Improving the Hyperpolarization of ³¹P Nuclei by Synthetic Design

Traditional ³¹P NMR or MRI measurements suffer from low sensitivity relative to ¹H detection and consequently require longer scan times. We show here that hyperpolarization of ³¹P nuclei through reversible interactions with <i>para</i>hydrogen can deliver substantial signal enhancements in a range of regioisomeric phosphonate esters containing a heteroaromatic motif which were synthesized in order to identify the optimum molecular scaffold for polarization transfer. A 3588-fold ³¹P signal enhancement (2.34% polarization) was returned for a partially deuterated pyridyl substituted phosphonate ester. This hyperpolarization level is sufficient to allow single scan ³¹P MR images of a phantom to be recorded at a 9.4 T observation field in seconds that have signal-to-noise ratios of up to 94.4 when the analyte concentration is 10 mM. In contrast, a 12 h 2048 scan measurement under standard conditions yields a signal-to-noise ratio of just 11.4. ³¹P-hyperpolarized images are also reported from a 7 T preclinical scanner

Illustration of the human-climate model dynamics for five different levels of responding.

<p> Economic productivity targets of 1, 2, 3, 4, and 5 percent are shown by the blue, green, red, cyan, and purple solid lines, respectively. Parameters governing the behavior of the model are shown above the panels.</p

Schematic of the interactive human-climate model.

<p>Participants set an economic productivity target, <i>R</i>, which drives economic growth, <i>E</i>, but also generates greenhouse gas emissions, <i>G</i>. Emissions accumulate in the atmosphere, subject to passive removal by natural carbon sinks at rate <i>κ</i>. CO₂ accumulation, <i>c</i>, generates warming, increasing the global mean temperature, <i>T</i>. Temperature has a negative effect on economic growth. Capacity for economic growth is reduced under increased warming.</p

System dynamics associated with the optimal response profile (solid curves) overlaid with data from informed and uninformed conditions (dashed lines).

<p>Data are collapsed across cover story conditions and across Experiments 1 and 2. The panels show trajectories for responses (a), economic index (b), excess CO₂ concentration (c), and warming (d). Error bars are one standard error of the mean.</p

Data from Experiment 1.

<p>Panels show group averaged data for participant responses (a), economic index (b), excess CO₂ concentration (c), and warming (d). Error bars are one standard error of the mean.</p

Iridium(III) Hydrido N‑Heterocyclic Carbene–Phosphine Complexes as Catalysts in Magnetization Transfer Reactions

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses <i>para</i>-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir­(H)₂(IMes)­(MeCN)₂(R)]­BF₄ and [Ir­(H)₂(IMes)­(py)₂(R)]­BF₄ [py = pyridine; R = PCy₃ or PPh₃; IMes = 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir­(H)₂(NCMe)­(py)­(IMes)­(PPh₃)]­BF₄ and [Ir­(H)₂(NCMe)­(py)­(IMes)­(PCy₃)]­BF₄, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir­(H)₂(NCMe)­(py)­(IMes)­(PPh₃)]­BF₄, [Ir­(H)₂(MeOH)­(py)­(IMes)­(PPh₃)]­BF₄, and [Ir­(H)₂(NCMe)­(py)₂(PPh₃)]­BF₄. Studies are also described that employ the deuterium-labeled substrates CD₃CN and C₅D₅N, and the labeled ligands P­(C₆D₅)₃ and IMes-<i>d</i>₂₂, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir­(H)₂(pyridine-<i>h</i>₅)­(pyridine-<i>d</i>₅)­(IMes)­(PPh₃)]­BF₄ is examined, its hydride ligand signals are shown to become visible through <i>para</i>-hydrogen-induced polarization rather than SABRE