The He, Li, and Li+ penetrations of the benzene ring are discussed from density functional theory (DFT) computations and intrinsic reaction coordinate (IRC) analyses. The high activation energy of more than 200 kcal/mol for the He penetration appears to derive mainly from the electrostatic repulsion between the benzene ring and the He atom. The activation energies decrease in the sequence of He > Li > Li+. The activation barrier for the Li+ penetration (160 kcal/mol) may be overcome under high-temperature and high-pressure conditions. If it occurs in preparing lithium intercalation graphite, we must change our concept of “staging” in graphite intercalation compounds.
The conformations of phenol, anisole and guaiacol, especially solvent effect on the intra-molecular hydrogen bond were computed with semi-empirical molecular orbital calculation, MOPAC2000. Solvent effect on the conformations of phenol and anisole appears minimal. However, the optimum conformation of guaiacol was transformed from cis- to trans-forms by increase of specific conductivity of the solvent. This effect would be due to the break of intra-molecular hydrogen bond between phenolic hydroxyl and methoxyl groups caused by perturbation from polar solvents.
The rate constants were experimentally obtained for the oxidative reaction of three monolignols (p-coumaryl, coniferyl and sinapyl alcohol) and four similar forms of 3-mono- or 3, 5-disubstituted p-coumaryl alcohols (3-ethoxy-, 3-propoxy-, 3-methyl- and 3, 5-dimethyl-p-coumaryl alcohol) by using horseradish peroxidase (HRP) -H2O2 as the oxidant. To investigate the factor determining reaction rate of each substrate, the molecular volume and the highest occupied molecular orbital (HOMO) energy were applied. The molecular volume was obtained by using optimum conformation data with MOPAC2000. And the HOMO energy was calculated with MOPAC2000 under four conditions, i.e., when the phenolic hydroxyl form of each compound is either neutral or anion, and when each substrate is at polar or nonpolar milieu. We infer that the reason why 3, 5-disubstituted p-coumaryl alcohol deviates from the profiles of others was due to local steric hindrance and distortion effect of the substituted groups. In assuming anion/water, the reaction rates of substrates correlated with HOMO energies excluding 3, 5-disubstituted p-coumaryl alcohols. In alternate form and milieu, no suitable relationship between HOMO energies and reaction rates was obtained. From these results, we suggest that the rate-determining step in the oxidative reaction for 3-substituted p-coumaryl alcohol is where one electron is withdrawn by oxidized HRP under anion/water condition.
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