Transfer of Parahydrogen-Induced Hyperpolarization to ¹⁹F

Abstract

Homogeneous hydrogenations of unsaturated substrates with parahydrogen yield strong NMR signal enhancements of the transferred 1H nuclei if the symmetry of H2 is broken in the resulting hydrogenated products. This chemically induced hyperpolarization known as Parahydrogen-induced polarization (PHIP) is also transferred to other protons and heteronuclei (2H, 13C, 29Si, 31P) when the hydrogenation is initiated at low magnetic fields. Hydrogenating various fluorinated styrenes and phenylacetylenes, we show that PHIP-derived hyperpolarization is transferred to 19F not only in the Earth's magnetic field (ALTADENA condition) but also in a strong magnetic field, e.g., when carrying out the reaction in the NMR spectrometer (PASADENA condition). Upon conducting a systematic analysis of the observed PHIP transfer to 1H, 13C, and 19F in the hydrogenation products to elucidate the mechanisms that govern this parahydrogen-aided resonance transfer (PART), we conclude that high- and low-field PHIP transfer mechanisms differ in detail depending on either through-bond or through-space interactions. Substrates with high hydrogenation rates and long spin-lattice relaxation times (T1) yield the highest degree of heteronuclear hyperpolarization. Possible medical applications for hyperpolarized 19F-containing molecules as "active" contrast agents for magnetic resonance imaging (MRI) are outlined.

Transfer of CIDNP among coupled spins at low magnetic field

“…As has been pointed out by de Kanter and Kaptein [10], such a coupling may lead to the transfer of CIDNP to nuclei which do not have any HFI in the transient radicals and therefore cannot be polarized at high magnetic fields. Such an effect of polarization transfer is not unique for CIDNP but is observed also with other methods of dynamic polarization sucia as para-hydrogen induced polarization (PHIP) [I1], when the hydrogenation reaction that is responsible for the formation of polarization is performed at low magnetic field [12][13][14]. Furthermore, in all experiments employing a variable magnetic field strength it is important to consider also the effects of sample transfer (field variation) from the polarization field to the detection field.…”

Photo‐CIDNP Studies of Hydroxy Ketones and Higher‐Molecular‐Weight Systems

“…There it introduces efficient stirring agitated by the spinning, in addition to allowing sufficient UV irradiation of the continuously stirred solution. This stirring effect is especially beneficial in 13 C-FT-NMR spectrometers and has its merit even without accompanying UV illumination. Recording of CIDNP spectra with FT-NMR spectrometers in combination with a pulsed-light source (laser) has certain advantages over a CW spectrometer in combination with a Hg/Xe arc lamp, because, in this way, the effects of NMR relaxation can be greatly reduced or even eliminated.…”

“…We attribute this phenomenon to the different T 1 nuclear-relaxation times, which are known to be short for macromolecular systems. A quite promising, potential alternative is to transfer the CIDNP-derived initial 1 H spin polarization to heteronuclei, in particular to 13 C. For certain heteronuclei, notably for 13 C, the T 1 nuclear-relaxation times are considerably longer than those of the corresponding protons. Hence, we have investigated the feasibility and rules for transferring nuclear polarization from 1 H to 13 C or to 19 F, respectively.…”

“…A quite promising, potential alternative is to transfer the CIDNP-derived initial 1 H spin polarization to heteronuclei, in particular to 13 C. For certain heteronuclei, notably for 13 C, the T 1 nuclear-relaxation times are considerably longer than those of the corresponding protons. Hence, we have investigated the feasibility and rules for transferring nuclear polarization from 1 H to 13 C or to 19 F, respectively. For this purpose, we have used another and much more efficient source of initial 1 H spin polarization, namely the so-called parahydrogen-induced polarization.…”

“…However, the subsequent behavior of the polarized spin system is identical to those where the initial polarization was attained by exploiting the CIDNP phenomenon. Various systems have been investigated in great detail, where we have successfully transferred the initial 1 H polarization to 13 C and, alternatively, to 19 F. For this purpose, we have also developed the appropriate pulse sequences, which differ significantly from those used typically for nonpolarized spin systems. The results of all of these studies have been published [8 -13].…”