Theoretical study on the effects of the substituent groups on the homolysis of the ether bond in lignin trimer model compounds
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摘要: 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.Abstract: The homolytic bond dissociation energies (BDEs) of Cα-O and Cβ-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 Cα-O and Cβ-O bonds were investigated. The results indicated that a single methoxyl group at R2 or R3 has a minor influence on the BDE of Cβ-O bond, whereas two methoxyl groups at R2 and R3 lead to an obvious decrease in the BDE of Cβ-O bond. The decrement in the BDE of Cβ-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 Cα-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 Cα-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 Cα-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 Cβ-O bond, which is however dramatically increased by the hydroxymethyl group at R1.
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图 1 木质素三聚体模型化合物的化学结构式及化学键均裂方式
Figure 1 Spatial structure and major homolytic cleavage ways of lignin trimer model compound
图 2 模型化合物MC1(a)、MC6(b)、MC7(c) 的几何结构示意图
Figure 2 Spatial structures of lignin model compounds
(a): MC1; (b): MC6; (c): MC7
表 1 R2、R3位甲氧基对Cβ-O键均裂解离能的影响
Table 1 Effects of methoxyl groups at R2 and R3 on the BDEs of the Cβ-O bond
NO. Substituent group Compound Cβ-O E/(kJ·mol-1) R1 R2 R3 R4 R5 BDE ΔBDE A1 H H H H H MC 1 294.3 - H OCH3 H H H MC 2 294.9 0.6 H OCH3 OCH3 H H MC 3 286.0 -8.9 A2 H H H OCH3 H MC 4 308.6 - H OCH3 H OCH3 H MC 8 291.7 -16.9 H OCH3 OCH3 OCH3 H MC 10 271.7 -20.0 A3 H H H OCH3 OCH3 MC 5 304.6 - H OCH3 H OCH3 OCH3 MC 9 279.8 -24.8 H OCH3 OCH3 OCH3 OCH3 MC 11 260.3 -19.5 A4 CH3 H H H H MC 6 294.9 - CH3 OCH3 H H H MC 14 293.9 -1.0 CH3 OCH3 OCH3 H H MC 15 286.8 -7.1 A5 CH3 H H OCH3 H MC 18 307.6 - CH3 OCH3 H OCH3 H MC 20 291.6 -16.0 CH3 OCH3 OCH3 OCH3 H MC 22 268.3 -23.3 A6 CH3 H H OCH3 OCH3 MC 19 311.0 - CH3 OCH3 H OCH3 OCH3 MC 24 294.0 -17.0 CH3 OCH3 OCH3 OCH3 OCH3 MC 26 270.5 -23.5 A7 CH2OH H H H H MC 7 302.3 - CH2OH OCH3 H H H MC 12 303.0 0.7 CH2OH OCH3 OCH3 H H MC 13 296.8 -6.2 A8 CH2OH H H OCH3 H MC 16 313.7 - CH2OH OCH3 H OCH3 H MC 21 298.6 -15.1 CH2OH OCH3 OCH3 OCH3 H MC 23 288.4 -10.2 A9 CH2OH H H OCH3 OCH3 MC 17 320.3 - CH2OH OCH3 H OCH3 OCH3 MC 25 305.3 -15.0 CH2OH OCH3 OCH3 OCH3 OCH3 MC 27 284.6 -20.7 
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表 2 R4、R5位甲氧基(OCH3) 对Cα-O键均裂解离能的影响
Table 2 Effects of methoxyl groups at R4 and R5 on the BDEs of the Cα-O bond
NO. Substituent group Compound Cα-O E/(kJ·mol-1) R1 R2 R3 R4 R5 BDE ΔBDE B1 H H H H H MC 1 252.1 - H H H OCH3 H MC 4 246.0 -6.1 H H H OCH3 OCH3 MC 5 237.6 -8.4 B2 H OCH3 H H H MC 2 273.4 - H OCH3 H OCH3 H MC 8 249.8 -23.6 H OCH3 H OCH3 OCH3 MC 9 233.4 -16.4 B3 H OCH3 OCH3 H H MC 3 279.0 - H OCH3 OCH3 OCH3 H MC 10 244.3 -34.7 H OCH3 OCH3 OCH3 OCH3 MC 11 228.4 -15.9 B4 CH3 H H H H MC 6 258.6 - CH3 H H OCH3 H MC 18 251.2 -7.4 CH3 H H OCH3 OCH3 MC 19 243.6 -7.6 B5 CH3 OCH3 H H H MC 14 273.2 - CH3 OCH3 H OCH3 H MC 20 250.9 -22.3 CH3 OCH3 H OCH3 OCH3 MC 24 242.2 -8.7 B6 CH3 OCH3 OCH3 H H MC 15 276.4 - CH3 OCH3 OCH3 OCH3 H MC 22 237.8 -38.6 CH3 OCH3 OCH3 OCH3 OCH3 MC 26 229.0 -8.8 B7 CH2OH H H H H MC 7 268.2 - CH2OH H H OCH3 H MC 16 258.2 -10.0 CH2OH H H OCH3 OCH3 MC 17 249.6 -8.6 B8 CH2OH OCH3 H H H MC 12 279.5 - CH2OH OCH3 H OCH3 H MC 21 253.6 -25.9 CH2OH OCH3 H OCH3 OCH3 MC 25 245.2 -8.4 B9 CH2OH OCH3 OCH3 H H MC 13 266.6 - CH2OH OCH3 OCH3 OCH3 H MC 23 236.7 -29.9 CH2OH OCH3 OCH3 OCH3 OCH3 MC 27 217.7 -19.0
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表 3 R1位取代基对Cα-O、Cβ-O键均裂解离能的影响
Table 3 Effects of substituent groups at R1 on the BDEs of the Cα-O and Cβ-O bonds
NO. Substituent group Compound Cα-O E/(kJ·mol-1) Cβ-O E/(kJ·mol-1) R1 R2 R3 R4 R5 BDE ΔBDE BDE ΔBDE C1 H H H H H MC 1 252.1 - 294.3 - CH3 H H H H MC 6 258.6 6.5 294.9 0.6 CH2OH H H H H MC 7 268.2 9.6 302.3 7.4 C2 H OCH3 H H H MC 2 273.4 - 294.9 - CH3 OCH3 H H H MC 14 273.2 -0.2 293.9 -1.0 CH2OH OCH3 H H H MC 12 279.5 6.3 303.0 9.1 C3 H OCH3 OCH3 H H MC 3 279.0 - 286.0 - CH3 OCH3 OCH3 H H MC 15 276.4 -2.6 286.8 0.8 CH2OH OCH3 OCH3 H H MC 13 266.6 -9.8 296.8 10.0 C4 H H H OCH3 H MC 4 246.0 - 308.6 - CH3 H H OCH3 H MC 18 251.2 5.2 307.6 -1.0 CH2OH H H OCH3 H MC 16 258.2 7.0 313.7 6.1 C5 H H H OCH3 OCH3 MC 5 237.6 - 304.6 - CH3 H H OCH3 OCH3 MC 19 243.6 6.0 311.0 6.4 CH2OH H H OCH3 OCH3 MC 17 249.6 6.0 320.3 9.3
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