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Epigallocatechin gallate (EGCG or (2S,3R)-5,7-dihydroxy-2-(3,4,5-trihydroxyphenyl)-3,4-dihydro-2H-chromen-3-yl 3,4,5-trihydroxybenzoate) and its isomers gallocatechin gallate (GCG or (2R,3R)-2-(3,4,5-trihydroxyphenyl)-3,4-dihydro-1(2H)-benzopyran-3,5,7-triol 3-(3,4,5-trihydroxybenzoate) are the main antioxidant components in tea polyphenols and tea. In this paper, the B3LYP density functional theory (DFT) was used to optimize the EGCG and GCG molecular configuration at the 6‑311G(d,p) level of theory implemented in Gaussian software. The result is convergent and has no imaginary frequency, indicating a stable structure that reaches minimum energy value. Multiwfn wave function analysis software was used to study the frontier orbitals of EGCG and GCG. The electrophilic and nucleophilic sites of the two molecules were predicted by the contribution of each atom to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The results show that the electrophilic reaction is mainly on No.1 benzene ring, and the main contribution is the C atom, especially the para‑C atom of the C atom connected by the two phenolic hydroxyl groups on the benzene ring. At the same time, the activation of the phenolic hydroxyl group on the para- and ortho-C atoms on the benzene ring was verified. When there is a phenolic hydroxyl group on the benzene ring, its para- and ortho-C atoms become more active, and they are easier to lose electrons. So substitution or other reactions occurs. This site is also the main site against oxygen free radicals which is an accelerated aging substance. The nucleophilic sites are mainly around C=O and no. 3 benzene rings, so it has certain ability to obtain electrons. Finally, the active sites of EGCG were analyzed by using the two methods of Fukui function and mean partial ionization energy, and the results were basically consistent with the above.

Based on DFT, B3LYP method was used to optimize the structure of 3-tert-butyl-4-hydroxyanisole (BHA) molecule at the level of 6-311g(d,p) set level. On this basis, the first 50 excited states were calculated by using B3LYP/def 2-TZVP method based on TD-DFT with ethanol as solvent. Multiwfn 3.6 software was used to plot the UV spectrum, and three methods including atomic charges, Fukui function (FF) and electrostatic potential (ESP) were used to predict its electrophilic reactive sites. Finally, bond energy of O10–H11 and C12–O13 were investigated by the ωB97XD/TZVP method. Comparing the experimental and theoretical UV spectra, it was found that the overall agreement was good. The oxygen atom on the phenolic hydroxyl group and the oxygen atom on the ether bond are the electrophilic reactive sites. And the C12–O13 bond on the ether bond is more prone to dissociation. They are easily reacted with peroxide radicals to have oxidation resistance.

Dibutyl phthalate DBP and dioctyl phthalate DOP are the main components of the plasticizers. In order to investigate their molecular structure, chemical bond and spectrum, the geometrical parameters of the ground state and infrared (IR) spectrum are calculated using the density functional theory B3LYP method at the level of 6-311++G(d,p). On this basis, the first twenty-six excited states and the UV-Vis absorption spectra of DBP and DOP are studied using the time-dependent density functional theory (TDDFT) in the same fundamental group and compared with the ultraviolet absorption peak of the molecules measured with UNICO UV-Vis spectrophotometer. The two kinds of molecular spectra are then classified and compared with that in reference. The results show that the strong absorption of IR spectra of DOP and DBP are produced by C-H bending in-plane vibration and C=O telescopic vibration producing. The most absorption of UV-Vis absorption spectra appears in the end absorption belt from n to σ* transition, and the stronger absorption in the E belt of benzene electronic transition from π to π*. There are blue shift for DOP end absorption belt from n to σ* transition and red shift for DOP E absorption belt from π to π* transition relative to that of DBP. This calculation results are better in accord with the spectral data measured by UNICO ultraviolet and visible spectrophotometer.