Strategies for the Hyperpolarization of Acetonitrile and Related Ligands by SABRE

详细信息    查看全文

• 作者：Ryan E. Mewis ; Richard A. Green ; Martin C. R. Cockett ; Michael J. Cowley ; Simon B. Duckett ; Gary G. R. Green ; Richard O. John ; Peter J. Rayner ; David C. Williamson
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：January 29, 2015
• 年：2015
• 卷：119
• 期：4
• 页码：1416-1424
• 全文大小：312K
• ISSN：1520-5207

文摘

We report on a strategy for using SABRE (signal amplification by reversible exchange) for polarizing ¹H and ¹³C nuclei of weakly interacting ligands which possess biologically relevant and nonaromatic motifs. We first demonstrate this via the polarization of acetonitrile, using Ir(IMes)(COD)Cl as the catalyst precursor, and confirm that the route to hyperpolarization transfer is via the J-coupling network. We extend this work to the polarization of propionitrile, benzylnitrile, benzonitrile, and trans-3-hexenedinitrile in order to assess its generality. In the ¹H NMR spectrum, the signal for acetonitrile is enhanced 8-fold over its thermal counterpart when [Ir(H)b>2b>(IMes)(MeCN)b>3b>]⁺ is the catalyst. Upon addition of pyridine or pyridine-db>5b>, the active catalyst changes to [Ir(H)b>2b>(IMes)(py)b>2b>(MeCN)]⁺ and the resulting acetonitrile ¹H signal enhancement increases to 20- and 60-fold, respectively. In ¹³C NMR studies, polarization transfers optimally to the quaternary ¹³C nucleus of MeCN while the methyl ¹³C is hardly polarized. Transfer to ¹³C is shown to occur first via the ¹H鈥?sup>1H coupling between the hydrides and the methyl protons and then via either the ²J or ¹J couplings to the respective ¹³Cs, of which the ²J route is more efficient. These experimental results are rationalized through a theoretical treatment which shows excellent agreement with experiment. In the case of MeCN, longitudinal two-spin orders between pairs of ¹H nuclei in the three-spin methyl group are created. Two-spin order states, between the ¹H and ¹³C nuclei, are also created, and their existence is confirmed for Me¹³CN in both the ¹H and ¹³C NMR spectra using the Only Parahydrogen Spectroscopy protocol.