IR spectroscopic and DFT investigations on molecular conformations of thio-free oxo technetium (V) benzamidoxime complexes

Journal of Molecular Structure 990 (2011) 152–157

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Journal of Molecular Structure

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IR spectroscopic and DFT investigations on molecular conformations of thio-freeoxo technetium (V) benzamidoxime complexes

Khajadpai Thipyapong a,b, Tomoya Uehara b, Keisuke Suzuki b,c, Yasushi Arano b, Vithaya Ruangpornvisuti d,⇑a Department of Chemistry, Faculty of Science, Burapha University, Chonburi 20131, Thailandb Graduate School of Pharmaceuticals Science, Chiba University, 1-8-1 Inohana, Chuo, Chiba 260 8675, Japanc Research Center, Nihon Medi-Physics Co. Ltd., 3-1 Kitasode, Sodegaura-City, 299-0266 Chiba, Japand Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

a r t i c l e i n f o

Article history:Received 1 November 2010Received in revised form 20 January 2011Accepted 20 January 2011Available online 26 January 2011

Keywords:Technetium (V) complexBenzamidoximeBenzohydroxamamide (BHam)IR spectraConformational analysisDFT calculations

0022-2860/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.molstruc.2011.01.034

⇑ Corresponding author. Tel.: +66 2218 7644; fax: +E-mail address: [email protected] (V. Ruangpo

a b s t r a c t

Synthesized 99m/99gTc complex with benzohydroxamamide (BHam), 99m/99gTc/BHam2 characterized withHPLC equipped with UV–Vis and ESI–MS, and FTIR spectroscopic measurements were carried out. Thestructures of BHam conformers its complexes with Tc(V) (Tc/BHam2) computed using the density func-tional theory (DFT) calculations were obtained. Six conformers of Tc/BHam2 complexes based on theimino-, amino- and hydroxo-BHam tautomers as cis- and trans-conformers were found. The computedIR spectra of all the conformers of Tc/BHam2 complex were analyzed and compared with experimentalspectrum. The IR spectrum of the trans-Tc/amino-BHam2 which is the most stable conformer is in goodagreement with the measured spectrum.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Benzamidoxime or benzohydroxamamide (BHam) is a thiol-freebidentate ligand used in the preparation of 99mTc-labeled com-pounds for molecular imaging. Such the radioactive compoundsshow high stability and radiochemical yields even at low BHamconcentrations [1,2]. Two forms of the 99mTc-labeled compoundhave been previously reported and this usually limits applicationsof the labeled compound. On the other hand, tetradentate BHamligands, i.e. N,N0-ethylene bis(benzohydroxamamide) [(C2(BHam)2)]and N,N0-propylene bis(benzohydroxamamide) [(C3(BHam)2)] pro-vide a single form of the 99mTc-labeled compound. The resultingcompound exhibits high stability, with over 95% radiochemicalyields over a wide pH range at ligand concentration as low as2.5 � 10�6 M [3]. In addition, the C3(BHam)2-based bifunctionalchelating agent has been used to produce 99mTc-labeled antibodies,with high stability and high specific activity under mild conditions[4]. To date, characterization data of the 99mTc-labeled compoundsformed by either bidentate or tetradentate BHam derivatives arenot well-established. Our earlier efforts to prepare rhenium com-plexes as technetium model compound were unsuccessful. TheHartree–Fock and DFT methods were used to evaluate the

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66 2254 1309.rnvisuti).

structures of BHam and its oxo technetium (V) complexes [5].The computational results suggested that two hydroxyimino andten hydroxyamino conformers were formed via tautomerization.The trans-[L2TcO]+ complex was also found to the most stable con-former. Recently, crystallographic study reveals that BHam mole-cules are connected via intermolecular NAH� � �O and OAH� � �Nhydrogen bonds, forming a two-dimensional supramolecularstructure [6] due to tautomerization [7–9]. The tautomerizationoccurring through migration of amine proton to imine leads to aformation of imino conformer which has lower acidity thanoximino form [10–12]. Based on this formalism, the presence ofTc/BHam conformers would be feasible [13].

Liquid Chromatography–Mass Spectrometry (LC–MS), whichallows detection of very low concentrations (up to nanomolar),has been used extensively in the structural characterization of99mTc complexes. In addition, the use of LC–MS equipped withUV- and radiometric detectors permits simultaneous measure-ments of MS-, UV-, and radio-chromatograms of 99mTc complexes.According to preliminary studies on well-established 99mTc radio-pharmaceuticals, LC–MS provided the data that supported thehypothesized structure of 99mTc complexes [14–17]. In order towell-characterize the mass of the selected peaks, addition of carrier99gTc to 99mTc complexes is necessary.

In the present study, the chemical identity and stoichiometryof 99m/99gTc complex with BHam have been investigated at

K. Thipyapong et al. / Journal of Molecular Structure 990 (2011) 152–157 153

nano-molar levels using High Performance Liquid Chromatography(HPLC) equipped with radiodetector and electrospray ionizationmass spectrometry (ESI-MS). All conformations of Tc complex withBHam and their geometries have been investigated using densityfunctional theory (DFT) calculations. The DFT-computed IR spec-troscopic spectra of conformers of the Tc complex with BHam havebeen used to compare with the experimental data in order to con-firm the existing conformer of the Tc/BHam2 complex in thenature.

2. Experimental

2.1. General procedures

All reactions were performed under nitrogen atmosphere unlessotherwise noted. Solvents and chemicals obtained from commer-cial sources were of analytical grade or higher and were used with-out further purification. BHam was synthesized according to theprocedure of Bedford et al. with slight modification [18]. Sodium[99mTc]pertechnetate was eluted from a commercial 99Mo/99mTcgenerator (Nihon Medi-Physics Co., Ltd., Nishinomiya, Japan orMallinckrodt-Tyco Inc., Petten, Netherlands). In house freeze–dry,a stannous glucoheptonate kit, containing SnCl2�2H2O 0.015 mg(0.066 lmol) and sodium glucoheptonate 10 mg (40 lmol), wasused through out this experiment. The labeling reactions were per-formed for all solutions under N2 atmosphere. 99gTc is a weak b�

emitter (E = 0.292 MeV; t1/2 = 2.12 � 105 years) and may only behandled in laboratories approved for use of low-level radioactivity.Ammonium [99gTc]pertechnetate was purchased from Oak RidgeNational Laboratory (TN, USA). Tetrabutylammonium tetrachloro-oxotechnetate (TBA[99gTcOCl4]) was synthesized following thepublished procedure [19]. The infrared (IR) spectra were recordedusing a Perkin Elmer BX II IR spectrometer (Perkin Elmer, MA,USA).

2.2. Preparation of 99m/99gTc-carrier for mass analysis

Na[99mTcO4] was eluted from 99MoA99mTc generator. The timeinterval from the preliminary elution to the elution in use was7 days in order to increase the amount of 99gTc carrier. Approxi-mately 1 mL of generator eluate containing ca. 1 GBq of Na[99mTc-O4] was added to a mixture consisting of glucoheptonate kit. 99mTclabeled glucoheptonate was obtained after 30 min at room temper-ature. 99m/99gTc labeled glucoheptonate (50 lL) was added to 50 lLof BHam (8.8 � 10�1 mM) in an acetate buffer (100 mM, pH 4.0)and 99m/99gTc/BHam was obtained after 1 h at room temperature.RP–HPLC–LCMS was carried out using a C18 Develosil ODS-UG-5column (4.6 � 250 mm, Nomura Chemical, Ltd., Seto, Japan). Themobile phase was a mixture of acetonitrile (A) and 25 mM ammo-nium acetate at pH 6.7 (B). The mobile phase condition for 99m/

99gTc/BHam was linear gradient; starting from 30% A and 70% Bto 40% A and 60% B for 20 min. Twenty percents of the HPLC eluate(0.2 mL/min) was injected to the MS part. The electrospray param-eters were as follows: capillary voltage = 3.5 kV, drying gas (nitro-gen) flow = 600 L h�1, drying temperature = 150 �C, and sourcetemperature = 80 �C, and cone voltage = 30 V. Mass spectra weredetected using a positive ion mode with a mass range of m/z 80–1000.

2.3. Synthesis of 99gTc/BHam2 complex

99gTc/ethyleneglycol was prepared and used as the precursor.Ten drops of triethylamine were added to a mixture of TBA[99gTc-OCl4] (160 mg, 320 lmol) and ethyleneglycol (10 drops) in THF(7 mL). A violet solution of 99gTc/ethyleneglycol was obtained

and chloride salt removed by filtration. Afterwards, a solution ofBHam (82 mg, 640 lmol, 2 eq) in THF (15 mL) was added dropwiseto the 99gTc/ethyleneglycol solution. The reaction mixture was keptstirring for 3 h under N2 atmosphere. The solution changed itscolor from violet to deep orange during the reaction. The solventwas removed in vacuo and the residue dissolved in methanol(2 mL). By H2O (2 mL) treatment, brown solid (65.1 mg, 52.9%)was precipitated. The RP–HPLC experiment was carried out usinga Nucleosil 100-5 C18 column (3 � 250 mm2, Macherey-NagelAGC, Oensingen, Switzerland) at a flow rate of 0.5 mL/min, with agradient mobile phase starting from 100% A (50 mM triethylam-monium phosphate buffer pH 2.5) and 0% B (acetonitrile) to 0% Aand 100% B at 30 min. The eluent was detected using an onlineultraviolet detector (250 nm) and a radiodetector (Model LB 508,EG &G BERTHOLD LB, Regensdorf, Switzerland).

3. Computational methods

All calculations were performed using the GAUSSIAN 03 soft-ware package [20]. GaussView 3.07 [21] was utilized to buildmolecular structures, display molecular geometry convergenceand generate molecular graphics of all related species. The fullgeometry optimizations of all complexes were carried out usingdensity functional theory (DFT) with the Becke’s three parameterhybrid functional (B3) with the Lee–Yang–Parr correlation func-tional (LYP) called as B3LYP [22,23] with the basis set LANL2DZ[24]. For a comparison, the two-layered ONIOM hybrid method[25,26] of calculations was also attempted. The two-layeredONIOM(B3LYP/6-311++G(d,p):B3LYP/LANL2DZ) approach appliedon the Tc/BHam2 complex conformers, two molecules of BHamligand in the coordination system and Tc (V) ion were treated asthe high and low levels of theory, respectively.

All the optimized structures, vibrational frequencies were sub-sequently computed and the vibrational modes were assigned withthe aid of GaussView. All computed IR spectra were plotted usingthe GaussSum 2.2 [27].

4. Results and discussion

4.1. Mass spectrometry of 99gTc complex with BHam

BHam ligand was prepared on a 0.10-molar scale. The crudeproduct was recrystallized from benzene/hexane, giving 9.69 g(57%) of white solid, mp 70–71 �C (lit. 70–71 �C) [18]. The99g/99mTc complex with BHam was synthesized according toligand exchange from 99g/99mTc/glucoheptonate. The radio-chromatograms of 99g/99mTc complex with BHam under the RP–HPLC–LCMS conditions is shown in Fig. S1b, Supplementary data.The two peaks of 99mTc detect by c-ray detector at Rt = 5.72 min(A) and 9.56 min (B) was found in qualitative agreement withthe previous study [1]. The former UV detector in Fig. S1a showspeak at Rt = 5.67 min (A) and 9.50 min (B) while the later massspectrometer shows peak on total mass chromatogram atRt = 6.02 min (A) and 9.82 min (B) see Fig. S1(c).

Total mass chromatogram, peaks A and B provided mass spectraas shown in Fig. S2a and b. Both spectra were shown most inten-sive signals at m/z of 385 of the formula C14H14N4O3

99Tc+ (MW.384) without solvent molecule participating in the technetiumcoordination. Significant fragments of the complex were observedat m/z = 354.01, 251.1, and 137.1 for C14H13N3O2

99Tc+,C7H8N2O2

99Tc+, and C7H9N2O+, respectively. The MS spectra of[M + H]+ ions show that the most favored decomposition was dueto the cleavage of the hydroxyamine CAN bond, resulting in a cat-ionic species with m/z = 354.18. This ion exhibits further loss ofbenzonitrile, causing the ions at m/z = 251 in accordance with the

Table 1IR vibrational absorption modes of BHam ligand and 99gTc/BHam2 complex.

Vibrational wave number (m, cm�1) Assignment for vibrational modes

BHam ligand Tc/BHam2 complex

3300–2500 (br) 3300–2500 (br) OAH stretch3453, 3360 (s) – NAH stretch1648 (s) 1635 (w) C@N stretch1558 (s) 1558 (w) NAH bend primary amine– 1549 (m) C� � �N stretch– 1480 (m) C� � �N stretch1449 (m) 1443 (m) CAH bending aromatic1386 (s) 1366 (s) CAN stretch– 948 (s) Tc@O stretch926 (s) 926 (m) N@O stretch817 (m) – CAH aromatic out-of-plane bending773 (m) 773 (m) CAH aromatic out-of-plane bending691 (s) 695 (s) CAH aromatic out-of-plane bending– 643 (s) TcAN stretch583 598 NAH out-of-plane bending– 558 TcAO stretch506 493 NAH out-of-plane bending

154 K. Thipyapong et al. / Journal of Molecular Structure 990 (2011) 152–157

mechanism shown in Fig. 1. These results illustrate that 99m/99gTccomplex with BHam is formed a conformer with mole ratio 1:2as [TcO(L�2)(LH�1)], where the neutral BHam is denoted as LH2

and the complex is therefore denoted as Tc/BHam2.The result was supported the earlier speculation for zero charge

and cis–trans conformer of 99mTc complex with BHam [1,2] andits stoichiometry was similar to trans-[Ni(BHam)2] and trans-[Co(BHam)2] complex [28].

4.2. Infrared spectroscopy of 99gTc complex with BHam

The synthesized complex was soluble only in dimethyl sulfoxide(DMSO) and 0.1% trifluoroacetic acid (TFA)/methanol. Therefore,99gTc complex in 0.1% TFA/methanol solution was prepared andanalyzed by the RP–HPLC eluted with triethylammonium phos-phate buffer pH 2.5 and acetonitrile under the gradient conditions.The retention time for 99gTc complexes with BHam were at 19.45and 25.82 min. They were similar to 99mTc complexes with BHam(21.03 and 26.92 min) monitored by a c-radioactive detector. Thissuggests that the structures of 99gTc complexes were equivalent tothose of 99mTc-labeled compounds prepared at very low techne-tium concentration. Vital IR absorption modes for the BHam ligandand its Tc complexes are listed in Table 1. The IR spectrum of 99gTccomplexes with BHam in Fig. 2 shows a vital band at 948 cm�1

attributable to Tc@O double bond which does not exist in thefree BHam ligand. This value is comparable to those observed forother oxo complexes of Tc containing r- and p-donating ligands[29,30].

Owing to the functional groups in BHam, the other bands werealso observed. The broad and intense band observed in 3300–2500 cm�1 as the stretching vibrations of the OAH groups suggestthe deposition of solvent (H2O) on complex to form hydrogenbonds. Significantly, the characteristic strong bands of C@N,C� � �N and NAH at 1648 and 1588 cm�1 in free BHam ligand weredecreased to weak band at 1635 and 1558 cm�1 while absorptionof CAN bond observed at 1386 cm�1 was shifted to 1366 cm�1

and increased to strongest band in Tc/BHam complex. The assignedvibrational modes for C� � �N in chelate ring appearing at 1549 and1480 cm�1 were found as the new bands.

Fig. 1. Proposed ionization mechanism for molecular i

In addition, broad medium bands of the BHam ligand in range690 to 400 cm�1 are found to be the new bands with slightly higherwave number and sharp-edged. The new two IR peaks of Tc com-plex with BHam at 643 and 558 cm�1 corresponding to TcANand TcAO asymmetric stretching modes, respectively, as shownin Fig. 4.

4.3. Geometry optimization of Tc/BHam2 complex

The initial cis and trans geometries of square-pyramidal struc-ture of TcO3+ complexes were built based on the formula[TcO(L�2)(LH�1)] and previous computational study [5,31]. Possi-ble mechanism of interconversion reaction of three conformers ofTc/BHam2 complex, due to their intramolecular proton transfer ofthree isomers of one of BHam ligand as imino-, amino- and hydro-xo-BHam is shown in Scheme 1. These three isomers were used tospecify type of Tc/BHam2 complex conformers. Due to the DFT-optimized structures of all the Tc/BHam2 complex conformers,

ons of positive mode ESI of the 99m/99gTc/BHam2.

Fig. 2. IR spectra of BHam ligand (dot line) and 99gTc/BHam2 complex (solid line).

Scheme 1. Possible mechanism of interconversion reaction of three conformers ofTc/BHam2 complex, due to their intramolecular proton transfer of three isomers ofone of BHam ligand as: (a) imino-, (b) amino- and (c) hydroxo-BHam.

K. Thipyapong et al. / Journal of Molecular Structure 990 (2011) 152–157 155

two BHam molecular ions coordinate to the Tc(V) can thereforeform cis and trans conformations with six conformers. The sixB3LYP/LANL2DZ-optimized structures of Tc/BHam2 conformersnamely cis-Tc/imino-BHam2 (CI), cis-Tc/amino-BHam2 (CA), cis-Tc/hydroxo-BHam2 (CH), trans-Tc/imino-BHam2 (TI), trans-Tc/amino-BHam2 (TA) and trans-Tc/hydroxo-BHam2 (TH) and theirrelative energies compared with the most stable conformer (TA)

Fig. 3. The structures of six conformers of Tc/BHam2, (a) cis-Tc/imino-BHam2 (CI), (b) ci(TI), (e) trans-Tc/amino-BHam2 (TA) and (f) trans-Tc/hydroxo-BHam2 (TH). Relative energLANL2DZ) (in parentheses) are in kcal/mol.

computed at the B3LYP/LANL2DZ and ONIOM(B3LYP/6-311++G(d,p):B3LYP/LANL2DZ) levels are shown in Fig. 3. Thesestructures are configurationally stabilized by intramolecularhydrogen bonds due to their imino, amino and hydroxo groups.

The stabilities of six conformers, in combination withthe B3LYP/LANL2DZ and ONIOM(B3LYP/6-311++G(d,p):B3LYP/LANL2DZ) computations, are in order: TA > CA > CH � TH �TI� CI. The most stable conformer (TA) is more stable than thesecond most stable (CA) by 4.70 kcal/mol (11.28 kcal/mol), atthe B3LYP/LANL2DZ and ONIOM(B3LYP/6-311++G(d,p):B3LYP/LANL2DZ) (in parenthesis) levels.

The reactivities for all the six conformers of Tc/BHam2 complexbased on their B3LYP/LANL2DZ energy gaps as shown in Table S1are in order: CI � CA � TA > TH > TI � CH. Nevertheless, energygaps for all the six conformers of Tc/BHam2 complex are quite highand within the range of 3.30–3.51 eV. Therefore, these six con-formers of Tc/BHam2 complex should be not the active forms.

The selected geometrical parameters for conformers ofTc/BHam2 complex, computed at the B3LYP/LANL2DZ level oftheory was listed in Table 2. It shows that all Tc bonds namely

s-Tc/amino-BHam2 (CA), (c) cis-Tc/hydroxo-BHam2 (CH), (d) trans-Tc/imino-BHam2

y (DE) optimized at the B3LYP/LANL2DZ and ONIOM(B3LYP/6-311++G(d,p): B3LYP/

Table 2Selected geometrical parameters for conformers of Tc/BHam2 complex, computed at the B3LYP/LANL2DZ level of theory.

Conformers TIa TAb THc CId CAe CHf

Bond length (A�)TcAO1 1.706 1.706 1.706 1.707 1.706 1.705TcAO2 2.000 2.034 2.079 2.002 2.036 2.060TcAO3 1.949 1.959 1.945 1.945 1.957 1.955TcAN1 2.164 2.040 1.991 2.180 2.050 2.004TcAN3 1.942 1.953 1.966 1.960 1.961 1.949

Bond angle (�)O2ATcAO1 115.79 111.02 108.36 115.68 111.33 111.05O3ATcAO1 114.45 109.20 106.79 109.42 111.15 113.82N3ATcAO1 110.79 112.56 113.72 112.65 109.41 107.79N1ATcAO1 111.77 114.24 115.47 107.77 111.26 113.62O2ATcAO3 129.73 139.76 144.31 83.70 84.79 84.66O3ATcAN3 77.95 77.65 77.64 77.84 77.56 77.81N3ATcAN1 137.37 133.12 130.71 91.12 91.05 92.44N1ATcAO2 77.71 77.72 74.02 77.31 77.55 74.23N1AC1AN2 117.50 114.19 121.48 117.29 114.30 121.80N3AC4AN4 117.53 117.48 117.52 117.14 117.26 105.25

Dihedral angle (�)C1AN2AO2ATc 9.17 11.06 9.15 10.12 10.77 6.39C4AN4AO3ATc 10.66 10.70 11.75 �12.75 �11.04 �9.21N1AC1AC2AC3 �176.85 143.21 149.76 177.59 140.44 148.28N3AC4AC5AC6 �22.72 �22.36 �23.47 �155.58 �156.33 �155.43

a trans-Tc/imino-BHam (TI).b trans-Tc/amino-BHam (TA).c trans-Tc/hydroxo-BHam (TH).d cis-Tc/imino-BHam (CI).e cis-Tc/amino-BHam (CA).f cis-Tc/hydroxo-BHam (CH).

156 K. Thipyapong et al. / Journal of Molecular Structure 990 (2011) 152–157

TcAO1, TcAO2, TcAO3, TcAN1 and TcAN3 bonds for all the con-formers are quite the same length. The N3ATcAN1 bond anglecan indicate conformers of Tc/BHam2 complex. The N3ATcAN1bond angle for trans- and cis-conformers are within the ranges of130.71–137.37� and 91.05–92.44�, respectively.

4.4. Vibrational frequency calculations

The IR spectra in the region of 1800–400 cm�1 for all the Tc/BHam2 conformers obtained using the ONIOM(B3LYP/6-311++G(d,p):B3LYP/LANL2DZ) computations are shown in Fig. S3.The computed IR spectra of the trans-Tc/amino-BHam2 (TA) whichis the most stable conformer is in good agreement with the ob-served spectrum. The IR spectra in the region of 1100–400 cm�1

for the Tc/BHam2 complex measured in solid phase and the TA con-former obtained from the ONIOM(B3LYP/6-311++G(d,p):B3LYP/LANL2DZ) computation are shown in Fig. 4. It shows that Tc@O

Fig. 4. IR spectra in the region of 1100–400 cm�1 for: (a) Tc/BHam2 complexmeasured in solid phase by KBr disk, (blue line) and (b) TA conformer obtainedusing the ONIOM(B3LYP/6-311++G(d,p):B3LYP/LANL2DZ) computation (red line).(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

symmetric stretching, TcAN and TcAO asymmetric stretchingmodes corresponding with the observed spectrum peaks at 948,643 and 558 cm�1, respectively are in agreement with theONIOM(B3LYP/6-311++G(d,p):B3LYP/LANL2DZ)-computed spectraas shown in Fig. 4.

5. Conclusion

Benzohydroxamamide (BHam) and 99m/99gTc complexes withBHam were synthesized and characterized with HPLC equippedwith UV–Vis and ESI–MS, and their FTIR spectroscopic measure-ments. It was concluded that the Tc complex with BHam ligandis formed as a conformer with mole ratio 1:2 as [TcO(L�2)(LH�1)],(denoted as Tc/BHam2). The six conformers of Tc/BHam2 obtainedusing DFT calculations as cis-Tc/imino-BHam2 (CI), cis-Tc/amino-BHam2 (CA), cis-Tc/hydroxo-BHam2 (CH), trans-Tc/imino-BHam2

(TI), trans-Tc/amino-BHam2 (TA) and trans-Tc/hydroxo-BHam2

(TH) were obtained and the TA is the most stable conformer ofwhich structure was characterized by the observed and DFT-computed IR spectroscopic spectra.

Acknowledgments

We gratefully acknowledge support of this work by Japan Soci-ety for the Promotion of Science (JSPS) Grants No. NRCT-10928.This work was supported in part by a Grant-in-Aid for ScientificResearch (B) and for Exploratory Research, and Special Funds forEducation and Research (Development of SPECT Probes for Phar-maceutical Innovation) from the Ministry of Education, Culture,Sports, Science and Technology, Japan. The authors are also gratefulto Professor Roger Alberto and his colleagues at Institute of Inor-ganic Chemistry University of Zürich, Zürich Switzerland for syn-theses and IR measurements of 99gTc complexes. The authorsacknowledge National Electronics and Computer TechnologyCenter, National Science and Technology Development Agency

K. Thipyapong et al. / Journal of Molecular Structure 990 (2011) 152–157 157

for providing computing resources that have contributed to the re-search results reported within this paper.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molstruc.2011.01.034.

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