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  • 1
    Online Resource
    Online Resource
    Coherent transfer of nuclear spin polarization in field-cycling NMR experiments
    AIP Publishing ; 2013
    In:  The Journal of Chemical Physics Vol. 139, No. 24 ( 2013-12-28)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 139, No. 24 ( 2013-12-28)
    Abstract: 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.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.4848699
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2013
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 2
    Online Resource
    Online Resource
    High resolution NMR study of T1 magnetic relaxation dispersion. IV. Proton relaxation in amino acids and Met-enkephalin pentapeptide
    AIP Publishing ; 2014
    In:  The Journal of Chemical Physics Vol. 141, No. 15 ( 2014-10-21)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 141, No. 15 ( 2014-10-21)
    Abstract: Nuclear Magnetic Relaxation Dispersion (NMRD) of protons was studied in the pentapeptide Met-enkephalin and the amino acids, which constitute it. Experiments were run by using high-resolution Nuclear Magnetic Resonance (NMR) in combination with fast field-cycling, thus enabling measuring NMRD curves for all individual protons. As in earlier works, Papers I–III, pronounced effects of intramolecular scalar spin-spin interactions, J-couplings, on spin relaxation were found. Notably, at low fields J-couplings tend to equalize the apparent relaxation rates within networks of coupled protons. In Met-enkephalin, in contrast to the free amino acids, there is a sharp increase in the proton T1-relaxation times at high fields due to the changes in the regime of molecular motion. The experimental data are in good agreement with theory. From modelling the relaxation experiments we were able to determine motional correlation times of different residues in Met-enkephalin with atomic resolution. This allows us to draw conclusions about preferential conformation of the pentapeptide in solution, which is also in agreement with data from two-dimensional NMR experiments (rotating frame Overhauser effect spectroscopy). Altogether, our study demonstrates that high-resolution NMR studies of magnetic field-dependent relaxation allow one to probe molecular mobility in biomolecules with atomic resolution.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.4897336
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2014
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 3
    Online Resource
    Online Resource
    Exploiting adiabatically switched RF-field for manipulating spin hyperpolarization induced by para hydrogen
    AIP Publishing ; 2015
    In:  The Journal of Chemical Physics Vol. 143, No. 23 ( 2015-12-21)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 143, No. 23 ( 2015-12-21)
    Abstract: 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.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.4937392
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2015
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 4
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    Online Resource
    Magnetic field and orientation dependence of solid-state CIDNP
    AIP Publishing ; 2019
    In:  The Journal of Chemical Physics Vol. 150, No. 9 ( 2019-03-07)
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    In: The Journal of Chemical Physics, AIP Publishing, Vol. 150, No. 9 ( 2019-03-07)
    Abstract: The magnetic field dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) in solid-state systems is analyzed theoretically with the aim to explain the puzzling sign change of polarization found at low fields [D. Gräsing et al., Sci. Rep. 7, 12111 (2017)]. We exploit the analysis of polarization in terms of level crossings and level anti-crossings trying to identify the positions of features in the CIDNP field dependence with specific crossings between spin energy levels of the radical pair. Theoretical treatment of solid-state CIDNP reveals a strong orientation dependence of polarization due to the spin dynamics conditioned by anisotropic spin interactions. Specifically, different anisotropic CIDNP mechanisms become active at different magnetic fields and different molecular orientations. Consequently, the field dependence and orientation dependence of polarization need to be analyzed together in order to rationalize experimental observations. By considering both magnetic field and orientation dependence of CIDNP, we are able to explain the previously measured CIDNP field dependence in photosynthetic reaction centers and to obtain a good qualitative agreement between the experimental observations and theoretical results.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.5077078
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2019
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 5
    Online Resource
    Online Resource
    Present status of the 2.45 GHz ECRIS for a light ion accelerator at IAP
    AIP Publishing ; 1996
    In:  Review of Scientific Instruments Vol. 67, No. 3 ( 1996-03-01), p. 1183-1185
    Details
    In: Review of Scientific Instruments, AIP Publishing, Vol. 67, No. 3 ( 1996-03-01), p. 1183-1185
    Abstract: This article presents a 2.45 GHz electron cyclotron resonance ion source which is under construction at the Institute of Atomic Physics Bucharest, Romania. It will be used as an injector into an existing linac for light ions and is a compact version of a similar source, for which we have obtained the magnetic system by superimposing an axial magnetic field generated by two solenoids and a radial magnetic field generated by a permanent magnet hexapole. For the compact version we have employed two rings made of permanent magnets for the axial magnetic field and the same type of hexapole for the radial field. This second version will work on a high voltage platform. The calculation, design, and construction of the two minimum magnetic field structures (B minimum) are presented. These sources have a special microwave antenna, of a helix type, allowing a much smaller diameter plasma chamber than is usually used for the employed microwaves. The microwave system is also presented. The first experiments were made with an Ar plasma.
    Type of Medium: Online Resource
    ISSN: 0034-6748 , 1089-7623
    URL: Article
    DOI: 10.1063/1.1146726
    Language: English
    Publisher: AIP Publishing
    Publication Date: 1996
    detail.hit.zdb_id: 209865-9
    detail.hit.zdb_id: 1472905-2
    SSG: 11
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  • 6
    Online Resource
    Online Resource
    High resolution NMR study of T1 magnetic relaxation dispersion. II. Influence of spin-spin couplings on the longitudinal spin relaxation dispersion in multispin systems
    AIP Publishing ; 2010
    In:  The Journal of Chemical Physics Vol. 133, No. 19 ( 2010-11-21)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 133, No. 19 ( 2010-11-21)
    Abstract: Effects of scalar spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of coupled multispin systems were analyzed. Taking spin systems of increasing complexity we demonstrated pronounced influence of the intramolecular spin-spin couplings on the NMRD of protons. First, at low magnetic fields where there is strong coupling of spins the apparent relaxation times of the coupled spins become equal. Second, there are new features, which appear at the positions of the nuclear spin level anticrossings. Finally, in coupled spin systems there can be a coherent contribution to the relaxation kinetics present at low magnetic fields. All these peculiarities caused by spin-spin interactions are superimposed on the features in NMRD, which are conditioned by changes of the motional regime. Neglecting the effects of couplings may lead to misinterpretation of the NMRD curves and significant errors in determining the correlation times of molecular motion. Experimental results presented are in good agreement with theoretical calculations.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.3495988
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2010
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 7
    Online Resource
    Online Resource
    Coherent manipulation of non-thermal spin order in optical nuclear polarization experiments
    AIP Publishing ; 2017
    In:  The Journal of Chemical Physics Vol. 146, No. 11 ( 2017-03-21)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 146, No. 11 ( 2017-03-21)
    Abstract: Time resolved measurements of Optical Nuclear Polarization (ONP) have been performed on hyperpolarized triplet states in molecular crystals created by light excitation. Transfer of the initial electron polarization to nuclear spins has been studied in the presence of radiofrequency excitation; the experiments have been performed with different pulse sequences using different doped molecular systems. The experimental results clearly demonstrate the dominant role of coherent mechanisms of spin order transfer, which manifest themselves in well pronounced oscillations. These oscillations are of two types, precessions and nutations, having characteristic frequencies, which are the same for the different molecular systems and the pulse sequences applied. Hence, precessions and nutations constitute a general feature of polarization transfer in ONP experiments. In general, coherent manipulation of spin order transfer creates a powerful resource for improving the performance of the ONP method, which paves the way to strong signal enhancement in nuclear magnetic resonance.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.4976990
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2017
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 8
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    Online Resource
    Level crossing analysis of chemically induced dynamic nuclear polarization: Towards a common description of liquid-state and solid-state cases
    AIP Publishing ; 2016
    In:  The Journal of Chemical Physics Vol. 144, No. 14 ( 2016-04-14)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 144, No. 14 ( 2016-04-14)
    Abstract: 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.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.4945341
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2016
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 9
    Online Resource
    Online Resource
    Coherent transfer of hyperpolarization in coupled spin systems at variable magnetic field
    AIP Publishing ; 2008
    In:  The Journal of Chemical Physics Vol. 128, No. 15 ( 2008-04-21)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 128, No. 15 ( 2008-04-21)
    Abstract: 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.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.2901019
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2008
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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  • 10
    Online Resource
    Online Resource
    High resolution NMR study of T1 magnetic relaxation dispersion. III. Influence of spin 1/2 hetero-nuclei on spin relaxation and polarization transfer among strongly coupled protons
    AIP Publishing ; 2012
    In:  The Journal of Chemical Physics Vol. 137, No. 9 ( 2012-09-07)
    Details
    In: The Journal of Chemical Physics, AIP Publishing, Vol. 137, No. 9 ( 2012-09-07)
    Abstract: 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 19F 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.
    Type of Medium: Online Resource
    ISSN: 0021-9606 , 1089-7690
    URL: Article
    DOI: 10.1063/1.4746780
    Language: English
    Publisher: AIP Publishing
    Publication Date: 2012
    detail.hit.zdb_id: 3113-6
    detail.hit.zdb_id: 1473050-9
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