Vol. 05 INTERNATIONAL JOURNAL OF PHOTOENERGY 2003

Spectroscopic studies of Ln(III) complexes withpolyoxometalates in solids, and aqueous

and non-aqueous solutions

Stefan Lis,1,† Slawomir But,1 Andrzej M. Kłonkowski,2 and Beata Grobelna2

1 Faculty of Chemistry, Adam Mickiewicz University, 60–780 Poznań, Poland2 Department of Chemistry, University of Gdańsk, 80–952 Gdańsk, Poland

Abstract. Chosen polyoxometalate (POM) anions and their lanthanide(III) complexes, LnPOM, havebeen synthesized and spectroscopically characterized in solid state, aqueous and non-aqueous solutions.POMs, such as Keggin’s, Dawson’s and Anderson’s type, Na9EuW10O36, compositions that function as in-organic cryptands ([(Na)P5W30O110]14−-Preyssler anion, and [(Na)As4W40O140]27−), containing inorganic(Na+,K+,NH+4 ) or organic (tetrabutylammonium, NBu

+4 ) counter cations were obtained and their Ln(III) com-

plexes (sandwiched and encapsulated) studied. The synthesized compounds were identified using elementaland thermogravimetric analysis, UV-Vis spectrophotometry and FTIR spectroscopy. The complexation stud-ies were carried out with the use Nd(III) and Er(III) optical absorption and Eu(III) luminescence spectroscopy.Luminescence characterization, including results of intensity, quantum yields and luminescence lifetimes ofEuPOM complexes in aqueous, non-aqueous solutions (DMF, DMSO, acetonitryle) and solid are discussed.Based on luminescence lifetime measurements of the Eu(III) ion the hydration numbers of its sandwiched(efficient emitters) and encrypted complexes have been determined and quenching effect discussed. TheEu(III) complexes entrapped in a xerogel matrix have been studied as luminescent materials. Luminescenceintensity, lifetime and quantum yield of the EuPOM materials and their photochemical stability, during con-tinuous UV irradiation, were tested.

1. INTRODUCTION

The majority POMs composed of molybdenum andtungsten polyhedrons, due to their interesting physic-ochemical properties, and biological importance, havebeen a subject of numerous studies. This class of com-pounds has received much attention in the last twodecades because of their wide applications in catalysis,material engineering, photochemistry and in medicine.Their attractive and often unusual physicochemicalproperties are related to their structure, shape, chargedensity, redox potential, acidic character and solubility.POMs with alkali counter cations such as Na+, K+ andNH+4 are well water-soluble compositions. They can besolved in solvents of different polarity, depending onthe kind of the counter cation occurring in the POMmolecule [1–4]. Generally POMs can be categorizedinto three structural groups, depending on the coordi-nation number of the heteroatom [1,2]. Our previousstudies concerned synthesis of POMs and their Ln(III)complexes, representing the three structural groups ofPOMs and additionally so-called inorganic analoguesof crown-ethers and cryptands (e.g. Preyssler’s anion).They have been characterized using spectroscopicmethods both in aqueous [2,5–8] and non-aqueoussolutions [9]. Some of LnPOM complexes show in-teresting and effective luminescence properties, i.e.high luminescence intensity, its quantum yields

† E-mail: blis@amu.edu.pl

and long lifetimes of Ln(III) excited states [10]. Suchcomplete inorganic materials, particularly EuPOMcomplexes, have been entrapped in xerogel matrices bysol-gel method and examined as luminescent materi-als. The resulting immobilization of Eu(III) complexesin xerogel matrices is known to enhance emissionintensity [11, 12]. These luminescent materials havebeen also tested for their photochemical stabilityunder continuous UV irradiation.

2. EXPERIMENTAL

2.1. Methods. All reagents used in these stud-ies were at least analytical grade, while Nd2O3 andEu2O3 were spectroscopically pure and non-aqueoussolvents (DMF, DMSO, acetonitryle, dichloromethanefrom Fluka) pure for UV spectroscopy (content ofH2O < 0.005%). Elemental analysis of the POMs andtheir europium complexes were made with the use ofan Elemental Analyser 2400 CHN, Perkin Elmer. Ther-mogravimetric (TG) and differential thermal (DTA) anal-ysis were conducted using a Shimadzu TGA-50H/A50thermoanalytic system, temperature interval was 293–823 K, heating rate 2 K/min in air atmosphere. Absorp-tion spectra were recorded on a UV-2401PC Shimadzuspectrophotometer. The IR spectra were obtained bymeans of FTIR Bruker JFS 113v spectrophotometer forthe samples (∼2mg) prepared in KBr. The correctedluminescence spectra of Eu(III) were recorded using

234 Stefan Lis et al. Vol. 05

Table 1. The luminescence data of Eu(III) ion in EuPOM complexes in solutions, λexc = 394 nm, CEu(III) = 0.001 mol/l.

CompoundLifetime

Hydration numberLuminescence Intensity [a.u.],

Quantum yield[ms] λmax = 615 nm

Eu(ClO4)3 0.11 9.3 0.01 2.60× 10−3Na9[EuW10O36] 2.29 0.2 2.0c 4.19× 10−3K17[Eu(P2W17O61)2] 3.08 0.1 0.4 1.38× 10−2Eu2TeMo6O24 0.19 5.1 0.08 –

K25[(Eu)As4W40O140] 0.25 3.7 0.02d 9.06× 10−3K12[(Eu)P5W30O110] 0.30 3.1 0.23e 9.6× 10−5(NBu4)12[(Eu)P5W30O110]a 0.79 0.6 0.37e 1.1× 10−4(NBu4)8H3[Eu(PMo2W9O39)2]b 1.11 0.2 0.95f –

K11,13[Eu((P/Si)MoxW11−xO39)2]1.2–3.5 0.1 0.0–4.8 10−4–10−2

Ref. [6] Ref. [6] Ref. [18]

a in acetonitryle, b in DMF, c λmax = 300 nm, d CEu(III) = 1× 10−4 mol/l, e CEu(III) = 1× 10−2 mol/l, f) λexc=342 nm.

Table 2. The luminescence characterization of EuPOM complexes entrapped to xerogels.

HPAS Type of matrixLifetime [µs], Luminescence Intensity [a.u.],

Quantum yieldλexc = 394 nm λmax = 615 nm

Na9EuW10O36 SiO2 642.8 23.7∗ 0.123SiO2 − PDMS 491.9 19.8∗ 0.104SiO2 − TEG 508.4 24.4∗ 0.066

Eu2TeMo6O24 SiO2 245.9 15.3+ 0.027SiO2 − PDMS 162.9 23.8++ 0.018SiO2 − TEG 275.4 37.6+++ 0.012

∗ λexc = 394 nm, + λexc = 320 nm, ++ λexc = 315 nm, +++ λexc = 330 nm.

a Perkin-Elmer MPF-3 spectrofluorimeter. The lumines-cence lifetime of Eu(III) was measured with the use ofthe detection system described earlier, consisting of anitrogen laser (KB6211) and a tunable dye laser [13].The luminescence decay curves observed in this workwere analyzed by a single exponential fitting, provid-ing the decay constants. The luminescence quantumyield, Φ, of the Eu(III) ion in solution was calculatedusing [Ru(bpy)3]Cl2 as a standard. In the case of pow-der samples Φ was determined based on the proceduredescribed by Wrighton et al. [14]. Measurement of lu-minescence quantum yields of the powder samples in-volved determination of the diffuse reflectance of thesample relative to a nonabsorbing standard (KBr) at theexcitation length, and then measuring the emission ofthe sample under the same conditions [15]. The quan-tum yield is the ratio of the emitted photons to thedifference in the number of diffuse reflected photonsfrom the sample and the nonabsorbing standard, hasbeen calculated using the equation (1):

Φ = ERstd − Rsmpl

(1)

where E is the area of the corrected emission curveof the sample, Rstd and Rsmpl are corrected areas un-der the diffuse reflectance curves of the nonabsorbingstandard and samples, respectively, at the excitation

wavelength. The calculated values of the luminescencequantum yields of the samples studied are given in Ta-bles 1 and 2.

The Eu(III) complexes entrapped in xerogels wereirradiated by means of a UV lamp of radiation power0.53 W/cm2.

2.2. Synthesis and identification of compounds.The lacunary structures of the heteropolyanions (forexample SiW11O

8−39 ) were obtained from the corre-

sponding plenary structures (SiW12O4−40 ) as a result

of their partial degradation as described earlier [16].Europium sandwiched complexes Eu(POM)2 were ob-tained according to the method described by Pea-cock and Weakley [17] and modified by us [18]. Thecryptand analogues of the heteropolyanions and theirEu(III)-encrypted derivatives were prepared as previ-ously described [5, 15, 19, 20]. Tetrabutylammoniumsalts of the chosen compounds was obtained with us-ing a literature procedure [21]. Identification of syn-thesized compounds was done by comparison of theIR spectra of obtained compounds with those previ-ously reported, elemental and thermogravimetric anal-ysis [5–7, 18] and our own spectrophotometric methodfor determination of tungsten and molybdenum con-tents. The spectrophotometric method enables thesimultaneous determination of W(VI) and Mo(VI) in

Vol. 05 Spectroscopic studies of Ln(III) complexes with polyoxometalates … 235

POMs and their Ln/POM complexes with disodium-1,2-dihydrobenzene-3,5-disulfate (Tiron), used as colori-metric reagent [22].

2.3. Synthesis of the POMs entrapped in sol-gelmatrices. A wet gel product was obtained after somedays by sol-gel process (hydrolysis and polyconden-sation) of tetramethoxysilane (TMOS) in mixture withmethanol and aqueous solution of EuPOM complex[23]. Methylated silicate xerogels was synthesized inthe similar way. A 1 : 1 mixture of TMOS (Aldrich)and polydimethylsiloxane (PDMS 200, Aldrich)) was dis-solved in methanol and water as reagent was added[24]. In the case of TMOS and triethylenglycol (TEG)with 1 : 1 molar ratio, the components were dissolvedin methanol, heated to 60 ◦C and stirred for ten min-utes [25]. Then aqueous solution of EuPOM complexwas added. In each gel preparation the molar ratio[alkoxidegroup] : [H2O] = 1 : 1 and the resulting wetgels were dried at room temperature to obtain xero-gels. The final concentration of the Eu(III) complexesin xerogels are 5× 10−5 mol/g xerogel.

3. RESULTS AND DISCUSSION

The compositions of POMs were determined based onresults of the elemental (C, N, H), thermogravimetricanalysis (H2O, [NH4]+, [NBu4]+, H+), spectrophotomet-ric determinations of Mo and W in the range of UV-Vis,and FTIR spectra analysis. The elemental analysis wasvery useful in the case of POM compositions containingan organic counter cation.

The use of the FTIR spectroscopy has shown to be avery useful tool in studies of POMs and their complexesdue to easy indication of the plenary, lacunary and ofsandwich complexes, based on characteristic featuresof the spectra [6]. The formation of the lacunary struc-ture (pH dependent degradation of an appropriate ple-nary structure) indicates a split of the P-Oa and W-Oc-Wband oscillations, as a consequence of existence oftwo different coordination environments of phospho-rus and oxygen atoms, Oc. FTIR spectroscopy has beenused to fingerprint the POM structures because of goodcorrelations between spectral peak positions, shapes,and relative intensities of the spectra, obtained for solidand solution. These correlations strongly indicate iden-tical structures [18]. The FTIR spectra of tetrabutylam-monium compositions of POMs have generally the samebands as the spectra recorded for the potassium or am-monium salts. Additional bands attributed to the C−Hand C− C oscillations of the (NBu4)+ organic species,in the range of 1300–1500 cm−1 and 2800–3000 cm−1,occur in this case.

Absorption spectra of Nd(III) and Er(III) in therange of their hypersensitive transitions 4I9/2 → 2H9/2and 4I15/2 → 2H11/2, respectively, were measured forvarious metal : ligand ratios in non-aqueous solutions.Positions and intensities of the spectra are sensitive to

the ligand field of POMs and can be used to evaluatethe formation of LnPOM complexes. Examples of thespectra, measured for various Ln(III) : POM molar ra-tios, ranging from 1 : 0 to 1 : 3, in DMSO and DMFsolutions, are presented in Figures 1 and 2.

560 570 580 590 600 610A

bso

rban

cenm

0.00

0.02

0.04

0.06

0.080.08

0.06

0 1 2 3CHPAS/CNd(III)

Nd : POM1 : 01 : 0.41 : 0.71 : 1.01 : 1.41 : 1.81 : 2.01 : 2.5–3.0

Figure 1. Absorption spectra of Nd(III) in the rangeof the hypersensitive transition (4I9/2 → 2H9/2) for var-ious Nd : [NBu4]6H[PMo2W9O39] molar ratios in DMSO,CEr(III) = 0.001 mol/l.

510 520 530

nm

Ab

sorb

ance

0.00

0.02

0.04

0.04

0.020 1 2 3

CHPAS/CEr(III)

λmax=521 nm

Er(III) : POM1 : 01 : 0.41 : 0.71 : 1.01 : 1.41 : 1.81 : 2.01 : 2.51 : 2.8

Figure 2. Absorption spectra of Er(III) in the range of

the hypersensitive transition (4I15/2 → 2H11/2) for var-ious Er : [NBu4]6H[PMo2W9O39] molar ratios in DMF,CEr(III) = 0.001 mol/l.

These spectra show an increase in absorption and ashift of their maxima consistent with formation of ML2complexes.

Recently, based on the analyses of the absorptionspectra of Nd(III) and Er(III) ions, we evidenced for-mation of the ML and ML2 complexes with Keggin’sand Dawson’s type of polyanions [7, 18, 26] and MLcomplexes with the polyanion [(Na)As4W40O140]27−

[18] in aqueous solutions. In the case of [MnMo9O32]6−

236 Stefan Lis et al. Vol. 05

(Anderson’s anion) with the Nd(III) ion spectroscopicstudies demonstrated that the M2L type of complexesoccur in solution [7].

Formation of the ML2 sandwiched complexes innon-aqueous solvents (DMSO, DMF, acetonitryle) asare shown in Figures 1 and 2. They were also ear-lier observed in aqueous solutions [7, 26]. Our stud-ies concerning chemometrics and factor analysis of theNd(III) absorption spectra with chosen POM complexesconfirmed formation of the M2L, ML, ML2 complexes[26, 27]. In spectrophotometric studies we also provedthat formation of LnPOM complexes is function of ionicstrength. For example, in equimolar solutions of Nd(III)and Keggin’s POM of high ionic strength, a preferentialformation of the sandwich complexes Nd(HPAS)2, in-stead of NdHPAS, were observed [6, 7].

Eu(III) luminescence studies (intensity, lifetimemeasurements and quantum yield) of the EuPOM sys-tems were used to investigate their emission proper-ties, effectiveness of energy transfer processes, hydra-tion numbers and complex compositions [6, 7].

The Eu(III) luminescence lifetimes measured for theEu/POM complexes were used to calculate the numberof water molecules present in the inner sphere of theEu(III) ion, using the following eq. [28]:

nH2O = 1.05τ−1H2O − 0.7 (2)

The lifetime of Eu(III) excited state is efficientlyquenched by OH oscillators of inner sphere H2Omolecules bound to the Eu(III) ion. Based on the Eu(III)luminescence lifetime measured for EuPOM complexesin non-aqueous solutions and solid the hydration num-bers of this ion were calculated as shown in Table 1and compared with those obtained earlier in aque-ous solution [5–7]. The measured Eu(III) luminescencelifetime gave the longest values for sandwiched com-plexes with inorganic counter cations in aqueous so-lution (from 1.2 to 3.5 ms) and with organic countercation (1.11 ms) in DMF solution. The hydration num-bers calculated from the measured lifetime values in-dicate no water molecules in the Eu(III) inner coordi-nation sphere of the complexes. In the case of Eu(III)encapsulated Preyssler’s complexes, the compositionwith organic counter ion (NBu4)12[(Eu)P5W30O110] hassmaller hydration number (∼ 0.5) than that with inor-ganic counter ion K12[(Eu)P5W30O110] having 3 watersof hydration. It is worth to mention that the solid Eu(III)complexes have coordination number of 8 or 9 in thecase of solution.

Examples of the luminescence excitation and emis-sion spectra of Eu/POM complexes recorded in non-aqueous solvents are presented in Figure 3.

The Eu(III) luminescence lifetimes obtained forEu/POMs in non-aqueous solvents with various molarratios of the components demonstrated the ML2 com-plex formation (see Figure 4).

250 300 350 400 560 600 640 680 7200.0

0.5

1.020000

10000

0

nm

Lum

ines

cen

ceIn

ten

sity

[a.u

.]

(a) (b)

Lum

ines

cen

ceIn

ten

sity

[a.u

.]

Figure 3. Luminescence excitation (a) and emission (b) spec-

tra of Eu(III) in (NBu4)8H3[Eu(PMo2W9O39)2] in DMF.

0 1 2 3 4CPOM/CEu(III)

CEu(III) = 0.002 mol/l(NBu4)5H2[PW11O39]

(NBu4)6H[PMo2W9O39]

Lum

ines

cen

ceLi

feti

mes

[µS]

500

1000

1500

Figure 4. Eu(III) luminescence lifetime as a function of thePOM : Eu(III) molar ratios in non-aqueous solvent (DMF).

The luminescence spectra of non-aqueous solu-tions are generally similar to those obtained in aque-ous solution [6]. Excitation spectra of the Eu(III)molib-dotungstate compounds show strong emission band(λmax ∼ 340 nm) due to ligand to metal energy trans-fer (LMET) from the tungstate group to the Eu(III) ion[29, 30].

Our previous studies have shown that the mostintense Eu(III) luminescence was observed for the(EuW10O36)9− and [Eu(SiMoW10O39)2]13− sandwichedcomplexes due to energy transfer from the tungstategroup to the Eu(III) ion. In the case of [Eu(SiW11O39)2]13−

and [Eu(P2W17O61)2]17− and the Eu-encrypted com-plexes a weak luminescence intensity was observed. Inlatter cases the transfer from ligand to Eu(III) does notoccur. Interesting pattern of the Eu(III) luminescencelifetime and quantum yield were observed in the case ofthe heterotungstomolybdate Eu-sandwiched complexes{[Eu(Si(P)MoxW11−xO39)2]13−}. A linear dependence

Vol. 05 Spectroscopic studies of Ln(III) complexes with polyoxometalates … 237

0 50 100 150 200 250 300

Exposure Time [min]

Emis

sion

Inte

nsi

ty[a

.u.]

0

10

20

30

40

50

60

70

Na9EuW10O36 ·19H2O in SiO2, conc.=5·10−5 mole/g SiO2

Eu2TeMo6O24 ·18H2O in SiO2, conc.=5·10−5 mole/g SiO2

Figure 5. The photochemical stability of EuPOMs com-

plexes.

of the Eu(III) luminescence lifetime, τ , and quantumyield, φ, on the content of Mo (number of atoms x,where x = 0–9) in the [Eu(Si(P)MoxW11−xO39)2]13−structures were observed. These dependences can beapplied for the determination contents of Mo in poly-tungstomolybdate complexes [6, 9, 18].

Based on luminescence lifetimes measured both forsolid and aqueous solutions, we found no H2O in Eu(III)inner sphere in the sandwiched complexes Eu(POM)2.Whereas the complexes Eu2POM have four or six watermolecules and europium-encrypted derivatives possessthree or four H2O’s in the Eu(III) inner coordinationsphere.

In order to increase photochemical stability andto minimise water (O−H oscillators) interaction, weused Eu(III) complexes with the heteropolyanions(Na9EuW10O36,Eu2TeMo6O24) entrapped in silica xe-rogel matrix. The luminescence characterization ofEuPOM complexes entrapped to xerogels are presentedin Table 2. The resulting encapsulation and immobiliza-tion of Eu(III) complexes with POMs in xerogel matricesenhanced emission intensities and luminescence quan-tum yield related to solid EuPOM and EuPOM dissolvedin the solutions.

The Eu(III) complexes entrapped in xerogels weretested for their photochemical stability during UV ir-radiation. As it is shown in Figure 5, the emission in-tensity (at λmax = 617 nm, when λexc = 273 nm) of thesilica xerogel doped with Eu(III) complexes during UVirradiation remains constant within experimental error.Thus, for the Eu(III) in inorganic environment there ispresent no photodegradation effect due to relative highenergy (UV) quanta used for the irradiation.

ACKNOWLEDGEMENT

This work was supported by the Polish State Committeefor Scientific Research, Grant No. 4 T08A 044 23.

REFERENCES

[1] M. T. Pope, Heteropoly and Isopolyoxometalates,Springer-Verlag, New York 1983.

[2] S. Lis, Acta Phys. Pol. A 90 (1996), 275.

[3] D. E. Katsoulis, Chem. Rev. 98 (1998), 359.

[4] M. T. Pope and A. Müler, Angew. Chem. Int. Ed.Engl. 30 (1991), 34.

[5] S. Lis, M. Elbanowski, and S. But, Acta Phys. Pol. A90 (1996), 361.

[6] S. Lis and S. But, Materials Science Forum 315–317(1999), 431.

[7] S. Lis and S. But, J. Alloys Comp. 300–301 (2000),370.

[8] A. Szyczewski, S. Lis, Z. Kruczynski, and S. But, J.Alloys Compd. 341 (2002), 307.

[9] S. But, Ph.D. Thesis, Adam Mickiewicz University,Poznan, Poland, 1999.

[10] R. Ballardini, E. Chiorboli, and V. Balzani, Inorg.Chim. Acta 95 (1984), 323.

[11] O. A. Serra, E. J. Nassar, G. Zapparolli, and I. L. V.Rosa, J. Alloys Compd. 207–208 (1994), 454.

[12] V. Bekieri, G. Pistolis, and P. Lianos, J. Non. Cryst.Solids 226 (1998), 200.

[13] Z. Stryla, S. Lis, and M. Elbanowski, Optica Appli-cata XXIII (1993), 163.

[14] M. S. Wrighton, D. S. Ginley, and D. L. Morse, J. Phys.Chem. 78 (1974), 2229.

[15] A. M. Klonkowski, S. Lis, M. Pietraszkiewicz, Z.Hnatejko, K. Czarnobaj, and M. Elbanowski, Chem.Mater. 15 (2003), 656.

[16] Y. Jeannin and J. Martin-Frere, Inorg. Synth. 27(1990), 71.

[17] R. D. Peacock and T. J. R. Weakley, J. Chem. Soc. A(1971), 1836.

[18] S. Lis and S. But, J. Inclusion Phenom. Molec. Recog-nition Chem. 35 (1999), 225.

[19] I. Creaser, M. C. Heckel, R. J. Neitz, and M. T. Pope,Inorg. Chem. 32 (1993), 1573.

[20] M. R. Antonio and L. Soderholm, Inorg. Chem. 33(1994), 5988.

[21] J. Bartis, S. Sukal, M. Dankova, E. Kraft, R. Kronzon,M. Blumenstein, and L. C. Francesconi, J. Chem.Soc., Dalton Trans. 11 (1997), 1937.

[22] S. Lis and S. But, J. Alloys Compds. 303–304 (2000),132.

[23] K. Czarnobaj, M. Elbanowski, Z. Hnatejko, A. M.Klonkowski, S. Lis, and M. Pietraszkiewicz, Spec-trochim. Acta A 54 (1998), 2183.

[24] Z. Hnatejko, A. M. Klonkowski, S. Lis, K. Czarnobaj,M. Pietraszkiewicz, and M. Elbanowski, Mol. Cryst.Liq. Cryst A. 354 (2000), 207.

[25] P. Judeinstein and H. Schmidt, J. Sol-Gel Sci. Tech.3 (1994), 189.

[26] G. Meinrath, S. Lis, S. But, and M. Elbanowski, Ta-lanta 55 (2001), 371.

238 Stefan Lis et al. Vol. 05

[27] G. Meinrath and S. Lis, Fresenius, J. Anal. Chem.369 (2001), 124.

[28] P. P. Barthelemey and G. R. Choppin, Inorg. Chem.23 (1989), 2044.

[29] G. Blasse, G. J. Dirksen, and F. Zonnevijlle, J. Inorg.,Nucl. Chem. 41 (1981), 2947.

[30] G. Blasse, Eur. J. Solid State Inorg. Chem. 28 (1991),719.

Submit your manuscripts athttp://www.hindawi.com

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014

International Journal ofPhotoenergy

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Carbohydrate Chemistry

International Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

Chemistry

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Advances in

Physical Chemistry

Hindawi Publishing Corporationhttp://www.hindawi.com

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

SpectroscopyInternational Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

Medicinal ChemistryInternational Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Chromatography Research International

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Applied ChemistryJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Theoretical ChemistryJournal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

Spectroscopy

Analytical ChemistryInternational Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Journal of

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Quantum Chemistry

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Organic Chemistry International

ElectrochemistryInternational Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

CatalystsJournal of