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

Abstract The homolytic bond dissociation energies (BDEs) of C α −O and C β −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 (CH 3 , CH 2 OH and OCH 3 ) at different positions on the BDEs of C α −O and C β −O bonds were investigated. The results indicated that a single methoxyl group at R 2 or R 3 has a minor influence on the BDE of C β −O bond, whereas two methoxyl groups at R 2 and R 3 lead to an obvious decrease in the BDE of C β −O bond. The decrement in the BDE of C β −O bond from the methoxyl groups at R 2 and R 3 can be enhanced by the methoxyl groups at R 4 and R 5 , but is hardly influenced by the substituent groups at R 1 . Meanwhile, the BDE of C α −O bond is gradually reduced when the H atoms at R 4 and R 5 are successively substituted with methoxyl groups; the decrement in the BDE of C α −O bond from the methoxyl groups at R 4 and R 5 can be strengthened by the methoxyl groups at R 2 and R 3 . Furthermore, the methyl and hydroxymethyl groups at R 1 can gradually increase the BDE of C α −O bond and this effect is weakened when the H atoms at R 2 and R 3 are successively substituted with methoxyl groups. The methyl group at R 1 has little influence on the BDE of C β −O bond, which is however dramatically increased by the hydroxymethyl group at R 1 .