取代基对木质素三聚体模型化合物醚键均裂反应影响的密度泛函理论研究

利用密度泛函理论M062X/6-31++G(d,p)方法,对27种具有不同取代基(甲基、羟甲基和甲氧基)的木质素三聚体模型化合物的Cα-O和Cβ-O键均裂解离能进行了理论计算,探究了不同位置取代基对醚键解离能的影响规律。结果表明,当R2或R3位氢原子仅有一个被甲氧基取代时,Cβ-O键解离能变化很小;当R2、R3位氢原子均被甲氧基取代时,Cβ-O键解离能明显降低;且R4、R5位甲氧基能强化R2、R3位甲氧基对Cβ-O键解离能的降低程度,而不受R1位取代基的影响。当R4、R5位氢原子相继被甲氧基取代时,Cα-O键解离能逐渐降低,且R2、R3位甲氧基也能强化R4、R5位甲氧基对Cα-O键解离能的降低程度。当R1位氢原子相继被甲基、羟甲基取代时,Cα-O键解离能逐渐升高,然而R2、R3位甲氧基会弱化R1位甲基、羟甲基对Cα-O键解离能的升高程度;R1位甲基不会影响Cβ-O键解离能,羟甲基却能明显提高Cβ-O键解离能。

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

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 (CH₃, CH₂OH and OCH₃) 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 R₂ or R₃ has a minor influence on the BDE of C_β-O bond, whereas two methoxyl groups at R₂ and R₃ 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 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 C_α-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 C_α-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 C_α-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 C_β-O bond, which is however dramatically increased by the hydroxymethyl group at R₁.