Para-hydrogen raser delivers sub-millihertz resolution in nuclear magnetic resonance

Abstract

The precision of nuclear magnetic resonance spectroscopy 1 (NMR) is limited by the signal-to-noise ratio, the measurement time T m and the linewidth ν = 1/(πT 2 ). Overcoming the T 2 limit is possible if the nuclear spins of a molecule emit continuous radio waves. Lasers 2,3 and masers 4-13 are self-organized systems which emit coherent radiation in the optical and microwave regime. Both are based on creating a population inversion of specific energy states. Here we show continuous oscillations of proton spins of organic molecules in the radiofrequency regime (raser 5 ). We achieve this by coupling a population inversion created through signal amplification by reversible exchange (SABRE) 14-16 to a high-quality-factor resonator. For the case of 15 N labelled molecules, we observe multi-mode raser activity, which reports di erent spin quantum states. The corresponding 1 H-15 N J-coupled NMR spectra exhibit unprecedented sub-millihertz resolution and can be explained assuming two-spin ordered quantum states. Our findings demonstrate a substantial improvement in the frequency resolution of NMR.Radio-wave masers (rasers) arise from the radiofrequency Zeeman splittings of nuclear spins such as 1 H, 3 He, 29 Al or 129 Xe. Rasers using 3 He and 129 Xe gas as the rasing medium employ spin exchange optical pumping 17 (SEOP) at T > 400 K to create sufficient population inversion 6-8 , whereas 29 Al solid 9 or 1 H Zeeman liquidstate masers 10 rely on dynamic nuclear polarization (DNP) techniques 18 or photochemical excitation 11 to invert populations. Solidstate maser action has been observed in pulsed mode at room temperature with pentacene 12 , and a continuous-mode solid-state maser based on nitrogen-vacancy centres in diamond has been proposed 13 .Here we report the observation of a liquid-state para-hydrogen pumped molecular raser that operates at 300 K with protons of organic molecules in solution and thereby avoids costly high magnetic fields, high vacuum, optical pumping or DNP techniques. We continuously supply para-hydrogen (p-H 2 ) gas into a solution containing the raser active molecules and an iridium-based SABRE catalyst [14][15][16] . This spin-order transfer catalyst creates population inversion, equivalent to a negative spin temperature, on target molecules without altering their molecular structure. Coupling of these hyperpolarized molecules to a high-Q resonator 19 produces a sustained raser signal comprised of frequencies that originate from the scalar couplings of nuclei within the molecule.The operating principles for the 3 He Zeeman maser 6-8 are a starting point to assess challenges associated with the design of a SABRE-pumped room-temperature proton raser working at low frequencies. Masing starts once the radiation-damping rate 1/τ rd , which quantifies the coupling between the resonator and the nuclear spins, satisfies the conditionHere the apparent transverse relaxation rate 1/T * 2 = 1/T 2 + 1/τ p is the sum of the transverse relaxation rate 1/T 2 and the pumping rate 1/τ p . The radiation-dampi...

Continuous Radio Amplification by Stimulated Emission of Radiation using Parahydrogen Induced Polarization (PHIP‐RASER) at 14 Tesla

“…Similar to SABRE‐RASER, this new effect is based on the spin order of parahydrogen ( p H 2 ) [Eq. (1)], …”

“…Using a sophisticated setup and continuously renewing polarization, a spectral resolution of 0.6 Hz was achieved at ≈3.8 mT (≈4 ppm). This unprecedented resolution of J‐couplings was reported, but chemical shift resolution was not possible because of the low magnetic field . A transfer of this method to higher fields was not feasible because the level anticrossing (LAC) is required for spontaneous polarization transfer to occur (for 1 H, ≈mT) .…”

Multiple Quantum Coherences Hyperpolarized at Ultra‐Low Fields

“…During the past decades, various hyperpolarization techniques have been developed [1][2][3][4][5][6][7][8][9][10][11][12][13][14] to boost the magnetic resonance (MR) signal in order to bring new applications to industry [15][16][17] and life-sciences. During the past decades, various hyperpolarization techniques have been developed [1][2][3][4][5][6][7][8][9][10][11][12][13][14] to boost the magnetic resonance (MR) signal in order to bring new applications to industry [15][16][17] and life-sciences.…”

“…[18][19][20][21][22][23][24][25] One of these methods is signal amplification by reversible exchange (SABRE), [26][27][28][29][30][31][32][33][34][35][36] where parahydrogen (pH 2 ) is used to polarize dissolved molecules by their mutual exchange with a transient complex. SABRE at high magnetic field for high-resolution NMR encounters difficulties due to magnetic field inhomogeneity caused by pH 2 bubbling; [38] this is not a problem for ultra-low field (ULF) NMR and even allows continuous SABRE, [39] radiowave amplification by stimulated emission of radiation (RASER) [13] and QUASi-Resonance SABRE (QUASR). SABRE at high magnetic field for high-resolution NMR encounters difficulties due to magnetic field inhomogeneity caused by pH 2 bubbling; [38] this is not a problem for ultra-low field (ULF) NMR and even allows continuous SABRE, [39] radiowave amplification by stimulated emission of radiation (RASER) [13] and QUASi-Resonance SABRE (QUASR).…”

Measuring molecular parity nonconservation using nuclear-magnetic-resonance spectroscopy

“…It would also be interesting to explore whether coherent experiments, like the NMR "RASER" of Ref. [46], could be beneficial for measuring PNC.…”