Molecular Modeling of 4-Ethyl-1-(pyridin-2-yl)thiosemicarbazide and its Cu(II) Complex Utilized for Separation of Cu(II) from Water Samples Mohamed M. Hassanien, † Mohamed. H. Abdel-Rhman, ‡ Issam M. Gabr, ‡ and Ahmed A. El-Asmy* ,‡ Chemistry Department, Industrial Education College, Beni-Suef University, and Chemistry Department, Faculty of Science, Mansoura University, Egypt Molecular modeling and spectroscopic characterization of 4-ethyl-1-(pyridin-2-yl) thiosemicarbazide (HEPTS) and its complex [Cu 4 (EPTS) 3 (H 2 O)(EtOH)(OH)Cl 4 ] 2 H 2 O are investigated. Structural, electronic, and vibrational features are discussed, and assignments were proposed on the basis of semiempirical quantum mechanics calculations (ZINDO/S and PM3). The simulated IR and UV-vis spectra are in reasonable accordance with the experimental data. Moreover, a rapid and selective procedure for extraction of Cu(II) at trace levels using silica gel immobilized with 4-ethyl-1-(pyridin-2-yl)thiosemicarbazide is proposed. The Cu(II)-EPTS complex was easily adsorbed on silica gel and eluted with 5.0 M HNO 3 . The pH, amount of HEPTS, sample volume, and detection limit were evaluated in synthetic, spiked, and tap water samples. The interfering effect of some cations and anions on the recovery of Cu(II) was studied. Under optimum conditions, the recovery of Cu(II) ions was (97 ( 1) % at the 95 % confidence level. The adsorption isotherm was studied, and the binding equilibrium constant and the adsorption capacity were 3.585 L · mg -1 and 520.8 μg · g -1 , respectively. The detection limit and relative standard deviation were 4 ng · L -1 and 1.5 %, respectively. The proposed method was applied for the separation of Cu(II) ions from water samples. Introduction Thiosemicarbazides are interesting because they form highly stable and intensely colored complexes that are used for the spectrophotometric determination of metal ions in different media 1-5 and show catalytic activity. 6,7 Also, they possess antibacterial and anticancer activity. 8,9 4-Ethyl-1-(pyridin-2- yl)thiosemicarbazide and its Cu(II) complexes with different anions have been synthesized and characterized 10 and used as a chelating agent for separation, preconcentration, and deter- mination of Cu(II) ions in saturated saline solutions by a cloud point extraction technique. 11 The high activity of many thiosemicarbazides to form stable complexes in solid and solution prompted us to use 4-ethyl-1- (pyridin-2-yl)thiosemicarbazide (HEPTS) as a chelating agent and silica gel as solid support for the separation, preconcen- tration, and determination of Cu(II) from water samples. Silica gel immobilized with various organic compounds has received great attention. 12-24 Solid phase extraction (SPE) is considered to be a powerful tool for the separation and enrichment of various inorganic and organic analytes. 25 The basic principle of SPE is to transfer the analyte from the aqueous mobile phase to bind to the active sites of the adjacent solid phase. SPE has several advantages such as high preconcentration factor, easiness of separation, enrichment under dynamic conditions, no need for organic solvents, and low consumption of reagents. Selective solid-phase extractors have been prepared by either physical loading or chemical binding of selected chelating reagents to different supports such as cellulose, fibers, activated carbon, sand, clay, zeolites, polymers, metal oxides, and highly dispersed silica. 25 1,5-Bis(di-2-pyridyl)methylene thiocarbohydrazide immobi- lized on silica gel was used for the determination of Pd(II) and Pt(IV) by a flow injection technique with a detection limit of 0.4 ng · mL -1 for Pd(II) 26 and 0.8 ng · mL -1 for Pt(IV). 27 The method has been applied for the determination of platinum in catalysts, vegetation, soils, and water samples 27 and Pt(IV) in spiked foods and beverages. 28 Separation, preconcentration, and determination of trace amounts of Cu(II) ions in aqueous solutions quantitatively during the passage of its solution through tetradecyldimethylbenzyl ammonium chloride (TDMBAC) treated with analcime pyro- catechol-immobilized zeolite were developed. The retained Cu(II) ions were desorbed with 5.0 mL of 4.0 M nitric acid and determined by flame atomic absorption spectrometry (FAAS) with a correlation coefficient of 0.9987. The detection limit was 0.05 ng · mL -1 . Copper was determined in alloys with a recovery of 97 %. 29 In a continuation of our pervious work, the molecular modeling of 4-ethyl-1-(pyridin-2-yl)thiosemicarbazide and its Cu(II) complex as well as the vibrational and electronic spectra simulated using semiempirical theoretical methods are described. * Corresponding author. Tel: +20 101645966. E-mail: aelasmy@yahoo.com. † Beni-Suef University. ‡ Mansoura University. Table 1. Calculated Frequencies (cm -1 ) of HEPTS and its Cu(II) Complex in the Gaseous State and Their Observed IR Spectra in KBr as well as Their Vibrational Assignment HEPTS HEPTS PM3 Cu(II) complex Cu(II) complex PM3 cm -1 cm -1 cm -1 cm -1 assignment 3443, 3365 3631 ν(OH) solv 3250 3350 3257 3353 ν(N 4 H) 3153 3255 3163 3175 ν(N 1 H) 1640 1641 ν(CdN 2 ) 1604 1745 1611 1746 ν(CdN) Py 1518, 1446 1525, 1496 1530, 1455 1512, 1484 thio I, III 800 842 ν(CdS) 772 772 778 781 F(NH) J. Chem. Eng. Data 2009, 54, 1277–1283 1277 10.1021/je800802c CCC: $40.75  2009 American Chemical Society Published on Web 03/19/2009