Theoretical study of bond dissociation energies for lignin model compounds

The bond dissociation energies (EB) of C–O and C–C bond in 63 lignin model compounds for six prevalent linkages (β–O–4, α–O–4, 4–O–5, β–1, α–1 and 5–5) were theoretically calculated by using density functional theory methods B3P86 at 6–31 G (d,p) level. The effect of various substituents on EB and the correlation between the bond lengths and the corresponding EB were analyzed. The calculation results show that C–O bond is generally weaker than C–C bond, and the average bond dissociation energy of Cα–O (182.7 kJ/mol) is the lowest, and that of Cβ–O is second lowest. The substituent group on both the aromatic and alkyl groups can substantially weaken C–O bonds, and C–O bonds do not exhibit a strong correlation between C–O bond lengths and BDE. Compared with C–O bonds, EB of C–C bonds are little affected by the substituent on the aromatic groups, but affected obviously by the substituent on alkyl groups. There is a strong linear relationship between C–C bond lengths and BDE. The EB are weak when the C–C bond lengths are long.