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On the thermal properties of metal (II) complexes of chalcone
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Materials Letters 62 (2008) 852 – 856 www.elsevier.com/locate/matlet
On the thermal properties of metal (II) complexes of chalcone J. Meena Devi a,⁎, P. Tharmaraj b , S.K. Ramakrishnan b , K. Ramachandran a a b
School of Physics, Madurai Kamaraj University, Madurai-625021, Tamilnadu, India Department of Chemistry, Thiagarajar College, Madurai-625009, Tamilnadu, India Received 23 May 2007; accepted 29 June 2007 Available online 5 July 2007
Abstract The preparation of chalcone ligand and complexes of zinc, cadmium, mercury with chalcone is described. Thermogravimetric measurements made on these samples show ligand to be more stable than the metal complexes as it exhibits higher initial decomposition temperature. Thermal diffusivity of these four samples is measured by photoacoustic technique at room temperature to find the influence of coordination of metal on the thermal properties of chalcones. Experimental results indicate that the thermal diffusivity of chalcone metal complexes is enhanced when compared to the parent ligand. If we consider the free electron density of metals, then the increase may be understood from the electronic contribution of metals to the thermal property. So among metal complexes, thermal diffusivity increase with the increase in the free electron density of the metal ion coordinated. © 2007 Elsevier B.V. All rights reserved. Keywords: Chalcone; Metal complex; Thermal properties; Photoacoustics
1. Introduction Chalcones [1] are the precursors of flavonoids and antiflavonoids, which are available in plenty in ferns and higher plants. These flavonoids could be identified in fruits and vegetables, using photoacoustic (PA) technique [2]. This way photocoustic technique is becoming a powerful tool for biomaterials particularly in photochemistry and photobiology. Chemically the chalcones consist of two aromatic rings with an unsaturated chain. Chalcone compounds have displayed wide array of pharmacological activities [3–6] such as anticancer, anti-inflammatory, cytotoxic in vitro, bactericidal, insecticidal, antifungal, antioxidant, anti-HIV and phytoestrogenic activities. Apart from its use in medical science, some chalcone derivatives [7] exhibit second harmonic generation (SHG), and hence they are also used in non-linear optical (NLO) applications. Already, synthesis of some metal complexes of chalcone and their biological activities are available in literature; for example, chalcone–tellurium complexes [8] are active in the inhibition of E. coli. Ru(II)/ Ru(III) polypyridyl complexes containing 2,6(2′-Benzimidazolyl)-pyridine/chalcone as co-ligand [9] have been found to exhibit anti-HIV and cytotoxic activities. ⁎ Corresponding author. Fax: +91 452 245918. E-mail address: [email protected] (J.M. Devi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.06.077
For the physical stability of the drug and its compatibility with potential excipients, thermal analysis plays a major role [10]. In this regard, we had measured the thermal diffusivity of cinnamoyl chalcones and difurans [11] by photoacoustics where it is found that the thermal diffusivity increases with the increase in the number of methoxy groups conjugated. In the present work, thermal properties of chalcone ligand and its three metal (Zn, Cd, Hg) complexes are investigated using photoacoustic (PA) technique. 2. Synthesis of chalcone ligand About 6.8ml of o-hydroxy acetophenone is mixed with 5.3ml of benzaldehyde in a conical flask, kept in a cold water bath. Around 20ml of ethanol is used as the solvent. Then 20ml of 40% NaOH is added dropwise with constant stirring until yellow solid product results. The product is then kept aside for 24h. This is neutralised with 20ml of 1:1 glacial acetic acid. As a result of this neutralization, a fine yellow precipitate (chalcone) is obtained. 3. Synthesis of zinc, cadmium and mercury complexes About 2.24g of the chalcone ligand is dissolved in alcohol. For preparing zinc complex, zinc chloride is dissolved in water and added to the ligand solution. Likewise for preparing cadmium
J.M. Devi et al. / Materials Letters 62 (2008) 852–856
853
molecular structure of chalcone ligand and its metal complexes is shown in Fig. 1. 4. IR spectra
Fig. 1. Molecular structure A) Chalcone ligand (C15H12O2) B) Chalcone metal complex.
complex, cadmium sulphate is dissolved in water and added to the ligand solution. Similarly to prepare mercury complex, anhydrous mercuric chloride is dissolved in water and added to the ligand solution. Then sufficient amount of NaOH is added for proper complex formation and the pH of 8 to 9 is maintained. The
The IR spectra (Fig. 2) of the synthesized ligand and complexes are recorded using Jasco Double beam Infrared Spectrophotometer in the range 400–4000cm− 1 by KBr pellet technique. The characteristic stretching frequencies observed are consistent with the functional groups present in the chalcone and the chalcone metal complexes. In the ligand and complexes, the IR absorption peaks at 1638 and 1640cm− 1 are assigned to C=O stretching vibration; absorptions in the region 1571, 1572, 1574cm− 1 are attributed to aromatic C=C stretching vibration; absorptions at 1339, 1340, 1368, 1369cm− 1 are assigned to C– O stretching vibration. Vibrations in the range 618, 621cm− 1 observed in the metal complexes indicate the presence of M–O bond (M-metal) in the molecules, which are not present in the original ligand. 5. Thermogravimetric measurement (TG) Thermogravimetric measurements are made from ambient to 800°C using STA 409C NETZCH-Geratebau GmbH thermal
Fig. 2. IR spectra A) Chalcone ligand B) Zn-complex C) Cd-complex D) Hg complex.
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J.M. Devi et al. / Materials Letters 62 (2008) 852–856
Fig. 3. TG curve A) Chalcone ligand B) Zn-complex C) Cd-complex D) Hg complex.
analyser with alumina as the reference material. These are given in Fig. 3. Temperature is increased in steps of 5°C. Thermal decomposition of ligand occurs in the temperature range 240– 395°C. Zinc complex decomposes in the temperature range 180–340°C, cadmium complex in the range 110–320°C and mercury complex in the range 160–280°C. Thermal analysis on these samples by TG measurements show ligand to be more stable than the complexes as it exhibits higher initial decomposition temperature. Arshad et al. [12] have reported from their thermal analysis on the 1, 2-dipiperidinoethane complexes of cobalt, nickel, copper, zinc and cadmium that the coordination of metal ion into ligand is the reason for the weakening of the system. 6. Thermal diffusivity Thermal diffusivity measurements are performed using photoacoustic technique. Chalcone samples for PA measurement are taken in pellet form. White light from 450W Xenon lamp chopped by an electromechanical chopper is focused onto the sample placed inside the photoacoustic cell. Photoacoustic signal generated is detected by a microphone and then fed to the
lock-in-amplifier for measurements. Photoacoustic signal is recorded as a function of chopping frequency and the log–log plots of photoacoustic amplitude vs chopping frequency of the all four samples are shown in Fig. 4. Thermal diffusivity (α) [13] is then calculated from the characteristic frequency (fc) of the sample (characteristic frequency is defined as the frequency at which the sample goes from thermally thick region to thermally thin region) where slope change is observed in Fig. 4 using the following relation a ¼ fc l 2
ð1Þ
where l is the thickness of the sample. Thermal diffusivity values thus determined by PA technique at room temperature are given in Table 1. 7. Results and discussions Thermal diffusivity of chalcone ligand and its metal complexes show systematic variation (Table 1) due to the substitution of metal in chalcone. Here the observed thermal diffusivity of zinc complex is higher than cadmium complex and which is higher than mercury complex. This suggests that thermal diffusivity of these complexes
J.M. Devi et al. / Materials Letters 62 (2008) 852–856
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Fig. 4. Log–log plot of photoacoustic amplitude vs chopping frequency for A) Chalcone ligand B) Zn-complex C) Cd-complex D) Hg-complex.
increase with the decrease in the mass and increase in the free electron density of the metal ion coordinated. This may be explained from the free electron theory and phonons in metals. Even though zinc (MZn = 65.37 amu), cadmium(MCd = 112.40 amu) and mercury(MHg = 200.59 amu) belong to group II, the free electron density of zinc (13.2 × 1022/cm3) is higher than cadmium(9.27 × 1022/cm3) and mercury(8.65 × 1022/cm3). So the increase may be understood from the electronic contribution of thermal conductivity. When the mass and atomic size of the metal ions increase, the mean free path would decrease and so a reduction in thermal conductivity among the metal complexes is expected in the sequence of Zn, Cd and Hg complex. Since this being the first report for these chalcone metal complexes we have compared our results with other organic compounds (Table 1) benzophenone [14] and amylopectin [15] as for the order of the thermal diffusivity is considered. The present measurements on metal-chalcone complexes show the same order for thermal diffusivity. A search through the literature reveals that very few PA studies have been carried out on chalcone metal complexes. Sankara Raman et al. [16] have measured the thermal diffusivity of CoL2X2 and CuL2X2 (where L = 1-nitrobenzyl-2-nitrophenyl benzimidazole (NBPBI) and X = Cl, Br, I) using photoacoustic technique. Thermal diffusivity increases with Cl, Br and I in CoL2X2 complex but decreases in CuL2X2 for the same X = (Cl, Br, I). This shows that the thermal diffusivity mainly depend on the ligand, metal and halogen part. Moreover the side chain in this basic ligand is more compared to the present ligand. So, the thermal diffusivity may be higher in benzimidazole complex when
compared to present chalcone complex. In our previous work [11] on chalcones, we have showed an increase in thermal properties as the side chain increases. The effect of metal ion on the thermo physical parameter has been observed in organic composites also. Rusu et al. [17] have reported that the incorporation of zinc powder in high-density polyethylene (HDPE) composite increases the thermal diffusivity. Duncan et al. [18] have observed improved heat transport in polytetrafluoroethylene (PTFE) composite on incorporating it with aluminium flakes. Weidenfeller et al. [19] have reported that by the addition of metal and oxide particles to plastics, thermal transport properties can be varied systematically. Hence in the present work also like reported in literature for other organic systems, introduction of metal in chalcone has influenced and enhanced the thermal diffusivity.
Table 1 Thermal diffusivity of chalcone ligand and its complexes Author
Sample
Thermal diffusivity (10− 7 m2/s)
Present work q q q Ref.[14] Ref.[15]
Ligand–chalcone Zn-complex Cd-complex Hg-complex Benzophenone Amylopectin
5.8 9.0 8.1 7.0 1.8 1.3
856
J.M. Devi et al. / Materials Letters 62 (2008) 852–856
8. Conclusion Thermal diffusivity of chalcone metal complexes is enhanced when compared to the parent ligand. If we consider the free electron density of metals, then the increase may be understood from the electronic contribution of thermal conductivity. So the thermal diffusivity values observed among the Zn, Cd, Hg complexes increase with increase in the free electron number density of metal ion coordinated. References [1] Yerra Koteswara Rao, Shih-Hua Fang, Yew-Min Tzeng, Bioorg. Med. Chem. 12 (2004) 2679–2686. [2] R.J.S. Lima, A.S. Vasconcelos, J.F. Suassuna, J. Phys. IV 125 (2005) 51–53 France. [3] Jiu-Hong Wu, Xi-Hong Wang, Yang-Hua Yi, Kuo-Hsiung Lee, Bioorg. Med. Chem. Lett. 13 (2003) 1813–1815. [4] Simon Feldbaek Nielsen, Thomas Boesen, Mogens Larsen, Kristian Schonning, Hasse Kromann, Biorg. Med. Chem. 12 (2004) 3047–3054. [5] Ohad Nerya, Ramadan Musa, Soliman Khatib, Snait Tamir, Jacob Vaya, Phytochem. 65 (2004) 1389–1395. [6] Guray Saydam, Hakan Aydin, Fahri Sahin, Ozlem Kucukoglu, Ercin Erciyas, Ender Terzioglu, Filiz Buyukkececi, Serdar Bedii Omay, Leuk. Res. 27 (2003) 57–64.
[7] Vincent Crasta, V. Ravindrachary, R.F. Bhajantri, Richard Gonsalves, J. Cryst. Growth 267 (2004) 129–133. [8] Lallan Mishra, Ragini Sinha, Hideji Itokawa, Kenneth F. Bastow, Yoko Tachibana, Yuka Nakanishi, Nicole Kilgore, Kuo-Hsiung Lee, Bioorg. Med. Chem. 9 (2001) 1667–1671. [9] W.E. Rudinski, T.M. Aminabhavi, Inorg. Chim. Acta 70 (1983) 175–178. [10] D.Q.M. Craig, V.L. Kett, C.S. Andrews, P.G. Royall, J. Pharm. Sci. 91 (2002) 1201–1213. [11] J. Meena Devi, K. Sithik Ali, V.R. Venkatraman, S.K. Ramakrishnan, K. Ramachandran, Thermochim. Acta 438 (2005) 29–34. [12] Muhammad Arshad, Saeed-ur Rehman, Shad Ali Khan, Khalid Masud, Nasima Arshad, Abdul Ghani, Thermochim.Acta 364 (2000) 143–153. [13] K.N. Madhusoodanan, J. Philip, Phys. Status Solidi, A Appl. Res. 108 (1988) 775–782. [14] B. Bonno, J.L. Laporte, Y. Rousset, J. Appl. Phys. 67 (1990) 2253–2256. [15] P. Rodriguez, G. Gonzalez de la Cruz, J. Food Eng. 58 (2003) 205–209. [16] Sankara Raman, V.P.N. Nampoori, C.P.G. Vallabhan, N. Saravanan, J. Mater. Sci. Lett. 15 (1996) 230–231. [17] Mihai Rusu, Nicoleta Sofian, Daniela Rusu, Polym. Test. 20 (2001) 409–417. [18] Duncan M. Price, Mark Jaratt, Thermochim. Acta 392 (2002) 231–236. [19] Bernd Weinfeller, Michael Hofer, Frank R. Schilling, Compos., Part A Appl. Sci. Manuf. 35 (2004) 423–429.
Materials Letters 62 (2008) 852 – 856 www.elsevier.com/locate/matlet
On the thermal properties of metal (II) complexes of chalcone J. Meena Devi a,⁎, P. Tharmaraj b , S.K. Ramakrishnan b , K. Ramachandran a a b
School of Physics, Madurai Kamaraj University, Madurai-625021, Tamilnadu, India Department of Chemistry, Thiagarajar College, Madurai-625009, Tamilnadu, India Received 23 May 2007; accepted 29 June 2007 Available online 5 July 2007
Abstract The preparation of chalcone ligand and complexes of zinc, cadmium, mercury with chalcone is described. Thermogravimetric measurements made on these samples show ligand to be more stable than the metal complexes as it exhibits higher initial decomposition temperature. Thermal diffusivity of these four samples is measured by photoacoustic technique at room temperature to find the influence of coordination of metal on the thermal properties of chalcones. Experimental results indicate that the thermal diffusivity of chalcone metal complexes is enhanced when compared to the parent ligand. If we consider the free electron density of metals, then the increase may be understood from the electronic contribution of metals to the thermal property. So among metal complexes, thermal diffusivity increase with the increase in the free electron density of the metal ion coordinated. © 2007 Elsevier B.V. All rights reserved. Keywords: Chalcone; Metal complex; Thermal properties; Photoacoustics
1. Introduction Chalcones [1] are the precursors of flavonoids and antiflavonoids, which are available in plenty in ferns and higher plants. These flavonoids could be identified in fruits and vegetables, using photoacoustic (PA) technique [2]. This way photocoustic technique is becoming a powerful tool for biomaterials particularly in photochemistry and photobiology. Chemically the chalcones consist of two aromatic rings with an unsaturated chain. Chalcone compounds have displayed wide array of pharmacological activities [3–6] such as anticancer, anti-inflammatory, cytotoxic in vitro, bactericidal, insecticidal, antifungal, antioxidant, anti-HIV and phytoestrogenic activities. Apart from its use in medical science, some chalcone derivatives [7] exhibit second harmonic generation (SHG), and hence they are also used in non-linear optical (NLO) applications. Already, synthesis of some metal complexes of chalcone and their biological activities are available in literature; for example, chalcone–tellurium complexes [8] are active in the inhibition of E. coli. Ru(II)/ Ru(III) polypyridyl complexes containing 2,6(2′-Benzimidazolyl)-pyridine/chalcone as co-ligand [9] have been found to exhibit anti-HIV and cytotoxic activities. ⁎ Corresponding author. Fax: +91 452 245918. E-mail address: [email protected] (J.M. Devi). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.06.077
For the physical stability of the drug and its compatibility with potential excipients, thermal analysis plays a major role [10]. In this regard, we had measured the thermal diffusivity of cinnamoyl chalcones and difurans [11] by photoacoustics where it is found that the thermal diffusivity increases with the increase in the number of methoxy groups conjugated. In the present work, thermal properties of chalcone ligand and its three metal (Zn, Cd, Hg) complexes are investigated using photoacoustic (PA) technique. 2. Synthesis of chalcone ligand About 6.8ml of o-hydroxy acetophenone is mixed with 5.3ml of benzaldehyde in a conical flask, kept in a cold water bath. Around 20ml of ethanol is used as the solvent. Then 20ml of 40% NaOH is added dropwise with constant stirring until yellow solid product results. The product is then kept aside for 24h. This is neutralised with 20ml of 1:1 glacial acetic acid. As a result of this neutralization, a fine yellow precipitate (chalcone) is obtained. 3. Synthesis of zinc, cadmium and mercury complexes About 2.24g of the chalcone ligand is dissolved in alcohol. For preparing zinc complex, zinc chloride is dissolved in water and added to the ligand solution. Likewise for preparing cadmium
J.M. Devi et al. / Materials Letters 62 (2008) 852–856
853
molecular structure of chalcone ligand and its metal complexes is shown in Fig. 1. 4. IR spectra
Fig. 1. Molecular structure A) Chalcone ligand (C15H12O2) B) Chalcone metal complex.
complex, cadmium sulphate is dissolved in water and added to the ligand solution. Similarly to prepare mercury complex, anhydrous mercuric chloride is dissolved in water and added to the ligand solution. Then sufficient amount of NaOH is added for proper complex formation and the pH of 8 to 9 is maintained. The
The IR spectra (Fig. 2) of the synthesized ligand and complexes are recorded using Jasco Double beam Infrared Spectrophotometer in the range 400–4000cm− 1 by KBr pellet technique. The characteristic stretching frequencies observed are consistent with the functional groups present in the chalcone and the chalcone metal complexes. In the ligand and complexes, the IR absorption peaks at 1638 and 1640cm− 1 are assigned to C=O stretching vibration; absorptions in the region 1571, 1572, 1574cm− 1 are attributed to aromatic C=C stretching vibration; absorptions at 1339, 1340, 1368, 1369cm− 1 are assigned to C– O stretching vibration. Vibrations in the range 618, 621cm− 1 observed in the metal complexes indicate the presence of M–O bond (M-metal) in the molecules, which are not present in the original ligand. 5. Thermogravimetric measurement (TG) Thermogravimetric measurements are made from ambient to 800°C using STA 409C NETZCH-Geratebau GmbH thermal
Fig. 2. IR spectra A) Chalcone ligand B) Zn-complex C) Cd-complex D) Hg complex.
854
J.M. Devi et al. / Materials Letters 62 (2008) 852–856
Fig. 3. TG curve A) Chalcone ligand B) Zn-complex C) Cd-complex D) Hg complex.
analyser with alumina as the reference material. These are given in Fig. 3. Temperature is increased in steps of 5°C. Thermal decomposition of ligand occurs in the temperature range 240– 395°C. Zinc complex decomposes in the temperature range 180–340°C, cadmium complex in the range 110–320°C and mercury complex in the range 160–280°C. Thermal analysis on these samples by TG measurements show ligand to be more stable than the complexes as it exhibits higher initial decomposition temperature. Arshad et al. [12] have reported from their thermal analysis on the 1, 2-dipiperidinoethane complexes of cobalt, nickel, copper, zinc and cadmium that the coordination of metal ion into ligand is the reason for the weakening of the system. 6. Thermal diffusivity Thermal diffusivity measurements are performed using photoacoustic technique. Chalcone samples for PA measurement are taken in pellet form. White light from 450W Xenon lamp chopped by an electromechanical chopper is focused onto the sample placed inside the photoacoustic cell. Photoacoustic signal generated is detected by a microphone and then fed to the
lock-in-amplifier for measurements. Photoacoustic signal is recorded as a function of chopping frequency and the log–log plots of photoacoustic amplitude vs chopping frequency of the all four samples are shown in Fig. 4. Thermal diffusivity (α) [13] is then calculated from the characteristic frequency (fc) of the sample (characteristic frequency is defined as the frequency at which the sample goes from thermally thick region to thermally thin region) where slope change is observed in Fig. 4 using the following relation a ¼ fc l 2
ð1Þ
where l is the thickness of the sample. Thermal diffusivity values thus determined by PA technique at room temperature are given in Table 1. 7. Results and discussions Thermal diffusivity of chalcone ligand and its metal complexes show systematic variation (Table 1) due to the substitution of metal in chalcone. Here the observed thermal diffusivity of zinc complex is higher than cadmium complex and which is higher than mercury complex. This suggests that thermal diffusivity of these complexes
J.M. Devi et al. / Materials Letters 62 (2008) 852–856
855
Fig. 4. Log–log plot of photoacoustic amplitude vs chopping frequency for A) Chalcone ligand B) Zn-complex C) Cd-complex D) Hg-complex.
increase with the decrease in the mass and increase in the free electron density of the metal ion coordinated. This may be explained from the free electron theory and phonons in metals. Even though zinc (MZn = 65.37 amu), cadmium(MCd = 112.40 amu) and mercury(MHg = 200.59 amu) belong to group II, the free electron density of zinc (13.2 × 1022/cm3) is higher than cadmium(9.27 × 1022/cm3) and mercury(8.65 × 1022/cm3). So the increase may be understood from the electronic contribution of thermal conductivity. When the mass and atomic size of the metal ions increase, the mean free path would decrease and so a reduction in thermal conductivity among the metal complexes is expected in the sequence of Zn, Cd and Hg complex. Since this being the first report for these chalcone metal complexes we have compared our results with other organic compounds (Table 1) benzophenone [14] and amylopectin [15] as for the order of the thermal diffusivity is considered. The present measurements on metal-chalcone complexes show the same order for thermal diffusivity. A search through the literature reveals that very few PA studies have been carried out on chalcone metal complexes. Sankara Raman et al. [16] have measured the thermal diffusivity of CoL2X2 and CuL2X2 (where L = 1-nitrobenzyl-2-nitrophenyl benzimidazole (NBPBI) and X = Cl, Br, I) using photoacoustic technique. Thermal diffusivity increases with Cl, Br and I in CoL2X2 complex but decreases in CuL2X2 for the same X = (Cl, Br, I). This shows that the thermal diffusivity mainly depend on the ligand, metal and halogen part. Moreover the side chain in this basic ligand is more compared to the present ligand. So, the thermal diffusivity may be higher in benzimidazole complex when
compared to present chalcone complex. In our previous work [11] on chalcones, we have showed an increase in thermal properties as the side chain increases. The effect of metal ion on the thermo physical parameter has been observed in organic composites also. Rusu et al. [17] have reported that the incorporation of zinc powder in high-density polyethylene (HDPE) composite increases the thermal diffusivity. Duncan et al. [18] have observed improved heat transport in polytetrafluoroethylene (PTFE) composite on incorporating it with aluminium flakes. Weidenfeller et al. [19] have reported that by the addition of metal and oxide particles to plastics, thermal transport properties can be varied systematically. Hence in the present work also like reported in literature for other organic systems, introduction of metal in chalcone has influenced and enhanced the thermal diffusivity.
Table 1 Thermal diffusivity of chalcone ligand and its complexes Author
Sample
Thermal diffusivity (10− 7 m2/s)
Present work q q q Ref.[14] Ref.[15]
Ligand–chalcone Zn-complex Cd-complex Hg-complex Benzophenone Amylopectin
5.8 9.0 8.1 7.0 1.8 1.3
856
J.M. Devi et al. / Materials Letters 62 (2008) 852–856
8. Conclusion Thermal diffusivity of chalcone metal complexes is enhanced when compared to the parent ligand. If we consider the free electron density of metals, then the increase may be understood from the electronic contribution of thermal conductivity. So the thermal diffusivity values observed among the Zn, Cd, Hg complexes increase with increase in the free electron number density of metal ion coordinated. References [1] Yerra Koteswara Rao, Shih-Hua Fang, Yew-Min Tzeng, Bioorg. Med. Chem. 12 (2004) 2679–2686. [2] R.J.S. Lima, A.S. Vasconcelos, J.F. Suassuna, J. Phys. IV 125 (2005) 51–53 France. [3] Jiu-Hong Wu, Xi-Hong Wang, Yang-Hua Yi, Kuo-Hsiung Lee, Bioorg. Med. Chem. Lett. 13 (2003) 1813–1815. [4] Simon Feldbaek Nielsen, Thomas Boesen, Mogens Larsen, Kristian Schonning, Hasse Kromann, Biorg. Med. Chem. 12 (2004) 3047–3054. [5] Ohad Nerya, Ramadan Musa, Soliman Khatib, Snait Tamir, Jacob Vaya, Phytochem. 65 (2004) 1389–1395. [6] Guray Saydam, Hakan Aydin, Fahri Sahin, Ozlem Kucukoglu, Ercin Erciyas, Ender Terzioglu, Filiz Buyukkececi, Serdar Bedii Omay, Leuk. Res. 27 (2003) 57–64.
[7] Vincent Crasta, V. Ravindrachary, R.F. Bhajantri, Richard Gonsalves, J. Cryst. Growth 267 (2004) 129–133. [8] Lallan Mishra, Ragini Sinha, Hideji Itokawa, Kenneth F. Bastow, Yoko Tachibana, Yuka Nakanishi, Nicole Kilgore, Kuo-Hsiung Lee, Bioorg. Med. Chem. 9 (2001) 1667–1671. [9] W.E. Rudinski, T.M. Aminabhavi, Inorg. Chim. Acta 70 (1983) 175–178. [10] D.Q.M. Craig, V.L. Kett, C.S. Andrews, P.G. Royall, J. Pharm. Sci. 91 (2002) 1201–1213. [11] J. Meena Devi, K. Sithik Ali, V.R. Venkatraman, S.K. Ramakrishnan, K. Ramachandran, Thermochim. Acta 438 (2005) 29–34. [12] Muhammad Arshad, Saeed-ur Rehman, Shad Ali Khan, Khalid Masud, Nasima Arshad, Abdul Ghani, Thermochim.Acta 364 (2000) 143–153. [13] K.N. Madhusoodanan, J. Philip, Phys. Status Solidi, A Appl. Res. 108 (1988) 775–782. [14] B. Bonno, J.L. Laporte, Y. Rousset, J. Appl. Phys. 67 (1990) 2253–2256. [15] P. Rodriguez, G. Gonzalez de la Cruz, J. Food Eng. 58 (2003) 205–209. [16] Sankara Raman, V.P.N. Nampoori, C.P.G. Vallabhan, N. Saravanan, J. Mater. Sci. Lett. 15 (1996) 230–231. [17] Mihai Rusu, Nicoleta Sofian, Daniela Rusu, Polym. Test. 20 (2001) 409–417. [18] Duncan M. Price, Mark Jaratt, Thermochim. Acta 392 (2002) 231–236. [19] Bernd Weinfeller, Michael Hofer, Frank R. Schilling, Compos., Part A Appl. Sci. Manuf. 35 (2004) 423–429.