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Buntkowsky G, Gutmann T. PASADENA NMR. Nat Catal 2022. [DOI: 10.1038/s41929-022-00859-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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2
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Buntkowsky G, Theiss F, Lins J, Miloslavina YA, Wienands L, Kiryutin A, Yurkovskaya A. Recent advances in the application of parahydrogen in catalysis and biochemistry. RSC Adv 2022; 12:12477-12506. [PMID: 35480380 PMCID: PMC9039419 DOI: 10.1039/d2ra01346k] [Citation(s) in RCA: 13] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2022] [Accepted: 03/23/2022] [Indexed: 12/15/2022] Open
Abstract
Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) are analytical and diagnostic tools that are essential for a very broad field of applications, ranging from chemical analytics, to non-destructive testing of materials and the investigation of molecular dynamics, to in vivo medical diagnostics and drug research. One of the major challenges in their application to many problems is the inherent low sensitivity of magnetic resonance, which results from the small energy-differences of the nuclear spin-states. At thermal equilibrium at room temperature the normalized population difference of the spin-states, called the Boltzmann polarization, is only on the order of 10-5. Parahydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, which has widespread applications in Chemistry, Physics, Biochemistry, Biophysics, and Medical Imaging. PHIP creates its signal-enhancements by means of a reversible (SABRE) or irreversible (classic PHIP) chemical reaction between the parahydrogen, a catalyst, and a substrate. Here, we first give a short overview about parahydrogen-based hyperpolarization techniques and then review the current literature on method developments and applications of various flavors of the PHIP experiment.
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Affiliation(s)
- Gerd Buntkowsky
- Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt Alarich-Weiss-Str. 8 D-64287 Darmstadt Germany
| | - Franziska Theiss
- Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt Alarich-Weiss-Str. 8 D-64287 Darmstadt Germany
| | - Jonas Lins
- Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt Alarich-Weiss-Str. 8 D-64287 Darmstadt Germany
| | - Yuliya A Miloslavina
- Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt Alarich-Weiss-Str. 8 D-64287 Darmstadt Germany
| | - Laura Wienands
- Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt Alarich-Weiss-Str. 8 D-64287 Darmstadt Germany
| | - Alexey Kiryutin
- International Tomography Center, Siberian Branch of the Russian Academy of Science Novosibirsk 630090 Russia
| | - Alexandra Yurkovskaya
- International Tomography Center, Siberian Branch of the Russian Academy of Science Novosibirsk 630090 Russia
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3
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Knecht S, Barskiy DA, Buntkowsky G, Ivanov KL. Theoretical description of hyperpolarization formation in the SABRE-relay method. J Chem Phys 2020; 153:164106. [PMID: 33138423 DOI: 10.1063/5.0023308] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
SABRE (Signal Amplification By Reversible Exchange) has become a widely used method for hyper-polarizing nuclear spins, thereby enhancing their Nuclear Magnetic Resonance (NMR) signals by orders of magnitude. In SABRE experiments, the non-equilibrium spin order is transferred from parahydrogen to a substrate in a transient organometallic complex. The applicability of SABRE is expanded by the methodology of SABRE-relay in which polarization can be relayed to a second substrate either by direct chemical exchange of hyperpolarized nuclei or by polarization transfer between two substrates in a second organometallic complex. To understand the mechanism of the polarization transfer and study the transfer efficiency, we propose a theoretical approach to SABRE-relay, which can treat both spin dynamics and chemical kinetics as well as the interplay between them. The approach is based on a set of equations for the spin density matrices of the spin systems involved (i.e., SABRE substrates and complexes), which can be solved numerically. Using this method, we perform a detailed study of polarization formation and analyze in detail the dependence of the attainable polarization level on various chemical kinetic and spin dynamic parameters. We foresee the applications of the present approach for optimizing SABRE-relay experiments with the ultimate goal of achieving maximal NMR signal enhancements for substrates of interest.
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Affiliation(s)
- Stephan Knecht
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
| | - Danila A Barskiy
- University of California at Berkeley, College of Chemistry and QB3, Berkeley, California 94720, USA
| | - Gerd Buntkowsky
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
| | - Konstantin L Ivanov
- International Tomography Center, Siberian Branch of the Russian Academy of Sciences, and Novosibirsk State University, Novosibirsk 630090, Russia
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4
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Pravdivtsev AN, Hövener JB. Coherent polarization transfer in chemically exchanging systems. Phys Chem Chem Phys 2020; 22:8963-8972. [DOI: 10.1039/c9cp06873b] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Simulation of the interplay of coherent polarization transfer and chemical exchange described by superoperators and Monte Carlo simulations alike.
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Affiliation(s)
- Andrey N. Pravdivtsev
- Section Biomedical Imaging
- Molecular Imaging North Competence Center (MOIN CC)
- Department of Radiology and Neuroradiology
- University Medical Center Kiel
- Kiel University
| | - Jan-Bernd Hövener
- Section Biomedical Imaging
- Molecular Imaging North Competence Center (MOIN CC)
- Department of Radiology and Neuroradiology
- University Medical Center Kiel
- Kiel University
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5
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Hadjiali S, Bergmann M, Kiryutin A, Knecht S, Sauer G, Plaumann M, Limbach HH, Plenio H, Buntkowsky G. The application of novel Ir-NHC polarization transfer complexes by SABRE. J Chem Phys 2019; 151:244201. [PMID: 31893872 DOI: 10.1063/1.5128091] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
In recent years, the hyperpolarization method Signal Amplification By Reversible Exchange (SABRE) has developed into a powerful technique to enhance Nuclear Magnetic Resonance (NMR) signals of organic substrates in solution (mostly via binding to the nitrogen lone pair of N-heterocyclic compounds) by several orders of magnitude. In order to establish the application and development of SABRE as a hyperpolarization method for medical imaging, the separation of the Ir-N-Heterocyclic Carbene (Ir-NHC) complex, which facilitates the hyperpolarization of the substrates in solution, is indispensable. Here, we report for the first time the use of novel Ir-NHC complexes with a polymer unit substitution in the backbone of N-Heterocyclic Carbenes (NHC) for SABRE hyperpolarization, which permits the removal of the complexes from solution after the hyperpolarization of a target substrate has been generated.
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Affiliation(s)
- Sara Hadjiali
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
| | - Marvin Bergmann
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
| | - Alexey Kiryutin
- International Tomography Center, Institutskaya 3A, Novosibirsk and Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia
| | - Stephan Knecht
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
| | - Grit Sauer
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
| | - Markus Plaumann
- Medical Faculty, Institute for Biometrics and Medical Informatics, Otto-von-Guericke University Magdeburg, 39120 Magdeburg, Germany
| | - Hans-Heinrich Limbach
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
| | - Herbert Plenio
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
| | - Gerd Buntkowsky
- Eduard-Zintl Institute for Inorganic and Physical Chemistry, TU Darmstadt, Darmstadt 64287, Germany
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6
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Pravdivtsev AN, Hövener JB. Simulating Non-linear Chemical and Physical (CAP) Dynamics of Signal Amplification By Reversible Exchange (SABRE). Chemistry 2019; 25:7659-7668. [PMID: 30689237 DOI: 10.1002/chem.201806133] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2018] [Revised: 01/18/2019] [Indexed: 01/30/2023]
Abstract
The hyperpolarization of nuclear spins by using parahydrogen (pH2 ) is a fascinating technique that allows spin polarization and thus the magnetic resonance signal to be increased by several orders of magnitude. Entirely new applications have become available. Signal amplification by reversible exchange (SABRE) is a relatively new method that is based on the reversible exchange of a substrate, catalyst and parahydrogen. SABRE is particularly interesting for in vivo medical and industrial applications, such as fast and low-cost trace analysis or continuous signal enhancement. Ever since its discovery, many attempts have been made to model and understand SABRE, with various degrees of simplifications. In this work, we reduced the simplifications further, taking into account non-linear chemical and physical (CAP) dynamics of several multi-spin systems. A master equation was derived and realized using the MOIN open-source software. The effects of different parameters (exchange rates, concentrations, spin-spin couplings) on relaxation and the polarization level have been evaluated and the results provide interesting insights into the mechanism of SABRE.
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Affiliation(s)
- Andrey N Pravdivtsev
- Section Biomedical Imaging, Molecular Imaging North Competence Center (MOIN CC), Department of Radiology and Neuroradiology, University Medical Center Schleswig-Holstein (UKSH), Kiel University, Am Botanischen Garten 14, 24118, Kiel, Germany
| | - Jan-Bernd Hövener
- Section Biomedical Imaging, Molecular Imaging North Competence Center (MOIN CC), Department of Radiology and Neuroradiology, University Medical Center Schleswig-Holstein (UKSH), Kiel University, Am Botanischen Garten 14, 24118, Kiel, Germany
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7
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Knecht S, Pravdivtsev AN, Hövener JB, Yurkovskaya AV, Ivanov KL. Quantitative description of the SABRE process: rigorous consideration of spin dynamics and chemical exchange. RSC Adv 2016. [DOI: 10.1039/c5ra28059a] [Citation(s) in RCA: 48] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
A consistent theoretical description of the spin dynamics and chemical kinetics underlying the SABRE (Signal Amplification By Reversible Exchange) process is proposed and validated experimentally.
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Affiliation(s)
| | - Andrey N. Pravdivtsev
- International Tomography Center
- Siberian Branch of the Russian Academy of Science
- 630090 Novosibirsk
- Russia
- Novosibirsk State University
| | | | - Alexandra V. Yurkovskaya
- International Tomography Center
- Siberian Branch of the Russian Academy of Science
- 630090 Novosibirsk
- Russia
- Novosibirsk State University
| | - Konstantin L. Ivanov
- International Tomography Center
- Siberian Branch of the Russian Academy of Science
- 630090 Novosibirsk
- Russia
- Novosibirsk State University
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8
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Pravdivtsev AN, Ivanov KL, Yurkovskaya AV, Petrov PA, Limbach HH, Kaptein R, Vieth HM. Spin polarization transfer mechanisms of SABRE: A magnetic field dependent study. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2015; 261:73-82. [PMID: 26529205 DOI: 10.1016/j.jmr.2015.10.006] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/17/2015] [Revised: 10/05/2015] [Accepted: 10/09/2015] [Indexed: 06/05/2023]
Abstract
We have investigated the magnetic field dependence of Signal Amplification By Reversible Exchange (SABRE) arising from binding of para-hydrogen (p-H2) and a substrate to a suitable transition metal complex. The magnetic field dependence of the amplification of the (1)H Nuclear Magnetic Resonance (NMR) signals of the released substrates and dihydrogen, and the transient transition metal dihydride species shows characteristic patterns, which is explained using the theory presented here. The generation of SABRE is most efficient at low magnetic fields due to coherent spin mixing at nuclear spin Level Anti-Crossings (LACs) in the SABRE complexes. We studied two Ir-complexes and have shown that the presence of a (31)P atom in the SABRE complex doubles the number of LACs and, consequently, the number of peaks in the SABRE field dependence. Interestingly, the polarization of SABRE substrates is always accompanied by the para-to-ortho conversion in dihydride species that results in enhancement of the NMR signal of free (H2) and catalyst-bound H2 (Ir-HH). The field dependences of hyperpolarized H2 and Ir-HH by means of SABRE are studied here, for the first time, in detail. The field dependences depend on the chemical shifts and coupling constants of Ir-HH, in which the polarization transfer takes place. A negative coupling constant of -7Hz between the two chemically equivalent but magnetically inequivalent hydride nuclei is determined, which indicates that Ir-HH is a dihydride with an HH distance larger than 2Å. Finally, the field dependence of SABRE at high fields as found earlier has been investigated and attributed to polarization transfer to the substrate by cross-relaxation. The present study provides further evidence for the key role of LACs in the formation of SABRE-derived polarization. Understanding the spin dynamics behind the SABRE method opens the way to optimizing its performance and overcoming the main limitation of NMR, its notoriously low sensitivity.
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Affiliation(s)
- Andrey N Pravdivtsev
- International Tomography Center SB RAS, Institutskaya 3a, Novosibirsk 630090, Russia; Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia
| | - Konstantin L Ivanov
- International Tomography Center SB RAS, Institutskaya 3a, Novosibirsk 630090, Russia; Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia.
| | - Alexandra V Yurkovskaya
- International Tomography Center SB RAS, Institutskaya 3a, Novosibirsk 630090, Russia; Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia
| | - Pavel A Petrov
- Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia; Nikolaev Institute of Inorganic Chemistry SB RAS, Acad. Lavrentiev Ave., 3, Novosibirsk 630090, Russia
| | - Hans-Heinrich Limbach
- Institut für Chemie und Biochemie, Freie Universität Berlin, Takustr. 3, Berlin 14195, Germany
| | - Robert Kaptein
- Utrecht University, Bijvoet Center, Padualaan 8, NL-3584 CH Utrecht, The Netherlands
| | - Hans-Martin Vieth
- International Tomography Center SB RAS, Institutskaya 3a, Novosibirsk 630090, Russia; Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, Berlin 14195, Germany
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9
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Salnikov OG, Burueva DB, Barskiy DA, Bukhtiyarova GA, Kovtunov KV, Koptyug IV. A Mechanistic Study of Thiophene Hydrodesulfurization by the Parahydrogen-Induced Polarization Technique. ChemCatChem 2015. [DOI: 10.1002/cctc.201500691] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Oleg G. Salnikov
- International Tomography Center SB RAS; Insitutskaya Street, 3 A 630090 Novosibirsk Russia
- Novosibirsk State University; Pirogova Street, 2 630090 Novosibirsk Russia
| | - Dudari B. Burueva
- International Tomography Center SB RAS; Insitutskaya Street, 3 A 630090 Novosibirsk Russia
- Novosibirsk State University; Pirogova Street, 2 630090 Novosibirsk Russia
| | - Danila A. Barskiy
- International Tomography Center SB RAS; Insitutskaya Street, 3 A 630090 Novosibirsk Russia
- Novosibirsk State University; Pirogova Street, 2 630090 Novosibirsk Russia
| | | | - Kirill V. Kovtunov
- International Tomography Center SB RAS; Insitutskaya Street, 3 A 630090 Novosibirsk Russia
- Novosibirsk State University; Pirogova Street, 2 630090 Novosibirsk Russia
| | - Igor V. Koptyug
- International Tomography Center SB RAS; Insitutskaya Street, 3 A 630090 Novosibirsk Russia
- Novosibirsk State University; Pirogova Street, 2 630090 Novosibirsk Russia
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10
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Buntkowsky G, Gutmann T, Petrova MV, Ivanov KL, Bommerich U, Plaumann M, Bernarding J. Dipolar induced para-hydrogen-induced polarization. SOLID STATE NUCLEAR MAGNETIC RESONANCE 2014; 63-64:20-29. [PMID: 25218522 DOI: 10.1016/j.ssnmr.2014.07.002] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/09/2014] [Revised: 07/17/2014] [Accepted: 07/28/2014] [Indexed: 06/03/2023]
Abstract
Analytical expressions for the signal enhancement in solid-state PHIP NMR spectroscopy mediated by homonuclear dipolar interactions and single pulse or spin-echo excitation are developed and simulated numerically. It is shown that an efficient enhancement of the proton NMR signal in solid-state NMR studies of chemisorbed hydrogen on surfaces is possible. Employing typical reaction efficacy, enhancement-factors of ca. 30-40 can be expected both under ALTADENA and under PASADENA conditions. This result has important consequences for the practical application of the method, since it potentially allows the design of an in-situ flow setup, where the para-hydrogen is adsorbed and desorbed from catalyst surfaces inside the NMR magnet.
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Affiliation(s)
- Gerd Buntkowsky
- Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Alarich-Weiss-Str. 8, D-64287 Darmstadt, Germany.
| | - Torsten Gutmann
- Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Alarich-Weiss-Str. 8, D-64287 Darmstadt, Germany
| | - Marina V Petrova
- International Tomography Center, Institutskaya 3a, Novosibirsk 630090, Russia
| | - Konstantin L Ivanov
- International Tomography Center, Institutskaya 3a, Novosibirsk 630090, Russia; Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia
| | - Ute Bommerich
- Leibniz-Institute for Neurobiology, Magdeburg, Germany
| | - Markus Plaumann
- Dept. of Biometry and Medical Informatics, Otto-von-Guericke University of Magdeburg, Leipziger Str. 44, 39120 Magdeburg, Germany
| | - Johannes Bernarding
- Dept. of Biometry and Medical Informatics, Otto-von-Guericke University of Magdeburg, Leipziger Str. 44, 39120 Magdeburg, Germany
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11
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Lego D, Plaumann M, Trantzschel T, Bargon J, Scheich H, Buntkowsky G, Gutmann T, Sauer G, Bernarding J, Bommerich U. Parahydrogen-induced polarization of carboxylic acids: a pilot study of valproic acid and related structures. NMR IN BIOMEDICINE 2014; 27:810-816. [PMID: 24812006 DOI: 10.1002/nbm.3123] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/02/2013] [Revised: 03/24/2014] [Accepted: 03/24/2014] [Indexed: 06/03/2023]
Abstract
Parahydrogen-induced polarization (PHIP) is a promising new tool for medical applications of MR, including MRI. The PHIP technique can be used to transfer high non-Boltzmann polarization, derived from parahydrogen, to isotopes with a low natural abundance or low gyromagnetic ratio (e.g. (13)C), thus improving the signal-to-noise ratio by several orders of magnitude. A few molecules acting as metabolic sensors have already been hyperpolarized with PHIP, but the direct hyperpolarization of drugs used to treat neurological disorders has not been accomplished until now. Here, we report on the first successful hyperpolarization of valproate (valproic acid, VPA), an important and commonly used antiepileptic drug. Hyperpolarization was confirmed by detecting the corresponding signal patterns in the (1)H NMR spectrum. To identify the optimal experimental conditions for the conversion of an appropriate VPA precursor, structurally related molecules with different side chains were analyzed in different solvents using various catalytic systems. The presented results include hyperpolarized (13)C NMR spectra and proton images of related systems, confirming their applicability for MR studies. PHIP-based polarization enhancement may provide a new MR technique to monitor the spatial distribution of valproate in brain tissue and to analyze metabolic pathways after valproate administration.
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Affiliation(s)
- Denise Lego
- Leibniz Institute for Neurobiology, Magdeburg, Germany
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12
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Nasibulov EA, Pravdivtsev AN, Yurkovskaya AV, Lukzen NN, Vieth HM, Ivanov KL. Analysis of Nutation Patterns in Fourier-Transform NMR of Non-Thermally Polarized Multispin Systems. Z PHYS CHEM 2013. [DOI: 10.1524/zpch.2013.0397] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Abstract
The complex spin order of hyperpolarized multispin systems giving rise to anomalous NMR spectral patterns that vary with the RF excitation angle is analyzed by decomposing its nutation behavior in a superposition of Fourier harmonics. The product operator formalism is applied to calculating the spectral contributions of the various mutual alignments of scalar coupled spins. Two cases are treated, namely systems exhibiting only differences in population of their spin states and systems showing in addition zero-quantum coherences between states, a situation often seen at hyperpolarization. After deriving the general solution a number of representative examples are discussed in detail. The theoretical treatment is applied to analyzing the spin order observed in a hyperpolarized two-spin system that is prepared in the singlet state by para-hydrogen induced polarization.
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Affiliation(s)
- Egor A. Nasibulov
- International Tomography Center SB RAS, Novosibirsk, 630090, Russische Föderation
| | | | | | - Nikita N. Lukzen
- International Tomography Center SB RAS, Novosibirsk, 630090, Russische Föderation
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13
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Gutmann T, del Rosal I, Chaudret B, Poteau R, Limbach HH, Buntkowsky G. From Molecular Complexes to Complex Metallic Nanostructures-2H Solid-State NMR Studies of Ruthenium-Containing Hydrogenation Catalysts. Chemphyschem 2013; 14:3026-33. [DOI: 10.1002/cphc.201300200] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2013] [Indexed: 11/08/2022]
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14
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Ivanov KL, Yurkovskaya AV, Vieth HM. Parahydrogen Induced Polarization in Scalar Coupled Systems: Analytical Solutions for Spectral Patterns and their Field Dependence. ACTA ACUST UNITED AC 2012. [DOI: 10.1524/zpch.2012.0269] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Abstract
We have obtained analytical solutions for Para-Hydrogen Induced Polarization (PHIP) for several types of coupled spin systems, namely, for AB-, ABX-, AA´A´´- and A2B-systems. Scalar spin-spin interactions were considered the factor, that determines the PHIP spectral pattern; it is the variation of the spin coupling regime (from strong coupling at low field to weak coupling at high field), which is responsible for the PHIP magnetic field dependence. The field dependence of polarization was considered in detail, general peculiarities of PHIP were found, PHIP patterns were calculated for the systems mentioned. Special attention was paid to the effects of field switching on PHIP.
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15
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Ratajczyk T, Gutmann T, Dillenberger S, Abdulhussaein S, Frydel J, Breitzke H, Bommerich U, Trantzschel T, Bernarding J, Magusin PCMM, Buntkowsky G. Time domain para hydrogen induced polarization. SOLID STATE NUCLEAR MAGNETIC RESONANCE 2012; 43-44:14-21. [PMID: 22365288 DOI: 10.1016/j.ssnmr.2012.02.002] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/02/2011] [Revised: 01/25/2012] [Accepted: 02/02/2012] [Indexed: 05/03/2023]
Abstract
Para hydrogen induced polarization (PHIP) is a powerful hyperpolarization technique, which increases the NMR sensitivity by several orders of magnitude. However the hyperpolarized signal is created as an anti-phase signal, which necessitates high magnetic field homogeneity and spectral resolution in the conventional PHIP schemes. This hampers the application of PHIP enhancement in many fields, as for example in food science, materials science or MRI, where low B(0)-fields or low B(0)-homogeneity do decrease spectral resolution, leading to potential extinction if in-phase and anti-phase hyperpolarization signals cannot be resolved. Herein, we demonstrate that the echo sequence (45°-τ-180°-τ) enables the acquisition of low resolution PHIP enhanced liquid state NMR signals of phenylpropiolic acid derivatives and phenylacetylene at a low cost low-resolution 0.54 T spectrometer. As low field TD-spectrometers are commonly used in industry or biomedicine for the relaxometry of oil-water mixtures, food, nano-particles, or other systems, we compare two variants of para-hydrogen induced polarization with data-evaluation in the time domain (TD-PHIP). In both TD-ALTADENA and the TD-PASADENA strong spin echoes could be detected under conditions when usually no anti-phase signals can be measured due to the lack of resolution. The results suggest that the time-domain detection of PHIP-enhanced signals opens up new application areas for low-field PHIP-hyperpolarization, such as non-invasive compound detection or new contrast agents and biomarkers in low-field Magnetic Resonance Imaging (MRI). Finally, solid-state NMR calculations are presented, which show that the solid echo (90y-τ-90x-τ) version of the TD-ALTADENA experiment is able to convert up to 10% of the PHIP signal into visible magnetization.
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Affiliation(s)
- Tomasz Ratajczyk
- Institute of Physical Chemistry, Technical University Darmstadt, Petersenstrasse 22, D-64287 Darmstadt, Germany
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16
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Trantzschel T, Bernarding J, Plaumann M, Lego D, Gutmann T, Ratajczyk T, Dillenberger S, Buntkowsky G, Bargon J, Bommerich U. Parahydrogen induced polarization in face of keto-enol tautomerism: proof of concept with hyperpolarized ethanol. Phys Chem Chem Phys 2012; 14:5601-4. [PMID: 22434387 DOI: 10.1039/c2cp40272f] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Hyperpolarization (HP) techniques are increasingly important in magnetic resonance imaging (MRI) and spectroscopy (MRS). HP methods have the potential to overcome the fundamentally low sensitivity of magnetic resonance (MR). A breakthrough of HP-MR in life sciences and medical applications is still limited by the small number of accessible, physiologically relevant substrates. Our study presents a new approach to extend PHIP to substrates that primarily cannot be hyperpolarized due to a steady intramolecular re-arrangement, the so-called keto-enol tautomerism. To overcome this obstacle we exploited the fact that instead of the instable enol form the corresponding stable ester can be used as a precursor molecule. This strategy now enables the hydrogenation which is required to apply the standard PHIP procedure. As the final step a hydrolysis is necessary to release the hyperpolarized target molecule. Using this new approach ethanol was successfully hyperpolarized for the first time. It may therefore be assumed that the outlined multi-step procedure can be used for other keto-enol tautomerized substances thereby opening the application of PHIP to a multitude of molecules relevant to analyzing metabolic pathways.
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Affiliation(s)
- Thomas Trantzschel
- Dept. for Biometry and Medical Informatics, Otto-von-Guericke-University Magdeburg, Germany.
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Gutmann T, Ratajczyk T, Dillenberger S, Xu Y, Grünberg A, Breitzke H, Bommerich U, Trantzschel T, Bernarding J, Buntkowsky G. New investigations of technical rhodium and iridium catalysts in homogeneous phase employing para-hydrogen induced polarization. SOLID STATE NUCLEAR MAGNETIC RESONANCE 2011; 40:88-90. [PMID: 21855300 DOI: 10.1016/j.ssnmr.2011.08.002] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/09/2011] [Revised: 07/30/2011] [Accepted: 08/01/2011] [Indexed: 05/31/2023]
Abstract
It is shown that the para-hydrogen induced polarization (PHIP) phenomenon in homogenous solution containing the substrate styrene is also observable employing simple inorganic systems of the form MCl(3)·xH(2)O (M=Rh, Ir) as catalyst. Such observation confirms that already very simple metal complexes enable the creation of PHIP signal enhancement in solution. This opens up new pathways to increase the sensitivity of NMR and MRT by PHIP enhancement using cost-effective catalysts and will be essential for further mechanistic studies of simple transition metal systems.
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Affiliation(s)
- Torsten Gutmann
- Laboratoire de Chimie de Coordination (LCC) CNRS, Toulouse, France
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Gutmann T, Ratajczyk T, Xu Y, Breitzke H, Grünberg A, Dillenberger S, Bommerich U, Trantzschel T, Bernarding J, Buntkowsky G. Understanding the leaching properties of heterogenized catalysts: a combined solid-state and PHIP NMR study. SOLID STATE NUCLEAR MAGNETIC RESONANCE 2010; 38:90-96. [PMID: 21435842 DOI: 10.1016/j.ssnmr.2011.03.001] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/21/2010] [Revised: 03/01/2011] [Accepted: 03/02/2011] [Indexed: 05/30/2023]
Abstract
Para-hydrogen induced polarization (PHIP) NMR in solution, combined with solid-state NMR, can be efficiently employed for the highly sensitive in-situ detection of the leaching properties of immobilized catalysts. The knowledge of this property is important for possible applications of PHIP experiments in medicine, biology or industry, where leached catalysts poison the solution of hyperpolarized products. As experimental example Wilkinson's catalyst RhCl(PPh(3))(3) (1) immobilized on mesoporous silica is chosen. As model reaction the hydrogenation of styrene in solvents with different polarities (methanol-d(4), acetone-d(6) and benzene-d(6)) is used. A (31)P solid-state MAS-NMR study reveals that there are two different species of catalysts on the silica, namely coordinatively bound catalysts and physisorbed catalyst. Only the second species exhibits substantial leaching, which is visible in a strong PHIP enhancement of the reaction product.
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Affiliation(s)
- Torsten Gutmann
- Institute of Physical Chemistry, Technical University Darmstadt, Petersenstrasse 22, D-64287 Darmstadt, Germany
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Hübler P, Natterer J, Bargon J. Indirect characterisation of hydrogenation intermediates using PASADENA NMR spectroscopy - evolution of zero-quantum coherence in AB spin systems. ACTA ACUST UNITED AC 2010. [DOI: 10.1002/bbpc.19981020313] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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20
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Buntkowsky G, Limbach HH, Walaszek B, Adamczyk A, Xu Y, Breitzke H, Schweitzer A, Gutmann T, Wächtler M, Amadeu N, Tietze D, Chaudret B. Mechanisms of Dipolar Ortho/Para-H2O Conversion in Ice. Z PHYS CHEM 2009. [DOI: 10.1524/zpch.2008.5359] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Abstract
In this paper a possible explanation for an unexpected ortho/para-water ratio in the gas clouds of comets is given. The description is based on the quantum-mechanical density matrix formalism and the spin temperature concept. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a four spin system, created by two nearest neighbour water molecules, spin eigenstates and their dynamics under the influence of their mutual dipolar interactions are studied. It is shown that a fast conversion between ortho- and para-states occurs on a msec time scale, caused by the intermolecular homonuclear magnetic dipolar interaction. Moreover the spin eigenstates of water in an ice crystal are determined by magnetic dipolar interactions and are not given by normal ortho- and para-H2O states of gaseous water. As a result of this the spin temperature of gaseous water evaporated from ice depends strongly on its evaporation history and the ortho/para-ratio of water molecules are only an indirect measure of the temperature of ice crystals from where they descend. This result could explain the unexpected experimentally observed ortho/para-ratios in the clouds of comets.
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Affiliation(s)
| | | | | | | | | | | | | | | | | | | | | | - Bruno Chaudret
- Laboratoire de Chimie de Doordination du CNRS, Toulouse Cedex 04, Frankreich
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Bouguet-Bonnet S, Reineri F, Canet D. Effect of the static magnetic field strength on parahydrogen induced polarization NMR spectra. J Chem Phys 2009; 130:234507. [DOI: 10.1063/1.3152843] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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22
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Gutmann T, Sellin M, Breitzke H, Stark A, Buntkowsky G. Para-hydrogen induced polarization in homogeneous phase—an example of how ionic liquids affect homogenization and thus activation of catalysts. Phys Chem Chem Phys 2009; 11:9170-5. [DOI: 10.1039/b908198d] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Duckett SB, Wood NJ. Parahydrogen-based NMR methods as a mechanistic probe in inorganic chemistry. Coord Chem Rev 2008. [DOI: 10.1016/j.ccr.2008.01.028] [Citation(s) in RCA: 103] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
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Ivanov KL, Miesel K, Vieth HM, Yurkovskaya AV, Sagdeev RZ. 2D NMR Nutation Analysis of Non-Thermal Polarization of Coupled Multi-Spin Systems. ACTA ACUST UNITED AC 2008. [DOI: 10.1524/zpch.217.12.1641.20479] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Abstract
A new, convenient method of analyzing the spin polarization of a non-equilibrium system of N coupled nuclei is described and applied to photo-reactions exhibiting chemically induced dynamic nuclear polarization (CIDNP). It is based on the Fourier analysis of the variation of NMR line intensities as a function of the radio frequency excitation pulse length. A relationship between the spectral components at various harmonic order and the alignment in the spin multiplet is established. In application to the Norrish type I photolysis of cyclodecanone we demonstrate that at low magnetic field the rate determining step in the reaction kinetics depends on the mutual orientation of at least four pairs of non-equivalent spins.
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Limbach HH, Buntkowsky G, Matthes J, Gründemann S, Pery T, Walaszek B, Chaudret B. Novel Insights into the Mechanism of the Ortho/Para Spin Conversion of Hydrogen Pairs: Implications for Catalysis and Interstellar Water. Chemphyschem 2006; 7:551-4. [PMID: 16493700 DOI: 10.1002/cphc.200500559] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
The phenomenon of exchange coupling is taken into account in the description of the magnetic nuclear spin conversion between bound ortho- and para-dihydrogen. This conversion occurs without bond breaking, in contrast to the chemical spin conversion. It is shown that the exchange coupling needs to be reduced so that the corresponding exchange barrier can increase and the given magnetic interaction can effectively induce a spin conversion. The implications for related molecules such as water are discussed. For ice, a dipolar magnetic conversion and for liquid water a chemical conversion are predicted to occur within the millisecond timescale. It follows that a separation of water into its spin isomers, as proposed by Tikhonov and Volkov (Science 2002, 296, 2363), is not feasible. Nuclear spin temperatures of water vapor in comets, which are smaller than the gas-phase equilibrium temperatures, are proposed to be diagnostic for the temperature of the ice or the dust surface from which the water was released.
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Affiliation(s)
- Hans-Heinrich Limbach
- Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, 14195 Berlin, Germany.
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Buntkowsky G, Walaszek B, Adamczyk A, Xu Y, Limbach HH, Chaudret B. Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion. Phys Chem Chem Phys 2006; 8:1929-35. [PMID: 16633680 DOI: 10.1039/b601594h] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
In this paper a quantitative explanation for a diamagnetic ortho/para H2 conversion is given. The description is based on the quantum-mechanical density matrix formalism originally developed by Alexander and Binsch for studies of exchange processes in NMR spectra. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a three spin system, the reactions of the hydrogen gas with the catalysts are treated as a phenomenological rate process, described by a rate constant. Numerical calculations reveal that for nearly all possible geometrical arrangements of the three spin system an efficient spin conversion is obtained. Only in the chemically improbable case of a linear group H-X-H no spin conversion is obtained. The efficiency of the spin conversion depends strongly on the lifetime of the H-X-H complex and on the presence of exchange interactions between the two hydrogens. Even moderate exchange couplings cause a quench of the spin conversion. Thus a sufficiently strong binding of the dihydrogen to the S spin is necessary to render the quenching by the exchange interaction ineffective.
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Affiliation(s)
- G Buntkowsky
- FSU Jena, Institut für Physikalische Chemie, Helmholtzweg 4, 07743, Jena, Germany.
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Matthes J, Pery T, Gründemann S, Buntkowsky G, Sabo-Etienne S, Chaudret B, Limbach HH. Bridging the gap between homogeneous and heterogeneous catalysis: ortho/para H(2) conversion, hydrogen isotope scrambling, and hydrogenation of olefins by Ir(CO)Cl(PPh(3))(2). J Am Chem Soc 2004; 126:8366-7. [PMID: 15237979 DOI: 10.1021/ja0475961] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Some transition metal complexes are known to catalyze ortho/para hydrogen conversion, hydrogen isotope scrambling, and hydrogenation reactions in liquid solution. Using the example of Vaska's complex, we present here evidence by NMR that the solvent is not necessary for these reactions to occur. Thus, solid frozen solutions or polycrystalline powdered samples of homogeneous catalysts may become heterogeneous catalysts. Comparative liquid- and solid-state studies provide novel insight into the reaction mechanisms.
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Affiliation(s)
- Jochen Matthes
- Institute of Chemistry, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany
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Hübler P, Giernoth R, Kümmerle G, Bargon J. Investigating the Kinetics of Homogeneous Hydrogenation Reactions Using PHIP NMR Spectroscopy. J Am Chem Soc 1999. [DOI: 10.1021/ja984353y] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Patrick Hübler
- Contribution from the Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany
| | - Ralf Giernoth
- Contribution from the Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany
| | - Günther Kümmerle
- Contribution from the Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany
| | - Joachim Bargon
- Contribution from the Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany
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29
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Limbach HH, Ulrich S, Gründemann S, Buntkowsky G, Sabo-Etienne S, Chaudret B, Kubas GJ, Eckert J. NMR and INS Line Shapes of Transition Metal Hydrides in the Presence of Coherent and Incoherent Dihydrogen Exchange. J Am Chem Soc 1998. [DOI: 10.1021/ja962903r] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Hans-Heinrich Limbach
- Contribution from the Institut für Organische Chemie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, the Laboratoire de Chimie de Coordination du CNRS (UP 8241), 205, route de Narbonne, F-31077 Toulouse-Cedex, France, and LANSCE, Los Alamos National Laboratory, Los Alamos, Mail Stop H805, New Mexico 87545
| | - Stefan Ulrich
- Contribution from the Institut für Organische Chemie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, the Laboratoire de Chimie de Coordination du CNRS (UP 8241), 205, route de Narbonne, F-31077 Toulouse-Cedex, France, and LANSCE, Los Alamos National Laboratory, Los Alamos, Mail Stop H805, New Mexico 87545
| | - Stephan Gründemann
- Contribution from the Institut für Organische Chemie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, the Laboratoire de Chimie de Coordination du CNRS (UP 8241), 205, route de Narbonne, F-31077 Toulouse-Cedex, France, and LANSCE, Los Alamos National Laboratory, Los Alamos, Mail Stop H805, New Mexico 87545
| | - Gerd Buntkowsky
- Contribution from the Institut für Organische Chemie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, the Laboratoire de Chimie de Coordination du CNRS (UP 8241), 205, route de Narbonne, F-31077 Toulouse-Cedex, France, and LANSCE, Los Alamos National Laboratory, Los Alamos, Mail Stop H805, New Mexico 87545
| | - Sylviane Sabo-Etienne
- Contribution from the Institut für Organische Chemie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, the Laboratoire de Chimie de Coordination du CNRS (UP 8241), 205, route de Narbonne, F-31077 Toulouse-Cedex, France, and LANSCE, Los Alamos National Laboratory, Los Alamos, Mail Stop H805, New Mexico 87545
| | - Bruno Chaudret
- Contribution from the Institut für Organische Chemie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, the Laboratoire de Chimie de Coordination du CNRS (UP 8241), 205, route de Narbonne, F-31077 Toulouse-Cedex, France, and LANSCE, Los Alamos National Laboratory, Los Alamos, Mail Stop H805, New Mexico 87545
| | - Gregory J. Kubas
- Contribution from the Institut für Organische Chemie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, the Laboratoire de Chimie de Coordination du CNRS (UP 8241), 205, route de Narbonne, F-31077 Toulouse-Cedex, France, and LANSCE, Los Alamos National Laboratory, Los Alamos, Mail Stop H805, New Mexico 87545
| | - Jürgen Eckert
- Contribution from the Institut für Organische Chemie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, the Laboratoire de Chimie de Coordination du CNRS (UP 8241), 205, route de Narbonne, F-31077 Toulouse-Cedex, France, and LANSCE, Los Alamos National Laboratory, Los Alamos, Mail Stop H805, New Mexico 87545
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31
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Bowers CR, Weitekamp DP. Nuclear magnetic resonance by measuring reaction yield of spin symmetry species. SOLID STATE NUCLEAR MAGNETIC RESONANCE 1998; 11:123-128. [PMID: 9650795 DOI: 10.1016/s0926-2040(97)00101-x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
It is proposed that the nuclear magnetic resonance of sites which release dihydrogen can be obtained by measuring the branching fraction to the ortho and para forms. The motivation is to transform the sensitivity problem from that of detecting magnetization into the more tractable one of establishing the para and ortho content of free H2. It is shown with a density operator formalism that the para mole fraction reports directly on the zero-quantum coherence of the precursor and that other spin operators may be observed indirectly. Spectra are simulated for the case of a surface site from which H2 is released.
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Affiliation(s)
- C R Bowers
- Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena 91125, USA
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32
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Buntkowsky G, Limbach HH, Wehrmann F, Sack I, Vieth HM, Morris RH. 2H NMR Theory of Transition Metal Dihydrides: Coherent and Incoherent Quantum Dynamics. J Phys Chem A 1997. [DOI: 10.1021/jp970103c] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Gerd Buntkowsky
- Institut für Organische Chemie and Institut für Experimentalphysik der Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
| | - Hans-Heinrich Limbach
- Institut für Organische Chemie and Institut für Experimentalphysik der Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
| | - Frank Wehrmann
- Institut für Organische Chemie and Institut für Experimentalphysik der Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
| | - Ingolf Sack
- Institut für Organische Chemie and Institut für Experimentalphysik der Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
| | - Hans-Martin Vieth
- Institut für Organische Chemie and Institut für Experimentalphysik der Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
| | - Robert H. Morris
- Institut für Organische Chemie and Institut für Experimentalphysik der Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
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