Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Phys Chem B. 2011 Mar 10;115(9):1958-70. doi: 10.1021/jp110330q. Epub 2011 Feb 14.
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 (1)H and (13)C chemical shifts for thousands of lignin oligomers. Herein, Boltzmann averaged (1)H and (13)C GIAO NMR calculations were performed on coniferyl alcohol and four of its dimers, β-O-4, β-β, β-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 (13)C results were more accurate than the (1)H results, due to poorer agreement between calculated hydroxyl-H results and observed data. Further Boltzmann-averaged, multistandard NMR calculations compared the (13)C and (1)H chemical shifts with experiment for the four stereoisomers of the β-O-4 dimer, as well as the 5-5, β-5, and β-β dimers of coniferyl alcohol. The (13)C 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 (1)H NMR was generally inferior to the (13)C NMR results for the dimers.
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。在木质素形成过程中，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，其中报告了数千种木质素低聚物的（1）H 和（13）C 化学位移。本文通过 Boltzmann 平均（1）H 和（13）C GIAO NMR 计算对松柏醇及其四种二聚体（β-O-4、β-β、β-5 和 5-5）进行了计算，将计算得到的化学位移与实验值进行了比较。对松柏醇的六个 B3LYP/6-311++G(d,p)能量最小化构象异构体进行了单点 GIAO NMR 计算。最初，比较了四种 NMR 位移计算方法：在 HF/6-311+G(2d,p)、B3LYP/6-311+G(2d,p)和 mPW1PW91/6-31G(d)理论水平上使用三种 TMS 标准方法，以及在 mPW1PW91/6-31G(d)理论水平上使用多标准方法。对于多标准方法，苯被用作芳香族 C 和 H 原子的标准，甲醇被用作脂肪族 C 和 H 原子的标准，甲醇的羟基-H 被用作羟基-H 原子的标准。六个构象的 Boltzmann 平均结果表明，与常用的 TMS 标准方法相比，多标准方法对于松柏醇及其二聚体更准确，基于平均未签名、均方根和最大误差，以及观察值与计算值之间的线性相关性。由于计算的羟基-H 结果与观察数据的一致性较差，因此（13）C 结果比（1）H 结果更准确。进一步的 Boltzmann 平均、多标准 NMR 计算比较了β-O-4 二聚体的四个立体异构体以及松柏醇的 5-5、β-5 和β-β 二聚体的（13）C 和（1）H 化学位移与实验值。对于所有二聚体，（13）C 结果与实验值高度相关（r(2)>0.99），并且与实验值相比具有较小的统计误差。对于二聚体，（1）H NMR 与实验值的相关性总体上不如（13）C NMR 结果。
