Quantum-chemical modelling and prediction of the structure and functional activity of free radicals and biradicals of N-phenyl-N

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1 Kauchuk i Rezina, No. 4, 2008, pp Quantum-chemical modelling and prediction of the structure and functional activity of free radicals and biradicals of N-phenyl-N -isopropylp-phenylenediamine and N,N -diphenyl-pphenylenediamine E.A. Mukhutdinov and A.A. Mukhutdinov Kazan State Technology University Selected from International Polymer Science and Technology, 35, No. 11, 2008, reference KR 08/04/06; transl. serial no Translated by P. Curtis Secondary aromatic diamines derivatives of p-phenylenediamine are used to protect tyre rubbers against heat ageing, ozone ageing, and fatigue. The high protective action of these compounds is due to the presence in their molecules of two active centres. Widely used inhibitors of this class are N-phenyl-N -isopropylp-phenylenediamine (Diafen FP) and N,N -diphenyl-pphenylenediamine (DPPD) [1]. In studies of the dependence of the properties of antioxidants on their structure [2 4] it has been shown that the structure of the antioxidant molecules must ensure high activity in interaction with peroxide radicals and high activity of the inhibitor radicals formed. Such properties are possessed by derivatives of aromatic amines that contain a mobile hydrogen atom in the NH group [2]. Diafen FP and DPPD are strong antioxidants. The protective action of these inhibitors is based on the high activity of the amino groups, which leads to shortening of the kinetic chains. Even small additions are sufficient to interrupt the chain reaction of oxidation [5]. However, in the literature there is no information on the comparative functional activity of these stabilisers; determining this activity would make it possible to predict the mechanism of synergism when they are used together in tyre rubbers. The aim of the present work is to conduct quantumchemical modelling of the structural parameters, electron structure, and energy characteristics and to predict the comparative functional activity of molecules of Diafen FP and DPPD, monoradicals of Diafen FP (I, II) and DPPD (III), and biradicals (IV, V): Ar N A r NH Pr-iso I Ar N A r NH Ar III Ar N A r N Ar V Ar NH Ar N Pr-iso II Ar N A r N Pr-iso where Ar = C 6 H 5 and Ar = C 6 H 4. The quantum-chemical calculations were conducted by the method of the density functional B3LYP in basal plane 6-311G(d, p) using the Gaussian program package [6]. The illustrations in this paper were generated using the ChemCraft program [7]. Figure 1 presents the three-dimensional structure of a Diafen FP molecule in different states. The initial molecule and the mono- and biradicals have a gauche gauche conformation, and the angles between the aryl groups are practically the same (44 48 ). As can be seen from Figure 1a, a labile hydrogen atom in the ground state of the molecule is positioned at the level of two neighbouring hydrogen atoms of the aryl groups. In the excited state, the torsion angle ArNAr increases, which makes it easier for free mono- IV 2009 Smithers Rapra Technology T/13

2 Figure 1. Three-dimensional structures of different states of Diafen FP: (a) ground state; (b) singlet biradical; (c) triplet biradical Figure 2. Three-dimensional structures of different states of DPPD: (a) ground state; (b) singlet biradical; (c) triplet biradical and biradicals to approach the labile hydrogen atom in processes of inhibition of the ozone and thermo-oxidative ageing of rubbers. It can therefore be assumed that the configuration interaction of Diafen FP molecules in the excited state is made easier. Figure 2 presents the structures of DPPD molecules in different states. A gauche gauche conformation is again characteristic of molecules and mono- and biradicals of DPPD, but the torsion angles of singlet and triplet biradicals differ appreciably (43 and 70 respectively). Of considerable interest is the change in entropy S and specific heat C v during the formation of biradicals from molecules of Diafen FP and DPPD in the singlet and triplet states. Increase in these quantities is due to the formation, by homolytic rupture of the N H bond, of an aminyl monoradical and a single hydrogen atom; with the formation of singlet and triplet biradicals, two hydrogen atoms break away, and, instead of a single initial molecule, three particles appear a single biradical and two hydrogen atoms. Furthermore, in the triplet state, on account of the repulsion of electrons with identical spins, there is a slight increase in the volume of the biradical, which leads to an increase in entropy by comparison with the singlet state for Diafen FP and DPPD, by 21 and 23 J/(mol K) respectively. It is of interest to compare the energy of the ground state and the energy necessary for the homolytic detachment of hydrogen from the NH group with the formation of free mono- and biradicals. These energies are related by the following equation: E detach = E radical E ground where E detach is the energy of homolytic detachment of hydrogen from the ground state of the molecule, E radical is the sum of the energies of formation of an inhibitor monoradical and the corresponding hydrogen radical, and E ground is the energy of formation of the ground state of the molecule. The values of energies E radical and E ground are presented in Tables 1 and 2. For Diafen FP, the E detach values calculated from them for the formation of monoradicals I and II amount to and kj/mol respectively; for DPPD, E detach = kj/mol. From the data given in Table 1 it can be seen that, for Diafen FP, the enthalpy of formation of monoradical II is higher than for monoradical I, which is due to the steric effect of the isopropyl group, which prevents the detachment of the hydrogen atom. The difference in the energies of formation, E, of T/14 International Polymer Science and Technology, Vol. 36, No. 7, 2009

3 Table 1. Comparative characteristics of Diafen FP molecules in different states Characteristics Diafen FP Diafen FP + H Diafen FP + Diafen FP + 2H biradical radical (aryl) H radical (isopropyl) singlet triplet E tot (Hartree) E, kj/mol H (exp.), kj/mol [9] Electron affi nity, ev 1.05 Entropy S, J/(mol K) Specifi c heat C v, J/(mol K) S, J/(mol K) C v, J/(mol K) Low-frequency shift NH, cm 1 : Ar NH Ar 14 Ar NH Pr iso 17 Surplus charges on fragments: Ar Ar Pr iso Surplus charges on groups: Ar NH Ar Ar NH Pr iso Dipole moment, debye Length of NH bond, Å: in Ar NH Ar fragment in Ar NH Pr iso fragment Angle between benzene rings in Ar NH Ar fragment, deg E LUMO, ev E HOMO, ev E LUMO HOMO, ev aminyl mono- and biradicals in the singlet and triplet states is extremely informative. The energies of formation of biradicals of Diafen FP and DPPD for the singlet state of the system are given in reference [8]. The formation of biradicals from Diafen FP and DPPD molecules in the singlet state requires an energy consumption respectively 59.5 and kj/mol lower than the sum of energies of formation of each radical individually. At the same time, for a biradical in the triplet state, the energy consumption for Diafen FP and DPPD molecules is respectively 42.3 and kj/mol greater than the sum of energies of formation of the two monoradicals individually. This may be due to triplet repulsion [9]. On the whole, differences E for Diafen FP and DPPD biradicals in the singlet and triplet states are similar and amount to and kj/mol respectively. The interaction of unpaired electrons in a biradical [10, 11] via indirect exchange interaction when the distance between the radical centres is fairly large must also be pointed out. These distances are identical for aminyl biradicals of Diafen FP and DPPD and are equal to 0.55 nm. Such exchange interaction may be caused by combination of excited states with the ground state, with the participation of electrons of the bridge (in our case, of the phenyl fragment) between the radical centres, and also by the energetic inhomogeneity of electrons with various spins [12]. Two types of excited state contributing to the electron transition energy can be distinguished [10]: the state with charge transfer from the bridge to the free orbital of the radical and the state with triplet excitation of the bridge (phenyl fragment). This enables us to assume that the difference in the enthalpies of dissociation of the N H bond during formation of the biradical comprises the electron transition energy. A low-frequency shift of the absorption band of NH groups on the IR Fourier spectra of Diafen FP during the formation of an NH N hydrogen bond (see Table 1) is characteristic of free mono- and biradicals. The appearance of a low-frequency shift during the formation of monoradicals is due to the redistribution of electron 2009 Smithers Rapra Technology T/15

4 Table 2. Comparative characteristics of DPPD molecules in different states Characteristics of molecules DPPD (gauche) DPPD + H radical DPPD + 2H biradical singlet triplet E tot (Hartree) E, kj/mol H (exp.), kj/mol [9] Electron affi nity, ev Entropy S, J/(mol K) Specifi c heat C v, J/(mol K) S, J/(mol K) C v, J/(mol K) Low-frequency shift NH, cm 1 : symmetric antisymmetric Surplus charges: on Ar fragment on Ar fragment on NH groups ;* Dipole moment, debye Length of NH bond, Å Angle between Ar NH Ar, deg ; ; Torsion angle between phenyl rings, deg E LUMO, ev E HOMO, ev E LUMO HOMO, ev * Charge on the nitrogen atom from which the hydrogen radical is separated density after detachment of the hydrogen atom and to the small increase in the lengths of the N H bonds. To predict the functional activity of the initial molecules of the inhibitors and aminyl mono- and biradicals, the difference in the values of E LUMO and E HOMO, E LUMO HOMO, is of considerable interest. The E LUMO value of the ground state (see Table 1) is slightly less than zero, which is advantageous for positioning on the given electron orbital. On the basis of the data in Tables 1 and 2 it can be observed that, with homolytic rupture of the electron pair, the reactivity of the molecule is determined by the values of E LUMO and E HOMO ; here, the lower the value of E HOMO and the higher the value of E LUMO, the higher is the reactivity of the inhibitor molecule. Thus, E LUMO HOMO can be used as the index of reactivity of the inhibitor molecules. The greater the energetic break, the higher is the kinetic stability and the lower is the chemical reactivity, and vice versa. A graphic example is the gradual reduction in E LUMO HOMO from the ground state to aminyl mono- and biradicals. The given data also indicate a higher reactivity of the formed aminyl mono- and biradicals by comparison with molecules in the ground state. The results obtained make it possible to predict some degree of synergism of Diafen FP and DPPD when they are introduced separately into rubber mixes as a result of likely collision of their molecules in the diffusion region. However, the greatest degree of synergism is possible only after the physicochemical modification of inhibitors in binary melts (the kinetic region). It must be noted that E LUMO HOMO values for free monoradicals I and II of Diafen FP (6.46 and 6.44 ev respectively) and also for a biradical in the singlet state (4.79 ev) are higher than for the free monoradical of DPPD (2.95 ev) and for the biradical of DPPD (4.64 ev), which are kinetically less stable and more reactive. However, the lowest E LUMO HOMO values are characteristic of the triplet state of biradicals (for Diafen FP 2.42 ev, and for DPPD 1.92 ev); consequently, triplet biradicals are the most reactive [13]. This makes it possible to explain the synergism of inhibitors in tyre rubbers from the viewpoint of physical and chemical mechanisms in the following way. If we consider that, during the oxidative degradation of polymer chains, free mono- and biradicals of different activity are formed, then it is logical to assume that the more active polymer radicals initially interact with the more reactive mono- and biradicals of DPPD, and then with the less reactive mono- and biradicals of Diafen FP. After this, the inhibitor radicals will interact in a similar way with the less active polymer radicals, and such T/16 International Polymer Science and Technology, Vol. 36, No. 7, 2009

5 inhibition on the whole will correspond to a chemical mechanism of synergism. Thus, during the inhibition of oxidative and ozone degradation of tyre rubbers, to begin with there is an improvement in dispersion and slower migration of hydrogen-bound inhibitor molecules from the tyre rubbers (physical mechanism), and then inhibition by mono- and biradicals of DPPD and Diafen FP of polymer radicals in the surface layer of the tyre rubbers (chemical mechanism). Comparison of E LUMO HOMO values for different states of the Diafen FP and DPPD molecules makes it possible also to suggest the appearance of dissimilar activity of their aminyl radicals during interaction with polymer macroradicals. Thus, owing to considerable surplus negative charge on the radical ( 0.348) and a comparatively large E LUMO HOMO value (6.46 ev), the interaction of aminyl radicals of Diafen FP may occur under charge control, whereas in the case of aminyl radicals of DPPD, owing to the small E LUMO HOMO value (2.95 ev), the occurrence of a reaction with macroradicals under orbital control can be expected. Here, the following must be pointed out. As, with orbital control, the energies necessary for the addition of an electron to the LUMO ( 4.64 ev) or for detachment of an electron from the HOMO ( 7.59 ev) differ in all by a factor of 1.6 and E LUMO HOMO does not exceed 2.95 ev, it is possible to assume a low kinetic stability and a higher functional activity of aminyl radicals of DPPD molecules than for Diafen FP, for which E LUMO = 0.67 ev, E HOMO = 7.13 ev, and E LUMO HOMO = 6.46 ev. Consequently, with identical values of surplus negative charges on =N ( 0.35 for both molecules), aminyl radicals of DPPD will be more reactive and will be able to interact with polymer radicals, even of low activity, formed by the thermo-oxidative decomposition of rubber macromolecules within the compound. At the same time, the less reactive aminyl radicals of Diafen FP, on account of their higher polarity by comparison with aminyl radicals of DPPD (the dipole moments are 5.34 and 3.60 debye respectively) and poor thermodynamic compatibility with non-polar rubber, will migrate more rapidly into the surface layer of the rubber compound and interact primarily with the more reactive macroradicals of the rubber that are formed under the action of ozone and solar radiation. On the whole, such interaction of aminyl and polymer radicals will not contradict the chemical mechanism of synergism of the Diafen FP DPPD binary mixture. The obtained results of quantum-chemical calculations make it possible to predict the following mechanism of inhibition of thermo-oxidative and ozone ageing of tyre rubbers by Diafen FP and DPPD molecules: 1. Under the action of heat or solar radiation, Diafen FP and DPPD molecules pass into the excited state with a reduction in E LUMO HOMO, which promotes a reduction in N H bond energy, as a result of which the hydrogen atom becomes labile. 2. Homolytic detachment of the labile hydrogen atom of the NH group occurs, with subsequent interaction of the formed free mono- and biradicals with free radicals of the rubber macromolecules, and, as a result, breaking of chain reactions of macromolecule breakdown by ozone with the formation of inactive compounds. REFERENCES 1. K.B. Piotrovskii and Z.N. Tarasova, Ageing and Stabilisation of Synthetic Rubbers and Vulcanisates. Khimiya, Moscow, 1980, 264 pp. 2. Ya.A. Gurvich et al., Phenolic Stabilisers. TsNIITEneftekhim, Moscow, 1978, 80 pp. 3. E.T. Denisov, Usp. Khimii, 47, No. 6, 1978, p E.T. Denisov, Khim. Fizika, 4, 1985, p M.E. Cain, Rubb. J., 150, No. 11, 1968, p M.J. Frisch et al., Gaussian 98. Gaussian, Inc., Pittsburgh, PA, G.A. Zhurko and D.A. Zhurko, ChemCraft: Tool for Treatment of Chemical Data. ChemCraftProg, Moscow, E.A. Mukhutdinov et al., Kauch. i Rezina, No. 3, 2007, p E.T. Denisov, Kinetika i Kataliz, 36, No. 3, 1995, p V.N. Parmon et al., Stable Biradicals. Nauka, Moscow, 1980, 240 pp. 11. A.L. Buchachenko, Weak exchange interactions in nitrate bi- and polyradicals, in Free Radical States in Chemistry, ed. by L.A. Blyumenfel d and Yu.N. Molin. Nauka, Novosibirsk, 1972, p K. Zhen, Electron paramagnetic resonance study of trapped radicals, in Formation and Trapping of Free Radicals, ed. by A.N. Bass and H.P. Broid. New York/London, 1960; Izdatinlit, Moscow, 1962, p. 250 (trans. from English). 13. Concise Chemical Encyclopaedia, Vol. 4. Sovetskaya Entsiklopediya, Moscow, 1965, 1178 pp. Received Smithers Rapra Technology T/17