A comparison of transition state of phenol in H-atom abstraction by methyl and methylperoxyl radicals

Abstract

DFT method was employed to locate transition state for H-atom transfer from phenol by methyl radical and methylperoxyl radical. The reaction pathway energy profiles and the structure of transition state show that a common feature is the out-of-plane structure of the transition state: in contrast to the energetic minima of a hydrogen-bonded intermediate, the hydrogen bond in transition structures is considerably twisted out of the aromatic ring. From the values of enthalpy (ΔH) and activation energy (E a ) obtained, it is found that the rate of the reaction of peroxyl radical with phenolic antioxidant is higher than that of alkyl radical with antioxidant. Spin density distributions show that the electron transmission is between methyl (methylperoxyl) radical and phenol. density functional calculations, phenols, radical, activation energy, hydrogen atom abstraction Free radicals play a significant role in causing many diseases, deteriorating foods, and degrading chemical materials. In biologic system, alkyl radicals (R·) and peroxyl radcials (ROO·) are the main chain-carrying radicals. Phenolic antioxidants (ArOH) can efficiently scavenge these radicals and have attracted wide attention [1][2][3] . For example, α-tocopherol is the major lipidsoluble, chain-breaking antioxidant in human blood plasma and low-density lipoprotein. Phenolic antioxidants mechanism is believed to transfer its phenolic H-atom to chain-carrying radicals and then break the chain process of autoxidants [4][5][6][7][8][9][10][11][12][13][14] . The main reactions can be represented by the following reactions (1) and (2) [13][14][15][16] : ROO·+ ArOH → ROOH + ArO· (1) R· + ArOH → RH + ArO·(2) Understanding of the mechanism of H-atom abstraction from phenols needs the knowledge of the transition state and reaction pathway. Since experimental studies cannot obtain directly transition-state information, high level calculations are necessary. In this paper, we locate the transition state for hydrogen atom transfer from phenol by methyl radical and methylperoxyl radical and compare these transition-states structures. With the calculations of intrinsic reaction coordinate (IRC), we compare the energy, structure and electronic density changes along the reaction pathway. Calculation methodsThe geometries were optimized for the reaction molecules, transition state and the product molecules using density functional theory with B3LYP function on the basis set of 6-311+G(d,p) [17][18][19] . All stationary points were positively identified as minima or first-order saddle points by evaluation of the frequencies and normal modes. Single-point energy calculations at higher level with B3LYP/6-311++G(3df,2pd) were done to draw a reliable conclusion. Further, pathways between the transition states and their corresponding minima were char-

Combined effect of heat shock protein inhibitor geldanamycin and free radicals on photodynamic therapy of prostate cancer

“…The choice of the initiator Computational chemistry guides the conduct of experiments by predicting the properties of compounds, thereby avoiding unnecessary attempts and reducing the cost of workforce and material resources, which is impossible in experimental research. The transition state theory 34,35 was used to discuss the decomposition reaction of three commercial initiators, V-50, V-086, and AIBI (Fig. S2, ESI †).…”

MPSC6 binuclear complexes immobilized on graphene oxide for oxidation of lignin model compounds and lignin

“…5) affording oxidation products such as vanillin (compound 2, Scheme 1) and vanillyl acid (compound 3, Scheme 1), which were most likely obtained via an oxidative hydrogen-abstracting to phenolic radicals catalyzed by oxocomplex followed by alkyl side-chain oxidation. 24,25 The efficient oxidation of vanillyl alcohol, under relatively mild reaction conditions, may be attributed to the fact that the porous structure of GO provides uniformly dispersion of active species and accessible voids for the substrate to approach the immobilized complex. 26 Furthermore, the catalysis results given in Fig.…”