Hans-Martin Vieth

Dynamic nuclear polarization at high magnetic fields in liquids.

MPI for Biophysical Chemistry Gottingen, Am Fassberg 11, 37077 Gottingen, Germany b Free University Berlin, Inst. of Experimental Physics, Arnimallee 14, 14195 Berlin, Germany Magnetic Resonance Center (CERM) and Department of Chemistry, University of Florence, Via Luigi Sacconi 6, 50019 Sesto Fiorentino, Italy Bruker Biospin GmbH, Silberstreifen 4, 76287 Rheinstetten, Germany e Technische Universitat Munchen, Department of Chemistry, Lichtenbergstr. 4, 85747 Garching, Germany Goethe University Frankfurt, Max von Laue Strasse 7, 60438 Frankfurt, Germany

Transient ESR nutation signals in excited aromatic triplet states

Abstract Transient ESR nutation signals following laser pulse excitation are analyzed for the case of the photoexcited triplet state of acridine in a molecular crystal matrix. Essential features, in particular the short rise-time ( −7 ) of the signals, can be related to the inhomogeneous nature of the ESR linewidth. Applications are discussed.

A simple method analyzing 2H nuclear magnetic resonance line shapes to determine the activation energy distribution of mobile guest molecules in disordered systems

We investigated the 2H nuclear magnetic resonance (NMR) line shape of deuterated benzene and hexamethylbenzene as guest molecules in organic glasses in the temperature range of 10–150 K. A broad distribution G(ln τ) of correlation times determines the slowing down of the molecular reorientation around the sixfold symmetry axis of the guests. The line shape is described by a superposition of temperature‐dependent fractions F(T) of only two subspectra corresponding to fast and slowly rotating molecules; no spectra characteristic for intermediate mobility as found in crystal matrices are observed. Assuming a thermally activated motional process, the temperature dependence of G(ln τ) comes from a temperature‐independent distribution of activation energies g(E). In this case, the derivative of the fraction dF(T)/dT yields directly the distribution g(E). Using this method an asymmetric distribution g(E) with its maximum at the low energy side is found for the glasses. While the general shape of g(E) is similar ...

Field cycling by fast NMR probe transfer: Design and application in field-dependent CIDNP experiments

A novel field-cycling unit with fast digital positioning of a high-resolution nuclear magnetic resonance probe in a spatially varying magnetic field is described and used to measure CIDNP spectra of the amino acid-dye (histidine-bipyridyl) photoreaction system in the range between 0 and 7 T. The pattern of nuclear polarization varies with the magnetic field. In particular, strong polarization with an emission/absorption pattern (multiplet effect) is found at low field for two histidine ringprotons with scalar coupling below 3 Hz visible only because of the high resolution made possible by the new field-cycling setup. Also for the CH2 protons in the β-position a multiplet effect is observed having a pattern changing with magnetic field. By analysis of the spin nutation the non-Boltzmann population differences of the nuclear levels have been determined.

Level Anti-Crossings are a Key Factor for Understanding para-Hydrogen-Induced Hyperpolarization in SABRE Experiments

Various hyperpolarization methods are able to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) by several orders of magnitude. Among these methods are para-hydrogen-induced polarization (PHIP) and signal amplification by reversible exchange (SABRE), which exploit the strong nuclear alignment of para-hydrogen. Several SABRE experiments have been reported but, so far, it has not been possible to account for the experimentally observed sign and magnetic-field dependence of substrate polarization. Herein, we present an analysis based on level anti-crossings (LACs), which provides a complete understanding of the SABRE effect. The field-dependence of both net and anti-phase polarization is measured for several ligands, which can be reproduced by the theory. The similar SABRE field-dependence for different ligands is also explained. In general, the LAC concept allows complex spin dynamics to be unraveled, and is crucial for optimizing the performance of novel hyperpolarization methods in NMR and MRI techniques.

Coherent transfer of hyperpolarization in coupled spin systems at variable magnetic field

A general theoretical approach to coherent transfer of hyperpolarization (HP) among coupled spins is suggested and applied to field-cycling experiments where the polarized spin system is prepared at low magnetic field while observation is performed at high field. The formalism works for an arbitrary mechanism of HP formation and takes into account finite periods of HP preparation, free evolution at the polarization field, and field variation, which was considered in the two limiting regimes of sudden and adiabatic field jump. The polarization transfer is of a coherent nature and proceeds due to strong coupling of spins at low field, where the difference of their Zeeman interaction with the field is less than or comparable to their scalar spin-spin coupling. The case of three nonequivalent coupled spins is studied in detail including the calculation of HP transfer efficiency in the range from zero to high field. Characteristic transfer times are found. New features in the field dependence of polarization are predicted that are due to avoided crossings of the nuclear spin eigenstates of the molecule. The feasibility of HP transfer via spacer spins among spins that have no direct coupling is explored. Another application is the description of para-hydrogen induced polarization in a three-spin system over a wide field range.

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 para-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.

Investigation of the magnetic field dependence of CIDNP in multinuclear radical pairs. 1. Photoreaction of histidine and comparison of model calculation with experimental data

Magnetic field effects on CIDNP formed in the diamagnetic products of geminate recombination of photo-generated radical pairs are calculated at arbitrary field strength. The simulations are based on a model that uses a full quantum mechanical description of the polarized nuclei and a semiclassical account of all other nuclear spins. The results are compared with CIDNP experiments on the photoreaction between N-acetylhistidine and 2,2′-dipyridine in fields between 0 and 7 T employing field cycling between a variable field of polarization and a fixed field of detection. To allow quantitative comparison the simulation takes into account the effects of adiabatic field change and of RF excitation pulse angle on the NMR intensity. Both net and multiplet polarization as observed in two coupled spin pairs (CH2 protons in β position of histidine, and protons in H2,H4 position of its ring) are well reproduced over the whole field range. The wide applicability of the theoretical model and the possibility to extend it to larger spin systems make it particularly useful for the analysis of CIDNP in protein systems.

Transfer of SABRE-derived hyperpolarization to spin-1/2 heteronuclei

In this paper, we describe a method of hyper-polarizing “insensitive” Nuclear Magnetic Resonance (NMR) nuclei by exploiting the SABRE (Signal Amplification By Reversible Exchange) technique and transferring spin order from protons originating from parahydrogen. We demonstrate that hyperpolarization transfer is due to a coherent mechanism, which is operative at (i) very low magnetic field; (ii) geomagnetic field; (iii) high field in the presence of a suitable radiofrequency-excitation scheme. Experiments are performed using 15N-labelled pyridine as the SABRE substrate; NMR enhancements achieved for 15N nuclei are more than 1000 for free pyridine in solution and more than 20 000 for pyridine bound to the SABRE complex. High-field SABRE experiments are particularly important for enhancing the sensitivity of NMR methods: they enable strong signal enhancements and avoid technically demanding field-cycling. Furthermore, such experiments use very low power for NMR excitation and make feasible continuous re-hyperpolarization of the substrate in high-field experiments: polarization can be quickly restored to the maximal level within only 15 seconds with the result that polarization levels stay constant over several hundred experiments. The techniques outlined are applicable to hyper-polarizing spin-1/2 hetero-nuclei, such as 13C, 19F, 31P, etc. Development of such methods opens new avenues in NMR spectroscopy and imaging, which were out of reach for sensitivity reasons.

Non-thermal nuclear magnetic resonance quantum computing using hyperpolarized xenon

Current experiments in liquid-state nuclear magnetic resonance (NMR) quantum computing are limited by low initial polarization. To address this problem, we have investigated the use of optical pumping techniques to enhance the polarization of a 2-qubit NMR quantum computer (13C and 1H in 13CHCl3). We have generalized the procedure for effective pure state preparation in order to efficiently use the increased polarization. With this more flexible scheme, an effective pure state is prepared with polarization enhancement of a factor of 10 compared to the thermal state. An implementation of Grover’s quantum search algorithm with a polarization-enhanced spin system is demonstrated.