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HUE UNIVERSITY UNIVERSITY OF EDUCATION -oOo - DINH QUY HUONG A RESEARCH ON ANTIOXIDANTS, METAL CORROSION INHIBITORS BY QUANTUM CHEMICAL CALCULATIONS AND EXPERIMENTAL METHODS Major: INORGANIC CHEMISTRY Code: 9440113 SUMMARY OF CHEMISTRY THESIS HUE, 2020 The work was completed at Department of Chemistry, University of Education, Hue University Supervisors: Assoc Prof Dr Tran Duong Assoc Prof Dr Pham Cam Nam Reviewer 1:……………………………………………… Reviewer 2:……………………………………………… Reviewer 3:……………………………………………… The thesis will be evaluated by Hue University Thesis Asessment Council at:………………………………………… At the time:……… day……… month………year…………… Thesis can be found at the Library of the University of Education, Hue University INTRODUCTION Materials oxidation and metal corrosion are problems that cause serious harm, affect to the national economy In the corrosion field, the study of measures to prevent metal corrosion is an urgent task, requiring scientists to focus on research There are many methods to prevent metal corrosion, in which the use of metal corrosion inhibitors is a low-cost way with a high efficiency Compounds containing sulfur or nitrogen are known to be highly effective corrosion inhibitors for steel in acid solutions In addition, the presence of benzene ring is also an important factor, it increases the electrostatic interaction between inhibitors and metal surfaces, enhances the ability to inhibit metal corrosion for a long time Therefore, compounds containing heteroatoms and benzene rings will be one of the options to be considered when studying corrosion inhibitors In terms of antioxidants, the use of antioxidants in the fields of food, medicine and industry are also issues to be investigated Antioxidants are usually compounds with low dissociation energy of N-H, O-H and S-H bonds Antioxidant capacity of a given compound can be evaluated by three mechanisms involving hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SETPT) and sequential proton loss electron transfer (SPLET)… Experimentally, 2.2-diphenyl-1-picrylhydrazyl and 2,2’-azinobis(3ethylbenzothiazoline-6-sulfonate) methods are widely used to quantitatively determine the antioxidant activity of the compound These methods are not only rapid, simple and inexpensive but also provide first-hand information on the overall antioxidant capacity of the test system The search for compounds that have the ability to inhibit metal corrosion and the antioxidant ability, plays an important role in life However, up to now, no studies have mentioned this issue Moreover, a combination of experimental methods and theoretical calculations is necessary in scientific research This is also one of the research orientations that attracts the attention of all scientists all over the world From the above scientific analysis, "A research on antioxidants, metal corrosion inhibitors by quantum chemical calculations and experimental methods" is selected as the research topic in this thesis New contributions of the thesis The thesis obtained some new findings as follows: - 1-phenyl-2-thiourea has a better ability to inhibit mild steel corrosion than 1,3-diisopropyl-2-thiourea in 1.0 M HCl solution with efficiencies of 92.00 % at 20 oC, 94.05 % at 30 °C, 96.95 % at 45 °C and 98.96 % at 60 °C at the concentration of 5.103 M - The mild steel corrosion inhibition ability of 1-phenyl-2-thiourea is better than urotropine in both 1.0 M HCl and 3.5 % wt NaCl solution in the same concentration and temperature conditions - The mild steel corrosion inhibition ability of 1-phenyl-2-thiourea in acidic environment is better than in salt environment - 1-phenyl-2-thiourea has a higher ability to capture DPPH• and ABTS•+ than 1,3-diisopropyl-2-thiourea in ethanol These results are also consistent with quantum chemical calculations - 1-phenyl-2-selenourea exhibits better antioxidant capacity than 1phenyl-2-thiourea In selenourea derivatives, compounds containing electron donating groups give better antioxidant capacity than compounds containing electron accepting groups - 1-(4-methoxyphenyl)-2-selenourea is selected as a potential steel corrosion inhibitor and a good antioxidant in research compounds CHAPTER OVERVIEW The thesis has conducted an overview of theoretical issues as follows: 1.1 OVERVIEW OF METAL CORROSION 1.1.1 Concept of metal corrosion 1.1.2 Classification of metal corrosion process 1.1.3 Harm of metal corrosion 1.1.4 Steel corrosion 1.1.5 Methods to prevent metal corrosion 1.1.6 Corrosion inhibitors 1.1.7 Activity mechanism of metal corrosion inhibitors 1.1.8 Requirements for metal corrosion inhibitors 1.1.9 Range of using corrosion inhibitors 1.1.10 Research situation on the metal corrosion inhibition ability 1.2 OVERVIEW OF ANTIOXIDANTS 1.2.1 Introduction about antioxidants 1.2.2 Anti-oxidation mechanism 1.2.3 Research situation of antioxidants 1.3 OVERVIEW OF RESEARCH METHODS 1.3.1 Experimental methods of metal corrosion inhibition research 1.3.2 Experimental methods of antioxidant research 1.3.3 Quantum chemical calculation methods 1.3.4 Quantum chemical calculation softwares CHAPTER RESEARCH CONTENTS AND METHODS 2.1 RESEARCH CONTENTS The thesis focuses on five main contents as follows: - Compare the steel corrosion inhibition of 1-phenyl-2-thiourea with 1,3-diisopropyl-2-thiourea - Compare the mild steel corrosion inhibition ability of 1-phenyl-2thiourea with urotropine - Research on the antioxidant capacity of 1-phenyl-2-thiourea and 1,3-diisopropyl-2-thiourea - Research on the antioxidant capacity of 1-phenyl-2-selenourea and its derivatives by quantum chemical calculations - Propose a potential compound that can inhibit steel corrosion and oxidation capacity by quantum chemical calculations 2.2 EXPERIMENTAL METHODS 2.2.1 Chemical compounds 2.2.2 Potentiodynamic polarization measurements (PDP) In 1.0 M HCl solution, potentiodynamic polarization curves were measured by scanning the potential from −0.55 V to 0.00 V with a sensitivity of In 3.5 % wt NaCl solution, the potential was scanned from −1.30 V to −0.80 V with a sensitivity of The scanning rate was mV.s−1 in both solutions The working electrode was low carbon steel with a surface area of 0.196 cm2, the rest was surrounded by an epoxy layer, leaving only the work surface in contact with the solution 2.2.3 Electrochemical impedance spectroscopy (EIS) EIS was performed at open-circuit potential with an alternating current amplitude of 10 mV using a frequency region of 10 mHz to 100 Hz The total number of points was 30 2.2.4 Scanning electron microscope analysis (SEM) Specimens of mild steel were immersed in 1.0 M HCl and 3.5 % wt NaCl solution with and without PTU (5.10−3 M) for 24 hours at room temperature Surface analysis of the steel was then carried out using a JSM-6010PLUS/LV scanning electron microscope with an energy dispersive Xray analyzer attached 2.2.5 2,2-diphenyl-1-picrylhydrazyl (DPPH•) assay DPPH• was diluted in ethanol at a concentration of 6.7.105 M The antioxidant with various concentrations was added into DPPH• with the volume scale of 3:1 The reaction mixture was stirred and kept in the dark for 30 minutes The absorbance of the resulting solutions was measured at 517 nm The DPPH• radical scavenging activity was determined via the IC50DPPH value 2.2.6 2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS•+) assay ABTS•+ radical cation was produced by the reaction of mM ’ 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium solution and 140 mM K2S2O8 The final concentration of K2S2O8 is 2.45 mM After 16 hours, ABTS•+ solution was diluted with ethanol to an absorbance of 0.70 ± 0.05 at 734 nm After addition of 1.0 mL of antioxidant to mL of diluted ABTS•+, the solution was kept for minutes at room temperature 2.3 Theoretical methods: 2.3.1 Quantum chemical calculations application to research on metal corrosion inhibitors 2.3.2 Quantum chemical calculations application to research on antioxidants CHAPTER RESULTS AND DISSCUSSION 3.1 INVESTIGATION OF STEEL CORROSION INHIBITION ABILITY OF THIOUREA DERIVATIVES 1-phenyl-2-thiourea (PTU) and 1,3-diisopropyl-2-thiourea (ITU) were chosen to investigate mild steel corrosion inhibition ability 3.1.1 Investigation of mild steel corrosion inhibition ability of PTU and ITU in 1.0 M HCl 3.1.1.1 Effect of inhibitor concentration on the mild steel corrosion inhibition ability Figure 3.1 showed that both cathode and anode current densities decreased in the presence of PTU and ITU The steel corrosion density depended on the concentration of inhibitors At 30 o C, inhibition efficiencies of PTU reached of 90.54 %; 91.11 %;  93.88 % and 94.95 % corresponding to the concentrations of 10 4,    5.10 4, 10 and 5.10 M (Table 3.1) However, inhibition efficiency  of ITU only changed much at the concentration of 5.10 M The steel corrosion inhibition efficiencies of ITU reached 75.56 %; 76.67    %; 77.78 % and 83.33 % at concentrations of 10 4, 5.10 4, 10 and  5.10 M, respectively 2 1 -1 -1 -2 1.0 M1,0 HCl HCl M -4 -4 10 PTUM10PTU M -4 -4 5.10 M PTU PTU 5.10 M -3 -3 10 PTUM10PTU M -3 -3 PTU 5.10 M 5.10 M PTU -3 -4 -5 -0.5 -0.4 -0.3 -0.2 -0.1 log i log i -2 HClM1,0 M 1.0 HCl -4 -4 10 ITUM 10ITU M -4 5.10 M ITU 5.10-4ITU M -3 -3 10 M ITU ITU 10 M -3 5.10 M -3ITU ITU 5.10 M -3 -4 -5 -6 0.0 -0.5 Ecorr (V) -0.4 -0.3 -0.2 -0.1 0.0 Ecorr (V) (a) PTU (b) ITU Figure 3.1 Polarization curves of mild steel in 1.0 M HCl with various concentrations of (a) PTU and (b) ITU in hour at 30 °C Table 3.1 Polarization parameters of mild steel in 1.0 M HCl at various concentrations of PTU and ITU at 20, 30, 45, and 60 °C Inhibitors Tempera ture(oC) 20 PTU 30 C (M) Ecorr (V)  104 5.104 103 5.103  104 5.104 103 0.23 0.34 0.37 0.39 0.40 0.24 0.33 0.35 0.37 βa -βc icorr (iinh) (mV.dec1) (mV.dec1) (mA.cm2) 32.50 34.70 30.80 33.40 31.20 36.40 48.00 33.10 39.30 20.90 21.50 20.20 22.40 19.30 24.80 21.70 20.60 26.00 0.25 0.05 0.04 0.03 0.02 0.90 0.09 0.08 0.06 H (%) 80.40 (1.27) 84.00 ((1.11) 88.00 (1.22) 92.00 (1.30) ( 90.54 (1.13) 91.11 (1.20) 93.88 (1.01) 45 60 20 30 ITU 45 5.103  104 5.104 103 5.103  104 5.104 103 5.103  104 5.104 103 5.103  104 5.104 103 5.103  104 5.104 103 5.103  60 104 5.104 103 5.103 0.38 0.25 0.27 0.33 0.38 0.32 0.24 0.30 0.33 0.30 0.32 0.35 0.44 0.45 0.47 0.49 0.24 0.38 0.39 0.39 0.42 0.34 0.35 0.36 0.39 0.39 0.35 0.37 0.36 0.39 0.37 33.00 35.70 36.40 40.40 41.90 21.60 37.10 30.50 37.90 40.10 39.60 32.50 26.30 30.10 30.30 30.20 36.40 28.60 34.10 34.60 34.50 35.70 18.40 20.60 25.60 20.50 37.10 38.40 37.40 40.20 34.00 20.80 30.00 32.10 21.20 22.90 32.60 30.40 30.40 22.40 22.30 26.50 20.90 16.50 17.20 16.40 16.30 24.80 17.70 19.60 17.20 17.20 30.00 24.00 17.30 20.30 17.90 30.40 37.70 28.50 41.50 32.00 0.05 2.69 0.14 0.13 0.11 0.09 7.35 0.29 0.13 0.08 0.08 0.25 0.12 0.09 0.07 0.05 0.90 0.22 0.21 0.20 0.15 2.69 0.43 0.37 0.32 0.26 7.35 0.81 0.73 0.60 0.54 94.95 (1.05) 94.80 (1.09) 95.17 (1.11) 95.91 (1.32) 96.65 (1.23) 96.10 (1.19) 98.17 (1.24) 98.95 (1.20) 98.96 (1.15) 53.20 (1.30) 62.40 (1.21) 71.60 (1.01) 80.80 (1.20) 75.56 (1.27) 76.67 (1.25) 77.78 (1.31) 83.33 (1.02) 84.01 (1.40) 86.25 (1.35) 88.10 (1.22) 90.33 (1.30) 88.98 (1.10) 90.07 (1.10) 91.84 (1.23) 92.65 (1.21) Values in parenthesis in the last column of this table are the mean absolute deviation Nyquist diagrams for steel in 1.0 M HCl with the presence of PTU and ITU were displayed in Figure 3.2 All the impedance spectra exhibited one single capacitive semicircle This showed that the charge transfer process of the corrosion process and double layer behavior mainly controlled the corrosion of carbon steel 1.0 M HCl 10-4 M PTU 5.10-4 M PTU 10-3 M PTU 5.10-3 M PTU 1.0 M HCl 10-4 M ITU 5.10-4 M ITU 10-3 M ITU 5.10-3 M ITU (a) PTU (b) ITU Figure 3.2 Nyquist plots of the corrosion of mild steel in 1.0 M HCl with different concentrations of (a) PTU and (b) ITU at 30 °C In general, in the presence of inhibitors in solution, Rct values increased, and Cdl values decreased (Table 3.2) These might suggest that the inhibitors formed a protective layer on the electrode surface This layer made a barrier for mass and charge transfer Moreover, Rct values of PTU were higher than that of ITU at the same concentration, which proved that PTU could inhibit better than ITU The best inhibition efficiency of PTU and ITU were 93.33 and 82.63 % according to the EIS method Table 3.2 EIS parameters for the corrosion of mild steel in 1.0 M HCl in the absence and presence of inhibitors at 30 °C Inhibitors C (M) HCl  PTU ITU 104 5.104 103 5.103 104 5.104 103 5.103 Rs (Ω.cm2) 3.52 2.34 2.30 4.68 2.27 2.45 2.52 2.49 2.65 CPE (μΩ.sn.cm2) 64.62 7.48 8.22 8.40 8.43 4.68 4.13 4.53 5.38 Rct (Ω.cm2) 125.10 937.70 1167.00 1690.00 1870.00 505.00 530.00 580.00 720.00 n 0.80 0.73 0.73 0.67 0.72 0.84 0.71 0.82 0.76 Cdl (μF.cm2) 29.14 4.24 4.15 4.04 3.99 9.64 8.22 6.44 6.13 H (%) 86.66 89.28 92.60 93.31 75.23 76.40 78.43 82.63 3.1.1.2 Effect of temperature on mild steel corrosion inhibition efficiency PTU exhibited an effective inhibitor with high inhibition performances of 92.00 % at 20 °C, 94.95 % at 30 °C, 96.65 % at 45 °C, and 98.96 % at 60 °C (Table 3.1) While inhibition performances of ITU were only 80.80 % at 20 °C, 83.33 % at 30 °C, 90.33 % at 45 °C, and 92.65 % at 60 °C 3.1.1.3 Adsorption isotherms and thermodynamic parameters 1.00 0.98 0.96 0.94 0.95 R2  0.913 0.90 R2  0.781 0.85 R2  0.963 0.80 0.90   0.92 R2  0.701 R2  0.888 R2  0.982 R2  0.792 R2  0.968 0.75 0.70 0.88 0.86 0.84 0.82 0.80 -10 -9 -8 lnC -7 -6 ITU-20 oC ITU-30 oC ITU-45 oC ITU-60 oC 0.65 PTU-20 oC PTU-30 oC PTU-45 oC PTU-60 oC 0.60 0.55 0.50 -10 -5 -9 -8 -7 -6 -5 lnC (a) PTU (b) ITU Figure 3.3 Temkin’s adsorption isotherms of (a) PTU and (b) ITU on the surface of mild steel in 1.0 M HCl According to Figure 3.3, the correlation coefficients of the plots between lnC versus θ were considerably different from unit (except for R2 at 20, 45 °C) They proved that adsorption of PTU and ITU on the steel surface did not follow the Temkin isotherm 0.006 0.007 y  1.08 x  4.17.10   R  0.999 C/ 0.004 y  1.05 x  1.43.10 R  0.999 0.003 y  1.03 x  6.43.106  2 5 R  0.999     y  1.01 x  2.84.106 R  0.999 0.002 0.000 0.001 0.002 0.003 0.004 0.005   R  0.999 0.005 y  1.19 x  5.20.105 R  0.999 0.004     0.003   0.002 PTU-20 oC PTU-30 oC PTU-45 oC PTU-60 oC 0.001 0.000 y  1.21 x  1.46.104 0.006 5 C/ 0.005 y  1.10 x  2.22.105 R  0.999 y  1.08 x  1.07.105 R  0.999 ITU-20 oC ITU-30 oC ITU-45 oC ITU-60 oC 0.001 0.000 0.006 0.000 0.001 0.002 0.003 0.004 0.005 0.006 C (M) C (M) (a) PTU (b) ITU Figure 3.4 Langmuir’s adsorption isotherms of (a) PTU and (b) ITU in 1.0 M HCl Next, Langmuir adsorption was applied for evaluation and was shown in Figure 3.4 The straight lines between C and C/θ were found with the correlation coefficients close to 1, and the slope log i -1 -2 3.5NaCl % wt.NaCl 3,5% 10-4PTU M PTU 10-4 M -4 -4 5.10 M5.10 PTU PTU M 10-3PTU M PTU 10-3 M -3 -3 5.10 M PTU PTU 5.10 M -3 -4 -5 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 Ecorr (V) Figure 3.5 Polarization curves of steel in 3.5% wt NaCl with different concentrations of PTU at 30 oC Table 3.4 Polarization parameters of mild steel in 3.5 % wt NaCl at different concentration of PTU Ecorr (V) Solution icorr (iinh) H 2 (mA.cm ) (%) 3.5 % wt NaCl 0.50 1.03 0.22 55.70 3.5 % wt NaCl + 5.103 M PTU 1.21 3 0.26 49.08 3.5 % wt NaCl + 10 M PTU 1.18 4 0.28 44.84 3.5 % wt NaCl + 5.10 M PTU 1.13 4 0.29 43.50 3.5 % wt NaCl + 10 M PTU 1.11 3.1.3 Comparison of metal corrosion inhibition between PTU and urotropine in acid and salt environments The inhibition efficiencies of urotropine were 65.11 % in acid solution and 46.56 % in salt solution (Table 3.5) Table 3.5 Polarization parameters of mild steel in 1.0 M HCl and 3.5 % wt NaCl in the presence of 5.103 M Urotropine Ecorr icorr (iinh) Solution H (%) (V) (mA.cm 0.31 65.11 1.0 M HCl + 5.103 M urotropine 0.25 ) 3 0.27 46.56 3.5 % wt NaCl + 5.10 M urotropine 1.12 10 3.1.4 Effect of molecular structure and environment on the corrosion inhibition ability of thiourea derivatives 3.1.4.1 Effect of inhibitor molecular structure on mild steel corrosion inhibition efficiency PTU and ITU were derivatives of thiourea, they were only different regarding the molecular structures of the substituents nearby the nitrogen atom The rest of the inhibitor molecule affected the electron density at the functional group; therefore, it also influenced the adsorption on the metal surface In the PTU molecule, an important structural factor was a benzene ring, it rose electrostatic interaction and gave the higher coverage between inhibitors and metal surface All these factors helped PTU to give the better inhibition performance than ITU Compared with urotropine, inhibition efficiency of PTU was higher This could be explained that PTU had a flat structure, so it was easily adsorbed onto the iron surface, while urotropine had a cage structure with four nitrogen atoms located at peaks, therefore, its adsorption was more difficult due to the bulky structure 3.1.4.2 Effect of environment on steel corrosion inhibition efficiency At the same concentration, the inhibition efficiency of PTU in 1.0 M HCl acid solution was higher than in 3.5 % wt NaCl solution (Figure 3.6) 100 In 1.0 M HCl In 3.5 % wt NaCl H (%) 80 60 40 20 0.000 0.001 0.002 0.003 0.004 0.005 Cinh (M) Figure 3.6 Comparison of inhibition efficiency of PTU in 1.0 M HCl and 3.5 % wt NaCl solutions 11 3.1.5 Investigation of mild steel corrosion inhibition ability of PTU and Urotropine by quantum chemical calculations 3.1.5.1 Optimal molecular structures, quantum chemical parameters of PTU and urotropine in neutral forms (a) PTU (b) Urotropine Figure 3.7 Optimal structures of (a) PTU and (b) Urotropine HOMO-PTU LUMO-PTU HOMO-Urotropine LUMO-Urotropine Figure 3.8 HOMO and LUMO of PTU and Urotropine The highest occupied molecular orbital (HOMO) could display the electron-donating position of the molecule Figure 3.8 showed that PTU could donate mainly electrons to metal surface on the S18 atom, while urotropine happened on almost all carbon and nitrogen atoms On the other hand, the LUMO represented the accepting electron ability of the molecule The most reactive sites were found at the whole of molecules for PTU and urotropine Bảng 3.6 Quantum chemical parameters for inhibitors in the gas phase at B3LYP/6-311G(d,p) Compounds PTU Urotropine EHOMO (eV) 5.84 6.04 ELUMO ΔE(L-H) (eV) (eV) 4.81 1.03 0.88 6.92 χ μ (eV) (eV) 3.43 3.43 2.58 2.58 η (eV) 2.41 3.46 S ΔN (eV1) 0.42 0.74 0.29 0.64 µD (D) 4.78 0.00 All quantum chemical parameters demonstrated that PTU was a better corrosion inhibitor than urotropine 3.1.5.2 Monte Carlo simulations Monte Carlo simulations were used for researching adsorption of PTU and urotropine molecules on the Fe(110) surface It could be seen clearly from Figure 3.15 that PTU adsorbed closely, and laid parallel to the surface of Fe(110), while urotropine had a cage structure so it had only one site close to the metal surface 12 (a) PTU (b) Urotropine Figure 3.9 Side and top views of the adsorption of the inhibitors on the Fe(110) via Monte Carlo simulations: (a) PTU; (b) Urotropine Adsorption energies of PTU and urotropine on the Fe(110) surface had the values of 92.47 57.88 kcal.mol1 The more negative adsorption energy of PTU proved that PTU adsorbed better on the metal surface than urotropine 3.1.6 Mild steel corrosion inhibition mechanism of thiourea derivatives in various environments In HCl solution, PTU was easily protonated to form an organic cation, while the steel surface tended to be electronegative due to the adsorption of Cl− ions in hydrochloric acid Hence, PTU first physically adsorbed onto the negatively charged surface of the steel through electrostatic interaction After H2 was released onto the metal surface, PTU was converted to its neutral state, and then a chemical adsorption process began The HOMO orbital of this compound was located mainly at the thiourea group, so this group was the main adsorption center of PTU, which could donate an electron to the vacant d-orbital of the iron atom to form a coordinatecovalent bond through a nucleophilic interaction In salt solution, the steel surface was oxide covered The cathodic reaction occured on most of the oxide surface while the anodic reaction occured on only a small area of the iron surface This led to the appearance of local corrosion sites, causing pit phenomena Chloride ions occupied these pits PTU inhibited the corrosion by forming a coordinate covalent bond between lone electron pairs of N and S atoms and π electrons in the benzene ring with unoccupied d orbitals of Fe This formed a protective barrier inside the pit, preventing Cl− adsorption 13 3.2 INVESTIGATION OF ANTIOXIDANT CAPACITY OF THIOUREA DERIVATIVES 3.2.1 Investigation of antioxidant capacity of thiourea derivatives by DPPH• assay IC50 of ITU and PTU were calculated to be 0.084 M and 5.5.104 M, respectively These proved that PTU could capture DPPH• free radicals better than ITU 3.2.2 Investigation of antioxidant capacity of thiourea derivatives by ABTS•+ assay IC50ABTS of ITU and PTU had values of 6.6.103 M and 3.4.104 M, respectively These results showed that PTU was able to neutralize ABTS•+ free radical cation stronger than ITU Both experimental methods showed that PTU had better antioxidant capacity than ITU 3.2.3 Investigation of the antioxidant ability of thiourea derivatives by quantum chemical calculations 3.2.3.1 Evaluation of the accuracy of ROB3LYP/6311++G(2df,2p)//B3LYP/6-311G(d,p) method in bond dissociation energy calculation Based on the studied results, ROB3LYP/6311++G(2df,2p)//B3LYP/6-311G(d,p) accurately generated the BDE values with the average fluctuation within 2.0 kcal.mol-1 Therefore, this method was reasonably chosen for further computational calculations of all other thermoparameters in next works 3.2.3.2 Evaluation of the antioxidant capacity of thiourea derivatives by HAT and SET mechanisms Figure 3.10 Optimized structure of ITU at B3LYP/6-311G(d,p) In case of ITU, C4H8 bond had the smallest bond dissociation energy value Meanwhile, N12H13 bond was most easily dissociated in PTU molecule Comparing these two values, 14 BDE (N12H13) of PTU was smaller than BDE (C4H8) in both gas and ethanol solvent Therefore, PTU was able to give hydrogen atoms more easily than ITU when considering HAT mechanism Table 3.7 Calculated BDE and IE values of ITU and PTU in gas and ethanol at ROB3LYP/6311++G(2df,2p)//B3LYP/6311G(d,p) ITU PTU Thermal parameters N2H9 C4H8 N12H13 N15H16 N15H17 (kcal.mol1) 94.74 93.56 86,13 99,53 94,01 (96.80) (92.11) (87,53) (101,11) (97,77) 174.29 IE 175.22 (118.86) (118.04) (Data in parenthesis were calculated in ethanol) As shown in Table 3.7, IE values of PTU and ITU were not significantly different in both gas and solvent phase It meant that the electron exchanging ability of these two substances were similar Therefore, BDE values decided the difference in antioxidant capacity of ITU and PTU The above theoretical calculation results were in accordance with the experimental results of DPPH• and ABTS•+ methods PTU always showed to be a better antioxidant than ITU Therefore, PTU was selected as an object for the next research 3.2.4 Comparison of antioxidant ability of 1-phenyl-2-thiourea (PTU) and 1-phenyl-2-selenourea (PSeU) Thermal parameters of PTU and PSeU were calculated in Table 3.8 Antioxidant ability of PTU and PSeU were compared and evaluated via three mechanisms HAT, SETPT, SPLET 3.2.4.1 Hydrogen atom transfer (HAT) In the gas phase, PseU’s BDE values of N12H13, N15H16, N15H17 bonds were 83.00; 91.40; 86.91 kcal.mol1, respectively Compared to PTU, these values were smaller 3.13; 8.13 and 7.10 kcal.mol1 In ethanol, BDE (NH) values of both PTU and PSeU increased slightly from 1.394.28 kcal.mol1 (compared to calculated values in the gas phase) And N12H13 was found to be the most susceptible bond with the smallest BDE values for both compounds BDE 15 3.2.4.2 Single electron transfer followed by proton transfer (SET-PT) Based on the data given in Table 3.8, IE values in the gas phase of the studied compounds were arranged in the following order: PTU (175.22 kcal.mol1)> PSeU (168,04 kcal.mol1) This order of these IE values was similar in ethanol solvent The PDE values in ethanol solvent were significantly smaller than these values in the gas phase This indicated that the proton dissociation of PTU and PSeU in ethanol solvent were preferred Table 3.8 Thermal parameters of the studied compounds in gas phase and ethanol phase at ROB3LYP/6311++G(2df2p)//B3LYP/6311G(d,p) in kcal.mol1 PTU PSeU Thermal parameters N12H13 N15H16 N15H17 N12H13 N15H16 N15H17 (kcal.mol1) BDE 86.13 (87.53) IE PDE PDE+IE PA ETE PA+ ETE 225.42 (14.45) 400.64 (133.31) 336.50 (42.57) 64.14 (90.74) 400.64 (133.31) 99.53 (101.11) 175.22 (118.86) 238.82 (28.03) 414.04 (146.89) 349.05 (50.90) 64.99 (95.99) 414.04 (146.89) 94.01 (97.77) 233.30 (24.69) 408.52 (143.55) 340.49 (47.23) 68.03 (96.32) 408.52 (143.55) 83.00 (86.18) 91.40 86.91 (93.89) (91.18) 168.04 (112.76) 229.47 237.87 233.37 (19.20) (26.91) (24.20) 397.51 405.91 401.41 (131.97) (139.67) (136.97) 332.66 345.35 336.48 (39.61) (48.68) (44.85) 64.85 60.56 64.93 (92.35) (91.00) (92.12) 397.51 405.91 401.41 (131.97) (139.67) (136.97) (Data in parenthesis were calculated in ethanol) 3.2.4.3 Sequential proton loss electron transfer (SPLET) The data in Table 3.8 showed that ethanol solvent significantly influenced PA values and proton separation in ethanol was preferred However, the increase of ETE values indicated that electron transfer of anion was not favorable for both PTU and PSeU Based on the calculated thermal parameters related to HAT, SPLET and SETPT mechanisms, PSeU appeared to be a stronger antioxidant than PTU and HAT mechanism was more favorable than SPLET and SETPT mechanisms 16 3.2.5 Investigation of the antioxidant ability of selenourea derivatives Figure 3.11 The general formula for C4 position substituted selenourea derivatives 3.2.5.1 Influence of para substituents on IE and (N12H13)BDE values) Electron donating groups tended to significantly reduce the bond dissociation energies and ionization energies Table 3.9 Corrected Hammett constants and BDE and IE values of selenourea derivatives Corrected IE BDE(N12H13) Hammett Compounds 1 1 + (kcal.mol ) (kcal.mol ) constants (σp ) 76.22 152.71 N(CH3)2PSeU 1.70 77.40 156.96 NH2PSeU 1.30 79.18 161.37 OCH3PSeU 0.78 80.83 165.35 CH3PSeU 0.31 80.89 170.45 FPSeU 0.07 0.11 80.88 170.51 Cl PSeU 0.61 81.29 174.70 CF3PSeU 0.66 81.10 177.33 CNPSeU 0.79 81.66 178.91 NO2PSeU Figure 3.12 showed that IE, BDE(N12H13) of selenourea derivatives and corrected Hammett constants had a linear relationship with the R2 determination coefficients of 0.982; 0.849, respectively Compared to BDE(N12H13), IE had a better linear relationship with σp+ These assessments could provide useful directions in the development of potential antioxidants 17 82 175 IE  10.19. p  169.66 170 R  0.982 BDE(N12-H13) (kcal.mol-1) IE (kcal.mol-1) 180 165 160 155 150 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 81 BDE  2.09. p  80.52 R  0.849 80 79 78 77 76 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -1 Relative energy (kcal.mol ) σ p σ p + + (a) σp and IE (b) σp and BDE(N12-H13) Figure 3.12 Relationship between the corrected Hammett constants and (a) IE values; (b) (N12H13) BDEs of selenourea derivatives 3.2.5.2 HOO• free radical scavenging capacity of selenourea derivatives Reactants Inter1 TS Inter Products Figure 3.13 Potential energy surface of the reaction between PSeU and HOO• M052X/6311++G(d,p) was applied to calculate the kinetics of the reaction between PSeU derivatives and HOO• PSeU 18 and its derivatives played a role in scavenging free radical via either H-abstraction at NH bonds Because BDE of (N12H13) had the lowest value, the following studies focused primarily on this bond The geometries of all intermediates and transition states involving in the reaction of PSeU and HOO• were illustrated in Figure 3.13 as a typical example for selenourea derivatives In addition, the effect of substituted groups on potential energy surface was investigated The reactions also started with the formation of Inter lying below reactants of 4.94,4.95, 8.85, 4.26; 4.42; 12.69; 5.31; 5.43 and 5.67 kcal.mol1 corresponding to para-substituted groups of F, Cl, CH3, OCH3, NH2, N(CH3)2, CF3, CN, NO2 Following this order of substituted groups, the reactions passed transition states (TS) at the energy barriers of 6.29; 6.28; 6.30; 4.68; 2.98; 3.68; 6.09; 6.29 and 6.23 kcal.mol1, respectively Obviously, transition state energy values of the donating groups were much lower than those of the withdrawing groups The arrangement was similar when the product complexes were calculated with energy values of 13.02; 11.72; 9.80; 10.65; 13.19; 12.58; 11.91; 13.23; 12.17 kcal.mol1 corresponding to substituted groups of F, Cl, CH3, OCH3, NH2, N(CH3)2, CF3, CN, NO2 3.3 DESIGN OF COMPOUNDS THAT CAN INHIBIT METAL CORROSION AND PREVENT OXIDATION CH3OPSeU was selected as an object to study the ability to inhibit metal corrosion and to prevent oxidation by quantum chemical calculations 3.3.1 Calculation of metal corrosion inhibition parameters Figure 3.14 Structure of CH3OPSeU Most quantum chemical parameters (except ELUMO) showed CH3OPSeU had better corrosion inhibition than PTU In acid solution, CH3OPSeU and PTU might undergo protonation at heteroatom sites (N, S) However, pPTUS18 and 19 pCH3OPSeUSe18 were the most stable protonated conformations of PTU CH3OPSeU due to their lowest energies Therefore, they were selected for the next research pPTUS18 pCH3OPSeUSe18 Figure 3.15 Protonated structures of PTU and CH3OPSeU The quantum chemical parameters of protonated conformations (except ELUMO) also showed that pCH3OPSeUSe18 was a better inhibitor than pPTUS18 Table 3.10 Quantum chemical parameters for neutral and protonated inhibitors in the gas and acid solution at B3LYP/6-311G(d,p) S EHOMO ELUMO ΔE(L-H) χ η ΔN Compounds (eV) (eV) (eV) (eV) (eV) (eV1) 2.98 2.20 0.45 0.91 CH3OPSeU 5.18 0.78 4.40 8.06 2.69 0.37 0.20 pPTUS18 10.75 5.37 5.37 7.39 2.27 0.44 0.09 pCH3OPSeUSe18 9.66 5.13 4.53 Molecular dynamics simulation was applied to study the interaction of pPTUS18 and pCH3OPSeUSe18 on Fe(110) in 1.0 M HCl solution at 303 K (a) pPTUS18 (b) pCH3OPSeUS18 Figure 3.16 Adsorption configuration of (a) pPTUS18 and pCH3OPSeUSe18 on Fe (110) surface in 1.0 M HCl solution Interaction energy of pPTUS18 and pCH3OPSeUSe18 had values of 558.40 and 560.46 kcal.mol1, respectively The more negative interaction energy of pCH3OPSeUSe18 showed that pCH3OPSeUSe18 adsorbed on the surface of Fe (110) more 20 strongly than pPTUS18 3.3.2 Calculation of specific quantities for antioxidants First, the antioxidant capacity of CH3OPSeU was assessed via HAT mechanism The energies of the transition states had values of 4.68; 5.31 and 5.43 kcal.mol1 at positions of N12H13, N15H16 and N15H17 at M052X/6311 ++G(d,p) The hydrogen transfer rate constants (kHAT) of CH3OPSeU were calculated by Eyringpy software with the values of 4.09.106 Ms at N12H13 bond, 8.43.103 Ms at N15H16 bond and 1.32.103 Ms at N15H17 bond Thus the total rate constant via HAT mechanism was 4.10.106 Ms Meanwhile, the reaction rate constant of Trolox - a typical antioxidant had a value of 2.74.106 Ms when participating in hydrogen atom transfer reaction with free radicals HOO• This proved that CH3OPSeU was able to react with HOO• free radicals 1.5 times faster than Trolox SET was the next mechanism used to assess the antioxidant capacity of CH3OPSeU in the gas phase The electron donating process (reaction 3.1), electron accepting process (reaction 3.2) between CH3OPSeU and HOO• could occur as follows: CH3OPSeU + HOO → CH3OPSeU+ + HOO (3.1) CH3OPSeU + HOO → CH3OPSeU + HOO+ (3.2) Enthalpy (ΔH0) and Gibbs free energy (ΔG0) at 298.15 K of reaction (3.1) had values of 143.58 and 143.30 kcal.mol, respectively, while these values of reaction (3.2) were 275.58 and 276.83 kcal.mol These proved that the reaction (3.1) was more thermodynamically favorable than the reaction (3.2) With this approach, electron transfer rate constant (kSET) had a value of 6.84.10-238 Msfor the reaction (3.1) and zero for the reaction (3.2) Although the reaction (3.1) predominated more than reaction (3.2), kSET value was very small when compared to kHAT This meant that the electron exchanging mechanism was negligible when CH3OPSeU reacted with HOO• Next, antioxidant capacity of CH3OPSeU was investigated through radical adduct formation mechanism (RAF) Among C2, C3, C5 and C6 positions of aromatic ring, the reaction at C6 position had 21 the lowest transition state energy with a value of 23.46 kcal.mol 1 (compared with the energy of the reactants) And this was also the position with the largest reaction rate constant with the value of 1.44.103 Ms The total rate constant via radical adduct formation reaction mechanism between CH3OPSeU and HOO• (kRAF) was 1.96.103 Ms This value was much smaller than kHAT The products of HAT mechanism accounted for 99.99 % of the total products created by HAT, SET and RAF mechanisms *Comments: - Regarding steel corrosion inhibition ability, CH3OPSeU showed to be a better inhibitor than PTU - a highly appreciated inhibitor in experiments - Regarding antioxidant capacity, thermal parameters and kinetic parameters showed that CH3OPSeU was a good antioxidant 22 CONCLUSIONS From the above results, we could draw the following main conclusions: Researched the effects of concentration, structure and temperature on the steel corrosion inhibition ability of PTU and ITU in 1.0 M HCl solution Steel inhibition efficiencies of 5.10 M PTU were 92.00 % at 20 °C, 94.05 % at 30 °C, 96.95 % at 45 °C and 98.96 % at 60 °C, while efficiencies of 5.10 M ITU were 80.80 % at 20 °C, 83.33 % at 30 °C, 90.33 % at 45 °C and 92.65 % at 60 °C PTU inhibited steel corrosion in acidic environment better than in salt environment Compared the steel corrosion inhibition ability of PTU with a traditional inhibitor - urotropine The results showed that PTU could inhibit steel corrosion better than urotropine in both 1.0 M HCl and 3.5 % wt NaCl Researched and compared ABTS+ and DPPH• free radical scavenging capacity of PTU and ITU in ethanol IC50DPPH and IC50ABTS of PTU were 5.5.104 M and 3,4.104 M, while these values of ITU were 0,084 M and 6,6.103 M, respectively These results were completely consistent with the quantum parameters calculated at ROB3LYP/6311++G(2df, 2p)//B3LYP/311G(d,p) Researched on the antioxidant capacity of PSeU and its derivatives By quantum chemical calculations, the anti-oxidation capacity of PSeU was better than PTU In selenourea derivatives, derivatives containing electron donating groups gave better antioxidant capacity than derivatives containing electron accepting groups Quantum chemical parameters of CH3OPSeU and pCH3OPSeU were calculated at B3LYP/6311G(d,p) Results showed that CH3OPSeU could inhibit better than PTU Molecular dynamics simulations also showed that pCH3OPSeU interacted strongly with the metal surface, made its adsorption process on Fe (110) better Among the surveyed antioxidant mechanisms, CH3OPSeU mainly reacted via HAT mechanism when scavenging HOO• free radicals with the fastest reaction rate constant of 4.09.106 Ms1 at N12H13 bond, and the amount of products via this mechanism made up 99.99 % of the total product amount at M052X/6311++G(d,p) CH3OPSeU was selected as a potential corrosion inhibitor and a good antioxidant 23 LIST OF ARTICLES RELATED TO THE THESIS International Journals: Dinh Quy Huong, Tran Duong, Pham Cam Nam (2019), Experimental and theoretical study on corrosion inhibition performance of N-phenylthiourea for mild steel in hydrochloric acid and sodium chloride solution, J Mol Model., 25(7) Dinh Quy Huong, Tran Duong, Pham Cam Nam (2019), Effect of the structure and temperature on corrosion inhibition of thiourea derivatives in 1.0 M HCl solution, ACS Omega, 4(11) Dinh Quy Huong, Tran Duong, Pham Cam Nam (2019), An experimental and computational study of antioxidant activity of N-phenylthiourea and N-phenylselenourea analogues, Vietnam J Chem., 57(4), 469-479 National Journals: Dinh Quy Huong, Tran Duong, Pham Cam Nam (2017), A computational study on para-substituted selenocarbamates as corrosion inhibitors, Hue university journal of science, Natural science, 126(1D), 53-62 Dinh Quy Huong, Tran Duong, Pham Cam Nam (2018), Computational study of antioxidant activity of selenocarbamate, selenothiocarbamate, diselenocarbamate compounds, Proceedings of the conference for young Scientist 2018, Hue University Publishing House, 247-254 Dinh Quy Huong, Tran Duong, Pham Cam Nam (2020), Research on reaction mechanism of 1-(4-methoxyphenyl)-2• selenourea and HOO free radical by quantum chemical calculation, Hue university journal of science, Natural science, 129(1C) (Accepted) 24 ... calculations is necessary in scientific research This is also one of the research orientations that attracts the attention of all scientists all over the world From the above scientific analysis, "A research... 1-phenyl-2-selenourea exhibits better antioxidant capacity than 1phenyl-2-thiourea In selenourea derivatives, compounds containing electron donating groups give better antioxidant capacity than... and ionization energies Table 3.9 Corrected Hammett constants and BDE and IE values of selenourea derivatives Corrected IE BDE(N12H13) Hammett Compounds 1 1 + (kcal.mol ) (kcal.mol ) constants