Temperature Dependence of the Proton Overhauser DNP Enhancements on Aqueous Solutions of Fremy’s Salt Measured in a Magnetic Field of 9.2 T

Abstract

The temperature dependence of the water-proton dynamic nuclear polarization (DNP) enhancement from Fremy's salt nitroxide radicals was measured in a magnetic field of 9.2 T (corresponding to 260 GHz microwave (MW) and 392 MHz NMR frequencies) in the temperature range of 15-65°C. The temperature could be determined directly from the proton NMR line shift of the sample. Very high DNP enhancements of -38 (signal integral) or -81 (peak intensity) could be achieved with a high-power gyrotron MW source. The experimental findings are compared with classical Overhauser theory for liquids, which is based on the translational and rotational motion of the molecules and with molecular dynamics calculations of the coupling factor.

Simultaneous DNP enhancements of 1H and 13C nuclei: theory and experiments

“…[25][26][27][28][29] This mechanism is responsible for DNP enhancements in solutions and conducting solids containing free radicals [30][31][32][33] and was recently shown to also exist in the MAS of dielectric solids. [25][26][27][28][29] This mechanism is responsible for DNP enhancements in solutions and conducting solids containing free radicals [30][31][32][33] and was recently shown to also exist in the MAS of dielectric solids.…”

A table‐top PXI based low‐field spectrometer for solution dynamic nuclear polarization

“…Temperature changes on the order of 50°C are observed for small samples at high field, which increases the coupling factor and result in a linear increase in ε versus ΔT. [31] Because the coupling factor in the low field regime is at the dipolar limit [12] , an increase in temperature is not likely to contribute in the same way for radicals in solution. The shift in resonance positon due to temperature is only on the order of a few ppm which is below the homogeneity of low field systems operating below~5 MHz (0.117 T).…”

“…Careful consideration of experimental and resonator conditions at high magnetic fields has shown sizable DNP enhancements can be achieved, even up to fields of 9 T. These very recent and exciting observations are in direct conflict with the “established” theory for solution OE DNP that predicts loss of enhancement at high magnetic fields, as the electron Larmor frequency becomes much larger than the timescale of the dipolar interaction modulating the polarization transfer. It turns out that fast local dynamics can increase the ρ to the point where sizable enhancements are achievable .…”

“…If B 0 homogeneity is high enough, the shift in resonance position of water or mixtures of water; ethylene glycol have been used to assess temperature change during DNP experiments. Temperature changes on the order of 50 °C are observed for small samples at high field, which increases the coupling factor and result in a linear increase in ε versus ΔT . Because the coupling factor in the low field regime is at the dipolar limit, an increase in temperature is not likely to contribute in the same way for radicals in solution.…”

“…Work at both intermediate (0.35 T) and high field (≥ 3 T) have studied the sensitivity of ε to small changes in sample temperature via modulation of f or ρ, respectively. At 0.35 T, temperature modulation of f was most pronounced at radical concentrations <1 mM .…”