Nuclear Magnetic Resonance Spectra and 207Pb Chemical-Shift Tensors of Lead Carboxylates Relevant to Soap Formation in Oil Paintings
Nuclear Magnetic Resonance Spectra and 207Pb Chemical-Shift Tensors of Lead Carboxylates Relevant to Soap Formation in Oil Paintings
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DOI：
10.1366/13-07209
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发表时间：
2014-03-01
影响因子：
3.5
通讯作者：
Dybowski, Cecil
中科院分区：
文献类型：
--
作者：
Catalano, Jaclyn;Yao, Yao;Dybowski, Cecil
在传统油画中，当含重金属的颜料如铅白、2PbCO(3)中心点铅(OH)(2)和铅锡黄I型、Pb2SnO4与结合介质中的脂肪酸反应时，就会形成肥皂。这些肥皂可能会形成直径可达100-200微米的聚集体，这些聚集体膨胀并突出穿过油漆表面，导致漆膜降解，损害艺术品的完整性。触发肥皂形成的因素和这个过程的机制(S)还不是很清楚。为了阐明这些问题，化学和结构信息是必要的，这些信息可以使用固态铅-207和碳-13核磁共振(核磁共振)获得。在这篇文章中，我们报道了与肥皂形成有关的硬脂酸铅、棕榈酸铅和固件酸铅，以及油酸铅和庚酸铅的固体核磁共振谱和铅-207化学位移张量。这些铅羧酸盐的~(13)C交叉极化魔角旋转(MAS)谱表明，离铅最近的碳原子发生了共振倍增，表明不对称单元中的脂肪链有两种不同的构象。用直接激发和自旋-温度交替、有和没有MAS以及宽带均匀速率平滑截断Carr-Purcell-Meiom-Gill脉冲序列获得了测定张量的核磁共振谱。这些实验结果表明，与棕榈酸铅和硬脂酸铅不同的是，月桂酸铅的局部配位环境不同于显示肥皂形成的漆膜中的配位环境。此外，比较了所研究的羧酸铅的核磁共振化学位移张量，发现酰链的晶体堆积可能是决定铅周围配位环境的一个因素。
Soap formation in traditional oil paintings occurs when heavy-metal-containing pigments, such as lead white, 2PbCO(3)center dot Pb(OH)(2), and lead tin yellow type I, Pb2SnO4, react with fatty acids in the binding medium. These soaps may form aggregates that can be 100-200 mu m in diameter, which swell and protrude through the paint surface, resulting in the degradation of the paint film and damage to the integrity of the artwork. The factors that trigger soap formation and the mechanism(s) of the process are not yet well understood. To elucidate these issues, chemical and structural information is necessary, which can be obtained using solid-state Pb-207 and C-13 nuclear magnetic resonance (NMR). In this article, we report Pb-207 and C-13 solid-state NMR spectra and Pb-207 chemical-shift tensors of lead carboxylates implicated in soap formation: lead stearate, lead palmitate, and lead azelate, in addition to lead oleate and lead heptanoate for comparison. The C-13 cross polarization with magic-angle spinning (MAS) spectra of these lead carboxylates show resonance doubling for the carbons closest to the lead, indicating two different conformations of the fatty acid chains in the asymmetric unit. The Pb-207 NMR spectra, from which tensors were determined, were obtained with direct excitation and spin-temperature alternation, with and without MAS, and with the wide band uniform rate smooth truncation Carr-Purcell-Meiboom-Gill pulse sequence. The results of these experiments show that the local coordination environment of lead azelate is different from lead palmitate and lead stearate and could thus be distinguished from these in a paint film displaying soap formation. In addition, comparing the Pb-207 NMR chemical-shift tensors of the lead carboxylates studied shows that crystal packing of the acyl chains may be a factor in determining the coordination environment around the lead.