木质素模化物键离解能的理论研究

采用密度泛函理论B3P86方法,在6-31G(d,p)基组水平上,对木质素结构中的6种连接方式(β-O-4、α-O-4、4-O-5、β-1、α-1、5-5)的63个木质素模化物的醚键(C-O)和C-C键的键离解能E_B进行了理论计算研究。分析了不同取代基对键离解能的影响以及键长与键离解能的相关性。计算结果表明,C-O键的键离解能通常比C-C键的小,在各种醚键中C_α-O键的平均键离解能最小,为182.7 kJ/mol;其次是β-O-4连接中的C_β-O键,苯环和烷烃基上的取代基对醚键的键离解能有较强的弱化作用,C-O键的键长和键离解能的相关性较差。与C-O键相比,C-C键的键离解能受苯环上取代基的影响很小,而烷烃基上的取代基对C-C键的键离解能有较大的影响,C-C键的键离解能和键长之间存在较强的线性关系,C-C键的键长越长,其键离解能越小。

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.