Theoretical study of bond dissociation energies for lignin model compounds

The bond dissociation energies (BDE) 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 6-31G(d,p) level. The effect of various substituents on BDE and the correlation between the bond lengths and the corresponding BDE were analyzed. The calculation results show that C-O bond is generally weaker than C-C bond, and the average bond dissociation energy of C_α-O (182.7 kJ/mol) is the lowest, and that of C_β-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 such a strong correlation between C-O bond lengths and BDE. Compared with C-O bonds, BDE 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 BDE are weak when the C-C bond lengths are long.