Transfer of1H and13C dynamic nuclear polarization from immobilized nitroxide radicals to flowing liquids

Abstract

In this study, ~H and 13C dynamic nuclear polarization (DNP) was generated at a magnetic field strength of 0.33 T utilizing silica phase immobilized nitroxide (SPIN) samples. The pola¡ was subsequently transferred to flowing liquids and monitored at a magnetic field strength of 4.7 T. These solid/liquid intermolecular transfer (SLIT) experiments provide efficient polarization transfer without the necessity of the free radical system present in the monitoring fluid. Specifically, ultimate ~H SLIT DNP Overhauser enhancements of -5 6 and --110 have been observed for benzene and chloroform in the presence of SPIN system 2, respectively.The ~3C SLIT DNP enhancement for benzene is dominated by three-spin effects and poor leakage factors (fe). However, a particularly favorable case is the chloroform/SPIN 2 system which exhibits a scalar dominated enhancement. For this case, positive erthancements 40--60 times the ~3C thermal Boltzmann magnetization at 4.7 T have been observed. The large scalar dominated 13C DNP enhancement for this system represents one of the largest experimental enhancements reported to date. The 13C DNP spectra for other samples which exhibit favorable scalar 13C dominated enhancements (e.g., Freon 113) are also presented. Three different SPIN systems were also prepared and characterized in the present smdy.

1H Dynamic Nuclear Polarization in Supercritical Ethylene at 1.4 T

“…In this way both the DNP and NMR setups can be optimized separately, and it also provides the possibility of performing NMR spectroscopy at a higher field than the DNP experiments, which further increases the NMR sensitivity as well as the spectral resolution (29,30). (4) The radicals can be immobilized in a solid matrix, e.g., by embedding the radicals in a solid such as silica gel (30). This eliminates a possible negative Overhauser enhancement due to dissolved radicals, which counteract the positive enhancement occurring at the fluid/solid interface.…”

“…This can be achieved by pumping the SCF from the DNP area into the NMR coil, which can be located outside the microwave device used for DNP. In this way both the DNP and NMR setups can be optimized separately, and it also provides the possibility of performing NMR spectroscopy at a higher field than the DNP experiments, which further increases the NMR sensitivity as well as the spectral resolution (29,30). (4) The radicals can be immobilized in a solid matrix, e.g., by embedding the radicals in a solid such as silica gel (30).…”

“…Moreover, it facilitates the transport of the polarized SCF molecules to areas outside the microwave region, as the relaxation times of the nuclei in these molecules are no longer shortened by the radicals. In liquids this approach has been used by for a low-field DNP investigation on aqueous char suspensions, and by Dorn et al (29,30) to perform NMR spectroscopy at a larger field than that used for DNP. When the radicals are present as a solid, DNP is governed by interactions between the SCF nuclei and the unpaired electrons at or near the solid/fluid interface, rendered time-dependent by molecular motions similar to those of dissolved radicals.…”

“…This approach has been used by Dorn and co-workers (28), who measured a proton Overhauser enhancement of Ϫ140 for benzene dissolved in supercritical CO 2 , doped with a nitroxide radical (and this enhancement is estimated to become at least Ϫ260 for s ϭ f ϭ 1). Second, solid free radicals can be used which will not dissolve in the SCF or are immobilized in a solid matrix (29,30). This method has the advantage that the radicals do not contaminate the SCF, which prevents possible alterations of chemical processes involving SCFs.…”

Trends in Spectroscopic Techniques for Process Control

“…Novel polarization transfer methods [22], spin-lattice relaxation time reduction techniques [23], and cryogenic NMR probes [24] have been utilized toward the improvement of NMR measurements. Because cryogenic magnets are not compatible with the industrial environment, the development of process capability of such instruments is highly appreciated, for example, by further developing MR NMR and microcoil capillary NMR techniques [25,26].…”