Preparation of highly polarized nuclear spin systems using brute-force and low-field thermal mixing

Abstract

Over the years, several strategies have been developed for generating highly polarized nuclear spin systems, including dynamic nuclear polarization, optical pumping, and methods exploiting parahydrogen. Here, we present an alternative strategy, using an enhanced 'brute-force' approach (i.e. exposure to low temperatures and high applied magnetic fields). The main problem with this approach is that it may take an excessively long time for the nuclear polarization to approach thermal equilibrium at low temperatures, since nuclear relaxation becomes exceedingly slow due to the loss of molecular motion. We show that low-field thermal mixing can alleviate the problem by increasing the rate at which slowly-relaxing nuclei reach equilibrium. More specifically, we show that polarization can be transferred from a relatively rapidly relaxing (1)H reservoir to more slowly relaxing (13)C and (31)P nuclei. The effects are particularly dramatic for the (31)P nuclei, which in experiments at a temperature of 4.2 K and a field of 2 T show a 75-fold enhancement in their effective rate of approach to equilibrium, and an even greater (150-fold) enhancement in the presence of a relaxation agent. The mixing step is also very effective in terms of the amount of polarization transferred-70-90% of the maximum theoretical value in the experiments reported here. These findings have important implications for brute-force polarization, for the problem becomes one of how to relax the solvent protons rather than individual more slowly-relaxing nuclei of interest. This should be a much more tractable proposition, and offers the additional attraction that a wide range of nuclear species can be polarized simultaneously. We further show that the (1)H reservoir can be tapped repeatedly through a number of consecutive thermal mixing steps, and that this could provide additional sensitivity enhancement in solid-state NMR.

Transfer of the Haupt-hyperpolarization to neighbor spins

“…1a the typical up and down pattern for the undecoupled 13 C NMR spectrum of 1 in natural abundance is shown. Due to the positive and negative sign of the quadruplet signals 1 H decoupling leads to a severe loss of intensity which is illustrated in Fig.…”

“…The phenomenon is related to the Haupt effect which was discovered in 1972 [2] and has since then very often been debated in the literature [3][4][5][6][7][8][9][10][11][12][13]. An obvious question after the observation of the Haupt effect in high resolution was, whether the hyperpolarization observed can be transferred from the methyl groups towards neighbor spins.…”

“…Very recently we have observed hyperpolarized up and down multiplets of methyl groups both for 13 C and 1 H in high resolution NMR spectra after first cooling in liquid helium and then rapid dissolution in deuterated solvents at room temperature [1]. The phenomenon is related to the Haupt effect which was discovered in 1972 [2] and has since then very often been debated in the literature [3][4][5][6][7][8][9][10][11][12][13].…”

Use of Nuclear Spin Noise Spectroscopy to Monitor Slow Magnetization Buildup at Millikelvin Temperatures

“…[5] We performed the experiments at 17.5 mK on the millikelvin NMR spectrometer described in Refs. [4,24,25],w hich employed a 3 He- 4 He dilution refrigerator and as uperconducting magnet with av ariable field strength of up to 15 T; we applied between 2.0 and 3.0 T. The NMR probe was tuned to af ixed frequency of 104.5 MHz, which corresponded to an approximate Larmor frequency of 1 Hat2 .5 T.…”

Time‐Resolved Chemically Induced Dynamic Nuclear Polarization of Biologically Important Molecules

“…Such an approach is experimentally challenging, [1] at the same time, it does not allow one to reach P $ 1 (an estimate show that this would require mK temperatures when B 0 % 10 Tesla). Further increase of the magnetic field strength is also problematic: presently high-resolution NMR experiments are limited to the field strength of 24 T [2] with a potential to reach 28:2 T corresponding to the 1:2 GHz proton NMR frequency; [3] by using series-connected hybrid magnets one can presently go up to 36.1 T NMR but even at such high fields we obtain that P eq ( 1. [4] For this reason, hyperpolarization methods do not exploit the brute-force approach and rely on different principles and physical phenomena.…”