New structural information on a humic acid from two-dimensional 1H-13C correlation solid-state nuclear magnetic resonance.
New structural information on a humic acid from two-dimensional 1H-13C correlation solid-state nuclear magnetic resonance.
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来自二维 1H-13C 相关固态核磁共振的腐殖酸新结构信息。
DOI：
10.1021/es0014988
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发表时间：
2001
影响因子：
11.4
通讯作者：
K. Schmidt
中科院分区：
文献类型：
--
作者：
J. Mao;B. Xing;K. Schmidt
通过一系列二维 1H-13C 异核相关固态 NMR (HETCOR) 实验，采用不同的接触时间，并通过偶极相移和 13C 横向弛豫滤波进行光谱编辑，获得了有关泥炭腐植酸化学结构的新信息。发现碳键合甲基 (C-CH3) 靠近脂肪族和 O-烷基，但不靠近芳香族基团。光谱证明大多数 OCH3 基团直接与芳香环连接，这在木质素中是典型的。因此，大约三分之一的芳香族C-O基团不是酚类C-OH，而是C-OCH3。质子化和非质子化异头 O-C-O 碳均在一维和二维光谱中被识别。 COO 基团主要存在于 OCHn-COO 环境中，但也有一些与芳香环和脂肪族基团键合。文献中的所有腐殖酸模型至少缺乏此处观察到的一些特征。通过将 1H 自旋扩散引入 HETCOR 实验来研究成分异质性。与合成聚合物聚碳酸酯的数据比较表明，腐殖酸中的 O-烷基和芳族基团之间的间距小于 1.5 nm。然而，1H解耦下的横向13C弛豫过滤揭示了纳米尺度的异质性，慢弛豫组分富含类木质素芳香族-C-O-CH3部分，而缺乏COO基团。
New information on the chemical structure of a peat humic acid has been obtained using a series of two-dimensional 1H-13C heteronuclear correlation solid-state NMR (HETCOR) experiments with different contact times and with spectral editing by dipolar dephasing and 13C transverse relaxation filtering. Carbon-bonded methyl groups (C-CH3) are found to be near both aliphatic and O-alkyl but not aromatic groups. The spectra prove that most OCH3 groups are connected directly with the aromatic rings, as is typical in lignin. As a result, about one-third of the aromatic C-O groups is not phenolic C-OH but C-OCH3. Both protonated and unprotonated anomeric O-C-O carbons are identified in the one- and two-dimensional spectra. COO groups are found predominantly in OCHn-COO environments, but some are also bonded to aromatic rings and aliphatic groups. All models of humic acids in the literature lack at least some of the features observed here. Compositional heterogeneity was studied by introducing 1H spin diffusion into the HETCOR experiment. Comparison with data for a synthetic polymer, polycarbonate, indicates that the separation between O-alkyl and aromatic groups in the humic acid is less than 1.5 nm. However, transverse 13C relaxation filtering under 1H decoupling reveals heterogeneity on a nanometer scale, with the slow-relaxing component being rich in lignin-like aromatic-C-O-CH3 moieties and poor in COO groups.