Quantum rotor induced hyperpolarization

Analysis of 1-aminoisoquinoline using the signal amplification by reversible exchange hyperpolarization technique

Signal amplification by reversible exchange (SABRE), a parahydrogen-based hyperpolarization technique, is valuable in detecting low concentrations of chemical compounds, which facilitates the understanding of their functions at the molecular level as well as their applicability in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). SABRE of 1-aminoisoquinoline (1-AIQ) is significant because isoquinoline derivatives are the fundamental structures in compounds with notable biological activity and are basic organic building blocks. Through this study, we explain how SABRE is applied to hyperpolarize 1-AIQ for diverse solvent systems such as deuterated and non-deuterated solvents. We observed the amplification of individual protons of 1-AIQ at various magnetic fields. Further, we describe the polarization transfer mechanism of 1-AIQ compared to pyridine using density functional theory (DFT) calculations. This hyperpolarization technique, including the polarization transfer mechanism investigation on 1-AIQ, will provide a firm basis for the future application of the hyperpolarization study on various bio-friendly materials.

Quantum-rotor-induced polarization

Quantum-rotor-induced polarization is closely related to para-hydrogen-induced polarization. In both cases, the hyperpolarized spin order derives from rotational interaction and the Pauli principle by which the symmetry of the rotational ground state dictates the symmetry of the associated nuclear spin state. In quantum-rotor-induced polarization, there may be several spin states associated with the rotational ground state, and the hyperpolarization is typically generated by hetero-nuclear cross-relaxation. This review discusses preconditions for quantum-rotor-induced polarization for both the 1-dimensional methyl rotor and the asymmetric rotor H₂¹⁷ O@C₆₀ , that is, a single water molecule encapsulated in fullerene C₆₀ . Experimental results are presented for both rotors.

Testing signal enhancement mechanisms in the dissolution NMR of acetone

In cryogenic dissolution NMR experiments, a substance of interest is allowed to rest in a strong magnetic field at cryogenic temperature, before dissolving the substance in a warm solvent, transferring it to a high-resolution NMR spectrometer, and observing the solution-state NMR spectrum. In some cases, negative enhancements of the ¹³C NMR signals are observed, which have been attributed to quantum-rotor-induced polarization. We show that in the case of acetone (propan-2-one) the negative signal enhancements of the methyl ¹³C sites may be understood by invoking conventional cross-relaxation within the methyl groups. The ¹H nuclei acquire a relative large net polarization through thermal equilibration in a magnetic field at low temperature, facilitated by the methyl rotation which acts as a relaxation sink; after dissolution, the ¹H magnetization slowly returns to thermal equilibrium at high temperature, in part by cross-relaxation processes, which induce a transient negative polarization of nearby ¹³C nuclei. We provide evidence for this mechanism experimentally and theoretically by saturating the ¹H magnetization using a radiofrequency field pulse sequence before dissolution and comparing the ¹³C magnetization evolution after dissolution with the results obtained from a conventional ¹H-¹³C cross relaxation model of the CH₃ moieties in acetone.

Dipolar relaxation mechanism of long-lived states of methyl groups

We analyze the symmetry properties of the dipolar Hamiltonian as the main relaxation mechanism responsible for the observed NMR spectra of long-lived states of methyl groups. Long-lived states exhibit relaxation times that are considerably longer than the spin-lattice relaxation time, T 1 . The analysis is complementary to previous studies and provides insight into the relaxation mechanism of long-lived states by focusing exclusively on the symmetry of the spin Hamiltonian. Our study shows that the dipole-dipole coupling between protons of a methyl group and between the protons and an external spin are both symmetry breaking interactions that can lead to relaxation pathways that transform the polarization from symmetry order to Zeeman order. The net contribution of the internal dipolar interaction to the NMR observation of long-lived states is zero. Our calculation is in good agreement with the reported features of the observed spectra and previous theoretical studies.

Methyl rotor quantum states and the effect of chemical environment in organic crystals: γ-picoline and toluene

Using a set of first-principles calculations, we have studied the methyl tunnel splitting for molecular crystals of γ-picoline and toluene. The effective rotational potential energy surface of the probe methyl rotor along the tunneling path is evaluated using first-principles electronic structure calculations combined with the nudged elastic band method. The tunnel splitting is calculated by an explicit diagonalization of the one-dimensional time-independent Hamiltonian matrix. The effects of chemical environment and rotor-rotor coupling on the rotational energy barriers were investigated. It is found that more dense packing of the molecules in toluene compared to that in γ-picoline gives rise to a larger rotational barrier which in turn yields a considerably smaller tunnel splitting. Moreover, it turned out that coupled motion of the face-to-face methyl groups in γ-picoline has a significant effect on the reduction of the rotational barrier. Our results are in good agreement with the experimentally observed tunnel splitting.

NMR of molecular endofullerenes dissolved in a nematic liquid crystal

We report the NMR of the molecular endofullerenes H₂@C₆₀, H₂O@C₆₀ and HF@C₆₀ dissolved in the nematic liquid crystal N-(4-methoxybenzylidene)-4-butylaniline (MBBA).

Aharonov-Bohm effect in the tunnelling of a quantum rotor in a linear Paul trap

Quantum tunnelling is a common fundamental quantum mechanical phenomenon that originates from the wave-like characteristics of quantum particles. Although the quantum tunnelling effect was first observed 85 years ago, some questions regarding the dynamics of quantum tunnelling remain unresolved. Here we realize a quantum tunnelling system using two-dimensional ionic structures in a linear Paul trap. We demonstrate that the charged particles in this quantum tunnelling system are coupled to the vector potential of a magnetic field throughout the entire process, even during quantum tunnelling, as indicated by the manifestation of the Aharonov-Bohm effect in this system. The tunnelling rate of the structures periodically depends on the strength of the magnetic field, whose period is the same as the magnetic flux quantum φ0 through the rotor [(0.99 ± 0.07) × φ0].

Spin-symmetry conversion in methyl rotors induced by tunnel resonance at low temperature

Field-cycling NMR in the solid state at low temperature (4.2 K) has been employed to measure the tunneling spectra of methyl (CH3) rotors in phenylacetone and toluene. The phenomenon of tunnel resonance reveals anomalies in (1)H magnetization from which the following tunnel frequencies have been determined: phenylacetone, νt = 6.58 ± 0.08 MHz; toluene, νt(1) = 6.45 ± 0.06 GHz and νt(2) = 7.07 ± 0.06 GHz. The tunnel frequencies in the two samples differ by three orders of magnitude, meaning different experimental approaches are required. In phenylacetone the magnetization anomalies are observed when the tunnel frequency matches one or two times the (1)H Larmor frequency. In toluene, doping with free radicals enables magnetization anomalies to be observed when the tunnel frequency is equal to the electron spin Larmor frequency. Cross-polarization processes between the tunneling and Zeeman systems are proposed and form the basis of a thermodynamic model to simulate the tunnel resonance spectra. These invoke space-spin interactions to drive the changes in nuclear spin-symmetry. The tunnel resonance lineshapes are explained, showing good quantitative agreement between experiment and simulations.

Long-lived nuclear spin states in methyl groups and quantum-rotor-induced polarization

Substances containing rapidly rotating methyl groups may exhibit long-lived states (LLSs) in solution, with relaxation times substantially longer than the conventional spin-lattice relaxation time T1. The states become long-lived through rapid internal rotation of the CH3 group, which imposes an approximate symmetry on the fluctuating nuclear spin interactions. In the case of very low CH3 rotational barriers, a hyperpolarized LLS is populated by thermal equilibration at liquid helium temperature. Following dissolution, cross-relaxation of the hyperpolarized LLS, induced by heteronuclear dipolar couplings, generates strongly enhanced antiphase NMR signals. This mechanism explains the NMR signal enhancements observed for (13)C-γ-picoline (Icker, M.; Berger, S. J. Magn. Reson. 2012, 219, 1-3).

Experimental boundaries of the quantum rotor induced polarization (QRIP) in liquid state NMR

The Haupt-effect is a rather seldom used hyperpolarization method. It is based on the interdependence between nuclear spin states and rotational states of nearly free rotating methyl groups having C3 symmetry. A sudden change in temperature from 4.2 K to room temperature by fast dissolution yields considerably enhanced (13)C and (1)H resonance signals. This phenomenon is now termed quantum rotor induced polarization. More than 40 substances have been studied by this approach in order to identify them as polarizable by the 'Haupt-effect in the liquid state'. Influencing factors have been analyzed systematically. It could be concluded that substances having a high tunneling frequency, which is due to a small and narrow potential barrier, are most likely to feature quantum rotor induced polarization-enhanced signals.

Transfer of the Haupt-hyperpolarization to neighbor spins

The NMR hyperpolarization observed for freely rotating methyl groups by exerting a temperature jump from 4.2 K to 298 K can be transferred to spins which have a spin, spin coupling with the carbon of the methyl group. First, a spin echo sequence readjusts the primary up/down signals to an in-phase multiplet. This in-phase magnetization is then decoupled and transferred by a simple COSY step using one scan. The polarization factors at the neighbor spins are about 50 by comparing their signal-to-noise ratio with the signal strength after full relaxation.

Unexpected multiplet patterns induced by the Haupt-effect

An NMR polarization up to a factor of 100 compared to the room temperature signal of a fully equilibrated sample and up/down multiplets are observed when 4-methyl-pyridine or toluene are taken rapidly from liquid helium temperatures to room temperature by dissolving in acetone-d6. These findings result from the inherent coupling between rotational and nuclear spin states in methyl groups which can act as quantum rotors. The temperature jump causes changes in rotational and spin energy level population due to symmetry rules that is known as the Haupt-effect.

Dynamic tunneling polarization as a quantum rotor analogue of dynamic nuclear polarization and the NMR solid effect

The populations of the tunneling states of CH(3) are manipulated by rf irradiation of weakly allowed sideband transitions within the manifold of tunneling-magnetic levels. Substantial positive and negative CH(3) tunneling polarizations are observed, providing a quantum rotor analogue of dynamic nuclear polarization and the solid effect in NMR. The field-cycling NMR technique used in the experiments employs level crossings between tunneling and Zeeman systems to report on the tunneling polarization. The tunneling lifetimes are measured and the field dependence investigated.

Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids

This article provides an overview of polarizing mechanisms involved in high-frequency dynamic nuclear polarization (DNP) of frozen biological samples at temperatures maintained using liquid nitrogen, compatible with contemporary magic-angle spinning (MAS) nuclear magnetic resonance (NMR). Typical DNP experiments require unpaired electrons that are usually exogenous in samples via paramagnetic doping with polarizing agents. Thus, the resulting nuclear polarization mechanism depends on the electron and nuclear spin interactions induced by the paramagnetic species. The Overhauser Effect (OE) DNP, which relies on time-dependent spin-spin interactions, is excluded from our discussion due the lack of conducting electrons in frozen aqueous solutions containing biological entities. DNP of particular interest to us relies primarily on time-independent, spin-spin interactions for significant electron-nucleus polarization transfer through mechanisms such as the Solid Effect (SE), the Cross Effect (CE) or Thermal Mixing (TM), involving one, two or multiple electron spins, respectively. Derived from monomeric radicals initially used in high-field DNP experiments, bi- or multiple-radical polarizing agents facilitate CE/TM to generate significant NMR signal enhancements in dielectric solids at low temperatures (<100 K). For example, large DNP enhancements (∼300 times at 5 T) from a biologically compatible biradical, 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL), have enabled high-resolution MAS NMR in sample systems existing in submicron domains or embedded in larger biomolecular complexes. The scope of this review is focused on recently developed DNP polarizing agents for high-field applications and leads up to future developments per the CE DNP mechanism. Because DNP experiments are feasible with a solid-state microwave source when performed at <20K, nuclear polarization using lower microwave power (<100 mW) is possible by forcing a high proportion of biradicals to fulfill the frequency matching condition of CE (two EPR frequencies separated by the NMR frequency) using the strategies involving hetero-radical moieties and/or molecular alignment. In addition, the combination of an excited triplet and a stable radical might provide alternative DNP mechanisms without the microwave requirement.