Abstract
This article reports the discovery of surface-mediated parahydrogen-induced alignment of the proton magnetic moments in liquid water as well as methanol and ethanol. In this SWAMP (surface waters are magnetized by parahydrogen) effect, the spin polarization of the solvent protons is enhanced simply by the bubbling of parahydrogen through a suspension of Pt3Sn intermetallic nanoparticles (iNPs) encapsulated within a protectective mesoporous silica shell (Pt3Sn@mSiO2). The conversion of singlet spin order into magnetization is mediated by symmetry-breaking interactions on the surface of the iNPs. Stimulated emission NMR signals of the exchangeable hydroxy protons are observed. Non-exchangeable methyl or methylene protons are also hyperpolarized, an observation that provides insight into the molecular mechanism. SWAMP has a myriad of potential applications, ranging from low-field MRI to drug discovery.
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•Heterogeneous catalyst aligns the proton magnetic moments in water and alcohols•Negative spin temperature induced by bubbling parahydrogen through the neat liquid•Hyperpolarization is mediated by intermolecular spin-spin couplings on the Pt3Sn surface•H/D isotope exchange between H2 and D2O is unproductive for hyperpolarization
Imagine a process in which the magnetic moments of hydrogen nuclei (i.e. protons) in liquid water molecules are aligned simply by the bubbling of a gas through a suspension of an insoluble solid catalyst in neat water in Earth's magnetic field. The existence of such a method would have far-reaching implications; for example, it could enable medical MRI without superconducting magnets in remote or impoverished regions. What special properties would the catalyst and the gas need to possess? By what catalytic mechanism can the proton alignment be realized without violating the postulates of thermodynamics and quantum mechanics? Answers to these questions are provided in this article.
We have discovered a heterogeneous catalyst that aligns the proton magnetic moments in liquid water, methanol, and ethanol molecules by using parahydrogen. In this SWAMP (surface waters are magnetized by parahydrogen) effect, hyperpolarization of the solvent protons is induced simply by the bubbling of parahydrogen gas through a suspension of the intermetallic nanoparticle catalyst in the neat liquid. The conversion of singlet spin order into magnetization is mediated by surface interactions and intermolecular spin couplings. The SWAMP effect has promising applications ranging from low-field MRI to drug discovery.