Substituent effects on the antioxidant capacity of monosubstituted diphenylaminesa DFT study

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 p

Substituent effects on the antioxidant capacity of monosubstituted

diphenylamines: a DFT study

Pham Thi Thu Thao 1,2 , Nguyen Minh Thong 3* , Quan V Vo 4 , Mai Van Bay 5 , Duong Tuan Quang 6 ,

Pham Cam Nam 1*

1

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

2

Department of Chemistry, Hue University of Sciences, Hue University, 77 Nguyen Hue Le Loi, Hue City

53000, Viet Nam

3

The University of Danang, Campus in Kon Tum, 704 Phan Dinh Phung, Kon Tum 58000, Viet Nam 4

The University of Danang, University of Technology and Education, 48 Cao Thang, Da Nang City

55000, Viet Nam

5

Department of Chemistry, The University of Danang, University of Science and Education, 48 Cao Thang,

Da Nang City 55000, Viet Nam 6

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 N  H 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(N  H)s values more accuracy Effects of substituents and substitution positions on the BDE(N  H)s were also examined Moreover, there is a good correlation of BDE(N  H)s with the Hammett's substituent constants Depending on the nature of substituents, electron withdrawing groups increase the BDE(N  H)s but electron donating ones reduce the BDE(N  H)s The hydrogen atom transfer processes from N  H 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

1 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 radical-trapping 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

aminyl radical (Ar2N), and the latter will react with

peroxide converted to the nitroxide form (Ar2NO)

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 NH BDEs of

diphenylamines

The B3P86/6-311G level of theory was tested

for accurately BDE(NH) 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

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 NH 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

2 COMPUTATIONAL METHODS The BDE(NH)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(NH) = H f(YC6H4NC6H5) + Hf(H) 

H f(YC6H4NHC6H5) (1)

where H f ’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

All rate constants (k) were estimated in the gas

phase by using conventional transition state theory

(TST) and 1 M standard state as:

(2)

where k B , 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]

3 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 mono- and di-substituted

diphenylamines, the available experimental

BDE(NH) 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 NH

bond In line with the basis set in combination with

B3P86 functional, we pre-evaluated the BDE(NH)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(NH) 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(NH)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(NH)s for a series

of mentioned diphenylamines and the obtained

values were given in table 1

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(NH)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(NH) for diphenylamine derivatives

seems to be rationalized

Table 1: Benchmark of the calculated BDE(NH)s for a small set of mono- and di-substituted

diphenylamines using B3P86/6-311G

Compounds* BDE(NH) (kcal/mol)

Calculated Expt.[46]

pCH3-Ar2NH 86.4( 0.5) 86.9

pOCH3-Ar2NH 85.0(  0.6)[  0.1] 85.6[85.1]

pNO2-Ar2NH 89.8(  0.6)[  1.2] 90.4[91.0]

pBr-ArNH-Ar-pBr

pCH3

-ArNH-Ar-pCH3

pCH3

O-ArNH-Ar-pOCH3

83.0(  0.3) 83.3

pN(CH3)2

-ArNH-Ar-pN(CH3)2

79.3(  0.2) 79.5

pC(CH3 ) 3

-ArNH-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(NH) of meta and para-monosubstituted 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 1 shows that mono-substitution can occur at the sites numbered from 2

to 6 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 NH bond Therefore, in this work we focused mainly on the BDE(NH)s and the substituent effect at meta and para positions Using B3P86/6-311G method, all

calculated BDE(NH)s values in the gas phase for the studied monosubstituted diphenylamines were given in table 2

The change of BDE(NH) values depends on the

type of substituents and the position of replacement

are shown in figure 2 At meta substitutions (3- and

5-position), the change of BDE(NH) values influenced by substituents is insignificant Halogens, EDG and EWG induce the NH 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(NH) value

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

NH 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(NH) up the amount

of 1.9, 1.4 and 2.6 kcal/mol respectively

Table 2: Calculated BDE(NH) for meta and para

monosubstituted diphenylamines using

B3P86/6-311G method (in kcal/mol)

Y

Substitution position

Figure 2: Change of the BDE (NH) of

monosubstituted diphenylamines by position and

nature of substituent Obviously, the variation in the homolytic bond

dissociation enthalpies of diphenylamines shown in

Figure 2 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 3

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 1 atm The TE is derived from the equation of TE =

RE – GE The calculated results using B3P86/6-311G 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)

(B)

Figure 4: Calculated GE and RE of Y-C6H4

-NH-C6H5 at meta- (A) and para- (B) positions

In the case of meta substitution, the ground

effect and radical effect could be hardly observed when substituents were at positions 3 and 5 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

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(NH)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(NH) values for pCH3,

pOCH3, pNH2, pN(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(NH)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(NH) of monosubstituted diphenylamines

In this section, we mainly try to answer how good

the linear correlations between the Hammett

parameter () with the BDE(NH)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,

Leo and Taft.[49] Plotting fitted values by calculated

values at para positions graphically illustrates

R-squared values for regression models (figure 5) Based on figure 5, a good correlation is observed between Hammett constants with BDE(NH) 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(NH) = 2.8003+

p + 87.0776 (3)

Figure 5: Correlation between BDE(NH)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 HOO 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 2 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+

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 6 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,

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 7

Figure 6: Potential energy surface of reaction of

diphenylamines with CH3OO at M052X/6-311++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

The highest occupied molecular orbital (HOMO)

density surfaces of the TSs in figure 7 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 NHO 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 4 (see table S5 of the SI for more

details)

The effects of substituents on the rate constants

are clear EDG groups reduce the BDE(NH) and

enhance the rate constants, whose values are in the

range of 3.97105 L.mol-1.s-1 and the opposite trends

are found for EWG groups The reaction rates are

about 5.06101 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/6-311++G(d,p) method at 298.15 K in the gas phase

Substituent

at para site,

Y

Reactions

∆G≠

(kcal/

mol)

k

(L.mol1.s1)

CH3OO

17.2 1.08103

N(CH 3 ) 2 p- N(CH3 ) 2

-Ar-2NH + CH3OO

11.5

3.97105

NO 2 p-NO2-Ar2NH +

CH3OO

19.9

5.06101

Trolox Trolox +

CH3OO

10.9

3.97106

BHT BHT + CH3OO 14.5 1.51104

4 CONCLUSIONS The B3P86/6-311G method has shown an excellent performance of accurate prediction of the bond energy for the NH bond This method can reproduce the BDE(NH)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(NH)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(NH) but EWGs increase the BDE(NH) The remarkable decrease of BDE(NH)

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(NH) of para

monosubstituted diphenylamines is also obtained

The potential energy surfaces of reactions of

para-substituted 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 1.08103-3.97105 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

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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