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Cite this paper: Vietnam J Chem., 2020, 58(6), 742-751 Article DOI: 10.1002/vjch.202000065 Substituent effects on the antioxidant capacity of monosubstituted diphenylamines: a DFT study Pham Thi Thu Thao1,2, Nguyen Minh Thong3*, Quan V Vo4, Mai Van Bay5, Duong Tuan Quang6, Pham Cam Nam1* Department of Chemistry, The University of Danang, University of Science and Technology, 54 Nguyen Luong Bang, Hoa Khanh Bac, Lien Chieu, Da Nang City 55000, Viet Nam Department of Chemistry, Hue University of Sciences, Hue University, 77 Nguyen Hue Le Loi, Hue City 53000, Viet Nam The University of Danang, Campus in Kon Tum, 704 Phan Dinh Phung, Kon Tum 58000, Viet Nam The University of Danang, University of Technology and Education, 48 Cao Thang, Da Nang City 55000, Viet Nam Department of Chemistry, The University of Danang, University of Science and Education, 48 Cao Thang, Da Nang City 55000, Viet Nam Department of Chemistry, University of Education, Hue University 34 Le Loi, Hue City 53000, Viet Nam Submitted April 28, 2020; Accepted August 11, 2020 Abstract There are undesirable effects leading to considerable changes in the properties of polymers and plastics since exposing to oxygen undergo oxidative degradation Therefore, investigation of the bond dissociation enthalpies (BDEs) of NH bond for a series of monosubstituted diphenylamines is great interest In this study, DFT-based method B3P86/6-311G was employed to perform this task In comparison with the available experimental data, this method could reproduce the BDE(NH)s values more accuracy Effects of substituents and substitution positions on the BDE(NH)s were also examined Moreover, there is a good correlation of BDE(NH)s with the Hammett's substituent constants Depending on the nature of substituents, electron withdrawing groups increase the BDE(NH)s but electron donating ones reduce the BDE(NH)s The hydrogen atom transfer processes from NH bond of these diphenylamines to the peroxyl radical (CH3OO) were also analyzed via potential energy surfaces and kinetic calculations Keywords Antioxidants, diphenylamine derivatives, DFT, substituent effects, Hammett’s constants INTRODUCTION In modern society, polymers and plastics are playing an increasingly important role and the products made from them are indispensable However, when being exposed to oxygen undergo oxidative degradation, there are undesirable effects leading to considerable changes in the properties.[1] Hence, preventing and decreasing the degradative changes in the properties are the challenges faced by researchers One of the solutions to retard the degradative process is to add small amounts of antioxidants into the polymer or plastic products Antioxidants can be broadly defined as compounds that can prevent or slow damage to cells caused by free radicals.[2] Based on their mechanism of interference, there are able to arrange into two types including: Preventive antioxidants and radicaltrapping antioxidants (or chain-breaking antioxidants) To retard or stop the propagation and autoxidation process, radical-trapping antioxidants and preventive antioxidants were added, they react with chain-carrying peroxyl radicals to yield unreactive radicals.[2-4] Diphenylamine (Ar2NH) and its derivatives are used as radical-trapping antioxidants that have the potential of prohibiting oxidation of lubricants, rubber, polymers, and biological materials.[5-13] The antioxidant mechanisms of diphenylamine behave as autoxidation inhibitors relate the hydrogen atom donating ability of the amino group to the peroxyl radicals carrying to yield a non–radical products and 742 Wiley Online Library © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Vietnam Journal of Chemistry aminyl radical (Ar2N), and the latter will react with peroxide converted to the nitroxide form (Ar2NO) The antioxidant capacity of diphenylamine might be explained by the formation of unreactive radicals (Ar2N) that cannot propagate the chain reaction,[6,14,15] or by Denisov reaction cycle.[14,15] Therefore, in this study, the hydrogen atom transfer (HAT) mechanism would be clarified because its antioxidant activity depended on the hydrogen donating ability Consequently, the N–H bond dissociation enthalpy (BDE) represents one of the essential descriptors in the estimation of their antioxidant action.[6,16-19] In general, the physicochemical properties of molecule change significantly when one atom in the molecule was substituted by another atom or functional group.[20] In the case of diphenylamine, substituent effects on the strength of the N–H bond are vital to predict several chemical and thermochemical properties and are still attracting much research attention.[21] In particular, Pratt et al showed that the BDE values of aromatic amines, including diphenylamines are affected by different substituents.[22] Later on, Poliak et al extensively studied the effects of substituent and substituted position on the N–H BDE values in diphenylamine derivatives using (U)B3LYP/6-311++G(d,p) approach.[23] Presently, several experimental methods[16,24-27] and high level computational chemistry approaches[23,28-35] have been used to determine the BDE(N–H) However, there remains a disadvantage because the computations for molecules with over eight heavy atoms spends a lot of time and requires ultrafast processing speed of computer As mentioned above, the previous study showed that the B3LYP method with unrestricted formalism and need to be further improved.[23] Therefore, the first aim of this work is to answer the question whether the low cost computational methods could predict accurately the NH BDEs of diphenylamines The B3P86/6-311G level of theory was tested for accurately BDE(NH) by comparing with the real BDE values Moreover, the effects of various electron donating or electron withdrawing group on the change of the BDE(N–H) of diphenylamine were also systematically studied when substitution occurred at the ortho, meta and para position for only one aromatic ring The relationships between the calculated BDEs and Hammett’s constant were also taken into account Obviously, it is well-known that due to the steric effect, the application of Hammett equation to the ortho substitution of the phenolic ring is unsuccessful.[36] Therefore, our Nguyen Minh Thong et al investigation was focused on the case when one substituent was placed at the meta and para site of one phenolic ring of diphenylamine The second major aim of this work is to understand the antioxidant mechanisms of the diphenylamines The potential energy surfaces (PES) of reactions between the substituted diphenylamines with CH3OO radical were calculated at M05-2X/6-311++G(d,p) level of theory Rate constants for hydrogen atom transfer processes at the NH bond were also computed at the same level of theory using the conventional transition state theory (TST) Figure 1: Diphenylamine and its meta and para monosubstituted derivatives COMPUTATIONAL METHODS The BDE(NH)s for a number of diphenylamines were accurately evaluated using the density functional of B3P86 with unrestricted formalism for open shell The obtained results were then compared with available experimental values.[37,38] The major factors of homolytic BDE used for determining antioxidant capacity are calculated using the equations (1): BDE(NH) = Hf(YC6H4NC6H5) + Hf(H) Hf(YC6H4NHC6H5) (1) where Hf’s are the enthalpies at 298.15 K of each species in the equation (1) The energy of hydrogen atom was calculated at the corresponding level of theory for B3P86 because the energy-lowering corrections for the hydrogen atom will considerably underestimate the BDE’s in this case are in much better agreement with the experimental values This result was consistent with the previous studies.[38,39] The global minima for reactants, pre-reactive complex (RC), product complex (PC) and products are checked with no imaginary vibrational frequency, whereas transition states (TS) were successfully obtained with one imaginary frequency with negative value and vibrational mode of above imaginary frequency should match the action of the reaction paths To build the potential energy surface then to calculate rate constants, all of species were performed at M05-2X/6-311++G(d,p) level of © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 743 Vietnam Journal of Chemistry Substituent effects on the antioxidant capacity of… theory.[40] All rate constants (k) were estimated in the gas phase by using conventional transition state theory (TST) and M standard state as: Table 1: Benchmark of the calculated BDE(NH)s for a small set of mono- and di-substituted diphenylamines using B3P86/6-311G (2) where kB, T, h, ΔG#, σ and in the equation (2) are the Boltzmann constant, the temperature, Planck constant, the gas constant, the Gibbs free energy of activation, the reaction symmetry number and accounts for tunneling corrections, respectively [41,42] All computational calculations were carried out using Gaussian 09 suit of program.[43] Rate constants in the gas phase were generated from output files of the Eyringpy program.[44,45] RESULTS AND DISCUSSION 3.1 Performance of the proposed DFT method for predicting bond dissociation enthalpies of a few diphenylamines with available experimental values Among the monoand di-substituted diphenylamines, the available experimental BDE(NH) values of diphenylamine derivatives were measured and estimated.[46] Therefore a brief comparison should be carried out to evaluate the reliable performance of these proposed methods when applying on these derivatives having the NH bond In line with the basis set in combination with B3P86 functional, we pre-evaluated the BDE(NH)s for diphenylamine (Ar2NH) using several basis sets then compared with the experimental value of Ar2NH (87.2 kcal/mol).[46] The discrepancy between the calculated BDE(NH) at each basis set and experimental one was shown figure S1 of Supporting Information - SI, indicating that the smallest discrepancy is at the basis set of 6-311G and 6-31G Whereas, BDE(NH)s are underestimated in the range of 2.0 to 4.2 kcal/mol when adding the polarized and diffuse functions To further test the performance of B3P86/6-311G method, we calculated the BDE(NH)s for a series of mentioned diphenylamines and the obtained values were given in table Based on the data in table 1, it is clear that B3P86/6-311G method was found to be appropriate for the prediction of BDE(NH)s with the mean of differences is only -0.2 kcal/mol Thus, the B3P86 functional with a small basis set 6-311G used for predicting BDE(NH) for diphenylamine derivatives seems to be rationalized Compounds* BDE(NH) (kcal/mol) Calculated Expt.[46] Ar2NH 87.2(0.0) 87.2 mF-Ar2NH 88.4 88.1(0.3) mCH3-Ar2NH 87.6(0.0) 87.6 pCH3-Ar2NH 86.9 86.4(0.5) pOCH3-Ar2NH 85.6[85.1] 85.0(0.6)[0.1] pNO2-Ar2NH 90.4[91.0] 89.8(0.6)[1.2] pBr-ArNH-Ar87.0(0.0) 87.0 pBr pCH3-ArNH-Ar85.6(0.2) 85.4 pCH3 pCH3O-ArNH-Ar83.3 83.0(0.3) pOCH3 pN(CH3)2-ArNH79.5 79.3(0.2) Ar-pN(CH3)2 pC(CH3)3-ArNH85.9(0.1) 85.8 Ar-pC(CH3)3 Data in parentheses (BDE = BDEcalc – BDEexpt.) *The information of Cartesian optimized geometries and energies of these compounds and the corresponding radicals can be found in table S1, SI 3.2 BDE(NH) of meta and paramonosubstituted diphenylamines and the effect of substituents The introduction of the substituents with different nature into an aromatic ring gives compounds with unique properties Concerning to monosubstituted diphenylamines, figure shows that monosubstitution can occur at the sites numbered from to on the benzene ring As mentioned in the introduction part, for an ortho substituted position, the rule of the substituent effect did not reveal due to the steric effect on the adjacent NH bond Therefore, in this work we focused mainly on the BDE(NH)s and the substituent effect at meta and para positions Using B3P86/6-311G method, all calculated BDE(NH)s values in the gas phase for the studied monosubstituted diphenylamines were given in table The change of BDE(NH) values depends on the type of substituents and the position of replacement are shown in figure At meta substitutions (3- and 5-position), the change of BDE(NH) values influenced by substituents is insignificant Halogens, EDG and EWG induce the NH BDEs change with the amount smaller than 1.6 kcal/mol However, the substituent effect is considerably observed at the para position The strong EDGs like NH2 and N(CH3)2 at the para site reduces BDE(NH) value © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 744 Vietnam Journal of Chemistry Nguyen Minh Thong et al remarkably and the differences compared with the parent diphenylamine are of 4.3 and 4.5 kcal/mol, respectively In contrast, the EWGs increase of the NH BDE values of para monosubstituted diphenylamines The stronger EWGs the larger enhancement of the BDE For instance, CF3, CN and NO2 groups increase the BDE(NH) up the amount of 1.9, 1.4 and 2.6 kcal/mol respectively Table 2: Calculated BDE(NH) for meta and para monosubstituted diphenylamines using B3P86/6311G method (in kcal/mol) Y H F Cl CH3 OCH3 NH2 N(CH3)2 CF3 CN NO2 3Y 87.2 87.9 87.7 87.2 87.7 87.4 87.4 88.0 88.0 88.1 Substitution position Meta para 5Y 4Y 87.2 87.2 88.1 86.7 87.9 87.2 87.6 86.4 88.0 85.0 87.0 82.9 86.8 82.7 88.3 89.1 88.2 88.2 88.8 89.8 Figure 3: Exchange reactions for GE, RE and TE Based on the thermodynamic viewpoint, the GE and RE are the enthalpies of the reaction of the first two reactions in Figure 3, one of which is the change in enthalpy of reaction calculated for 298.15K and atm The TE is derived from the equation of TE = RE – GE The calculated results using B3P86/6311G for GE, and RE were drawn in Figure 4, in which the upper is the data for meta sites (3Y and 5Y) and the lower is for the para site (4Y) (A) Figure 2: Change of the BDE (NH) of monosubstituted diphenylamines by position and nature of substituent (B) Figure 4: Calculated GE and RE of Y-C6H4-NHC6H5 at meta- (A) and para- (B) positions Obviously, the variation in the homolytic bond dissociation enthalpies of diphenylamines shown in Figure depends robustly on the position and nature of substituent and needs to be quantified The change of the BDEs can be explained in terms of ground effect (GE), radical effect (RE) and total effect (TE) These parameters are calculated from the isodesmic reactions between monosubstituted diphenylamines and related species and expressed in figure In the case of meta substitution, the ground effect and radical effect could be hardly observed when substituents were at positions and on the aromatic ring Figure 4A indicates the change of neutral and radical derivatives in comparison to the diphenylamine and its radical when substituent Y is at 3- and 5-ring sites Generally, they change inconsiderably the stabilization of the neutral and the radical species Both EDG and EWG substituents slightly stabilize the parent diphenylamine and the © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 745 Vietnam Journal of Chemistry calculated GEs are just smaller than 0.6 kcal/mol For radical species, EWGs destabilize the radical species but the largest calculated RE values are just within 0.7-1.2 kcal/mol Generally, it can be stated that with the “O pattern”, the ground and radical effects are insignificant when both EDG and EWG substituents are at the meta position Consequently, this causes the BDE(NH)s to slightly change only from 0.0 to 1.6 kcal/mol The effect trend is more striking when substituents are at the para position Figure 4B shows that all substituents stabilize the corresponding radicals, except for Y = F, Cl and CF3 It should emphasize that a negligible impact was observed for halogen and all EWG substituents but the significant effect for EDG: The stronger donating electron group, the higher stabilization of the radical However, there is a clearly opposite impact of the EDG and EWG on the stabilization of ground states EWGs stabilize the ground species, but destabilization is found for EDGs Based on the data in figure 4, in case of EDG the calculated enthalpies of the ground stabilization were around of +1.0 kcal/mol and -1.7 to -3.1 kcal/mol for EWG The calculated BDE(NH) values for pCH3, pOCH3, pNH2, pN(CH3)2 are 86.4, 85.0, 82.9 and 82.7 kcal/mol, respectively The behavior of the EDG and EWG can be explained that nitrogen atom possesses an electron lone pair, diphenylamines belong to the so called the “Class O” category,[47,48] in which a radical is stabilized by the electron donating substituent at the para position and destabilized by the electron withdrawing one It also means that the 4-EDG diphenylamine derivatives are slightly more active than the parent diphenylamine but their radical forms are more stable than that of diphenylamine However, strong electron donating groups substituted at the para positions induce, with a sharp decrease of BDE(NH)s, meanwhile these compounds enrich electron density at the phenolic rings and easy react with oxygen to produce hydroperoxides, rendering them pro-oxidants It is considered as an important remark for design and synthesize of potential antioxidant 3.3 Correlation of Hammett parameter with BDE(NH) of monosubstituted diphenylamines In this section, we mainly try to answer how good the linear correlations between the Hammett parameter () with the BDE(NH)s can be found when substitution takes place at the para site of the phenol ring in which p+ values were taken from the compilations of Hammett parameters by Hansch, Substituent effects on the antioxidant capacity of… Leo and Taft.[49] Plotting fitted values by calculated values at para positions graphically illustrates Rsquared values for regression models (figure 5) Based on figure 5, a good correlation is observed between Hammett constants with BDE(NH) values in case of para substitution with the R-squared of 0.9681 The linear equation from straight line fitting of the para monosubstituted diphenylamines is expressed in the equation (3): 4-position: BDE(NH) = 2.8003+p + 87.0776 (3) Figure 5: Correlation between BDE(NH)s vs Hammett constants at para monosubstituted diphenylamines 3.4 The radical scavenging activity of the studied compounds 3.4.1 Mechanism evaluating It is generally observed that the radical scavenging was mainly focused on the HOO and HO radicals, however the high reactivity of HO and H-bond interactions of HOO with antioxidants may affect the results.[50] In the case of the B3P86 functional a paper by Pereira and co-workers showed this functional has a good performance in geometry optimizations but underestimate the activation barriers by kcal/mol.[51-53] Therefore, in this paper, the antiradical activity of the monosubstituted diphenylamines was investigated against CH3OO radical at the M05-2X/6-311++G(d,p) level[40] with several possible reaction mechanisms, according to the following expressions: i) Formal hydrogen transfer – FHT: Y-Ar2NH + CH3OO Y-Ar2N + CH3OOH ii) Single electron transfer - SET: Y-Ar2NH + CH3OO Y-Ar2NH+ + CH3OO iii) Proton transfer - PT: Y-Ar2NH + CH3OO Y-Ar2N + CH3OOH+ © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 746 Vietnam Journal of Chemistry iv) Radical adduct formation – RAF: Y-Ar2NH + CH3OO [Y-Ar2NH-OOCH3] To find the most likely pathway for antiradical activity of the studied compounds, the Gibbs free energies for each mechanism was calculated in vacuum a reaction with CH3OO radical For the RAF mechanism, because there are six positions in each ring of diphenylamines therefore it should be determined the favored site for formation of [Ar2NH-OO CH3] adduct Obviously, this reaction is favor at the site of more atomic charge For instance in Ar2NH, C6, C2 and C4 are more negative charges than other sites, in which the CH3OO adduct reactions will have the transition states lying lower than the remains on the PES Indeed, PES of adduct reaction between CH3OO and Ar2NH shown in figure S2 of the SI has reconfirmed this observation Therefore, in the substituted diphenylamines we mainly focused on addition reactions of CH3OO radical on the C6 site of p-Y-Ar2NH Based on the values of Gibbs free energies (ΔG) obtained from table S3 of SI, only the reactions following FHT pathway yielded exothermic and spontaneous reactions On the contrary, SET, PT, and RAF mechanisms are not spontaneous in the studied environment Hence the FHT mechanism is favored for the ROO radical scavenging activity of the monosubstituted diphenylamines Thus this mechanism was further study in the kinetic calculations 3.4.2 Potential Energy Surface (PES) In this section, PES of reactions between CH3OO radical and the monosubstituted diphenylamines (Y = H, N(CH3)2, and NO2) was investigated following the HAT mechanism at the M05-2X/6-311++G(d,p) level because of the high recommendation.[40] The details of the Cartesian coordinates of all structures in the selected reaction of Y-Ar2NH + CH3OO were shown in Table S4 of the SI Figure shows that all reaction paths of the studied compounds with CH3OO tend to be similar The first pre-reactive complex (RC) was formed, whose relative energy is lower than that of reactants The transition state (TS) that decribes the hydrogen donating process from diphenylamines to the radical end of CH3OO was found, lying higher than that of RC After passing the TS, the product complex (PC) is produced, whose structure indicates that the H atom completely tranfers to CH3OO In figure 6, to clarify the effect of substituents on the reaction path, the substituents were devided into three groups (X, Nguyen Minh Thong et al EDG, and EWG), in which X includes H; EDG stands for the electron donating substituents (N(CH3)2) and EWG is the electron donating ones (NO2) The effects of the substituents on the reaction channels are clearly described in figure 6, in which the EDG substituents reduce energy barriers of the TS larger than the EWG ones This observation is quite consistent with the effects of substituents on the BDE(N-H)s To gain further insight into mechanism of the radical scavenging, the frontier orbitals for TS structures were used to analyse the single entity (H) or proton coupled electron (one H+ and one e) transfer process,[54, 55] and shown in figure Figure 6: Potential energy surface of reaction of diphenylamines with CH3OO at M052X/6311++G(d,p) (Y = H, N(CH3)2 and NO2) Figure 7: Structure, the frontier orbitals density surfaces of transition states for the selected compounds reaction with CH3OO● radical © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 747 Vietnam Journal of Chemistry The highest occupied molecular orbital (HOMO) density surfaces of the TSs in figure show that there is an overlap between a delocalized -orbitals of the rings and a lone pair on the central peroxyl O of methylperoxyl radical This overlap involves the electron transfer between the N lone pair - ring in the TS structures and central O atoms of methylperoxyl radical As can be seen in figure 7, in the singly occupied molecular orbital (SOMO) density surfaces a significant atomic orbital density oriented along the NHO transition vector is observed at the TSs It means that the proton is transferred along the line connecting the two O and N centers That appears to suggest that the CH3OO scavenging reaction of studied compounds may occur following the PCET mechanism That is also consistent with the previous studies.[56, 57] 3.4.3 Kinetic study The kinetics of the reactions between diphenylamines and CH3OO were also performed for further insights into their radical scavenging activity The Gibbs free energy of activation (ΔG≠) and rate constants (k) were calculated at the M052X/6-311++G(d,p) level at 298.15 K between the studied compounds with the CH3OO radical and given in table (see table S5 of the SI for more details) The effects of substituents on the rate constants are clear EDG groups reduce the BDE(NH) and enhance the rate constants, whose values are in the range of 3.97105 L.mol-1.s-1 and the opposite trends are found for EWG groups The reaction rates are about 5.06101 L.mol-1.s-1 for NO2 In compared with the rate constants of potential antioxidants link Trolox and BHT, these diphenylamines may be considered as the promising radical trapping antioxidants Table 4: The calculated ∆G≠ and k at the M052X/6311++G(d,p) method at 298.15 K in the gas phase Substituent ∆G≠ k at para site, Reactions (kcal/ (L.mol1.s1) Y mol) H-Ar2NH + 17.2 H 1.08103 CH3OO p- N(CH3)2-Ar- 11.5 N(CH3)2 3.97105 2NH + CH3OO p-NO2-Ar2NH + 19.9 NO2 CH3OO 5.06101 10.9 Trolox Trolox + CH3OO 3.97106 BHT BHT + CH3OO 14.5 1.51104 Substituent effects on the antioxidant capacity of… CONCLUSIONS The B3P86/6-311G method has shown an excellent performance of accurate prediction of the bond energy for the NH bond This method can reproduce the BDE(NH)s in monosubstituted diphenylamines to be in agreement with the experimental data Applying the latter approach, the calculated BDE of meta- and para- monosubstituted diphenylamines were predicted and the change of the BDE(NH)s along with the substituents and the substituted position were also been examined At meta position, the change of BDE is within 0.0-2.0 kcal/mol A clear effect trend is found when substituent at the para position Halogens and EDGs reduce the BDE(NH) but EWGs increase the BDE(NH) The remarkable decrease of BDE(NH) is observed when substituents are NH2 and N(CH3)2 The effect of substituents is also explained in terms of radical effect, ground state effect and total effect In addition, the good linear correlation between the Hammett constants and BDE(NH) of para monosubstituted diphenylamines is also obtained The potential energy surfaces of reactions of parasubstituted diphenylamines with CH3OO radical and the rate calculations using TST theory are also performed at M052X/6-311++G(d,p) level of theory It was showed that the diphenylamine derivatives can act as radical trapping antioxidants with the rate constants in the range of 1.08103-3.97105 L.mol-1.s-1 in the gas phase Acknowledgements This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.06-2018.42 REFERENCES I Puškárová, Z Cibulková, M Breza On NMR prediction of the antioxidant effectiveness of heterocyclic nitrogen compounds and substituted amines in styrene-butadiene rubber, Polym Degrad Stabil., 2017, 144, 1-6 L Valgimigli, D A Pratt Antioxidants in Chemistry and Biology, Encyclopedia of radicals in chemistry, biology and materials, 2012 K U Ingold Inhibition of the Autoxidation of Organic Substances in the Liquid Phase, Chem Rev., 1961, 61(6), 563-589 B Li, D A Pratt Methods 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important role in proton-coupled electron transfer reactions, J Am Chem Soc., 2007, © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 750 Vietnam Journal of Chemistry 129(19), 6199-203 55 L Muñoz-Rugeles, A Galano, J Raúl AlvarezIdaboy Non-covalent π–π stacking interactions turn off non-adiabatic effects in proton-coupled electron transfer reactions, Phys Chem Chem Phys, 2017, 19(10), 6969-6972 56 J J Hanthorn, L Valgimigli, D A Pratt Incorporation of ring nitrogens into diphenylamine Nguyen Minh Thong et al antioxidants: striking a balance between reactivity and stability, J Am Chem Soc., 2012, 134(20), 8306-8309 57 E A Haidasz, R Shah, D A Pratt The catalytic mechanism of diarylamine radical-trapping antioxidants, J Am Chem Soc., 2014, 136(47), 16643-16650 Corresponding authors: Nguyen Minh Thong The University of Danang, Campus in Kon Tum, 704, Phan Dinh Phung, Kon Tum City 58000, Viet Nam E-mail: nmthong@kontum.udn.vn Pham Cam Nam Department of Chemistry, University of Science and Technology 54, Nguyen Luong Bang, Hoa Khanh Bac, Lien Chieu The University of Danang, Da Nang City 55000, Viet Nam E-mail: pcnam@dut.udn.vn © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 751 ... 1.51104 Substituent effects on the antioxidant capacity of? ?? CONCLUSIONS The B3P86/6-311G method has shown an excellent performance of accurate prediction of the bond energy for the NH bond This... electron donating substituents (N(CH3)2) and EWG is the electron donating ones (NO2) The effects of the substituents on the reaction channels are clearly described in figure 6, in which the EDG substituents... barriers of the TS larger than the EWG ones This observation is quite consistent with the effects of substituents on the BDE(N-H)s To gain further insight into mechanism of the radical scavenging, the
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