Dynamic Nuclear Polarization of ¹³C Nuclei in the Liquid State over a 10 Tesla Field Range

Abstract

Nuclear magnetic resonance (NMR) techniques play an essential role in natural science and medicine.Inspite of the tremendous utility associated with the small energies detected, the most severe limitation is the low signal-to-noise ratio. Dynamic nuclear polarization (DNP), at echnique based on transfer of polarization from electron to nuclear spins,h as emerged as atool to enhance sensitivity of NMR. However,the approach in liquids still faces several challenges.H erein we report the observation of room-temperature,l iquid DNP 13 C signal enhancements in organic small molecules as high as 600 at 9.4 Tesla and 800 at 1.2 Tesla. Amechanistic investigation of the 13 C-DNP field dependence shows that DNP efficiency is raised by proper choice of the polarizing agent (paramagnetic center) and by halogen atoms as mediators of scalar hyperfine interaction. Observation of sizable DNP of 13 CH 2 and 13 CH 3 groups in organic molecules at 9.4 To pens perspective for abroader application of this method.Improving the sensitivity of NMR and magnetic resonance imaging (MRI) remains am ain target of research aiming to expand the scope of NMR applications.Besides technological developments in spectrometer construction, the most powerful approaches rely on hyperpolarization techniques that increase the population difference (or polarization) of nuclear spins far from the polarization at thermal equilibrium. [1] Dynamic nuclear polarization (DNP) takes advantage of the higher polarization of ap olarizing agent and transfers it to nuclear spins via their hyperfine interaction and microwave (MW) irradiation. [2] Themethod has historically worked well in solid-state NMR experiments but turned out to be much more challenging in liquids,b ecause of mechanistic and technical reasons.F irst, the mechanism in liquids requires electron-nuclear hyperfine interaction to be modulated at the time scale of the electron-spin resonance frequency. [3] At low magnetic fields ( 1T esla), molecular diffusion with correlation times on the order of tens of ps provides the suited time scale for this modulation, but it does not for resonance frequencies in the high field range (! 3T esla). [2,3b] Moreover, 13 C-DNP performed up to medium fields revealed an intriguing dependence of the signal enhancements on the 13 C chemical environment, which hampered quantitative mechanistic descriptions. [4] Secondly,i mplementing liquid DNP at high magnetic fields and ambient temperatures is aggravated by dielectric losses.T his effect can be reduced by using MW resonant structures where the sample is placed in aminimum of the electric field of the standing wave. [5] Progress in this direction has been shown in recent reports. [6] We have recently reported the observation of athousandfold 13 CD NP signal enhancement in the liquid state at 3.4 Te sla. [4f] Our report raised the question whether the method is compatible with magnetic fields and requirements used in high-resolution NMR spectroscopy. [7] Herein, we explore the capabilities of 13 Cl iquid DNP over...

Single-Chip Dynamic Nuclear Polarization Microsystem

“…However, there are also situations where the sample is volume limited. For volume-limited samples, the use of an inductive detector having a sensitive volume matched to the volume of the sample under investigation results in a significant improvement of the SNR. − For samples in the nanoliter and subnanoliter range, noninductive detection methods have been also proposed, such those based on nitrogen vacancies in diamond − and magnetic resonance force microscopy. − Another approach to increase the SNR, applicable to samples of any volume, is to increase the nuclear spin polarization, e.g., by microwave-, optical-, and chemistry-based methodologies. − In the microwave DNP approach, the sample under investigation contains unpaired electron spins which are excited into electron spin resonance (ESR). The electron spin excitation allows one to enhance the nuclear magnetization of several orders of magnitude above its thermal value, improving the SNR in the NMR experiment by the same factor and reducing the required experimental time as the square of this factor.…”