Oxalate ligands are found in many classes of materials, including energy storage materials and biominerals. Determining their local environments at the atomic scale is thus paramount to establishing the structure and properties of numerous phases. Here, we show that high-resolution ¹⁷O solid-state NMR is a valuable asset for investigating the structure of crystalline oxalate systems. First, an efficient ¹⁷O-enrichment procedure of oxalate ligands is demonstrated using mechanochemistry. Then, ¹⁷O-enriched oxalates were used for the synthesis of the biologically relevant calcium oxalate monohydrate (COM) phase, enabling the analysis of its structure and heat-induced phase transitions by high-resolution ¹⁷O NMR. Studies of the low-temperature COM form (LT-COM), using magnetic fields from 9.4 to 35.2 T, as well as ¹³C–¹⁷O MQ/D-RINEPT and ¹⁷O{¹H} MQ/REDOR experiments, enabled the 8 inequivalent oxygen sites of the oxalates to be resolved, and tentatively assigned. The structural changes upon heat treatment of COM were also followed by high-resolution ¹⁷O NMR, providing new insight into the structures of the high-temperature form (HT-COM) and anhydrous calcium oxalate α-phase (α-COA), including the presence of structural disorder in the latter case. Overall, this work highlights the ease associated with ¹⁷O-enrichment of oxalate oxygens, and how it enables high-resolution solid-state NMR, for “NMR crystallography” investigations.

中文翻译：

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高分辨率 17O 固态 NMR 作为研究材料中草酸盐结合模式的独特探针：草酸钙生物矿物质的案例研究

草酸盐配体存在于许多类别的材料中，包括储能材料和生物矿物。因此，在原子尺度上确定它们的局部环境对于建立众多相的结构和性质至关重要。在这里，我们证明高分辨率¹⁷ O 固态 NMR 对于研究结晶草酸盐系统的结构是宝贵的资产。首先，使用机械化学证明了草酸配体的有效¹⁷ O 富集过程。然后，¹⁷ O 富集草酸盐被用于合成生物学相关的一水草酸钙 (COM) 相，从而能够通过高分辨率¹⁷ O NMR 分析其结构和热诱导的相变。使用 9.4 至 35.2 T 的磁场以及¹³ C– ¹⁷ O MQ/D-RINEPT 和¹⁷ O{ ¹ H} MQ/REDOR 实验对低温 COM 形式 (LT-COM) 进行研究，使得草酸盐的8个不等价氧位点待解析并暂定。 COM 热处理后的结构变化也通过高分辨率¹⁷ O NMR 进行跟踪，为高温形式 (HT-COM) 和无水草酸钙 α 相 (α-COA) 的结构提供了新的见解，包括后一种情况存在结构紊乱。总体而言，这项工作强调了草酸氧¹⁷ O 富集的简便性，以及它如何实现高分辨率固态 NMR，用于“NMR 晶体学”研究。