Adiabatic Passage through Level Anticrossings in Systems of Chemically Inequivalent Protons Incorporating Parahydrogen: Theory, Experiment, and Prospective Applications

Abstract

Level anticrossings (LACs) are ubiquitous in quantum systems and have been exploited for spin-order transfer in hyperpolarized nuclear magnetic resonance spectroscopy. This paper examines the manifestations of adiabatic passage through a specific type of LAC found in homonuclear systems of chemically inequivalent coupled protons incorporating parahydrogen (pH 2 ). Adiabatic passage through such a LAC is shown to elicit translation of the pH 2 spin order. As an example, with prospective applications in biomedicine, proton spin polarizations of at least 19.8 ± 2.6% on the methylene protons and 68.7 ± 0.5% on the vinylic protons of selectively deuterated allyl pyruvate ester are demonstrated experimentally. After ultrasonic spray injection of a precursor solution containing propargyl pyruvate and a dissolved Rh catalyst into a chamber pressurized with 99% para-enriched H 2 , the products are collected and transported to a high magnetic field for NMR detection. The LAC-mediated hyperpolarization of the methylene protons is significant because of the stronger spin coupling to the pyruvate carbonyl 13 C, setting up an ideal initial condition for subsequent coherence transfer by selective INEPT. Furthermore, the selective deuteration of the propargyl side arm increases the efficiency and polarization level. LAC-mediated translation of parahydrogen spin order completes the first step toward a new and highly efficient route for the 13 C NMR signal enhancement of pyruvate via side-arm hydrogenation with parahydrogen.

Spying on parahydrogen-induced polarization transfer using a half-tesla benchtop MRI and hyperpolarized imaging enabled by automation

“…To hydrogenate 50 mM of the precursor, we needed about 30 s. This period is longer than reported before (5 s) 33 and partially explains the lower-than-expected polarization. The use of higher temperatures, a higher pH 2 flow or more advanced pH 2 mixing may improve the matter 33 , 36 , 66 .…”

“…The straightforward way to address this would be to supply more hydrogen, e.g., by using larger tubings or higher flow. In the literature, however, alternative methods were reported that should also be considered, including semipermeable membrane reactors, ideal for disturbing the magnetic field less and continuous polarization 36 , 75 , 76 , as well as the ultrasonic spraying of the solution into a reaction chamber pressurized with pH 2 66 .…”

Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies

“…Second, that all sites in the resulting free propene show polarization enhancement from para-H 2 incorporation necessitates a mechanism in which both a slower isomerization, via a 1,3-hydride shift (complex 1′), 15,66 and rate-determining reductive elimination from a propyl hydride intermediate (C) operates for propyne hydrogenation. Finally, the enhancement of the methyl signal in the first measurement indicates that processes akin to level anti-crossing effects also operate under these ALTADENA condition, 67 as this behavior cannot be accounted for by an isomerization process. 68 While using hyperpolarization to report on mechanism in solution-based catalysis has been established for over 30 years, 69 our observations here open up further the possibility for the informative interrogation of mechanism in molecular solid-state catalysis through a simple approach that does not perturb the thermodynamic or kinetic profile by labeling.…”

StereoPHIP: Stereoselective Parahydrogen‐Induced Polarization

“…[42] The differences in the signal patterns and intensities with and without MFC in low and high field are the consequence of the different mechanisms (net polarization transfer during MFC where level anticrossing plays an important role vs. multiplet polarization transfer in higher fields). [9,36,37,[44][45][46][47][48][49] In addition, the magnetic field profile of the 9.4 T and 1 T magnet is very different along the sample insertion path. The B 0 field in the 1 T Magritek magnet is horizontal (i.e.…”