Comparison of Multistandard and TMS-Standard Calculated NMR Shifts for Coniferyl Alcohol and Application of the Multistandard Method to Lignin Dimers
Comparison of Multistandard and TMS-Standard Calculated NMR Shifts for Coniferyl Alcohol and Application of the Multistandard Method to Lignin Dimers
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DOI：
10.1021/jp110330q
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发表时间：
2011-03-10
影响因子：
3.3
通讯作者：
Kubicki, James D.
中科院分区：
文献类型：
--
作者：
Watts, Heath D.;Mohamed, Mohamed Naseer Ali;Kubicki, James D.
松柏醇是木质素的单体结构单元，木质素是第二丰富的生物聚合物。在木质化过程中，单体通过自由基加成形成各种键。已经构建了一个大型NMR数据库，该数据库报告了数千种木质素低聚物的H-1和C-13化学位移。本文中，对松柏醇及其四种二聚体β-O-4、β-β、β-5和5-5进行玻尔兹曼平均H-1和C-13 GIAO NMR计算，以将计算的化学位移与实验进行比较。采用B3 LYP/6-311++G（d，p）方法对松柏醇的6个能量最小构象异构体进行了单点GIAO NMR计算.首先，比较了四种NMR位移计算方法：三种使用TMS标准方法在HF/6-311+G（2d，p），B3 LYP/6-311+G（2d，p）和mPW 1 PW 91/6- 31 G（d）理论水平上进行，第四种使用mPW 1 PW 91/6- 31 G（d）理论水平上进行。对于多标准方法，苯用作芳香族C和H原子的标准，甲醇用于脂肪族C和H原子。甲醇的羟基-H用作羟基-H原子的标准。六个构象的玻尔兹曼平均结果表明，多标准的方法是更准确的松柏醇和它的二聚体比常用的TMS标准方法，平均无符号的基础上，均方根，最大误差，以及观测值和计算值之间的线性相关性。C-13结果比H-1结果更准确，因为计算的羟基-H结果与观察数据之间的一致性较差。进一步的玻尔兹曼平均，多标准NMR计算比较了C-13和H-1化学位移与实验的四个立体异构体的β-O-4二聚体，以及5-5，β-5，和β-β二聚体的松柏醇。对于所有二聚体，C-13结果与实验相关良好（r（2）> 0.99），并且与实验相比显示出小的统计误差。对于二聚体，H-1 NMR与实验的相关性通常劣于C-13 NMR结果。
Coniferyl alcohol is a monomeric building block of lignin, the second most abundant biopolymer. During lignification, the monomer forms a variety of linkages through free radical additions. A large NMR database has been constructed that reports the H-1 and C-13 chemical shifts for thousands of lignin oligomers. Herein, Boltzmann averaged H-1 and C-13 GIAO NMR calculations were performed on coniferyl alcohol and four of its dimers, beta-O-4, beta-beta, beta-5, and 5-5, to compare the calculated chemical shifts with experiment. Six B3LYP/6-311++G(d,p) energy-minimized conformational isomers of coniferyl alcohol were subjected to single-point GIAO NMR calculations. Initially, four NMR shift calculation methods were compared: three were performed using the TMS-standard method at the HF/6-311+G(2d,p), B3LYP/6-311+G(2d,p), and mPW1PW91/6-31G(d) theory levels, and the fourth was performed with a multistandard approach using a mPW1PW91/6-31G(d) theory level. For the multistandard method, benzene was used as the standard for aromatic C and H atoms and methanol was used for aliphatic C and H atoms. The hydroxyl-H of methanol was used as the standard for hydroxyl-H atoms. The Boltzmann averaged results for six conformers showed that the multistandard method is more accurate for coniferyl alcohol and its dimers than the often used TMS-standard method, based on the mean unsigned, root-mean-squared, and maximum errors, as well as linear correlations between observed and calculated values. The C-13 results were more accurate than the H-1 results, due to poorer agreement between calculated hydroxyl-H results and observed data. Further Boltzmann-averaged, multistandard NMR calculations compared the C-13 and H-1 chemical shifts with experiment for the four stereoisomers of the beta-O-4 dimer, as well as the 5-5, beta-5, and beta-beta dimers of coniferyl alcohol. The C-13 results correlated well with experiment (r(2) > 0.99) for all dimers and showed small statistical errors, compared with experiment. The correlation with experiment for H-1 NMR was generally inferior to the C-13 NMR results for the dimers.