Prediction of oxidative reaction of methoxybenzenes by molecular orbital calculation
2002
Shigematsu, M. (Gifu Univ. (Japan). Faculty of Agriculture) | Kawai, S. | Hirai, H.
SUMMARY To predict the oxidation ability of lignin degrading enzymes without any experiments, the highest occupied molecular orbital energy (HOMO) of methoxybenzens (MBs) were calculated with semi-empirical molecular orbital calculation (RHF/PM3), with assumption in water circumstance (RHF/PM3+COSMO) and ab initio molecular orbital calculation (RHF/6-31G sup*). Calculated HOMOs were compared with the experimental data of half-wave oxidation potential (E sub(1/2))' For RHF/PM3 method, good correlation between E sub(l/2) and HOMO was obtained within methoxybenzene and dimethoxybenzenes, but not for the larger MBs, which include trimethoxybenzenes, tetramethoxybenzenes, pentamethoxybenzene and hexamethoxybenzene. For RHF/PM3+COSMO and RHF/6-31G sup* methods which are higher accuracy of calculation, the correlations were comparatively better than RHE/PM3 method, but insufficient for prediction of oxidation. It suggests that the effect of solvation energy caused by methoxyl groups cannot be disregarded. To solve this problem, the solvation energy term, which could be calculated from the neutral and the cation radical forms with RHF/PM3+COSMO method, was introduced as compensation term to HOMO energy. By this compensation, good correlation (R > 0.9) was obtained for all MBs except hexamethoxybenzene in RHF/6-31G sup* calculation. Consequently, it was found that this calculation was applicable to prediction of oxidation of MBs without any experiments.
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