RF-SABRE: A Way to Continuous Spin Hyperpolarization at High Magnetic Fields

A new technique is developed that allows one to carry out the signal amplification by reversible exchange (SABRE) experiments at high magnetic field. SABRE is a hyperpolarization method, which utilizes transfer of spin order from <i>para</i>-hydrogen to the spins of a substrate in transient iridium complexes. Previously, it has been thought that such a transfer of spin order is only efficient at low magnetic fields, notably, at level anti-crossing (LAC) regions. Here it is demonstrated that LAC conditions can also be fulfilled at high fields under the action of a RF field. The high-field RF-SABRE experiment can be implemented using commercially available nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) machines and does not require technically demanding field-cycling. The achievable NMR enhancements are around 100 for several substrates as compared to their NMR signals at thermal equilibrium conditions at 4.7 T. The frequency dependence of RF-SABRE is comprised of well pronounced peaks and dips, whose position and amplitude are conditioned solely by the magnetic resonance parameters such as chemical shifts and scalar coupling of the spin system involved in the polarization transfer and by the amplitude of the RF field. Thus, the proposed method can serve as a new sensitive tool for probing transient complexes. Simulations of the dependence of magnetization transfer (i.e., NMR signal amplifications) on the frequency and amplitude of the RF field are in good agreement with the developed theoretical approach. Furthermore, the method enables continuous re-hyperpolarization of the SABRE substrate over a long period of time, giving a straightforward way to repetitive NMR experiments

Evidence for Coherent Transfer of <i>para</i>-Hydrogen-Induced Polarization at Low Magnetic Fields

We have investigated the mechanism of <i>para</i>-hydrogen-induced polarization (PHIP) transfer from the original strongly aligned protons to other nuclei at low external magnetic fields. Although it is known that PHIP is efficiently transferred at low fields, the nature of the transfer mechanism, that is, coherent spin mixing or cross-relaxation, is not well established. Polarization transfer kinetics for individual protons of styrene was, for the first time, measured and modeled theoretically. Pronounced oscillations were observed indicating a coherent transfer mechanism. Spin coherences were excited by passing through an avoided level crossing of the nuclear spin energy levels. Transfer at avoided level crossings is selective with respect to spin order. Our work provides evidence that the coherent PHIP transfer mechanism is dominant at low magnetic fields

Nuclear Spin Singlet Order Selection by Adiabatically Ramped RF Fields

We describe an NMR method to generate singlet order in spin pairs from longitudinal spin magnetization and suppress residual background signals. This method can also be used for generating and observing long-lived spin states. A singlet order selection (SOS) filter is proposed, which allows us to find signals of the spin pair of interest buried in a crowded NMR spectrum. Likewise, SOS filtering enables proton NMR measurements in H₂O without pulse sequences for solvent suppression. We demonstrate that the method works perfectly for both weakly and strongly coupled spin pairs. Furthermore, it can be combined with standard NMR pulse sequences: in this way, <i>T</i>₁- and <i>T</i>₂-relaxation times for spin pairs of interest can be measured. The power of the SOS-filter is demonstrated by relaxation studies in biomolecules

Highly Efficient Polarization of Spin-1/2 Insensitive NMR Nuclei by Adiabatic Passage through Level Anticrossings

A method is proposed to transfer spin order from <i>para</i>-hydrogen, that is, the H₂ molecule in its singlet state, to spin-1/2 heteronuclei of a substrate molecule. The method is based on adiabatic passage through nuclear spin level anticrossings (LACs) in the doubly rotating frame of reference; the LAC conditions are fulfilled by applying resonant RF excitation at the NMR frequencies of protons and the heteronuclei. Efficient conversion of the <i>para</i>-hydrogen-induced polarization into net polarization of the heteronuclei is demonstrated; the achieved signal enhancements are about 6400 for ¹³C nuclei at natural abundance. The theory behind the technique is described; advantages of the method are discussed in detail

Parahydrogen Allows Ultrasensitive Indirect NMR Detection of Catalytic Hydrogen Complexes

The ¹H NMR signal of dissolved molecular hydrogen enriched in parahydrogen (<i>p</i>-H₂) exhibits in the presence of an organometallic hydrogenation catalyst an unusual, partially negative line shape (PNL). It results from a strongly enhanced two-spin order connected to the population of the <i>T</i>₀ level of orthohydrogen (<i>o</i>-H₂). This two-spin order is made visible by a slow asymmetric exchange process between free hydrogen and a transient catalyst-hydrogen complex. By Only Parahydrogen Spectroscopy (OPSY) it is possible to selectively detect the two-spin order and suppress the signal from the thermal <i>o</i>-H₂. The intensity of the PNL can be strongly affected by the PArtially NEgative Line (PANEL) experiment, which irradiates a long narrow-band radio frequency (RF) pulse. When the RF is in resonance with the chemical shift values of the hydrogen bound to the elusive catalyst or of the free hydrogen, a strong intensity reduction of the PNL is observed. Numerical simulations of the experiments performed at 500 and 700 MHz proton frequency show that the indirect detection has at least 3 orders of magnitude higher sensitivity than the normal NMR experiment. A theoretical model, including reversible binding and <i>S</i> – <i>T</i>₀ evolution, is developed, which reproduces the NMR line shape, the nutation angle dependence and the dependence on the frequency of the irradiation field of the PNL and permits the determination of the proton chemical shift values and the sign of the scalar coupling in the transient NMR invisible complex where singlet–triplet conversion take place

Nucleophilic Ring Opening of Donor–Acceptor Cyclopropanes with the Cyanate Ion: Access to Spiro[pyrrolidone-3,3′-oxindoles]

The nucleophilic ring opening of donor–acceptor cyclopropanes with the cyanate ion is reported for the first time. Cyclopropanes, spiro-activated with oxindole fragments as acceptors, are shown to undergo transformations into biologically relevant spiro­[pyrrolidone-3,3′-oxindoles] while being treated with potassium cyanate under microwave assistance

Lewis and Brønsted Acid Induced (3 + 2)-Annulation of Donor–Acceptor Cyclopropanes to Alkynes: Indene Assembly

(3 + 2)-Annulation of donor–acceptor cyclopropanes to alkynes induced by both Lewis and Brønsted acids has been developed. The reaction provides a rapid approach to functionalized indenes displaying intense visible emission (λ_max = 430 nm, Φ = 0.28–0.34)