were of reagent grade and used

without further purification. Benzimidq,zole was

CCNI ')-N=No N &H -::?' R

~I

(a -NaBenz) (L ")

1 H-2(naphthyl a-azo) Benzimidazole

(L)

1H-2(arylazo)Benzimdazole

purchased from Aldrich. 1H-2-(naphthyl a-azo)Benzimidazole (II) and 1H-2-(arylazo)benzimidazoles (HaaBenz.R), where [R=H (IlIa), -CH3 (llIb), -N02 (I1Ic) and -OH (IIId)] weresynthesized as previously reported 16,17. The organicsolvents were purified and dried according to thecon ventional methods I R.

Elemental analyses (C, H, N and X) wereperformed by a Perkin-Elmer CHNS-lO elementalanalyzer at the Chemistry Department, Faculty ofScience, Cairo University. Infrared spectroscopicanalysis was done by uSll1g Perkin-Elmerspectrophotometer (model 1430; KBr disk,4000-200 cm'l) and the 70 eV electron impact-massspectra of mercury(II)-complexes were performed bya Finnigan Mt SSQ 7000 mass spectrometer at the

ational Research Center, Cairo, Egypt. IH NMRspectra of benzimidazole, a-hydroxy arylazo (benz.)and their mercury(II) complexes in cf-DMSO wererecorded on a Varian EM-390MHz NMRspectrometer. The chemical shift (8) was measureddown field from tetramethylsilane (TMS) as aninternal reference and the data are considered to beaccurate to ±0.02 ppm. The thermogravimetricanalysis (TGA) and thermoanalytical curves wereobtained using a Perkin-Elmer TGA 7 thermobalancewith the operating conditions: (20-1000°C); purenitrogen atmosphere with a flow rate of 20 mL min'l;lOoC min'l heating rate and sample mass is 5-12 mg.

Synthesis of mercUl'y (II) complexesTo an ethanolic solution of either benz. (I), 1H-2-

(naphthyla-azo) or benzimidazole (a-NaBenz) (II) or

R I H I CH 31 NO 2 OH

L I a I b I c d

1H-2-(arylazo) benzimidazole (III) [0.15, 0.3 and0.28-0.34 g, respectively 1.27 mmol] was addedHgClz [0.36, 1.33 mmoll in ethanol. The reactionmixture was stirred for 30 min. Hg(II) complexeswere precipitated immediately, filtered, washedseveral times with water and ethanol.

Analysis of complexing water molecules and metal contentHg(II) was determined complexometrically as

reported 19. Karl Fisher titration method was used todetermine the number of water molecules accordingto the following reaction:

h + 2H20 + S02 ---> 2Hl + H2SO-l

The amount of water in the sample IS mainlydetermined on the basis of titrated iodine.

Results and DiscussionMode of bonding IIg(II)-complexes via infrared spectral data

The newly synthesized Hg(II)-complexes areslightly soluble in cornman organic solvents and allare colored except Hg(II)-Benz. (I) as given inTable 1. Benzimidazole showed strong intermolecularhydrogen bonding in the solid state due to the strongand broad absorption infrared bands in the region3600-2400 cm'l (ref. 20). The band assignment(cm'l) of benzimidazole and its metal complexes arelisted in Table 2. The infrared spectra ofbenzimidazole. (arylazo) benz., a-nitro (arylazo)benz., a-hydroxy (arylazo)benz.. anda-methyl(arylazo)benz. metal complexes showed onlyweak absorption band -3020-3100 cm'l which isprobably due to the C-H stretching vibration of thearomatic ring. The complete absence of both N-H

Table 1 - Analytical and physical data of benzimidazole, a-substituted (arylazo) benzimidazole, u(naphthyl azo bcnz)ligands and their Hg(ll) complexes

M.pt.(OC)

170 71.0071.1819.2819.7670.44702733.5033.3571.4071.1822.702208582058.4229.4729.9065.0465.5421.402096745075.0045.724596

5.075.082.503.054.054.502.902.964.805081.291.703.003.372052103.804.201.401.613804.412.903.60

237023736.066.58

25.5025.22108010.3723.4023.726.907.36

25.8026.2113.0013.40237023527.107.52

20.7720.5812.4012.60

8.00835

47.1347.2

13501315

37553717

14.371400

526352.74

6.40681

38.4938.00

9309.54

53.4253.91

7.787.99

22.802260

Benzi midazole(L')

[HgL'CI.2H20]2H20(Aryl azo )benz.(Lu)

[HgLuCI2]C2HsOH

a-Methyl (arylazo)bcnz.(Lh)

[Hg2L',C!3·H20]a-Nitro (ary!azo)benz.(Lc)

[HgLcCI.1-!20]

a-I-!ydrox y(ary lazo )benz. (Lei)

[Hg2LelC!2·21-!20]

(u-Naphthylazo )benz.(L")

[HgL2"CI2].41-!20

3112-2856 1617

2919 16003113-2942 1617 1409

2919 1600 1395

3112-2882

29183113-2858

2917

3112-2885

2919

3112-2856

3110-2849

1618

16091619

1601

1601

1606

1616

1609

stretching frequencies near 3200 cm-I and the N-Hbending frequency at 1595 em-I indicates the absenceof the Imll10 hydrogen. Thus, the coordination ofHg(II) to ligand takes place at N1 position formingtetrahedral structures. The infrared spectrum ofo-hydroxy(aryl azo)benz showed a strong band at3372 cm-I which could be attributed to -OH of thephenolic group. This band is absent on complexation

1459

14291457

1414

1458

1407

1408

J390

indicating coordination through the deprotonatedphenolic -OH group21

The spectrum of (a-naphthyl)azo benzi midazoleshowed the presence of N-H stretching and N-Hbending modes indicating that the ligand protonremaltls bound at the N I position and, thus thecomplex bond formation between metal and ligandtakes place at the N2 position. The spectra of the

Hg(II)-complexes showed sharp intense band at1395-1414 cm-I corresponding to the -N=N-stretching mode and was red shifted when comparedto free ligand values22 (1408-1458 cm-I). Theelemental analysis and infrared spectroscopic data areconsistent with the structures assigned in Scheme 1(Electron impact mass spectral fragmentationpathways of IHgL'CI.2H20].2H2.0).

Stoichiometry of mercury (ll) complexesBased on the data of elemental analysis and metal

contents given in Table 1 and the results of infraredanalysis (Table 2), the stoichiometric ratio for thecomplex formation of mercury(II) with the studiedIigands can be described as follows. Benzimidazole asa coordinating ligand was characterized by therormation of (1:1)-type of complex[HgL'C1 .2H20].2H20, suggesting the monodentatebehavior via N1 atom. The three categories based onthe stoichiometry of the studied and identified

Hg(II)-complexes of azo-derivatives are:(i) [HgL"C12]C2HsOH and [HgLcCI.H20J, exhibitingthe (l: 1) stoichiometric ratio leading to a bidentatebehavior of the interacting ligand via azo-N moietywith the presence of chloride ion in the inner sphere;(ii) lHg2LbCl3.H20] and [Hg2LclCl2.2H20 j arecharacterized by a (2: 1), stoichiometric ratio. Thesetwo Hg(II)-complexes suggest binding of the testedligands via (-N=N-, N1, N3, and o-OH) in atetradentate fashion. However, two types of ringformation are characterized in [Hg2LbCl3.H20j. fiveand four-membered ring, which is common inpyrimidine complexes23

; and, (iii) IHgL2"C1j.4H20has been found to form a (1:2) stoichiometic ratio.suggesting the ability of this ligand to bind withHg(II) as a bidentate via azo-imine moiety, forming ahexa coordinate Hg(II)-complex.

The structure of these Hg(II) complexes is shownin Scheme 1.

CI CI" I

(Y~tO~N

\H

Mass spectra of mercury(lI) complexesThe [HgL'CI.2H20j.2H20 complex was found to

exhibit several characteristic mass spectral fragmentions2-l. These include a fragment ion at mlz 271-273corresponding to [HgCI.(H20)2] ion with relativeabundance of (15-12%) due to isotope contribution ofHg. This ion was further fragmented to yield threeother characteristic peaks at m./z 253-255, 235-237and 198-202 due to successive losses of H20, H20and CI, respectively. Another fragment ion at mlz 117was found as the base peak in the mass spectrum ofthis Hg-complex due to the loss of L' from themolecular ion. This fragment ion produces anothermass spectral peak at mlz 90 due to the information ofthis fragment ion [CGH-lNt. The fragmentation patternof [HgL'CI.2H20].2HzO is shown in Scheme 2.

This Hg-complex, [HgL"CI2].C2HsOH, gave thehighest fragment ion at mlz 270-272(5%) due to theformation of (HgClzt, and another fragment ion atml:: 200-202 due to the presence of Hg ion (7%). Theion related to the Iigand was identified at nyz 223(8%) and 222 (4%). Other produced fragment ionsfrom the ligand were identified at mlz 77 (13%), 90(27%) and 118 (100%). The mass spectrum of

Nr('1~N~H/CI

;f g,H

20 01-12

l+CCN 1I ')~ N

Ill/z 117 (100%)

[Hg2LbCb.H20] complex was found to exhibit thefollowing fragment ions with their relativeabundance; [HgClzt at nyz 270-272 (-12%),[HgCI.H20]+ at mlz 253 (-2%), ILbt· at mlz 236 and[Hgt at 200-202 (-12%). In addition, other fragmentions are produced at mlz 134 (6%), 117 (100%). 91(30%) and 90 (6%) that can be correlated to the ligandstructure.

The mass spectrum of this complex [HgLcCI.HzOjwas identified by several characteristic fragment ionsincluding mainly the ligand. The base peak wasrelated to mlz 117 and another ion at mlz 150 due tothe formation of [Nz-ph-NOzt with a relativeabundance of 5%. Several fragments at mlz 122 (5%)and 76 (-12%) due to successive losses of Nz andNOz molecules from the fragment ion at ml:: 150 werealso characterized. Two weak abundant fragment iOlisat 253-255 «1%) and 198-202 (-3-5%) are directlyrelated to the presence of [HgCI HzOr and Hg ionmoieties in the complex.

The 70-eV mass spectra of the two remaining Hg-complexes, rHg2LclCl2.2H20] and [HgLz"Cbl.4HzOwere also found to be similar to in their fragmentationpathways to the above mentioned four mercury(Il)

CI l+/

H,O-I-I2:

- "01-12

Ill/z 271-273(-15-12%)

l+1-120-Hg-C1

Ill/z 253-255( -2-3%)

+I-I"C~+

b

Ill/z 235 -237( -1-2%)

il+'

I-Ig

complexes. The major mass spectral fragment peaksand ions are mainly due to the Hg(II) part and ligandmoiety.

Thus, the mass spectral data and interpretations of thestudied Hg-complexes confirm the assigned structuresgiven to these complexes as shown in Scheme l.

IIl-NMR spcctnlThe IH-NMR spectra of benzimidazole, a-hydroxy

(arylazo) benz. and their Hg(II)-complexes wererecorded in {t-DMSO, and assignments of the major

Table 3 - I/-I-NMR signals of benzimidazole, o-hydroxy(arylazo)benz. and their /-Ig(1 I)-complexes

0- /-IydJ"Oxy(arylazo)benl-imidazole

/-Ig-o-hydrox y(arylazo)benzi midazole

Signals(0. ppm)

7157.5982512.137.327818576547.1576089512.436.707.277.85

C4TH = 2/-1+CS.6-/-I = 2/-1+Cz-/-I = I WN-/-I= I WC4T/-I = 2/-1+CS.6-/-I = 2/-1+Cz-/-I = I /-1+Phenyl protons = 4 /-1+C4T/-I = 2I-tCS•6-/-I = 2/-1+Phenolic proton = I /-1+N-H = I /-1+Phenyl protons = 4 /-1+C4T/-I = 2H+CS.6-/-I = 2/-1 +

peaks are given in Table 3. These can be listed andcorrelated to the following signals: (I) Phenyl protonssignals are located in the range 6.54-6.70 ppm. Allsignals are splitted with total number of protons=4;(2) The a-hydroxy (arylazo)benz. and the parentbenzimidazole gave no NMR detection to -H groupand absence of oNH in both complexes suggestingthat coordination is possible through the I {ref. 25):(3) In case of a-hydroxy (arylazo)benz., the absenceof signals due to phenolic -OH and -NH protonssuggested the coordination of metal to ligand via bothcenters; (4) Benzimidazole protons are shifted by0.16-0.52 ppm to higher 0 in coordinationcompounds; and, (5) All protons in benzi midazoleand a-hydroxy(arylazo)benz suffer downfield shiftingin Hg-complexes compared to the free ligand values.

Thermal studiesThermogravimetric data of benzimidazole, IH-2-

(arylazo) benz., a-nitro(arylazo)benz., a-hydroxy(arylazo)benz., and their mercury(II)complexes, are given in Table 4.

The method described by Borchardt and Daniels2(,

was applied to determine order of reaction (1/) andactivation energy (Ea) for the thermal steps, using theequation of rate constant, k=r!J(doJdT)!f(o) J. where fJis the constant heating rate and j(a) = (i-O) ". Thecriterion of best linearity indicates the most probable

Table 4 - Kinetic and thermodynamic parameters from thermal studies

Camp. Stcp Temp DTGIll:l.x Wtloss 1/ En -LlS* R Ln (A) Assignmellts

range (K) (K) Obs (calc) ('Yo) (kJmor') (J mor'K")

Bcnzimie!azole '+63-543 503 98.48(98.73) I 83.08 -99 176 0.95540 18.052 Loss of bcnz. molccule(L')IllgL'CL21-1,01. 21-1,0 '+53-593 518 89.40(90.10) 0.67 69.44 -137'+71 097396 13.'+7 Loss of 21-1,0 molccule;,

ane! 2Coord.I-I,0. bcnz.molccule, CI and 0.8(l-Ig)

(Arylazo)bcnz. I 403-453 428 1794( I8.00) 2 3720 - I71.19 051633 923 Loss of -N=N- and C(L,) 2 453-553 503 73.05(72.9) 0.67 44.85 - I73.83 0.95226 907 Loss of bcnz. molecule.

C,I-I, and 1.5(C)

Il-lgL,CI,I. C,II., Oil '+33-503 468 3033(3000) 2 6700 -124.15 0.80118 1497 Loss of C,I-I,OI-l molccule573 6.+.78 ane! Ph- ,

2 513-633 -+3.50(43.40) 2 -1'+761 0.92547 1235 Loss of benz. molecule ane!0.5 (I-Ig)

o-Nitro (arylazo) benz. 393-443 418 903(10.40) 2 11076 1714 0.93150 3185 Loss of -N=N- molccule(L,) Loss of bcnz. molccule.

2 463-533 493 84.95(85.07) 2 186.98 115.03 0.97252 43.79 NO" 2(C,I-I,) and CII-IgL,CL1-1,0I I ,+53-(,33 543 80.49(80.75) 1 4485 -18626 0.98249 765 Loss of coore!. 1-1,0

molccule. -N=N-. NO,.benz. moleculc. (',,1-1,-. ('Iand 0.5 (I-Ig)

o-llye!roxy (ar)'lazo)benz. I '+13-533 473 83.67(83.68) 2 8237 -85.15 098071 19.67 Los;, of benl.. molecule.(L,,) N=N-. 2(C,I-I,) and 1-1,gas

[l-Ig,L"C1,.21-I,O I 473-563 518 48.35(48.01) 2 120.25 -3187 0.96124 2617 Loss of 2 coord. 1-1,0molccules. -N=N-. benz.moleculc. C"I-I, ane!05(l-Ig)

2 583-663 623 26.64(26.95) 2 140.51 -2439 087349 27.26 Loss of I-If!

order by plotting In(k) versus lIT for different valuesof 11.=0, 0.33, 0.5. 0.67, 1.00 or 2.00 (Table 4). Theregression values R2 is given by:

The values of R2 nearest to unity in most cases. Onapplying this procedure, the regression line having avalue of R2 nearest to unity is selected as shown in thefigures. The values of E([ and A have been calculatedand the entropy change L1S* was calculated using the

. "7eguatton- .

whereas, R is the ideal gas constant, A is the pre-exponential factor, h is the Plank constant, K is theBoltzmann constant and T,II is the temperature atmaximum decomposition rate. All the studiedcompounds exhi bited negati ve L1S*-values, except ino-nitro Benz. ligand. A comparison of L1S* values forthese complexes indicates that the thermaldecomposition of a-nitro (arylazo) benz. complexshowed the highest value, while thea-hydroxy(arylazC' )benz. complex showed the lowest.Thus. the fonner decomposes with the lowest degreeof randomness while the latter with the greatest.

From the energy of activation point of view, Hg(II)-complex of a-nitro exhibited the lowest value whileHg(II) complex of a-hydroxy showed the higher one.This may be explained by the electron withdrawingeffect of the nitro group, which leads to a weakening ofAr-Hg-bond, giving rise to a relatively easy thermaldegradation process. In case of a-hydroxy-Hg complex.the phenolic group is an electron donating and the Ar-Hg-bond is strengthened. Therefore, the value ofactivation energy in this case is higher than in theunsubstituted phenyl azo Benz-Hg complex. Thus, thefollowing order can be set up: a-OH (Hg) > Phenylazo-(Hg) > Benz-Hg > a-N02(Hg). In the case of theo-nitro(arylazo)benz. ligand, abnormal feature isoutlined due to the higher value of E([. This could beassigned to probable existence of nitro group indifferent geometrical planes. Such a group is twistedand reoriented. The activation energy of the ligands isin the order:

x= a-Nitro> Benz> a-OH > PhenyJazoIt is. therefore, evident that the nature of substituent

on the rhenyl group contributes to the variation in

activation energy for the thermal degradation of thecomplexes. The energy of activation in turn reflectsthe kinetic lability of the complexes. The compoundswith lower E([ are more labile as compared to thosewith higher E([ values2

.!.

ReferencesI Gosselin R E, Smith R P & Hodge H C. Clinical Toxicologr

of COllllllercial Prodllcls, 5th Edn (Williams and Wilkins.Baltimore), 1984.

2 Friberg L, Nordberg F. Kessler E & Vouk V B. lIandbook ofthe Toxicology of Melals. 2'''' Edn (Elsevier SciencePublishers B V. Amsterdam) 1986.

3 Gilman A G, Goodman L S & Gilman A. Goodman andGilman's The Pharlllacological Basis Therapelilics. 71h Edn(Macmillan Publishing, Ncw York) 1985

4 Gong J F, Wang D W, Zhang Y H, Zhu Y & Wu Y J. InorgChillI ACla, 359 (2006) 2115.

5 Lopez-Torres E. Mendiola A & Pastor C .I, IJol."hedron. ::~(2006) 1464.

6 Liu L. Lam Y W & Wang W Y, J Organolllel Cheln, 691(2006) 1092.

7 Casas J S, Castellano E E, Couce M D. Ellena J. Sanchez A,Sordo J & Taboada C, ./Inorg Chelll, 100 (2006) 124

8 Rekha S & Nagasundara K R, Indian ./ Chelll, 45A (2006)2421.

9 Sharma R Bansal A K & Nagar M. Indian ./ ('helll. ·UA(2005) 2255

10 Halli M B & Qureshi Z S. Indian J Chelll, ·BA (200'+) 23'+7.II Sharma R & Kaushik N K. IndiclIl./ Cheln, .+3A (::00'+) 769.12 Mahmoud M E. Haggag S S & Hegazi A H . ./ Coilliller Sci,

300 (2006) 94.13 Chand B G. Ray U S. Mostafa G, Cheng J. Lu T H & Sinha

C. Inorg Chilli ACla, 358 (2005) 1927.14 Podunavec-Kuzmonovic S O. Leovac L M. PerisicjanJilic N

U & Rogand J . ./ Serb Chelll Soc. 64 (1999) 381.15 Ahujia I S & Prasad I. Inorg Nile! Cheln Lell. 12 (1976) 777.16 Braga D. ./ Chelll Soc, Oalloll TraIlS, (2000) 3705.17 Mathur T. Jasimuddin S K. Milton H. Woollins J D & Sinha

C. Inorg ChillI /\cla, 357 (2004) 3503.18 Dinda J. Das D. Santra P K. Sinha C & Falvello R. ./

Organolllel Chelll, 629 (2001) 28.19 Schwarzenbach G. COlllple.wlllelric Tilralion (Methuen Co .

London) 1957.20 Xue G. Juenlong Z, Shi G. Wu Y & Shuen B. ./ ehelll Soc

Perkin II, (1989) 33.21 Zaki Z M, Haggag S S & Soayed A A. Spec/rosc Lell. 31

(1989) 757.22 Bag K. Misra T K & Sinha C. ./ Indian Chelll Soc. 75 (1998)

421.23 Masoud M S. Khalil E A. Hindawy A M. Ali A E &

Mahmoud E F. Speclrochilll ACla. 60A (2004) 2807,24 Marrwaha S S, Kaur J & Sodhi G. ./ Inorg Biachelll. 54

(1994) 6725 Zaki Z M, Haggag S S & EI-Shabasy M. Speclroscopr Lell.

28 (1995) 489.26 Borchardt H J & Daniels F J, ./ Alii Chelll Soc, 79 (1957)'+ I.27 Aravindakshan K K & Muralcedharan K. Thel"lllOchilll /\cw.

159 (1990) 101.