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Download fileStrategies for the Hyperpolarization of Acetonitrile and Related Ligands by SABRE
journal contribution
posted on 2015-12-17, 07:09 authored by 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. WilliamsonWe 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)2(IMes)(MeCN)3]+ is the catalyst. Upon addition
of pyridine or pyridine-d5, the active
catalyst changes to [Ir(H)2(IMes)(py)2(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–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.