<br>13C NMR 作为碳酸检测和 pKa 测定的分析工具,Magnetic Resonance in Chemistry

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13C NMR 作为碳酸检测和 pKa 测定的分析工具
Magnetic Resonance in Chemistry ( IF 1.9 ) Pub Date : 2024-01-23 , DOI: 10.1002/mrc.5430
Daniel Rossado Oliveira ₁ , Eric Tavares da Costa ₁ , Leonardo Araujo Schenberg ₁ , Lucas Colucci Ducati ₁ , Claudimir Lucio do Lago ₁

Affiliation

核磁共振波谱已成为碳捕获和储存研究的标准技术。 ¹³ C NMR 可以检测碳酸水样的两个峰：一个用于 CO _2(aq) ，另一个用于 H ₂ CO ₃ 、HCO ₃ ^-和 CO ₃ ²⁻物质 - 此处统称为 H _x CO ₃ ^{x -2} 。第二个峰的化学位移取决于平衡中三种物质的摩尔分数，并已用于评估 HCO ₃ ^-和 CO ₃ ²⁻之间的平衡。由于当介质酸化时同时释放CO ₂ ，因此阻碍了低pH溶液中H ₂ CO ₃的检测。在此，带阀的NMR管有助于在宽pH范围内检测H _x CO ₃ ^x-2峰，甚至在主要物质为H ₂ CO ₃的pH 1.8下也是如此。该方法采用在管内形成NaH ¹³ CO ₃和酸溶液的冷冻层，当管达到室温时将其混合。此时，管子已被牢固密封，防止 CO ₂损失到大气中。分光光度法可以测量加压 NMR 管内的实际 pH 值。 H ₂ CO ₃的化学位移测定为160.33 ± 0.03 ppm，这与DFT计算结合Car-Parrinello分子动力学获得的值非常吻合。对于15％D ₂ O水介质和0.8mol/L离子强度，通过本方法测定的H ₂ CO ₃ pK a_值为3.41±0.03。所提出的方法可以扩展到有关烷基碳酸和氨基甲酸等类似物的研究。

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13C NMR as an analytical tool for the detection of carbonic acid and pKa determination

NMR spectroscopy has become a standard technique in studies both on carbon capture and storage. ¹³C NMR allows the detection of two peaks for carbonated aqueous samples: one for CO_2(aq) and another one for the species H₂CO₃, HCO₃⁻, and CO₃²⁻—herein collectively named H_xCO₃^x-2. The chemical shift of this second peak depends on the molar fraction of the three species in equilibrium and has been used to assess the equilibrium between HCO₃⁻ and CO₃²⁻. The detection of H₂CO₃ at low pH solutions is hindered, because of the concurrent liberation of CO₂ when the medium is acidified. Herein, a valved NMR tube facilitates the detection of the H_xCO₃^x-2 peak across a wide pH range, even at pH 1.8 where the dominant species is H₂CO₃. The method employed the formation of frozen layers of NaH¹³CO₃ and acid solutions within the tube, which are mixed as the tube reaches room temperature. At this point, the tube is already securely sealed, preventing any loss of CO₂ to the atmosphere. A spectrophotometry approach allowed the measurement of the actual pH inside the pressurized NMR tube. The chemical shift for H₂CO₃ was determined as 160.33 ± 0.03 ppm, which is in good agreement with value obtained by DFT calculations combined with Car–Parrinello molecular dynamics. The H₂CO₃ pK_a value determined by the present method was 3.41 ± 0.03, for 15% D₂O aqueous medium and 0.8 mol/L ionic strength. The proposed method can be extended to studies about analogs such as alkyl carbonic and carbamic acids.

更新日期：2024-01-23

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