Theoretical study on the effect of the substituent groups on the homolysis of the ether bond in lignin trimer model compounds

The homolytic bond dissociation energies (BDEs) of Cα−O and Cβ−O bonds in 27 lignin trimer model compounds were calculated by employing density functional theory methods at M062X level with 6−31++G(d,p) basis set; the effects of various substituent groups (CH3, CH2OH and OCH3) at different positions on the BDEs of Cα−O and Cβ−O bonds were investigated. The results indicated that a single methoxyl group at R2 or R3 has a minor influence on the BDE of Cβ−O bond, whereas two methoxyl groups at R2 and R3 lead to an obvious decrease in the BDE of Cβ−O bond. The decrement in the BDE of Cβ−O bond from the methoxyl groups at R2 and R3 can be enhanced by the methoxyl groups at R4 and R5, but is hardly influenced by the substituent groups at R1. Meanwhile, the BDE of Cα−O bond is gradually reduced when the H atoms at R4 and R5 are successively substituted with methoxyl groups; the decrement in the BDE of Cα−O bond from the methoxyl groups at R4 and R5 can be strengthened by the methoxyl groups at R2 and R3. Furthermore, the methyl and hydroxymethyl groups at R1 can gradually increase the BDE of Cα−O bond and this effect is weakened when the H atoms at R2 and R3 are successively substituted with methoxyl groups. The methyl group at R1 has little influence on the BDE of Cβ−O bond, which is however dramatically increased by the hydroxymethyl group at R1.