untitled European Journal of Chemistry 2 (2) (2011) 229‐234 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2011 EURJCHEM DOI:10.5155/eurjchem.2.2.229‐234.317 European Journal of Chemistry Journal homepage: www.eurjchem.com Thermal decomposition of N‐(salicylidene)‐L‐leucine in static air atmosphere Munusamy Vennila, Govindasamy Manikandan, Venugopal Thanikachalam and Jayaraman Jayabharathi* Department of Chemistry, Annamalai University, Annamalai Nagar 608 002, India *Corresponding author at: Department of Chemistry, Annamalai University, Annamalai Nagar 608 002, India. Tel.: +91.4144.239523; fax: +91.4144.238080. E‐mail address: pvta1998@yahoo.co.in (J. Jayabharathi). ARTICLE INFORMATION ABSTRACT Received: 14 November 2010 Received in revised form: 05 January 2011 Accepted: 26 January 2011 Online: 30 June 2011 KEYWORDS The thermal degradation of N‐(salicylidene)‐L‐leucine was studied under non‐isothermal conditions in air atmosphere. For kinetic analysis, the TG/DTA/DTG data obtained at three different heating rates were processed by Friedman, Kissinger‐Akahira‐Sunose, Flynn‐Wall‐ Ozawa and Kissinger methods. The analysis indicates a complex reaction process which can be best described by the three dimensional (Ginstling‐Brounshtein) model D4. Schiff base Thermal degradation Isoconversional Crystal deformation Non‐isothermal Activation energy 1. Introduction Schiff base ligands have significant importance in chemistry. Schiff bases derived from salicylaldehyde can function as polydendate ligands and form stable complexes with transition metal ions [1‐3]. Many Schiff base complexes show excellent catalytic activity in a number of reactions [4‐7]. Wang et al. have reported L‐‐amino acid Schiff base [8,9] which has chirality and different properties depending on the substituent. The salicylidine‐amino acid Schiff bases were assembled using Cu2+ ion with neutral planar chelating ligand, phen (or bipy) and the chemical nuclease property of Cu‐phen entity that intercalates into DNA groove [10] would be introduced. Combining with the available medico radionuclide 64Cu one kind of potential pharmaceutical has been investigated and found to possess antitumour activity and tumour accumulation in vivo‐R and S‐configuration of Cu‐phen complexes were theoretically constructed. The geometries of the complexes were optimized using PM3 method, then ab initio B3LYP 6‐ 31/G* calculation was performed to describe the molecular properties. The single point calculation of the three single crystals of the complexes was carried out with the theory of B3LYP 6‐31/G*, the structures with and without solvents were treated separately. All calculations were carried out employing the GAUSSIAN 98 program [11]. The present article is to investigate the kinetics of thermal degradation of N‐(salicylidene)‐L‐leucine in air atmosphere, under non‐isothermal conditions. The kinetic parameters for the decomposition were calculated using Friedman, Kissinger‐ Akahira‐Sunose (KAS), Flynn‐Wall‐Ozawa (FWO) and Kissinger methods. 2. Materials and methods All the chemicals were of analytical quality and have been used without further purification. Elemental analysis (carbon, hydrogen and nitrogen) has been performed using a Heraeus Carlo Erba 1108 model at Central Drug Research Institute, Lucknow, India. FT‐IR spectrum of the compound was recorded on a AVATAR model 330 using KBr pellet. Thermogravimetric analysis were carried out using a NETZSCH‐Geratebare GMBH thermal analysis, STA 409 PC. The weight of the sample was constant (10 mg) for all the heating rates of 10, 15 and 20 C/min, upto a temperature of 800 C and air flow of 50 mL/min. 2.1. Preparation of N‐(salicylidene)‐L‐leucine This compound was synthesized by mixing salicylaldehyde in ethanol and sodium salt of L‐leucine in ethanol‐water (50% v/v) [12]. The mixture was heated and refluxed on a mantle for about 5 hours. The reaction mixture was cooled to room temperature and neutralized with 1:1 HCl. The colorless Schiff base was separated, filtered off, washed thoroughly with deionised water‐ethanol mixture followed by ether. The product obtained was dried in a vacuum desiccator. The melting point of the compound was 114 C (Lit. 114 C) [12]. Anal. Calcd. for C13H17NO3; C, 66.38; H, 7.23, N, 5.95. Found C, 66.23; H, 7.20; N, 5.85%. FT‐IR (KBr disc, υ, cm‐1): 2958 (C‐H), 1622 (C=N), (C=O), (C‐N), 1073 (C‐O) and 698 (C‐H). 2.2. Rate equation Usually the change in extent of reaction () is used to study solid state reaction kinetics 230 Vennila et al. / European Journal of Chemistry 2 (2) (2011) 229‐234   mm mm t 0 0=  (1) where m0, mt and m are initial mass, mass at time t and mass at the end of reaction, respectively. Several reaction models [13] using f() or g() are reported in literature. Under non‐ isothermal conditions in which a sample is heated at a constant rate, the explicit temperature dependence of the rate equation is given by, )( exp           f RT EA dT d a (2) upon integration, equation (2) gives          T 0 exp = )g( dT RT EA a (3) If Ea/RT is replaced by x and integration limits transformed equation (3) becomes,     0 2 a )exp( AE = )g( dx x x R  (4) Equation (4) can be written as p(x) AE = )g( a R  (5) p(x) has no analytical solution but has many approximations [14‐16] and one of the most popular being the Coats‐Redfern method [17]. This method utilizes the asymptotic series expansion for approximating the exponential integral in equation (5) giving RT E E RT E AR T g a aa                 2 1ln )( ln 2 (6) Plotting the left hand side of equation (6), ln[g()/T2] versus 1/T gives Ea and A from the slope and intercept, respectively [17‐26]. The model that gives the best linear fit is selected as the correct model. 2.3. Isoconversional method According to the results of International Congress on Thermal Analysis and Calorimetry (ICTAC) kinetic project, isoconversional methods can match up to this challenge among other methods [27]. In non‐isothermal kinetics, the Friedman (FR) [28], Flynn‐Wall‐Ozawa (FWO) [29,30] and Kissinger‐ Akahira‐Sunose (KAS)[31‐33] methods are the most popular representatives of the isoconversional methods. 2.3.1. Friedman’s isoconversional method This method [28] was one of the earliest isoconversional methods, according to which the non‐isothermal rate law, )(    fAe dT d RT Ea (7) gives          RT E fA d a,)( ln dt ln  (8) Hence, a plot of ln(d/dT) versus 1/T at each  gives Ea from the slope of the plot. 2.3.2. Kissinger‐Akahira‐Sunose method The Kissinger‐Akahira‐Sunose (KAS) method [31‐33] was based on the following equation           RT E gE AR T a a )( lnln 2 (9) The Ea for different conversion values can be calculated from the linear plots of         2T ln versus 1/T. 2.3.3. Flynn‐Wall‐Ozawa method The Flynn‐Wall‐Ozawa (FWO) method [29,30] was based on the following equation RT E Rg AE aa        0516.1 )( 0048.0 ln = ln  (10) for  = constant, ln  versus 1/T obtained at several heating rates yields a straight line whose slope allows evaluation of the apparent activation energy. 3. Results and discussion Figure 1 shows TG‐DTA‐DTG curves corresponding to the Schiff’s base. A weak endothermic effect followed by intense endothermic effect at about 285.40, 288.71 and 292.34 C are observed at different heating rates (10, 15 and 20 K/min). The first two peaks are not accompanied by weight loss which is attributed to melting and crystal deformation of the Schiff base. The weight loss of 97.5, 98.4 and 93.0 % are observed at different heating rates. This total decomposition is accompanied by an endothermic effect with a maximum at 310 C and the decomposition is completed after this period. 3.1. Isoconversional kinetic analysis Friedman, KAS and FWO methods are used to determine the energy of activation (Ea) at constant several conversion degrees () (Table 1). The plots of ln (d/dT) versus 1/T, ln(/T2) versus 1/T and ln  versus 1/T, corresponding to several conversion degrees () were constructed. In the present study, three different heating rates were used (10, 15 and 20 K/min). Different heating rates give different Arrhenius plots, therefore a series of Ea values can be determined from the slopes of the straight lines at conversion degrees (Table 1). According to the Kissinger‐Akahira‐Sunose (KAS) isocon‐ versional method, straight lines with the angular co‐efficient ‐E/R were obtained and then a series of E values can be calculated by using equation (9). The values of the apparent activation energies obtained by Friedman method are lower than that of KAS and FWO methods. The average values of Ea in the range 0.2    0.9 were 251.61  0.58 kJ/mol (Friedman), 267.92  1.91 kJ/mol (KAS) and 263.84  1.90 kJ/mol (FWO) methods. The apparent activation energy sharply decreases with increase in the degree of conversion (0.01    0.2) (Table 1; Figure 2). The data show that energy of activation independent of conversion (), decomposed product not equilibrium with solid surface. Then, the energy of activation was found to be independent of conversion upto  = 0.94 which indicates that only one mechanism is involved for the decomposition of N‐(salicylicdene)‐L‐leucine in air atmosphere. Vennila et al. / European Journal of Chemistry 2 (2) (2011) 229‐234 231 Figure 1. TG‐DTG curves of N‐(salicylidene)‐L‐leucine at different heating rates a) 10, b) 15 and c) 20 K/min in dynamic air atmosphere. Figure 2. Isoconversional activation energy corresponding to the linear non‐isothermal decomposition in dynamic air atmosphere of the N‐(salicylidene)‐L‐ leucine . 232 Vennila et al. / European Journal of Chemistry 2 (2) (2011) 229‐234 Table 1. Temperatures corresponding to the same degree of conversion at different heating rates for N‐(salicylidene)‐L‐leucine.  Heating Rates Ea (kJ/mol) 10K 15K 20K Friedman method KAS method FWO method 0.10 524.28 526.44 528.60 ‐ 361.02 351.57 0.12 528.20 530.45 533.13 257.06 318.06 310.79 0.14 529.60 531.75 534.64 257.23 310.15 303.29 0.16 532.80 535.24 538.11 259.37 300.25 293.94 0.18 532.80 535.93 538.17 254.12 298.80 292.55 0.20 535.60 538.20 541.18 252.11 288.94 283.23 0.22 537.40 540.78 543.10 250.54 285.70 280.18 0.24 539.30 542.50 545.13 250.70 282.25 276.93 0.26 541.40 543.34 547.07 249.95 277.31 272.26 0.28 542.60 545.90 548.53 254.24 280.76 275.56 0.30 544.10 547.20 550.12 252.93 277.89 272.86 0.34 546.60 549.30 552.64 254.48 276.47 271.55 0.36 547.70 550.43 553.79 254.11 275.35 270.51 0.40 550.30 552.90 556.46 253.17 272.71 268.03 0.44 552.00 554.91 558.28 253.31 272.05 267.43 0.46 553.20 556.02 559.54 250.98 269.36 264.90 0.48 554.60 557.20 560.99 247.30 265.37 261.13 0.50 555.35 558.16 561.75 251.30 268.52 264.13 0.52 556.20 558.88 562.58 252.03 268.66 264.28 0.54 557.20 559.95 563.71 247.44 264.34 260.18 0.56 557.90 560.79 564.42 249.85 266.17 261.94 0.58 559.30 561.81 565.75 250.12 265.61 261.43 0.60 560.40 562.51 566.66 253.76 268.12 263.84 0.62 560.70 563.52 567.26 251.28 266.24 262.05 0.64 561.80 564.24 568.24 253.14 267.27 263.04 0.66 563.20 565.10 569.50 249.59 263.47 259.46 0.68 563.50 566.00 570.06 250.16 264.13 260.09 0.70 564.20 566.80 570.86 247.65 261.76 257.84 0.72 564.53 567.58 571.24 251.81 265.52 261.43 0.74 565.00 568.18 571.80 249.17 263.11 259.15 0.76 566.20 569.00 572.90 251.64 264.73 260.71 0.78 567.60 570.50 574.36 251.86 264.58 260.58 0.82 569.00 571.78 575.72 254.16 266.19 262.13 0.84 570.00 572.61 576.63 257.49 268.82 264.65 0.86 570.80 573.67 577.76 245.79 258.33 254.69 0.88 571.60 574.52 578.67 242.26 254.99 251.53 0.90 572.60 575.35 579.57 245.83 257.87 254.28 0.92 574.10 576.65 581.16 239.85 252.09 248.81 0.94 575.90 577.87 582.74 239.44 251.13 247.93 3.2. Invariant kinetic parameters (IKP) method The kinetics parameters are calculated using equation (6) and values are listed in Table 2. Lesnikovich and Levchik [34] suggested that correlating these values by the apparent compensation effect, ln A = a + b Ea, one obtains the compensation effect parameters, a and b, which strongly depends on the heating rates () as well as on the considered set of conversion functions. The straight lines ln A versus Ea for three constant heating rates should intersect at a point (isoparametric point [35]) which corresponds to the true values of the activation energy and pre‐exponential factor. These were named as invariant kinetic parameters. Invariant kinetic parameters Einv and Ainv are determined according to literature method, using various combination models and listed in Tables 3 and 4 (Figure 3). The Ea calculated by Friedman method coincided with AKM (all kinetic models). By introducing the reaction model,   3/21 3 2 1)(g        in to equation (5), equation (11) is obtained           R xpAEa )( 1 3 2 1 3/2 (11) The plot of   3/21 3 2 1        against R )x(pEa at the different (Figure 3) heating rates is considered. By using equation (11), the A value was determined from the slope of the line shown in Figure 4. Figure 3. Kinetic compensation effect for the decomposition of N‐ (salicylidene)‐L‐leucine in static air atmosphere. By applying the three‐dimensional diffusion controlled (Ginstling‐Brounshtein) D4 model Ea = 251.61  0.58 kJ/mol, the pre‐exponential (frequency) factor A = 1.29  1022 1/min (ln A = 50.91). The obtained value of ln A is in good agreement with that from the invariant method (ln A = 53.20). Therefore, the corresponding kinetic equation for describing the non‐isothermal decomposition process of N‐ (salicylidene)‐L‐leucine is given by Vennila et al. / European Journal of Chemistry 2 (2) (2011) 229‐234 233 Table 2. Arrhenius parameters for non‐isothermal decomposition of N‐(salicylidene)‐L‐leucine obtained from model fitting method. Kinetic model  = 10 K/min  = 15 K/min  = 20 K/min Ea (kJ/mol) ln A (A/s) r Ea (kJ/mol) ln A (A/s) r Ea (kJ/mol) ln A (A/s) r P2 47.16 8.17 ‐0.995 46.03 8.23 ‐0.995 44.90 8.17 ‐0.994 P3 28.36 3.71 ‐0.994 27.58 3.86 ‐0.994 26.81 3.91 ‐0.992 P4 19.01 1.34 ‐0.992 18.42 1.55 ‐0.992 17.82 1.63 ‐0.990 F1 151.23 31.98 ‐0.998 148.25 31.50 ‐0.998 145.26 30.90 ‐0.998 F2 219.14 47.65 ‐0.981 215.10 46.84 ‐0.981 211.01 45.91 ‐0.981 F3 303.73 66.99 ‐0.958 298.38 65.78 ‐0.958 292.93 64.44 ‐0.958 D1 229.56 55.74 ‐0.996 225.15 54.84 ‐0.996 220.76 53.85 ‐0.996 D2 241.96 50.52 ‐0.999 237.19 49.52 ‐0.999 232.42 48.41 ‐0.999 D3 275.50 56.71 ‐1.000 270.18 55.54 ‐1.000 264.86 54.27 ‐1.000 D4 270.98 55.43 ‐0.998 265.50 54.23 ‐0.998 260.04 52.93 ‐0.998 A2 71.05 13.98 ‐0.998 69.53 13.92 ‐0.998 68.00 13.75 ‐0.998 A3 43.85 7.65 ‐0.998 42.83 7.74 ‐0.998 41.80 7.71 ‐0.998 A4 30.96 4.53 ‐0.997 30.17 4.68 ‐0.997 29.37 4.72 ‐0.998 R2 124.89 25.15 ‐1.000 122.34 24.79 ‐1.000 119.78 24.32 ‐1.000 R3 117.53 23.70 ‐0.999 115.10 23.38 ‐0.999 112.66 22.95 ‐0.999 Table 3. Compensation effect parameters for several combinations of kinetic models for N‐(salicylidene)‐L‐leucine.  (K/min) AKM AKM{D1; D3; D4} a , A/s b /mol/J r a , A/s b /mol/J r 10 ‐1.72733 0.22207 0.995 ‐2.61283 0.22641 0.999 15 ‐1.97715 0.22332 0.995 ‐2.24555 0.2252 0.995 20 ‐2.34321 0.22456 0.995 ‐1.99356 0.22391 0.999  (K/min) AKM{F2; D1; D2; D3; D4} AKM ‐ {P4; F2; D1; D3; D4; A1; A2} a , A/s b /mol/J r a , A/s b /mol/J r 10 ‐2.73824 0.22882 0.999 ‐2.81444 0.22914 0.999 15 ‐2.36953 0.22765 0.999 ‐2.44544 0.22791 0.999 20 ‐2.11633 0.22646 0.999 ‐2.18782 0.22676 0.999 Table 4. IKP for several combinations of kinetic models for N‐(salicylidene)‐ L‐leucine. Kinetic model Einv (kJ/mol) ln Ainv ‐r AKM 247.27 53.20 0.994 AKM ‐ {D1; D3; D4} 244.13 53.36 0.992 AKM ‐ {F2; D1; D2; D3; D4} 263.37 57.55 0.993 AKM ‐ {P4; F2; D1; D3; A1; A2} 262.99 57.47 0.993  1)1(2/3 61.251 exp.1029.1 3/122           RT x dT d (12) where  1)1(2/3 3/1   represents the differential form of three dimensional diffusion (D4) controlled reaction. However this is further conformed by masterplot method (Figure 5). The overall masterplot of the compound shown in Figure 6. Figure 4. Plots of [1‐2/3‐(1‐)2/3against Eap(x)/R for the decomposition of N‐(salicylidene)‐L‐leucine at heating rates of 10, 15 and 20 K/min. Figure 5. Theoretical and experimental master plots of N‐(salicylidene)‐L‐ leucine at 10 K heating rate (non‐isothermal). Figure 6. The overall theoretical and experimental masterplots of N‐(salicylidene)‐L‐leucine at 10 K heating rate (non‐isothermal). 234 Vennila et al. / European Journal of Chemistry 2 (2) (2011) 229‐234 Table 5. Determination of kinetic parameters by Kissinger method. Peak Temperature (K) Ea (kJ/mol) ln A (A/s) r G (kJ/mol) H (kJ/mol) S (J/mol.K) 558.40 561.71 565.34 242.31 52.02 0.9914 135.15 237.4 ‐182.57 The activation energy and pre‐exponential factor are also calculated by Kissinger single point method [31] and the thermodynamic parameters of activation can be calculated [36‐ 39] and data are listed in Table 5. A exp(‐Ea/RTp) =  exp(‐G/RTp) (13) H = Ea ‐ RTp (14) G =  pH ‐ TpS (15) where G is the Gibbs free energy of activation, H the enthalpy of activation, S the entropy of activation and  the Einstein vibrational frequency,  = kBT/h (where kB and h are Boltzmann and Planck’s constants, respectively). The values are calculated at the peak temperature Tp in the DTG curve for the corresponding stage. Table 5 reveals that the value of S is negative. It means that the activated complexes have greater degree of arrangement than the initial stage. In terms of the theory of activated complex [36‐39], the thermal decomposition of N‐ (salicylidine)‐L‐leucine may be interpreted as slow. This was confirmed by the very high value of activation energy (Ea = 242.31 kJ/mol). The positive values of H and G showed that the processes in highly endothermic and is non‐ spontaneous. 4. Conclusion The thermal decomposition of N‐(salicylididene)‐L‐leucine was investigated in detail by TG, DTA and DTG. The process involved melting, solid‐solid phase transition and decomposition. The kinetic parameters of decomposition were obtained by the isoconversional and invariant methods. The decomposition reaction is endothermic as shown by the positive value of G. The three dimensional model D4 can be the most probable model which can give adequate kinetic description for the thermal decomposition of the compound chosen for study. 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