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Berlin Brandenburg

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  • Polarization
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  • 1
    Article
    Article
    Exploiting level anti-crossings for efficient and selective transfer of hyperpolarization in coupled nuclear spin systems (2013)
    Details
    In: Physical Chemistry Chemical Physics, 2013, Vol.15(35), pp.14660-14669
    Description: Spin hyperpolarization can be coherently transferred to other nuclei in field-cycling NMR experiments. At low magnetic fields spin polarization is redistributed in a strongly coupled network of spins. Polarization transfer is most efficient at fields where level anti-crossings (LACs) occur for the nuclear spin-states. A further condition is that field switching to the LAC positions is non-adiabatic in order to convert the starting population differences into spin coherences that cause time-dependent mixing of states. The power of this method has been demonstrated by studying transfer of photo-Chemically Induced Dynamic Nuclear Polarization (photo-CIDNP) in N -acetyl-tryptophan. We have investigated the magnetic field dependence and time dependence of coherent CIDNP transfer and directly assessed nuclear spin LACs by studying polarization transfer at specific field positions. The proposed approach based on LACs is not limited to CIDNP but is advantageous for enhancing NMR signals by spin order transfer from any type of hyper-polarized nuclei.
    Keywords: Magnetic Fields ; Coherence ; Dynamic Tests ; Polarization ; Nuclei ; Nuclear Magnetic Resonance ; Nuclear Spin ; Time Dependence ; Miscellaneous Sciences (So);
    ISSN: 1463-9076
    E-ISSN: 1463-9084
    DOI: 10.1039/c3cp52026a
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  • 2
    Article
    Article
    Coherent transfer of nuclear spin polarization in field-cycling NMR experiments (2013)
    Details
    Language: English
    In: The Journal of Chemical Physics, 28 December 2013, Vol.139(24)
    Description: Coherent polarization transfer effects in a coupled spin network have been studied over a wide field range. The transfer mechanism is based on exciting zero-quantum coherences between the nuclear spin states by means of non-adiabatic field jump from high to low magnetic field. Subsequent evolution of these coherences enables conversion of spin order in the system, which is monitored after field jump back to high field. Such processes are most efficient when the spin system passes through an avoided level crossing during the field variation. The polarization transfer effects have been demonstrated for N-acetyl histidine, which has five scalar coupled protons; the initial spin order has been prepared by applying RF-pulses at high magnetic field. The observed oscillatory transfer kinetics is taken as a clear indication of a coherent mechanism; level crossing effects have also been demonstrated. The experimental data are in very good agreement with the theoretical model of coherent polarization transfer. The method suggested is also valid for other types of initial polarization in the spin system, most notably, for spin hyperpolarization.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
    DOI: 10.1063/1.4848699
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  • 3
    Article
    Article
    Level crossing analysis of chemically induced dynamic nuclear polarization: Towards a common description of liquid-state and solid-state cases (2016)
    Details
    Language: English
    In: The Journal of Chemical Physics, 14 April 2016, Vol.144(14)
    Description: Chemically Induced Dynamic Nuclear Polarization (CIDNP) is an efficient method of creating non-equilibrium polarization of nuclear spins by using chemical reactions, which have radical pairs as intermediates. The CIDNP effect originates from (i) electron spin-selective recombination of radical pairs and (ii) the dependence of the inter-system crossing rate in radical pairs on the state of magnetic nuclei. The CIDNP effect can be investigated by using Nuclear Magnetic Resonance (NMR) methods. The gain from CIDNP is then two-fold: it allows one to obtain considerable amplification of NMR signals; in addition, it provides a very useful tool for investigating elusive radicals and radical pairs. While the mechanisms of the CIDNP effect in liquids are well established and understood, detailed analysis of solid-state CIDNP mechanisms still remains challenging; likewise a common theoretical frame for the description of CIDNP in both solids and liquids is missing. Difficulties in understanding the spin dynamics that lead to the CIDNP effect in the solid-state case are caused by the anisotropy of spin interactions, which increase the complexity of spin evolution. In this work, we propose to analyze CIDNP in terms of level crossing phenomena, namely, to attribute features in the CIDNP magnetic field dependence to Level Crossings (LCs) and Level Anti-Crossings (LACs) in a radical pair. This approach allows one to describe liquid-state CIDNP; the same holds for the solid-state case where anisotropic interactions play a significant role in CIDNP formation. In solids, features arise predominantly from LACs, since in most cases anisotropic couplings result in perturbations, which turn LCs into LACs. We have interpreted the CIDNP mechanisms in terms of the LC/LAC concept. This consideration allows one to find analytical expressions for a wide magnetic field range, where several different mechanisms are operative; furthermore, the LAC description gives a way to determine CIDNP sign rules. Thus, LCs/LACs provide a consistent description of CIDNP in both liquids and solids with the prospect of exploiting it for the analysis of short-lived radicals and for optimizing the polarization level.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
    DOI: 10.1063/1.4945341
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  • 4
    Article
    Article
    Exploiting adiabatically switched RF-field for manipulating spin hyperpolarization induced by para hydrogen (2015)
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    Language: English
    In: The Journal of Chemical Physics, 21 December 2015, Vol.143(23)
    Description: A method for precise manipulation of non-thermal nuclear spin polarization by switching a RF-field is presented. The method harnesses adiabatic correlation of spin states in the rotating frame. A detailed theory behind the technique is outlined; examples of two-spin and three-spin systems prepared in a non-equilibrium state by Para -Hydrogen Induced Polarization (PHIP) are considered. We demonstrate that the method is suitable for converting the initial multiplet polarization of spins into net polarization: compensation of positive and negative lines in nuclear magnetic resonance spectra, which is detrimental when the spectral resolution is low, is avoided. Such a conversion is performed for real two-spin and three-spin systems polarized by means of PHIP. Potential applications of the presented technique are discussed for manipulating PHIP and its recent modification termed signal amplification by reversible exchange as well as for preparing and observing long-lived spin states.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
    DOI: 10.1063/1.4937392
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  • 5
    Article
    Article
    High resolution NMR study of T 1 magnetic relaxation dispersion. III. Influence of spin 1/2 hetero-nuclei on spin relaxation and polarization transfer among strongly coupled protons (2012)
    Details
    Language: English
    In: The Journal of Chemical Physics, 07 September 2012, Vol.137(9)
    Description: Effects of spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of protons were studied in a situation where spin ½ hetero-nuclei are present in the molecule. As in earlier works [K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, J. Chem. Phys. 129 , 234513 ( 2008 ) 10.1063/1.3040272 ; S. E. Korchak, K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, J. Chem. Phys. 133 , 194502 ( 2010 ) 10.1063/1.3495988 ] , spin-spin interactions have a pronounced effect on the relaxivity tending to equalize the longitudinal relaxation times once the spins become strongly coupled at a sufficiently low magnetic field. In addition, we have found influence of 19 F nuclei on the proton NMRD, although in the whole field range, studied protons and fluorine spins were only weakly coupled. In particular, pronounced features in the proton NMRD were found; but each feature was predominantly observed only for particular spin states of the hetero-nuclei. The features are explained theoretically; it is shown that hetero-nuclei can affect the proton NMRD even in the limit of weak coupling when (i) protons are coupled strongly and (ii) have spin-spin interactions of different strengths with the hetero-nuclei. We also show that by choosing the proper magnetic field strength, one can selectively transfer proton spin magnetization between spectral components of choice.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
    DOI: 10.1063/1.4746780
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  • 6
    Article
    Article
    Exploiting level anti-crossings (LACs) in the rotating frame for transferring spin hyperpolarization (2014)
    Details
    In: Physical Chemistry Chemical Physics, 2014, Vol.16(35), pp.18707-18719
    Description: A method of transferring hyperpolarization among scalar-coupled nuclear spins is proposed, which is based on spin mixing at energy Level Anti-Crossing (LAC) regions. To fulfill LAC conditions a resonant RF-field was applied with properly set frequency and amplitude. In this situation LACs occur between the nuclear spin levels in the rotating doubly tilted reference frame. The validity of the approach is demonstrated by taking as an example the transfer of para -hydrogen induced polarization in a symmetric molecule, whose coupled spin network can be modeled as a four-spin AAMM-system with two pairs of isochronous spins. For this spin system LAC positions have been identified; rules for the sign of spin polarization have been established. The dependence of the polarization transfer efficiency on the RF-field parameters and on the time profile of switching off the RF-field has been studied in detail; experimental results are in excellent agreement with the theory developed. In general, exploiting LACs in the rotating doubly tilted frame is a powerful tool for manipulating hyperpolarization in multispin systems.
    Keywords: Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
    DOI: 10.1039/c4cp01445f
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  • 7
    Article
    Article
    Manipulating spin hyper-polarization by means of adiabatic switching of a spin-locking RF-field (2013)
    Details
    In: Physical Chemistry Chemical Physics, 2013, Vol.15(34), pp.14248-14255
    Description: We propose a technique for transferring the multiplet spin polarization (CIDNP or PHIP, or one created by any other method), which is the mutual entanglement of spins, into net hyper-polarization with respect to the direction of a high magnetic field by slowly (adiabatically) switching-off a strong external RF-field with a specially selected frequency. The net hyper-polarized molecules can then be used in NMR spectroscopy or imaging for strong signal enhancement.
    Keywords: NMR Spectroscopy ; Magnetic Fields ; Switching ; Physical Chemistry ; Entanglement ; Polarization ; Imaging ; Adiabatic Flow ; Miscellaneous Sciences (So);
    ISSN: 1463-9076
    E-ISSN: 1463-9084
    DOI: 10.1039/c3cp52061g
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  • 8
    Article
    Article
    Field-cycling NMR experiments in an ultra-wide magnetic field range: relaxation and coherent polarization transfer (2018)
    Details
    In: Physical Chemistry Chemical Physics, 2018, Vol.20(18), pp.12396-12405
    Description: An experimental method is described allowing fast field-cycling Nuclear Magnetic Resonance (NMR) experiments over a wide range of magnetic fields from 5 nT to 10 T. The method makes use of a hybrid technique: the high field range is covered by positioning the sample in the inhomogeneous stray field of the NMR spectrometer magnet. For fields below 2 mT a magnetic shield is mounted on top of the spectrometer; inside the shield the magnetic field is controlled by a specially designed coil system. This combination allows us to measure T 1 -relaxation times and nuclear Overhauser effect parameters over the full range in a routine way. For coupled proton-carbon spin systems relaxation with a common T 1 is found at low fields, where the spins are strongly coupled. In some cases, experiments at ultralow fields provide access to heteronuclear long-lived spin states. Efficient coherent polarization transfer is seen for proton-carbon spin systems at ultralow fields as follows from the observation of quantum oscillations in the polarization evolution. Applications to analysis and the manipulation of heteronuclear spin systems are discussed.
    Keywords: Magnetic-Field ; Oscillation ; Spinning-System ; Inhomogeneous-Field ; Experimental-Method ; Hybrid-Technique ; Carbon ; NMR-Spectrometers ; Magnetic-Shielding ; Magnetic-Shields ; Spectrometers ; Relaxion-Time ; Magnetic-Resonance ; Magnetisches Feld ; Oszillation ; Spinnsystem ; Inhomogenes Feld ; Experimentalmethode ; Hybridtechnik ; Kohlenstoff ; NMR-Spektrometer ; Magnetische Abschirmung ; Magnetschild ; Spektrometer ; Relaxationszeit ; Magnetische Resonanz ; Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
    DOI: 10.1039/c7cp08529j
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  • 9
    Article
    Article
    Transfer of SABRE-derived hyperpolarization to spin-1/2 heteronuclei (2015)
    Details
    In: RSC Advances, 2015, Vol.5(78), pp.63615-63623
    Description: 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 15 N-labelled pyridine as the SABRE substrate; NMR enhancements achieved for 15 N nuclei are more than 1000 for free pyridine in solution and more than 20000 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 13 C, 19 F, 31 P, etc. Development of such methods opens new avenues in NMR spectroscopy and imaging, which were out of reach for sensitivity reasons.
    Keywords: NMR Spectroscopy ; Magnetic Fields ; Exchange ; Pyridines ; Constants ; Polarization ; Nuclei ; Nuclear Magnetic Resonance ; Chemical and Electrochemical Properties (MD) ; Chemical and Electrochemical Properties (Ep) ; Chemical and Electrochemical Properties (Ed) ; Chemical and Electrochemical Properties (EC);
    ISSN: 2046-2069
    E-ISSN: 2046-2069
    DOI: 10.1039/c5ra13808f
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  • 10
    Article
    Article
    Complete magnetic field dependence of SABRE‐derived polarization (2018)
    Details
    In: Magnetic Resonance in Chemistry, July 2018, Vol.56(7), pp.651-662
    Description: Signal amplification by reversible exchange (SABRE) is a promising hyperpolarization technique, which makes use of spin‐order transfer from parahydrogen (the H molecule in its singlet spin state) to a to‐be‐polarized substrate in a transient organometallic complex, termed the SABRE complex. In this work, we present an experimental method for measuring the magnetic field dependence of the SABRE effect over an ultrawide field range, namely, from 10 nT to 10 T. This approach gives a way to determine the complete magnetic field dependence of SABRE‐derived polarization. Here, we focus on SABRE polarization of spin‐1/2 hetero‐nuclei, such as C and N and measure their polarization in the entire accessible field range; experimental studies are supported by calculations of polarization. Features of the field dependence of polarization can be attributed to level anticrossings in the spin system of the SABRE complex. Features at magnetic fields of the order of 100 nT–1 μT correspond to “strong coupling” of protons and hetero‐nuclei, whereas features found in the mT field range stem from “strong coupling” of the proton system. Our approach gives a way to measuring and analyzing the complete SABRE field dependence, to probing NMR parameters of SABRE complexes and to optimizing the polarization value. An approach is proposed to measuring the magnetic field dependence of the SABRE effect. SABRE‐derived polarization is measured in the magnetic field range 10 nT‐10 T and analyzed theoretically. C and N nuclei are efficiently hyper‐polarized.
    Keywords: Reservation Systems ; Magnetic Fields;
    ISSN: 0749-1581
    E-ISSN: 1097-458X
    DOI: 10.1002/mrc.4694
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