The complexes of zinc with L-a-amino acids as additives have been widely applied in medicine, foodstuff and cosmetics ^[1-3]. The preparation methods of making zinc amino acids have been comprehensively reviewed ^[4-5]. The solubility properties of ZnAc₂-Val-H₂O [Val = Valine (C₅H₁₁O₂N)] system at 298.15 K has been investigated by the semimicro-phase equilibrium method ^[6]. The phase diagram is a simple one, in which the phase region of the complexes does not exist. The preparation of the crystal of Zn(Thr)SO₄·H₂O was reported in literature [7] : to the reaction liquid of ZnSO₄ and L-a-Thr with the molar ratio of 1:1, three times volume acetone was added, and the product was obtained.
    In this paper, the solid complex of Zn(Val)Ac₂ was prepared in the mixture solvent of water-acetone ratio of 1:10. The composition of the complex was identified as Zn(Val)Ac₂ by chemical and elemental analyses. The bonding characteristic and thermo stability of the title complex were investigated by IR, DSC and TG – DTG. Its enthalpies of decomposition were obtained from the DSC curve. The constant-volume combustion energy of the compound was determined by a precise rotating-bomb calorimeter at 298.15 K; its standard enthalpies of combustion and standard enthalpies of formation were calculated.

1 EXPERIMENTAL
1.1 Reagents and equipments
ZnAc₂ · 2H₂O, A. R. (Xi'an Chemical Company); L-a-Val, B. R. (Shanghai Kangda Company), purity > 99.95%; acetone, A. R. (Xi'an Chemical Company), the others are of A. R. grade. Zn²⁺ was determined with EDTA by complexometric; Val by formalin' method, the Zn²⁺ was removed by precipitating with K₂C₂O₄ before it was titrated. Carbon, hydrogen and nitrogen analyses were carried out on a 2400 type elemental analyzer of PE Company, DSC experiment was performed with a Model CDR-1 thermal analyzer made in the Shanghai Balance Instrument Factory. TG-DTG curve were recorded on a TG-7 type thermobalance under the heating rate of 10ºC·min^-1 and the flow rate of N₂ of 60 mL · min^-1, the sample weight is about 1mg. IR analyses were carried out by a BEQ, UZNDX-550 IR spectrophotometer with KBr pellet. The constant-volume combustion energy of compound was determined by a precise rotating-bomb calorimeter ( RBC-type ) ^[8].The experimental method and steps were the same as the literature ^[9].The initial temperature was regulated to (25.0000 ±0.0005)ºC by a super constant temperature thermostat and the initial oxygen pressure was 2.5 MPa. The final products were analyzed by method in literature [9]. The correct value of the heat exchange was calculated according to the Linio-Pyfengdelel-Wsava formula ^[10]. The calorimeter was calibrated by benzoic acid of 99.999% purity from Chengdu Chemical reagent Factory. Benzoic acid had an isothermal heat of combustion at 25 ºC of (-26434 ± 5.80) J·g^-1. The energy equivalent of calorimeter was (17936.01 ± 9.08) kJ·K^-1. The precision was 5.06×10^-4.
1.2 Preparation of complex            
ZnAc₂·2H₂O and L-a-Val with the molar ratio of 1:1 were dissolved in an appropriate amount of water, kept the reaction at 60-70ºC for 8 hours, and allowed to cool. To the solution, 10 times volume acetone was added, and large amount white precipitate was obtained. The precipitate was filtered off, washed with acetone, and kept in vacuum over P₄O₁₀ to dryness for being used. The yield is 33 % (Table 1). Anal. Calc. for Zn(Val)Ac₂ : Zn²⁺ 21.71, Val 38.97, C 35.96, H 5.70 , N 4.66 ; found Zn²⁺ 21.78, Val 38.56, C 36.01, H 5.80, N 4.68.

Table1 Experimental results of the preparation of the complex with different volume ratios of water and acetone

2 RESULTS AND DISCUSSION
2.1 IR spectroscopy              
Data of IR absorption for main groups of ligand and complex are listed in Table 2. Non-existing the characteristic absorption bands of -COOH group at 1700-1750 cm^-1 for complex reveals that Val still keeps zwitter-ion structure ^[11]; wide shifts of nand d for complex contrasting with ligand indicates that Val coordinates Zn²⁺ through N and O atom as a bidentate ligand; The wide characteristic absorption band at 3440 cm^-1 of the asymmetric vibration of OH^- does not present, which reveals that there is no H₂O molecule in the complex.

2.2 Thermal behavior
The TG-DTG curves of the complex are showed in Fig.1. On the basis of the thermal decomposition results (calculated values) of the complex, the thermal decomposition mechanism can be postulated as follows (the middle is the peak temperatures of DTG ):

    From the curves, it can be seen the mass residues of every stage of the complex are rather close to the calculated values. The DTG curve shows the thermal decomposition process of Zn(Val)Ac₂ can be divided into two stages. In the first stage, the skeleton of the complex is partly broken and decomposed into ZnO. The characteristic absorption peaks of the complex and ZnO (356 cm^-1) are observed in the spectra of decomposition products. In the second stage, the complex is completely decomposed into ZnO. Its IR spectrum coincides with the standard IR spectrum of ZnO. (Fig.2).

Figure 1 TG-DTG curve for Zn(Val)Ac₂                           Figure 2 IR spectra of ligand and its metal compounds
                                                                                         a. ZnO, b. 4Zn(Val)Ac₂·ZnO, c. Zn(Val)Ac₂, d. Val

    The DSC curve of the complex is showed in Fig.3. Their decomposition temperatures coincide basically with the TG-DTG thermal-analytical ones. The two peaks in the figure can be considered as the enthalpy changes of the decomposition of the complex. As shown in the figure, = 21.50 J·g ^-1, = 376.60 J·g ^–1.

Figure 3 DSC curve for Zn(Val)Ac₂

Table 3 The experimental results of constant-volume combustion energy for Zn(Val)Ac₂

2.3 Standard enthalpy of formation
The experimental data of constant volume combustion energy of Zn(Val)Ac₂ are showed in Table 3.
    The standard enthalpy of combustion,, is referred to the combustion enthalpy change of the following ideal combustion reactions at 298.15 K and 0.1MPa:
      (1)
=+DnRT (2)
is calculated by Eq.(2) as the value of (-5701.97±7.89) kJ·mol^-1.
    The standard enthalpy of formation of the complex, , is calculated by Hess's law according to Eq. (1):
( Zn(Val)Ac₂,s,298 K ) = [(ZnO,s,298 K )+9(CO₂,g,298 K ) +(H₂O,l,298 K )] - (Zn(Val)Ac₂,s,298 K)= (-619.64±2.59 ) kJ·mol^-1  (3)
where，(ZnO,s,298 K )=（-350.46 ± 0.27）kJ·mol^-1 ， (CO₂,g,298 K ) =（-393.51 ± 0.13）kJ·mol^-1， (H₂O,l,298 K ) =（-285.83 ± 0.04）kJ· mol^{-1 [12]}.
    The standard mole enthalpy of formation is calculated as（-619.64 ± 2.59 ）kJ·mol^-1.

REFERENCES
[1] Mahmoud M, Abdel-monem S, Paul M. US Patent 4 039 681, 1977-08-02.
[2] Taguchi S, Inokuchi M, Nakajima N. WO Patent 10 178, 1992-06-25.
[3] Harvey H, Ashmed , Kaysville utah. US Patent 4 830 716, 1989-05-16.
[4] Gao S L, Liu J R, Ji M, Yang X W, et al. Chinese Sci. Bull. (Kexue Tongbao), 1998, 43 (14): 1496.
[5] Gao S L, Hou Y D, Liu J R,et al. Chinese Chem. Bull.(Huaxue Tongbao), 1999, 11:30 .
[6] Guo L J, Zhang F X, Tang X Q, et al. Journal of Northwest University (Natural Science Edition), (Xibei Daxue Xuebao) 2002, 23(1): 29.
[7] Gao S L, Zhang X Y,Yang X W, et al. Acta Chmica Sinica (Huaxue Xuebao), 2001, 59 (1): 73.
[8] Gao S L, Chen S P, Yang X W,et al. Chin. J. Chem.(Zhongguo Huaxue), 2001,19 (11): 1037.
[9] Yang X F, Yang X W, Sun L Z,et al. Chem.J. Chin, Univ.(Gaodeng Xuexiao Huaxue Xuebao), 1986, 7: 363.
[10] Popov M M, Thermometry and Calorimetry, Moscow University Press Moscow, 1954,382.
[11] Niu C J, Zhang S G, Wang Z L, et al. Chem. J. Chin. Univ. (Gaodeng Xuexiao Huaxue Xuebao), 1991, 12 (10): 1386.
[12] Cox J. D. J. Chem. Thermodyn. 1978, 10: 903.

[ Back ] [ Home ] [ Up ] [ Next ]Mirror Site in USA  Europe  China  GBNet